Hydrogen is now confirmed as a key component of a CO₂-neutral economy, we need to transition towards. The production of large quantities of hydrogen now requires breakthroughs in finding new catalysts that are efficient, stable and cheap, i.e. based on abundant elements. Indeed fuel formation involves multi-electron multi-proton reactions that are inherently kinetically sluggish. Efficient catalysts can be found in living micro-organisms producing or metabolizing hydrogen thanks to hydrogenases. Catalysis in these enzymes only requires Earth-abundant metal centers, the reactivity of which is enhanced thanks to the presence of basic sites acting as proton relays [1] at their vicinity. Such active sites have been used as an inspiration to design new synthetic catalysts for H₂ evolution [2-4] and oxidation [5-6]. Specification, catalytic platforms with installed proton relays display bidirectional [7] and, in rare cases, reversible catalysis [5]. In this presentation we will show how a detailed molecular electrochemistry study can help understanding and quantifying the role of the protons relays related to these remarkable behaviors.

Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Initially, research on hydrogenation reactions focused on the use of noble metal complexes based on Ru, Rh, Ir, Pd, Pt and remarkable achievements have been made using such complexes. However, recently the focus shifted towards the use of base metal catalysts, as these metals exhibit a higher availability in earth’s crust.[1] In the first instance, the direct reduction of C=O bonds with H₂ seems to be a very atom and redox economic approach. However, the reaction protocols for base-metal catalyzed hydrogenation reactions require often harsh reaction conditions and high H2 pressure to activate H2 and each ton of H2 that is formed via steam reforming produces about 12 tons of CO₂.[2]

I will present an electrochemical approach utilizing electrons and protons for the catalytic hydrogenation of polar double bonds, such as C=O in CO₂, ketones, and aldehydes.[3] In a proof of principle study we demonstrated that in-situ generated Mn hydride species hydrogenate CO₂ forming formic acid. This protocol was later expanded to aromatic, and aliphatic ketones as well as aldehydes. The method is selective for C=O bonds over the thermodynamically favored hydrogenation of C=C bonds. The competing hydrogen evolution reaction was suppressed successfully, and the reactions proceed with high Faraday efficiencies. A large variety of substrates was accessible by this method and in-depth mechanistic studies gave valuable insights for catalyst improvement.

With a two million ton global production hydrogen peroxide is a valuable product.^[1] Yet it is produced via a non-catalytic and energy intensive anthraquinone process rather than via a sustainable electrochemical procedure.

We have shown that Cu-tmpa can produce hydrogen peroxide upon electrochemical reduction of dioxygen at rates of 1.8 x 10⁶ s^–1  and in significant amounts providing that no mass transport limitations in O₂ occur.^[2] When H₂O₂ starts to accumulate and O₂ concentrations drop overreduction of peroxide to water becomes significant.

We have shown that the rate determining step for O₂ reduction is binding of dioxygen to Cu(I), in line with studies by the Karlin group,^[3] while reduction of H₂O₂ appears to proceed via Fenton-like chemistry, showing similarities with lytic polysaccharide monooxygenase chemistry.^[4] Despite that strongly oxidative reactive oxygen species are likely to be involved turnover numbers exceed 1000 without any sign of catalyst deactivation.

Detailed mechanistic studies, structure-correlation studies, comparisons with Cu-monooxygenase chemistry, DFT calculations, and experiments to boost the hydrogen peroxide production employing flow cell chemistry and gas diffusion electrodes are in progress.

In the early studies, on Oxygenic Photosynthesis, the “quantasome hypothesis” led to seminal discoveries correlating the structure of natural photosystems with their complementary photo-redox functions.[1,2] Indeed, and despite the vast bio-diversity footprint, just one protein complex is used by Nature as the H₂O-photolyzer: photosystem II (PSII). Man-made systems are still far from replicating the complexity of PSII, showing the ideal co-localization of Light Harvesting antennas with the functional Reaction Center (LH-RC). Here we report the design of multi-perylenebisimide (PBI) networks shaped to function by interaction with a polyoxometalate water oxidation catalyst (Ru₄POM).[3-5] Our results point to overcome the classical “photo-dyad” model, based on a donor-acceptor binary combination, with integrated artificial “quantasomes” formed both in solution and on photoelectrodes, showing a: (i) red-shifted, light harvesting efficiency (LHE>40%), (ii) favorable exciton accumulation and negligible excimeric loss; (iii) a robust amphiphilic structure; (iv) dynamic aggregation into large 2D-paracrystalline domains.[5] Photoexcitation of the PBI-quantasome triggers one of the highest driving force for photo-induced electron transfer applied so far.[5-7]

[1] Scheuring, S., Sturgis, J. N. Chromatic Adaptation of Photosynthetic Membranes. Science 309, 484–487 (2005);

[2] Sartorel, A., Carraro, M., Toma, F. M., Prato, M., Bonchio, M. Shaping the beating heart of artificial photosynthesis: oxygenic metal oxide nano-clusters. Energy Environ. Sci. 5, 5592 (2012);

[3] Piccinin, S.; Sartorel, A.; Aquilanti, G.; Goldoni, A.; Bonchio, M.; Fabris, S. Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium-oxo molecular complex. Proc. Natl. Acad. Sci. 110, 4917–4922 (2013)

[4] Toma, F. M.; Prato, M.; Bonchio, M. et al. Efficient water oxidation at carbon nanotube–polyoxometalate electrocatalytic interfaces. Nature Chemistry 2, 826-831 (2010).

[5] Bonchio, M.; Sartorel, A.; Prato, M. et al. Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation. Nature Chemistry 11, 146-153 (2019).

[6] Gobbo, P.; Bonchio, M.; Mann, S. et al. Catalytic processing in ruthenium-based polyoxometalate coacervate protocells Nature Commun 11, 41 (2020).

[7] GobbatoT.; Rigodanza F.; Benazzi, E.; Prato, M.; Bonchio M. et al. J. Am. Chem. Soc.144,14021-14025 (2022).

Tailoring and understanding the structure of the electrochemical interface at the atomic level is key to elucidating the design principles for the discovery of new electrocatalysts for renewable energy conversion. This talk will focus on our recent work on new catalyst (nano)materials and engineered interfaces for electrochemical energy conversion and storage. In particular, I will discuss structure sensitivity and electrolyte effects for oxygen and carbon dioxide/carbon monoxide electrocatalysis.

First, I will present our work toward understanding and tuning the structure-activity-stability relations for oxygen reduction [1] and oxygen evolution [2], for applications in fuel cells and water electrolysers, respectively. Then, I will discuss our model studies on Cu-based surfaces to understand structure-properties relations for CO₂ reduction [3-4]. We have investigated the effect of pH, specific anion adsorption, and potential dependence for CO reduction on Cu single crystals. We show how well-defined studies are essential to design efficient electrocatalysts to produce renewable fuels.

Catalysis is key in the future development of green chemistry and in the renewable production of chemicals and fuels. However, this transition requires a new approach to catalysis, as central chemical reactions cannot be catalyzed with existing materials. The grand challenge is to discover new catalyst materials, which are both stable and active. We identify three energy conversion reactions, which are corner stones for the green transition and in urgent need of new catalysts.

High Entropy Alloys are solid solutions where five or more elements are mixed randomly together. The realization of HEAs has opened for a vast composition space with a practically infinite number of new not yet explored catalyst materials. We can tune their properties by smoothly change their composition. This has led to the statement that: HEA is a shift of paradigm “from using the materials we have, to engineer the materials we need”. The hypothesis is that among the HEAs there are catalysts with superior stability and activity for the important green reactions.

will present the challenges regarding simulations and prediction of the relation between surface structure and catalytic activity on HEA surfaces. Further, I will show studies where the flexibility of the vast chemical space of HEAs is utilized to predict new catalysts.

Hydrogen and other solar fuels have been highlighted as one of the future energy vectors. Having natural photosynthesis as inspiration, we can develop a device capable to split water using sunlight, obtaining oxygen and hydrogen. [1], [2] Different strategies can be used to achieve this: from separated light harvesting and catalytic systems to all integrated devices able to transform directly sunlight into fuels. In both approaches the catalyst is pivotal to improve the system efficiency. Although rapid progress is being made in the field, understanding of the limiting factors of these catalysts has allowed remarkable improvements in their performance.

In this talk I will discuss different approaches to modify the activity of metal/metal oxide nanoparticles towards redox catalytic processes for water splitting. Nanoparticles are excellent catalysts due to their high surface/volume ratio and electronic and chemical surface properties.[3],[4] Consequently, several strategies can be used to modulate the activity of NP towards catalytic processes. In this talk I will discuss how doping, NP size and surface functionalization effects the catalytic activity of Ni/NiOOH NPs for oxygen evolution in the first case and Ru NP for hydrogen in the other two.[5] I will discuss these effects in terms of electrocatalytic activity and thoughtful characterisation of the system: spectroelectrochemical experiments, XPS, TEM, SEM, TOF-Sims, ATP, etc. All this acquired knowledge will help to systematically improve the next generation of catalysts for water splitting.

The construction of carbonyl compounds via carbonylation reaction using a safe CO source remains a long-standing challenge to synthetic chemists.[1] In this work, we explored the electrosynthetic approach using CO₂ as a CO surrogate in the carbonylation of benzyl chlorides, utilizing a combination of two earth-abundant metal catalysts. As proof of concept, the protocols allow the synthesis of symmetric ketones from good to excellent yields in an undivided cell with non-sacrificial electrodes. The mechanistic studies suggested a synergistic effect between the two metal complexes. Based on the mechanistic studies, we propose the coupling of the electrocatalytic CO formed from CO₂ reduction with the benzyl chlorides activated by the Ni-catalyzed. Our approach that has proven more challenging is the use of an undivided cell to perform tandem CO₂ reduction to CO, followed by CO utilization in carbonylation reactions. While this approach has the potential to offer several benefits such as simple experimental setups, straightforward synthetic protocols, direct utilization of the CO formed, decrease of the employed toxic CO, and facilitation of the formation of labeled compounds. This is complex due to the need to balance all required catalytic cycles. Few methods used CO₂ as a CO source because of its inertness but also due to its inherent reactivity that delivers preferably carboxylation products. In this line, Skrydstrup, Daasbjerg and co-workers developed a set of classical Pd-catalyzed carbonylation reactions using the CO evolved from the electrochemical CO₂ reduction in a two-compartment setup.[2] Before that, Perichon and co-workers reported a stepwise electrochemical/chemical method for the stoichiometric synthesis of ketones.[3]

Porphyrins have been actively investigated as oxygen evolution reaction (OER) catalysts, yet only a few studies refer to their integration into electrode devices as heterogeneous catalysts [1-2], mainly due to processability constraints. Moreover, the catalytic activity of porphyrin films can be substantially improved by their modification in the form of conjugated polymers. The enhanced conductivity of metalloporphyrin conjugated polymers, resulting from the extended delocalization of π-electrons in the conjugated system, may promote a faster charge transfer to the active sites, improving the catalytic performance. Recently, our group developed a methodology allowing the straightforward preparation of fused metalloporphyrins conjugated polymers thin films over a wide variety of substrates, based on the oxidative chemical vapour deposition (oCVD) [3,4]. In this way, a new venue for the study of the electrocatalytic activity of fused metalloporphyrin conjugated polymers bearing different metal centres and substituents was opened. Consequently, we have investigated the electrocatalytic activity of Ni(II) and Co(II) fused porphyrins conjugated polymers on FTO substrates towards the OER. We found out that there is a significant role of the direct fusion reaction to form organized electrocatalysts able to operate at lower overpotentials than the monomeric counterparts, resulting in enhanced electrocatalytic performance. Nonetheless, previous reports on Ni and Co complexes, including porphyrins, have demonstrated the decomposition of the molecular matrix to form a material thin film[2,5], possibly a metal oxide specie, acting as the true catalyst for water oxidation at the electrode surface. Therefore, we have conducted a deep-through investigation of the possible conversion of fused metalloporphyrin polymers into a material catalyst, using the combination of structural and compositional analysis with theoretical studies. We pay special attention to the role of polymerization and the presence of substituents of different natures on the transformation kinetics and resulting electrocatalytic activity.

Electrochemical water oxidation (2e^- WOR) to hydrogen peroxide (H₂O₂) is a topic of growing interest. Still material development, mechanistic understanding and process conditions are yet to be defined to allow for efficient and durable hydrogen peroxide production at industrially-relevant current densities [1]. Carbonate ions (CO₃²⁻) are suggested to enable indirect water oxidation to H₂O₂ and alkali metal cations (Li⁺, Na⁺, and K⁺) are known to influence electrochemical activity of various electrochemical reactions including the oxygen evolution reaction [2,3]. Also, in electrocatalysis the electrolyte composition is known to influence electrode stability. Material stability in the context of anodic H₂O₂ production is barely addressed in recent literature. Moreover, the influence of the electrolyte used for selective electrochemical water oxidation is yet to be fully disclosed [4].

Herein, we discuss the effect of different electrolytes (K₂CO₃ and Na₂CO₃) on the H₂O₂ production and electrode stability using commercially available fluorine-doped tin oxide (FTO) anodes frequently used as active material and support. The determined faradaic efficiencies (FE) suggest that the use of K₂CO₃ over Na₂CO₃ electrolyte (FE_H2O2 = 34.8 % in K₂CO₃ and FE_H2O2 = 25.3 % in Na₂CO₃ at 5 mA/cm²) is preferred. We observe a maximum production rates of 0.158 mmol min⁻¹cm⁻² and 0.118 min⁻¹cm⁻² at a current density of 100 mA cm⁻² in K₂CO₃ and Na₂CO₃, respectively highlighting the influence of alkali metal cations on the H₂O₂ formation. Moreover, we will highlight the influence of electrolyte composition on the stability of the FTO electrodes using ICP-MS and SEM analysis. Overall, in this contribution we will disclose our understanding of the role of carbonate on the H₂O₂ formation in the context of the reaction parameter space.

Hydrogen peroxide is known as an essential green chemical oxidant for advanced oxidation processes such as wastewater treatment, sterilization (which includes the pathogenic SARS-CoV-2), decolorization of waste dyes, and as an oxidant in chemical reactions. Besides its oxidizing ability, hydrogen peroxide is considered an emerging green fuel and energy career. Because of these properties, hydrogen peroxide is an ideal green energy carrier comparable to that of compressed hydrogen gas and a medium for energy storage. It is also set to play an important role in energy conversion technologies such as fuel cells and batteries. These emergent energy aspects promise an even higher global demand for this valuable chemical. In this talk, I will discuss our efforts in electrosynthesis of hydrogen peroxide, as well as the potential applications of hydrogen peroxide in battery technologies, and how computational material design can lead to the design and discovery of novel materials for a revolutionary new application.

Transition-metal phosphides are of high interest for electrochemical applications, owing to their chemical stability, composition and phase tunability, and economic potential.  
In this talk, I will discuss our new route for metal phosphides synthesis, utilizing a direct reaction of transition-metal nitrate salts with triphenylphosphine (PPh₃) in molten-state. This method was found straightforward, benign and scalable. We demonstrated this method for nanoscale transition-metal phosphides synthesis, as an attractive alternative to common solvothermal or gas-solid phosphidation synthetic methods, allowing tunable phase composition of the produced nanoparticles, along with significant simplification of procedures and equipment.

We initially demonstrated the new method for nickel-phosphides molten-state synthesis from Ni(NO₃)₂^∙6H₂O and triphenylphosphine (PPh₃)[1]. By combining analytical and computational methods, we elucidated the reaction mechanism, indicating that the reaction propagates dominantly through favored Ni-P bonding and consecutive cleavage of phenyl–P bonds of PPh₃. Interestingly, we found both by DFT and XRD that an intermediate formation of metallic nickel nanoparticles, originating from ligand-to-metal charge transfer at the Ni-P bond, is essential for in-situ production of phosphides.

Furthermore, we exemplified the advantages of this simple and controllable method for the investigation of phase/composition-activity correlation of different Ni/P products for hydrogen evolution reaction (HER) and as anode-materials for Li-ion batteries.
We found a clear composition-activity trend for both applications, providing high performance which is comparable to nickel phosphides produced by other common methods. We found Ni₃P products favorable as HER electrocatalysts, with 145 mV overpotential at 10 mA/cm², and Ni₂P was found favorable as LIBs anode materials, with 206 mAh g^-1 capacity; both with high morphological and electrochemical stability.

This new simple synthetic path offers new possibilities for low-cost synthesis and design of other transition-metal based materials. I will present and discuss our latest results of an improved and generalized synthesis method based on the above detailed molten-state method, allowing the production of a variety of single and multi-component transition-metal phosphides for electrochemical applications.

Perovskite oxides are commonly viewed as excellent electrocatalysts for oxygen evolution reaction (OER). However, OER conditions can substantially damage the perovskite structure and can lead to a wide range of phenomena, including cation leaching, dissolution, oxygen intercalation and modification of electronic structure.

In this talk, using single-crystalline perovskite electrocatalysts, I will demonstrate how operando electrochemical atomic force microscopy (EC-AFM) can be used to record nanoscale changes in the surface morphology during anodic corrosion of the perovskite surface. I will also show that perovskite surfaces can drive a spontaneous ORR in an aqueous environment and, through this reaction, become substantially oxidized, reaching potentials close to the OER, which results in cation leaching, dissolution and oxygen intercalation. Finally, I will discuss the effect of oxygen intercalation and how it can lead to extreme oxidation of a perovskite oxide, which we observe for the first time using resonant inelastic X-ray scattering and rationalize using DFT and molecular dynamics.

Ultimately, our results highlight the complexity of the electrochemical and structural behavior of perovskites and the importance of taking their structural evolution into account when considering elementary reaction steps on such surfaces.

Solar panels can not only produce electricity, but are also in early-stage development for the production of sustainable fuels and chemicals. They can therefore mimic plant leaves in shape and function as demonstrated for overall solar water splitting for green H2 production by the laboratories of Nocera and Domen.[1,2] This presentation will give an overview of our recent progress to construct prototype solar panel devices for the conversion of carbon dioxide and solid waste streams into fuels and higher-value chemicals through molecular surface-engineering of solar panels with suitable electrocatalysts. Specifically, a standalone ‘photoelectrochemical leaf’ based on an integrated lead halide perovskite-BiVO4 tandem light absorber architecture has been built for the solar CO2 reduction to produce syngas using an integrated Co porphyrin catalyst.[3] Recent advances in the manufacturing have enabled the reduction of material requirements to fabricate such devices and make the leaves sufficiently light weight to even float on water, thereby enabling application on open water sources.[4] The tandem design also allows for the integration of biocatalysts as ideal model catalysts and the selective and bias-free conversion of CO2-to-formate has been demonstrated using enzymes.[5] The versatility of the integrated leaf design has been demonstrated by replacing the perovskite light absorber by BiOI for solar water and CO2 splitting.[6] An alternative solar carbon capture and utilization technology is based on co-deposited semiconductor powders on a conducting substrate.[2] Modification of these immobilized powders with a molecular Co bis-terpyridine catalyst provides us with a photocatalyst sheet that can cleanly produce formic acid from aqueous CO2.[7] CO2-fixing bacteria grown on the photocatalyst sheet enable the production of multicarbon products through clean CO2-to-acetate conversion.[8] The deposition of a single semiconductor material on glass gives panels for the sunlight-powered conversion plastic and biomass waste into H2 and organic products, thereby allowing for simultaneous waste remediation and fuel production.[9] The concept and prospect behind these integrated systems for solar energy conversion, related approaches,[10] and their reliance on suitable transition metal catalysts will be discussed.

References
[1] Reece et al., Science, 2011, 334, 645–648.
[2] Wang et al., Nat. Mater., 2016, 15, 611–615.
[3] Andrei et al., Nat. Mater., 2020, 19, 189–194.
[4] Andrei et al., Nature, 2022, 608, 518–522.
[5] Moore et al., Angew. Chem. Int. Ed. 2021, 60, 26303–26307.
[6] Andrei et al., Nat. Mater., 2022, 21, 864–868.
[7] Wang et al., Nat. Energy, 2020, 5, 703–710.
[8] Wang et al., Nat. Catal., 2022, 5, 633–641.
[9] Uekert et al., Nat. Sustain., 2021, 4, 383–391.
[10] Wang et al., Nat. Energy, 2022, 7, 13-24.

Molecular electrocatalysts have received a renewed interest due to their capabilities towards sustainable and energy–efficient redox chemical transformations.¹ Particularly, those involved in new strategies towards energy storage application.^{1a, 2} Along these lines, I will present our first means of molecular cooperativity under electrochemical reduction conditions targeting carbon dioxide reduction.³

First, I will provide direct experimental evidence on the correlation of remote interactions between a synthesized MnI-complex and different alkali cations with redox potential tuning. Additionally, the electrochemical behavior of the Mn-complex towards carbon dioxide will be discuss, including the effects of added alkali salts using cyclic voltammetry.^3a

Later, I will present the formation, characterization and use of a dinuclear cobalt complex bearing a pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions, for the electrocatalytic reduction carbon dioxide to carbon monoxide in the presence of Brönsted acids (94 % selectivity towards CO formation, at –1.35 V vs Saturated Calomel Electrode in DMF/TFE mixtures).^3b Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere of this complex indicate pairwise reduction processes. And, infrared spectro-electrochemical studies under carbon dioxide and carbon monoxdie atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of carbon dioxide. Additionally, our theoretical results indicate the cooperativity of the ligand platform during the reduction process, delocalizing the electron density at the ligand and reducing the overpotential of the reduction reaction.

References

(1) a) N. W. Kinzel, C. Werlé, W. Leitner, Angew. Chem. Int. Ed. 2021, 60, 11628–11686; b) N. Wolff, O. Rivada‐Wheelaghan, D. Tocqueville, ChemElectroChem 2021, 8, 4019–4027.

(2) a) T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G. Nocera, Chem. Rev. 2010, 110, 6474–6502.

(3) a) A. Srinivasan, J. Campos, N. Giraud, M. Robert, O. Rivada–Wheelaghan, Dalton Trans. 2020, 49, 16623–16626; b) A. Bohn, J. J. Moreno, P. Thuéry, M. Robert, O. Rivada-Wheelaghan, 2022, 10.1002/chem.202202361.

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Enzymatic systems have evolved complex strategies to maximize the efficiency and product selectivity in small molecule activation, among which CO2 reduction. Beside unique active sites containing by definition earth-abundant elements, enzyme further control catalytic activity through second sphere interactions and a fine control of electron transfer chains. In this talk, we will introduce a series of bio-inspired strategies for the design of electrocatalytic systems for CO2 reduction. We will highlight a series of earth-abundant metal based molecular catalysts inspired by the active sites of enzymatic systems and will introduce a new strategy for the electrocatalytic metal hydride generation using synthetic Fe4S4 clusters acting as concerted proton electron transfer (CPET) mediators. We will demonstrate that the combination of synthetic Fe4S4 clusters with the CO2 electroreduction catalyst [MnI(bpy)(CO)3Br] (bpy = 2,2’-bipyridine) allows the preparation of a benchmark catalytic system for HCOOH generation. We will finally introduce the preparation of the first complete redox series of Fe4S4 complexes and their use to generate potent strong reducing agents via entatic activation strategies. References: 4. L. Grunwald, M. Clémancey, D. Klose, L. Dubois, S. Gambarelli, G. Jeschke, M. Wörle, G. Blondin, V. Mougel* A complete biomimetic iron-sulfur cubane redox series PNAS, 2022, 119 (31), e2122677119 3. S. Dey, F. Masero, E. Brack, M. Fontecave, V. Mougel* Electrocatalytic metal hydride generation using CPET mediators Nature, 2022, 607 (7919), 499-506 2. S. Dey, T. Todorova, M. Fontecave*, V. Mougel* Electroreduction of CO2 to Formate with low overpotential using Cobalt Pyridine Thiolate Complexes Angew. Chem. Int. Ed., 2020, 59(36), 15726-15733 1. A. Mouchfiq, T. Todorova, S. Dey , M. Fontecave*, V. Mougel* A Bioinspired Molybdenum-Copper Molecular Catalyst for CO2 Electroreduction Chem. Sci., 2020, 11, 5503-5510

A current chemistry challenge is the (photo)electrochemical production of ammonia (NH₃) from atmospheric N₂ without using H₂ or emitting CO₂, as an alternative to the current Haber-Bosch process which consumes 1% of the world energy and emits nearly 1.5% of the CO₂. The nitrogen reduction reaction (N₂ + 6H⁺ + 6e^- -> 2NH₃) implies a cascade of elementary steps that require a delicate balance of reactivity at the metal active site M (cleavage of the N-N triple bond without creating a too strong, poisoning M-N interaction; avoiding competitive reduction of protons to H₂ …) [1,2]. To date, examples of molecular electrocatalysts including abundant metals remain scarce, although recent spectacular progress has been made, notably with Fe and Mo based catalysts [3,4]. In this presentation, we will present and discuss reactivity and mechanistic aspects of a Mn catalyst able to reduce dinitrogen to ammonia in homogeneous as well as in electrochemical conditions [5].

Climate change concerns have spurred a growing interest in developing environmentally friendly technologies for energy generation, including green H₂ production from water splitting. Moreover, concerted attempts are also being made to re-utilize CO₂ by electrocatalytically converting it into value-added chemicals and fuels, offering the possibility to store renewable energy into chemical bonds. Thus, efficient, selective and durable electrocatalysts that can operate under mild reaction conditions (atmospheric pressure and low overpotential) are urgently needed. However, the latter challenging goal can only be achieved when fundamental understanding of the electrocatalyst structure and surface composition under reaction conditions becomes available. It should also be kept in mind that even morphologically and chemically well-defined pre-catalysts are commonly susceptible to drastic modifications under operando conditions, especially when the reaction conditions themselves change dynamically. Here, a synergistic experimental approach taking advantage of a variety of cutting-edge microscopy (EC-AFM, EC-TEM), spectroscopy (NAP-XPS, XAS, Raman Spect., GC/MS) and diffraction (XRD) has been employed to unveil the complexity of energy conversion electrocatalysts.

In particular, I will provide new insights into the electrocatalytic reduction of CO₂ as well as the oxygen evolution reaction using operando characterization methods while targeting model pre-catalyst systems ranging from single crystals to thin films and metal nanoparticles. Some of the aspects that will be discussed include: (i) the design of size- and shape-controlled catalytically active nanoparticle pre-catalysts (Cu, Cu₂O cubes, ZnO@Cu₂O cubic NPs, CoOx NPs), (ii) the understanding of the active state formation and (iii) the correlation between the dynamically evolving structure and composition of these electrocatalysts under operando reaction conditions and their activity, selectivity and durability.

Our results are expected to open up new routes for the reutilization of CO₂ through its direct conversion into industrially valuable chemicals such as ethylene and ethanol and the generation of H₂ through water splitting.

[1] Grosse, P.; Yoon, A.; Rettenmaier, C.; Herzog, A.;. Chee, S.W.; and Roldan Cuenya, B., Nature Comm. 2021, 12, 6736

[2] Timoshenko, J.; Bergmann, A.; Rettenmaier,  C.; Herzog, A.; Aran Ais, R.; Jeon, H.; Haase, F.; Hejral, U.; Grosse, P.; Kühl, S.; Davis, E.; Tian, J.; Magnussen, O.; Roldan Cuenya, B.; Nature Catalysis 2022, 5, 259.

[1] Haase, F.;  Bergmann, A.; Jones, T.; Timoshenko, J.; Herzog, A.; Jeon, H.; Rettenmaier, C.; Roldan Cuenya, B., Nature Energy (2022) 7, 765.

Correlating activity, selectivity and stability with the structure and composition of catalysts is crucial to advancing the knowledge in chemical transformations which are essential to move towards a more sustainable economy. Among these, the electrochemical CO₂ reduction reaction (CO₂RR) holds the promise to close the carbon cycle by storing renewable energies into chemical feedstocks. Yet it still suffers from the lack of efficient, selective and stable catalysts.

In this talk, I will showcase a few examples which highlight how shape-controlled nanocrystals can contribute to address the selectivity challenge in CO₂RR. First of all, I will discuss how size control of Cu nanocubes and Cu octahedra has revealed the importance of facet-ratio to maximize the selectivity towards ethylene and methane, respectively. Second, I will describe our recent efforts towards using shape-controlled NCs beyond pure Cu, in the framework of tandem schemes and bimetallics. Finally, I will present our strategies to enhance the stability of these Cu catalysts during operation.

CO2 reduction to long-chain compounds has the potential to close the carbon cycle and if using excess renewables be a route to sustainability. However, most of the materials we have only produce C2. I will analyze theoretically some of the aspects behind these difficulties in going higher in the C-C bond forming strategies. On one side I will describe why easy one material one orientation do not provide enough flexibility to generate large chains and why the role of the microenvironment and the materials complexity is so intrinsically crucial to the improvement of the electrocatalytic activity. At the same time I will demonstrate how mastering the microenvironment properties improvements can be introduced following different strategies. The work presented will illustrate how much complexity both at the electrocatalytic site but also in the solution and with different operational modes are needed and how the integration of different methodologies is crucial.

The electrocatalytic CO₂ reduction reaction (CO₂RR) powered by renewable energy and promoted by transition metal catalysts, will play an important role in the global decarbonization process of the chemical industry, since represents a sustainable route for the production of value-added chemicals using CO₂ as a feedstock. [1]-[2] However, despite the significant progress made in the field, selectivity, durability and intrinsic activity of the catalysts are still key challenges to achieve an efficient CO₂RR. For organometallic molecular systems, characterized by a well-defined chemical environment of the active site, a rational tuning of the CO₂RR efficiency and selectivity can be precisely controlled through a rational modification of the ligand scaffold. [3]-[5] Moreover, the encapsulation of molecularly defined active units into reticular frameworks was recently shown as an effective strategy to boost the CO₂RR performances of molecular catalysts, but also to alter their redox behavior. [6]

In recent years, the synergy between molecules and nanostructured materials has been proposed as a promising approach to design efficient heterogeneous catalysts for CO₂ electroreduction. For instance, the presence of organic modifiers on metallic surfaces was found to tune the stability of key reaction intermediates or the local surface microenvironment, thus altering the product selectivity. [7]-[8] In this contribution, we will discuss novel strategies and approaches to form hybrid electrocatalysts based on the utilization of reticular or molecular chemistry as tools to steer the CO₂RR selectivity and activity of transition metal-based nanostructured catalysts. The discussion will focus on providing a molecular perspective towards a rational design of heterogeneous catalysts.  

Global warming, climate change and our over-dependence on non-renewable fossil fuels demand long-term solutions to reduce CO₂ emissions and develop sustainable energy technologies. The electrochemical CO₂ reduction has the potential to accomplish a “carbon-neutral energy cycle”, which incorporates CO₂ as the unlimited carbon source for the production of high-density fuels, and renewable energy as the driving force behind the process.[1] However, for an industrial application, there are still challenges to overcome, such as low selectivity, short durability and low current densities along with high overpotentials. New sustainable, modular, robust and efficient catalytic platforms are needed. In this regard, this work entails a fundamental understanding of the CO₂ mechanisms using Single Atom Catalyst (SAC) within Covalent Organic Frameworks (COFs). This work focuses on the investigation of the electrochemical CO₂ reduction (CO₂RR) in water using new Manganese based-Covalent Organic Frameworks with emphasis on understanding the relationship between structure and electrocatalytic activity. The initial center of interest was to accomplish active performance and durability for CO₂RR using highly-organized {Mn(CO)₃} active sites within COFs. The catalytic activity of the materials was benchmarked against other molecular supported catalysts reported in the literature. Compared to equivalent Mn derivates, COFs exhibited higher selectivity and activity towards CO₂ reduction [2]. Additionally, mechanistic studies based on in situ / in operando spectroelectrochemical techniques (ATR-IR, UV-vis, SEIRA, EPR) together with DFT calculations were used to detect key catalytic intermediates and correlate the catalytic activity with the mechanical constraints impose to the {Mn(CO)₃} active sites by reticular framework. Of particular note is the detection of a radical intermediate within a Mn based COFs avoiding the detrimental formation of a dimeric specie determined as a resting state in the catalytic cycle. In addition, the study of the Mn centers within the COF was expanded and focused on the understanding of the mechanism and dynamic processes at the electrode interface. This study show case the richness and complexity of reticular materials and can serve as a guide to investigate the dynamics of these organic frameworks.

Copper (Cu) is an excellent catalyst capable to convert CO₂ beyond CO and produces fuels and hydrocarbons. However, it is low product-selective.(1) One strategy to improve product-selectivity on Cu is tuning the geometry of the surface active sites positions, i.e., the group of atoms in the catalyst surface that catalyse the CO2 conversion. In this communication, I show an easy and feasible strategy to tune and quantify the number and geometry of the surface active sites on Cu nanostructures. (2) At first, we have assessed the reversible adsorption/desorption of lead (Pb), or Pb under potential deposition (UPD), on different well-ordered Cu single facets (Figure 1). The lead adsorption/desorption on Cu, is monitored by a cyclic voltammetry technique (CV), which provides distinguishable peaks on each single facet, i.e., it is highly sensitive to the active site´s geometry (Figure 1). Then, we have nanostructured a Cu electrode using and electroplating method. We have recorded the Pb UPD on the nanostructured Cu surface, which voltammetric shape has several peak-contributions. Using the Pb UPD voltammetric data recorded on Cu single facets, we decouple the different facet contributions on our nanostructured Cu surfaces. This voltammetric analysis can be used to rationally design nanostructured copper catalysts with tailored geometry at the active sites positions, which is relevant to improve the CO₂ reduction reaction.

Figure 1 shows the voltammetric Pb UPD profiles recorded on Cu single facets at pH 3 and from a 0.1 M KClO₄ + 1mM NaCl solution.

In recent years, two dimensional metal organic frameworks (2D-MOF) have attract much interest not only for the ease of their synthesis, but also for their semiconducting properties and catalytic activities. The appeal of these materials is that they are layered and can be easily exfoliated to obtain a mono (or few) layer material with interesting optoelectronic properties. Moreover, they have great potential for CO2 reduction to obtain solar fuels with more than one carbon atom, such as ethylene and ethanol, in addition to methane and methanol. The production of ethylene has been reported in a recent communication, but its mechanism of action is still unknown.

In this talk, I will explore how a particular class of 2D-MOF based on a phthalocyanine core can be the reactive center for the production of ethylene and ethanol, focusing on the mechanism of action by mean of a novel computational method called 'Gran Canonical Potential Kinetic', in which an applied potential is explicitly considered to obtain accurate results which are directly comparable with experimental electrocatalytic measurements.

We observed that not only methane and ethylene can be formed, along with methanol, acetylene, ethanol and other different alcohols, but also that the key reaction step -the insertion of CO-  occur without the presence of a CO-CO dimerization, as commonly observed on metal surfaces, but with a novel method which can still lead to the formation of C2 products, making the 2D-MOF behave like a single atom catalyst.

The electrochemical reduction reaction of CO₂ (CO₂RR) into valuable chemical products is foreseen as a promising lever to move from fossil to renewable energy with the benefit of closing the carbon look. Copper is a unique catalyst for this reaction as it generates products beyond CO, such as CH₄ or C₂H₄. While polycrystalline copper is unselective, faceting copper at the nanoscale has proven to improve its selectivity while retaining attractive activity.^1,2 However, its severe lack of stability during operation dramatically hampers its further use on the industrial scale. Indeed, Cu surfaces suffer from drastic restructuring which often results in a performance loss.^3,4

Herein, we propose the encapsulation of Cu nanocatalysts with a thin amorphous oxide coating as one promising solution against their structural changes during CO₂RR. Specifically, we utilize colloidal atomic layer deposition (c-ALD) to grow an alumina (AlOx) shell with tunable thickness around 7 nm copper spheres to form well-defined Cu@AlOx core@shell  structures. We find that the improved morphological stability of the Cu@AlOx is reached when full encapsulation is achieved. To understand the mechanisms behind the observed behavior, we correlate the shift in product distribution with observations from ex situ electron microscopy and operando X-ray absorption spectroscopy. Overall, this work offers the big opportunity to create stable Cu catalysts during CO₂RR which is currently one of the most important challenges in the field.

Electrochemical CO₂ reduction is an appealing approach to diminish CO₂ emissions, while obtaining valuable chemicals and fuels from renewable electricity. However, efficient electrocatalysts exhibiting high selectivity and low operating potentials are still needed. Herein , we present a preparation method for the obtention of Cu and Fe nanoparticles supported on porous N-doped graphitic carbon matrix as efficient and selective electrocatalysts for CO₂ reduction to CO at low overpotentials. XRD and Raman spectroscopy confirmed independent Cu and Fe metals as the main phases. HRSEM and HRTEM images show the coral-like morphology of the porous N-doped graphitic carbon matrix supporting Cu and Fe metal nanoparticles (about 10 wt. %) homogeneously distributed with an average size of 1.5 nm and narrow size distribution. At the optimum Fe/Cu ratio of 2, this material present high activity for CO₂ reduction to CO at -0.3 V vs RHE with a Faradaic efficiency of 96 %. Moreover, at -0.5 V vs RHE this electrocatalyst produces  27.8 mmol of CO x gcat-1 x h-1, the production rate being stable for 17 h. A synergy between Cu and Fe nanoparticles due to their close proximity in comparison with independent Cu or Fe electrocatalysts is discussed.

Artificial photosynthesis requires a hybrid assembly that integrates light absorption, electron transfer, and catalysis. Covalent anchoring of a molecular catalysts to a light harvesting semiconductor results in a photocathode that is able to perform selective photoelectrocatalytic CO₂ reduction (PEC-CO₂RR). In hybrid materials, the molecular catalyst provides with selective catalytic activity whereas the photoactive semiconductor material is used as support and for light harvesting [1]. In these systems, charge accumulation and kinetics of photoinduced electron transfer from the photocathode to the molecular catalyst and to the CO₂ are crucial for an efficient CO₂ conversion [2, 3]. In this work, a mesoporous anatase TiO₂ layer is used as a solid photosensitizer support to anchor iron porphyrin-based molecular catalysts for CO₂ to CO obtention. Micro to mili second Transient absorption spectroscopy (TAS) is employed to investigate the accumulation and lifetime of photogenerated electrons in TiO₂ by controlling the applied bias and in the presence of CO₂. By probing the transient signal of TiO₂ conduction band electrons, we observe a decrease in the accumulation of charges in non-functionalized TiO₂ under applied bias when CO₂ is present (at -0.5 V vs. Ag/AgCl, by 35%), which proves the transference of electrons from the photocathode to the CO₂. Furthermore, we observe a shorter lifetime of photogenerated electrons when the TiO₂ is functionalized with iron porphyrins (10-fold decrease) derived from a fast electron transfer from the photocathode to the molecular catalyst. Ultimately, we correlate the results to the potentiostatic electrocatalysis performance of TiO₂ and TiO₂-Iron porphyrin hybrid materials under CO₂; with bare TiO₂, CO and CH₄ along with H₂ are produced, whereas TiO₂-Iron porphyrin shows 100% selective CO formation. However, we identify moderate CO₂RR performances caused by interfacial charge recombination between the oxidized iron porphyrin and conduction band electrons of TiO₂ which decrease the charge transfer process to the CO₂ molecule.

Global warming, as a consequence of the dependence on the use of fossil fuels, is one of the major problems that our current society is facing. For this reason, the production of energy by renewable sources has become a great challenge for the scientific community. Thus, researchers have recently focused their efforts on the development of new technologies that guarantee the production of high amounts of energy with low to minimal carbon footprint. Intermittence of renewable energies is a present challenge that requires to develop technologies able to store huge amounts of energy to be easily supplied at periods of low or zero energy production. In this scenario, (photo)electrochemical technologies have become an attractive candidate with the purpose of producing green fuels in order to store energy at the moments of excess of production. Research in the field of (photo) electrocatalysis has mainly been focused on the generation of green H₂, from the oxygen evolution reaction (OER) through the oxidation of H₂O; and the reduction of CO₂ (CO2RR) to obtain green fuels.[1] in these studies, notable advances have been made in recent years, both in terms of understanding the mechanisms of reactivity, as well as the selectivity of the processes activated by sunlight.[2,3]  In both cases, O₂ is obtained as by-product at the anode, with a very low price at the market, thus reducing the economic viability and the interest in this technology.[4] With the aim to store energy at the same time that obtaining compounds with added value, several organic transformations have been pointed out as alternative to the OER^.at the anode and H₂ production or CO2RR at the cathode. [5,6]

One very interesting alternative anodic reaction is based on the oxidation of lignin or compounds from biomass, since these species also allow reducing overpotentials while obtaining valuable products for the chemical industry, with the subsequent biomass revalorization. 5-hydroxymethylfurfural (HMF) is a biomass derived species, which can be easily obtained from C6 natural sugars. The selective oxidation of one of the hydroxyl groups of HMF leads to 2,5-furandicarbox-aldehyde (DFF), which can be used as a monomer in the synthesis of various polymers based in furan; [7,8] while further oxidation of HMF gives rise to 2,5-furandicarboxylic acid (FDCA), a very important component in the pharmaceutical and polymer industry, which can replace the monomers traditionally used in the synthesis of ethylene terephthalate (PET).[9] Besides, the reduction of HMF can lead to 2,5-bi(hydroxymethyl)furan (BHMF), a species that can be used as a precursor for the synthesis of many bio-based polymers, [10] consequently making this process very interesting as alternative to H₂ production or CO2RR in photoelectrochemical processes.

In our project we have performed the transformation of HMF to added value products by means of electrochemistry. The electrochemical oxidation of HMF to 2,5 FDCA has been achieved using easily made and inexpensive NiO-OH electrodes, obtained by Ni electrodeposition on pencil graphite rods (Ni/PGR). We have optimized the reaction conditions to prevent HMF degradation, leading to a nearly complete conversion of HMF into FDCA, with 88% Faradaic Efficiency (FE) and very low degradation in less than 2 hours. The electrochemical reduction of HMF to BHMF has been achieved using Cu foils electrodes decorated with Ag deposition using the galvanic replacement technique. BHMF has been obtained with 89% yield and 87% FE at pH 9 media in less than 3 hours. Impedance Spectroscopy (IS) has been analysed in both electrochemical processes to elucidate the electron transfer mechanism and the adsorption of reactants and intermediate species on the surface of the electrodes.

Photocatalytic liposomes are a promising supramolecular platform for artificial photosynthesis,^[1] but the understanding of reaction mechanisms and surface dynamics between membrane-bound molecular components still remain a challenge. In this presentation, we will report the photocatalytic CO₂ reduction performance of a series of 3d transition metal terpyridine and porphyrin catalysts immobilised on liposomes,^[2-4] and briefly about their use in (photo)electrocatalytic CO₂ reduction.^[5] Time-resolved spectroscopy was utilised to provide new insights into electron transfer processes taking place between the electron donor (sodium ascorbate) and membrane-bound photosensitiser and catalyst molecules. The most active molecular photocatalyst system, containing a cobalt porphyrin, was studied by (spectro)electrochemistry and density functional theory to gain a better understand of its enhanced performance compared to the other studied assemblies, and to propose a possible reaction mechanism for CO₂ reduction.^[4] Furthermore, the same cobalt porphyrin catalyst is successfully utilised in a (photo)electrochemical device that can simultaneously reduce CO₂ and reform plastic waste.^[5] Therefore, our findings demonstrate the great potential of liposomes as versatile photocatalytic scaffolds. We also show how the same molecular catalyst can be utilised successfully in photocatalytic and (photo)electrocatalytic systems, and illustrate the power of combining time-resolved and in situ spectroscopic techniques to understand molecule-based systems.

References:

[1]         Pannwitz, A.; Klein, D. M.; Rodríguez-Jiménez, S.; Casadevall, C.; Song, H.; Reisner, E.; Hammarström, L.; Bonnet, S., Roadmap towards solar fuel synthesis at the water interface of liposome membranes. Chem. Soc. Rev. 2021, 50, 4833-4855.

[2]         Wang, Q.; Warnan, J.; Rodríguez-Jiménez, S.; Leung, J. J.; Kalathil, S.; Andrei, V.; Domen, K.; Reisner, E., Molecularly engineered photocatalyst sheet for scalable solar formate production from carbon dioxide and water. Nat. Energy 2020, 5, 703–710.

[3]         Zhang, X.; Cibian, M.; Call, A.; Yamauchi, K.; Sakai, K., Photochemical CO₂ Reduction Driven by Water-Soluble Copper(I) Photosensitizer with the Catalysis Accelerated by Multi-Electron Chargeable Cobalt Porphyrin. ACS Catal. 2019, 9, 11263-11273.

[4]         Rodríguez-Jiménez, S.; Song, H.; Lam, E.; Wright, D.;Pannwitz, A.; Bonke, S. A.; Baumberg, J. J.; Bonnet, S.; Hammarström, L.; Reisner E., Self-Assembled Liposomes Enhanced Electron Transfer for Efficient Photocatalytic CO₂ Reduction. J. Am. Chem. Soc. 2022, 144, 9399-9412.

[5]         Bhattacharjee, S.; Rahaman, M.; Andrei, V.; Miller, M. Rodríguez-Jiménez, S.; Lam, E.; Pornrungroj C.; Reisner, E., Photoelectrochemical CO₂-to-fuel conversion with simultaneous plastic reforming. Nat. Synth (2023). in print (https://doi.org/10.1038/s44160-022-00196-0).

Electrocatalytic CO2 reduction to renewable fuels has drawn increasing attention due to its great potential in addressing the issues of the greenhouse effect, energy crisis, and industrial transformation thus having the utmost importance in attaining a carbon-neutral society. Nevertheless, there are general practical limitations such as poor selectivity for multi-carbon products and most importantly the electrocatalysts suffer from high overpotentials while directly converting the CO2 to C2+ products. To overcome these challenges and enhance the ECO2R, a cascade mechanistic approach is explored as an alternative to the direct conversion of CO2 to C2+ products. A two-step tandem set-up is employed where CO2 is converted to CO in the first step, enabled with a highly efficient Ni single-atom catalyst on activated carbon black, and sequentially CO is converted to multi-carbon products in the second step. The single-atomic catalytic sites yield a nearly 100 % faradaic efficiency towards CO at an average current density of over -40 mA·cm-2. The as-produced product stream is directly supplied as the reactant stream to a second electrolyzer that is equipped with oxide-derived copper supported on carbon black. Initially, it is noticed that the presence of unreacted CO2 in the second cell has an adverse effect and results in poor performance. When the unreacted CO2 is trapped by using an absorption chamber in between these two electrolyzers, a surge in the faradaic efficiency towards ethylene, ethanol, and n-propanol thus accounting for more than 80% total faradaic efficiency at an average current density of over 120-130 mA·cm-2 and the unwanted yet competitive H2 is limited to just 10-12 % faradaic efficiency. The aforementioned approach is found to be highly efficient not only in terms of faradaic efficiency but also energy efficiency. It is evident that multi-step electrolysis yields a highly promising conversion of CO2 to multi-carbon products. On the flip side, direct or indirect routes are also explored to electrocatalytically reduce the carbonated solvent stream placed in between the electrolyzers. The observed results clearly are imperative by enabling a direct cascade approach and exclusive separation of unreacted feed gas aid in attaining higher electro-conversion of CO2 to alternative fuels.

Electrocatalytic hydrogen production or C-based fuels synthesis via water splitting and  CO2 reduction, respectively, is considered a promising technology to store renewable energy into chemical bonds when using Earth-abundant electrocatalysts. The efficiency of these catalysts does not only depend on the nature of the metal centre, but also on the morphology and crystalline structure. However, chemical and morphological changes in the catalysts take place in operando conditions that are poorly understood.

In this poster, I will present recent advances on the understanding of morphological transformations of Cu-based electrocatalysts for the hydrogen evolution and CO2 oxidation reactions during catalysis and the effect of such transformations in the reactions kinetics. In particular, the effect of morphological transformations on increasing the electrochemically active surface area (ECSA) will be discussed. In this poster, I will show that such increase in the ECSA is found to increase the efficiency of the catalyst noting this work is performed in mild pH conditions. 

Energy storage plays, undoubtedly, a fundamental role in the process of total decarbonization of the global economy that is expected to take place in the coming decades. The energy transition to a renewable and sustainable generation is the solution to reduce greenhouse gas emissions and thus achieve the European Commission´s goal of becoming the world´s first decarbonized economy. In this transition, beyond li-ion technologies will play a key role to meet the increasing energy demand that cannot be covered by Li-ion batteries solely. In this scenario, sodium-oxygen and zinc air batteries have become an attractive alternative due to their high gravimetric energy densities (1605 or 1108 Wh/kg based on Na₂O₂ or NaO₂, respectively; and 1086 Wh/kg based on ZnO discharge products) resulting from the use of an oxygen-based phase-change reaction (potentially reducing the weight and freeing up space for other components).

In this talk, a general overview of these systems will be given along with the materials requirement and system performance. These batteries have been considered as the holy grail of battery research due to their high theoretical energy densities; however, several challenges remain to be solved before commercialization.

Renewables are essential for the energy transition to a sustainable and eco-friendly energy system that avoids the use of fossil fuels. One of the major limitations of renewables is their intermittent character which depends on climate conditions. This climate dependence leads to a complete disconnection between energy production and demand. Electrochemistry, fed by renewables, has emerged as a smart procedure that can pave the way for the energy transition. The main chemical procedures studied in electrochemistry are water splitting and CO₂ reduction. In both cases, the oxygen evolution reaction (OER) takes place at the anode, providing the electrons and protons needed in the cathode for the generation of H₂ in water splitting or the conversion of CO₂ into valuable species, such as CH₄ or CH₃OH. The OER produces O₂ that despite being a very important compound, its price in the market is very low, which limits the economic viability of the process and ultimately reduces the interest in this technology.¹ Furthermore, this reaction exhibits large overpotentials when using catalysts based on earth abundant materials, limiting energy conversion efficiency.² For these two reasons, there is a growing interest to find alternative reactions to OER at the anode that, by one side, reduce the overpotentials needed and by the other, produce compounds with higher added-value and interest for the chemical industry.^1,3 In this framework biomass valorization, has emerged as an attractive substitute to the oxidation of H₂O.⁴ Herein we present our more recent results in the electrochemical transformation of biomass, as well as the use of electrochemistry for energy storage.

Solid oxide fuel cells (SOFCs) are capable of providing high energy efficiency and fuel flexibility leading to use of hydrogen and hydrocarbons due to high operation temperature (600–800°C). Recently, ammonia (NH₃) is considered as one of the most promising hydrogen carriers because of great advantages of zero-carbon emission, a high hydrogen content and worldwide infrastructures established for production, transport and storage. Thus, much effort has been given to directly utilize ammonia as a fuel for SOFCs, so called direct ammonia (DA)-SOFCs in which the fuel is decomposed hydrogen and nitrogen, and the released hydrogen is electrochemically oxidized in the anode consecutively. Thus, DA-SOFCs are believed to accomplish a CO₂-free power generation with high efficiency. In this study, the current status on the development of SOFCs fed with ammonia, and strategical approaches to overcome technical issues limiting the development of DA-SOFCs will be introduced. In addition, results on the application of an exsolution catalyst to decompose the fuel in the anode of DA-SOFCs are present, which has proven the feasibility of ammonia fuel for high performance and reliable SOFCs.

Abstract

One of the challenges faced by photoelectrochemical (PEC) water splitting technology is the relatively high levelized cost of hydrogen (LCOH) compared with other hydrogen generation methods. A potential strategy to increase its competitiveness is to couple PEC water splitting with hydrogenation reactions that produce valorized chemicals[1][2]. In this study, we evaluate the economic potential of co-producing hydrogen and methylsuccinic acid (MSA) by coupling the hydrogenation of itaconic acid (IA) inside a PEC water splitting device based on BiVO₄ and silicon heterojunction (SHJ) absorbers[3][4].

This study examines the economics of a 1000 kg H₂/day rated capacity PEC plant with the coupled co-production feature by conducting a techno-economic assessment (TEA) under a base case scenario where PEC devices with solar-to-hydrogen (STH) efficiency of 10% and longevity of 20 years are considered. When H₂ is the only product, the obtained cradle-to-gate LCOH is 22.3 €/kg. However, with the coupled hydrogenation reaction, the LCOH can be reduced to 2.8 €/kg when only 3.9% of the generated H₂ molecules are converted to MSA. This LCOH value is already competitive with the benchmark H₂ derived from steam methane reforming (SMR)[5]. This means that the technical advancements based on currently demonstrated coupled catalyst PEC technology can provide sufficient cost reductions to allow solar hydrogen to directly compete on a levelized cost basis with hydrogen produced from fossil energy.

Our sensitivity analysis further indicates that the H₂-to-MSA conversion efficiency and MSA sales price have the greatest impact on the reduction of LCOH. Therefore, the system can take advantage of its flexibility to maximize benefits by adjusting the conversion rates accordingly to market conditions. Further optimization is suggested regarding the substitution of expensive system components, i.e., PV absorber and membrane, with envisioned cheaper alternatives, which might substantially lower the overall system cost.

Key words:  water splitting, (photo)electrochemistry, technoeconomic analysis, coupled catalysis, hydrogenation

Oxygen reduction reaction activity is governed by the oxygen adsorption/dissociation, proton conduction, and electron transfer kinetics in protonic ceramic fuel cells (PCFCs). Various strategies have been explored to enhance the proton and electron conductivity via tuning oxygen vacancy concentration in the electrode materials and introducing electronic conducting agents. However, there are few studies on improving surface exchange reaction (oxygen adsorption/dissociation) kinetics in protonic ceramic fuel cells. In this study, we report uniformly distributed nickel oxide (NiO) nanoparticles as a catalyst to enhance the electrochemical performance of BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O₃ (BCFZY-BZCYYb) composite cathode by improving surface exchange reaction kinetics. The 0D NiO nanoparticles with high adsorption and fast dissociation ability of oxygen could enlarge the active sites for surface exchange reaction without fading the BCFZY surface and triple-phase boundaries where H₂O formation reaction occurs. The cathode employing NiO nanoparticles exhibits largely reduced polarization resistance and superior power density of 780 mW/cm² at 600 °C.  In addition, the water-stable characteristic of NiO and reduced polarization resistance of composite cathode drastically improved the cell stability.

Much research has shown that crystal structure and dimensionality in materials play an important role in determining their functional properties besides material composition itself. From this aspect, attention to two-dimensional (2D) materials with different three-dimensional (3D) crystallography has been given. The initial boost in research of 2D materials is largely attributed to graphene, which shows interesting phenomena such as super-electrical conductivity and high flexibility with good strength that are absent in bulk graphite. After the discovery of graphene, relevant research on single layers of transition metal dichalcogenides (TMDCs) with lamellar structures similar to that of graphite generated a second wave of interest. Accordingly, many unusual physical, chemical, or electronic properties compared to their bulk counter-parts have been reported. On the other hand, whereas much research effort has been devoted to graphene and TMDC, to date, relatively little research has been conducted in the area of 2D oxide nanosheets because most oxides have 3D crystal structures (it is noted that graphene and TMDC are inherently 2D structures, which are strongly bonded in plane but weakly bonded out of plane with van der Waals force). However, oxides have advantages such as chemical stability and low cost compared to the above materials, and thus, if synthesized to 2D structures, they can be another potential group with valuable functionality. In this presentation, the synthesis of 2D materials based on oxides and, further, their usage for various applications such as energy, sensor, and display will be dealt with.  

Characterization of multi-component slurry such as cathode and anode slurries of Li-ion battery is important, however, it is very difficult task. Thus, we investigated both the flow ability and the packing (settling) ability to discuss the particles dispersion and aggregation state for the cathode slurry of Li-ion battery. The slurries were prepared by changing the mixing condition of acetylene black particles and then the prepared slurry was casted and dried to fabricate the cathode layer. The flow curve of the slurry was measured by a rheometer and the time change of settling hydrostatic pressure was measured by a hydrostatic pressure analyzer. The density and electric resistance of the fabricated cathode layer were measured as well. It was found that a slurry in which acetylene black particles forms a network structure, with sufficient strength and the ability to rapidly recover after breaking by an external force, yields a cathode with comparatively high density and comparatively low volume resistivity [1]. It was also found that the normalized settling time of a cathode slurry determined from its change in hydrostatic pressure over time correlates well with both the density and volume resistivity of a resulting as-cast cathode [1].

The demand for rechargeable Li batteries having more safety and higher energy density has been increased to meet the strong demand of novel applications such as electric vehicles and energy storage system. In this aspect, Li ion batteries containing typical liquid electrolytes have fundamental limitations because liquid electrolytes can act as fuels in thermal runaway behavior leading to a fire or an explosion of battery and can be decomposed at high potential (> 4.5V) leading to the restricted use of high potential cathodes. To address these problems, there are several approaches. One of promising approaches is to apply proper oxide-based solid electrolytes (SEs) instead of liquid electrolytes because they can enable to deliver superior safety with Li metal and to achieve high energy density simultaneously.

In this talk, I will discuss about the progress of the development of the oxide-based SEs, especially, focusing on the newly developed garnet-type SE that has superior electrochemical/chemical properties with respect to the wettability with Li metal, ionic conductivity, and chemical stability with air, and the revisited Lisicon-type SE that has superior compatibility of most of active materials. Also, I will talk about the efforts in our group to build up all solid-state battery by using the developed oxide-based SEs.

Solid oxide regenerative fuel cells (SORFCs), which perform the dual functions of power generation and energy storage at high temperatures, offer one of the most efficient and environmentally friendly options for future energy management systems. Although the functionality of SORFC electrodes could be significantly improved by reducing the feature size of electrode to nanoscale, the practical use of nanomaterials has been limited due to the lack of stability and controllability at high temperatures. Herein, we demonstrate an advanced infiltration technique that allows nanoscale control of highly active and stable catalysts at elevated temperatures. Homogeneous precipitation in chemical solution, which is induced by urea decomposition, allows the precise tailoring of the phase purity and geometric properties. Particularly, effective complexing followed by instantaneous precipitation enables the atomic-scale dispersion of active catalysts on support. Controlling the key characteristics of nanocatalysts yields an electrode that is very close to the ideal electrode structure. Consequently, outstanding performance and durability are demonstrated in both fuel cell and electrolysis modes [1, 2].

References

[1] K.J. Yoon, M. Biswas, H.-J. Kim, M. Park, J. Hong, H. Kim, J.-W. Son, J.-H. Lee, B.-K. Kim, H.-W. Lee, Nano Energy 2017, 9.

[2] J. Shin, Y.J. Lee, A. Jan, S.M. Choi, M.Y. Park, S. Choi, J.Y. Hwang, S. Hong, S.G. Park, H.J. Chang, M.K. Cho, J.P. Singh, K.H. Chae, S. Yang, H.-I. Ji, H. Kim, J.-W. Son, J.-H. Lee, B.-K. Kim, H.-W. Lee, J. Hong, Y.J. Lee, K.J. Yoon, Energy & Environmental Science 2020 13 4903

Irreversible phase transformations of layered oxide cathodes during charging have been detrimental for most of them. Even if a lot of efforts have been made to relieve this highly irreversible phase transformation, there have been just a few successful results, which definitely limit the amount of extracted alkali ions and therey the available capacitie of the layered oxides. So, this presentation will suggest two strategies to get over the limitation of previous researches.  

As an inverse conceptual strategy, we first observed the possibility to make this irreversible phase transformation extremely reversible by utilizing crystal water as a pillar. Although we found a few cyrstal structures working with this reversible phase transition, the works using Na-birnessite (Na_xMnO₂•yH₂O; Na-bir) or Li-birnessite (Li-bir) as basic structural units will be highlighted here. The crystal water in the structure contributes to generating metastable spinel-like phase, which is the key factor for making this unusual reversibility happen. The reversible structural rearrangement between layered and spinel-like phases during electrochemical reaction could activate new cation sites and enhance ion diffusion with higher structural stability. This unprecedented reversible phase transformation between spinel and layered structure was maximized through modulating the steric coordination or amount of cyrstal water in the lattice resultantly optimizing the electrochemical performances of the birnessite layered oxides.

Pseudo Jahn-Teller effect (Pseudo JTE) will be also stressed out as a fundamental reason behind lattice distortion of layered oxide cathodes. Even though Jahn-Teller effect (JTE) has been regarded as one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge, there have been many reports that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (Pseudo JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and anti-bonding molecular orbitals in transition metal octahedra. As a model case, some of layered oxide cathodes including P2-type Mn-rich cathode (Na_3/4MnO₂) will be dealt with in this presentation

The two new insights provide deep insight into novel class of intercalating materials which can deal with highly reversible framework changes, and thus it can break up some typical prejudices which we have about the layered cathodes for Li or Na secondary batteries.

The intermittent nature of renewable energies inevitably leads to energy surpluses to maintain energy demand at peak times. Consequently, the storage of energy surpluses from renewables becomes a necessity. To tackle this problem, in this work we propose an efficient way of energy storage based on using green hydrogen as an energy vector. Green hydrogen can be store using different technologies but among them, the storage forming chemical bonds represents an unexplored technology that will have an important impact in handling, management and transporting hydrogen. Our aim is to develop efficient systems for hydrogen storage in the form of chemical bonds based on Liquid Organic Hydrogen Carriers (LOHCs) [1].

The key point of efficient LOHCs lies in the design of suitable catalysts and processes for the conversion and reconversion of hydrogen into chemical bonds. In this work, we describe the state of the art in the field [2] [3] and our last results in for hydrogen storage in the form LOHCs.

All-Inorganic Halide Perovskite Nanocrystals (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different fields like photovoltaics, lasing and emission. In the present talk, we will describe our efforts towards the application of these materials for solar-driven hydrogen production coupled to other processes like organic transformations and waste valorization.[1-2] We will discuss on the rational design of these fascinating materials towards photoelectrochemical processes, and the importance of extracting basic electronic and optical information to understand the carrier dynamics,[3] the influence of trap states and to define adequate defect passivation strategies to maximize the performance and stability of these materials.[4] Moreover, proper interrogation tools are needed to validate their photoelectrocatalytic activity and selectivity. The need for integration of the developed materials into tailored photoelectrochemical devices highlight the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments.

All-solid-state batteries (ASSBs) with sulfide-based solid electrolytes (SEs) have been considered the most promising next-generation energy storage system due to their high energy density. Silicon (Si) has been intensively studied as an anode material due to the high theoretical capacity (3572 mAh g^-1, Li₁₅Si₄) and a relatively low working voltage (~0.5 V vs Li/Li⁺) in ASSBs. However, poor electrochemical properties caused by the large volume change (~ 300%) and low electronic conductivity of Si have limited its practical use. Here, we report Si nanoparticles embedded in carbon nanofiber (CNF) coated with solid electrolyte (LPSCl) (Si/CNF@LPSCl) as anode material to achieve high energy density and cycle stability for ASSBs. The CNF is a desirable host matrix for the Si nanoparticles due to its good mechanical property and high electronic conductivity. The conformal coating of SE on the surface of Si/CNF composite enhances the interfacial stability between the active material and the SE, which leads to the improvement in electrochemical properties by suppressing the contact loss. The Si/CNF@LPSCl composite electrode exhibits a reversible capacity of ~ 1172 mAh g^-1 at 0.1C and stable cyclability of ~ 84.3% at 0.5C after 50 cycles.

Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Attaining pure single-photon emission is key for many quantum technologies, from optical quantum computing to quantum key distribution and quantum imaging.  Across single CsPbX3 NCs (X: Br and I) of different sizes and compositions, we find that increasing quantum confinement is an effective strategy for maximizing single-photon purity due to the suppressed biexciton quantum yield. We achieve 98% single-photon purity (g(2) (0) as low as 2%) from a cavity-free, nonresonantly excited single 6.6 nm CsPbI3 NCs, showcasing the great potential of CsPbX3 NCs as room-temperature highly pure single-photon sources for quantum technologies [1]. In another study, we address the linewidth of the single-photon emission from perovskite NCs at room temperature. By using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr3 NCs, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35−65 meV (vs. initial values of 70–120 meV), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20−60 meV) [2]. NC self-assembly is a versatile platform for materials engineering, particularly for attaining collective phenomena with perovskite NCs, such as superfluorescence [3, 4, 5]. The NC shape anisotropy leads to structures not observed with spherical NCs. We present a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr3 NCs (and FAPbBr3 NCs) co-assembled with the spherical, truncated cuboid, and disk-shaped NC building blocks. CsPbBr3 nanocubes combined with Fe3O4 or NaGdF4 spheres and truncated cuboid PbS NCs form binary SLs of six structure types with high packing density; namely, AB2, quasi-ternary ABO3, and ABO6 types as well as structures already known in all-spheres systems [NaCl, AlB2, and CuAu types]. In these structures, nanocubes preserve orientational coherence. Combining nanocubes with large and thick NaGdF4 nanodisks results in the orthorhombic SL resembling CaC2 structure with pairs of CsPbBr3 NCs on one lattice site. Also, we implement two substrate-free methods of SL formation. Collective electronic states arise at low temperatures from the dense, periodic packing of NCs, observed as sharp red-shifted bands at 6 K in the photoluminescence and absorption spectra and persisting up to 200 K. Co-assembly of CsPbBr3 nanocubes with very thin disk LaF3 nanodisks (9.2–28.4 nm in diameter, 1.6 nm in thickness) yields six columnar structures with AB, AB2, AB4, and AB6 stoichiometry, not observed before and in our reference experiments with NC systems comprising spheres and disks. Perovskite SLs exhibit superfluorescence, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behaviour with a strongly accelerated build-up time.

[1] Chenglial Zhu et al. Nano Lett. 2022, 22, 3751−3760

[2] Gabriele Raino et al. Nat. Commun., 2022, 13, 2587

[3] Ihor Cherniukh et al. Nature, 2021, 593, 535–542.

[4] Ihor Cherniukh et al. ACS Nano, 2021, 15, 10, 16488–16500

[5] Ihor Cherniukh et al. ACS Nano, 2022, 16, 5, 7210–7232

We present Pulsed Laser Deposition (PLD) as a physical vapor deposition of halide perovskites, allowing near stoichiometric transfer and multi-element film formation independent of the relative volatility of the elements from a single solid target. We will discuss the effects of deposition pressure, deposition rate and PLD target composition on the formation of stoichiometric and phase-pure films of CsSnI₃, MAPbI₃, MA_1-xFA_xPbI₃ and Cs₂AgBiBr₆. Microstructural, compositional and optoelectronic characterization of the films confirmed that PLD allows control on polymorph formation, optical properties, thickness, and conformal coating on textured substrates [1,2,3]. At the device level, we discuss the influence of the bottom contact layers on the first stages of the perovskite film growth, final film morphology and solar cell performance. Proof-of-concept n-i-p and p-i-n solar cells with > 14 % efficiency are demonstrated with as-deposited room-temperature grown PLD-MA_1-xFA_xPbI₃ films. All these are important steps forward in the controlled growth and future scalability of vapor-deposition methods for perovskite solar cells.[4]

Halide perovskite materials find applications in solar cells, light emitting diodes or memory storage devices. This family of materials are mixed electronic and ionic conductors and the ionic conductivity is responsible for the hysteresis observed in the electrical characterization.¹ Here we explain how this ion migration can be used to our advantage to promote formation of conductive and insulating states making them useful as resistive memories (memristors). We show that the working mechanism and performance of the memory devices can be tuned and improved by a careful selection of each structural layer. Several configurations are  evaluated in which structural layers are modified systematically: formulation of the perovskite², the nature of the buffer layer³  and the nature of the metal contact⁴. In addition, we develop an electrical model to account for the observed j-V response.⁵ Overall, we provide solid understanding on the operational mechanism of halide perovskite memristors that unveils the connection between electronic and ionic conduction.

Besides the outstanding properties such as high absorption coefficient,  bandgap tunability and long diffusion length, organometal halide perovskite devices (MHPs) continue to suffer from the consequences of their dual ionic/electronic nature. Hysteresis behaviour during IV measures is the direct sentinel of the ion migration or defect drift occurring in these devices. But this defect drift or ionic migration in MHPs can be positively utilized to design a novel memory device.  In fact, perovskites are emerging as promising candidates for the next generation of resistive random-access memory (ReRAM) devices or memristors. ReRAM have been developed as promising next-generation of nonvolatile memory devices due to the simple device architecture, fast operation, and low power consumption[1].  Perovskite memristors are being reported lately with remarkable success due to their good electrical performances, meeting the requirements for memory technology; i.e:  non-destructive reading process, nonvolatility, high ON/OFF ratio, and fast switching speed. [2]. Due to their solution processability, perovskite memristor devices have been fabricated both in their polycrystalline and monocrystalline forms, as thin-film and bulk configurations respectively. Outstanding results have been obtained for both.[3] However, the mechanisms behind their good performance and how their crystalline nature affects them are still under discussion. In this work, we report the first memristor device based on MAPbBr₃ thin-film single crystal. The synergy between its thin-film configuration and its monocrystalline nature,  allows displaying very high stability during operation conditions and massive operative currents, exhibiting an outstanding ON/OFF ratio of 10⁶. The mechanisms governing the operation of this device are analysed in depth by impedance spectroscopy measures, contributing to a better understanding of the physics governing these devices.

Radiation detection is of utmost importance in fundamental scientific research, as well as medical diagnostics, homeland security, environmental monitoring and industrial control [1]. Lead halide perovskites (LHPs) are attracting growing attention as high-atomic-number materials for next-generation scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional materials [2], it is necessary for LHPs to conjugate high scintillation yields with emission stability under high doses of ionizing radiation. To date, no definitive solution has been devised to optimize the scintillation efficiency and kinetics of LHPs and nothing is known of their radiation hardness for doses above a few kiloGrays, to the best of our knowledge. Here we demonstrate that CsPbBr3 nanocrystals exhibit exceptional radiation hardness for γ-radiation doses as high as 1 MGy [3]. Spectroscopic and radiometric experiments highlight that despite their defect tolerance, standard CsPbBr3 nanocrystals suffer from electron trapping in dense surface effects that are eliminated by post-synthesis fluorination. This results in >500% enhancement in scintillation efficiency, which becomes comparable to commercial scintillators, and still retaining exceptional levels of radiation hardness. These results have important implications for the widespread use of LHPs in ultrastable and efficient radiation detectors for strategic applications such as national and industrial security, high-energy physics and nuclear power plant control, from space exploration to medical imaging diagnostics such as CT and PET.

The detection of ionising radiation is the focus of many strategic applications in both science and technology. Typically, the detection is carried out by scintillator materials that emit light following interaction with ionising radiation, which is detected by highly sensitive photodetectors. The fundamental characteristics of scintillators are the probability of interaction with ionising radiation, scintillation yield, scintillation rate, and stability to high doses of absorbed radiation, commonly referred to radiation hardness. For large scale applications, it is essential that scintillators can be manufactured in large sizes and massive quantities with cost-effective techniques in terms of both energy and raw material consumption.

Recently, lead halide perovskite nanocrystals (LHP-NCs)^[1] have emerged as promising nanoscintillators prized for their tuneable optical properties, radiation stability, high average Z-number and defect tolerance that enables high light yields and resistance to radiation damage up to extreme radiation levels^[2]. However, hot-injection synthesis methods used for high-optical-quality LHP NCs are not suitable for mass production and scalable LARP approaches are limited by concentration gradients in the reaction environment resulting in poorer optical performance. LHP-NCs are also susceptible to the fabrication protocols for polymeric matrices mostly due to the polarity of monomers, the presence of initiators and the polymerization temperature that lead to surface damage, NC dissolution and/or phase shift to non-emissive allotropes.

In this talk, I will report the fabrication of new ultrafast and radiation hard plastic scintillators based on CsPbBr₃ NCs formed within a polyacrylate matrix from precursor nanoparticles synthesized by a high-throughput, large scale turbo-emulsion method. Importantly, the formation of CsPbBr₃ NCs is accompanied by the gradual curing of quenching defects, resulting in large size nanocomposites with photoluminescence quantum yield of ~90% and light yield of about 1000 ph/MeV for NC loading of only 0.2wt%. Radiometry experiments after as much as 1 MGy of certified γ-ray dose from ⁶⁰Co source reveal perfect radiation hardness. Also fundamentally, pulsed X-ray scintillation measurements demonstrate ultrafast <60 ps scintillation due to multi-excitonic radiative decay as further confirmed by transient absorption measurements, indicating that CsPbBr₃ NCs are efficient scintillators despite the negative effects of nonradiative Auger decay.

The combination of fabrication scalability with ultrafast and stable scintillation under extreme operational conditions represents offers a valid platform for future advancements in radiation detection schemes, in particular for time-of-flight radiometry applications in high-energy physics and medical diagnostics.

In the last decades, the interest in nanoparticles in the biomedical field experienced a rapid growth because of the tunability of their physical and chemical properties and their rich surface chemistry that enables specific functionalization by design. Different classes of functional nanoparticles, including metals, semiconductors, metal/lanthanide oxides and organic or hybrid systems have found successful application in several medical branches, such as nano-therapy, diagnostics and imaging. Today, one of the most advanced biomedical uses of nanoparticles is offered by their strong interaction with ionizing radiation, which makes it possible to improve the effectiveness of conventional cancer treatments, imaging techniques and radiation dosimetry [1]. In oncological therapies, one of the most adopted medical treatment is radiotherapy (ca. 50% of total cases), a non-invasive technique typically consisting in the local release of the energy of X-rays to stop tumor cells proliferation, either by directly damaging their DNA, or indirectly by forming cytotoxic free radicals [2]. Among the different material classes with potential application in this field, metal halide nanocrystals (NCs) have recently attracted substantial attention for ionizing radiation detection, prized for their high average atomic number (Z) that enhances the interaction probability with ionizing radiation and strong robustness to prolonged exposure to ionizing radiation [3][4][5]. However, despite such promise, very few examples of medical diagnostic and therapeutic strategies based on metal halide NCs have been proposed mainly due to their low stability in aqueous environment resulting in their rapid dissolution and further consequent release of potentially harmful Pb²⁺ ions.

In this talk, we show that multicomponent systems consisting of lead halide perovskite NCs (CsPbX₃-NCs, X=Br, I) grown inside mesoporous silica nanospheres (NSs) with selectively sealed pores are potentially promising candidates for enhanced radiotherapy and radio-imaging strategies, yhanks to the intense scintillation and strong interaction with ionizing radiation of CsPbX₃ NCs with the chemical robustness in aqueous environment of silica particles. We demonstrate that CsPbX₃ NCs boost the generation of singlet oxygen species (¹O₂) in water under X-ray irradiation and the encapsulation into sealed SiO₂ NSs warrants perfect preservation of the inner NCs after prolonged storage in harsh conditions. We find that the ¹O₂ production is triggered by the electromagnetic shower released by the CsPbX₃ NCs with a striking correlation with the halide composition (I₃>I_3-xBr_x>Br₃), without quenching their radioluminescence. This opens to the possibility of designing multifunctional radio-sensitizers able to reduce the local delivered dose and the undesired collateral effects in the surrounding healthy tissues by improving a localized cytotoxic effect of therapeutic treatments and concomitantly enabling optical diagnostics by radio imaging.

One of the main thrusts of medical X-ray imaging is to minimize the X-ray dose acquired by the patient, down to the fundamental limit set by the Poisson photon statistics. Such low-dose X-ray detection characteristics have been demonstrated with only a few direct-detection semiconductor materials such as Si and CdTe; however, their industrial deployment in medical diagnostics is still impeded by elaborate and costly fabrication processes. Hybrid metal halide perovskites – newcomer semiconductors -– make for a viable alternative owing to their scalable, inexpensive, robust, and versatile solution growth and recent demonstrations of single gamma-photon counting under high applied bias voltages. The major hurdle with perovskites as mixed electronic-ionic conductors, however, arises from the rapid material's degradation under high electric field, thus far used in perovskite X-ray detectors. Here we discuss the negative effects of the ion migration on X-ray detection performance and demonstrate the mitigation path by utilizing the perovskite X-ray detectors in the photovoltaic mode of operation at zero-voltage bias. We show both countings of almost every single incoming photon and long-term stable performance, by employing thick and uniform methylammonium lead iodide single crystal films, solution-grown directly on hole-transporting electrodes The operational device stability is equivalent to the intrinsic chemical shelf lifetime of MAPbI₃, being at least one year in the studied case. Furthermore, direct readout array integration of detectors is demonstrated as well. A high spatial resolution of 11 lp mm^-1 is obtained with a linear detector array. We also comment the lack of a clear commonly accepted X-ray detection characterization, approach, particularly in material research community, often leads to a misguiding assessment of performance.  We propose the guidelines for the determination of figures of merit for the low-dose X-ray imagers. These findings pave the path for the implementation of hybrid perovskites in low-cost and low-dose commercial detector arrays for X-ray imaging.

Metal halide perovskites have now achieved impressive power conversion efficiencies (PCEs) in both single junction and tandem solar cells, making them promising candidates to solve energy and environment crisis. Record efficiencies of perovskite solar cells (PSCs) are obtained with doped spiro-OMeTAD as the hole transport layer (HTL). Conventionally, spiro-OMeTAD is doped by hygroscopic lithium salts with the assistance of volatile 4-tert-butylpyridine, which, however, brings a time-consuming doping process as well as poor device stability. To exclude the aforementioned disadvantages, researchers have tried to replace the LiTFSI with other dopants, like other metallic salts, F4TCNQ or organic radicals, which could also help to generate radicals improving the conductivity and get rid of the moisture sensitive byproduct LixOy. However, all the recipes with varied dopants can only achieve high PCE with tBP, but the mechanism behind have not been fully discussed to the point. Here, we develop an instantly effective (without post oxidation) doping strategy for spiro-OMeTAD, by employing stable organic radicals and ionic salts (referred to as ion-modulated (IM) radical doping). We achieve high power conversion efficiencies (PCEs) over 25% and much improved device stability under harsh conditions. In addition, the IM doping mechanism was revealed through advanced characterizations and theoretical analysis: electron transfer from neutral spiro-OMeTAD to radicals provide free holes to instantly increase the conductivity and work function (WF); the electrostatic interactions from ionic salts further modulate the WF by affecting the electron transfer activation energy. Our IM radical doping strategy proves effective with a variety of ionic salts, and the doped spiro-OMeTAD demonstrates universal applicability in different PSCs. The understandings on the doping mechanism also solve the mystery of tBP in conventional spiro-OMeTAD doping. Last but not least, the IM doping strategy addressed the importance of localized electrostatic environment in organic doping process, offering new insights for organic semiconductor doping by decoupling the conductivity and WF tunability, and can find applications in a number of optoelectronic devices.

Since the emergence of hybrid perovskite photovoltaics in 2009 the scientific community has witnessed disruptive progress of the technology in terms of solar to electric power conversion efficiency (25.7%) [1] accompanied by advancement in addressing stability issues and developing a deeper understanding of fundamental structure-processing-property relationships. Merely a decade after discovery, the commercialization of organic-inorganic metal halide perovskites photovoltaic devices is just around the corner! Many issues had to be tackled to meet industrialization, among them large area deposition techniques, safety standards and material wastage without compromising efficiency and stability. Saule technologies has been developing a fully scalable inkjet printing process of perovskite solar cell modules on lightweight flexible substrates. This talk will focus on recent advancements in the technology and the product development process required to bring inkjet printed perovskite modules closer to market entry. Moreover, it will underline the unique properties of the inkjet printing technique, which allow scalable solution-based fabrication of high-quality perovskite films and devices, processed in ambient atmosphere. [1] https://www.nrel.gov/pv/cell-efficiency.html

Recent experiments involving a new carbon electrode demonstrate a true fully roll to roll coated perovskite solar cell via a continous slot die coating method. All previous reports of roll to roll coated perovskite solar cells have completed the device off-line with an evaporated metal contact. The application of a wet carbon film continuously and compatibly with an underlying perovskite device stack in a moving web at manufacturing speeds is complex but game-changing. The ability to sequentially deposit all layers of the device stack culminating in a fully working device entirely in-line means that the promise of high volume “liquid in/solar cell out” can be realised.

This multifunctional carbon electrode can be safely deposited on top of a layered solar cell without any deformation or dissolution of the underlying layers. The new contact material overcomes issues of solvent incompatibility, interface incompatibility and narrow rheology and heating process windows.

In this talk we will present the development journey of this new electrode material and the recent succesful pilot run of the material. In particular we will show how we formulate a new carbon ink with solvent compatible with the perovskite stack that crucially has suitable boiling point for low temperature, high speed processing coupled with very low toxicity (no work place exposure limit). The solid loading of the ink is optimised for a rheological profile suitable for slot die and we demonstrate the roll to roll slot die coating of the electrode sequentially following the roll to roll coating of the NIP device stack incorporating a low temperature processed p type interlayer. The carbon ink is formulated with solvent system orthogonal with the device stack and with no detrimental action on the perovskite active layer as shown through X-ray diffraction analysis. Electrochemical impedance spectroscopy and steady state photoluminescence analysis reveal that charge transfer at the interface is equivalent to evaporated gold electrodes. Further, the device stack is demonstrated to have no detrimental effect on stability, unencapsulated cells retained 90% of original PCE at atmospheric temperature for 1000 hours and outperform gold electrode cells at elevated temperature.

This work introduces the very first entirely roll to roll devices achieving efficiency matching evaporated gold electrodes. This first fully roll-to-roll coated perovskite prototype promises the possibility of transferring to industrially efficient PV production in the near future.

Since 2014, Oxford PV has been focused on the integration of perovskite PV in tandem configuration with silicon, holding the world record in large area at 26.8%. Less than a decade later, perovskite-on-silicon modules are close to enter the market. This achievement is the result of an accelerated effort to address efficiencies, technology fundamentals, manufacturing processes and supply chain, all under a sustainable perspective.

Energy security concerns have accelerated the deployment of solar, this opportunity for a faster transition to clean energies, comes with the added responsibility to making sure it considers the needs of generations to come. In this presentation, I will show Oxford PV’s sustainable journey, where environmental considerations go in parallel with the selection of materials, product designs and manufacturing processes, together with operational factors. Perovskite PV market integration comes with many challenges, but also the great opportunity to be the most efficient and sustainable solar energy source

Abstract:

Wide bandgap perovskite materials show promising potential as tandem top cells to pair with silicon bottom cells and achieve power conversion efficiencies (PCEs) over 32%, while the fabrication costs are likely to stay low. To date, most of the efficient wide bandgap perovskite layers are fabricated by spin coating, which is difficult to scale up to large area and the crystallization mechanism remains unknown. In this work, we report on slot-die coating for an efficient, wide bandgap triple-halide perovskite, (Cs_0.22FA_0.78)Pb(I_0.85Br_0.15)₃ + 5 mol% MAPbCl₃^{1 2}. A suitable solvent system was designed and optimized specifically for the slot-die coating technique. We demonstrate that with this perovskite, our fabrication route enables a bandgap of 1.68 eV which is suitable for tandem solar cells, and without phase segregation typically observed for high Br loadings. The slot-die coated wet perovskite film was dried using a stream of nitrogen (N₂) from an ''N₂ knife'' with high reproducibility, and avoiding the need to use antisolvents.

We explored varying drying and annealing conditions from 100°C to 170°C and measured absolute as well as transient photoluminescence (PL) to extract information about the perovskite bandgap, quasi Fermi level splitting (QFLS) and charge carrier lifetimes. We find parameters allowing to crystallize the perovskite film into large grains reducing charge collection losses and thus enabling higher current density in solar cells (Fig. 1B). With annealing at 150 °C, an optimized tradeoff between crystallization and the detrimental formation of PbI₂ aggregates on the film’s top surface is found. Insitu Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) investigation of the solution intermediate and film annealing at various stages has also unveiled the perovskite crystallization and PbI₂ formation processes. With the optimized annealing conditions, we improve the cell stability and performance of perovskite single junction cells towards a stabilized power output of up to 19.4 %.

By integrating the optimized perovskite fabrication with commercial saw damage etched Czochralski silicon bottom cells, a two-terminal monolithic tandem solar cell with a PCE of 25.2 % on 1 cm² active area is demonstrated with fully scalable processes. Note that this is reached for a wafer thickness of around 120 µm, not enabling the full photocurrent potential in the NIR wavelength regime compared to thicker wafers (> 250 µm) that are typically used in literature. Furthermore, a 4 cm² tandem solar cell has been developed with this fabrication route and using screen printed silver front grids, yielding PCEs up to 24 %. 

Finally, we show a detailed comparison between spin coated and slot-die coated perovskite films. For the solar cells, we present the loss mechanisms as well as guidelines for further improving the printed films. With that, we highlight the high potential for slot-die coating as fabrication route for scalable and industrially relevant perovskite/silicon tandem solar cells.

While light-emitting diodes (LEDs) made from lead halide perovskites have demonstrated external quantum efficiencies (EQEs) well over 20% [1]–[4], their electrical stability must be addressed before they are seriously considered for commercial applications [5]–[8]. In an effort to improve the optoelectronic properties of lead halide perovskites for light emission, many researchers have investigated introducing both alkaline-earth metal ions [9], [10] (e.g., Ba²⁺ and Sr²⁺) and transition metal ions [11]–[13] (e.g., Mn²⁺, Zn²⁺, Cd²⁺, and Ni²⁺) into the B-site of the perovskite’s ABX₃ structure. Additionally, the factors that limit the electrical stability of perovskite LEDs remain under investigation [5]–[7], [14]–[16]. In this work, we dope Mn²⁺ ions into an organic-inorganic hybrid quasi-bulk 3D perovskite resulting into (PEABr)_0.2Cs_0.4MA_0.6Pb_0.7Mn_0.3Br₃ thin films with the addition of tris(4-fluorophenyl)phosphine oxide (TFPPO) dissolved in a chloroform antisolvent to achieve an EQE of 13.4% and a peak luminance of 95,400 cd/m². While the inclusion of TFPPO into the chloroform antisolvent dramatically increases the EQE of perovskite LEDs, the electrical stability is severely compromised. At an electrical bias of 5 mA/cm², our perovskite LED fabricated with a pure chloroform antisolvent (2.5% EQE) decays to half of its initial luminance in 90.68 minutes. Alternatively, our perovskite LED fabricated with TFPPO (13.4% EQE) decays to half of its initial luminance in 2.07 min. In order to investigate this trade-off in EQE and electrical stability, we study both photophysical and electronic characteristics before and after electrical degradation of the perovskite LEDs. We find that given identical electrical degradation conditions, the TFPPO-based device’s turn on voltage and overall electrical resistance increases in a much larger fashion as compared to the pure chloroform-based device. While the EQE characteristics of this Mn²⁺-doped perovskite LED show promise for B-site engineered perovskites, there is still large concern to simultaneously achieve both energy-efficient and electrically stable perovskite-enabled lighting. Uncovering the effects from the TFPPO additive on perovskite LEDs will reveal pathways on how to mitigate their negative consequences on electrical stability while retaining their energy-efficiency boosting properties.

After establishing themselves as promising active materials in the field of solar cells, perovskites are currently being explored for fabrication of low-cost, easy processable and highly efficient light emitting diodes (LEDs). Even though higher efficiencies are reported for perovskite-based LEDs (PeLEDs), the fabrication technique used is spin coated or vacuum evaporation. Herein, we show a successful incorporation of air stable inkjet-printed (IJP) NiOX as an electron blocking layer (EBL) in addition to poly(3,4‑ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) as hole injection layer. 2,4,6-tris[3(diphenylphosphinyl)phenyl]-1,3,5-triazine (POT2T) or inkjet-printed (IJP) SnO2 as hole blocking layer (HBL). Individual layer properties and the morphology of IJP NiOX were analysed through X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Vis spectroscopy. Comparing p-n junction diodes with and without IJP NiOX as EBL shows a low leakage current and good rectification behaviour. Owing to its higher valence band levels, high hole mobility, low trap density at its interface with perovskite, PeLEDs with IJP NiOX and POT2T achieved higher luminance values of 19230 cd m-2 and an EQE of ~ 2.5 % using IJP of high purity CsPbBr3 layers. Furthermore, all inkjet printed PeLEDs replacing IJP SnO2 reached a luminance of 324 cd/m2 with an EQE of 0.017 %. Thus, an ambient processed PeLED with inorganic perovskite sandwiched between inorganic charge injection layers (CILs) which are also inkjet-printed is demonstrated. The results potentially lay a route towards ambient processed air stable all inorganic fully inkjet-printed PeLEDs.

Engineering the chemical composition of metal-halide perovskites via halide mixing allows a facile bandgap modulation but renders perovskite materials prone to photoinduced halide segregation.[1] Triple-halide alloys containing Cl, I,
and Br were recently reported as a means to stabilize Cs_yFA_1–yPb(Br_xI_1–x)₃ perovskite under illumination.[2] Herein, these triple-halide alloys are found to be intrinsically less stable with respect to the reference I-Br in ambient conditions. By exploiting the influence of low-molecular-weight organic gelators on the crystallization of the perovskite material, a triple-halide alloy with improved moisture tolerance and thermal stability at temperatures as high as 120 °C is demonstrated.
The hydroxyl-terminated organic gelators are found to aggregate into nanoscale fibers and promote the gelation of the solvent inducing the formation of a 3D network, positively interfering with perovskite solidification. The addition
of a tiny amount of organic gelators imparts a more compact morphology, higher crystallinity, and compositional stability to the resulting triple-halide polycrystalline films, making them more robust over time without compromising the
photovoltaic performance.[3] Overall, this approach offers a solution toward fabrication of active perovskite materials with higher energy gap and improved stability, making these triple-halide alloys truly exploitable in solar cells.

Inorganic cesium lead iodide (CsPbI₃) perovskite solar cells (PSCs) have attracted enormous attention due to their promising thermal stability and optical bandgap (~1.73 eV), making them particularly well-suited for tandem device applications. Although the performance of CsPbI₃ PSCs has surged to over 20%, most high-performance devices are fabricated using the dimethylammoniumiodide (DMAI)-assisted method, which hinders the options for mass production. Furthermore, the potential presence of the organic component DMA in the final CsPbI₃ films remains under debate. Therefore, it is imperative to develop a plethora of methods to fabricate highly phase-pure CsPbI₃ thin films. However, many methods of fabricating such CsPbI₃ films require processing at high-temperatures (~340 ℃), which limits their applicability on flexible substrates. Thus, it is still challenging to achieve high-performing photovoltaic devices processed at low temperatures.

Here we reported a new method to fabricate high-efficiency and stable γ-CsPbI₃ PSCs at low temperatures (~180 ℃) by introducing long-chain organic cation salt ethane-1,2-diammonium iodide (EDAI₂) and regulating the content of lead acetate (Pb(OAc)₂) in the perovskite precursor solution. By optimizing the excess amount of Pb(OAc)₂ in the precursor solution, we demonstrate improved crystallinity, morphology and reduced carrier recombination are observed in the EDAI₂ and Pb(OAc)₂ synergistically stabilized CsPbI₃ films. By optimizing the hole transport layer of CsPbI₃ inverted architecture solar cells, we demonstrate efficiencies of up to 16.6%, which surpass previous reports examining γ-CsPbI₃ in inverted PSCs. Notably, the encapsulated solar cells maintain 97% of their initial efficiency at room temperature and dim light for 25 days, demonstrating the synergistic effect of EDAI₂ and Pb(OAc)₂ in stabilizing γ-CsPbI₃ PSCs.

Metal halide perovskite photovoltaics (MHP) faces fundamental challenges which hinder their tangible impact on the final applications, with performance and stability being critical limiting parameters. Adjusting the perovskite grain boundaries and the crystalline surface are the primary tools to overcome these issues. Additionally, the research in new selective transport materials also plays a key role in improving optoelectrical devices' stability and performance to their theoretical limit to extend the perovskite application range.

Beyond efficiency and stability issues, perovskite photovoltaics must rely on sustainable technology for a feasible and fast green transition. Sustainability simultaneously considers environmental, economic, and social dimensions. Any sustainable technology must: a) generate enough power with reduced space usage (efficiency), b) be cost-effective, and c) not be detrimental to the environment or society. The environmental impact of the device fabrication is seriously affected not only by harmful materials (heavy metals and solvents) but also by the energy consumption of the synthetic and deposition routes used at the laboratory scale, which are not realistic for scaled-up manufacturing. Therefore, developing low-demanding synthetic routes is compulsory for a fast green transition.

Here, we present the research already performed by our team towards the synthesis and passivation methods of device-oriented metal halide perovskites (monocrystals at the macro and nanoscale) and metal oxides as selective transport materials. Our synthetic routes are mainly based on the in-situ synthesis approach. This one-step, low-cost, and low-demanding approach offers the possibility of modifying the precursor solution formulation very easily to adapt them to commercial-available printing techniques, one of the reasons behind MHP's success. Regarding sustainability, the in-situ synthesis approach is a low-carbon footprint synthetic route compared to traditional wet chemistry and colloidal routes. Following the global tendency to develop solution-processed materials, we have been involved for years in the in-situ synthesis approach of different materials, from MHP [1,2] to metal oxides [3] and conducting polymers [4]. We will show our work with solution-processed MHP and transparent metal oxides formulated as precursor inks compatible with roll-to-roll printing for large-scale production. The working principle analysis, including electrical and optical techniques, displays how the optoelectrical response can be adjusted by controlling the crystal growth and synthesis conditions and surface passivation, which can be linked to fundamental variations in the surface and bulk structure and composition.

_{1. Low-demanding in situ crystallization method for tunable and stable perovskite nanoparticle thin films}

_{J. Noguera-Gómez, I. Fernández-Guillen, P. F. Betancur, V. S. Chirvony, P. P. Boix, R. Abargues.}

_{Matter. 2022, 5, 3541-3552}

_{https://doi.org/10.1016/j.matt.2022.07.017}

_{2. Spray-driven halide exchange in solid-state CsPbX3 nanocrystal films}

_{RI Sánchez-Alarcón, J Noguera-Gomez, VS Chirvony, H Pashaei Adl, Pablo P Boix, G Alarcón-Flores, JP Martínez-Pastor, R Abargues.}

_{Nanoscale. 2022, 14 (36), 13214-13226}

_{https://doi.org/10.1039/D2NR03262G}

_{3. Solution-processed Ni-based nanocomposite electrocatalysts: an approach to highly efficient electrochemical water splitting.}

_{J. Noguera-Gómez, M. García-Tecedor, J. F. Sánchez-Royo, L. M. Valencia Liñán, M. Herrera-Collado, S. I. Molina, R. Abargues, and S. Giménez.}

_{ACS Appl. Energy Mater. 2021, 4, 5255-5264}

_{https://doi.org/10.1021/acsaem.1c00776}

_{4. In Situ Synthesis of Polythiophene and Silver Nanoparticles within a PMMA Matrix: A Nanocomposite Approach to Thermoelectrics}

_{J F Serrano-Claumarchirant, A Seijas-Da Silva, J F Sanchez-Royo, M Culebras, A Cantarero, C M Gómez, R Abargues}

_{ACS Applied Energy Materials 2022, 5 (9), 11067-11076}

The mixture of inorganic quantum dots and hybrid perovskite in one nanocomposite is considered a favorable approach to overcome restrictions of metastable perovskite. However, to date only few examples of improved opto-electronic perovskites have been realized with such materials. Here, we show a systematic approach to characterize the standard methylammonium lead iodide (MAPbI3) perovskite system by: (i) the substitution of some MA by guanidinium (Gu); (ii) the incorporation of PbS quantum dot (QD) additives and (iii) addition of both Gu and PbS at the same time. We studied the effect of the incorporations of the big cation on the film strain and crystal cell unit volume, and on the solar cell device efficiency and stability. With the control of Gu and PbS QD content, higher performance and longer solar cell stability are obtained. PbS QDs aid Gu incorporation, resulting in an expected more stable material and devices with high amount of guanidnium. From the optical point view, the tuning of the energy levels of perovskite nanocomposite with PbS with different ligands will be also exemplified, to show that a slight shift of the energy level leads to improved efficiencies especially in the case of formamidinium/PbI₂ capping ligand. With this study, we demonstrated the reason why the solar cells performances are improved up to 20%, even when the optical properties are detrimental.

Since the seminal report on the 9.7% efficient and 500 h-stable solid-state perovskite solar cell (PSC) in 2012 based on methylammonium lead iodide, power conversion efficiency (PCE) was swiftly increased to 25.7% due to unique photophysical property of halide perovskite. According to Web of Science, number of publications on PSCs increases exponentially since 2012, leading to the accumulated publications of more than 30,900 as of December 2022. PSC is regarded as a game changer in photovoltaics because of low-cost and high efficiency surpassing the conventional high efficiency thin film technologies. High photovoltaic performance was realized by compositional engineering, device architecture and fabrication methodologies for a decade. Toward theoretical efficiency over 30% along with long-term stability, exquisite control of light management and photo-excited charges are highly required, along with thermodynamic phase stability. In this talk, facet engineering of perovskite films is reported. A specific crystal facet was found to have strong interaction with photon, leading to high photocurrent. Furthermore, a certain facet was found to be quite stable under moisture and PSC based on a perovskite film with abundant moisture-tolerant facet was remarkably stable in humid atmosphere even without encapsulation.

Halide perovskites constitute a fascinating family of materials that has revolutionized the optoelectronic field in the last decade. Halide perovskite are soft materials presenting a benign defect physics that can be prepared a relative low temperature. This appealing property, from the point of view of industrialization, also makes of material and device stability the Achiles heel of the optoelectronic halide perovskite technology. Consequently, the development of stabilization procedures is probably the current main topic of these systems. The soft nature that limits stability also allows the easy combination of halide perovskites with other materials. In this talk we show as an accurate choice of additives can promote interesting synergies boosting the long term stability of halide perovskites devices. We show how the use of PbS quantum dots increases significantly the stability of FAPbI3. Incorporation of PbS QDs allows the dramatic decrease of the annealing temperature for the formation of black FAPbI3 phase perovskite thin film, from the 170ºC required without QDs to 85ºC when QDs are present. We have also verified the synergic combination of different additives for a significant increase of device stability of Pb free Sn-based solar cells, LEDs and lasers. Eventually, stabilization of halide perovskite nanoparticles is a necessary step for the development of high performance Halide Perovskite LEDs. Control of post synthetic washing processes as well as nanoparticle capping allows the preparation of LEDs with enhanced performance and stability.

Mixed Tin/Lead (Sn/Pb) perovskites have the potential to achieve higher performances in single junction solar cells than Pb-based compounds. The best Sn/Pb based devices are fabricated in the p-i-n structure and frequently PEDOT: PSS is utilized as hole transport layer, even if there are many doubts on a possible detrimental role of this conductive polymer. Here, we propose the use of [2-(9H-Carbazol-9-yl)ethyl]phosphonic acid (2PACz) and the functionalized [2-(3, 6-dibromo-9H-carbazol-9-yl) ethyl] phosphonic acid (Br-2PACz) version, as substitutes for PEDOT: PSS. By using Br-2PACz as HTL we achieve record efficiency (19.51%) with the perovskite composition Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ without any anti-reflective coating. The halogen functionalization of the SAMs is an efficient way to improve both the device performances and stability. Several factors seem to determine these improvements. The two carbazole-based molecules are able to form a self-assembled monolayer which show minimal parasitic absorption and low charge recombination when compared to PEDOT: PSS films. Additionally, the perovskite layer deposited on SAMs show an higher crystallinity, with reduced pinhole density and larger grains. The defect density for the perovskite films was also reduced when deposited on Br-2PACz or 2PACz compared to PEDOT: PSS. PL and TRPL measurements further confirmed the better perovskite film quality when deposited on SAMs, with reduced charge recombination.

Finally, the wettability of the perovskite precursor solution on top of the SAMs is a problem which limits SAMs application in Sn- or mixed Sn/Pb- perovskite solar cells on a large scale, limiting enormously the yield of device fabrication. I will present a solution for this issue, that may allow to improve both the scalability and reproducibility of the SAMs-based perovskite solar cell.

Metal-Halide Perovskite (MHP) photovoltaic (PV) modules are at the cusp of commercialization. One major hurdle that remains is establishing confidence in long-term field performance and durability of MHP modules. A lot of progress has been made in addressing many performance stability issues in MHP cells and modules; however, there is still work to be done to understand degradation and demonstrate real world operation of this technology. In this talk I will share examples of both lab-scale and field studies to improve our understanding of MHP solar cell and module degradation mechanisms. First, I will discuss an application of X-ray scattering methods to probe the nanoscale heterogeneity of metal halide perovskite absorber layers and couple these results to device level stability studies to understand the role heterogeneity plays in device performance. I will then present a brief overview of an initial field demonstration of MHP modules as part of the Perovskite PV Accelerator for Commercializing Technologies (PACT) program. Together this work aims to improve our confidence in real world MHP module performance.

In this work I will present two new concepts related to hybrid perovskite synthesis and devices.

(i) Chiral molecules were implemented into hybrid perovskite forming 2D hybrid perovskite with chirality properties. We used the two enantiomers R)-(+)-α-Methylbenzylamine (R-MBA) and, (S)-(-)-α-Methylbenzylamine (S-MBA). The chirality is manifested at low n values and pure 2D structure measured by circular dichroism (CD). The anisotropy factor (gabs) decreased by an order of magnitude when decreasing the n value achieving 0.0062 for pure 2D. Ab initio many-body perturbation theory successfully describes the band gaps, absorbance and CD measurements. For the first time these quasi 2D chiral perovskites were integrated into the solar cell. Using circular polarization (CP) and cut off filter we were able to distinguish the chirality effect from the solar cells photovoltaic response.

(ii) In the second part we developed unique fully printable mesoporous indium tin oxide (ITO) perovskite solar cell. In this structure, the perovskite is not forming a separate layer but fills the pores of the triple-oxide structure. One of the advantageous of this solar cell structure is the transparent contact (mesoporous ITO) which permit the use of this cell structure in bifacial configuration without the need for additional layers or thinner counter electrode. We performed full characterizations on both sides (i.e. ITO-side and glass-side) and elucidate the solar cell mechanism, where the glass side show 15.3% efficiency compare to 3.8% of the ITO-side. Further study of the mechanism shows that the dominant mechanism when illuminating from the glass-side is Shockley-Read-Hall recombination in the bulk, while illuminating from the ITO-side show recombination in multiple traps and inter gap defect distribution which explain the poor PV performance of the ITO-side. Electrochemical impedance spectroscopy shed more light on the resistance and capacitance. Finally, we demonstrate 18.3% efficiency in bifacial configuration. This work shows a fully printable solar cell structure which can function in bifacial configuration.

We fabricate and characterize carbon-based  lead halide perovskite solar cells composed of a mesoscopic scaffold of metal oxides that is screen printed and infiltrated with a lead halide perovskite precursor solution with a methylammonium cation.(MAPbI3)  We characterize the cells over time to investigate degradation pathways and improve fabrication methods. We measure the current produced by the cells under various illumination conditions as a function of applied bias voltage (IV curves) as well as spatially mapping both the structure and the photovoltaic performance of our cells to track and classify defects using both optical micrographs and Light Beam Induced Current (LBIC) imaging.   Observations of  the spectral response of the cells enables us to determine the External Quantum Efficiency (EQE) of our devices as well.  

In an undergraduate laboratory environment we fabricate and characterize Screen-Printed Mesoporous Carbon Perovskite Solar Cells (CPSCs).  The fabrication is based on pioneering work by Hongwei Han's research group.[1]  We adapt our methods from those developed by Trystan Watson's group [2].  We start with FTO coated glass substrates and laser engrave isolation lines.  We spay coat a compact titania layer followed by screen printing a mesoporous layers of titania (for electron transport), zirconia (for a spacer), and Carbon (for hole transport and a back contact).  A Methylammoina Lead Iodide (MAPbI) perovskite precursor is then inflitrated into the mesoporous layers crystalizing to form the perovskite semiconductor structure.  Silver contact electrodes are added to complete the devices.  The fabrication is performed in an ambient environment and no encapsulations are added to the devices.  Each substrate has 36 devices each with an active area of 0.49 sq. cm.  We have produced over a thousand devices with 4 generations of students in the lab. 

We characterize our devices with a wide range of techniques.  Current Voltage characteristics are measured for all devices.  Hero cells have power conversion efficiencies over 12%.  The devices show negligible hysteresis, and are limited in performance by moderate shunt and series resistances.  New higher conductivity Carbon and Silver ink formualations have been tested with significantly better conductivities that have improved Fill Factors and reduced series resistance.  We observe a variety of spatial defects by both LBIC and optical micrographs in the printing process and do statistical analysis of yields on every run to optimize our initial device performance.  We measure EQEs with peaks approaching 80% for freshly made devcies.

We have performed dark storage shelf life measurements over two years.  While devices still work after two years in dark storage, the performance decreases and defects clearly evolve as seen in our spatial imaging over time.  We also perform light soaking studies of both individual cells and modules of cells in series.  We record not only the IV characteristics and the IV curve evolution, but also the changes in EQE and spatial imaging as a function of light soaking under 1 sun conditions with no encapsulation or UV filtering.  We will present these data and discuss what this says about the degradation of these CPSCs.    

In semiconductors, exciton or charge carrier diffusivity is typically described as an inherent material property. Here, we show that the transport of excitons (i.e., bound electron-hole pairs) in CsPbBr3 perovskite nanocrystals (NCs) depends markedly on how recently those NCs were occupied by a previous exciton. Using fluence- and repetition-rate-dependent transient photoluminescence microscopy, we visualize the effect of excitation frequency on exciton transport in CsPbBr3 NC solids. Surprisingly, we observe a striking dependence of the apparent exciton diffusivity on excitation laser power that does not arise from nonlinear exciton-exciton interactions nor from thermal heating of the sample. We interpret our observations with a model in which excitons cause NCs to undergo a transition to a metastable configuration that admits faster exciton transport by roughly an order of magnitude. This metastable configuration persists for ~microseconds at room temperature, and does not depend on the identity of surface ligands or presence of an oxide shell, suggesting that it is an intrinsic response of the perovskite lattice to electronic excitation. The exciton diffusivity observed here (>0.15 cm2/s) is considerably higher than that observed in other NC systems on similar timescales, revealing unusually strong excitonic coupling in a NC material. The finding of a persistent enhancement in excitonic coupling between NCs may help explain other extraordinary photophysical behaviors observed in CsPbBr3 NC arrays, such as superfluorescence. Additionally, faster exciton diffusivity under higher photoexcitation intensity is likely to provide practical insights for optoelectronic device engineering.

Sub-nanosecond radiative decay time was first observed for weakly bound excitons in bulk semiconductors and was understood in terms of the phenomenon known as giant oscillator strength (GOS).¹ GOS is a quantum phenomenon connected with coherent excitation of excitons over the entire volume of exciton localization, and is counterintuitive because the radiative decay time is inversely proportional to this volume. Consequently, it was predicted theoretically² that an exciton weakly confined in a nanocrystal (NC), with radius a much larger than the exciton radius a_ex, is characterized by GOS with magnitude f_NC = f₀(a/a_ex)³ >> f₀, where f₀ is the exciton oscillator strength. Indeed ~100 ps radiative decay times were observed in large-size CsPbX₃ (X = Cl, Br, I) perovskite NCs.³

The development of organic-inorganic perovskite photovoltaics (PV) and radiation detectors (RD) has been particularly impressive. In the past 10 years, perovskite-based PV cells have reached a certified efficiency of 25.2% and perovskite-based RDs have demonstrated comparable progress by combining remarkable defect tolerance, large mobility-lifetime products, tunable band gaps, crystal growth from low-cost solution processes, and strong stopping power from Pb. We connect this progress with suppressed recombination in this material. Our analysis of the best PV cells, including perovskites, shows that their efficiencies are very close to ultimate PV limits in the absence of carrier recombination.⁴ The recent data on perovskite RDs⁵ also indicate, from our point of view, the advent of ultimate collection efficiency. For such RDs, we provide a theory that describes current collection efficiency in the absence of the carrier recombination.⁵

¹ E. I. Rashba and G. E. Gurgenishvili, Edge absorption theory in semiconductors. Sov. Phys. Solid State, 4, 759-760 (1962).

² Al. L Efros and A. L. Efros, Interband absorption of light in a semiconductor sphere, Sov. Phys.

³ M. A. Becker, et al. “Bright triplet excitons in caesium lead halide perovskites,” Nature, 553, 189-193 (2018).

⁴Al. L. Efros and V. G. Karpov “Electric power and current collection in semiconductor devices with suppressed electron−hole recombination”, ACS Energy Lett. 2022, 7, 3557−3563

⁵ M. Kovalenko, et al. Stable Near-to-Ideal Performance of a Solution grown Single-Crystal Perovskite X-Ray Detector. 2022, https://doi.org/10.21203/rs.3.rs-1117933/v1

Halide perovskites are exciting materials ushering in a new wave of photovoltaic and light-emitting technologies, with efficiencies already comparable to or beating more traditional semiconducting systems. However, a full understanding of carrier recombination and how it relates to performance losses and device operation in various device structures remains elusive. Here I will present results where we exploit luminescence from these materials to understand carrier recombination. I will give examples of different dimentionality systems and where carrier/exciton funnelling is critical for device operation. The luminescence approaches also allow tracking of device operation over time, and a decoupling of the impact of the active layer and contacts in understanding performance losses. These results allow us to draw generalised conclusions about carrier recombination and device performance and instability pathways driven by carrier trapping. Finally, I will show how we are exploiting this understanding to develop new solar cell and lighting devices, further pushing performances up.

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies.^1,2 While strongly confined III–V and II–VI QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band degeneracies are absent around the bandgap reducing the complexity of optical spectra. We show that for spherical-like CsPbBr₃ QDs with diameters >6 nm,³ excitons confine with respect to their center-of-mass motion leading to well-pronounced resonances in their absorption spectra. Optical pumping of the lowest-confined exciton with femtosecond laser pulses not only bleaches all excitons but also reveals a series of distinct induced absorption resonances which we attribute to exciton-to-biexciton transitions and are red-shifted by the biexciton binding energy (∼40 meV).⁵ The temporal dynamics of the bleached excitons further support our exciton confinement model. Our study provides the first insight into confined excitons in CsPbBr₃ QDs and gives a detailed understanding of their linear and nonlinear optical spectra.

All-inorganic lead-halide perovskite (CsPbX₃, X = Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser)^[1], as well as for devices exploiting strong light-matter interaction and operated at room-temperature.^[2] Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, e.g., by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity^{[3, 4]} in quantum light sources. Here, by combining single-QD optical spectroscopy performed at cryogenic temperatures in combination with configuration interaction (CI) calculations, we address the trion and biexciton binding energies and unveil their peculiar size dependence. We find that trion binding energies increase from 7 meV to 17 meV for QD sizes decreasing from 30 nm to 9 nm, while the biexciton binding energies increase from 15 meV to 30 meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. Our findings provide a deep insight into the multiexciton properties in all-inorganic lead-halide perovskite QDs, essential for classical and quantum optoelectronic devices.

Lead-based semiconductors are among the most explored compounds for the synthesis of colloidal nanomaterials, mainly due to the appealing optoelectronic properties demonstrated by lead halide perovskites in the UV-VIS and by lead chalcogenides in the IR spectral ranges. The mature research on both classes of materials has recently led to the exploration of systems where such properties can coexist and interact. One promising direction is that of heterostructures, which are composite materials formed by intimately connected domains of two different compounds. They are generally challenging to obtain, mostly because of the poor compatibility in terms of required synthetic conditions and crystal structures of the two materials. Another direction to investigate is represented by compounds that are chemically related to both lead halides and lead chalcogenides, namely the lead chalcohalides. These materials, with general formula PbaEbXc ,(E=S, Se, Te, X=F, Cl, Br) are expected to show intermediate properties in between those of lead halides and chalcogenides, and more importantly might feature the chemical and structural compatibility needed to interface with both. We pioneered the investigation of lead chalcohalides at the nanoscale, discovering additional colloidal nanocrystals (NCs): the new compounds are semiconductors with a band gap in between those of lead halide perovskites and of lead sulfide, were obtained in relatively mild reaction conditions and feature a remarkable chemical stability. Moreover, we achieved the synthesis of colloidal heterostructures formed by epitaxially connected domains of Pb₄S₃Br₂ sulfobromide and CsPbX₃ perovskite, thus demonstrating a synthetic and structural compatibility between lead halide perovskites and lead chalcohalides. Stability of the system is highly increased respect to CsPbX3 NCs We take advantage of the domains? compatibility and exploit CsPbCl₃ perovskite NCs as disposable and phase-selective epitaxial templates to drive the synthesis of lead sulfochloride NCs. As a result, we expand the family of lead chalcohalides with two new phases: Pb₃S₂Cl₂ and Pb₄S₃Cl₂.The perovskite domain is called a disposable template because, at a later stage, it can be etched from the heterostructures by exploiting the solubility of CsPbCl₃ in polar solvents, while leaving the Pb₄S₃Cl₂ domains intact. Hence, the full procedure delivered colloidally stable Pb₄S₃Cl₂ NCs that could not be obtained by direct synthesis due to the competitive nucleation. Our use of perovskite NCs as disposable and phase-selective epitaxial templates parallels that of reaction-directing groups in traditional organic chemistry and catalysis. Such an approach to a deterministic synthesis of NCs might be extended to other pairs of materials with known or predictable epitaxial relations, taking advantage of the vast library of already reported nanomaterials as starting templates. This approach could open new routes for the colloidal syntheses of materials which are now hindered by an excessive activation energy for the homogeneous nucleation, or by the competitive formation of undesired phases

Perovskite nanocrystals are highly advantageous semiconductor materials for tailored light applications [1]. Large quantum yields, narrow emission and broad spectral tunability are only some of the unique optoelectronic properties at the center of their beneficial performance. On the other hand, flat patterned microstructures have emerged as powerful platforms for controlled light-matter interactions [2]. When asymmetrically shaped nano-objects are chosen as the unit cell of the array structure, these near-field interactions get dependent on the circular polarization state. Particularly nonlinear optoelectronic devices governed by multi-photon processes could benefit from such a near-field control.

With the aim to equip perovskite nanocrystals with chiral effects, we have built a hybrid perovskite-metasurface system [3]. A suitable chiral z-shaped Si nanoantenna array was coated with a monolayer of cubic all-inorganic lead halide perovskite nanocrystals. The nanoantenna array exhibits pronounced chiral resonances in the visible to IR region which allow to confine the excitation light in the fabricated nanostructures. We demonstrate that the chiral near-field interactions can serve to induce polarization effects in the two-photon absorption process of the perovskite nanocrystals. By tuning the thickness of the perovskite film down to one monolayer, we restricted the interactions exclusively to the near-field regime.  We show that the layer’s two-photon excited luminescence is enhanced by up to one order of magnitude in this configuration. In particular, the enhancement is controllable by the excitation wavelength and by its polarization, revealing a pronounced fluorescence detected circular dichroism of the hybrid emitter-antenna system. Altogether, our findings present a pathway to control perovskite light emission via polarization sensitive near-field interactions, highlighting the potential of this hybrid system for sensing applications and display technologies.

In recent years, scintillating nanoparticles have been recognized as valid potential alternatives to inorganic and organic bulk scintillators, since they meet the needs and the demanding requirements of cutting-edge applications, such as nuclear and homeland security technologies, clinical and imaging devices. Nanoscintillators feature adaptable luminescence and scintillation properties, tuned by their physical and chemical characteristics, like the electronic structure, the dimensionality, and the defectiveness [1]. Most importantly, nanoscintillators can be embedded in suitable polymeric hosts to create composite materials and produce fast, efficient, and more sensitive detectors in a cost-effective way, thanks to reduced time-consuming synthesis procedures, to meet the specific demands of all up-to-date technological and medical applications [2].

The modern research is considering scintillating nanocomposites based on lead halide perovskite (LHP) nanocrystals (NCs) for the next generation of scintillation detectors [3,4]; when embedded in polymeric matrices, LHP NCs favors the enhancement of the interaction cross-section with ionizing radiation, thanks to their high atomic number [5], and retain exceptional levels of radiation hardness [6]. The implementation of this novel class of scintillating composites is imperative and aims at the achievements of scintillators with improved performances. This goal is essentially linked to the fundamental understanding of the correlation between the physical-chemical properties and the luminescence features and to the comprehension of the scintillation mechanism in nanosystems, from the primary interaction with the ionizing radiation, through energy transfer and trapping processes, to the emission of light. A fundamental stage in the scintillation process is the transport of free carriers generated upon the interaction between ionizing radiation and the scintillating material: it is often largely affected by the presence of trapping sites, which can capture migrating charge carriers and either delay their radiative recombination or decrease the overall scintillation efficiency, according to the characteristics of the traps involved.

In this work, we disclose the role of trapping defects and their interplay with the scintillation properties of LHP NCs and nanocomposites. Specifically, we present a thorough investigation of the tight correlation existing between delayed scintillation processes and defects acting as carrier traps, as well as of the competition between trapping sites and luminescent centers in free carrier capture. To these purposes, steady-state radio-luminescence as a function of both temperature and cumulated X-ray dose is combined with time-resolved photo-luminescence measurements and wavelength-resolved thermally stimulated luminescence (TSL) at cryogenic temperatures. The performances and defectiveness of CsPbBr₃ NCs prepared by hot-injection method are investigated and compared with those of CsPbBr₃ NCs produced by ligand-assisted reprecipitation synthetic approach. Our results suggest that shallow trap states, likely related to bromine vacancies, capture and slowly release electrons via a-thermal tunnelling to spatially correlated emissive centers responsible for delayed emission: because of the relatively large concentration of such defects, electron trapping in shallow defects is the main competitive channel to radiative exciton decay. The presence of surface-related defects is common in this class of materials, although advanced strategies for their passivation are continuously improved to enhance the emission efficiency. In addition, we prove that CsPbBr₃ NCs can be effectively embedded into polymethylmethacrylate matrix to obtain a high optical quality flexible and smooth scintillating nanocomposite film, whose defectiveness resembles that of LHP bulk crystals [7], featuring isolated energetically deep defects states that trap carriers which, upon heating, recombine in a specific intragap emission center, as revealed by TSL measurements. The effectiveness of this investigation approach coupling scintillation and TSL measurements, traditionally exploited only for classical single component bulk scintillators, is therefore here demonstrated also for nanosized materials.

Lead halide perovskite nanocrystals (LHP NCs) are an emerging class of light-emitting semiconductors owing to their remarkable optoelectronic properties such as their high photoluminescence quantum yield (PLQY), tunable emission across the visible spectrum emission, and facile synthesis. The optical properties of LHP NCs are not only tunable by their halide composition, but also through doping with metal cations. Generally, doping not only improves the stability of LHP NCs but also reduces Pb-related toxicity by replacing them with non-toxic dopants. Among all, Mn2+-doped LHP NCs have received significant interest to understand the exciton-to-dopant energy or electron transfer process.1-4 The Mn2+-doping in CsPbCl3 NCs results in the transfer of the exciton energy from CsPbCl3 to the dopants leading to orange emission from a spin-forbidden Mn d–d transition. However, the energy transfer efficiency not only depends on the amount of the dopant and band alignment of the dopants with respect to the excitons but also on the surface traps. In this presentation, I will discuss our findings on the enhanced exciton-to-dopant energy transfer by post-synthetic surface treatment with didodecyldimethylammonium chloride (DDAC), a tightly binding ligand. The DDAC ligands have the ability to replace the weakly bound oleyalammonium cations and reduce the density of chlorine vacancies on the surface, resulting in an increase in the PLQY of Mn2+ ions. Ultrafast time pump-probe studies and time-resolved luminescence of dopants revealed that the DDAC ligands remove the surface traps and thus promote energy transfer as well as reduce the quenching of Mn2+ emission by the surface traps.

Perovskite material appeared to be perspective and promising in photovoltaic application, such as solar cells, photodetectors, light emitting diodes and etc. due to outrageous properties like high carrier mobility, effective light absorption, mechanical flexibility, cheapness and simplicity of production makes them worthy opponent to crystalline silicon in creation less environment polluted world for our children. Although perovskites are already started to be used in solar cell fabrication on the market level, the extrinsic and intrinsic stability of them can be greatly improved. The most discussed issue nowadays is halide movement under continuous light illumination for I-rich and Br-rich domains. I-rich domain act as recombination centers in halide perovskites what impedes the carrier generation in the bromine-rich domain and blocks the electron flow through the device and reduces the efficiency of solar cells.

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. In this work, we quantitatively compare experimentally the variety of dimensionality materials (such as bulk thin films, 2D Ruddlesden-Popper and 0D nanocrystals (NCs)) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate carrier diffusion lengths. Further highlighting the importance of these studies are prior results, which suggest increased stabilities of mixed-anion perovskite NCs to irradiation. Observed qualitative and quantitative agreement with theory support a band gap-based model for anion photosegregation. More importantly, they suggest that mixed-halide perovskite photostabilities can be predicted using local gradients of (empirical) Vegard’s law expressions of composition-dependent band gaps. We have recently tested this alternative photostability metric on a mixed-cation/mixed-anion system, MA_0.5Cs_0.5Pb(I_1−xBr_x)₃ compared to MAPb(I_1−xBr_x)₃ and predicted that smaller local band gap gradients indeed correspond to improved anion photostabilities. Thus, not only is the developed band gap-based photosegregation model predictive but future extensions may rationalize remaining unexplained phenomena in lead halide perovskites such as halide remixing under large excitation intensities.

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers.  Remarkably the organic spacers play crucial role in optoelectronic properties of these compounds. The characteristic for ionic crystal coupling of excitonic species to lattice vibration became particularly important in case of soft perovskite lattice. The nontrivial mutual dependencies between lattice dynamics, organic spacers and electronic excitation manifest in a complex absorption and emission spectrum which detailed origin is subject of ongoing controversy. First, I will discuss electronic properties of 2D perovskites with different thicknesses of the octahedral layers and two types of organic spacer.  I will demonstrate that the energy spacing of excitonic features depends on organic spacer but very weakly depends on octahedral layer thickness. This indicates the vibrionic progression scenario which is confirmed by high magnetic fields studies up to 67T. Next, I will show that in 2D perovskites, the distortion imposed by the organic spacers governs the effective mass of the carriers.  As a result, and unlike in any other semiconductor, the effective mass in 2D perovskites can be easily tailored. In the end, I will discuss exciton fine structure. The bright-dark splitting is also of paramount importance for light emitters which rely on the radiative recombination of excitons, since the excitons usually relax to the lowest lying dark state, which is detrimental for the device efficiency. I will discuss our optical spectroscopy measurements with an applied in-plane magnetic field to mix the bright and dark excitonic states of (PEA)₂PbI₄, providing the first direct measurement of the bright-dark splitting. The induced brightening of the dark state allows us to directly observe an enhancement of the absorption at the low-energy side of the spectrum related to the dark state. The evolution of the PL signal in the magnetic field, suggests that at low temperatures the exciton population is not fully thermalized due to the existence of a phonon bottleneck, which occurs due to the specific nature of the exciton-phonon coupling in soft perovskite materials.

Halide perovskites possess outstanding optoelectronic properties favorable for applications as highly efficient absorbers in perovskite photovoltaics and as bright emitters in perovskite light emitting devices, lasers as well as quantum emitters. Underpinning this spectacular rise are their exceptional properties such as large absorption cross-sections, defect tolerance, large spin-orbit coupling, long balanced charge diffusion lengths, slow hot carrier cooling, ion migration, radiation tolerance etc. Hence, there has been a rapid proliferation of their applications beyond solar cells and light-emitting devices to fields such as spintronics, radiation detectors, memristors, bioimaging etc. Their versatile structures and diverse dimensionalities afford new levers for tunning their photophysical properties. For instance, the vast library of large organic cations allows one to tune the energy landscape in layered perovskites. In this talk, I will distill some of the underpinning photophysical mechanisms in low-dimensional emitters such as layered Ruddlesden Popper perovskites and colloidal perovskite nanocrystals. I will also take the opportunity to highlight some of our latest works on this topic.

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 25%.

On issue that still limits the implementation of silicon-perovskite tandem cells in particular, is the peculiar mechanisms underlying detrimental halide segregation in mixed iodide-bromide lead perovskites with desirable electronic band gaps near 1.75eV.[1,2,3,4] We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities.[2] However, remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains leads to enhanced radiative recombination.[3] Performance losses in photovoltaic devices may therefore potentially be mitigated by deployment of careful light management strategies. We further demonstrate[3] how a combination of simultaneous in-situ photoluminescence and X-ray diffraction measurements is able to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. While MAPb(Br0.5I0.5)3 exhibits rearrangement of halide ions only in localized volumes of perovskite, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation.[4] We further explore the influence of a hole-transport layer, necessary for a full device. We show that top coating FA0.83Cs0.17Pb(Br0.4I0.6)3 perovskite films with a poly(triaryl)amine (PTAA) hole-extraction layer surprisingly leads to suppression of halide segregation because photogenerated charge carriers are rapidly trapped at interfacial defects that do not drive halide segregation.[4]

We also discuss the charge-carrier dynamics in layered, 2D perovskites that have been found to improve the stability of metal halide perovskite thin films and devices.[5] We show that the 2D perovskite PEA2PbI4 exhibits an excellent long-range mobility of 8.0 cm2 (V s)–1, ten times greater than the long-range mobility determined for a comparable 3D material FA0.9Cs0.1PbI3. These values shows that the polycrystalline 2D thin films already have single-crystal-like qualities. We further demonstrate that these materials exhibit unexpectedly high densities of sustained populations of free charge carriers.

[1] A. J. Knight and L. M. Herz, Energy Environmental Science 13, 2024 (2020).

[2] S. G. Motti, J. B. Patel, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, L. M. Herz, Nature Communications 12, 6955 (2021).

[3] A. J. Knight, J. Borchert, R. D. J. Oliver, J. B. Patel, P. G. Radaelli, H. J. Snaith, M. B. Johnston, and L. M. Herz, ACS Energy Letters 6, 799 (2021).

[4] Impact of hole-transport layer and interface passivation on halide segregation in mixed-halide perovskites, V. J.-Y. Lim, A. J. Knight, R. D. J. Oliver, H. J. Snaith, M. B. Johnston, and L. M. Herz, Advanced Functional Materials 32, 2204825 (2022).

[5] Excellent long-range charge-carrier mobility in 2D perovskites, M. Kober-Czerny, S. G. Motti, P. Holzhey, B. Wenger, J. Lim, L. M. Herz, and H. J. Snaith, Advanced Functional Materials 32, 2203064 (2022).

Lead halide perovskite nanocrystals are promising candidates for applications in light-emitting devices due to their high exciton binding energy, exceptionally high quantum efficiency, and good stability. Depending on crystal symmetry and shape anisotropy, a distinct exciton fine structure controls their emission properties [1]. Here, we demonstrate the strength of polarization-resolved micro-photoluminescence (PL) spectroscopy on a single nanocrystal level for getting insight into exciton fine structure states and their relation to crystal symmetry and shape anisotropy.

In nearly cubic FAPbBr₃ nanocrystals, the degeneracy of the bright exciton triplet is lifted leading to three bright states with transition dipoles oriented along the orthorhombic crystal symmetry at cryogenic temperatures. Depending on the orientation of the nanocrystals with respect to the optical axis between one and three polarized emission lines are visible [2]. Magneto-PL and time-resolved-PL experiments on single nanocrystals demonstrate that the dark singlet exciton is energetically below the bright one, shifted by about 2.6 meV.

In highly anisotropic CsPbBr₃ nanoplatelets (NPLs) either one, two or three resolvable emission lines (case I, II and III, respectively) with significantly different polarization patterns are found. In case a single peak is observed, the emission is mostly unpolarized and linewidths generally exceed 1 meV. In contrast, the polarization of the two emission lines of case II NPLs are oriented orthogonally with respect to each other. In case III NPLs, the lowest and highest energy peaks are polarized collinearly, while the central emission line is polarized in a direction orthogonal to the former two. This quite characteristic polarization pattern can be explained by the occurrence of orthorhombic CsPbBr₃ NPLs with two different orientations of the crystal axes [3]. In case II NPLs, one of the orthorhombic crystal axes is aligned with the NPL thickness and the observation direction and the radiation emitted by the corresponding dipole cannot be detected. In case III NPLs, in contrast, all orthorhombic crystal axes have a finite projection perpendicular to the observation direction allowing the observation of all emission lines of the fine structure split triplet. The negligible fine structure splitting in case I NPLs is consistent with the coexistence of these different lattice polymorphs within a single NPL. Our findings not only allow the unambiguous identification of the crystal configuration of individual CsPbBr₃ nanoplatelets from pure optical measurements, but in addition facilitate the determination of a nanoplatelets’ absolute spatial orientation with respect to the lab coordinates via its characteristic polarization pattern.[4]

2D perovskites are often used to stabilize perovskite solar cells. By adding a thin layer of 2D perovskite on top of a 3D bulk solar cell, stability can be massively improved. This effect has lead to the impression that 2D perovskites are stable materials. However, we show that these materials are extremely unstable under illumination, especially when combined with air. 

We quantify the decrease in photoluminescence after illumination. The photoluminescence first increases (photobrightens) and then decays dramatically. We show that this decay in luminescence coincides with a loss of material and a decomposition into gasses and precursors. We also show some initial mechanistic explanation for a better stability under nitrogen atmosphere.

Our results indicate that 2D perovskites are not intrinsically stable, but rather protect the underlying 3D perovskite from ambient influences, presumably by forming a hydrophobic barrier. Our results further illustrate the caution required when studying 2D perovskites because they continuously change during the measurement.

Recently, microcavity exciton polariton research has attracted considerable interests in a number of excellent optical gain materials that demonstrate unique properties compared with conventional III-V or II-VI semiconductor quantum wells and organic semiconductors. Those materials include transition metal dichalcogenides (TMDs), and certain halide perovskite semiconductors. Particularly, those materials exhibit large exciton binding energies (much larger than thermal fluctuation energy ~ 26 meV), large oscillator strength and peculiar electronic band structures such as valley polarization or encoded chiroptical responses. In this talk, we will discuss our recent effort in manipulating exciton polariton condensates in halide perovskite semiconductors microcavities, for instance using artificial lattices to engineer the strong optical responses including topological properties, and their ultrafast propagation. Finally, we will briefly discuss the nonlinear optical properties in polariton condensate trapped in artificial potential landscapes, by a pump-probe transient spectroscopy at momentum space. Our results demonstrate a promising perspective of polaritonics in a wide range of ultrafast optical and photonic applications at room temperature.

1. S. Ghosh, R. Su*, J.X. Zhao, A. Fieramosca, J.Q. Wu, T.F. Li, Q. Zhang, F. Li, Z.H. Chen, T.C.H. Liew, D. Sanvitto, Qihua Xiong*, “Microcavity exciton polaritons at room temperature”, Photonics Insights 1, R04 (2022) (review)
2. J.Q. Wu^#, S. Ghosh^#, R. Su*, A. Fieramosca, T.C.H. Liew*, and Qihua Xiong*, “Nonlinear parametric scattering of exciton polaritons in perovskite microcavities”, Nano Lett. 21, 3120–3126 (2021)
3. J.G. Feng et al., “All-optical switching based on interacting exciton polaritons in self-assembled perovskite microwires”, Science Advances 7, eabj6627 (2021)
4. R. Su et al., “Optical control of topological polariton phase in a perovskite lattice”, Science Advances 7, eabf8049 (2021)
5. R. Su et al., “Observation of exciton polariton condensation in a perovskite lattice at room temperature”, Nature Physics 16, 301-306 (2020)
6. R. Su et al., “Room temperature long-range coherent exciton polariton condensate flow in lead halide perovskites”, Science Advances 4, eaau0244 (2018)
7. R. Su et al., “Room-temperature polariton lasing in all-inorganic perovskite nanoplatelets”, Nano Lett. 17, 3982–3988 (2017)

Increasing the efficiency of light-emitting devices is currently a hot topic in the field of semiconductor research. Here, perovskites are of special interest due to their excellent optical properties. Additionally, the possibility of sample preparation by vacuum deposition makes this class of material suitable for future large-scale industrial production. It is known that light-emission efficiency can benefit from an island-type active layer structure. If the lateral dimension of the structures is sufficiently small, excitons are confined and thus are far less likely to dissociate prior to radiative recombination.

In this work, we investigate the impact of the environment on the island formation in thermally deposited bilayers of CsPbBr₃/LiBr. We demonstrate that the island formation occurs only in humid environments, leading to an enhancement of the photoluminescence quantum yield by a factor of 350. Time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments document the island growth process and reveal that the LiBr has already changed the perovskite crystal orientation before the island formation.

Despite the relatively early level of development, Perovskite light-emitting diodes (PeLEDs) have reached outstanding luminance and radiative efficiency levels that roughly graze the maximum theoretical efficiency limits. Unfortunately, the complete understanding of the working principles and the photo-electrochemical mechanisms involved in the charge carrier generation/recombination dynamics is still a conundrum. Additionally, the strong ionic character of MHPs enables the migration of ions and the gradual formation of crystalline defects upon exposing to light and/or to an external electric field, which aggravate the complexity of these systems. In fact, these ionic processes are apparently coupled with those electrical involved in the generation of light, and seem to be connected with the widely reported limited long-term stability of the devices. Here, I will discuss on the exploitation of a new methodology based on the combination of two frequency-domain modulated techniques, i.e. electrochemical impedance spectroscopy (EIS) and light emission voltage-modulated spectroscopy (LEVS), aimed at reaching a full understanding of the working principles of perovskite LEDs. Particularly important is the deconvolution of the electrical, optical and ionic processes that are involved in the current-to-photon conversion, heat generation and/or degradation of the materials employed. We propose a new theoretical model and an equivalent circuit that considers both the non-radiative and radiative contributions, as a new tool for the advanced characterization of perovskite-based optoelectronic devices. 

Metal-Halide perovskite materials are known to be of high potential for solar and LED applications thanks to their very high absorption coefficients, remarkably long carrier-lifetime and high photoluminescence quantum yield in comparison to traditional semiconductor absorbers [1], [2]. This is particularly remarkable given that high materials quality can be obtained even when samples are grown from solutions. It seems that not only their defect tolerance but, even more so, the self-healing ability of halide perovskites are a fundamental reason for their excellent opto-electronic properties. Nevertheless, dynamic effects also give rise to a number of meta-stabilities. Materials can be degraded and temporarily perturbed under external stressors, such as light and bias [3],[4],[5]. In this work, we focus on methyl-ammonium lead tri-bromide microplatelets, prepared following the work from Mao et. al. [6]. We use these microplatelets to study fundamental dynamics of charge carriers interacting with physical or light-induced defects generated in these crystals by external perturbation. The samples can be considered an idealized model-system that exclude typical intra-grain boundaries of polycrystalline thin film samples, thus removing the different phenomena that could occur at these local sites.

Within these model systems, we show a correlation between time-resolved photoluminescence decay dynamics and sample properties, perturbed by the history of the micro-sized sample to illumination, generating light-induced defects; or mechanical stress, generating structural defects. We utilized excitation-intensity and repetition rate-dependent photoluminescence measurements [7] to provide a "finger print" of samples exposed to different degrading conditions to investigate the characteristic difference between light- and mechanically-induced defects affecting the charge carrier dynamics within the crystal.

Finally, we refined a numerical model to fit photoluminescence lifetime data based on population rate equations, including trapped populations [8],[9]. A fit of the presented data is used to see how material properties, such as trap densities, are affected by illumination intensity and/or repetition rate.

Photoexcitation of semiconductors by photons of higher energy than the bandgap creates ‘hot-charge carriers’ (i.e. different thermal energy w.r.t. lattice), which further cool down to the band-edge state by releasing their excess energy as a waste form of heat. Harvesting the excess energy of these hot carriers (HC) could eventually boost the performance of solar cells by manifold, while practical realization of this is still limited owing to the rapid cooling of these HC [1, 2]. On the contrary, in light-emitting applications, rapid HC cooling is highly desired to enable efficient radiative recombination by preventing carrier trapping. Therefore, detailed understanding of the HC cooling mechanisms and further controlling their dynamical pathways is prerequisite for engineering the semiconductor optoelectronics.

Herein I will explore this key area on CsPbX3 perovskite nanostructures of various dimensions and compositions by employing novel pump-push-probe based transient spectroscopy [3], which provides direct access to selectively control the hot carrier density and study their influence. This experimental finding unravels the role of carrier-carrier, carrier-phonon interactions to carrier-impurity (defect) scatterings in tailoring the hot carrier cooling dynamics in perovskite nanosystems. Our results of HC cooling dynamics among halide-composition space with controlled defect densities reveals that this dynamics is defect-tolerant for pure-iodide based perovskites unlike pure-Br and mixed (Br/I)-systems. Importantly, we also examine the slowing down pattern of HC cooling dynamics under high HC-density (termed as hot-phonon bottleneck effect), whose effect is significantly suppressed with the increase in quantum confinement (from 3D to 2D systems) due to reduced screening by phonons and enhanced Coulombic interaction between electron and holes for the latter systems. Although halide-vacancy related defects accelerate the HC cooling dynamics for pure Br-and mixed (Br/I)-systems, but they do not induce any such effect on the hot-phonon bottleneck process possibly due to saturation of the trap-states. This detailed understanding of the key routes that control the HC cooling dynamics would be instrumental for prospective applications of the next-generation perovskite optoelectronics.

Lead halide perovskites (LHP) are rapidly emerging as efficient, low-cost, solution-processable scintillators for radiation detection [1], [2]. Carrier trapping is arguably the most critical limitation to the scintillation performance [3], [4]. Nonetheless, no clear picture of the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process has been reported to date, as well as on the role of the material dimensionality. In this talk, this issue is addressed by performing, for the first time, a comprehensive study using radioluminescence and photoluminescence measurements side-by-side to thermally-stimulated luminescence (TSL) and afterglow experiments on CsPbBr₃ with increasing dimensionality, namely nanocubes, nanowires, nanosheets, and bulk crystals. All systems are found to be affected by shallow defects resulting in delayed intragap emission following detrapping via a-thermal tunneling. TSL further reveals the existence of additional temperature-activated detrapping pathways from deeper trap states, whose effect grows with the material dimensionality, becoming the dominant process in bulk crystals. These results highlight that, compared to massive solids where the suppression of both deep and shallow defects is critical, low dimensional nanostructures are more promising active materials for LHP scintillators, provided that their integration in functional devices meets efficient surface engineering.

Organic-inorganic hybrid low-dimensional perovskites are attracting significant attention for optoelectronic applications due to their higher stability, uncomplicated manufacturing process, and wide tunability of the optical and electronic properties. The use of various organic cations leads to the formation of two-dimensional (2D) layers, one-dimensional (1D) chains, or isolated zero-dimensional (0D) clusters depending on how the metal halide octahedra are connected. Even minimal differences in the structure of the organic cations can cause remarkable changes in the structural arrangement and, as a consequence, a modification of the optoelectronic properties of the perovskite system.

The incorporation of functionalized ammonium cations in perovskites allows the design of a variety of new organic-inorganic hybrid materials, in which functionalized and even semiconducting organic layers are assembled with a semiconducting inorganic layer at the molecular scale. In recent works devoted to low-dimensional inorganic-organic halide perovskites much attention has been paid to the broadband emission properties primarily originating from the self-trapped excitons (STEs) [1,2]. High structural distortion of low-dimensional perovskite structures induces electron-phonon coupling and enhances the STE process, resulting in an enhanced broadband emission. In addition, it has been demonstrated that perovskites grown in a form of atomically thin sheets possess unique features compared to their bulk counterparts [3]. For example, hybrid perovskite sheets exhibit an unusual structural relaxation, which leads to a band gap shift. Consequently, the controllable synthesis of ultrathin perovskite sheets has gained a great research interest for implementation in high-performance optoelectronic devices.

In this work, a series of low-dimensional hybrid organic-inorganic metal halides, based on multiple-ring aromatic ammonium cations and lead iodide were fabricated and their structures and properties were investigated. Examination of the influence of organic cations on the structural properties reveals that the number and position of amino groups at the aromatic ring affect the dimensionality of the perovskite, resulting in 2D and 1D corner- and face-sharing structures with different optical behavior. Highly distorted 1D perovskites show broadband emissions originating from STEs. Theoretical results reveal, that the organic cations in the 1D compounds strongly contribute to the band structure resulting in strong orbitals hybridization. Using a facile and fast crystallization method we also synthesized organic-inorganic hybrid perovskite few-layer free-standing nanosheets containing the naphthalene diammonium-based linker. Varying the synthetic conditions, we can modulate the thickness and lateral sizes of the nanosheets.

The success of the colloidal semiconductor quantum dots (QDs) field is rooted in the precise synthetic control of QD size, shape, and composition, enabling electronically well-defined functional nanomaterials that foster fundamental science and motivate diverse fields of applications. While the exploitation of the strong confinement regime has been driving commercial and scientific interest in InP or CdSe QDs, such a regime has still not been thoroughly explored and exploited for lead-halide perovskite QDs, mainly due to a so far insufficient chemical stability and size monodispersity of perovskite QDs smaller than about 7 nm. Here, we demonstrate chemically stable strongly confined 5 nm CsPbBr₃ colloidal quantum dots via a post-synthetic treatment employing didodecyldimethylammonium bromide ligands. The achieved high size monodispersity (7.5%±2.0%) and shape-uniformity enables the self-assembly of QD superlattices with exceptional long-range order, uniform thickness, an unusual rhombic packing with an obtuse angle of 104º, and narrow-band cyan emission. The enhanced chemical stability indicates the promise of strongly confined perovskite QDs for solution-processed single-photon sources, with single QDs showcasing a high single-photon purity of 73% and minimal blinking (78% “on” fraction), both at room temperature.

Hybrid perovskites have recently emerged as a promising material for optoelectronic applications due to their unique combination of properties. In particular, their 2D crystalline form is of great interest since the alternating inorganic and organic layers naturally form a multiple quantum-well structure, leading to the formation of stable excitonic resonances at room temperature. However, a controlled modulation of the quantum well width, which is defined by the number of inorganic layers (n) between two organic ones, is not straightforward and represents the main synthetic challenge in the field. This study presents a novel approach to easily tune the number of inorganic layers (n) in lead iodide perovskite single-crystalline flakes. This approach exploits an iodide salt as an additive that acts as an additional source of I−, inducing the generation of iodoplumbate species and resulting in an increase of the monomer density, resulting in the fine-tuning of the n value. Unlike other methods, our protocol allows us to fix the molar ratio of precursors and to control the "n" value by simply modulating the amount of potassium iodide (KI) added to the solution. The excellent optical quality of the synthesized flakes enables an in-depth analysis by Fourier-space microscopy, which reveals that the excitons orientation can be manipulated by modifying the number of inorganic layers. It is found that the excitonic out-of-plane component is enhanced when n is increased. This combined advance in synthesis and optical characterization provides a better understanding of the exciton behavior in low-dimensional perovskites, opening up possibilities for the design of materials with improved optoelectronic characteristics.

Tin halide perovskites display similar or even superior electronic and optical properties when compared to well-established Pb-based perovskites. Despite their favorable optoelectronic properties, the tin perovskite solar cells still exhibit power conversion efficiency (PCE) much lower than that of their lead analogs. This is mainly attributed to the fact that metastable Sn(II) in perovskite lattice is prone to the oxidation. The presence of Sn(IV) in the material results in self-doping and formation of non-radiative recombination centers. One of the strategies to overcome this obstacle is addition of bulky A-site cations which stabilize the resulting 3D films in a quasi-2D fashion or preparation of 2D tin halide perovskites. Especially in recent years, single crystal 2D-tin halide perovskites have attracted significant attention. However, the synthesis and growth of such materials high-quality remain very challenging. In this work, we report a comprehensive study of the synthesis and the growth of single-crystal 2D tin halide perovskites (Ruddlesden-Popper), which is still poorly explored (excluding extensively studied materials containing PEA derivatives acting as an A-site cations). By careful control the growth conditions (temperature, and composition), we obtained mm-sized (up to 10 mm), high-quality single crystals of 2D tin halide perovskites. The single crystals were characterized structurally and optically by means of inter alia X-ray diffraction, steady-state photoluminescence, and optical absorption.

The introduction of chiral organic spacers in 2D and 1D perovskites breaks the inversion symmetry operation in the crystal, due to the asymmetric hydrogen bonding between the H atom of the ammonium group in the organic cation and the halide atom (Cl, Br, or I) in the lead octahedra. [1,2] The absence of inversion symmetry operation results in the emergence of Rashba/Dresselhaus spin-splitting, i.e. the splitting of the double degenerated electronic band into two bands with the same energy, but shifted in k-space,[3,4] triggering spin-dependent properties even in the absence of magnetic field. Such a feature has been utilized for applications in polarized light detectors,[5] spin-dependent charge transport,[6] and polarized light-emitting diodes.[7] In this work, we investigate how the processing of chiral 2D perovskites impacts their optical properties, microstructure, and phase purity. Specifically, we synthesized the R-, S-, and rac-MBA2PbI4 (MBA: methylbenzylammonium) and varied the solvent used for film deposition (dimethylformamide, DMF, or acetonitrile, ACN), and the subsequent thermal annealing procedure. We found that the processing conditions strongly impact the optical properties and microstructure of the chiral 2D perovskites. Moreover, we identified the anisotropic emergence of a 1D phase, which is dependent on the molecule's chirality and seems more prone to occur with pure enantiomers than in the racemic mixture. We demonstrate that its formation is suppressed when the 2D perovskite is processed from DMF, due to its higher boiling point and a stronger interaction between the solvent and Pb2+ ions, thus enabling a higher phase purity. These observations give fundamental insights into the film formation processes of chiral 2D perovskites and offer processing strategies to control anisotropic growth in 2D perovskite and to tune their chiroptical response.

Hybrid organic-inorganic perovskites have received tremendous research attention over the past decade for use in optoelectronic applications such as solar cells and photodetectors. Despite their excellent optoelectronic properties, their commercialization is hindered by their limited intrinsic stability. A prime route to obtain more stable perovskite materials is the addition of a large organic ammonium cation to form quasi-2D perovskites.^[1-2]

We employ a functionalized benzothienobenzothiophene (BTBT) organic cation either as an additive in the precursor solution to obtain a quasi-2D perovskite for use in photodetectors^[1], or as an interlayer in perovskite solar cells.

The BTBT cation was added to a CsPbI₃ precursor solution to obtain a n = 2 quasi-2D perovskite film. We compared the stability and performance of this film to that of a state-of-the-art (BA)₂CsPb₂I₇ (n = 2) film. While the (BA)₂CsPb₂I₇ film starts to degrade at 130 °C, the thermal stability of the film containing the BTBT cation is significantly enhanced to 230 °C. Furthermore, while the (BA)₂CsPb₂I₇ film degrades into (BA)₂PbI₄ and other compounds after 1 day of storage in air at 77% relative humidity, the film containing the BTBT cation shows excellent resistance against humidity, with no apparent degradation after 1 year of storage at high relative humidity. We fabricated planar photodetectors using both films. The specific detectivity of the photodetectors with films containing our tailored cation is similar in magnitude compared to those containing the state-of-the-art (BA)₂CsPb₂I₇ films. In summary, we prepared a quasi-2D perovskite with a similar optoelectronic performance as the literature reference but with significantly enhanced stability.

The same BTBT cation was also used as an interlayer in a solar cell between a NiO_x hole-transporting layer and a triple-cation 3D perovskite leading to an enhanced lifetime of photo-generated charge carriers based on TRPL measurements. Furthermore, a significant V_OC improvement is achieved in solar cells leading to a higher power conversion efficiency. The moisture stability of the solar cells was enhanced with the presence of the interlayer.

Starting from the well-studied ABX₃ (A = CH₃NH₃⁺, Cs⁺; B = bivalent metal cation, X = Cl^-, Br^-, I^-) perovskite, new derivates can be obtained with the partial or full substitution of the A and B cations with different metal ions [1]. Besides a change in the elemental composition, the dimension of the ion in the A site enlarges the formation of polymorphs of a lower dimensionality (2D or 0D). Indeed, the use of an organic cation, usually bigger than the inorganic metal ion, is one of the most promising strategies to tune and confer new optoelectronic properties in metal halide compounds [2]. For example, the 2D layered lead halides have excellent emission comparable with the 3D counterpart but with better electronic conductivity and quantum confinement effect observable even in the bulk materials [3].

Here, we present our recent results on the preparation of emissive 2D (Pb,Mn)-based hybrid metal halide as single crystals. Starting from the reported synthesis of Bz₂PbBr₄ (Bz⁺ = (C₆H₅CH₂NH₃)⁺), the Pb²⁺ was progressively replaced by Mn²⁺, even if the percentage of Pb substitution strongly depends on the metal ratio in the starting materials. The presence of Bz cation in the structure is confirmed by FTIR analyses. The emission varies from the light blue of Bz₂PbBr₄ to the typical orange one of the Mn. The PL spectrum exhibits a weak emission for the excitonic peak at 410 nm and a strong Mn emission, centered at 610 nm, ascribable to the spin-forbidden Mn²⁺ d−d transition (⁴T₁ → ⁶A₁). The ABS spectra demonstrate that even at low Pb concentrations it is observable an energy-transfer process from the Pb²⁺ to Mn²⁺ emitter. Interestingly, the emission intensity reaches the maximum for the experimental Mn composition of 8.0% (PLQY » 47.9%), even if a notable value is measurable also for the Mn-rich composition (PLQY » 37.2% for Mn = 66.5%).

Incorporating organic semiconductor (OS) spacer cations into layered lead halide perovskites provides a powerful approach to tune the optoelectronic properties of the resulting organic-inorganic hybrid materials. Forming type-II nano heterostructures using OS spacers, for example, allows the partitioning of charge carriers either in the organic or inorganic layer and provides a potential strategy to overcome intrinsically high exciton binding energies in layered perovskites by facilitating charge separation.[1] However, large OS cations with suitable energy levels remain challenging to incorporate into a layered perovskite structure resulting in relatively few systems reported and characterized.

In this regard, our lab developed novel OS cations and optimized processing conditions to afford incorporation in layered perovskites.[2,3] In this presentation, we elucidate novel OS cations based on rylene dyes chromophores incorporation into a layered perovskite and elaborate the size limitations of the chromophore when it comes to the formation of (2D) layered perovskite structure. Using transient absorption (TA) and time resolved photoluminescent spectroscopy (TRPL) we indicate the formation of charge-transfer excitons at the perovskite-OS interface and further investigate the effect of the distance of the chromophore to the perovskite layer on the electron-transfer process from the perovskite layer to the OS cation. Time resolved microwave conductivity (TRMC) measurements and terahertz spectroscopy was used to elucidate the effect of the type-II nano heterostructure on the photogenerated free charge-carrier showing enhanced carrier lifetimes compared to layered perovskites incorporating aliphatic spacers cations. The presented work implies the possibility to mitigate the inefficient charge-separation process due to the high exciton-binding energy in lead-halide layered perovskite by forming OS-perovskite nano heterostructures opening the door for a new class of material.

Double perovskites are promising candidates for less toxic and highly stable metal halide perovskites, but their optoelectronic performances still lag behind those of the lead halide counterpart, due to the indirect nature of the bandgap and the strong electron-phonon coupling. Reducing the dimensionality of Cs₂AgBiBr₆ down to a 2D layered form is strategic in order to tune the band gap from indirect to direct and provides new insights into the structure-property relationships of double perovskites. Herein, we report on a series of monolayer 2D hybrid double perovskites of formula (RA)₄AgBiBr₈, where RA represents different primary ammonium large cations with alkyl- and aryl-based functionalities.[1] An in-depth experimental characterization of structure, film morphology and optical properties of these perovskites is carried out. Interestingly, the variation of the ammonium cation and the inter-planar distance between adjacent inorganic monolayers has peculiar effects on the film-forming ability and light emission properties of the perovskites. Experiments have been combined with DFT calculations in order to understand the possible origin of the different emissive features. Our study provides a toolbox for future rational developments of 2D double perovskites, with the aim of narrowing the gap with lead halide perovskite optoelectronic properties. We further discuss on the achievement of a bathochromic shift in the absorption features of a butylammonium-based silver-bismuth bromide monolayer double perovskite through doping with iodide and study the optical properties and stability of the resulting thin films in environmental conditions.[2] Finally, we report on recent, on-going work on the 2D/3D interface engineering to tune the efficiency of solar cells based on the double perovskite and carbon electrode charge extracting layers.[3] 

Epitaxial heterostructures based on oxide perovskites and III–V, II–VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics. Halide perovskites—an emerging family of tunable semiconductors with desirable properties—are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration. Atomically sharp epitaxial interfaces are necessary to improve performance and for device miniaturization. However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility and their poor chemical stability. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance. In this talk I will present an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. Highly stable and tunable lateral and vertical epitaxial heterostructures, multiheterostructures and superlattices will be demonstrated. Furthermore, using these 2D heterostructures as a new platform, I will present our recent efforts in 1) quantitatively understanding the anion inter-diffusions and migrations, and 2) controlling and manipulating exciton transport and light-emission in halide perovskites.

Metal halide perovskites are emerging materials for optoelectronics due to their excellent optoelectronic properties, such as direct and tunable band gaps, large absorption cross section, long lifetimes and diffusion paths of the charge carriers, coupled to an elevated defects tolerance.[1] The band gaps and the exciton binding energies of these materials can be tuned by varying the chemical composition of the inorganics or by reducing dimensionality by introducing large cations. Interestingly, reduction in dimensionality leads to the emergence of novel optical features, such as the sub-gap broad emission (BE) in 2D perovskites, whose origin is hotly debated.[2] Contrasting hypotheses assign BEs to the recombination of intrinsic self-trapped excitons (STEs) or to emission from native defects.[3-4] 

In this presentation the defect chemistry and photophysics of PEA2MX4 (PEA=phenethylammonium; M=Pb, Sn; X=I, Br, Cl) <100> 2D perovskites is discussed by a computational perspective, in order to provide a microscopic picture of the defects processes taking place in these quantum confined materials. The trends in the defects chemistry moving from 3D to 2D and the effects of the metal by replacing lead with tin will be discussed. Hence, by comparing predictions with experiments, the diverse hypothesis about the origin of the BE in this class of materials will be analyzed. DFT results show that sensibly lower defect densities are expected in 2D perovskites compared to 3D analogues, but a deepening of defect charge transitions in the band gap is reported due to QC. Despite the low calculated defect densities, emission from halide vacancies is compatible with the experimentally observed sub-gap features in the relatively large set of compounds considered in the study, suggesting that the density of these optically active centers is modulated by the crystallization kinetics. On the other hand, the simulation of STEs indicates that the self-trapping of holes and electrons is a feasible process only in the wide band gap Br- and Cl-based 2D perovskites, even though the process is thermodynamically hindered by the shallow nature of the transition.[5] 

Our work provides useful insights into the intrinsic and extrinsic mechanisms generating BEs in 2D perovskites and it demonstrates that a control of the crystallization process, e.g. by tuning PEA-halide stoichiometry in the precursors, is key to selectively control the optical features of these materials.

2D perovskites have emerged as more stable analogues for photovoltaic applications as their 3D counterparts. However, in contrast to the free charge carriers in 3D perovskites, the confined space of the layered 2D perovskites leads to the formation of excitonic excited states. To harvest the energy of the excitonic excited state in a solar cell, transport of the excitons to charge-separating interfaces is required. Understanding the transport of excitons in 2D perovskites is therefore a crucial step for the development of perovskite photovoltaics containing 2D phases which benefit from both improved stability and maximized efficiency. 

In this talk I will present our recent efforts to visualize exciton diffusion dynamics in a variety of 2D perovskite materials using Transient Photoluminescence Microscopy (TPLM). [1-4] TPLM combines diffraction limited excitation with time and spatially resolved detection of excitonic emission to reconstruct a movie-like representation of exciton transport with sub-nanosecond and few-nanometer resolution. Using this technique, a number of important structure-property relationships for exciton transport in 2D perovskites have been revealed, including the importance of lattice rigidity [1], the influence of trap states [3] and halide mixing [4], as well as the influence of doping strategies. I will highlight the importance of our use of Brownian dynamics simulations to explain the various anomalous transport regimes that can be encountered in these materials. 

In recent years, the interest in 2D materials and low-dimensional perovskites (LDP) as building blocks for photovoltaic devices has flourished. This is due to their chemical and electronic properties, which enable them to minimise performance losses and improve the stability of the devices. In this talk, it is presented an innovative strategy to improve the performances of inverted perovskite solar cells (PSC) by incorporating high-quality graphene flakes (GF) in the fullerene-based electron transporting layer (ETL) [1]. It has been proved that the so-formed ETL can be processed as a composite (Graphene:Fullerene) ultra-thin film atop the perovskite, reducing the defect-mediated recombination while creating preferential paths for the extraction of the electrons towards the current collector. Moreover, the properties and the synthesis of LDP based on thiophene cations will be discussed. At first, it will be shown how to control the properties of LDP: the optical and structural ones, and the segregation of different phases upon the modification of the halide composition. Indeed, the rational substitution of halides led to a spatial distribution of phases in the thin film, where the minor phase was responsible for the light emission while the major one gave a human-eye transparent appearance to the material [2]. Lastly, it will be discussed the implementation of this LDP in highly efficient PSC, giving rise to multidimensional 2D/3D perovskite heterostructure by means of two different passivation approaches.

[1] A.Zanetta, I.Bulfaro, F.Faini, M.Manzi, G.Pica, M.De Bastiani, S.Bellani, M.I.Zappia, G.Bianca, L.Gabatel, J.K.Panda, A.E.Del Rio Castillo, M.Prato, S.Lauciello, F.Bonaccorso, G.Grancini; Journal of Materials Chemistry A, 2023.

[2] A.Zanetta, Z.Andaji‐Garmaroudi, V.Pirota, G.Pica, F.U.Kosasih, L.Gouda, K.Frohna, C.Ducati, F.Doria, S.D.Stranks, G.Grancini; Advanced Materials, 34 (1), 2105942.

Metal halide perovskites are hot contenders as next generation of light emitters. Bright and colour-pure light-emitting diodes (LEDs) were demonstrated based on bulk 3D, nanocrystals, and quasi-2D structures (Ruddlesden-Popper phases) with targeted compositions that allow for accessing different spectral regions. Green and red LEDs have reached external quantum efficiencies exceeding 20%, but success in the blue spectral region has so far been limited.

Here we demonstrate blue LEDs with a peak wavelength of 481 nm, a colour purity of up to 88 % (CIE coordinates (0.1092, 0.1738)), an external quantum yield of 5.2 % and a luminance of 8260 cd m^-2. These devices are based on quasi-2D PEA₂(Cs_0.75MA_0.25)Pb₂Br₇, which is cast from solutions containing isopropylammonium (iPAm). The iPAm-modified sample when deposited on a PEDOT:PSS coated substrate, displays an exceptional PLQY as high as 64%. Cross-correlation of the optical and structural investigations indicate that the RP phase is composed of domains of n=3 phases surrounded by higher dimensionality phases, which allow the efficient transport of charge carriers towards the low dimensional domains. Interestingly, the energy transfer of the photoexcitations towards the low dimensional phases is blocked in samples using the iPAm additive, mostly due to the random orientation of very small crystalline domains. These interesting features present in our iPAm-modified system allowed us to fabricate bright blue-emitting PeLEDs with an average wavelength of 483 nm and FWHM of the electroluminescent of 25 nm for our champion device.

Our work demonstrates the great potential to tailor the composition and the structure of thin films based on Ruddlesden-Popper phases to boost performance of optoelectronic devices – specifically for blue-LEDs.

Low-dimensional perovskite structures are at the forefront of light emitting applications, mainly owing to high carrier and exciton confinement. The ability to tune the emission wavelength through halide alloying and addition of organic cations promoting formation of 2D structure enables fabrication of light emitting diodes (LEDs) with external quantum efficiency comparable with solutions based on organic semiconductors, with the benefit of high colour purity. One of the challenges in the development of perovskite LEDs is their fundamental absorption-related optical characterisation, including assessing the influence of possible processes at the interfaces between electrodes and the active layer compared to the pristine material. Photocurrent spectroscopy remains the standard high-sensitivity technique for probing defect-related absorption and material disorder, applicable but limited to fully contacted devices, using LEDs in reverse as photodetectors. Here we demonstrate a complementary approach of incorporating modulated photocurrent and photothermal deflection spectroscopy (PDS) experiments for characterisation of 2D perovskite LEDs on different fabrication stages: bare perovskite thin films, the same deposited on top of transparent transport layers from the substrate-side (half-devices), and full LED structures. The results confirm that in optimised device preparation arrangements the sandwiched perovskite layer can fully preserve its optical quality, confirmed by quantifying absorption related to sub-gap trap states and sharpness of the absorption edge (the Urbach energy), revealing a crucial information for future development of LEDs.

Among hybrid perovskites (HP), 2D are playing a major role in many optoelectronics applications including solar cells, thanks also to their stabilizing effect. Despite their huge success, the dynamics of photogenerated carriers in these materials still needs to be fully assessed. In this work we report the result of ultrafast optical spectroscopy measurement in  “tandem” configuration, combining transient absorption and time resolved photoluminescence in the same excitation conditions, as already applied to 3D and 2D HP thin films [1]. To get clearer information about intrinsic property, for the first time we apply our technique on single crystals of 2D HP (phenethylammonium (PEA)2PbI4), comparing nonresonant excitation, generating hot carriers, with resonant excitation, directly injecting cold excitons in the material. The results of our measurements show that both in the tihn film and in the crystal the photophysics consists in an initial massive formation of excitons, followed by a spontaneous ultrafast exciton splitting, occuring on a sub ps timescale. The evidences of this phenomenon are shown both in thin film and crystals wiht different excitation condition, eventually at low temperature, demonstrating that is an intrinsic effect. We believe that this results provide a better understanding in the photophysics laying under the exceptional optoelectronic properties of 2D HPs. 

 

Layered metal-halide perovskites have shown great promise for applications in optoelectronic devices, where a large number of suitable organic cations give the opportunity to tune their structural and optical properties. However, especially for Sn-based perovskites, a detailed understanding of the impact of the cation on the crystalline structure is still missing. By employing two cations, 2,2’-oxybis(ethylammonium) (OBE) and 2,2’-(ethylenedioxy)bis(ethylammonium) (EDBE), we obtain a planar <100> and a corrugated <110>-oriented perovskite, respectively, where the hydrogen bonding between the EDBE cations stabilises the corrugated structure. We will show that OBESnI₄ exhibits a relatively narrow band gap and photoluminescence bands compared to EDBESnI₄. An in-depth analysis shows that the markedly different optical properties of the two compounds have an extrinsic origin. Interestingly, thin films of OBESnI₄ can be obtained both in black and red colours. This effect is attributed to a second crystalline phase that can be obtained by processing the thin films at 100 °C. Our work highlights that the design of the crystal structure as obtained by ligand chemistry can be used to obtain the desired optical properties, whereas thin film engineering can result in multiple crystalline phases unique to Sn-based perovskites.

In order to further improve the efficiencies and in particular the open-circuit voltages of perovskite solar cells it is important to understand non-radiative recombination and their relation to defect densities. Here, the charge-carrier lifetime is frequently measured using transient spectroscopic techniques to provide an assay of charge carrier recombination. Frequently, the expectation is that at sufficiently low excitation conditions the experimentally observed lifetime should be a constant value that scales inversely with the defect density. We show that in (our) triple-cation perovskite films, layer stacks and devices, recombination is actually dominated by shallow defects, implying that the lifetime becomes a quantity that is continuously changing with carrier density. We thereby show that (i) it is not the low density of deep defects that limits the performance in typically triple-cation perovskites but a quite high density of shallow defects. Furthermore, we show that lifetime cannot be used anymore as a figure of merit if it is not plotted vs. carrier density, Fermi level splitting or voltage [1].

For the continuous improvement of optoelectronic devices such as organic light-emitting diodes and solar cells in terms of e.g. stability and efficiency, a comprehensive model including all major physical processes is crucial. Further, the combined analysis of measurement and simulation paves the way for the understanding of novel devices such as perovskite solar cells. In this work, an up-scaled R&D solar cell is investigated. Once a model and material & device parameter set describing the cell is found, the optimization of the cell performance can start. The initial challenge, however, is to determine such a set of parameters since the availability of e.g. material parameters is limited and is usually obtained by tailored measurements or taken from literature. The values for certain material parameters can vary depending on the literature source or the sequence of layers in the experimental stack. A way to determine the parameters is fitting. In least-square algorithms the sum of the squared differences between the measurement and simulation is minimized by varying the material parameters until an optimal set is found. This task however can be cumbersome, especially with a large number of unknown parameters. Often the parameters are also correlated which again increases the complexity of the problem. In such situations, domain knowledge is required to facilitate the search for the minimal error.
In this contribution, we discuss traditional and machine-learning assisted approaches [1] to determine the model parameters for perovskite solar cells based on electroluminescence images and current-voltage measurements.

Organic semiconductor-based photovoltaic (OPV) devices have many advantageous properties which makes them attractive for future energy harvesting technologies [1]. Owing to the low charge carrier mobilities in OPVs, a substantial built-in voltage is generally required within the active layer to ensure efficient charge carrier extraction [2,3]. Nevertheless, reliable methods to determine the built-in voltage in thin film OPVs are currently lacking. Standard Mott-Schottky analysis of capacitance-voltage (C-V) characteristics has been frequently used in the past to determine the built-in voltage in organic semiconductor diode devices. However, since OPVs are typically dominated by contact-induced background carriers with highly non-uniform charge distributions [4], this technique is generally not applicable for thin film OPVs [5]. In this work, we present an alternative method based on C-V characteristics, which accounts for the influence of injected carriers. Guided by device simulations, we derive a theoretical framework which describes the relationship between the capacitance and the built-in voltage in thin diode devices. We further use numerical drift-diffusion simulations to validate the theoretical framework. Finally, we substantiate the method experimentally on organic solar cells. Based on these findings, we clarify the role of ohmic contacts and the meaning of the built-in voltage in OPVs and related devices.

Halide perovskite semiconductors have captured significant research attention for application in optoelectronic devices, in particular solar cells, due to their outstanding properties and straightforward material synthesis. Amongst these favourable properties, the seemingly “slow” cooling of hot (nonequilibrium, high energy) carriers—which distinguishes halide perovskites as candidate materials for the light-absorbing layer in hot carrier solar architectures—is the subject of an increasing share of the community’s focus. The dynamics of hot carriers are commonly studied experimentally via transient absorption spectroscopy (TA), which allows for observation of the cooling of hot carriers following photoexcitation by an ultrashort laser pulse, and the advanced two-dimensional electronic spectroscopy technique (2DES), which can resolve shorter timescales and, thus, additionally enables scrutiny of carrier thermalization. These techniques are powerful, but there is generally a limit to the strength of the conclusions that can be drawn concerning the physical processes underlying the carrier dynamics in the probed material by virtue of these experimental measurements alone. Consequently, the origin of observations of long hot carrier cooling times in halide perovskite materials remains debated, despite the increase in traction of the “hot phonon bottleneck” hypothesis. Here, we present semiclassical simulation of semiconductor charge carrier dynamics—using the ensemble Monte Carlo method to solve the Boltzmann transport equation for non-equilibrium carrier distributions—as a valuable partner to experiment in elucidating the transient carrier dynamics in halide perovskites and other semiconductors. We include results from such simulations and draw qualitative inferences about the carrier dynamics in halide perovskites, also highlighting the care that must be taken in analysing TA experiments. We discuss remaining challenges and opportunities to develop our approach to allow experimental measurements to be reproduced.

Semitransparent organic solar cells (STOSC) exhibit promising applications as power-generating windows in buildings and agricultural greenhouses. Due to their widely adjustable optical band gap, organic semiconducting materials have the potential to efficiently utilize near infrared light while maintaining semitransparency in the visible light range. One challenge in improving the overall performance of STOSCs is maximizing both power conversion efficiency (PCE) and average visible transmittance (AVT) simultaneously, since intrinsically they are often in a trade-off relationship.^[1] A key factor in optimizing both values, is choosing the right material for each intermediate layer of the cell. One well-established material used for electrodes in semitransparent cells is Indium tin oxide (ITO). Although known for its high transparency and good conductivity, it is considered brittle, and its production is cost- and energy-intensive. Furthermore, Indium is a metal that rarely occurs in nature.^[2] Lastly ITO exhibits a comparably low reflection in the infrared region.^[3] One alternative for these problems is using multiple interlayers of silver and Al-doped ZnO (AZO) as a distributed Bragg reflector on the backside of the solar cell. By varying the layer thicknesses, the optical properties of this electrode can be modified to maximize reflection in the infrared range. This way the current generation of the photoactive layer can be improved while simultaneously maintaining a high transparency for the cell.  

Herein we report recent progress in ITO-free semitransparent organic solar cells by implementing a multilayer silver back electrode as an infrared mirror, achieving a power conversion efficiency of 10.2% for a cell stack based on PV-X Plus with an average visible transmittance of 33.1%. Both optical modelling and experimental findings were used to maximize the photocurrent generation and the average visible transmission of the solar cell. The back electrode consists of alternating layers of Al-doped ZnO and thin silver. As the reflected part of the light passes the photoactive layer a second time, an increase in absorption is achieved. Optical simulations show that the distance between the two silver layers has a strong influence on the wavelength range of the reflected light and thus determines the generation of the photocurrent in the cell. By optimizing the spacing between the two silver layers, it was possible to increase the photocurrent for a solar cell based on PV-X Plus by 9.8% (from 19.3 mA/cm² to 21.2 mA/cm²). At the same time, it was found that the fill factor decreases with an increase in the thickness of the interlayer. When using a single silver layer of 14 nm, the fill factor could be kept reasonably high (71.3%) that despite a comparably lower current generation (17.0 mA/cm²) a LUE (PCE x AVT) of 3.38% was achieved. 

Thermally Activated Delayed Fluorescence (TADF) process has appeared as the most popular design strategy towards reaching 100% internal quantum efficiency for Organic Light-Emitting Diodes (OLEDs). TADF consists in promoting upconversion of triplet excited states into emissive singlet ones through Reverse InterSystem Crossing (RISC), a process driven by spin-orbit coupling (SOC) and requiring a small singlet-triplet gap dE_ST. The advancement of the TADF field occurred essentially through materials design, the first strategy, as proposed by C. Adachi and co, consisting in connecting electron donating and accepting units to decrease the dE_ST. However, in doing so, the lower-lying singlet and triplet excited states bear a dominant charge transfer character that translates into a broad emission spectrum.

In this contribution, we will discuss based on computational considerations how doped triangle-shaped molecules can lead to (i) concomitant narrow emission, high quantum yield of emission and small dE_ST resulting in a whole new generation of TADF emitters, the multi-resonant TADF emitters and to (ii) a new family of compounds with an inverted singlet-triplet gap and potentially, a downwards energy RISC. To do so, we rely on high level quantum chemical calculations and show that an accurate description of electron correlation effects is key to correctly predict the excited states ordering as well as the optical properties of these compounds.

Excited state dynamics play key roles in numerous molecular and nanoscale materials designed for energy conversion. Controlling these far-from-equilibrium processes and steering them in desired directions require understanding of material’s dynamical response on the nanometer scale and with fine time resolution. We couple real-time time-dependent density functional theory for the evolution of electrons with non-adiabatic molecular dynamics for atomic motions to model such non-equilibrium response in the time-domain and at the atomistic level. The talk will introduce the simulation methodology [1] and discuss several exciting applications among the broad variety of systems and processes studied in our group [2,3], including metal halide perovskites, transition metal dichalcogenides, semiconducting and metallic quantum dots, metallic and semiconducting films, polymers, molecular crystals, graphene, carbon nanotubes, etc. Photo-induced charge and energy transfer, plasmonic excitations, Auger-type processes, energy losses and charge recombination create many challenges due to qualitative differences between molecular and periodic, and organic and inorganic matter. Our simulations provide a unifying description of quantum dynamics on the nanoscale, characterize the timescales and branching ratios of competing processes, resolve debated issues, and generate theoretical guidelines for development of novel systems.

Perovskites is a class of materials with notable crystal structure of ABX3, exceptionally wide tunability in chemical compositions and dimensionality in crystal-structures. Due to their exceptional optoelectronic properties, they are widely used for converting and storing (solar) energy, e.g. oxide perovskites as photocatalysts, halide perovskites as absorbers in solar cells, nitride perovskites as

mechanical energy harvesters. In these applications, the understanding of optoelectronic properties, chemical stability and their changes upon external stimuli (light excitation, mechanical, thermal and chemical stress) are paramount.

In this talk, I will show how our research group investigate these properties using atomistic multiscale modelling by combing electronic structure calculations with reactive molecular dynamics simulations. Her focus is on halide perovskites and the impact of defects on the efficiency and the stability of perovskite solar cells. We identify harmful defects which lead to either recombination losses and/or chemical degradations and show several strategies to mitigate and passivate these defects. These include engineering the composition of perovskite absorbers, optimizing interfaces with contact materials, and finetuning growth conditions. The atomistic insights provide a basis for further improving the efficiency and stability of perovskite materials and devices. The multiscale computational framework can be enhanced with The emerging data-driven approach and straightforwardly applied to other materials and applications.

In this talk, I will discuss the employment of state-of-the-art electronic structure theory for the modelling of novel perovskite-like materials within the Ag/Bi double salt and the vacancy ordered double perovskite (VODP) lattices. I will first discuss the case of halide double perovskites, and explore the link between the type and position of the atomic orbitals in the crystal and their electronic structure, and connect this established structure with the so-called rudorffites or Ag/Bi double salts. I will demonstrate the necessity of proper modelling of these, and present a symmetry-based method we developed [1], which correctly describes the exhibited electronic and optical properties of AgBiI₄, a prototypical material. I will employ this to probe other materials within the same family of double-salts, and further use the model to investigate the ideal (i.e., maximum limits) photovoltaic properties that these materials could achieve. In the second part, I will thoroughly analyze the properties of the VODP family of materials by employing state-of-art ab initio calculations and unveil key details of the electronic structure and the effects of electron-hole coupling on the optical properties [2]. I will discuss prototypical VODP structures by selecting members based on the electronic configuration of the tetravalent metal at the B-site (e.g. Sn, Te, Zr). I will go through their structural properties, and show that the size of the vacant site, can be tuned solely by the halogens. The electronic structures are investigated within the GW many-body green’s function method, and we obtain band-gaps spanning a range of 1.5-5.0 eV at G₀W₀. More importantly, the optical and excitonic properties of these compounds are probed within the independent particle approach, and by solving the Bethe-Salpeter equation (BSE). The exciton binding energies and dark-bright exciton exchange splitting are calculated for each type of VODP. Finally, the excitonic fine structure is unveiled by performing a complete symmetry analysis of the compound’s band structure and excitonic wavefunctions, on which a direct link between these and the metal site species is established. Overall, here I comment on the suitability and the prospect of the application in opto-electronics of Ag/Bi double salts, like AgBiI₄, and VODP, like Cs₂TeBr₆, might exhibit. I will identify the most and least promising materials that could act as photo-active materials or for selective charge transport layers, for light-emitting and solar-cell applications.

Lead halide perovskites have revolutionized the scenario of photovoltaics due to their excellent and tunable optoelectronic properties leading to solar cell efficiencies comparable to traditional high-quality silicon materials.[1] Issues concerning the presence of lead, however, have led researchers to find less toxic candidates possibly replacing lead in the perovskite, particularly tin. The performance of tin halide perovskites, however, is strongly limited by an enhanced self p-doping and the easy oxidation of Sn(II) to Sn(IV).[2] To guide the development of lead-free materials withenhanced efficiency and stability, a deep understanding of the defect chemistry and photophysics of these materials is needed. Ab-initio simulations represent a powerful tool to investigate defect properties and processes at the atomistic level. In this presentation a theoretical tour of the defect activity in lead- and tin-based perovskites is provided. We will show how DFT calculations may be applied to study defects in materials, the useful quantities obtained by simulations and the best practices in the case of metal halide perovskites. Hence, the trend in defect activity by moving from lead to tin will be discussed.[3] The nature of most abundant defects, their trapping activity and the impact on the doping of the materials will be illustrated. Defect driven mechanisms possibly activating the degradation of these perovskites will be discussed based on the defect analysis.[4] Furthermore, the effects of quantum confinement on the optoelectronic features and defect properties moving to low dimensional 2D perovskites will be showed, also in relation to emission features emerging in these quantum confined systems, such as broad emission, whose origin is hotly debated.[5] This work aims to provide a unified picture of defect chemistry of metal halide perovskites with emphasis on the factors governing the stability and the efficiency of these materials. It also aims to stimulate the community to a synergistic use of computer simulations to support the design of more
efficient and long-term stable devices.

Device modeling and energy yield (EY) calculations are essential tools to optimize solar cell architectures. Device modeling through opto-electrical simulations allows to analyze device performance under standard conditions. In perovskite solar cells (PSC); the energy alignment at the interfaces, ion migration and potential distribution along the device play a critical role on device performance. To investigate them, we have characterized horizontal PSC microstructures[1]. We were able to draw the potential distribution along the solar cell structure by coupling X‐ray photoelectron spectroscopy (XPS) and drift-diffusion modeling[1]. Furthermore, in this work we considered the role of ion migration for the analysis of the band device structure. On the other hand, EY calculations estimate the total output generated energy of a solar cell after one year in a specific place. EY calculations also allow the analysis of device stability which is highly affected by device temperature[2]. Therefore, it is crucial to determine accurately the device temperature. To estimate cell temperature, we propose a thermal model which is a function of device parameters, environmental variables, and is strongly linked with the experimental optical-electrical-thermal performance[2]. We studied the effect of realistic temperature conditions on the performance of PSCs and their transient response to environmental external changes using a theoretical-experimental combined approach. Linking the experimental results and our model, we were able to evaluate the most sensible device layers that increment device temperature affecting device stability.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying physical behaviours that may lead to developing new applications. Beyond the well-established characteristics of regular impedance arcs, we address the appearance of inductor effect at high voltage in perovskite solar cell. We present a physical model in terms of delayed recombination current that explains the evolution of impedance spectra and the evolution of current-voltage curves. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin. Here, it is termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. Therefore, it is generally defined in dynamical terms rather than by a specific physicochemical mechanism. The impedance spectra announce the type of hysteresis, either regular for capacitive response or inverted hysteresis for inductive response. We apply the dynamic picture based on a few neuron-like equations to the characterization of halide perovskite memristors. Memristors show an intense hysteresis that can be characterized in terms of the emergence of inductive components. We address the characterization of electron diffusion and radiative emission in halide perovskites using a range of light stimulated techniques as IMPS, IMVS, and voltage controlled light emission technique (LEVS).

Laser-induced transient grating (LITG) spectroscopy has been used to measure the carrier transport properties of perovskite materials and devices, including the diffusion coefficient and carrier lifetime [1]. These measurements are important for understanding the electronic properties of materials and how they can be optimized for different applications. LITG has the advantage of being a non-destructive method, which can be important for protecting the integrity of the material during measurement.

LITG method itself was realized several decades ago [2, 3]. The principle scheme of a LITG measurement is depicted in Fig.1. A pair of ultrashort pulses are overlapped both spatially and temporally to create an interference pattern on the sample – a transient grating with a spatial period Λ that depends on the pump wavelength and the beam intersection angle. Excitation by periodic pattern excites a spatially-modulated carrier distribution and, effectively, a periodic modulation of the refractive index, thus, diffracting the delayed probe beam.

Over time, the laser-induced grating decays due to carrier recombination (electronic decay with the rate of τ_R) and carrier diffusion (spatial decay with the rate of τ_D). The diffusion term depends on the transient grating period: fine gratings diffuse faster than coarser ones. Accordingly, if we measure the temporal behavior of the diffracted signal over a series of different periods Λ, we can determine the carrier diffusion coefficient D [2].

The device presented in this paper allows automated continuous tuning of the excitation grating period Λ and selection of excitation wavelength from 340 to 560 nm, which allows measuring carrier diffusion in the range from 0.1 to 50 cm²/s and carrier lifetime in the range from 1 ps to ca. 80 ns.

Exemplary carrier diffusion and lifetime measurements were performed with a thin film of metal-halide perovskite Cs_0.05(FA_0.85MA_0.15)Pb(I_0.85Br_0.15)₃ with Et2NH additive in a wide range of excitation densities. The results showed that the charge carriers of the material are in a trap-filling and de-trapping diffusion regime, from which carriers easily escape with increasing excitation density.

The extensive experience in optomechanical engineering and high-level automation enabled the all-optical measurement for carrier diffusion and carrier lifetime. It was extensively tested with different samples, including but not limited to the aforementioned perovskites. The results will be presented in detail during the conference.

The photovoltaic (PV) effect provides the most efficient means of converting the overwhelming amount of freely available energy from the sun (ca. 1000 times greater than the total energy consumption worldwide) into electricity. Emerging PV technologies based on materials such as organic, transparent oxides, kesterites, quantum dots and hybrid halide perovskites are increasingly gaining in importance nowadays.[1] Within the emerging PV technologies, many of the breakthroughs achieved in the power conversion efficiency and in device lifetime have been obtained through intensive materials research.[1, 2, 3] Current trends in PV research focus on the search for higher efficiencies through multi-junction concept and the expansion of the range of applications beyond standard solar farms. In this context, material screening and characterization requires many different pieces of equipment (one solar simulator, another light source for indoor, another set for EQE measurements, etc.). In cases like lateral tandem devices, the required setups are simply not commercially available.

We report a multi-purpose spectrum-on-demand light source (SOLS), conceived, primarily, but not exclusively, for the multiple and advanced characterization of photovoltaic (PV) materials and devices. The apparatus is a spectral shaper illumination device, providing a tunable, spectrally shaped and focused light beam, modulated in intensity and/or in a wavelength range with respect to a primary light source. SOLS stands out from the state of the art because it produces almost any spectrum on demand and delivers two types of output: a spectrally shaped and spatially homogeneous beam over its cross section for areal illumination; or a spatially and spectrally split beam into its wavelength components, a unique capability suited to characterize lateral-tandem (Rainbow) solar cells. The tuneability from broadband to narrow band illumination enables two characterization devices into one, namely, a solar simulator for the determination of the power conversion efficiency and an external quantum efficiency measuring system. We expect the SOLS setup to accelerate material screening, enabling the discovery and optimization of novel multi-component materials and devices, in particular, for emergent PV technologies like organic or metal halide perovskites PV, indoors and building integrated PV, agrovoltaics, multi-junction, etc.

Over the past 40 years, scientists have learned to synthesize colloidal nanocrystals of different composition with tunable size and shape. These features dictate the properties of these nanomaterials. Thus, their control is crucial for the discovery of phenomena, many of which contribute to technological advances. One example of the societal impact of this class of materials is the use of semiconductor nanocrystals as the active component in displays with the best color purity on the market. A second example is their use as materials platforms to advance catalyst development. Yet, the synthesis of colloidal nanocrystals still proceeds by trial and error. The search for the reaction conditions to obtain the nanocrystals with the desired composition, size and shape is time consuming and can fail in delivering the target product.

In this talk, I will highlight the importance of identifying the reaction intermediates during the formation of colloidal nanocrystals for the development of a more predictive synthesis to these nanomaterials. By discussing specific examples, I will illustrate that the chemical nature of these intermediates is diverse and that state-of-the-art in-situ techniques combined with theory are often needed to capture those transient species during the synthesis of nanocrystals. Nevertheless, I will use concrete examples to demonstrate that such efforts are worth it as the discovered mechanistic pathways pinpoint the critical steps to enhance the tunability of existing materials and to accelerate the discover of new ones.

In the same way metal complexes are converted into well-defined nanoparticles, nanoparticles can be used as tunable precursors capable of evolving into macroscopic solids with specific structural features. A consolidation step is usually required to produce the macroscopic solid from nanoparticles. The consolidation process defines the density and microstructure of the solids, which directly correlate with the electronic, thermal, and mechanical properties of the resulting material. In order to provide the material with densities as close as possible to the respective theoretical density, pressure-assisted sintering techniques are preferred, such as hot pressing or spark plasma sintering.

Characteristics of the particles such as size, shape, composition, and surface chemistry determine the sintering process and therefore dictate densification, grain growth, and materials' final microstructure. Consequently, using carefully-curated nanoparticles provides a unique opportunity to control material microstructure. In our framework, a nanoparticle is a multi-structured system consisting of an inorganic nanocrystalline domain, named the inorganic core, surrounded by surface species. Both the inorganic and surface species are tunable parameters in the design of nanoparticle-based precursors. Herein, we will employ solution-based synthesis procedures to produce precisely defined nanoparticles and explore their surface chemistry to adjust nanoparticles' reactivity during sintering to achieve a solid with specific targeted features.

Two very different approaches to controlling particles' surface chemistry need to be separated. One refers to the particle termination atoms, the other to the connected adsorbates that can be covalently bonded molecules or electrostatically adsorbed ionic groups. In this talk, we will show how surface adsorbates, intentionally or unintentionally introduced, are critical to controlling densification, grain growth, and materials' final microstructure. Furthermore, we will present different surface functionalization strategies that allow defining: atomic defects, grain boundary complexions, crystalline domain size, and the formation of secondary phase inclusions in the macroscopic solid.

Finally, we will discuss the effect of the different structural properties introduced in the macroscopic solids on their electrical and thermal transport properties and evaluate their potential as thermoelectric materials.

Photocatalytic sheets have disruptive potential in the field of solar hydrogen production from water. Recent demonstrations have shown the feasibility of their safe operation on a large scale.[1] The key challenge is the development of visible light absorbing photocatalytic particles that are efficient and stable for at least one year. In this talk, I will discuss recent efforts in our laboratory towards adapting our thin film semiconductor water splitting materials towards stable, photocatalytic particles. The methods for the synthesis of the particles and particle sheets will be highlighted, as well as the challenges for the stringent stability requirements (in water, under illumination). Proof-of-principle systems involving hydrogen evolution coupled with the oxidation of various organic molecules (so-called "value added oxidations") will be highlighted. Initial efforts with BiVO4 will be discussed, as this is a well-studied particle for the oxygen evolution reaction. Subsequently, copper oxide and antimony selenide-based systems will be discussed.

The chemistry of metal-organic and covalent organic frameworks (MOFs and COFs) is perhaps the most diverse and inclusive among the chemical sciences, and yet it can be radically expanded by blending it with nanotechnology. The result is reticular nanoscience, an area of reticular chemistry that has an immense potential in virtually any technological field.
In this talk, we explore the extension of such an interdisciplinary reach by surveying the explored and unexplored possibilities that framework nanoparticles can offer. We localize these unique nanosized reticular materials at the juncture between the molecular and the macroscopic worlds, and describe the resulting synthetic and analytical chemistry, which is fundamentally different from conventional frameworks. Such differences are mirrored in the properties that reticular nanoparticles exhibit, which we described while referring to the present state-of-the-art and future promising applications in medicine, catalysis, energy-related applications, and sensors. Finally, the bottom-up approach of reticular nanoscience, inspired by nature, is brought to its full extension by introducing the concept of augmented reticular chemistry. Its approach departs from a single-particle scale to reach higher mesoscopic and even macroscopic dimensions, where framework nanoparticles become building units themselves and the resulting super-materials approach new levels of sophistication of structures and properties.

Aliovalent I-V-VI semiconductor nanocrystals represent promising candidates for non-toxic and earth-abundant thermoelectric and infrared optoelectronic applications. Among them, famatite Cu3SbSe4 material stands out due to high absorption coefficients and narrow band gap in the mid-infrared spectral range. In this presentation we combine experiments and theory calculations to study the effects of composition and defects on optical properties of Cu3SbSe4 nanocrystals. Using fast and modular hot-injection colloidal synthesis approach, we achieve size-uniform and stoichiometric Cu3SbSe4 nanocrystals and study the influence of reaction parameters such as reaction temperature, growth time, and precursor concentrations. While a reduced activity of the Cu precursor induces the formation of Cu-deficient Cu3SbSe4 nanocrystals, an increased anion concentration or a balancing of cation precursor concentrations result in stoichiometric Cu3SbSe4 products with an absorption onset of 0.2 eV. Characterizing the products, we discover a broad solid solution for nanocrystals (nominally, CuxSbSe4, where x is 2.4–3.0), which is significantly larger than for the bulk Cu3SbSe4 phase. Within this CuxSbSe4 solid solution, the optical band gap widens to 0.35 eV as the amount of Cu decreases. Tight-binding simulations point to the existence of large amount of Cu vacancies, which are non-harmful for the optical performance of CuxSbSe4 nanocrystals. In contrast, even a small amount of CuSb and SbCu antisite defects creates intrinsic doping states, which are detrimental for Cu3SbSe4 properties. We affirm tunable band gaps by infrared spectroscopy and observe a good agreement to the theory calculations. We further report the energy-resolved photoelectric response of Cu3SbSe4 nanocrystals and show their excellent resilience towards oxidation. Our results will provide a launch platform for Cu3SbSe4 nanocrystals as one of very few non-toxic alternatives for mid-infrared applications.

Nickel boride Ni_xB_y has shown potential as an efficacious catalyst for a broad range of systems (hydrogenations, hydrogen, and oxygen evolution reactions) under challenging conditions (such as high pH or high temperatures). Preparing the nanoscale analogues of nickel borides in an attractive prospect as the increased surface area and creation of more active sites will enhance their catalytic activity. However, the nanostructure of Ni_xB_y remains underdeveloped mainly due to the notoriously difficult and energy demanging synthetic process which is not industrially compatible.^[1-3] Recent works have shown that synthetic methods at lower temperatures (<400 ºC) yield amorphous polydisperse nanocrystals (NCs), while phase purity remains an issue at higher temperatures. Here, a simple, scalable synthesis is demonstrated to obtain a phase-pure nanocrystalline Ni_xB_y. Through the solid-state reaction of either metallic Ni⁰ or NiCl₂ with NaBH₄ at a relatively low temperature (400^oC) under atmospheric pressure, crystalline nanosized Ni₂B and Ni₃B could be obtained with high yield in pure phase and with narrow size distribution (15-30 nm). Through extensive mechanistic studies, we show that Ni nanoclusters (approx. 1 nm) are intermediate in the boriding process, while the halide precursors lower the decomposition temperature of the NaBH₄ (used as a reducing agent and B source). We further explore the size control using reaction mediators, and we probe the differential nucleation and growth of Ni (clusters) or Ni_xB_y NCs while using L (amine, phosphine) and X-type (carboxylate) mediators. The synthesized Ni₃B nanopowder can undergo surface functionalization with inorganic ligands in polar solvents, forming a stable ink. Furthermore, the Ni₃B nanocrystals ink can be ligand-exchanged with organic ligands in a non-polar low-boiling point solvent. This work opens the door for the large-scale production of Ni_xB_y nanocrystals solution-processable inks to make it a commercially viable alternative catalyst to noble-metals and pave the way for other metal borides colloidal nanostructures. 

Colloidal semiconductor nanocrystals are opening new routes for a wide range of optoelectronic applications, spanning from lighting to photodetection, as their electronic properties can be tuned by changing their size and shape[1]. Core-shell nanocrystals further widen the tunability of their optoelectronic properties, due to the various energy alignments between the core and shell  [2].
In this contribution, we show how the interplay between density functional theory (DFT) simulations and various experimental techniques can provide fundamental insights into the atomistic and electronic structures of core-shell InAs@ZnSe semiconductor nanocrystals. In particular, we carry out a systematic comparison between various structural models simulated with DFT and the results of photoluminescence as well as x-ray diffraction and electron microscopy. This detailed comparison unveils information that is not available to either theory or experiment alone. We demonstrate that the composition of the core-shell structure does not switch abruptly between the core and the shell, but rather displays an In-Zn concentration gradient. Moreover, only by considering atomistic reconstructions on the surface of the nanocrystal as well as on the external core layer is it possible to obtain an electronic structure that matches the experimentally observed optical properties. Finally, we derived general insights to explain the observed boost in the photoluminescence quantum yield when the InAs particles are coated with the ZnSe shell.

Cardiovascular diseases are the leading cause of death worldwide. New treatments are continuously developed but require an in-depth understanding of cardiovascular morphology. Vascular corrosion casting provides 3D knowledge of anatomical structures by injecting a polymer resin and subsequently removing the surrounding tissue via chemical maceration, i.e. corrosion. [1] This process often leads to deformations or fragmentation of the fragile cast, resulting in a loss of information. In-situ high-resolution computed tomography (micro-CT) scans could provide detailed information on the vascular architecture without corroding the tissue. Unfortunately, distinguishing the polymer cast from the animals’ surrounding soft tissue is impossible due to a lack of CT contrast. To improve this, we introduce hafnium oxide nanocrystals (HfO₂ NCs) as contrast agents to the polymer resin. Here we communicate our insights on HfO₂ NC synthesis, their surface chemistry and their application as CT contrast agents.

We synthesize 5 – 10 nm HfO₂ NCs starting from HfCl₄.2THF in benzyl alcohol. Initially identified as a purely nonaqueous sol-gel route [2], we find the in-situ water formation to be responsible for gelation of the reaction mixture prior to particle crystallization. Through mechanistic investigation using in-situ Pair Distribution Function (PDF) analysis, Nuclear Magnetic Resonance (NMR), Extended X-ray Absorption Fine Structure (EXAFS) and rheology measurements we study this rapid precursor-to-gel conversion and subsequent crystallization. Using our new-found insight we gain better reaction control over the reaction and scale-up the synthesis, yielding gram-scale HfO₂ NCs with narrow size distribution.

To obtain a stable and homogeneous dispersion of the synthesized NCs in the casting resin, we optimized the particle’s surface chemistry. The ideal ligand is found to be a combination of a strong binding group (phosphonate), while matching the resin’s polarity via its organic tail (ethylene glycol oligomers). We demonstrate that the NCs remain stable during resin curing and homogeneously improve the contrast with concentrations as low as 5 m% of NCs.

Finally, we perform ex-vivo injections of both zebrafish and mouse models with the NC-doped resin and obtain high-quality cast visualization via segmentation of the obtained scans without having to adapt the existing injection methods. We are able to differentiate even µm scale details. This confirms the application potential of HfO₂ NCs as CT contrast agents in corrosion casting, while paving the way to obtain full cardiovascular information from casts without the need for tissue corrosion. Our results emphasize that through mechanistic insight and control over the nanocrystal synthesis, combined with an optimized surface chemistry, we can create high quality stable nanocomposites.

AECs are composed of a II^a metal (Mg, Ca, Sr) and S or Se (X), feature large bandgaps positioned in the UV and are commonly used as hosts for emissive ions. AEC nanocrystals could therefore be used as UV emitters, or scatter-free emitters based on lanthanide ions. Moreover, while most AEC crystallize in the rocksalt structure, MgX can also be grown as zinc blende crystals with lattice parameters that come close to those of commonly examined and used II^b-VI chalcogenides or III-V pnictides, such as CdSe and InP. Hence, AECs could extend the range of materials to form core/shell heterostructures out of these compounds. However, such implementations of AECs are hampered by the limited knowledge of the colloidal synthesis and the surface chemistry of these compounds. Here, we report on possible routes to synthesize these materials and the resulting surface termination of AECs by organic ligands, for which we take the formation of CaS as a starting point. We show that ~12 nm large CaS nanocubes can be formed by reacting calciumacetate and diphenylthiourea in a mixture of oleylamine (OLA), trioctylamine and oleic acid (OA)[1]. Using Nuclear Magnetic Resonance (NMR) and infrared (IR) spectroscopy, we demonstrate that such as-synthesized CaS nanocubes are terminated by a dense shell of oleate ligands, which are unequally packed at the surface, and small traces of OLA. Addition of a carboxylic acid shows the dynamic behavior of this dense shell and induces a slow reorganization of the ligands by breaking up the dense packing and replacing hydroxides at the surface by oleates. Apart from providing detailed insight in the surface chemistry of CaS NCs, this work shows that known approaches and concepts to analyze and rationalize the interaction between colloidal nanocrystals and surface-active ligands can be extended to AEC nanocrystals.

The pulmonary administration of nanomedicines such as viral vaccines is a promising delivery route because it is direct, noninvasive, and it can be used for both systemic delivery and lung targeting. However, the same mechanisms that protect us from pathogens entering via the inhalation route lead to the rapid elimination or degradation of nanomedicines. Most nanomedicines designed for pulmonary administration do not reach clinical practice and this is reflected in the low number of newly approved inhalation drugs.[1] This reality contrasts with the increase in lung diseases due to global ageing and new infectious diseases.[2] Since one of the advantages of pulmonary administration is the direct targeting of the lung, the challenges of improving this route of administration are worthy of being addressed towards overcoming unmet clinical needs. In this context, we will demonstrate that it will be key to study and understand the nano-bio interaction with lung barriers to design better drug nanovectors. To this end, we will give examples of how multifunctional nanoparticles capable of encapsulating drugs and being labeled with contrast agents to perform multiscale studies will be an essential tool to characterize these interactions.[3] We will provide examples of how different coating agents determine lung retention time, clearance by alveolar macrophages, penetration into the lung, etc. Finally, we will show how this may impact the therapeutic efficiency of some of these nanomedicines.

Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Severe challenges LHP NCs form by sub-second fast and hence hard-to-control ionic metathesis reactions, which severely limits the access to size-uniform and shape-regular NCs in the sub-10 nm range. We show that a synthesis path comprising an intricate equilibrium between the precursor (TOPO-PbBr₂ complex) and the [PbBr₃^-] solute for the NC nucleation may circumvent this challenge [1]. This results in a scalable, room-temperature synthesis of monodisperse and isolable CsPbBr₃ NCs, size-tunable in the 3-13 nm range. The kinetics of both nucleation and therefrom temporally separated growth are drastically slowed, resulting in total reaction times of up to 30 minutes. The methodology is then extended to FAPbBr₃ (FA = formamidinium) and MAPbBr₃ (MA = methylammonium), allowing for thorough experimental comparison and modeling of their physical properties under intermediate quantum confinement. In particular, NCs of all these compositions exhibit up to four excitonic transitions in their linear absorption spectra, and we demonstrate that the size-dependent confinement energy for all transitions is independent of the A-site cation.  We then show that this synthesis – relying on the labile ligand capping with TOPO-phosphinic acid mixture – makes for a convenient platform for the subsequent surface functionalization with diverse capping ligands [2]. Robust surface functionalization of highly ionic surfaces, as is the case of LHP NCs, has remained a formidable challenge due to the inherently non-covalent weak surface bonding. Leveraging the vast and facile molecular engineering of phospholipids, we present their efficacy as surface capping ligands for LHP NCs. Molecular dynamics simulations and solid-state NMR confirm that the surface affinity of these zwitterionic molecules is primarily governed by the geometric fitness of their anionic and cationic moieties. Judicious selection of the ligands yielded colloidally robust FAPbBr₃ and MAPbBr₃ NCs and enabled colloids in a variety of solvents, from n-hexane to acetone. Robustness of the surface capping is also reflected in optical properties: NCs exhibit PL quantum yield (QY) above 96% after numerous purifications. NCs are essentially blinking-free at a single particle level.

We will discuss the atomistic details of the ligand surface binding, ranging from conventional alkylammonium ligands [3] to diverse zwitterionic moeities. In particular, we show that solution and solid-state NMR readily captures the surface binding motifs and the impact of the ligands on the surface structure of CsPbBr₃ NCs [4].

Q. Akkerman et al. Science 2022, 377, 1406-​1412

V. Morad et al. submitted

A. Stelmakh et al. Chem. Mater. 2021, 33, 5962−5973

M. Aebli et al. submitted

Colloidal synthesis routes allow precise control over material parameters at the nanometer scale with moderate capital or operating costs and with high-throughput production and material yields. Stimulated by its simplicity and huge potential, countless groups all around the world have developed an extensive library of synthetic strategies and routes to produce nanocrystals with almost any composition, size, and shape. However, for such control over material parameters at the nanoscale to truly impact real applications, colloidal nanocrystals need to be arranged into functional patterns, thin films, porous nanomaterials, or highly dense nanocomposites, depending on the application. Additive manufacturing technologies based on layer-by-layer deposition of material ejected from a nozzle in the form of an ink are well-suited to produce macroscopic structures from colloids but are limited in terms of printing speed and resolution. Electrohydrodynamic (EHD) jetting uniquely allows the generation of submicrometer jets that can reach speeds above 1 m s^-1, but such jets cannot be precisely collected by too slow mechanical stages. In this talk, I will present our progress in the control of the jet trajectory in EHD jetting technologies through a voltage applied to electrodes located around the jet. This method allows to continuous adjust the jet trajectory with lateral accelerations up to 10⁶ m s^-2. Through electrostatically deflecting the jet, 3D objects with submicrometer features can be printed by stacking material layers on top of each other at layer-by-layer frequencies as high as 2000 Hz. The fast jet speed and large layer-by-layer frequencies achieved translate into printing speed up to 0.5 m s^-1 in-plane and 0.4 mm s^-1 in the vertical direction, three to four orders of magnitude faster than techniques providing equivalent feature sizes. This technique is applied to the production of electrode materials for energy conversion and storage.

Epitaxial growth methods usually need dedicated equipment, high energy consumption to maintain pure vacuum conditions and evaporation of source materials, and elevated substrate temperatures. Solution epitaxial growth requires nothing of that but is rarely used because the achieved microstructures are of low quality, not homogeneous, and finally exhibit worse performances in devices. We have introduced several growth methods including ink-jet printing [1], drop-casting [2] or antisolvent-vapor-assisted-crystallization [3] to obtain epitaxial perovskite micro-crystallites whose shapes and sizes can be controlled by the synthetic conditions. These micro-crystallites are not only oriented on the substrates such as mica or PbS single crystals, but also exhibit smooth side walls, which coincide with crystalline facets. Thus, they are acting as optical micro-resonators, supporting lasing under optical excitation. The obtained threshold powers as well as the environmental stability are competitive to those obtained by vapor deposition methods. The solution epitaxial growth can also be performed to obtain closed epitaxial films, instead of arrays of ordered micro-crystals, representing a common goal for epitaxial growth. Thus, solution epitaxy applied to metal-halide perovskites and performed in almost ambient conditions is rivaling with vapor deposition methods, even though it is achieved by cost-effective and simple methods and on cheap and commercially available substrates.

III-V quantum dots are printable semiconductors free of hazardous substances. In this talk, recent progress in III-V QDs synthesis at Ghent University and the device implementation of these materials is discussed. First, we show that the formation of InP-based QDs with near-unity photoluminescence quantum yield across the visible spectrum is now possible. This result is achieved by implementing a core/shell/shell structure, in which the composition of the inner shell is gradually changed from Se-rich to S-rich Zn(Se,S) with decreasing core QD size. We demonstrate that such application grade QDs can be used as color convertors to make on-chip QD-LEDs with +50% colo conversion efficiency. Second, we discuss how rational adaptations to the synthesis of In(As,P) QDs result in one-batch-one-size protocols yielding In(As,P) QD batches with a band-edge absorption up to 1600 nm, and we demonstrate the formation of QD photodiodes sensitive up to 1400 nm from these QDs. Both examples highlight how III-V QDs are evolving from a material for lab-scale proof-of-principle to devices ready of the consumer-market

Operando characterization techniques are indispensable for understanding the catalysts evolution during operation. Electrochemical liquid-phase transmission electron microscopy (ec-LPTEM) grants the capability of real-time imaging of electrochemically-induced processes in liquid media. Herein, we describe the advancements of ec-LPTEM towards studying the solid-liquid-gas interfacial processes occurring on Co-based oxide oxygen evolving catalysts. By performing real-time measurements, we can associate the potential-dependent variation of the local contrast to the effects that precede the electrocatalytic reaction such as electrowetting and redox-induced reactions. Further, through optimization of the microcell, we report on the direct probing of the evolution of molecular oxygen by operando electron energy loss spectroscopy (EELS). Similar experiments on IrO2 particles indicate the capability to separate the contribution of different components in the EEL spectra, providing qualitative maps of O2 and liquid electrolyte. Our work on catalytic particles exemplifies the crucial role that surface sensitive and electrochemical microcell TEM techniques can play in detailing the mechanism of electrocatalytic processes and can provide a new characterization framework aiding development of novel design routes for targeted nanocatalyst preparation.

Lead halide perovskites successfully advance towards applications in solar cells, light-emitting devices, and high-energy radiation detectors. Recent progress in understanding their uniqueness highlights the role of optoelectronic tolerance to intrinsic defects, particularly long diffusion lengths of carriers, and highly dynamic 3d inorganic framework. This picture indicates that finding an analogous material among non-group-14 metal halides can be very challenging, if possible at all. On the other hand, a judicious choice of chemistry made it possible to noticeably increase the performance of formamidinium tin iodide perovskites when integrated into thin-film photovoltaic devices. The main challenge with this material originates from the easiness of the trap states generation, which are typically ascribed to the oxidation of Sn(II) to Sn(IV). In this work, we describe the synthesis of colloidal monodisperse FASnI₃ NCs, whereby thorough control of the purity and redox chemistry of the precursors allows the concentration of Sn(IV) to be reduced to an insignificant level, to probe the intrinsic structural and optical properties of these NCs. Intrinsic FASnI₃ NCs exhibit unusually low absorption coefficients of 4•10³ cm^-1 at the first excitonic transition, a 190 meV increase of the bandgap as compared to the bulk material, and a lack of excitonic resonances. These features are attributed to a highly disordered lattice, distinct from the bulk FASnI₃ as supported by structural characterizations and first-principles calculations.

Self-assembly of colloidal nanocrystals into long-range-ordered structures through a bottom-up approach is highly promising for creating metamaterials with programmable functionalities arising from various synergistic effects and emergent interactions between neighboring NCs. Both single-component and binary superlattices reported so far were mainly composed of spherical NCs limiting the variety of obtained structures to the ones isostructural with the known atomic lattices [1]. A far greater structural diversity is accessible via assembling NCs of different shapes, e.g. nanoplates, nanorods, nanocubes, or nanodiscs. Notably, lead halide perovskite nanocrystals are characterized by remarkable optical properties together with being synthetically available in the monodisperse form making them attractive candidates as building blocks for self-assembly. A new step toward unique positional and orientational order would be to combine perovskite NCs with dielectric NCs of anisotropic shape. To this end, we have chosen LaF₃ and NaGdF₄ NCs as a second component for SLs due to their synthetic accessibility and well-defined, ensemble-uniform morphology.

Co-assembly of cesium lead halide nanocubes with disc-shaped LaF₃ NCs (9.2-28.4 nm in diameter) leads to the formation of six columnar structures with AB, AB₂, AB₄, and AB₆ stoichiometry as well as to noncolumnar lamellar and ReO₃-type SLs by employing larger perovskite NCs [2]. The latter two SLs proved to exhibit characteristic features of the collective ultrafast emission – superfluorescence. Intending to broaden the variety of building blocks for NC SLs, we utilized organic-inorganic perovskite NCs, namely FAPbBr₃ NCs, for the formation of binary SLs with spherical NaGdF₄ NCs (b-ABO₃-, AlB₂-, AB₂-, NaCl-type structures) and LaF₃ nanodiscs (columnar AB-type and lamellar SLs). Combining larger and thicker NaGdF₄ nanodiscs (18.5 nm in thickness) with cesium lead halide NCs resulted in the CaC₂-like and AB₃-type SLs. For CaC₂- and ABO₃-type structures, we expanded the scope of the available dimensionalities of SLs by employing microemulsion-templated self-assembly which allowed for the formation of three-dimensional binary supraparticles [3].

2D materials shows great promise for use in flexible electronics because their atomic thickness allows for maximum electrostatic control, optical transparency, sensitivity and mechanical flexibility. In addition, different 2D crystals can be easily combined in one stack, offering unprecedented control on the performance and functionalities of the resulting heterostructure device [3]. Solution processing of 2D materials allows simple and low-cost techniques, such as ink-jet printing, to be used for fabrication of heterostructure-based devices of arbitrary complexity.

Our group has developed highly concentrated, defect-free, biocompatible and inkjet printable and water-based 2D crystal formulations, by exploiting a supramolecular approach based on non-covalent functionalization of 2D materials with pyrene derivatives [1]. Examples of printed heterostructures, such as arrays of photosensors, programmable logic memories, transistors and memristors will be discussed [2-3]. I will show that inkjet printing can be easily combined with semiconducting 2D materials produced by chemical vapor deposition, allowing simple and quick fabrication of complex circuits on paper, compatible with CMOS technology [4-5] and also offers a simple way to integrate 2D materials into Si-based technology [6].

In contrast to the tremendous efforts dedicated to the exploration of graphene and inorganic 2D crystals such as metal dichalcogenides, boron nitride, black phosphorus, metal oxides, and nitrides, there has been much less development in organic 2D crystalline materials, including the bottom-up organic/polymer synthesis of graphene nanoribbons, 2D metal-organic frameworks, 2D polymers/supramolecular polymers, as well as the supramolecular approach to 2D organic nanostructures. One of the central chemical challenges is to realize a controlled polymerization in two distinct dimensions under thermodynamic/kinetic control in solution and at the surface/interface. In this talk, we will present our recent efforts in bottom-up synthetic approaches toward novel organic 2D crystals with structural control at the atomic/molecular level. On-water surface synthesis provides a powerful synthetic platform by exploiting surface confinement and enhanced chemical reactivity and selectivity. We will particularly present a surfactant-monolayer assisted interfacial synthesis (SMAIS) method that is highly efficient in promoting the programmable assembly of precursor monomers on the water surface and subsequent 2D polymerization in a controlled manner. 2D conjugated polymers and coordination polymers belong to such material classes. The unique 2D crystal structures with possible tailoring of conjugated building blocks and conjugation lengths, tunable pore sizes and thicknesses, as well as impressive electronic structures, make them highly promising for a range of applications in electronics, optoelectronics, and spintronics. Other physicochemical phenomena and application potential of organic 2D crystals, such as in membranes, will also be discussed.

The last few years have seen the increasing occurrence of water contamination by pharmaceuticals, personal care products, plastic and their additives among the other,^1-5 that are not satisfyingly removed by conventional drinking water treatment technologies.^{6, 7} In some cases, as for the endocrine disrupting agents such as per and polyfluoroalkyl substances (PFAS) and bisphenol A (BPA), the eco and human toxicity has been demonstrated ^8-11 calling for the urgent development of new technologies for detection, early warning and remediation of such  emerging contaminants.

Due to its commercial availability at good standard quality and processability in composite or membranes, graphene is one of the most promising nanomaterials for the realization of new technologies for water purification. In this talk,  I will present a selection of case studies of graphene-and chemically modified based materials and devices realized at CNR to detect and remove emerging contaminants including pharmaceuticals, personal care products and PFAS), from drinking water.[1,2,3] I will discuss about performance, sustainability issues in comparison with standard technologies and future challenges to respond to the sustainable development goal n6 of the UN ‘ensuring clean water and sanitation to all’.

The synthesis of different hydrophilic polymeric networks, by in situ radical polymerization in the presence of 2D materials, gives rise to three-dimensional soft structures. The role of the nanomaterial within the polymeric network is mainly intended for reinforcement (i.e. stiffness and toughness enhancement). However, we have shown that the presence of 2D materials, such as graphene, can also enhance features such as biocompatibility [1], sensing [2] or self-healing capability [3], giving rise to truly hybrid composites [4]. Moreover, the ability of these soft materials to respond to different stimuli, such as electric and magnetic fields, to behave as sensors, and the possibility to prepare them following 3D printing methodologies, open the way to applications in soft robotics [5]. The preparation of these smart soft systems requires the production of large quantities of 2D materials easily dispersible in water; ball milling methods developed in our laboratories have proven to be both sustainable and easily scalable technologies. [6], [7].

This talk aims to show the latest advances we have made on two-dimensional (2D) materials for energy storage and conversion. Specifically, we will show the great promise of a disruptive family of 2D transition metal layered hydroxides (LH), which exhibit a wide chemical tunability and can be produced in large scale. Indeed, a series of single and layered double hydroxides will be discussed including the Simonkolleite phases of cobalt-LH or the NiFe-LDHs of great interest in electrocatalysis, or the hybrid organic-inorganic LDHs of interest as precursors for the preparation of electrodes for rechargable alkaline batteries. Along with their synthesis and characterization, we will show the performance of these wonderful materials in the development of new battery prototypes or better electrocatalysts for the production of green hydrogen. Last but not least, we will show a new family of hybrid 2D-LHs with great potential as electrode materials: the so-called "meta-layered hydroxides".

A precise knowledge of the optical and electrical properties of 2D nanomaterials, is essential for pushing forward these materials in energy applications. In this regard, synthesis and studies of nanohybrids at single-crystal scale is a promising alternative to study the intrinsic properties of a photoactive material, since it is avoided the effect of the grain boundaries and amorphous domains present in polycrystalline films using for that purpose a homemade setup. [1, 2]

In this communication, we present the optical study of different microstructured materials such as hybrid perovskites for the development materials with applications in the field of energy conversion, optoelectronics and lighting. For instance, the incorporation of SubPc in the interlayer space of 2D–3D perovskites expands the photoresponse in the visible region [3]. SubPc incorporation was achieved by selectively controlling the composition during the synthesis. Photocurrent spectra on isolated single crystals indicates that SubPc molecules are participating in the charge generation process upon illumination, and acts as light harvester for long wavelength radiations. All these studies provide relevant information for device manufacturing in the development of high-efficiency solar cells, photocatalysts and optoelectronic devices. [1-4]

Heterogeneous photocatalysis by readily available, metal-free catalysts is of great appeal in view of the increasing pressure on industry to move towards sustainable schemes of chemical production. Graphitic carbon nitride (g-CN) is a versatile semiconductor nanomaterial, well known for applications in energy, such as H₂ photocatalytic production and CO₂ reduction. We recently highlighted the key role that g-CN could play in the realm of photocatalytic organic synthesis, [1] and showed that tailoring the structure of g-CN by means of minimally invasive post-synthetic protocols could be the solution to tackle challenging coupling reactions with great efficiency.[2,3] Through international collaborations, our group embarked in the in depth investigation of the structure/activity relationship of new g-CN derivatives related to photocatalysis, by means of a combination of advanced spectroscopic techniques [4] and computational methods.[5] The final goal is aims at establishing an approach based on rational design of modified CN materials, with properties tailored to the specific photocatalytic process. Apart from the strict metal-free photocatalysis, an additional possibility arises from the inclusion of single metal atoms into the CN scaffold, whereby the opportunely adjusted CN structure can interact and be in synergy with the metal site. This can be of high relevance both in dual photoredox catalysis for organic synthesis and in energy conversion processes.

In recent years, a plethora of material systems have been designed and prepared to increase the performance of light harvesting and light-emitting technologies, and to develop new and attractive applications. Limitations of state-of-the-art devices based on organics (both conjugated polymers or small molecules/oligomers) derive largely from material stability issues after prolonged operation. This challenge could be tackled by leveraging the enhanced stability of carbon nanostructures (CNSs, including carbon nanotubes and the large family of graphenebased materials) in carefully designed nano-hybrid or nano-composite architectures to be integrated within photo-active layers, paving the way to the exploitation of these materials in contexts in which their potential has not been yet fully revealed. In this talk, I will discuss the theoretical background behind CNSs hybridization with other materials such as graphene with donor-acceptor molecules, for the establishment of novel optoelectronic properties and provide an overview of new optoelectronic and transfer properties of twisted graphene nanoribbons with controlled peripheral size that allow to forecast interesting future perspectives for use in real devices.

Since the first discovery of graphene by Novoselov in 2004, graphene and in general 2D materials showed an increasing interest in the scientific community. Specifically Transition Metal Dichalcogenides (TMDC) showed a surge in interest, due to their different properties and flexibility due to the large amount of variability in the group. Not only this, but they can be easily engineered in different ways to tune their properties, examples are doping, functionalization and hybridation with other material¹. In this last context, Conductive Polymers (CP)² show complementary electrochemical properties to the 2D TMDC materials and can be exploited to further improve their performances. Here, we report on the synthesis of hybrids based polyaniline (PANI) and 2D transition metal dichalcogenides (TMDCs), employing different methodologies and formulations. In particular, we resort to liquid phase exfoliation (LPE)³ to produce 2D TMDCs in both the 2H and 1T phase and to in-situ polymerization to produce PANI chains directly on the surface of these nanomaterials to further amplify the electrochemical properties of the 2D material. Then the as obtained material can be further characterized electrochemically and can be also used in conjunction with flexible polymer scaffold in order to obtain piezo- active hydrogels. The as-obtained hydrogels are characterized through a combination of techniques and their swelling behaviour and mechanical properties are investigated.

Due to their outstanding electronic, thermal, optical, chemical, and mechanical properties, 0/1/2D carbon nanostructures (CNSs) have attracted great interest in the last two decades.^[1] Among CNSs, different procedures have been developed to synthesize carbon nanodots (CNDs) starting from a large pool of small molecules through a bottom-up approach, and several applications for these nanomaterials have been investigated.^[2,3] CNDs can be further decorated through chemical functionalization for sensing, catalysis, and optoelectronic applications. For example, light-conversion processes in covalently functionalized CNDs with donor-acceptor organic dyes can be investigated.^[4] Therefore, it is important to analyze the reactivity and accessibility of the surface functional groups, as well as their total quantities. This contribution discusses the characterization and quantification of the terminal functional groups of four different bottom-up synthesized CNDs.^[5] We resorted to pH back-titrations, Kaiser tests, X-ray photoelectron spectroscopy (XPS), and quantitative ¹⁹F-NMR of incorporated fluorine atoms to quantify the terminal amine content in the prepared CND samples. XPS provides both the surface functional group content and, together with elemental analysis (EA), the elemental composition of the synthesized CNDs. The amount of ethylene diamine used as starting material governs the fraction of functional groups on the surface. This quantification, before and after functionalization, provides useful information about the reactivity and accessibility of the terminal functional groups. Hereby, the yields of reactions can subsequently be determined. Based on the different amounts of terminal amino groups of these four CNDs, it is possible to determine which is most suitable for further functionalization.

Colloidal CdTe nanoplatelets featuring a large absorption coefficient and ultrafast tunable luminescence coupled with heavy-metal-based composition present themselves as highly desirable candidates for radiation detection technologies [1]. Historically, however, these nanoplatelets have suffered from poor emission efficiency, hindering progress in exploring their technological potential [2], [3]. In this talk, we report the synthesis of CdTe nanoplatelets possessing a record emission efficiency of 9%. This enables us to investigate their fundamental photophysics using ultrafast transient absorption, temperature-controlled photoluminescence, and radioluminescence measurements, elucidating the origins of exciton- and defect-related phenomena under both optical and ionizing excitation. For the first time in CdTe nanoplatelets, in this talk is reported the cumulative effects of a giant oscillator strength transition (GOST) and exciton fine structure. Simultaneously, thermally stimulated luminescence measurements reveal the presence of both shallow and deep trap states and allow us to disclose the trapping and detrapping dynamics and their influence on the scintillation properties [4].

Silver phenylselenolate (AgSePh) and silver phenyltellurolate (AgTePh) are novel two-dimensional (2D) van der Waals semiconductors. In contrast to 2D layered perovskites and transition metal dichalcogenides, AgSePh and AgTePh have strong covalent interaction between organic and inorganic components, becoming truly hybrid organic-inorganic semiconductors. However, despite having the similar crystal structure, composition and absorption characteristics, AgSePh and AgTePh exhibit strikingly different light emission characteristics. Whereas AgSePh exhibit narrow, fast luminescence with a minimal Stokes shift that tracks the temperature-dependent shifting of the lowest-energy excitonic absorption resonance, AgTePh exhibits comparatively slow, significantly broadened luminescence with large Stokes shift that does not track the shifting of excitonic absorption resonance peak with changing temperature. In this presentation, we will present the synthesis, structure and excitonic optical properties of AgSePh and AgTePh films. Furthermore, we will discuss different physical mechanisms underlying light emission in AgSePh and AgTePh. Using time-resolved and temperature-dependent optical spectroscopy, combined with sub-gap photoexcitation spectroscopy, we will show that exciton dynamics in AgTePh are dominated by intrinsic self-trapping behavior, whereas dynamics in AgTePh are dominated by interaction of band edge excitons with extrinsic defect states. Finally, we will show tunable excitonic properties in AgSe_1-xTe_xPh alloys depending on composition.

Two-dimensional (2D) materials can provide a suitable platform to develop new technologies for energy applications. In my group we are combining functional molecules with 2D materials with the idea of tuning/improving the properties of the “all surface” 2D material via an active control of the hybrid interface. This concept, which goes much beyond the conventional chemical functionalization of a 2D material, can provide an entire new class of hybrid molecular / 2D heterostructures, which may be at the origin of a novel generation of materials and devices of direct application in electronics, spintronics, molecular sensing and energy. Here this concept will be presented and used in the design of low energy consumption spintronic devices as well as in energy storage and conversion. In the first part I will show that 2D magnetic materials as well as hybrid molecular /2D antiferromagnets can be used to store and transport information with extremely low energy disipation and tunability. In the second part, hybrid materials formed by electroactive inorganic layers will be used as electrodes in supercapacitive devices and as electrocatalysts.

Photovoltaic and solar fuels technologies rely upon the intentional movement of charge carriers (i.e. electrons and holes) and/or energy (i.e. excitons) in prescribed directions to convert sunlight into electricity or fuels. Two-dimensional semiconductors have several advantages for PV and (photo)catalytic technologies, due to their large absorption coefficients, high mobilities for charge carriers and excitons, and catalytic activity for important fuel-forming reactions. To realize the full potential of 2D nanomaterials and related heterostructures for sustainable energy technologies, fundamental studies are needed to probe the key photochemical processes that occur upon photon absorption, including exciton diffusion and dissociation, interfacial charge and energy transfer, and charge recombination. In this presentation, I will highlight our ongoing studies probing charge and energy transfer across heterojunctions formed between monolayer transition metal dichalcogenides (TMDCs) and other nanoscale semiconductors such as semiconducting single-walled carbon nanotubes, small molecules, and nanocrystals. Appropriate tuning of the interfacial band alignment can enable rapid exciton dissociation and exceptionally long-lived charge-separated states that are essential for PV and catalytic applications. Tuning the binding motif of molecular species between van der Waals association and covalent bonds can also influence the mechanisms of interfacial charge/energy transfer. If time permits, I will also discuss how certain 2D nanomaterials can facilitate charge/energy transfer while simultaneously blocking undesired ion movement across the heterojunction.

2D organic semiconducting polymers have emerged as an important class of materials that offer high potential for a variety of applications, such as energy storage, sensing or organic electronics. However, the design of novel materials generally involves an expensive and environmentally unfriendly methodology that strongly contrasts with the ecological transition spirit. In this sense, computational design offers a green alternative to experimental laboratory research. On the other hand, Raman spectroscopy is a fast and nondestructive characterization tool widely used to evaluate the structural and electronic properties of pi-conjugated materials. In this study, we combine an experimental and theoretical approach that links DFT calculations with Raman spectroscopy aiming to control the electronic and structural properties of conjugated organic materials  Overall, our findings open the door to the control of the degree of the π-conjugation of 2D organic polymers for their subsequent synthesis and real applications ranging from sensing to electronics.  

Electron tomography enables one to measure the morphology and composition of nanostructures in three dimensions (3D), even at atomic resolution. However, an emerging challenge is to fully understand the connection between the 3D structure and properties under realistic conditions, including high temperatures as well as in the presence of liquids and gases. Our recent experiments demonstrate the progress that can be obtained by accelerating both the acquisition and reconstruction during electron tomography [1,2]. In this manner, we were able to perform a dynamic characterisation of shape transformations of metal nanoparticles at high temperatures [1,3]. Moreover, we measured the elemental diffusion dynamics of individual anisotropic bimetallic nanoparticles in 3D and determined the effect of parameters such as type of interfacial facets, aspect ratio, shape and presence of defects [4]. Finally, by combining aberration corrected electron microscopy with a quantitative interpretation, we can provide quantitative measurements of the coordination numbers of the surface atoms of catalytic nanoparticles at high temperatures and in gaseous environments [5].

II-VI semiconductor nanoplatelets are colloidal nanoparticles confined in one dimension which gives them exceptionally narrow optical properties. We are showing that through the design of core/crown/crown heterostructures, it is possible to synthesize nanoplatelets with green and red emissions from a single population. In the CdSe/CdTe/CdSe core/crown/crown NPLs, the exciton either recombine at the interface between 2 semiconductors due to the type II band alignment or in the CdSe area. The ratio of the two emissions can be tuned by the incident power. With increased power, the red emission saturates due to non-radiative Auger recombination thus affecting its emission much stronger than the green one. These NPLs can be introduced in LED. So, the design of the NPLs inorganic core enables to tune their optical properties but their shape can be further modified through the surface chemistry. NPLs with their large lateral dimensions can be considered as flexible substrate for the self-assembly of ligands which originally ensure the colloidal stability of NPLs. The stress brought by surface ligands and the specific zinc blende crystal structure of the NPLs induce a folding of NPLs as helices or twists. We show that an exchange from carboxylate ligands to halides ligands and thiolate ligands enable to unfold CdSe NPLs and to invert the chirality of the helices. Finally, this surface chemistry also enables to control the kinetic of a cation exchange.

Colloidal semiconductor heteronanocrystals (HNCs) exhibit unique optoelectronic properties that are inaccessible to single-component NCs, making them promising materials for a wide range of applications. The optoelectronic properties of HNCs are determined not only by the bandgap and band alignment of its constituent materials but also by their morphology and heteroarchitecture.¹ Applications requiring efficient charge separation (e.g., photovoltaics and photocatalysis) and polarized emission greatly benefit from anisotropic morphologies, such as heteronanorods. Most of the work on heteronanorods has focused on Cd-chalcogenide-based HNCs. However, given the toxicity of Cd, the potential of these materials for large scale applications is limited.

Copper-chalcogenide based NCs have attracted increasing attention as promising alternatives for Cd- and Pb-chalcogenide NCs due to their low toxicity, large absorption cross-sections across a broad spectral range, composition-dependent band gaps in the 1 to 2.5 eV range, and photoluminescence tunability, spanning a spectral window that extends from the UV to the NIR depending on the NC size and composition.^2,3

Several synthetic strategies have been used in the quest for high-quality colloidal copper chalcogenide based HNCs. The most promising ones are based on the multistage approach, which allows the combination of different synthesis techniques (e.g., cation exchange^4,5 or seeded growth^6-9) in a sequential manner in order to achieve the targeted preparation of colloidal HNCs. In this talk, I will discuss a selection of recent examples, chosen to illustrate specific synthesis strategies: CuInSe₂/CuInS₂ dot core/rod shell heteronanorods,⁴ Cu_1.8S-based multicomponent axially segmented heteronanorods,⁵ CuInS₂/ZnS dot core/rod shell heteronanorods,⁶ and Janus-type Cu_2‒xS/CuInS₂ and Cu_2−xS/ZnS heteronanorods.^8,9 I will show that by combining the design principles of post-synthetic heteroepitaxial seeded growth and nanoscale cation exchange into multistage synthesis strategies one can potentially gain access to a plethora of Cu-chalcogenide-based multicomponent heteronanorods with diameters in the quantum confinement regime.

Discovering and developing novel nanomaterials to improve the sustainability of optoelectronic devices, while maintaining reasonable efficiency, is of key importance to drive future research and make commercialization possible. With the criteria of earth-abundant, stable, and low-toxicity elements for nanocrystal (NC) synthesis, we focus on ABX₃ chalcogenides, where A is an alkaline earth metal and B a group four transition metal. We seek to exploit the unique properties of NCs and the perovskite crystal structure while shifting from lead (Pb) and cadmium (Cd) containing materials to more sustainable ternary chalcogenides of low toxicity. Specifically, in this class of materials, BaTiS₃ (in the hexagonal phase) and BaZrS₃ (in the perovskite structure) are promising. A flexible method for controlling the physical and chemical properties of these materials is through colloidal synthesis with organometallic precursors. Synthetic routes using different sulfur and barium sources are investigated. Characterization by TEM, XRD, and absorbance measurements reveal how the shape of the nanocrystals can be tuned from isotropic to anisotropic with aspect ratios ranging from 1:1 to 1:100 by changing the barium precursor from a methylamino compound to a halide salt. Alternatively, adjusting the sulfur source between various thioureas alters the crystallinity and reaction temperature needed to achieve NCs. Transient absorption (TA) spectroscopy shows long excitation lifetime (ca. 1 µs) and clear spectral features resembling PbS quantum dots with respect to spectral width of the band-edge bleach and photoinduced absorption signal. For the application in optoelectronic devices, it is necessary to exchange the long insulating ligands used during synthesis for shorter or more conductive ones. We have developed a ligand exchange procedure to cap the NCs with halides and process the NC solutions as thin films. With the gained knowledge from TA and film formation strategies, first work on the usage of BaTiS₃ NCs in photovoltaic devices and field-effect transistors will be presented.

Ternary alkali-metal dichalcogenides {AMeE (A = Li, Na, K, Rb, Cs, Cu, Ag, Tl; Me= Metals E = S, Se, Te} are an intriguing class of semiconductors with tremendous technological potential.[1,2]  For more than a decade, the compounds of these structural types have been explored as potent thermoelectric materials.[3] High-temperature solid-state synthesis is typically employed to synthesize alkali metal-based chemicals.  However, it naturally forms highly aggregated, polydisperse particles with little control over different phases and understanding in terms of mechanistic insights.  In this case, solution phase synthesis can be advantageous to form uniform nanocrystals.  Herein, we develop a facile colloidal hot-injection strategy to synthesize shape and size-tunable caesium copper selenide nanocrystals comprising earth-abundant and non-toxic elements.  Ex-situ mechanistic investigation reveals that the NCs formation is driven by the dissolution of binary Cu2-xSe, followed by the incorporation of Cs+ to form the ternary CsCu5Se3.  A thorough case study on the addition of alkyl acid and amine ligands indicates that the involvement of both acid and amine functionality is required to synthesize pure-phase NCs.  The current study also reveals that variation in the alkyl chain length of amines influences the size, shape, and formation of distinct phases. Furthermore, the nanocrystals were consolidated as pellets to study thermoelectric transport properties showing ultra-low thermal conductivity of 0.65 W/mK at 728 K, promising for improved thermoelectric applications. Altogether, the present study provides a mechanistic understanding of the factors that impact the synthesis of materials based on alkali metal chalcogenides and exhibit their promising thermoelectric properties.

The activity of electrocatalysts during water electrolysis is controlled by their surface chemistry. The control of the surface structure and composition of an electrocatalyst requires understanding of the solid state reactions that govern the formation of the surface and bulk during synthesis. Probing these reactions requires characterization that can cover multiple length scales, from the nanoscale near-surface layer to the mesoscale phase behavior.

In my talk, I will discuss how the phase evolution of oxide electrocatalysts across different length scales can be probed using a combination of scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM). Specifically, my talk will address the formation of (La,Sr)CoO₃ nano- and meso-scale particles that are often used as electrocatalysts for oxygen evolution reaction.  The impact of the size and phase state of precursors as well as the time-temperature heat treatment profile on the resulting surface and bulk chemistry of the nanoparticles will be discussed.

Colloidal semiconductor nanocrystals (NCs) such as chalcogenides and perovskites have outstanding optical and electronic properties that make them interesting for multiple applications that involve, sensing, solar cells, light-emitting device technology, and more recently photocatalysis. ¹

Tailoring the surface chemistry of colloidal metal halide perovskite NCs (AMX₃; A= monovalent metal such as CH₃NH₃⁺, or Cs⁺; M= Pb²⁺, Sn²⁺; and X= Cl^-, Br^-, I^-) is very challenging and crucial to determine their final properties. Therefore, the nature of the organic capping agents and their interaction with the surface atoms will be relevant to control the photophysical properties of the nanocrystals and prepare novel assemblies.  The most common ligands, according to the covalent bond classification, used to obtain highly emissive and stable colloidal perovskite NCs are X-type and L-type binding ligands (Figure 1).

Several examples of the role of those types of ligands in i) the emissive properties of the NCs²; ii) the formation of blue emissive 2D low-dimensional materials³; iii) the photocatalytic ability of the metal halide perovskite NCs,⁴ and iv) the preparation of nano-heterostructures for NIR light-harvesting applications ⁵ will be discussed.

Ligands play a crucial role in the synthesis and/or stabilization of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, of which oleic acid being the most typical example. Here we show that monoalkyl phosphinic acids are another ligand class.

We put forward monoalkyl phosphinic acids as an alternative ligand class, similar to carboxylic acids and phosphonic acids. After presenting the ligand synthesis for several selected substrates, we proceed to show the intermediate reactivity of the phosphinic acids in CdSe quantum dot syntheses. The nanocrystals synthesized with phosphinic acids are also easier to purify since there is no gel formation. Very small (2–3 nm) CdSe quantum dots with low polydispersity and high photoluminescence quantum yields can be easily accessed with phosphinic acid ligands. CdSe and CdS nanorods were also synthesized using phosphinic acids, whereby the rods showed high purity an d uniformity.

In addition, we investigate the ligand-NC interaction on both metal oxide (i.e., HfO₂) as metal chalcogenide (i.e., CdSe and ZnS) NCs. Given their intermediate acidity, phosphinic acids (pKa ≈ 3.08) bind to the surface with an affinity between carboxylic and phosphonic acids. Using solution NMR, we quantify the X-for-X ligand exchange via the alkene resonance of the oleyl chain (carboxylic and phosphonic acid), and the ether resonance of the 6-(hexyloxy)hexyl phosphinic acid. We conclude that the monoalkyl phosphinic acids quantitatively displace carboxylate ligands and are in equilibrium with phosphonates (although phosphonate binding is favored). This results show that monoalkyl phosphinic acids are suitable reagents to efficiently functionalize nanocrystal surfaces.

In conclusion, by careful design new type of ligands can be created, and can tailored towards specific functionalities such as solubility matching or intermediate reactivity and binding strength.

The detection of high-energy radiation has gained relevant interest thanks to its application in technologic-relevant fields, such as particle physics, astronomy, geology, medical diagnostics, nuclear monitoring and space exploration [1][2]. In all these areas, the most widely used detectors are scintillating materials that convert the energy deposited by incoming ionizing radiation into visible photons which is finally revealed by coupled photodiodes [3]. Beside typical inorganic materials based on high-Z elements, an alternative class of scintillators which further widens their applicability is plastic scintillators, which exhibit the advantages of a large sizes production, low weight and affordable costs making them particularly adapt for radiation monitors in border and industrial control[4]. Despite these advantages, the low density of plastic materials compared to inorganic crystals limits their interaction with ionizing radiation and typically requires doping these systems with high-Z components, such as organometallic complexes, perovskite or heavy metal chalcogenides nanocrystals (NCs). However, the intrinsic small Stokes shift of these materials represents an issue when used as nanoscintillators in highly dense or large volume detectors because of the strong reabsorption of the scintillation light along its path to the waveguide edges. We aim to takle this issue by developing a new strategy of scintillator consisting in a polymeric scintillating matrix incorporating reabsorption free Cd_0.5Zn_0.5S/ZnS core/shell NCs doped with manganese ions. Doping NCs with Mn is an established approach to activate efficient luminescence at intragap energy arising from the ⁴T₁ →⁶A₁ optical transition, yielding the characteristic Mn-emission at ~ 580-600 nm. Crucially, since such a transition is spin-forbidden, the corresponding optical absorption features negligible oscillation strength, resulting in an apparent Stokes shift between the band-edge absorption of the NCs host and the Mn-related luminescence. In our approach, we further adopted a synergic strategy in which both the plastic matrix waveguide and the NCs interact with incoming ionizing radiation, while the propagating emission is generated by the sole NCs, whose optical properties have been properly engineered to efficiently down-convert matrix emission. The emission efficiency and compatibility of the NCs with the polymer host have been optimized resulting in high optical quality nanocomposites completely transparent in the spectral region of their own emission, with scintillation efficiency comparable to commercial plastic scintillators.

Lead halide perovskites are emerging materials that can be synthesised at low cost on a large scale and have potentially disruptive optical performances in photonic and scintillation applications [1]. On the other hand, these materials are inherently toxic, due to the presence of Pb, and generally unstable, especially when exposed to heat, air and moisture. Therefore, there is vivid research aimed to replace Pb-based perovskites with non-toxic metal halide alternatives that may exhibit similar optical properties and, ideally, greater stability. In this context, the broad family of double perovskites (DPs) is particularly promising and offers fertile ground for new discoveries. Among the various structures, Sb³⁺-doped DPs are attracting increasing interest due to their bright and exceptionally large (more than 1.2eV) Stokes-Shifted PL [2], [3]. Here, we present colloidal nanocrystals (NCs) of Rb3InCl6, composed of isolated metal halide octahedra ("0D"), and dual perovskites of Cs2NaInCl6 and Cs2KInCl6, in which all octahedra share angles and are interconnected ("3D"), with the aim of elucidating and comparing their optical characteristics once doped with Sb3+ ions [4]. Our optical and computational analyses demonstrate the photophysical mechanism underlying PL in these systems, and that it is possible to double the quantum yield through localisation of the exciton in 0D structures by preventing its migration to trap states at the surface. Scintillation properties are evaluated by means of radioluminescence experiments and waveguide performance without resorption in large-area plastic scintillators is assessed by means of Monte Carlo ray-tracing simulations.

Electrically pumped lasers or laser diodes based on solution-processable materials have been long-desired devices for their compatibility with virtually any substrate, scalability, and ease of integration with on-chip photonics and electronics. Such devices have been pursued across a wide range of materials including polymers, small molecules, perovskites, and colloidal quantum dots (QDs). The latter materials are especially attractive for implementing laser diodes as in addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional character of their electronic states. These include a size-tunable emission wavelength, a low optical-gain threshold, and high temperature stability of lasing characteristics stemming from a wide energy separation between their atomic-like discrete energy levels.

Several challenges complicate the realization of colloidal QD laser diodes (QLDs). These include extremely fast nonradiative Auger recombination of optical-gain-active multicarrier states, poor stability of QD solids under high current densities required to achieve lasing, and unfavorable balance between optical gain and optical losses in electroluminescent (EL) devices wherein a gain-active QD medium is a small fraction of the overall device stack comprising multiple optically lossy charge-transport layers.

Here we resolve these challenges and achieve electrically driven laser action due to amplified spontaneous emission (ASE) in a colloidal-QD optical-gain medium. To demonstrate this effect, we employ compact, continuously graded QDs with strongly suppressed Auger recombination incorporated into a low-loss photonic waveguide integrated into a pulsed, high-current density light-emitting diode. These prototype QLDs exhibit strong, broad-band optical gain and demonstrate low-threshold, room-temperature laser action which leads to intense edge-emitted EL with intensity of more than 100 microwatts.

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Nanostructured semiconductors, or quantum dots (QDs), are heavily investigated for their applications in light emission such as light emitting diodes and lasers. The premise of cost-effective solution processing of such devices based on nanocrystals has recently driven research towards electrically pumped population inversion in laser diode structures. Challenges however remain to achieve net light amplification in the cavity due to a balance between limited material gains and lossy electrical contacts. Further reductions in threshold current densities, mainly limited by the non-radiative cap of ca. 1 nanosecond on the gain lifetime, are also required to achieve stable operation. Finally, color tunability is limited to the red by the gain bandwidth of the red-emitting CdSe/CdS QDs or even core/shell nanoplatelets used.
Here, we show that weakly confined charge carriers in giant CdS quantum dots display disruptive optical gain metrics that could alleviate these remaining issues. Being active in the green part of the spectrum, their properties match and even outcompeting state-of-the-art colloidal materials in the red. Material gain coefficients up to 50.000 cm^-1 combined with a broad gain window of 160 nm are found. Also, a very promising gain lifetime close to 3 ns is found. Invoking a model of stimulated emission based on bulk semiconductor physics, we are able to explain all of these remarkable gain metrics, yet only if a large band gap renormalization effect is invoked. Based on these unique materials, we demonstrate amplified spontaneous emission and lasing action under nanosecond optical excitation. Our results show that weakly confined nanomaterials are excellent gain materials, combining straightforward wet chemical synthesis and the promise of solution processability with beyond state-of-the-art gain metrics.

In condensed matter physics, a stepwise reduction of the dimensionality from bulk to the nanoscale is known to lead to striking electronic and optical properties such as the giant oscillator strength transition for two- dimensional (2D) semiconductor quantum wells and the spin-charge separation in one-dimensional (1D) systems. However, in contrast to metals, the lateral confinement in semiconductor materials competes with the exciton Coulomb interaction, which can be magnified by dielectric mismatch. Due to the complexity of these interactions, the degree of freedom in the electron motion remains poorly understood, whereas its clear circumscription could significantly improve the optoelectronic performances of quasi-2D systems such as CdSe colloidal nanoplatelets (NPLs).

Since the distribution of the quantized states, the so-called density of states (DOS), reflects the restrictions of the electron motion with dimensionality, it can serve as a genuine fingerprint to unambiguously disclose the confinement experienced by the charge carriers. A unique way to probe the DOS consists in measuring single-particle excitation spectra with scanning tunnelling spectroscopy (STS). Ji et al. have demonstrated that the hole DOS is consistent with a free in-plane motion, while the electrons have a more complex DOS that deviates from the simple 2D model. When the width (W) or/and the length (L) are smaller than ten to five times the exciton Bohr radius, the electron behavior, which has a smaller effective mass than the hole, becomes affected, with a significant impact on its interaction with the hole and the dielectric image charges. The already present complexity in understanding electron−hole correlation in these box-shaped nanostructures is further enhanced with the existence of electrical surface traps, which are caused by the sporadic absence of ligands. It makes the electron confinement in the NPLs still unknown, leading to intense debates about their true dimensionality. 

Here we used this technique to study electron confinement in colloidal CdSe nanoplatelets (NPLs), which have a quantized thickness d due to a discrete number of monolayers (MLs) and a rectangular shape with finite length L and width W. The observation of Van Hove singularities in the conduction band implies a paradigm shift on the electronic structure of typical CdSe NPLs considered in the literature. As the electron DOS exhibits a striking modulation that is directly related to the length of the NPLs, delineating the in-plane electron motion at low temperature has important conconsequences for a deeper understanding of the exciton dissociation, diffusive transport, and annihilation in NPLs.
Moreover, it is shown that the side facets of NPLs host electronic deep trap states, which cause a Coulomb blockade in the tunnelling current. As they could be fully removed by the formation of core-crown NPL, our results anticipate a genuine boost for NPL-based lateral heterostructures that will notably allow for the mixing of the dimensionalities to favor specific electronic or optical properties.[1]