The electrocatalytic reduction of CO2 is very sensitive to the nature of the electrolyte near the electrode surface. The presence of cations and anions, and how they are indirectly affected by the (local) pH, play a crucial role, both for CO2 reduction and for the competing hydrogen evolution reaction. In my talk, I will demonstrate these effects for CO2 reduction on flat and nanoporous gold electrodes, and show how mass transport regulates them. I will discuss the effect of monovalent cations, multivalent cations, bicarbonate concentration (and how bicarbonate responds to the presence of cations), and varying mass transport rates as regulated by a rotating disc electrode.  

      In the quest for developing a sustainable energy economy, the electrocatalytic reduction of carbon dioxide (CO₂RR) into value-added chemicals and fuels offers the potential to close the anthropogenic carbon cycle and store renewable energy into chemical bonds. It is therefore of particular interest to develop efficient and selective electrocatalysts, which reduce the reaction overpotential and steer the reaction toward hydrocarbons and multicarbon oxygenates. Nonetheless, in order to tailor the chemical reactivity of CO₂RR nanocatalysts at the atomic level, fundamental understanding of their structure and surface composition under reaction conditions must be obtained. The latter requires a synergistic experimental approach taking advantage of a variety of cutting-edge microscopy (EC-AFM, EC-TEM, LEEM) and spectroscopy (XPS, XAS, MS/GC, XPEEM, Raman spect.) methods.

This talk will provide new insights into the CO₂RR and CORR, with special focus on: (i) the evolution of the surface structure of well-oriented single crystal Cu catalysts [Cu(100), Cu(111), Cu(310)]; (ii) the design of size- and shape-controlled catalytically active nanoparticle pre-catalysts (Cu, Cu₂O, Ag-Cu₂O cubes, Au-Cu₂O cubes, ZnO/Cu₂O cubes), and (iii) the correlation between the dynamically evolving structure and composition of the electrocatalysts under operando reaction conditions, including pulse electrolysis treatments, and their activity and selectivity. The results are expected to open up new routes for the reutilization of CO₂ through its direct conversion into valuable chemicals and fuels such as ethylene and ethanol.

Electrochemical reduction of carbon dioxide (CO₂) to carbon monoxide (CO) and other C₁ products under mild conditions opens a way to produce fuels and chemical feedstock. Molecular catalysts made of Earth-abundant metals may be operated in homogeneous conditions or as supported heterogeneous systems.

Co and Fe complexes demonstrate excellent conversion of CO₂ to CO with a selectivity reaching 99 %, in organic solvent as well as in water. By setting proper conditions, we recently discovered that formaldehyde and methanol (Co complex) as well as methane (Fe complex) could be obtained. In other words, CO₂ could be reduced with 2, 4, 6 and 8 electrons with molecular catalysts. Our most recent results will be presented.

In this work, we present a mechanistic study of the electrochemical CO₂ reduction catalyzed by a highly active fac‐[Mn^I(CO)₃(bis‐MeNHC)MeCN]⁺ complex (Mn-MeCN⁺).^[1] By combining in-situ IR-SEC, chemical synthesis of intermediates and DFT calculations, we have identified and fully characterized Mn-CO⁺ (under CO₂) and Mn-H (under Ar) as key intermediates in the CO₂ reduction mechanism. Our observations confirm that, in the presence of low water concentrations, the CO₂ reduction to CO occurs through a protonation-first pathway. However, at higher concentrations of water, Mn-H is formed and easily inserts CO₂ to give the corresponding Mn-O₂CH intermediate. The formate release from the latter intermediate is a slow process which explains the decrease of the catalytic activity in the presence of high concentrations of added acid.

Greenhouse gases have maintained the conditions on Earth habitable. Nonetheless, the
atmospheric level of these gases, in particular carbon dioxide (CO2), has increased considerably in
the latest years. One way to cope with this increase is by electrochemically reduce CO2 into other
building blocks or fuels. However, the kinetic inertness and the thermodynamic stability of this
molecule makes its activation rather difficult, requiring the use of catalysts. Molecular catalysts,
such as iron porphyrins, are among the most efficient, robust, and selective catalysts towards the
reduction of CO2 to CO, by also being optimal catalysts in both organic and aqueous solutions.1,2 In
order to consider the reduction of CO2 by iron porphyrins as an interesting path to re-use and take
advantage of a waste product, fundamental knowledge is a pre-requisite to better understand this
reaction, improve their performance and orient their selectivity.

Even if electrochemical methods have allowed the determination of kinetic parameters,3,4 a
complete picture of the mechanism is still missing due to the lack of spectroscopic evidence. By
coupling electrochemical techniques with X-Ray absorption spectroscopy, we aimed at obtaining
a complete view of the modifications undergone by the catalyst during electroreduction and
catalysis. In this context, we have recently developed a spectroelectrochemical set-up for in situ
CO2RR studies by iron porphyrins in organic media. We present here the spectral changes
observed at the pre-edge and XANES region. This has allowed to give insights on the structural
and electronic changes that occur at the iron center and its interactions with the CO2, proposing
some possible intermediate species which are crucial to describe the CO2 reduction mechanism.

References:
(1) Costentin, C.; Robert, M.; Savéant, J.-M.; Tatin, A. Proc Natl Acad Sci USA 2015, 112 (22), 6882–6886.
(2) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M. Science 2012, 338 (6103), 90–94.
(3) Bhugun, I.; Lexa, D.; Savéant, J.-M. J. Am. Chem. Soc. 1996, 118 (7), 1769–1776.
(4) Costentin, C.; Passard, G.; Robert, M.; Savéant, J.-M. J. Am. Chem. Soc. 2014, 136 (33), 11821–11829.

Effective large-scale CO₂ conversion to fuels or value-added chemicals using renewable energies is critical to reduce our environmental impact [1]. To this end, better understanding of the CO₂ mechanism is needed to develop efficient and selective catalysts that operates in water controlling H₂ evolution. Covalent Organic Frameworks (COFs) are reticular materials, which can be used to combine the advantages of the well-defined molecular catalysts and the heterogeneous ones [2]. In this work, we present the first COF based on tricarbonyl Mn units (COF_bpyMn), that by π-π stacking is attached to MWCNTs form electrocatalytic electrodes active for CO₂ reduction in neutral water. The activity of the catalyst was evaluated by electrochemical techniques with good stability in aqueous solution. The COF_bpyMn shows a low CO2RR onset potential (η = 190 mV) and high current densities (>12 mA·cm^–2, at 550 mV overpotential) in water. TOF_CO and TON_CO values are as high as 1100 h^–1 and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent manganese-based molecular catalyst. The encapsulation of  tricarbonyl Mn single-atom centers within the reticular covalent organic structure plays an important role by favouring the electrocatalytic CO₂ reduction over competitive H₂ evolution reaction. The spectroelectrochemical studies evidence the formation of five-coordinate species in the catalytic cycle for CO formation. The COF imposes mechanical constraints on the {fac-Mn(CO)₃S} centers.

Cytochrome c nitrite reductase (CcNiR) is a multi-heme enzyme that catalyzes the six-electron, seven-proton reduction of nitrite to ammonia as part of the global nitrogen cycle. In this talk, the activity of iron and cobalt complexes that perform the multielectron reduction of nitrite, mimicking the activity of CcNiR, will be described. One complex is a small cobalt-peptide complex (CoGGH) that catalyzes the complete reduction of nitrite to ammonium with an onset potential at -0.65 V vs Ag/AgCl (1 M KCl) at pH 7.2. CoGGH also catalyzes the reduction of proposed intermediates nitric oxide and hydroxylamine to ammonium. The second catalyst is an iron complex featuring a macrocyclic redox-active ligand (FeN₅H₂), which catalyzes nitrite reduction with an onset potential of -0.90 V to form hydroxylamine. FeN₅H₂ is unusual for being an iron-based catalyst for multielectron nitrite reduction that functions near neutral pH. CoGGH also catalyzes proton reduction to H₂; the study of this reaction yields insights into proton handling by this catalyst relevant to understanding the nitrite reduction mechanism.

Mediated electrochemistry complements conventional electrocatalysis and introduces flexibility in catalyst design by enabling electrode-driven reactions to take place off-electrode under more conventional thermochemical conditions.[1] This talk will highlight the development and application of electron-proton transfer mediators that support redox processes relevant to fuel cells, emphasizing the oxygen reduction reaction (ORR).[2-4] Off-electrode mediated oxygen reduction proceeds effectively with non-precious-metal M-N-C (M = Fe, Co) catalysts that are similar or identical to those commonly used for direct electrocatalytic ORR. Mechanistic studies have been conducted to probe the mechanism of the thermochemical ORR with a hydroquinone mediator. In principle, this mediated ORR could proceed on the catalyst surface via the coupling of two independent half-reactions (IHR), consisting of hydroquinone oxidation and O₂ reduction, or by direct inner-sphere reaction (ISR) between O₂ and the hydroquinone. Data have been obtained that support the inner-sphere reaction pathway,[5] and these results have important implications for mediated electrochemical processes. Different mechanisms have different linear-free energy relationships (i.e., Tafel slopes) that offer potential advantages in overpotentials and/or rates of (net) electrochemical reactions.

In this famous quote that most surface scientists love and hate in equal proportions, Pauli said: „God made the bulk; surfaces were invented by the devil“.

In this talk,  I will show you how even the most well designed and atomically controlled 2D catalysts, and even the most common metal surfaces suffer from dynamic structural transformations when you look at them at the nanoscale, under electrochemical reaction conditions. The „devil“  face of electrocatalysts can affect the product selectivity trends in detrimental ways, but it can also be tuned to increase their performance in key reactions in artificial photosynthesis schemes. I will show you a few examples on the surface dynamics of Cu electrodes involved in CO₂ electroreduction (CO₂RR), and 2D transition metal-based materials that catalyze oxygen evolution reactions (OER). I will discuss the use of scanning probe microscopy and solid state spectroscopy to track the surface dynamics of electrocatalysts in-situ, and present a few unconventional strategies to reach significant boosts in the catalytic activity of Earth-abundant 2D materials (using spin-selective mechanisms).

I will present the most recent advances on the role of reconstructions and electrolytes and how they affect the modeling of electrocatalytic systems.

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. There are several 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”. I the talk I will present a simulation approach to discover new catalysis materials based on HEAs.

Hydrogen plays a critical role in the energy transformation to a sustainable future, due to its widespread applications as a fuel for transport, feedstock to chemical industries and a heat source for buildings. Low temperature water electrolysis can enable hydrogen production renewably and at scale, but device efficiencies are currently limited by poor kinetics of water oxidation at the anode [1]. The low operating pH and high oxidizing conditions experienced at the anode in proton exchange membrane water electrolyzers limits the choice of materials to catalyse the oxygen evolution reaction to oxides of iridium, which is highly scarce [1],[2]. Mechanistic understanding of the water oxidation reaction on iridium oxides can enable rational development of more active catalysts with lesser precious metal loading.
Iridium oxides exhibit complex redox chemistry, which cannot be understood using electrochemical measurements alone. Particularly, the redox transitions on iridium oxides can exhibit non-Nernstian behavior, resulting in the redox transitions being dependent on pH and cations present in the electrolyte. In this talk, I will demonstrate the power of optical spectroscopy to track potential-dependent electroabsorption changes as a function of electrolyte pH from 1 to 13 on iridium based oxides. These measurements enable a direct comparison of the nature of redox active species in acidic and alkaline solution and also provide insights about the electrolyte environment on the redox potential, and consequently binding energetics of oxygenated species on IrOx. Using complementary stepped potential spectroelectrochemical measurements, the density of these redox active sites can also be computed as a function of potential [3] and the kinetics of water oxidation in acid and base can be linked directly to the density of oxidized species [4], [5]. Furthermore, in addition to quantitative detection of redox states, measurements of the kinetics of charge accumulation in natural and deuterated electrolyte provides insights into the role of ion transport within the oxide and the degree of bulk iridium redox. Therefore, based on our spectroscopic results on iridium-based oxides, we can experimentally probe the effect of interfacial pH and ions on charge accumulation and its impact on water oxidation kinetics. 
 

Iridium oxide is the state-of-the-art electrocatalysts for water oxidation in polymer electrolyte membrane (PEM) electrolysers for green hydrogen production. Hydrous, amorphous iridium oxides (IrO_x) show far higher activity than the rutile crystalline iridium oxides (IrO2), but the reason why they are more active is still under intense debate. Various differences between IrO_x and IrO₂, including the differences in active surface area, chemical state of Ir, fractional coverage of -OH groups and the range of ordered structure, renders it difficult to identify the origin of the differing activity.[1], [2] Thus far, ambiguities in measuring the number of sites participating in the reaction have prevented the accurate measurement of intrinsic catalytic activity, or turnover frequency, for amorphous iridium oxides and rutile iridium oxide.

In this talk, I will present the results from a range of different spectroscopic techniques and electrochemical measurements to identify the redox species present in IrO_x and IrO₂ and establish how these control the reaction kinetics. The techniques include (i) time-resolved UV-vis absorption spectroscopy (ii) on chip electrochemical mass spectrometry (iii) time of flight secondary ion mass spectrometry.

We have identified the same short-lived and long-lived oxidised species for both IrO_x and IrO₂ at potentials from 1.4 V to 1.55 V. We have correlated these species to the O₂ evolution kinetics. We attribute the lower activity of rutile IrO₂ to a lower concentration of the species responsible for driving water oxidation. The electrochemical mass spectrometry allows us to separate capacitive currents from O₂ evolution on the two catalysts. Combining TOF-SIMs with a paraffin wax cover method,[3] the depth of proton penetrating into the bulk catalysts was also revealed.

By comparing the concentration of oxidised species to reaction rates, we can directly measure turnover frequency (TOF) for IrO_x and IrO₂ as a function of applied potential. In summary, this study quantitively measured the intrinsic activity of amorphous and crystalline iridium oxides without any assumption of surface area and therefore shines insights into the origin of activity difference between amorphous and rutile iridium oxides.

A carbon-neutral and sustainable future requires advanced electrocatalysis systems[1]. Magnetism-induced enhancement of electrocatalytic reactions has been gaining increasing attention [2],[3],[4]. A simple and efficient way to achieve an enhancement is to apply an external magnetic field to the reaction[5]. Various mechanisms concerning different aspects of the reactions, e.g., reaction kinetics[5],[6] and mass transfer processes[7], have been introduced. However, the enhancement is likely a result of a synergistic effect of at least two mechanisms. Here, using key electrocatalytic reactions, i.e., oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), and homogeneous magnetic fields provided by an electromagnet, we demonstrate the importance of experimentally decoupling the mechanisms affecting the electrocatalytic reactions at electrodes with different magnetic properties. Our results show that the effect on the reaction kinetics and selectivity is exclusive to ferromagnetic catalysts and electron spin-dependent reactions under sufficient magnetic field strength; however, an enhancement of the mass transfer is universal. We provide a versatile strategy to optimize the magnetic enhancement that can be implemented to boost electrocatalytic reactions.

The conversion of renewable energy into storable, high value fuels is a key outstanding challenge in the transition to a carbon neutral economy. The primary source of electrons for the production chemically reduced fuel compounds is from the electrocatalytic oxidation of water. However, large quantities of energy are currently lost in the form of catalytic overpotential during water oxidation. This creates a challenge in scaling up electrolysis technology. To surmount these challenges, water oxidation activity must be improved using earth abundant catalysts. However, progress towards this goal is stymied by the complex ways in which applied voltage can drive water oxidation. This complexity is a result of the cooperative effects arising from interactions between oxidized species when large fraction of an electrocatalytic surface becomes oxidised. Competing, but not mutually exclusive, hypotheses, point to changes in activation energy¹ and changes in the catalytic mechanism involving the coupling of multiple oxidising equivalents during catalysis² as driving changes in rate constant and Tafel slope as a function of potential.

Cobalt compounds are used as water oxidation electrocatalysts for water oxidation across a wide rage of pH. However, the convolution of overlapping,  non-Nernstian redox processes and changes in catalytic mechanism complicate interpretation of Tafel slopes. In-situ spectroelectrochemistry can be used to separate redox driven changes in surface electronic structure from catalytic effects³. In this talk I demonstrate the construction and capabilities of a new potential and time operando spectroelectrochemical system to help resolve these questions in two model cobalt catalysts. First,  cobalt-iron hexacyanoferrate⁴, in which catalytic Co sites should be physically isolated from one another,^5,6 and secondly, amorphous cobalt phosphate oxide, in which Co centres interact during catalysts by oxo coupling and also via non-nernstian effects.^7,8 Using global analysis of a rich, 3d spectreoelectrochemial data set,  the potential dependence of the density of catalytically active states in both materials can quantified and compared. Time resolved spectral measurements as the catalysts relax from an operating potential to rest under open circuit conditions are used to access the kinetics of water oxidation.  These results show that contrast to cobalt phosphate oxide, catalysis in cobalt hexacyanoferrate shows negligible cooperative effects, both in terms of an enthalpic change in surface reactivity and multi-metallic catalysis, indicating a simple connection between the density of catalytic states and the Tafel slope.

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. We will show how such active sites can be used as an inspiration to design new synthetic catalysts for H₂ evolution [2-4] and oxidation [5-6] and how the introduction of proton relays significantly impacts the catalytic properties of such catalytic platforms, opening in some cases the possibility for bidirectional and even reversible catalysis.

Electrosynthesis strategies have seen a resurgence over the last decade, although most electrocatalysts for electrosynthesis are either traditional metal catalysts (i.e. Pt, Pd, or alloy catalysts) or molecular electrocatalysts. However, biocatalysts can be utilized for improving the selectivity of electrosyntheses. This talk will discuss the use of microbial and enzymatic bioelectrocatalysis for electrosynthesis. It will compare and contrast microbial and enzymatic bioelectrocatalysis. Then, the talk will show the use of enzymes and microbes for mediated and direct bioelectrosynthesis of ammonia and value-added chemical products (i.e. chiral amines). Finally, the talk will discuss the future directions in bioelectrocatalysis for electrosynthesis.

Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher-efficiency catalysts, challenging. Measurements of intrinsic activity show that, by far, the most-active phases for OER under alkaline conditions are Fe-containing mixed-metal oxyhydroxides, but exactly how they function remains controversial. I will discuss our work to understand the key properties of these catalysts, including morphology, composition, and molecular/electronic structure, and how they evolve and are dynamic under active catalytic conditions. Specifically, I will highlight evidence for the critical role of “surface” Fe-O clusters on CoOOH and NiOOH porous supports as the key active site motif for OER. These concepts inform design strategies for higher-performance catalyst architectures and for their incorporation into practical electrolyzer devices to make clean hydrogen fuel from inexpensive renewable electricity for green long-distance transportation and long-duration energy storage.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals (NCs) have gained significant attention for both fundamental research and applications of optoelectronic devices owing to their appealing optoelectronic properties and excellent chemical processability. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality and stability is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus quenching luminescence. Very recently, we have suceeded in synthesis of phase stable and less defect preovksite QDs, including APbX3 NCs (A: FA, MA, Cs; X: I, Br, Cl) and Sn-Pb alloyed NCs [1-5]. We have demonstrated that a high room-temperature photoluminescence quantum yield (PL QY) of close to 100% can be obtained in the APbX3 perovskite QDs, signifying the achievement of ignorable less trapping defects in the APbX3 QDs. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. In addtion, photoexcited hot and cold carrier dynamics as well as charge transfer at the heterojunction of QD/metal oxide were systematically investigated [4]. Solar cells based on these high-quality perovskite QDs exhibit power conversion efficiency of over 12%, showing great promise for practical application. On the other hand, through incorporation of alkali ion Na+, we have realized for the first time efficient near-infrared emission from highly defective Sn-Pb perovskite QDs with substantially improved PL QY from ~0.3% to 28% [5]. Our findings provide new insights into the materials design strategies for improved optoelectronic properties of Sn-containing perovskites. We anticipate their use in near-infrared devices is very promising if issues of the sustainability of PL QY can be fully addressed in the near future.   

Over the past years we have developed various approaches to fabricate materials with sophisticated chemical and structural complexity. We have focused on synthesis in non-aqueous solution since this approach is not limited to one particular class of materials. Thus, it gives us flexibility to tailor the composition and properties of materials in respect to the application, for examples carbon dioxide sensors and photo-electrochemical devices.

In this talk, I will present how X-ray synchrotron methods, far from merely providing new tools, are extending the ways we study, understand and design such complex structures. A combination of spectroscopic, scattering and microscopic X-ray methods and rapid data acquisition help to uncover the complex chemical world behind the synthesis of functional materials. It gives complementary information about chemical reaction in solution and nucleation, growth and crystal phase transition of nanoparticles. [1-4] Moreover, on the selected examples, I will discuss how the possibility to select with high-energy resolution the incident and emission hard X-ray energies offers unprecedented site selectivity and give access to determine structure – function relationship of photo-, electroactive materials and devices. [5-6] Finally, will discuss the advantages and the pitfalls of the synchrotron methods for in situ and operando studies.

Solar-assisted carbon dioxide reduction reaction (CO2RR) offers the possibility to use atmospheric CO2
and sunlight to produce highly-valuable hydrocarbons such as methane, ethylene, and ethanol. So far, the
majority of the CO2RR investigations were carried out in water environments. Although this approach is
environmental-friendly and constitutes a promising strategy for future implementation with solar electro-
lyzers, three main drawbacks exist: the limited solubility of CO2 in water (~ 35 mmol/L at room tempera-
ture) (i), the competitive hydrogen evolution reaction (HER) occurring when performing CO2RR in water
that limits the overall Faradaic efficiency (ii), and the difficulty to use molecular homogeneous (pho-
to)catalysts that boost the unless poor selectivity of the reaction (iii). Therefore, the mid-term strategy is
to develop a double-compartment photoelectrochemical device where the anolyte and catholyte sides are
separated by polymeric membranes. Here, solar-driven oxygen evolution reaction (OER) occurs at the
water-based anolyte side, while CO2RR undergoes at the non-aqueous catholyte side of the cell. In this talk, we will present our recent in situ hard X-ray ambient pressure photoelectron spectroscopy
(AP-HAXPES) investigations of electrified solid/liquid interfaces. The experiments were performed at the
new SpAnTeX (Spectroscopic Analysis with Tender X-rays) end-station operating at the BESSY II syn-
chrotron facility. The seminar will focus on three activities performed at the SpAnTeX end-station:
• First, we will show how the chemistry of BiVO4 photoanode surfaces can be tuned when the sem-
iconducting material is in contact with an aqueous phosphate buffer solution;
• Second, we will present the results obtained on a gold polycrystalline surface/N,N-
dimethylformamide interface, as a function of the applied overpotential. At such an interface, io-
dine electrooxidation and CO2RR were studied under in situ conditions;
• Third, we will show some recent results obtained on newly-developed in situ cell for multimodal
in situ investigations, equipped with a gas diffusion layer and a polymeric membrane. We will conclude this contribution by discussing about future perspectives and technical implementations
for multimodal in situ investigations of (photo)electrocatalytic processes.

While the performance of catalysts for electrochemical CO₂ reduction reaction (CO₂RR) strongly depends on the nature of electrode-electrolyte interfaces, it remains a key challenge to spatially resolve interfacial physical-chemical properties under liquid phase reaction conditions. In this talk, recent advances in electrochemical atomic force microscopy (EC-AFM) to decipher in situ an electrocatalyst’s morphology, structure and  reaction-rate determining local charge transfer variations will be discussed. Selected results include deciphering the potential-dependent morphology of copper-based electrodes during CO₂RR. Distinct surface morphologies are revealed for a copper(100) electrode over wide potential ranges upon electrode exposure to electrolytes and during CO₂RR, which can be understood by surface mobility and specific adsorption. Decisively different behavior of copper electrocatalysts in the presence of halides compared to plain electrolytes is observed. Conductive AFM will be demonstrated as a correlative tool for revealing simultaneously the nanoscale electric current, morphology and friction force of electrocatalysts in relevant aqueous electrolytes. The broader relevance of implemented in situ EC-AFM methods will be discussed in the context of establishing reliable structure-property relationships for the rational design of advanced catalysts and electrochemical interfaces.

Hydrogen production through water splitting has attracted great interest in recent years due to its potential of generating energy without causing pollution. One of the best candidates for water splitting is nickel oxyhydroxide with iron content (Ni 1− x FexOOH) that has excellent efficiency at alkaline conditions and is now studied widely. But pure NiOOH has poor efficiency unless doped with Fe and Co. We explore the influence of co-doping on the efficiency of NiOOH in the process of water oxidation by using Density Functional Theory +U (DFT+U). We also test the effect of strain on catalytic efficiency by modeling water oxidation on expanded and contracted surfaces of NiFeCoOOH. We find that several doping locations of Co have a similar result as if NiFeOOH had no Co content. Iron is responsible for the high activity at the Fe active site due to the low energy required for charge extraction. Yet, the valence band edge includes Fe, Co and Ni states hybridized which allows better charge extraction during deprotonation. The valence band edge position is higher upon Co-doping, which should allow better hole transport toward the surface. Hence, the presence of Ni and Co atoms surrounding the active site is vital for better efficiency. Moreover, we found that applying strain does not improve the efficiency and therefore a substrate with minimal mismatch should be used for NiFeCoOOH electrocatalysis.

Machine learning accelerated catalyst discovery efforts have seen much progress in the last few years. Datasets of computational calculations have improved, models to connect surface structure with electronic structure or adsorption energies have gotten more sophisticated, and active learning exploration strategies are becoming routine in discovery efforts. However, there are several large challenges that remain: to date, models have had trouble generalizing to new materials or reaction intermediates and applying these methods requires significant training. I will briefly introduce the Open Catalyst Project and the Open Catalyst 2020 dataset, a collaborative project to span surface composition, structure, and chemistry and enable a new generation of deep machine learning models for catalysis. I will then discuss initial results for state-of-the-art deep graph convolutional models and significant recent progress from others in the community, many of which are likely to improve models in related materials science areas. 

Transition metal oxides are promising photoelectrode materials for solar-to-fuel conversion applications. However, their performance is limited by the low carrier mobility (especially electron mobility) due to the formation of small polarons. Recent experimental studies have shown improved carrier mobility and conductivity by atomic doping; however the underlying mechanism is not understood. A fundamental atomistic-level understanding of the effects on small polaron transport is critical to future material design with high conductivity. In this talk, we will discuss the effect of small polaron formation on optical and carrier transport properties of transition metal oxides from first-principles calculations.

First, we resolve the conflicting findings that have been reported on the optical gap of a well-known catalysis Co₃O₄ as an example[1]. We confirm that the formation of small hole polarons significantly influences the optical absorption spectra and introduces extra spectroscopic signature below the intrinsic band gap, leading to a 0.8 eV transition that is often misinterpreted as the band edge that defines the fundamental gap.

Then we discuss the formation of small polarons' effect on carrier concentration, by resolving the controversy of nature of "shallow" or "deep" impurities of intrinsic oxygen vacancies in BiVO₄ as an example[2], i.e. how to unify different experiments with the correct definition of ionization energy in polaronic oxides. We further discuss why certain dopants can have very low optimal concentrations (or very early doping bottleneck) in polaronic oxides such as Fe₂O₃, through a novel "electric-multipole" clustering between dopants and polarons[3]. These multipoles can be very stable at room temperature and are difficult to fully ionize compared to separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications. In addition, we show the importance of synthesis condition such as synthesis temperature and oxygen partial pressure on dopant and polaron concentrations, and how to optimize the synthesis condition based on theoretical predictions.

At the end, we show different theoretical models for polaron mobility calculations from a macroscopic dielectric continuum picture with an example of spin polarons in CuO[4] and a microscopic polaron hopping picture by combining generalized Landau-Zener theory and kinetic Monte-Carlo samplings for doped oxides[5].

Our first-principles calculations provide important insights and suggest design principles for optimal optical and transport properties of polaronic oxides.

Interfaces play an essential role in nearly all aspects of life and are critical for electrochemistry. Electrochemical systems ranging from high-temperature solid oxide fuel cells (SOFC) to batteries to capacitors have a wide range of important interfaces between solids, liquids, and gases which play a pivotal role in how energy is stored, transferred, and/or converted. This talk will focus on our use of ambient pressure XPS (APXPS) to directly probe the solid/gas and solid/liquid electrochemical interface. APXPS is a photon-in/electron-out process that can provide both atomic concentration and chemical-specific information at pressures greater than 20 Torr. Using synchrotron X-rays at Lawrence Berkeley Nation Laboratory, the Advanced Light Source has several beamlines dedicated to APXPS endstations that are outfitted with various in situ/operando features such as heating to temperatures > 500 °C, pressures greater than 20 Torr to support solid/liquid experiments and electrical leads to support applying electrical potentials support the ability to collect XPS data of actual electrochemical devices while it's operating in near ambient pressures. This talk will introduce APXPS and provide several solid/gas and solid/liquid interface electrochemistry examples using in situ and operando APXPS including the probing a Pt metal electrode undergoing a water-splitting reaction to generate oxygen[1], utilization of theory and experiment to understand CO₂’s interaction with Cu and Ag metal surfaces [2], [3], [4], and the ability to probe the electrochemical double layer (EDL) [5]. Gaining new insight to guide the design and control of future electrochemical interfaces.

The properties of emerging non-precious metal (oxy)hydroxide electrocatalysts for the oxygen evolution reaction (OER) evolve dynamically with voltage during operation. Understanding how material properties govern the kinetics of electron and ion transfer during the OER requires integrating electrochemical cells into advanced characterization methods to study reactivity during operation. Further, the sub-particle spatial heterogeneity of electrocatalysts during operation requires that these operando methods must be able to image the local chemical, physical, and electronic structure at the nanoscale and link these properties to the local electrochemical activity.

In this talk, I will describe a suite of correlative operando microscopy techniques we have developed to study the OER on faceted Co (oxy)hydroxide nanoplatelets.^[1] Operando Scanning X-Ray Transmission Microscopy (STXM) and Electrochemical Atomic Force Microscopy (EC-AFM) are used to map the local Co oxidation state and particle morphology across voltages spanning the bulk ion insertion reactions and OER regimes. These results are correlated to local electrochemical activity maps obtained through Scanning Electrochemical Cell Microscopy (SECCM) providing insight into where and why oxygen is evolved on the Co (oxy)hydroxide platelets. These experimental material properties and local reactivity measurements are integrated into a computational model that combines thermodynamic adsorption energetics of OER surface intermediates and microkinetic modeling to derive the experimental electrochemical Tafel behavior through first principles.^[2] The results from our multi-modal correlative approach alter long standing assumptions about the OER-active properties of the Co (oxy)hydroxide system and provide an improved methodology towards developing predictive electrocatalytic theories based on material properties that govern reactivity away from open circuit conditions.

Some Cu(I)-based oxides with delafossite structure have attracted attention as promising photocathode materials for photoelectrochemical water splitting due to their intrinsic p-type conductivity and good stability against photo-corrosion.  Beside CuFeO₂, which has been intensely studied and found to have some limitations, CuRhO2 is also of interest for its considerable stability and favorable energy gap.  To obtain insight into the fundamental bulk and surface properties of CuRhO2 that are relevant to such application, we have studied the bulk and majority (001) surface of CuRhO₂ in vacuum and electrochemical environment using first principles calculations.  We examine the bulk electronic structure and intrinsic defects, the surface stability diagram and band alignment at the oxide-water interface, and the energetics of the hydrogen evolution reaction at the interface. Based on these results, a comparative analysis of the properties of CuRhO₂ and CuFeO₂ is presented.

Polymer blends, which harness desirable properties from two or more complementary materials, have numerous applications in aerospace, pharmaceuticals, medical devices, electronics, and clean energy. In this work we investigate how polymer-polymer and polymer-solvent interactions affect the phase morphology and performance of polymer composites comprised of a majority non-conjugated polymer matrix component and a dispersed minority conjugated (semi-conductive) polymer additive. These blends are of interest for numerous organic electronic device applications as the matrix component can add mechanical durability, environmental stability and, in some cases, ionic conductivity. Meanwhile the minority conjugated component has been shown to retain optoelectronic performance at loading fractions as low as 1-10 wt%. Small angle and ultra-small angle neutron and X-ray scattering are particularly useful to characterize these composite blends and the self-assembled conjugated polymer components over length scales spanning from micrometers to fractions of a nanometer. We use small angle X-ray and neutron scattering and electrochemical analysis to develop a deep understanding of how blend morphology affects performance as a function of processing conditions and composition. This talk first provides a review of past work focused on blends of commodity homopolymers (i.e. polystyrene) and conjugated polymers (i.e. polythiophene derivatives). It will then cover more recent work focused on structured composite blends where the non-conjugated component is a block copolymer capable of self-assembly into mesoscale structures with long range structural order. These new structured composites include thermoplastic elastomers with high elasticity as well as water soluble block copolymers capable of forming templated hydrogel organic mixed ionic electronic conductors (OMIECs). In these cases, scattering experiments demonstrate that the block-copolymer templates can induce the organization of the conjugated polymer component over long length scales at significant loadings (up to 5wt%).

The emergence of electrically conductive metal-organic frameworks (MOFs) has been one of the most paradoxical developments in the field in the last few years. Indeed, how can one transport charges through a material that is mostly “empty” space? In this sense, MOFs made from layers of organic ligands connected by (typically) square-planar metal ions have shown particularly good electrical conductivity. However, a precise mechanism for charge transport is still the subject of debate, with various experimental and computational reports describing these materials as metals, semiconductors, semimetals, or even borderline insulators. Most of the discussion on this point has been focused on the effects of in-plane metal-ligand conjugation and the efficiency of in-plane transport. This lecture will describe the latest efforts from our group to understand the intrinsic properties of these materials, especially as related to single-crystal electrical measurement studies, and will discuss in particular the unexpectedly large influence of out-of-plane transport. Time allowing, I will discuss unexpected results stemming from the behavior of these materials as 1D metals, and applications in energy storage and conversion.

The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidize water and generate carbohydrates (a solar fuel) for over a billion years. Inspired, but not constrained by nature, artificial systems^[1] can be designed to capture light and oxidize water and reduce protons, CO₂ or other compounds to generate useful chemical fuels and feedstocks. In this context, this contribution will describe the preparation of efficient molecular water oxidation catalysts both in homogeneous phase and confined into solid conductive or semiconductive supports. Further the nature of the anchoring strategy on the performance of these molecular (photo)anode will be further discussed as well their implications for the generation of solar fuels.^[2]

[1] (a) Berardi, S.; Drouet, S.; Francàs, L.; Gimbert-Suriñach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Chem. Soc. Rev., 2014, 43, 7501-7519. (b) Matheu, R.; Ertem, M. Z.; Gimbert-Suriñach, C; Sala, X.; Llobet, A. Chem. Rev. 2019, 119, 3453–3471. (c) Matheu, R.; Garrido Barros, P.; Gil Sepulcre, M.; Ertem, M. Z.; Sala, X; Gimbert-Suriñach, C; Llobet, A. Nat. Rev. Chem. 2019, 3, 331–341.

[2] (a) Matheu, R.; Gray, H. B.; Brunschwig, B. S.; Llobet, A; Lewis, N. S. et al., J. Am. Chem. Soc. 2017, 139, 11345-11348. (b) Garrido-Barros, P.; Gimbert-Suriñach, C.; Llobet, A. et al., J. Am. Chem. Soc. 2017, 139, 12907–12910. (c) Hoque, Md. A.; Gil-Sepulcre, M.; Llobet, A. et al. Nat. Chem. 2020, 12, 1060–1066. (d) Schindler, D.; Gil‐Sepulcre, M.; Llobet, A.; Würthner, F. Adv. Energy Mater. 2020, 2002329. (e) Gil-Sepulcre, M.; Lindner, J. O.; Schindler, D.; Velasco, L.; Moonshiram, D.; Rüdiger, O.; DeBeer, S.; Stepanenko, V.; Solano, E.; Würthner, F.; Llobet, A. J. Am. Chem. Soc. 2021, 143, 11651–11661.

In a world that is running out of natural resources, there is a growing need to design and develop sustainable and green energy resources. In that respect, photo-electrocatalytically driven reactions for the production of alternative fuels (such as water splitting or CO₂ reduction) hold the potential to provide a route for future carbon neutral energy economy. Nevertheless, the slow kinetics of those catalytic reactions demands the development of efficient catalysts in order to drive it at lower overpotentials. Indeed, a variety of molecular catalysts based on metal complexes are capable of electrochemically reducing CO₂ and/or protons. Yet, despite the significant progress in this field, practical realization of molecular catalysts will have to involve a simple and robust way to assemble high concentration of these catalysts in an ordered, reactant-accessible fashion onto a conductive electrode. 

Our group utilizes Metal-Organic Frameworks (MOFs) based materials as a platform for heterogenizing molecular electrocatalysts. Their unique properties (porosity and flexible chemical functionality), enables us to use MOFs for integrating all the different functional elements needed for efficient catalysts: 1) immobilization of molecular catalysts, 2) electron transport elements, 3) mass transport channels, and 4) modulation of catalyst secondary environment. Thus, in essence, MOFs could possess all of the functional ingredients of a catalytic enzyme.  

In this talk, I will present our recent study on (photo)-electrocatalytically active MOFs incorporating molecular catalysts for solar fuel reactions.

A long-standing challenge is the development of artificial photosynthetic approaches for capturing and storing the energy of sunlight in the chemical bonds of a fuel, e.g. molecular hydrogen, offering a clean, sustainable and abundant source of energy. In this respect, colloidal nanostructured semiconductor materials are explored extensively as photosensitizers and photocatalysts. To push forward the development of functional materials based on semiconductor nanocrystals, function immanent exciton and charge-separation/recombination dynamics in relation to structural parameters need to be understood. Factors like composition, structure and dimensions of the semiconductor particles, the nature of cocatalysts and the type of surface ligands are severely influencing the efficiencies for solar to hydrogen conversion. By applying time-resolved transient absorption and photoluminescence spectroscopy we strive to reveal the connections between structure, charge carrier dynamics and the targeted function. To illustrate this approach, I will give a brief overview on the results of our studies on CdSe@CdS seeded nanorods functionalized with metal particles of varying composition and morphology as cocatalyst and the insights gained on the connection between charge separation at the nanosized semiconductor/metal interface and activity for hydrogen evolution.[1, 2] Besides the transfer of electrons to the cocatalyst, a key step in these heterostructures is hole localization in the CdSe seed which enables the formation of long-lived charge separation supporting charge accumulation at the catalytic reaction centers necessary to drive multi-electron redox reactions. Latest results revealed the influence of the nature of the surface ligands on the efficiency of the hole localization process.[3] Finally, interfacing semiconductor nanocrystals with molecular reaction centers presents an attractive alternative to metal particles and equally high charge transfer rates as at semiconductor/metal interfaces can be reached in principle.[4] Nevertheless, anchoring strategies with sufficient stability and supporting efficient transfer of multiple charge carriers and reducing unwanted charge recombination, e.g. by anchoring units serving as electron relay, still need to be established. First results indicate that bioinspired redox-active polymer matrices could play an important role in the future design of photocatalytically active inorganic/organic hybrid materials.[5]

Transparent conductive oxides are well-known inorganic materials that have found wide use in various fields of both research and industry. In particular, doped metal oxides have aroused great interest in power generation applications thanks to a very convenient combination of optical and electronic properties. To the effective absorption of light usually in the ultraviolet frequency range and to the typical optical transparency window in the visible, a tunable feature is added that enriches the versatility of these materials. It is the plasmon resonance in the infrared. This property is the result of the aliovalent doping of the oxide, which induces the typical conductive character thanks to the excess of extrinsic charges [1]. Further advantages in terms of preparation, costs and applications derive from processing in the nanoscale and, among the nanocrystalline forms of these oxides, one of the most widely used species is indium tin oxide (ITO) [2]. More recently, the possibility of a further increase in charge density through a zero environmental impact post-synthetic treatment has renewed the attraction for these materials and intensified their study. Many metal oxides nanocrystals have in fact demonstrated an exponential increase in charges through photo-doping induced by multiple events of light absorption [3]. This phenomenon encourages even more the use of these semiconductors in the field of solar energy conversion, but also in related fields such as photoelectrochemistry and photocatalysis. In each of these applications, one of the most current research lines in fact concerns the analysis of the multiple charges accumulated in the light-driven process and the possibility of accessing processes involving multiple charge transfers [4]. The consequences would concern both the increase in efficiency and the widening of the application window of these materials. In this work,[5] we report a first evidence, in the case of photodoped ITO nanocrystals, of the possible transfer process of more than one electron by means of a gradual oxidative treatment using an electron-acceptor molecule, the F4TCNQ. The spectroscopic analysis of the titration allows shedding light on the reactivity of the nanocrystals induced by light and on the potential transfer of multiple charges.

Nanocarbon in Photochemistry and Electrochemistry

Artificial photosynthesis, a process in which the light energy is converted into the chemical bond, represents an important area of research aimed toward generation of Sun-derived fuels and value-added chemicals. To successfully utilize photons from Sun, such solar reactors need to contain light-absorbing chromophores that efficiently and rapidly channel the absorbed energy toward desired catalytic sites where useful chemistry can takes place with high selectivity and low kinetic barriers. The Glusac group investigates molecular chromophores and catalysts for artificial photosynthesis. We utilize advanced time-resolved laser spectroscopy techniques to investigate mechanisms of energy and charge migration in molecular excited states and evaluate the parameters that control undesired energy losses through fast charge recombination. We also explore molecular electrocatalysts that are able to receive electrons and holes from excited chromophores and covert then into desired products.

Three projects are currently under investigation in our labs: (i) Bio-inspired CO2 reduction using metal-free NAD+/NADH analogs, where we look for strong hydride donors that can selectively reduce CO2 to methanol and that can be recycled photochemically; (iii) Light-harvesting by graphene quantum dot assemblies, where we explore excited-state energy and charge redistributions in chromophore-catalyst assemblies using advanced time-resolved laser spectroscopy methods; (iii) Carbon-based platforms for electrocatalysis. In this project, we investigate methods to decorate carbon electrodes with molecular catalytic motifs that can perform useful chemistry.

Syngas, an industrially relevant mixture of CO and H₂, is a high-priority intermediate resource for producing several commodity chemicals, e.g., ammonia, methanol, and synthetic hydrocarbon fuels. Accordingly, parallel photocatalytic conversion of CO2 and protons to syngas is a key step in fostering a sustainable energy cycle. State-of-the-art catalytic systems often fall short as concurrent CO and H₂ evolution requires challenging reaction conditions which can hamper stability, selectivity, and efficiency under application-oriented conditions. Here a light-harvesting metal-organic framework and two molecular catalysts are associated to design colloidal, water-stable, versatile nanoreactors for photocatalytic syngas generation with highly controllable product ratios. The host provides efficient directional light-harvested energy transport to the active sites, yielding sustained CO₂ reduction and H₂ evolution with incident photon conversions up to 36% and turnover numbers setting a benchmark, thus paving the way for application in solar energy-driven syngas generation. System bottlenecks, such as limited light absorption and kinetics, were identified, and proven addressable upon modularly co-hosting molecular additives owing to the system’s intrinsic versatility. Based on our findings, we highlight the uniqueness of MOF materials’ coordination space for rational designing and directing of such multicomponent supramolecular assemblies including antenna chromophores, auxiliary photosensitizers, and several catalysts of orthogonal photo-physical and chemical properties to minimize energy loss. This opens perspectives to tackle scalability, synthesize value-added products from the full redox cycle, harness the full solar spectrum energy, and precisely implement active sites with co-factors to mimic natural enzyme functionalities.[1-3]

Metal–organic frameworks (MOFs) represent a new class of materials, and one of their striking features lies in the tunable, designable, and functionalizable nanospace. The nanospace within MOFs allows the designed incorporation of different functionalities for targeted applications, such as gas storage, gas separation, sensing, drug delivery, environmental remediation, heterogeneous catalysis; and it has also provided plenty of opportunities for photocatalysis application. We will discuss a series of strategies to tailor MOFs as photocatalysts for photocatalytic small molecule activation under visible light irradiation including hydrogen production from water and water oxidation coupled carbon dixoide reduction reminiscent of photosynthesis in nature.

Carbon dioxide reduction by porphrynic metal-organic frameworks has been widely studied. Changes in porphyrin moiety (non-metallated, different central metal atoms) and three-dimensional structures result in putative changes in mechanism and resulting products, namely carbon monoxide and formate. There are two prevailing mechanisms that are debated in the literature (1) the formation of a porphyrin-node charge-separated state, where the node acts as the catalytic site, (2) porphyrin driven chemistry where the linker acts as both the photo-active species and catalytic site with the nodes simply serving as structural supports. Our collaborators have systematically explored the reduction rates and products for different non-metallated and metallated MOF-525. To support the mechanistic implications, we conducted photophysical studies, including transient absorption spectroscopy. Interestingly, the results do not support either of the prevailing mechanisms. Transient absorption spectroscopy eliminates the possibility for the formation of a charge-separated state. EPR spectroscopy confirms that the photogenerated Fe(II) species is not capable of carbon dioxide activation, consistent with previously published molecular mechanisms. Therefore, we turned to theory to provide insight. The computational results support a mechanism where the sacrificial donor plays a key role in the catalytic mechanism. Other recent mechanistic studies on molecular carbon dioxide reduction catalysts have put forth similar stories with sacrificial donors playing multiple roles in catalysis. The implications of sacrificial reducing agents in the field will be mentioned, as well as potential pathways for future research.

Despite a decade of research, much remains unknown about the role that grain boundaries play in impacting the photophysics of lead halide perovskites and influencing their performance in solar cells. In this talk, I will describe how by combining experimental studies with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and depends on the grain area with small grains acting as recombination hot spots. We show that the distribution of grain sizes not only influences the overall performance of perovskite solar cells, but also leads to significant current exchange between small and large grains at open-circuit conditions. 

On the wave of their success in strategic fields of application, as energy conversion and light-emission, great effort was devoted to better understand and clarify the native semiconducting properties of halide perovskite materials. As a result, the electronic properties of these systems are nowadays well understood, with clear identification of the influence of quantum confinement and chemical tuning on the corresponding band structure, as relevant in the case of dimensionally tailored materials,[1] and in mixed halide or double perovskites,[2] respectively. The situation however becomes more complex when one goes beyond the single particle picture. The Coulomb interaction between the photogenerated electron-hole pairs in fact may result in the formation of stable excitons, with distinct properties (energetics, polarization, degeneracy, etc.) with respect to the corresponding single particle transitions.

Here, we aim to provide a general frame for the discussion of the exciton properties of metal halide perovskite frames, which encompasses effects like quantum confinement and chemical composition. Symmetry-based, group theory analysis will provide a sounded ground for the discussion, with clear indication about the expected splitting of the excitonic features in the materials, when going from 3D, to 2D and to other systems of practical relevance.[3] These results will be then referred to recent spectroscopic experiments[4-6] and to cutting edge first-principle calculations,[7] so demonstrating the possibilities of modern first-principle simulations tools in supporting experimental investigations and predicting the excitonic properties of halide perovskite materials. Excitons dominate the optical response of two-dimensional layered perovskite materials, where quantum and dielectric confinement enhance the electron-hole interactions;[1] still, they are shown to play a fundamental role also for double perovskites,[8] with serious impact on the relative role of dimensional confinement.[9] In addition, also for 3D lead-based halide perovskites, featuring exciton binding as small as 14-25 meV at room temperature,[10] the clear understanding and assignment of the spectral signatures must rely on an excitonic picture.[4-5]

Two-dimensional (2D) hybrid organic-inorganic perovskites are among the most promising materials for optoelectronic applications thanks to the great synthetic versatility that allows tailoring their structural and photophysical properties. The alternation between organic ligands and inorganic layers creates a natural multiple quantum well (MQW) structure where the inorganic layers act as potential ‘wells’ while organics serve as energetic ‘barriers’. This structure provides strong electronic confinement and, as a consequence, creates stable excitons with high binding energy at room temperature. [1]

In this talk I will show that large single-crystal flakes of 2D perovskite are able to establish strong light-matter coupling with the generation of exciton−polaritons. These half-light half-matter quasi-particles have unique properties: they possess strong intrinsic nonlinearities, inherited from their excitonic component and extremely small effective mass and long coherence length, inherited from their photonic component.

By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton–photon coupling. [2]

Moreover, 2D single-crystals perovskite are able to sustain strong polariton nonlinearities at room temperature, with exciton-exciton interaction energies remarkably similar to the ones known for inorganic quantum wells at cryogenic temperatures and more than one order of magnitude larger than alternative room temperature polariton devices reported so far. [3]

Because of their easy fabrication, large dipolar oscillator strengths, and strong nonlinearities, these materials ideal candidate for integrated photonic circuits and electro-optical devices working at room temperature. [4 - 6]

Hybrid perovskites have generated a lot of interest in the field of photovoltaics during the past decade. In recent years, perovskite solar cells regularly output over 20 % of photoconversion efficiency, approaching conventional Silicon based devices.

Even though, remarkable optoelectronic properties of hybrid perovskites and easy fabrication routs have enabled fast development of devices, the common fabrication methods still result in formation of substantial density of defect states in hybrid perovskite thin films [1]. These defects when act as charge trapping centers, have been known to induce non-radiative losses and limit the maximum photoconversion efficiency of perovskite solar cells [2]. The progress in fundamental understanding of these defects, in particular about their formation sites and their exact impact on device efficiency, has been slower. Such knowledge, however, is critical to design successful passivation strategies and improve performance and viability of perovskite photovoltaic devices.

Here we elucidate the very origin of nanoscale defects that form in state-of-the-art solution processed triple cation mixed halide perovskite thin films, and evaluate their roles in charge trapping processes. We employed photoemission electron microscopy to directly image the distribution of defects on nanoscale [3] and revealed their spatial arrangement with respect to surface morphology. By adding the time resolution to our photoemission experiments, we accessed the exact roles of these defects in performance of perovskite thin films by monitoring charge trapping processes that occur at defect sites. We thus uncovered the presence of multiple types of nanoscale defect clusters, and found that depending on their origin, they play very different roles in photoexcited hole trapping – from very detrimental to relatively benign [4]. Our results highlight the need to develop targeted approaches to remove each undesired type of defect.

Spatial photoluminescence heterogeneity is a frequently recognized phenomenon in lead halide perovskite thin films meant for efficient photovoltaic devices. Deep trap states associated with local variation in structure, composition and strain are the main players dictating this photoluminescence heterogeneity and the device performance. These trap states can appear as nanoscale clusters and majorly reside in the grain boundaries.¹ Despite notable progress in understanding the fundamental aspects of these traps such as origin, location, and distribution, real time response of these trap states remains elusive. Temporal photoluminescence heterogeneity or intensity fluctuation introduced by the traps can be a major loss channel in the film and would require detailed investigation. In this context several questions that can arise are: (i) Are these photoluminescence quenching traps static or metastable? (ii) what are the quenching domains of these traps? (iii) Do they have any correlation with grain size and grain boundaries? (iv) Do they have any working timescale? (v) How does the surrounding environment influence formation and annihilation of these traps?

We observed large photoluminescence fluctuation in MAPbI₃ thin films when investigated in real time. The fluctuation varies as a function of grain size (500 nm-several µm) and surrounding environment. This fact indicates presence of metastable traps which can deplete significant amount of photogenerated carriers even in good quality films. Unlike spatially isolated crystals, proximity of the grains and crystallites in thin films provide a mixed photoluminescence signal making the trapping dynamics and carrier recombination more complex for individual grains. We have developed an advanced photoluminescence mapping technique based on correlation of the intensity fluctuation which divide the image into microscale clusters having high intra-cluster and low inter-cluster correlation, respectively. Size of the clusters changes as the grain size or the surrounding environment changes from air to inert. By correlating these clusters with electron microscopy image, we obtain important insight about the microscale quenching domain of these metastable traps. Power spectral density estimation of the intensity fluctuation of each cluster reveals existence of different types of metastable traps with timescales ranging from hundreds of milliseconds to tens of seconds.² Distinctly different response time of these quenching traps are noted when grains size is varied, and porosity is introduced into the sample. This approach demonstrates a methodology to understand several structural (quenching domain, grain size dependence) and dynamical aspects (metastability and working timescale) of the photoluminescence quenching traps in any thin films.

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. Furtheremore, 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. Finally I will discuss the exciton fince structure. Optically inactive dark exciton states play an important role in light emission processes in semiconductors because they provide an efficient nonradiative recombination channel. Understanding the exciton fine structure in materials with potential applications in light-emitting devices is therefore critical. Our studies of the exciton fine structure in the family of two-dimensional (2D) perovskites show that in-plane magnetic field mixes the bright and dark exciton states, brightening the otherwise optically inactive dark exciton. The bright-dark splitting increases with increasing exciton binding energy. Hot photoluminescence is observed, indicative of a non-Boltzmann distribution of the bright-dark exciton populations. We attribute this to the phonon bottleneck, which results from the weak exciton–acoustic phonon coupling in soft 2D perovskites. Hot photoluminescence is responsible for the strong emission observed in these materials, despite the substantial bright-dark exciton splitting.

Quasi-two-dimensional (Q2D) organic-inorganic halide perovskites are an exciting family of heterogeneous layered semiconductors, which combine chemical and structural versatility of 3D organic-inorganic metal-halide perovskites with the ability to design light-matter interactions driven by structural and dielectric confinement [1]. They are a broad and versatile playground not only for materials discovery, but also for exploring fundamental excited state physics and development of novel optoelectronic applications. In particular, the structural and chemical diversity of this family of materials offers a unique opportunity to understand the role of chemistry and structure on photoexcited states of complex semiconductors in a systematic way; first principles computational modeling takes a key role in this context.
In this talk, I will present our recent work understanding optical excitations complex Q2D organic-inorganic halide perovskites using state-of-the-art first principles computational modeling techniques, such as the GW method [2] and the Bethe-Salpeter equation [3]. I will present our first principles calculations of optical excitations in single-layered Q2D halide perovskites, revealing the role of the organic cations in enhancing the dielectric screening of these complex systems, and show our recent work on understanding the optoelectronic properties of self-assembled perovskite-non-perovskite self-assembled interfaces [4].

References:
[1] Smith, Crace, Jaffee & Karunadasa, Annu. Rev. Mater. Res., 48, 111-136 (2018).
[2] Hybertsen & Louie, Phys. Rev. B 34, 5390 (1986).
[3] Rohlfing & Louie, Phys. Rev. Lett. 81, 2312 (1998).
[4] Aubrey, Valdes, Filip, Connor, Lindquist, Neaton & Karunadasa, Nature, 597, 355-359 (2021).

Two-dimensional Ruddlesden-Popper metal halides (2D-RPMHs) are materials composed of quasi-2D layers of metal-halide octahedra separated by long (~1nm) organic cationic layers. The latter facilitate electron and hole quantum confinement within the metal-halide layers resulting in a quantum-well like structure. Properties of excitons (i.e., the  electron-hole bound states) in such structures are characterized by strong binding energy (>200 meV) arising from the dynamically screened Coulomb interactions [1]. We have experimentally observed that polaronic effects arising from the lattice dressing of the carriers, are not only active but that they fundamentally define excitons in 2D-RPMHs [2]. We thus refer to such excitons as the exciton-polarons, with properties that are measurably distinct than those of free excitons in semiconductors [1]. In this talk, I will discuss the quantum dynamics of exciton-polarons and provide spectroscopic insights into the peculiar phonon-phonon [3], exciton-phonon and exciton-exciton [4] interactions. I will present our perspective on how the coherent optical response of 2D perovskites can be effectively rationalized within the “exciton-polaron” framework, in which lattice dressing of photo-carriers constitute an integral component of excitonic wavefunction [1], with consequences on exciton recombination dynamics and diffusion.

References

[1] A. R. Srimath Kandada and C. Silva, J. Phys. Chem. Lett., 11, 3173-3184 (2020).

[2] F. Thouin, D. Valverde-Chavez, C. Quarti, D. Cortecchia, I. Bargigia, D. Beljonne, A. Petrozza, C. Silva and A. R. Srimath Kandada, Nature Materials, 18, 349-356 (2019).

[3] E. Rojas-Gatjens, C. Silva-Acuna and A. R. Srimath Kandada, Peculiar anharmonicity of Ruddlesden Popper metal halides: Temperature dependent dephasing, Materials Horizons (2022).

[4] A. R. Srimath Kandada, H. Li, F. Thouin, E. R. Bittner and C. Silva, Stochastic scattering theory for excitation-induced dephasing: Time dependent nonlinear coherent exciton lineshapes, J. Chem. Phys., 153, 164706 (2020).

Metal halide double perovskites, especially the Cs₂AgBiBr₆, have attracted the attention of researchers in the search for less toxic candidates as active materials in optoelectronic devices.^[1,2] From this 3D structure, low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic-octahedra layers separated by organic cations.^[3] Here, we synthesize a series of layered double perovskites from Cs₂AgBiBr₆ that consist of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different size and chemical structure. By carrying out temperature-dependent Raman spectroscopy measurements, we highlight phase transition signatures in both inorganic lattice and organic moieties by detecting changes in the position and linewidth of the vibrational modes. Variations in the conformational arrangement of the organic cations from an ordered to disordered state match with a phase transition in the 1L systems with propyl- and butyl-ammonium moieties. Density functional theory calculations of the band structure reveal that the bandgap is direct only for the 1L crystal structures, and it becomes indirect in 2L systems. The direct bandgap character in 1L compounds stems from an extremely flat lowest conduction band, which enables small octahedral distortions to produce significant changes in the band structure across the bandgap. This translates to the optical properties, where we observe significant changes of photoluminescence intensity around the transition temperature as result of octahedral tilting rearrangement emerging at the phase transition.^[4] Our work provides novel insights into the structure-optical properties relationship in layered double perovskites, and demonstrates how the proper selection of the organic cations in the layered perovskites is relevant in terms of thermal stability or the use of the phase transition for active switching in future applications.

Solution processable semiconducting perovskites hold great promise for demanding applications involving light emission, such as printable lasers or quantum light sources. A case example is that of fully inorganic CsPbBr₃ quantum wells (CQWs) which display high quantum yield at room temperature. Less studied than their organic-inorganic counterparts, these CQWs sustain high single exciton binding energies and luminescence quantum yields, yet little is known on the nature of the exciton and multi-excitons states required for advanced applications. Here, we show that charge carriers in fully inorganic 2D perovskites exist as stable exciton - polarons, a complex between a charge neutral exciton and a lattice deformation. Next, we show that these unique species can fuse together to form a hereto unexplored bi-exciton polaron state, i.e. a two-particle complex bound by attractive Coulomb attraction whilst simultaneously being strongly coupled to the lattice. Finally, we show that net stimulated emission occurs through radiative recombination from this unique bi-exciton polaron state to a single free exciton polaron, showing the stability of the newly found species. Consequences of the polaronic character are identified as a low threshold for stimulated emission but with limited optical gain coefficients, both of which we can fully reproduce using a thermodynamic gain model. As such, our results provide a general framework to understand and predict the behavior of not only single, but also multi-exciton polaron states in perovskite materials.

Two-dimensional (2D) hybrid organic−inorganic perovskites are attracting interest thanks to their wide range of applications for solar energy conversion and beyond. Among their advantages, the tunability of their structural and optical properties is unique. For example, as the structural features of the organic spacers vary, the distancing between the inorganic layers can be affected – which affects the quantum confinement and hence the bandgap of the 2D perovskite. To experimentally elucidate the effect of the organic spacer on the optical and structural properties of perovskites, we applied hydrostatic pressures up to 375 MPa onto 2D materials with different spacers - e.g. butylammonium, benzylammonium, phenylehtylammonium, naphthalene-O-propylammonium, and perylene-O-ehtylammonium. In addition to a different degree of rigidity and extent of bandgap shift of the 2D materials for different spacers, we found that when butylammonium is used as the spacer, the 2D material undergoes a first-order phase transition at 350 MPa. By studying the specific effect of aromaticity and alkyl chain length of the organic spacers on the overall rigidity (hence, bandgap shift) of the 2D material and finding a pressure-dependent phase transition in one of the studied materials, our work is essential to designing 2D perovskites with finely tuned optical and structural properties.

Halide perovskites are the new wonder material of the optoelectronics community due totheir outstanding photoluminescence quantum yield, tunable emission wavelength and simplesolution or vapor-phase deposition. At the same time, their facile ion migration andtransformation under optical, electrical and chemical stress are seen as a major limitation fordevice implementation. Mixed halide perovskites are particularly problematic sinceoptical excitation can cause changes in the band gap that are detrimental for solar cell and light-emitting diode efficiency and stability. In this work, instead of preventing suchchanges, we exploit photo-induced halide segregation in perovskites to enable responsive,reconfigurable and self-optimizing materials. We show how a mixed halide perovskite film can betrained to give highly directional light emission using a nanophotonic microlens: through a self-optimized process of halide photosegregation, the system mimics the training stimulus. Longertraining leads to more highly directional emission, while the different halide migration kineticsin the light (fast training) and dark (slow forgetting) allow for material memory. This self-optimized material performs significantly better than lithographically aligned quantum dots[1], because it eliminates lens-emitter misalignment and automatically corrects for lens aberrations.Our system shows a combination of mimicking, improving over time, and memory, which make itcompatible with the basic requirements for learning,[2,3] and give the intriguing prospectof intelligent optoelectronic materials

Hybrid perovskite photovoltaic devices have rapidly emerged as promising contenders for next generation, low-cost solar cell technology. Yet, the presence of defect states critically impacts device operation, including device efficiency and potentially long-term stability. Understanding the nature of these defects and their role in photocarrier trapping, requires techniques that are capable of probing ultrafast photocarrier dynamics at the nanoscale.

            In this talk, I will discuss the development of time-resolved photoemission electron microscopy (TR PEEM) techniques in my lab [1], [2], applied to hybrid perovskite solar materials. Thereby, we directly visualize the presence of the performance limiting nanoscale defect clusters and elucidate the role of diffusion in the charge carrier trapping process [3]. By correlating PEEM measurements with other spatially resolved microscopies, we identify different types of defects that form, and study how passivation strategies may have a varied impact on them [4].

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) quantum dots (QDs) are subject of intense research due to their exceptional properties as both classical¹ and quantum light sources.^2-4 Here^5,6 we present perovskite-type (ABO₃) binary nanocrystal superlattices, created via the shape-directed co-assembly of steric-stabilized, highly luminescent cubic CsPbBr₃ nanocrystals (which occupy the B and/or O lattice sites), assembled in combination with spherical Fe₃O₄ or NaGdF₄ nanocrystals (A sites). These ABO₃ superlattices, as well as the binary NaCl and AlB₂ superlattice structures that we demonstrated, exhibit a high degree of orientational ordering of the CsPbBr₃ nanocubes which preserve their high oscillator strength and long exciton coherence time in the assembly. Such superlattices exhibit superfluorescence—a collective emission that results in a burst of photons with ultrafast radiative decay (22 picoseconds) that could be tailored, by structural engineering of the nanoparticle assembly, for use in ultrabright (quantum) light sources. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures.

References

[1] Akkerman et al., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 17, 394–405 (2018).

[2] Becker et al., Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189–193 (2018).

[3] Rainò et al., Superfluorescence from lead halide perovskite quantum dot superlattices. Nature 563, 671–675 (2018).

[4] Utzat et al., Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science 363, 1068–1072 (2019).

[5] Cherniukh et al., Perovskite-type superlattices from lead halide perovskite nanocubes. Nature 593, 535–542 (2021).

[6] Cherniukh et al., Shape-Directed Co-Assembly of Lead Halide Perovskite Nanocubes with Dielectric Nanodisks into Binary Nanocrystal Superlattices ACS Nano 15, 10, 16488–16500, (2021).

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains debated.

Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes that occur within these materials. At low temperatures the emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.

In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation. Spectral multiplets assigned to the biexciton recombination are also revealed at the single nanocrystal level, which directly provides the biexciton binding energy and evidence for extremely slow spin relaxation between the exciton fine structure sublevels.

Lead halide perovskites (LHP) nanomaterials have emerged as an exciting new class of semiconductor materials with outstanding electronic and optical properties, easy synthesis, and high photoluminescence quantum yield. Beyond their solar cell application, they have been extensively exploited in diverse fields such as LED technology, laser, and more recently in photocatalysis. [1]

They respond to the general formula APbX₃, where A is an organic cation such as methylammonium, formamidinium or inorganic cation such as Cs⁺, and X is the halide anion (Cl^-, Br^- or I^-). The key role of the organic ligands for surface passivation and colloidal dispersion has been demonstrated in the preparation of the first CH₃NH₃PbBr₃ nanocrystals in 2014, using octylammonium bromide and oleic acid as capping agents. [2] The nature of the organic ligands can determine the dimensionality of the inorganic framework and the dimension of the perovskite material, and therefore their optical properties.

The synthesis of colloidal low dimensional perovskites with deep blue-emissive will be presented. In particular, ultrathin [CsPbBr₃]PbBr₄ nanoplatelets and pure two-layered L₂[CH₃NH₃PbX₃]PbX₄ perovskite nanosheets were synthesized by thermal/ultrasound methodology and ligand assisted techniques, respectively. [3, 4] Their emissive properties and chemical/photochemical stability will be discussed. The long-term stability in the solid-state together with the high processability of the colloidal nanosheets enabled the preparation of blue-emissive solid films, which may be suitable for assembling on a large area on-demand and, consequently, be useful for preparing high-quality films for flexible and ultrathin optoelectronic devices, or they could be combined with other types of 2D materials.

Are all important non-radiative recombination centres in perovskites metastable?

Ivan G. Scheblykin

Chemical Physics and Nano Lund, Lund University, Sweden

Despite of very substantial efforts, it is still difficult to quantitatively account for charge dynamics in metal-halide perovskites over a broad range of excitation condition by the standard theoretical approaches of semiconductor physics.¹

One aspect which has never been taken into account in any modelling of this kind is defect metastability. A defect state is metastable if, for example, it undergoes transition from its active state, in this state it induces strong non-radiative (NR) recombination, to its passive state where it does not influence charge dynamics at all.² Obviously, defect metastability must influence charge carrier dynamics substantially, especially if switching of the defects is light or/and charge carrier concentration dependent. Evidence for the defect metastability is overwhelming:

1. Photoluminescence (PL) blinking of individual microcrystals and even grains of polycrystalline films.³

2. Reversible PL enhancement and PL bleaching^4,5

3. Self-healing property of metal-halide perovskites in general terms⁶

I will discuss how we can see the metastable NR centers in action by observation of PL blinking. I will argue, that most probably all important defect states causing NR recombination, which scientists from all fields from fundamental to applied care about, are, in fact, metastable and see then in PL blinking experiments. So far, I do not see any experimental observations which would disprove to this hypothesis.

Despite that PL blinking in perovskites is known from 2015,⁷ so far there had been no real connection drawn from the results obtained on individual crystals and properties of films and devices. If all important defects are metastable, understanding this metastability becomes very crucial with PL micro spectroscopy as an ideal tool.

Metal-halide perovskites (MHPs) hold great potential for next-generation optoelectronic technologies due to their remarkable characteristics. While the influence of strain on the intrinsic properties of MHPs is gaining interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent PL intensity study on heteroepitaxially strained surface guided CsPbBr₃ nanowires. We reveal an unexpected linear coupling between the alternating field and the PL intensity with a significant modulation depth (up to 40%), stemming from an induced internal dipole. Low-frequency polarized-Raman spectroscopy shows structural modifications under an external field, associated with the observed polarity. These results reflect the important interplay between an external field and strain in MHPs and offer new insight into the design of novel MHP-based nano-optoelectronics.

Perovskite solar cells, with solution-based, cheap synthesis methods and a rapid increase in power conversion efficiency, are a promising candidate for future solar cells. However, a major hurdle for commercialization remains, namely the intrinsic instability of these systems. Ion migration, the process by which the A, B and ions of the ABX₃ structure become mobile in the perovskite layer, represents a key challenge to tackle.

MAPbBr₃ was shown to be more stable under environmental conditions when compared to MAPbI₃. Using transient ion drift, we show that this stems from key changes in ion migration when going from MAPbI₃ to MAPbBr₃: methylammonium migration is suppressed, while bromide migration is reduced. Nowadays, state-of-the-art perovskite devices combine multiple ions: we therefore extend our study to ion migration in mixed-halide perovskites with varying ratios of iodide to bromide, and find interesting dynamics regarding the phase segregation phenomenon. In order to benefit both from the high efficiency of the 3D perovskites and from the stability of the 2D perovskites, new device architectures composed of a 2D layer on top of a 3D layer have emerged. We quantify the ion migration dynamics in these mixed-dimensionality perovskites, and find that ion migration is hindered in all systems incorporating a 2D layer. The specific hindering mechanism is however dependent on the 2D spacer molecule. Finally, we investigate the evolution of ion migration in different MAPbBr₃ solar cells as a function of the grain size of the active perovskite film. We show that beyond composition engineering, crystallinity can be another effective tool to control ion migration.

The broad implementation of halide perovskites into electronic applications beyond photovoltaics relies on effectively tuning free carrier concentrations in this class of semiconductors. However, doping in these materials remains difficult and is far from being fully understood. Specifically, detailed knowledge on the perovskite-dopant chemical interactions is key to enable effective doping strategies. This talk will cover our recent work on carrier compensation in inherently p-type MASn_xPb_1-xI₃ (where MA is methylammonium) by employing an n-type molecular dopant, i.e. n-DMBI. We observe a decrease in free hole concentration of nearly one order of magnitude, an increase in Seebeck coefficient and a shallower perovskite Fermi level, in consistence with electron donation from n-DMBI. This is shown to occur via dopant hydride loss and subsequent radical transfer to perovskite. We then find that only oxidised molecular dopant binds onto perovskite surface Sn sites via Lewis acid-base interactions, suggesting that charge transfer is facilitated by dopant/perovskite chemical coordination. We expect the detailed chemical insight on charge transfer doping provided herein to give design guidelines towards future molecular dopants for perovskite-based technologies.

In this work we show that thin polycrystalline films of spin-coated formamidinium tin triiodide (FASnI₃) perovskite demonstrate efficient random lasing (RL) possessing distinguished optical properties such as high mode stability, very high quality factor (~ 10⁴) and low threshold (≈ 2 and ≈ 25 microJ/cm² for 15 and 300 K, respectively). While usually RL is characterized by chaotic changes of the narrow lines spectral positions, the found in our case RL mode stability is a unique property, which is observed here for the first time for polycrystalline semiconductor systems. We demonstrate that the efficient random lasing and mode stability are due to the high efficiency of light scattering by FASnI₃ grains, which is a result of high refractive index of the material (higher than in the case of Pb-based halide perovskites) as well as of optimized grain size distribution (average size is close to the emission wavelength). We demonstrate also that different excitation conditions are required for generation of ultra-narrow RL lines and rather broad-band Amplified Spontaneous Emission (ASE): RL lines dominate when excitation spot size is a few tens of micrometers, while ASE dominates in case of large size excitation spot of a few hundreds of micrometers. From the above, it follows that the stability in time of narrow RL lines is ensured by the strong space localization of RL modes and the absence of interaction between them [1].

[1] Chirvony, V. S.; Suárez, I.; Sanchez-Diaz, J.; Sánchez, R. S.; Rodríguez-Romero, J.; Mora-Seró, I.; Martínez-Pastor, J. P. Stable non-chaotic high Q near-infrared random lasing in thin polycrystalline films of lead-free formamidinium tin triiodide perovskite. To be submitted.

Transient photoluminescence (trPL) is a powerful technique to study charge carrier dynamics and identify carrier recombination channels in luminescent thin films and devices. It has been widely used to quantify carrier radiative and non-radiative lifetimes,^[1] to compute the charge extraction velocity ^[2], and to study the influence of the interfaces on the recombination mechanisms.^[3]

Data fitting helps you extract this information from experimental trPL decays, but considering the number and nature of involved variables, the set of extracted values is most of the time not unique. Indeed, the trPL decay is ruled by the radiative and non-radiative recombination processes which follow, respectively, a quadratic and a linear dependence on the photogenerated carriers. The concentration of these carriers depends on several parameters, such as the injection level, the doping of the emitting material, the radiative recombination coefficient, and the trap density. Because of this interdependence, it is easy to obtain comparable trPL decay fits by using different combinations of values and, hence, interpreting incorrectly the recombination events. To obtain reliable information, it is better to use a robust simulation approach where the parameters are distinctly assessed and recursively optimized according to experimental data from the literature.

We demonstrate how to simulate experimental trPL curves of a halide perovskite on glass by fitting the data with a fully-coupled 1D optoelectronic simulation. The validity of the simulated trPL curves is also verified by fitting the decay with a bi-exponential function, which allows estimating the lifetimes of the different recombination mechanisms. We will present a robust routine to ascertain the reliability of the obtained results. This routine is based on experimentally measured material properties and on the analysis of the band diagram evolution during the PL transient. Our approach is not limited to the analysis of bare perovskites. We will show also a first analysis of the influence of the absorber interfaces on the trPL. The mentioned routine for trPL simulation can be further extended with the inclusion of photon recycling.^[4]

The renaissance of interest in halide perovskites, triggered by the discovery of their unprecedented performance in opto-electronic applications, elicited worldwide efforts to uncover a variety of intriguing physical properties, with a special interest about spin-orbit and Rashba/Dresselhaus effects. The current work presents for the first-time magneto-optical experimental evidence for non- uniaxial Rashba field with surplus contribution by a Dresselhaus effect arising from the bulk of MAPbBr3. Magneto-photoluminescence (MPL) spectra, monitored along several different crystallographic directions, were dominated by dual exciton emission peaks, while each exhibited a highly non-linear response to a magnetic field, with distinguished behavior at extreme points. Moreover, these plots depicted anisotropy from-B0 to +B0, with strong dependence on the axis of observations. Furthermore, ground state electron spin resonance spectroscopy, illustrated resonances bands coinciding with the extreme points in the MPL spectrum. Those resonance appeared only after illumination and disappeared a few hours later, designating an association with electron-nuclear coupling (the Overhauser effect. A theoretical model corroborating the experimental results, implementing anisotropic Rashba/Dresselhaus terms, anisotropy in the Landé g-factors, and a lesser contribution from an Overhauser effect, corroborated the experimental results. The origin of the Rashba/Dresselhaus effects in the bulk of halide perovskites is assumed to be attributed to the presence of grain boundaries. The research discoveries put a basic ground for expansion of possible applications of the halide perovskites toward spin-base devices.

During recent years, the picture of dynamic charge carrier screening in lead halide perovskites (LHPs) evolved to explain long carrier lifetimes and diffusion lengths, leading to the nowadays famous optoelectronic properties of this material class. However, the most important prerequisite for such anticipated structural screening, the direct observation of related lattice- or organic cation motion within the estimated polaron formation time (<1ps), remains elusive. By developing two-dimensional optical Kerr effect (2D-OKE) spectroscopy, we unveil that the ultrafast polarization response to near-bandgap excitation pulses is overwhelmed by the instantaneous electronic polarizability [1]. In this case, the nonlinear mixing of highly anisotropic and dispersive light propagations is responsible for oscillatory Kerr-type modulations at terahertz (THz) frequencies [1, 2]. Therefrom-arising 2D-OKE fingerprints can be harnessed to trace lattice anisotropy across non-trivial phase transitions or in multi-cation alloyed LHPs [3]. Despite this useful characterization of static lattice distortions, the dynamic lattice response seems elusive for the OKE with excitation pulses in the visible spectral range.

Ultimately, to study the ultrafast lattice response to a transient electric field, we move the excitation to the THz spectral region, resonant to the optical modes of the lead halide lattice, by employing intense phase-stable THz fields. Conceptually, this close to single-cycle electric field with a rise-time below 1 ps, can be seen as proxy for ultrafast electron-hole separation during optical excitation and consecutive thermalization. By probing the THz-induced Kerr effect (TKE), we observe a strong THz polarizability in both inorganic CsPbBr₃ and in the hybrid MAPbBr₃ materials. Also here, we show that it is crucial to account for dispersion and optical anisotropy in interpreting the transient responses via rigorous four-wave mixing simulations. Nevertheless, we finally observe a contribution from the lattice polarizability by witnessing the nonlinear excitation of coherent phonons, in full agreement with static Raman spectra. Moreover, we pinpoint a single phonon mode which dominates the lattice polarizability, providing another stepping stone towards a better understanding of how lattice dynamics enable charge carrier protection in this class of defect tolerant semiconductors.

References:

[1] S.F. Maehrlein et al., PNAS, 118,7, e2022268118 (2021)

[2] L. Huber et al., J. Chem. Phys., 154, 9, p. 094202 (2021)

[3] F. Wang et al., J. Phys. Chem. Lett., 12, 20, 5016–5022 (2021)

Highly performing mixed Sn/Pb-perovskite solar cells (PSCs) are among the most promising options to reduce Pb content in perovskite devices and enable the fabrication of all-perovskite tandem solar cells owing to their reduced band gap. Here, I will show that the introduction of 2,3,4,5,6-pentafluorophenethylaminium cations in a perovskite active layer of composition (FASnI₃)_0.5(MAPbI₃)_0.5 enhances the crystallinity of the active layer, reduces the voltage losses and increases the material stability. The addition of the fluorinated cations allows the fabrication of highly oriented films with improved thermal stability. Moreover, the treated films exhibits merged grains with no evidence of 2D structures, which we believe leads to the passivation of trap states at the grain boundaries. Solar cells fabricated adding to the active layer the fluorinated cation display reduced trap-assisted recombination losses and lower background carrier density, which led to enhanced open-circuit voltage than the reference sample using phenethylammonium cations. The best perovskite solar cell showed an efficiency of 19.13%, with an open-circuit voltage of 0.84 V, which is substantially improved respect to the reference sample which exhibits 17.47% efficiency and 0.77 V as open-circuit voltage. More importantly, the fluorinated cations' addition help to improve the device's thermal stability, maintaining 90.3 % of its initial efficiency after 90 min of thermal stress at 85 C° in Nitrogen atmosphere.

Nowadays, overcoming the stability issue of perovskite solar cells (PSCs) while keeping high efficiency has become an urgent need for the future of this technology. We will show that getting stable PSC begins with the preparation of high-quality perovskite layers and that employing additives in the perovskite precursor solution allows to reach this objective. Two families of additives will be developed in the talk: halides and nanoparticles. We have highlighted the effect of chloride additives on the formation mechanism of methylammonium-free Cs0.1FA0.9PbI3 films upon annealing. By composition profile evolution tracking, two formation steps are distinguished: first the superficial residual solvent is eliminated, second, the solvent in the depth of the film is evaporated. The elimination profile signs the final morphology of the layer. The downward (top-down) growth has been encountered for the pristine and potassium chloride additive cases. It led to the formation of multiple boundaries and middle-sized grain morphology. We unveil that employing both potassium chloride and ammonium chloride additives forces the homogeneous elimination of the solvent across the layer, and then the lateral growth of the grains. It results in large size, monolithic and defect-poor grains with good coverage of the substrate which are the targeted properties for high efficiency and stability. Similar film formation steps and growth direction control have been highlighted by comparing the formation mechanism of MAPbI3 films with and without gold nanoparticles. The pristine layer grows in the top-down direction, while the presence of nanoparticles results in a lateral growth and formation of a monolithic structure.

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher performing solar cells. In this presentation, three engineering approaches are proposed to enhance the photovoltaic efficiency and stability of PSCs: (1) On the first method, a series of highly oriented vertical TiO₂ nanocolumn electron-transporting photonic structures were intentionally fabricated on half of the compact TiO₂-coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. (2) On the second method, we investigated the effect of removing the excess PbI₂ at the interface between the triple-cation Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite and the Spiro-OMETAD hole-transport layer. For this purpose, four different organic salts, including methylammonium iodide (MAI), formamidinium iodide (FAI),  methylammonium bromide (MABr) and methylammonium chloride (MACl) were applied and compared. (3) The last aspect that will be presented involves the application of the downshifting optical property of colloidal carbon quantum dots to enhance the stability of perovskite solar cells against UV degradation.

On the above-mentioned engineering methods, different characterization methods, including far-field and near-field optical experiments, structural and spectroscopic investigations, impedance spectroscopy, together with solar cell efficiency and in  particular device stability measurements are presented together in order to understand the underlying origins of the efficiency and stability enhancement observed in triple-cation perovskite solar cells.

While organic-inorganic (hybrid) perovskites achieved competitive power conversion efficiency within an impressive short time [1], they still suffer from insufficient stability [2]. Instability of hybrid perovskite thin films under thermal stress and/or in humid air is one of the major obstacles for the commercialization of perovskite-based solar modules [2,3]. One of the most promising approaches to improve the long-term stability is the incorporation of bulky, hydrophobic molecules into the perovskite layer [4]. While the beneficial effects of these post-deposition treatments are generally accepted, there is an ongoing discussion about the mechanism of this passivation effect. Particularly, it is under debate when optimum surface passivation is reached using different concentrations of bulky molecules where a transition is supposed to happen from the surface attached bulky molecule itself to a 2D layer formation with general structure  R₂A_n−1B_nX_3n+1 [5], which forms via intermixing of the bulky molecules and PbI₂ from the 3D perovskite. Here, A denotes the cation, B the metal ion, and X the halogen anion of the 3D perovskite, R is the bulky organic cation.

Here, we use in situ photoluminescence (PL) measurements during spin-coating and annealing to probe the dynamic deposition of 2-thiophenemethylammonium iodide (2-TMAI) and phenylethylammonium (PEAI) with varied concentration on 3D triple cation perovskites. For both molecules, we find the transition from molecular passivation to the formation of an R₂A_n−1B_nX_3n+1 layer at concentrations around 4-10 mmol/L. Using higher concentrations, we see the formation of a distinct surface layer. During spin-coating and annealing of these higher concentrated solutions, we furthermore monitor the transition from a single inorganic layer spaced by the bulky cations (n=1) to mixed 2D layers (n=2 and higher) and, in the case of 2-TMAI, to the formation of a mixed disordered phase [6]. The latter may negatively affect the electronic properties of the perovskite layer [7]. Our results illustrate how in situ PL can be used to gain mechanistic understanding on the 2D layer formation, its interaction with the 3D perovskite, and its transformation to the disordered phase. Therefore, it can be utilized to deliberately optimize the annealing sequence targeting an ideal 2D/3D interface satisfying enhanced charge transport and stability.

Combining abovementioned results with in situ GIWAXS measurements, we propose a model for the surface passivation mechanism either via molecular passivation or 2D layer formation, compare the mechanisms for both investigated molecules, and correlate them with device stability proposing an optimized stabilization treatment.

Lead halide perovskite absorber materials have attracted enormous attention from academia and industry due to their fascinating optoelectronic characteristics featuring multiple possible applications. In particular, the solution-processed perovskite solar cells demonstrated certified power conversion efficiencies of >25.5% thus coming very close to the best crystalline silicon solar cells. Low operational stability of perovskite solar cells is currently the main obstacle to their practical implementation.

Overcoming stability issues requires a very deep understanding of the compositional and structural dynamics of the perovskite absorber materials under the action of stress factors such as light, heat and electric field. Furthermore, the impact of defects and impurities on such dynamics is of crucial importance. These aspects remain insufficiently investigated, whereas many of the proposed aging pathways are still controversial.

               In this lecture, we will discuss compositional and structural inhomogeneities in solution-cast thin films of classical 3D and mixed-dimensional 2D/3D perovskites as revealed by advanced microscopy: SEM, AFM, KPFM and IR s-SNOM. A particular focus will be made on the light-induced phase segregation effects induced by both anion and cation (de)mixing. Furthermore, we will consider typical impurities present in the perovskite absorber films, their localization, effects on the grain boundaries and impact on the operational stability of solar cells.

Single-junction halide perovskite solar cells (PSCs) have already achieved a certified power conversion efficiency (PCE) above 25 %, making them one of the most promising emerging photovoltaic technologies. One of the main bottlenecks towards their commercialization is their long-term stability, which should exceed the 20-year mark. Many are the strategies applied to extend device lifetime, among them are the use of additives, the optimization of the fabrication process of perovskite thin films or the replacement of unstable organic transport layers such as Spiro-OMeTAD. Although most of these approaches can effectively improve device efficiency, they frequently fail at providing stable PSCs as defined as those able to display less than 10 % degradation after 1000 h of continuous illumination under 1 sun. In this respect, the understanding of defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on the device efficiency and stability is currently a challenge. In this talk, we will show our most recent results on additive engineering to enhance the stability of highly efficient PSCs. We demonstrate that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous impact on the device stability. We obtained PSCs with ~21 % efficiency that retain ~100 % of the initial efficiency after 1000 h at the maximum power point under simulated AM1.5 illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H^…I and O^…H), results in shallow point defect passivation that has a significant impact on the device stability but not on the nonradiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs.

I will talk about our work on understanding intrinsic stability of perovskites and meta-stability of perovskite solar cells and strategies for boosting perovskite solar cells’ durability against thermal extremes and humidity. Our perovskite solar cells encapsulated by a polymer-based encapsulation scheme were the first to exceed the strict requirements of International Electrotechnical Commission standards for thermal cycling damp heat and humidity freeze. I will then review some of the glass-to-glass bonding technologies that can be carried out at low-temperatures suitable for perovskite solar cells. This will be followed by our recent results of developing polymer-free glass-to-glass bonding technique for encapsulating perovskite solar cells.

Nanometer-sized colloidal metal halide perovskite semiconductors have emerged and brought unique opportunities for photovoltaic application due to the high defect tolerance of perovskite and many features that emerge at the nanoscale. Perovskite quantum dots (QDs) or more broadly, nanocrystals (NCs), show high photoluminescence (PL) quantum yields, spectrally tunable bandgap, flexible compositional control, and crystalline strain benefits. Metal halide perovskite nanocrystals are readily synthesized with exceptional optoelectronic quality, opening a route for next generation light emitters, as well as exploring LHP physics at the nanoscale. CsPbX₃ (X=Cl^-, Br^-, I^-, or mixed halide) QDs exhibit PL tunable from ultraviolet to near-infrared wavelengths by adjusting the halide composition and/or QD size. In 2016, CsPbI₃ QDs also became a point of interest in PV research. Currently CsPbI₃ holds the record efficiency for QD solar cells (16.6%) proving better than any previous QD material composition. This talk will highlight the unique potential of perovskite QD (PQD) solar cells, from synthesis to devices. We will discuss current state of the art and lay out many open opportunities in perovskite QD solar cells, and the related present and future pursuits in QD preparation and device architecture.

Reference:

Protesescu, L. et al. Nanocrystals of cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 15, 3692-3696 (2015).

Swarnkar, A. et al. Quantum dot-induced phase stabilization of α-CsPbI₃ perovskite for high-efficiency photovoltaics. Science 354, 92-95 (2016).

Yuan, J. et al. Band-aligned polymeric hole transport materials for extremely low energy loss α-CsPbI₃ perovskite nanocrystal solar cells. Joule 2, 2450-2463 (2018).

Mixed-dimensional perovskites containing mixtures of organic cations hold great promise to deliver highly stable and efficient solar cells. When used in small amounts, the low-dimensional species can act as a dopant to passivate defects at the grain boundaries of the 3D counterpart, leading to improved performance and stability. However, although a plethora of relatively bulky organic cations have been reported for such purpose, a fundamental understanding of the materials' structure, composition, and phase, along with their correlated effects on the corresponding optoelectronic properties and degradation mechanism remains elusive. Herein, we systematically engineer the structures of bulky organic cations to template the formation of low-dimensional perovskites with contrasting inorganic framework dimensionality (2D to 0D), connectivity (flat vs. corrugated), and coordination deformation. By combining X-ray single-crystal structural analysis with various spectroscopic techniques, such as depth-profiling XPS, solid-state NMR, and femtosecond transient absorption, it is revealed that not all low-dimensional species work equally well as dopants. Instead, it was found that inorganic architectures with lesser structural distortion tend to yield a less disordered energetic and defect landscape in the resulting mixed-dimensional perovskites, augmented in materials with higher PLQY (up to 11%), better solar cell performance (PCE more than 19%) and improved thermal stability (T70 up to 1000 hrs, unencapsulated). Our study highlights the importance of designing templating organic cations that yield low-dimensional materials with much less structural distortion profiles to be used as additives in stable and efficient perovskite solar cells.

Recently, organometallic hybrid perovskite materials are experiencing a real progress for solar cell applications. Due to particularly interesting properties: adaptable band gap, high crystallinity, high charge transport capacity and high thin film efficiency, these materials have the potential to exceed the performance limits of current technologies. They also combine a low cost and processing versatility. Among alternative device structures, carbon-based perovskite solar cells (C-PSCs) look highly promising due to their low cost and abundantly available materials (TiO2, ZrO2, carbon black and graphite powders), cost-efficient scalable fabrication methods and the inherent high stability. A one step (CH3NH3)x(AVA)1-xPbI3 perovskite solution (with AVA= ammonium valeric acid additive) was pipetted to infiltrate mp-TiO2/mp-ZrO2 through a thick porous carbon layer [1][2]. In order to reveal their maximum photovoltaic performance, these devices should be first matured under humidity and temperature. This step lasts of approximately 100-150 h and improves of the cell’s performance. To further investigate their stability, aging campaigns at 85°C/85%RH have been conducted during 1000 h. The macroscopic observations show an inhomogeneous degradation of the perovskite layer, the interfaces and the electrodes, mainly located at the edges. This inhomogeneity probably results from the pipetting process used to infiltrate the perovskite. This was confirmed by the variation of PV parameters during aging, which showed an important decrease in performance close to 50% after 1000 h of aging. In this study, a basic encapsulated system based on glass and a surlyn gasket was used, enabling the humidity permeation up to solar cells and inducing probably an accelerated degradation of devices. Thanks to dedicated characterization techniques, such as laser beam induced current (LBIC) measurements and photoluminescence imaging, the local performances have been correlated to the degradation inhomogeneity. The modifications of the perovskite layer have been evaluated with others more common techniques (X-Ray diffraction, UV-visible absorption and photoluminescence spectroscopy). Thanks to this multiscale approach, a degradation mechanism could be proposed highlighting the role playing by the prior maturation step. Today, others technological solutions are tested such as the inkjet printing for the perovskite infiltration and more advanced encapsulation systems, to improve the stability of these PV devices.

Abstract: Despite of huge success in photovoltaic community, Pb based halide perovskites have energy band gap higher than the optimal band gap required for single junction solar cells, governed by the Shockley–Queisser radiative limit. Sn based perovskites emerge as an alternative with rapidly improving efficiencies. Due to comparable optoelectronic properties as the Pb counterpart and their ability to lead to lower band gaps, Sn based perovskites open a new door to all-perovskite tandem solar cells. Along with efforts for efficiency, it is worth analysing the in-depth recombination dynamics for further development of Sn based perovskite solar cells (PSCs). The long photo-excited charge carrier lifetime (τ) is often attributed for the high performance of PSCs. Herein, we study the effect of ‘B’ cation in ABX₃ (A = MA⁺, FA⁺, and Cs⁺; B = Pb²⁺, Sn²⁺, and X = I⁻) perovskite structure on the charge carrier recombination dynamics to understand the possible recombination mechanism. We fabricated PSCs with p-i-n configuration based on FA_0.95Cs_0.05PbI₃ (pure Pb), MA_0.20FA_0.75Cs_0.05SnI₃ (pure Sn), and (MAPbI₃)_0.4(FASnI₃)_0.6 (Pb-Sn mixed) perovskitres. We optimized the Sn based perovskite thin film (pure Sn) in terms of moisture and thermal stability in order to minimize the error due to perovskite degradation. Further, we compared the charge carrier recombination dynamics of the three devices through transient photovoltage (TPV) measurement.¹ TPV is used to establish the relation between charge carrier density (n) and recombination rate constant (k) under different DC background intensities.² This study establish the rate law of charge carrier decay in the three devices and reveals the nonlinear dependence of k on n in Sn based PSCs, which will be correlated to the slow relaxation lifetime of the charge carrier and their recombination through the defect states in the perovskite thin films.

Keywords: Pb-free perovskite, rate law of charge carrier decay, transient photovoltage, defects, bimolecular recombination

One of the most promising third generation photovoltaic technologies is Perovskite Solar Cells (PSC). This perovskite family presents outstanding properties such as high absorption coefficient, long diffusion lengths, low-cost technology and ease of scalability. Currently, the efficiency record is set to 25.6 % [1]. On the other hand, the development of new materials combining multiple cations and multiple halides is one of the main research strategies to achieve high performance.

In this work, we have analysed the temperature behaviour of perovskite based solar cells with structure FTO/c-TiOx/m-TiOx/RbCsFAMAPbIBr/Spiro/Au. We have obtained the temperature dependence of the solar cell parameters (Voc, Isc, FF and efficiency) from -20 ºC up to 50 ºC. Impedance Spectroscopy (IS) has been carried out at room temperature and Cole-Cole spectra have been fitted using the Matryoshka circuit. Ideality factor has been obtained both from Voc vs light and from the recombination resistance extracted from IS measurements. In order to determine the dominant recombination mechanism, the activation energy has been estimated from the extrapolation of Voc down to 0 K [2, 3]. 

Hybrid tin halide perovskite solar cells stand as the most promising alternative to their toxic lead-based counterparts, although addressing their poor ambient stability remains as the main challenge to make this technology competitive. Hence, detailed knowledge of their chemical degradation pathways is essential to mitigate their decomposition. This talk will cover our recent findings on the degradation mechanism of tin perovskite thin films (i.e. (PEA)_0.2(FA)_0.8SnI₃, where PEA is phenylethylammonium and FA is formamidinium) via a combination of spectroscopy, diffraction and ab initio simulation techniques [1]. We find that SnI₄, a product that forms as a result of the oxygen-mediated decomposition of perovskite, reacts further to form iodine via the combined action of atmospheric water and oxygen. Iodine is then shown to be a highly aggressive species that rapidly leads to further perovskite degradation to give more SnI₄, establishing a cyclic degradation mechanism. We then find the ambient stability of tin perovskite films to be highly dependent on the hole transport layer chosen as the substrate, which is used to tackle the oxidative degradation of the material and increase its ambient stability. We expect the findings presented herein to provide key design rules towards stable, lead-free tin perovskite solar cells.

In the past decade, metal halide perovskite (MHP)-based solar cells marked a breakthrough in photovoltaic technologies and reach power conversion efficiencies exceeding 25%. While MHPs exhibit a remarkable defect tolerance, film degradation will eventually deteriorate the optoelectronic properties and hence device performance. A key strategy to substantially enhance the stability is to tailor the interfaces in the device.¹

Here, I will discuss the impact of interface formation on device performance also considering the effect of chemical reactions on interface energetics and durability,² particularly for our recent research activities on oxide buffer- and transport layers.³ In particular, I will describe our  use of surface-sensitive photoemission spectroscopy (PES) as a primary tool to provide guidelines for controlling the chemistry and optimize the electronic properties of MHP interfaces. In combination with further advanced characterization techniques, such as Kelvin probe force microscopy (KPFM) to determine surface photovoltage of the investigated layer stacks or quantitative photoluminescence imaging for the analysis of bulk optical properties, we specifically use PES methods to put the role of charge selective contacts in the focus of our research.⁴

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells. However, the mechanisms that allow for carriers to be transferred across these interfaces are still unknown.

In this presentation, I will discuss the role of crystal surface structural defects on optoelectronic properties of lead halide perovskites through synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to understand how these defects affect long term structural stability and thus optoelectronic performance over the long term.

Perovskite/silicon tandem is a new high-efficiency photovoltaic platform that converts sunlight into electricity with excellent performance. In few years, this driving factor have pushed the tandem technology already at the entrance of the photovoltaic market. However, to succeed towards their commercialization, the tandems need to secure the stability of the performances, in line with the warranty certificates of commercial crystalline silicon modules. Enhancing the stability while preserving record performances is a challenge that can be addressed with different approaches. In this direction, bifacial tandem solar cells are a potential solution. In this configuration, light is absorbed both at the sun-side and at the rear-side of the device by exploiting the albedo - the scattered and reflected photons from the ground. Thanks to the additional photons arising from the albedo, the bifacial configuration enhances its current above the conventional configuration, resulting in even better performances. The current enhancement is achieved by tailoring the perovskite bandgap, resulting in bromide-free composition. This composition significantly improve the stability of the performances, by suppressing the mechanism of halide segregation. Overall, the bifacial configuration combine together enhanced efficiency and better stability, promising the technology at the utility scale, towards a carbon-sustainable economy.

The usage of self-assembled monolayers (SAMs) as hole-selective layers was a key advancement for achieving >29%-efficient monolithic perovskite/silicon and >24%-efficient perovskite/CIGSe tandem solar cells. The SAMs also enabled model systems for a systematic study of charge extraction by transient methods.

This talk covers a short introduction into the development of SAMs for perovskite solar cells (PSCs), shows their application in perovskite tandem solar cells and shows which design guidelines could be concluded for a high-performance hole-selective interface. A combination of time-resolved and absolute photoluminescence spectroscopy, together with solar cell characterization identified how non-radiative recombination losses were minimized at the hole-selective interface, leading to high open-circuit voltages. The SAM model system further allowed an analysis of the influence of hole extraction speed on the fill factor of the PSCs. We found that hole extraction is around 100-times slower than electron extraction in p-i-n PSCs, which still leaves room for further fill factor gains. The dissection of charge extraction and non-radiative recombination in a perovskite absorber deposited on hole-selective layers was further detailed by a transient surface photovoltage study, confirming and completing the photoluminescence and solar cell analysis.

Finally, an overview of the wide variety of use cases for the SAMs in perovskite solar cells is shown, underlining their versatility, simplicity and robustness.

Perovskite-based multijunction photovoltaics are about to surpass power conversion efficiencies of 30% on a laboratory scale and thereby have irrevocably developed into one of the most promising next-generation photovoltaic technologies with enormous market potential. Unfolding this potential requires solving scientific and technological challenges related to high-efficiency device architectures, the development of stable perovskite materials, light management, and scalable fabrication processes of the perovskite top solar cell. In this contribution, our recent progress on novel passivation strategies using 2D/3D heterostructures, optimization of light management textures, and scalable fabrication processes (evaporation, lamination, and solution-based coating) for perovskite-based tandem photovoltaics will be presented. Prototype perovskite/Si and perovskite/CIGS tandem solar cells in 4-terminal architecture, as well as 2-terminal architecture, will demonstrate the maturity of these developments.

Tandem solar cells based entirely on thin-film metal halide perovskite absorbers hold the potential of high efficiencies at low cost and large flexibility.^[1] Prototypes of such technology have been fabricated successfully, and efficiencies exceeding 26% have been reached.^[2] However, compared to the more established combination of metal halide perovskite top cells with high-efficiency silicon bottom cells, the performance potential and limitations of all-perovskite tandems are less explored. One major difference with respect to the perovskite-silicon reference is the relevance of wave optical effects and of photon recycling for both, top and bottom absorbers. Hence, the optical coupling of the subcells needs to be considered rigorously in a simulation-based assessment of the performance potential.    

Here, we present an extension of our recently introduced model for photon recycling in thin-film perovskite absorbers^[3] to the situation of all-perovskite tandem solar cells, including a consistent consideration of luminescent coupling due to re-absorption in the bottom perovskite cell of the light emitted by the top perovskite absorber. The impact of photon-recycling and luminescent coupling on the photovoltaic device performance is assessed in the limit of ideal transport – using detailed balance – and under consideration of realistic charge transport based on a drift-diffusion model. The latter includes also losses due to both non-idealities in the subcell connection via the recombination junction and non-radiative recombination in the bulk and at interfaces.

This fully coupled optoelectronic approach enables an in-depth analysis of subcell performance issues and their propagation to the performance of the tandem device.^[4]

Solution-processed all-perovskite tandem solar cells have the potential to surpass the efficiency limits for single-junction devices by using complementary ~1.8 eV and ~1.2 eV absorbers, thereby reducing thermalization and transmission losses. However, both absorber categories, mixed-halide wide-bandgap and lead-tin narrow-bandgap, suffer from significant non-radiative recombination losses that can limit the overall open-circuit voltage of the multijunction device. Additionally, the complex optical stack in a tandem device can introduce optical losses due to parasitic absorption and reflection of incident light. This work presents an integrated all-perovskite tandem where the sub-cells use surface passivation strategies to reduce non-radiative recombination at the perovskite-fullerene charge-selective interfaces, yielding a high open-circuit voltage. Further, by using optically benign transparent electrode and charge-selective layers, the external quantum efficiency in the narrow-bandgap sub-cell is improved leading to reduced current-mismatch between sub-cells. Cumulatively, these strategies allow the development of a monolithic tandem solar cell exhibiting a power-conversion efficiency of over 23%.

Meeting the ambitious challenge of net-zero greenhouse gas emissions by 2050 and holding the average increase of global temperature below 1.5°C necessitates the upscaling of readily available renewable energy sources, the window of time to achieve this goal is closing fast, so it is key to accelerate the decarbonisation of the global energy system by increasing the power output of solar cells. In this presentation, I will show how the integration of perovskites into the well-established silicon production infrastructure can raise the rate of solar deployment. Perovskite/silicon tandems deliver a technological disruption in efficiency while maintaining compatibility with the present photovoltaics industry, making it the fastest route to enhance the silicon market and rapidly address climate change.

Tandem solar cells based on Perovskite with silicon or another perovskite can further reduce the cost of photovoltaics due to their high efficiency, and low cost (including low material and fabrication cost). To realize their full potential, the perovskites need to be deposited by scalable fabrication process, and the efficiency of each subcells need to reach their maximum. In this talk, I will present our recent progress in understanding the issues that limit open circuit voltage in wide bandgap perovskites and solutions to address them.  I will also discuss the progress in stabilize narrow bandgap Sn-based narrow bandgap perovskites based on the understanding of its degradation mechanism.  Finally, I will present the demonstrate of large area perovskite/silicon and perovskite/perovskite minimodules with high efficiency.

Tandem solar cells made of perovskite semiconductors offer unique advantages over existing PV technologies. Specifically exciting are opportunities to integrate high tandem PV into novel form factors and enable integration of PV into new areas. Here, we will give an update on R&D progress at Swift, focusing on developing scalable fabrication methods well suited to all kinds of perovskite tandem solar cell manufacturing. We will discuss the development of flexible perovskite tandems and show some early product prototypes and highlight manufacutirng and stability challenges unique to applications in the consumer electronics space as well as for integration into electric vehicles.

Hybrid halide perovskites are excellent building-blocks for multi-junction architectures, that provide the prospect to overcome fundamental efficiency limits of single-junctions. While perovskite/silicon or all-perovskite tandem cells have shown some remarkable progress, as of yet, perovskite/organic tandem cells show subpar efficiencies of ~20 per cent, limited by the low open circuit voltage of wide-gap perovskite cells, and serious optical/electrical losses introduced by the interconnect between the sub-cells [1].

Organic and perovskite semiconductors share similar processing technologies, which makes them attractive partners in multi-junction architectures. More importantly, the introduction of non-fullerene acceptors has boosted the efficiency of organic solar cells to levels beyond 18 per cent [2].

Here, we demonstrate perovskite/organic tandem cells with an efficiency up to 24 per cent, setting a new milestone for perovskite/organic tandem devices, which now for the first time outperform the most efficient single junction perovskite cells in p-i-n architecture [3].

This achievement draws from progress in all parts of the tandem:

Firstly, our ternary organic sub-cells (PM6:Y6:PC61BM) provide an enhanced efficiency in the near infrared spectral region and complement the perovskite cell. Most strikingly, under the filtered illumination conditions in the tandem, where excitons are solely generated on the Y6 acceptor, they are outstandingly stable even under long-term continuous operation of more than 5000 hours. This result is also in notable contrast to all-perovskite tandems, where stability issues of Sn-based narrow‑gap perovskite cells are a very serious issue.

Secondly, we managed to overcome interfacial losses, that are the predominant factors limiting the performance of wide-gap perovskite cells with high Br content. This allows us to access previously unreached territory of combined high open circuit voltage and fill factor.

Finally, we introduce a novel recombination interconnect for the two sub-cells, that is based on an ultra-thin (~1.5 nanometers) ALD grown indium oxide layer, that shows metallic properties and offers unprecedented low optical and electrical losses.

In an optimistic scenario, we envision that perovskite/organic tandem architectures bear a realistic prospect to reach efficiencies above 31 per cent.

Metal halide perovskite solar cells have gained tremendous attention due to their high power conversion efficiencies and potential for low‐cost manufacturing. Their wide and tunable bandgap makes perovskites ideal candidates for tandem solar cells with narrow bandgap photovoltaic technologies, such as crystalline silicon, Cu(In,Ga)Se₂, and low-bandgap perovskite, to go beyond the efficiency beyond the Shockley–Queisser single-junction limit. The past 5 years have witnessed unprecedented advancements in perovskite-based tandems, with lab-scale devices (up to around 1cm²) approaching or even surpassing efficiency records of their single-junction building blocks. Currently, the majority of the perovskite-based tandems are fabricated on rigid glass substrates. It is highly desirable to develop perovskite tandem solar cells on flexible and lightweight substrates. This would open up the possibilities for high throughput roll-to-roll manufacturing and various attractive applications including the portable and wearable electronics, smart buildings, transportations, etc., where a combination of high efficiency, lightweight, and flexibility are important considerations. In this contribution, we will present our progress on developing highly efficient flexible wide-bandgap perovskite solar cells with high near-infrared transmittance and flexible narrow-bandgap CIGS and Pb-Sn perovskite solar cells. We demonstrate over 22% and 24% in all-perovskite and perovskite-CIGS flexible thin film tandems, respectively.

Organic-inorganic halide perovskites have received widespread attention thanks to their strong light absorption, long carrier diffusion lengths, tunable bandgaps, and low temperature processing. Single-junction perovskite solar cells (PSCs) have achieved a boost of the power conversion efficiency (PCE) from 3.8% to 25.5% in just a decade. With the continuous growth of PCE in single-junction PSCs, exploiting of monolithic all-perovskite tandem solar cells is now an important strategy to go beyond the efficiency available in single-junction PSCs. However, their actual efficiencies today are diminished by the subpar performance of mixed lead-tin narrow-bandgap subcells. We firstly reported a strategy to reduce Sn vacancies in mixed Pb-Sn perovskites that use metallic tin to reduce the Sn to Sn via a comproportionation reaction. We increased thereby the charge carrier diffusion length in narrow-bandgap perovskites from sub-micrometer to 3 micrometers. We further reported simultaneous enhancements in the efficiency, uniformity and stability of narrow-bandgap subcells using strongly-reductive surface-anchoring zwitterionic molecules. The zwitterionic antioxidant inhibits Sn oxidation and passivates defects at the grain surfaces in mixed lead-tin perovskite film, enabling an efficiency of 21.7% (certified 20.7%) for single-junction solar cells. We obtained a certified efficiency of 24.2% in 1-cm -area all-perovskite tandem cells, and in-lab PCEs of 25.6% and 21.4% for 0.049 and 12 cm devices, respectively. We further obtained record-performing, 26.4%-efficient tandem devices by increasing the thickness of narrow-bandgap subcell without sacrificing the electrical performance. We recently also achieved a certified PCE of 21.7% for 20-cm2 all-perovskite tandem solar modules fully using scalable processing techniques. The encapsulated tandem devices retain 88% of their initial performance following 500 hours of operation under one-sun illumination in ambient conditions.

Resource limitations are becoming increasingly important for Terawatt-scale photovoltaics.  Reducing the CO2-emissions from processing, substantially reducing the silver demand and ramping up flat-glass production capacity must become key priorities. Furthermore, high-efficiencies obtainable by perovskite-silicon tandem solar cells are one strong leverage to reduce overall resource demand. Here, we report on record high currents for planar perovskite silicon tandem devices by more transparent TCOs and adapted layer thicknesses.  Using a TCO stack design, soft damage free processing could be combined with low parasitic absorption. We outline how further layer and interface optimization, as well as the application of textured bottom solar cells can lead to over 30% efficiency. We discuss, how usage of critical materials and costs can be reduced. First by the implementation of TOPCon-based bottom solar cells, with a PERC-like rear structure and a silicon tunnel junction front. Second, by the first-time successful demonstration of copper plating on perovskite solar cells.

As the efficiency of 2-terminal PK/Si tandem devices continues to increase towards 30% in the laboratory, the question of manufacturability of these devices must be addressed. This is important as the efficiency gap between what is being currently made in laboratory and what is possible using more scalable methods and materials is a critical issue for the future of this technology. Specifically, the integration of these devices into established PV process flows must be investigated to understand where adaption is possible and where new processes must be made. To this end, we present a proposed process flow for the fabrication of large area 2-terminal PK/Si tandems based on a heterojunction bottom cell. Methods to adapt chemical etch, metallization, and encapsulation are presented. Additionally, a high throughput meniscus coating approach for the perovskite layer will be demonstrated and the integration challenges discussed. Using this process flow, we demonstrate that PK/Si tandem solar cells based on commercially available CZ M2 wafers can be fabrication with efficiencies of over 22% on an active area of 100 cm².

Halide perovskite nanocrystals (NCs) have emerged as an intriguing material for optoelectronic applications, most notably for light-emitting diodes (LEDs), lasers, and solar cells. Their emission wavelength depends not only on material composition but also on size and dimensionality, as in the case of two-dimensional (2D) nanoplatelets (NPLs). These colloidal quantum wells have additional appeal for light emission, as the one-dimensional quantum confinement enhances their radiative rates and enables directional outcoupling. On top of this, due to a monolayer-precise control over their thickness, they constitute an intriguing system for spectroscopic studies on their fundamental optical, phononic, and energetic properties.

In this talk, I will explore our recent results on halide perovskite NPLs, including their synthesis. I will focus on their interesting excitonic properties, such as the energetic fine structure¹ with a strong thickness-dependent bright-dark exciton splitting and on exciton-phonon coupling.² Further, I will highlight their advantages and disadvantages for integration into optoelectronic applications.

The urgency for affordable and reliable detectors for ionizing radiation in medical diagnostics, nuclear control and particle physics is generating growing demand for innovative scintillator devices combining efficient scintillation, fast emission lifetime, high interaction probability with ionizing radiation, as well as mitigated reabsorption to suppress losses in large volume/high-density detectors. Prized for their solution processability, strong light-matter interaction, large electron-hole diffusion length and tunable, intense luminescence at visible wavelengths, lead halide perovskite nanocrystals (LHP-NCs) are attracting growing attention as highly efficient emitters in artificial light sources and as high-Z materials for next generation scintillators and photoconductors for ionizing radiation detection. Nonetheless, several key aspects, such as the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process and the radiation hardness of LHP NCs under high doses of ionizing radiation are still not fully understood, leaving scientists without clear indications of the suitability of LHP-NCs in real world radiation detectors or design strategies for materials optimization. In this talk I will present on our recent strategies for high performance radiation detection schemes and will report recent spectroscopic results of the scintillation process and its competitive phenomena, ultimately offering a possible path to the realization of highly efficient and extremely radiation hard LHP-NCs.

Over the last few years, halide perovskite nanocrystals (NCs) have received great attention in many fields ranging from chemistry to physics and engineering owing to their extremely interesting properties such as defect tolerance, high photoluminescence quantum yield (PLQY), tunable optical bandgap not only by their dimensions but also by composition, enhanced stability compared to thin-film perovskites, and ease of synthesis. In general, ion exchange reactions offer access to NCs with precise chemical compositions that are inaccessible by direct synthetic routes. While anion (halide)-exchange in halide perovskite NCs has been greatly exploited to tune their optical properties, cation exchange has been less explored. This talk will be focused on post-synthetic cation-composition tuning in lead halide perovskite NCs for fine-tuning their optical properties.

Semiconducting lead halide perovskites are excellent candidates for realizing low threshold light amplification due to their tunable and highly efficient luminescence, ease of processing and strong light-matter interactions. Indeed, several solution processable lasers have been demonstrated, even operating under nanosecond and continuous wave optical pumping. All of these examples use perovskite architectures with little to no confinement, for example using bulk films, nanowires or quantum dots (QDs) with average sizes well above the Bohr diameter. As such, there is no clear picture whether the use of perovskites in the strong 3D confinement regime would be beneficial, nor is there any insight in how optical gain would develop under those conditions. Here, we show through a combination of quantitative transient absorption and femtosecond fluorescence spectroscopy, that optical gain in strongly confined perovskite 0-dimensional QDs develops at remarkably low average exciton numbers per nanocrystal (<N> << 1). The gain magnitude is however smaller compared to bulk-like perovskite systems and the lifetime is capped by rapid Auger recombination. We are able to explain our observations using a 3-level model that takes both the strong exciton-exciton repulsion and Stokes shift due to electron-phonon coupling into account of strongly confined QDs. The enhanced repulsions and shifts act as a double edge sword, reducing the gain threshold but decreasing gain magnitude and lifetime. The concepts shown here provide a rational approach to look for low threshold optical gain materials based on an optimal interplay between electronic and vibrational degrees of freedom.

Self-assembly of colloidal nanocrystals into long-range ordered superlattices holds great promise in the multiscale engineering of solid-state materials with controlled and programmed functionalities which result not only from combination and enhancement of size-dependent properties of constituent building blocks but also from synergistic effects and novel interactions between neighboring nanocrystals. Thus far, the reports had mainly focused on single-component and binary systems of spherical NCs, yielding SLs isostructural with the known atomic lattices [1]. Far greater structural space is anticipated from combining nanocrystals of various shapes. Caesium lead halide perovskite nanocrystals, possessing unique optoelectronic properties (narrow-band tunable bright emission, high oscillator strength of bright triplet excitons, slow dephasing) and being synthetically available as uniform, monodisperse cubes, are promising building blocks for creating superlattice structures that exhibit collective optical properties.

We show that broad structural diversity enabling effective tuning of the relative position and orientation of sub-10 nm CsPbBr₃ nanocubes can be achieved by their co-assembly with spherical, truncated cuboid and disk-shaped building blocks into long-range ordered multicomponent superlattices. CsPbBr₃ nanocubes combined with spherical Fe₃O₄ or NaGdF₄ nanocrystals and truncated cuboid PbS nanocrystals form binary SLs of six structure types, namely, NaCl-, AlB₂-, CuAu- as well as uncommon to all-sphere assemblies novel AB₂-, quasi-ternary ABO₃- and ABO₆-types. In these structures, nanocubes preserve orientational coherence. Co-assembly of CsPbBr₃ nanocudes with larger disk-shaped LaF₃ NCs (1.6 nm in thickness) results in the formation of seven columnar and lamellar structures with A₂B, AB, AB₂, AB₄ and AB₆ stoichiometry, not observed before for systems comprising spheres and disks. We rationalize the effect of the cubic shape on assembly outcome using packing-density calculations. In the systems with comparable dimensions of nanocubes (8.6 nm) and nanodisks (6.5–12.5 nm), other, non-columnar structures are observed, such as ReO₃-type SL, featuring intimate intermixing and face-to-face alignment of disks and cubes.

Recently, lead halide perovskites (LHP) have attracted a lot of attention in the field of optoelectronics, inter alia, the application in light emitting devices (LED) is of enormous interest.^[1,2] One promising approach to increase the efficiency is the utilization of nanocrystals (NC) due to their ability of tailoring the ligand shell to specific needs.^[3]

Therefore, the ligand dynamics at the NC surface was examined on soy-lecithin covered LHPs and a highly dynamic surface equilibrium was discovered. It was found that depending on the surface ligand density the lateral diffusion, i.e., the movement of the ligand along the NC surface can be tuned. This behavior can be rationalized by the zwitterionic nature of lecithin and the binding situation at the surface. Precisely, as more ligand is added, less surface area per ligand is available and, thus, the molecules can only attach with one functional group rather than both, which results in significantly faster dynamics.

In a subsequent step, ligand exchange on CsPbBrI₂@SiO_x LHPs was performed utilizing organic semiconductors, namely metal derivatives of mono-carboxy tetraphenylporphyrin (mMTPP). By tethering the novel ligand to the NCs, the electronic conductivity could be increased whilst simultaneously suppressing ionic currents. The underlying phenomenon of this effect was the decrease of the charging energy. Upon examining the NCs in electroluminescent devices it was found that the current efficacy and turn-on voltage was improved by the semiconducting ligand almost fivefold.^[4]

However, an especially important downside of perovskite-based devices is the stability of the material against environmental influences. From the temperature-dependent conductivity of CsPbBrI₂ NCs an increased phase stability was found by introducing mZnTPP as a ligand.^[4] Upon exposing the system to X-ray radiation, we found a radiation-induced, isotropic contraction of the crystal lattice which in turn resulted in an altered electrostatic potential. This could be correlated to a characteristic shift in X-ray photoelectron spectroscopy signals, i.e., the electronic structure. Furthermore, these structural changes could be identified as a very likely driving force for the decomposition, which appears to be ligand dependent. For an oleic acid / oleylamine ligand shell, the decomposition into the corresponding salts was found, whereas the exchanged system exhibited a novel, significantly slower decay in the form of a disproportionation of Pb²⁺ to Pb⁰ and Pb³⁺.

Metal halide perovskites are one of the most investigated semiconductor materials, with the 3D CsPbX₃ structure being renowned for its enhanced optoelectronic performance. Multiple studies are currently attempting to substitute lead with other elements while still retaining the original properties of this material. This effort has led to the fabrication of metal halides of lower dimensionality. Recently, the layered perovskite structures have captured attention for their potential as an emerging class of colloidal semiconductors.

Here we report the colloidal synthesis of the pure Ruddlesden – Popper phase Cs₂CdCl₄:Sb³⁺, using a facile hot injection approach under atmospheric conditions. Through strict adjustment of the synthesis parameters with emphasis on the ligand ratio, we obtained nanoplatelets with a well-defined size and morphology. The particles underwent extensive structural characterization through synchrotron X-ray diffraction, pair distribution function analysis and transmission electron microscopy. Rietveld Refinements assigned a 14/mmm tetragonal space group and confirmed the aforementioned phase. Bright field imaging revealed nanoplatelets with a natural tendency to stack along the ab surface, self-assembling into long chains. Size distribution analysis assigned a length of 22.9 ± 3.8 nm and a thickness of 3.7 ± 0.8 nm, which were consistent with the dimension predictions given by the texture analysis from the x-ray diffraction data [1].

Spectroscopic characterization revealed an intense cyan emission, centered at 510 nm, and with a measured absolute PLQY of 20 ± 5% [1]. The emission is ascribed to the doped Sb³⁺ within the structure and is attributed to an s-p transition that originates from self-trapped excitons from the Sb³⁺ hosts [1][2]. Time-resolved photoluminescence measurements gave a double exponential decay, characterized by a fast (∼1.3 ns) and a long (∼1626 ns) component [1]. The existence of these two components is thought to result from two different recombination routes taking place from the excited Sb³⁺ states. A similar behaviour was previously reported in Sb³⁺ doped double-layered perovskites and is consistent with the findings from the respective bulk phase [2] [3] [4].

This work proves as a confirmation that colloidal synthetic approaches can give access to a new generation of RP phase nanocrystals through halides tuning, metal alloying, the replacement of Cs⁺ by alternative counterions and the introduction of various dopants, such as Bi³⁺ or Mn²⁺.

Despite the increased efforts to replace the Pb in the lead halide perovskites-based solar cells with Sn analogs materials, in the nanocrystal field, limited progress is reported. The intrinsic chemical instability of the Sn²⁺ hinders a clear understanding of the origin of the chemical instability and poor optical performance.

Here we present the Sn halide perovskites nanomaterials with tunable size and dimensionality (3D - Sn halide perovskites: ASnX₃, A=Cs⁺, FA⁺, X=I^-, Br^- and 2D - Ruddlesden-Popper (R-NH₃⁺)₂Cs_n-1Sn_nX_3n+1), and characteristic optical properties. The formation mechanism of the perovskite nanocrystals is based on the pre-formation of the 1D chained of [SnI₆]²- octahedra. Similar to Pb-based perovskites, the stability of the NCs is increased for 10 nm particles while the optical performance remains inferior to the Pb cousins.

The relation between the kinetic parameters during the synthesis processes and the chemical and optical stability of the obtained 3D and 2D perovskites nanostructures will be discussed.

A common strategy for downshifting luminescence is based on a trivalent lanthanide (Ln³⁺) ion as emission center and a sensitizer as light absorption center. Lead halide perovskites can be used as sensitizer to meet the demands for downshifting applications. However, perovskite sensitizers suffer from low visible absorption, poor stability, and toxicity. In this study, we demonstrate a downshifting configuration based on lead-free cesium manganese bromide nanocrystals enabling efficient transfer of high energy absorbed photons to the low energy emission centers in the near-infrared spectral region. For that, we obtained phase-pure CsMnBr₃ and Cs₃MnBr₅ nanocrystals via a controlled synthesis and successfully doped them with Er³⁺, Yb³⁺, Tm³⁺ and Nd³⁺. The correlation of the CsMnBr₃ broadband visible absorption to the lanthanides emission was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements. This work provides a lead-free material as an efficient sensitizer, which can help to develop and design visible to NIR downshifters.

A new synthetic method for colloidal perovskite nanocrystals has been designed, which offers slow thermodynamic control [1] instead of conventional kinetic growth [2].  The reaction time is increased up to 30 minutes while a wide size range of nanoparticles, some even reaching the strong confinement regime, is obtained with high level control of size and shape [1].  The synthesized quantum dots (QDs) turn out to have a spheroidal shape on average with remarkably well-separated higher absorption peaks. For the first time, this allows for a direct comparison between theory and experimental data related to the transitions beyond the lowest absorption line. Using empirical modelling with second-order many body perturbation theory, we are able to predict the energy positions as well as the oscillator strength of not only the lowest 1s-1s exciton but also of the higher excitonic transitions [3]. The calculated values are in fair agreement with the experimental data. Besides, by taking into consideration the spherical and cuboidal confining potentials, our theory offers an explanation for the well-defined higher transitions in the spheroidal QDs compared to cuboidal ones obtaining from more standard synthetic approaches [4]. The accuracy of the theoretical methods will be also critically discussed

Lead-halide perovskite (LHP) thin-films and nanocrystals have advanced to the forefront of materials research for a wide array of applications from solar-cells and related optoelectronic devices, near unity quantum yield light sources, to coherent single-photon emitters for quantum information processing. Electron-phonon coupling (EP-coupling) plays a critical role in LHPs, proposed to both enhance performance metrics in some cases, e.g. ‘polaronic protection’ of charge carriers, and limit them in others, e.g. exciton coherence loss and broadened emission in perovskite nanocrystals. At the root of EP-coupling is a shift of the equilibrium atomic coordinates of the atoms in a material, a lattice reorganization, upon a change of the electronic configuration. While various time-resolved spectroscopies have shed light on the phonons involved, the nature of the lattice reorganization, and therefore the mechanisms underlaying EP-coupling in LHPs, remains unclear. Valuable insight can be provided through physical characterization of the inherent excited-states structural dynamics of these materials. In principle, EP-coupling-driven lattice reorganizations can be directly measured through time resolved diffraction, and NCs are an attractive system to probe these dynamics. Here, we perform femtosecond-resolved optical pump diffraction probe measurements to quantify the suprisingly large lattice reorganization occurring as a result of exciton-phonon coupling to the interband transition in formamidinium-lead-bromide (FAPbBr₃, FA = CH₅N₂) NCs. A variety of Ab Intio techniques coupled with modelling are employed to understand the observed effect. Our findings provide an intuitive explanation for the origin of lower energy optical phonon coupling, and provide insight into both the excited state and equilibrium structure of LHPs.

Two-dimensional (2D) lead halide perovskite nanoplatelets are particularly promising for display light sources and single-photon emitters given the large exciton binding energies and the phenomenon of giant oscillator transition strength expected in these confined systems.  Here, we describe the thickness-dependent level structure and fine structure of excitons in colloidal 2D lead halide perovskite nanoplatelets.  We examine the effects of dielectric and quantum confinement on the exciton binding energy in individual and stacked nanoplatelets while accounting for quantum-confinement-induced effective mass anisotropy. We calculate the exciton fine structure and optical selection rules in an electron-hole exchange model, elucidating the roles of intrinsic crystal field splitting [1] and quantum-confinement-induced Bloch function mixing [2] in conjunction with the effects of shape anisotropy and image charges via the long range exchange interaction.  

Metal halide perovskites have attracted substantial interest due to their accessible fabrication process, promising properties for optoelectronic applications. The doping of cations in lattice of perovskite can therefore endow host many novel properties. However, the post-synthetic treatment of perovskite ABX3 nanocrystals (e.g., FAPbI3 or CsPbCl3) for doping remains elusive. The straightforward doping method for both A and B sites are rarely reported, which hinders the further application of optimized doping materials for optoelectronic devices. Herein, employing hybrid water-oil phase (water-hexane), we have partially replaced of Cs or Pb with FA and Mn by similar method for the first time, respectively. Taking advantage of in-situ photoluminescence tracking, we have distinguished the doping process and proposed a model to provide better understanding for the doping process. What’s more, this method could not only be use for doping, but also for synthesis of new materials. This study hence provides a controllable way for doping metal halide perovskites or even synthesis new materials to meet more complexed and specific requirements for different applications.

Lead (Pb) halide based perovskites nanocrystals show outstanding optoelectronic properties. But Pb-toxicity remains an issue for their widespread application. Therefore, search for Pb-free metal halide perovskites is natural. The fundamental question is how to identify alternative metal halides that could replicate optoelectronic properties of the Pb-halide perovskites.[1,2] I will address this question in the first part of my talk. Unfortunately, till date, Pb-free perovskites could not compete with optoelectronic properties Pb-halide perovskites. Why so? This question will also be discussed in the talk. Finally, in the last 3-4 years, we changed the goal post of our research. Instead of attempting to replicate the optoelectronic properties of Pb-halide perovskites, we started to explore newer directions. For example, we dope lanthanides like Yb³⁺ and Er³⁺ in Cs₂AgInCl₆ double perovskites to develop short-wave-infrared (SWIR) light emitting materials.[3] Furthermore, doping and/or codoping ions like Bi³⁺ and Sb³⁺ (that has two electrons in the outermost s-orbiatl) tailor both optical excitation and emission in the UV-visible region.[4,5] In the last part of my talk, I will discuss about such exploration.  

Magnetic doping in halide perovskite semiconductors is of timely interest with prospective for their use in opto-electronic and spin-related devices. Here, we report a thorough investigation of the optical and magneto-optical properties of Ni2+-doped caesium lead halide perovskite with a chemical formula CsPb(Br1-xClx)3, implementing steady-state and transient polarized photoluminescence (PL), and optically detected magnetic resonance (ODMR) spectroscopies. ODMR measurements probe magnetic resonance transitions of photo-generated electrons and holes in these emissive excited states, exposing phenomenological g-factors that deviate from those of band-edge charge carriers. Simulations of the ODMR spectra suggest carrier trapping in shallow traps with a slight anisotropic surrounding and with weak electron-hole exchange coupling. Furthermore, we observe substantial broadening of the hole resonance, associated with magnetic exchange coupling between Ni2+ unpaired spins and the trapped hole spin. Overall, these ODMR measurements uncover the role of the dopant in localizing photo-generated carriers while stiffening the crystal structure, and for the first time, provide a direct observation of carrier-dopant spin exchange interactions in metal-halide perovskite nanocrystals. These results offer insight into the influence of magnetic dopants on the electronic structures of metal-halide perovskites, with a view toward emerging spin-based devices made from perovskites.

Despite extensive reports on red and green perovskite-based LEDs (PeLEDs), development of white PeLEDs remains limited by the low photoluminescence quantum yield of white-emitting perovskites and the undesired energy-transfer (ET) process occurring in multidomain Ruddlesden–Popper perovskites. While ET is beneficial for achieving efficient monochromatic emissions, the broadband spectrum required for white electroluminescence makes this phenomenon undesirable. Processing-induced physical separation of emitters has been proposed as an effective way to curb ET. Here, it is shown that by adopting a bilayered emitter configuration, achieved through a facile antisolvent-assisted spin-coating process, an increase in spatial separation between the blue perovskite and red emitting organic species employed can be realized. This, in turn, has allowed for effective reduction of ET efficiency, leading to a record efficiency of 1.3%, the highest achieved to date from a perovskite-based white electroluminescent device.

The unique properties of lead halide perovskite nanocrystals (NCs) have drawn major attention of researchers over the last decade. However, the high toxicity of the lead in the composition and the relatively low phase stability are delaying their commercialization, therefore, diminishing their advantages [1]. These challenges are one of the major focuses of the community leading to the ongoing search for new stable lead-free compositions. However, up to date, there are only a few promising candidates to surpass the characteristics of lead halide perovskite NCs [2].

So far, no attention to cesium manganese chloride (CsMnCl₃) NCs was given likely due to the absence of promising properties [3]. We are the first ones to synthesize and characterize emissive colloidal CsMnCl₃ NCs through the conventional hot-injection route. The optical, structural, and morphological properties of CsMnCl₃ NCs were studied by the means of steady-state-, time-resolved, and low-temperature spectroscopy, transmission electron microscopy (TEM), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray diffractometry (XRD) respectively. The synthesized CsMnCl₃ NCs display  photoluminescence at 670 nm, large Stokes shift (250 nm), broad emission (FWHM = 97 nm), lifetime decay in the millisecond range, and fairly high quantum yield of 25%.

Moreover, Rietveld structural refinement was carried out to confirm the adopted structure of the NCs and investigate the relationship between synthesis conditions, crystal structure, and photoluminescence. It was found that when the synthesis temperature was tuned, the CsMnCl3 adopt different crystal structures (i.e. cubic and rhombohedral). By optimizing the temperature and the precursors' ratio, a pure rhombohedral structure is formed. It is worth nothing, that up to date, there is no work showing the luminescent rhombohedral CsMnCl₃ as well as the dependence between the synthesis conditions, structure, and optical properties.

In addition, the synthesized NCs show moderate phase and optical stability (more than 2 weeks) under the ambient conditions (dark, RH 40-60%, RT). Together with its unique optical properties, CsMnCl₃ is a promising candidate to substitute lead-halide perovskite NCs. The results of our work provide important insights into the photochemistry of manganese-based materials.

Abstract:

The fast rising demand for the ultralow detection limit of ionizing radiation in medical radiography, high-energy physics and security screening, has led to extensive research on X-ray imaging scintillators and detections. However, high-performance scintillators consist mainly of ceramic that needs harsh and costly preparation conditions. Therefore, searching for new scintillation materials is of great interest to material scientists, chemists, and engineers. Organic emitters and perovskites, are excellent candidates as scintillation materials due to their good processability and low-fabrication cost. In this talk, we will present the room-temperature synthesis of a colloidal scintillator comprising CsPbBr3 nanosheets of large concentration (up to 150 mg/mL). We found that the CsPbBr3 colloid exhibits a light yield (∼21000 photons/MeV) higher than that of the commercially available Ce:LuAG single-crystal scintillator (∼18000 photons/MeV). Interestingly, we reveal that the energy transfer process inside those stacked thin and thick nanosheet solids is indeed responsible for their superb scintillation performance. Moreover, we will present a highly efficient energy transfer strategy between the interfaces of these CsPbBr3 perovskite nanosheets and thermally activated delayed fluorescence (TADF) to obtain an efficient and reabsorption-free organic X-ray imaging scintillator with excellent performance. More specifically, the fabricated nanocomposite scintillators exhibit a high X-ray imaging resolution of around 100 μm and a low detection limit of 38.7 nGy/s. This detection limit is about 142 times lower than a typical dose of X-ray medical imaging, making this composite an excellent candidate for X-ray radiography.

Lead halide perovskites (LHP) are long-known crystalline materials with ABX3 general formula (where A=Cs+, MA:CH3NH3+ or FA:CH(NH2)2+, B=Pb2+ and X=Cl-,Br-,I-), characterized by a three-dimensional [PbX6]4- framework and a large A cation residing the cuboctahedra cavities. These materials in form of nanocrystals (NCs) are considered ideal candidates to be integrated in television displays and LEDs.[1] Due to the dynamic nature of the perovskite lattice, preventing the charge carriers from trapping, LHP NCs are highly tolerant to structural defects and surface states, which are considered benign with respect to their electronic and optical properties.[2]

The flexible nature of the perovskite framework, very prone to structural defectiveness, coupled with the reduced size of crystalline domains, makes these materials unsuitable for conventional crystallographic methods. At this purpose total scattering techniques based on the Debye Scattering Equation (DSE), have been established as effective methods for characterizing nanoscale materials and taking into account size-induced structural defects, emerging upon downsizing.[3] Through the DSE-based method developed by some of us,[4] starting from real space atomistic models, structural and microstructural information on NCs can be simultaneously derived within a unified approach, with all the well-known advantages associated to the use of reciprocal space methods.

In this talk, experimental and modelling aspects, related to the DSE approach and applied to provide atomic-to-nanometer scale insights on key nanoscale features of LHP NCs, will be presented.

A broad spectrum of structural and morphological features of LHP NCs, unveiled through a synergic combination of reciprocal space methods based on the DSE, will be analysed, from their peculiar defectiveness,[5] to faceting and surface termination,[6] to the formation of self-organized superstructures.

Lead-halide perovskite nanocrystals (NCs) have emerged as attractive nano-building blocks for photovoltaics and optoelectronic devices. Optimization of perovskite NC-based devices relies on a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. This talk will give an overview of our recent magneto-optical spectroscopic studies [1-5] revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of the inhomogeneities in the NC morphologies and crystal structures. It will highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of the thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton-phonon coupling, which sheds light on the remarkable photovoltaic properties of these materials and provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. These findings make single perovskite NCs attractive for a potential use as quantum light sources.

- Fu, M ; Tamarat, P. ; Huang, H. ; Even, J. ; Rogach, A. L. ; Lounis, B. Neutral and charged exciton fine structure in single lead halide perovskite nanocrystals revealed by magneto-optical spectroscopy, Nano Letters (2017) 17, 2895-2901

10.1021/acs.nanolett.7b00064

- Fu, M ; Tamarat, P. ; Trebbia, J.-B. ; Bodnarchuk, M. I. ; Kovalenko, M. V. ; Even, J. ; Lounis, B. Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons, Nature Communications (2018), 9, 3318

10.1038/s41467-018-05876-0

- Tamarat, P. ; Bodnarchuk, M. I. ; Trebbia, J.-B. ; Erni, R. ; Kovalenko, M. V. ; Even, J. ; Lounis, B. The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state, Nature Materials (2019) 18, 717-724.

10.1038/s41563-019-0364-x

- Tamarat, P. ; Hou, L. ; Bodnarchuk, M. I. ; Trebbia, Swarnkar, A. ; Biadala, L. ; Louyer, Y. ; Bodnarchuk, M. I. ; Kovalenko, M. V. ; Even, J. ; Lounis, B. The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals, Nature Communications (2020) 11, 6001.

10.1038/s41467-020-19740-7

- Hou, L. ; Tamarat, P. ; Lounis, B. Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals, Nanomaterials (2021) 11, 1058.

10.3390/nano11041058

Perovskite photovoltaics based on metal halide perovskites is a most promising photovoltaic
technology, but further advance demands improved device stability. Here we show the importance of
perovskite composition engineering by demonstrating long-term stable perovskite solar cells. A high-
throughput robotic approach is used to screen 160 mixed-cation mixed-halide perovskites based on
several optical characterizations, such as UV-vis absorption and photoluminescence spectra. Such
automated big data approaches allow to uniquely identify the most photo-thermal-stable perovskites
under elevated temperature and 1-Sun illumination. Most interestingly, while several perovskite
compositions are found to be stable against high temperatures of up to 140 C and are capable of
passing the damp/heat test without being packaged, the situation is more complex in working devices,
as a stable device requires both - a stable semiconductor layer and stable interfacesi. A p-type interface
consisting of polymeric multi-layers with variable doping is introduced as a robust anode interfaceii.
By integrating the most stable perovskite into this structure, we achieve stable devices that attain
about 100% of their initial efficiency after > 1000 hrs of continuous mpp operation at low
temperatures of 30-40 degrees Celsius. Increasing the temperature to beyond 65 °C caused interface
corrosion, as clearly seen by the formation of s-shaped jV curves. Meticulous engineering of the
dopand / host interactions allowed to overcome this instability, and most recent devices pass up to
2000 hrs under 1 sun @ 65 C without notable degradationiii. Nevertheless, degradation is still
observable when the operation temperature is further raised towards 85 C and above, which
highlights that the microscopic degradation mechanisms in perovskite devices are still not fully
understood. This work introduces into the fundamentals how to accelerate the screening for stable
perovskites layer and devices for operation at elevated temperatures.

REFERENCES:

i Y. Zhang et al and C. J. Brabec , Nature Communication 2021.
ii Y. Hou et al and C. J. Brabec, Science 2017.
iii Y. Zhang et al and C. J. Brabec, Nature Energy 2021, in print.

Engineering interfaces in perovskite solar cells is nowadays paramount in the optimization of multilayer perovskite device stack. This stem true for multi-dimensional (2D/3D) perovskite based solar cells, where high efficiency can be combined with promising device durability. However, the exact function of the 2D/3D interface in controlling the device behaviour and the interface physics therein are still vague.

Here I will discuss the 2D/3D functions which can simultaneously act as surface passivant, electron blocking layer, and driving efficient and selective charge extraction. In particular, I will demonstrate that the exact knowledge on the interface energetics is crucial to obtain for a smart interface engineering. As an example, I will discuss the case of thiophene-based 2D perovskite/ 3D perovskite interfaces forming a p-n junction. This leads to a reduction of the electron density at the hole transport layer interface and ultimately suppress the interfacial recombination. As a consequence, we demonstrate that photovoltaic devices with enhanced fill factor (FF) and open-circuit voltage (VOC) of 1.19V which approaches the potential internal Quasi-Fermi Level Splitting (QFLS) voltage of the perovskite absorber, nullifying the interfacial losses. We thus identify the essential parameters and energetic alignment scenario required for 2D/3D perovskite systems in order to surpass the current limitations of hybrid perovskite solar cell performances. This knowledge turns fundamental for device design, opening a new avenue for perovskite interface optimization. In addition, I will discuss the role of 2D cations versus 2D cation passivation to state the exact role of having a 2D perovskite layer on top if necessary – or maybe not – to push device performances.

The halide perovskite photovoltaic technology can be scaled to large area modules and panels using printing processes and laser patterning. Here, we will present the progresses made to scale up from small area solar cells to modules and panels up to a dimension of 0.5 sqm. By working in controlled atmosphere (Glove Box with nitrogen) and apply conventional spin-coating technique it Is possible to easily scale up from 9 to 140 cm² active area with efficiencies above 20% for the smallest modules and 14.7% for the largest. Specific efforts have been devoted to developing a deposition process out of the glove box (GB) in conventional ambient air. We transfer out of the GB several coating technologies, including blade coating and slot-die. To do this without penalizing efficiency and stability, a specific formulation of perovskite absorber and doping strategies of transporting layer have been formulated together with specific quencing techniques based on air, vacuum and solvents.  These optimizations permitted to realized perovskite solar modules with an efficiency of > 17% on an active area of 43 cm², keeping above 90% of the initial efficiency after 800 h thermal stress at 85 °C.  One of the critical issues scaling the cell to module size is the control of interface properties. We demonstrated that  tuning of interface properties can be successfully obtained by applying two-dimensional (2D) materials, such as graphene, functionalized MoS₂, MXenes as well as 2D Perovskite.  This permitted also to increase the stability of the cell (T80) well beyond 1000h under light soaking and thermal stress tests.

The discovery of a family of semiconductor materials based on the complex halides of group 14 elements opened a big research arena and led to the emergence of a new perovskite photovoltaic technology. Impressive photovoltaic performances were demonstrated for perovskite solar cells based on lead halides, whereas their practical implementation is still severely impeded by the low device operational stability. Most importantly, complex lead halides were found sensitive to both light and heat, which are unavoidable satellites under the realistic solar cell operational conditions. Suppressing these intrinsic degradation pathways requires a thorough understanding of their mechanistic aspects. Herein, we explored the temperature effects in the light-induced decomposition of the model systems represented by MAPbI3 and PbI2 thin films under well-controlled anoxic conditions. We show that decreasing the sample temperature from 55 oC to 30 oC can extend the perovskite lifetime spectacularly by a factor of >10-100 and also alter the material decomposition pathway. The analysis of the aging kinetics revealed that MAPbI3 and PbI2 photolysis have quite high effective activation energies of ~85 and ~106 kJ mol-1, respectively, which explain the observed strong effect of the temperature on the rate of the material photodecomposition. These findings suggest that controlling the temperature of the perovskite solar panels might be a key factor for reaching their long operational lifetimes (>20 years) required for the practical implementation of this promising technology.

Despite the tremendously rapid development of perovskite photovoltaics (PV) in terms of power conversion efficiency (PCE) the stability of such devices typically still does not fulfill the requirements for commercialization of this technology. Up to date, PV devices with carbon-based back-electrodes (CBEs) have demonstrated the longest operational stability, surpassing several IEC 61215 standards [1]. Moreover, recently we reported that such modules could withstand requirements of the IEC hot-spot test (evaluating the stability of PV devices against reverse-bias degradation),[2] which is one of the most crucial hurdles not only for perovskite PV, but even for such well-established PV technology as c-Si. [3] Thus, devices with CBEs represent a promising method for up-scalable manufacturing of stable and low-cost perovskite PV devices.[4,5]

However, besides cost and stability, we must also consider greenhouse gas (GHG) emissions coming from PV manufacturing, which are projected to grow beyond national emissions of countries like France or Germany in the next 10 years.[6] Although the production of perovskite PV with CBEs has the potential to reach the lowest CO₂-footprint limit possible for large scale PV applications,[4] one of the methods to reduce the GHG emissions of PV industry further is the development of effective recycling strategies, where perovskite materials offer unique advantage of liquid processing.

Up to now, only studies on the recyclability and life-cycle-assessments of non-encapsulated devices have been demonstrated. To show that perovskite PV devices with CBEs can not only be stable but also recyclable, we manufactured such solar cells and modules, encapsulated with thermoplastic polyolefin and polyisobutylene edge-seal, which provide sufficient protection against moisture, passing the IEC damp-heat test. [7] Through life-cycle assessment, we show that most of the negative environmental impact comes from the layer deposition, rather than the environmental footprint of material itself, making the re-use of as many layers as possible the most preferable option. We demonstrate an effective mechanochemical method to remove the edge-seal and encapsulant, as well as degraded perovskite and CBEs, leaving metal oxide layers intact making them available for re-use. Our novel recycling method of such devices results in minimal performance loss, which helps to reduce the negative environmental impact of such devices (global warming potential in kg CO₂-eq./kW_p) by > 40%. Such strong reduction in the global-warming potential of perovskite solar modules with CBEs qualifies them as truly sustainable PV technology meeting all requirements for its introduction to the PV market.

Organic-inorganic perovskites have the strong potential to replace the mainstream silicon photovoltaic technology due to their extensive research. The power conversion efficiency (PCE) of perovskite solar cells of just below 26% is advancing to that of silicon photovoltaics. The advancements make it a highly viable technology for broad applications especially building-integrated PVs and Tandems. Nevertheless, the efficiency of these cells is recorded on a small area. Scaling the small area cells into modules with minimal loss is a major challenge to be addressed. We will here, present our work on the optimization strategies of the semitransparent modules to achieve a power conversion efficiency slightly below 15% scaled from the semitransparent cell of 19.7%. The transparent electrode  plays a major role in the performance of the semitransparent module. Our work addresses the challenges in reducing the loss in PCE between small area cells and modules.

Deposition of two-dimensional perovskite layers between perovskite absorber and hole-transporting layer is considered to be an essential strategy to reduce defects in state-of-the-art perovskite solar cells (PSCs). This strategy, however, suffers from the inevitable formation of in-plane favoured two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up. Therefore, to overcome this limitation, we designed and investigated various cations that form 2D perovskite layers on top of the 3-dimensional perovskite layer. The ensuing PSCs achieve an efficiency of over 24% with long-term operational stability (over 1000 hours). Notably, a record efficiency of 23% for the perovskite module with an active area of 26 cm2 was achieved using compositionally engineered perovskite. In this talk, we will present strategies to enhance PSC's power conversion efficiency and stability.

Halide perovskite solar cells (PSCs) have emerged as a competitive photovoltaic technology with power conversion efficiencies (PCEs) surpassing the 22 % mark. One of the main bottlenecks of the technology is their long-term stability. Understanding the different degradation mechanisms of the constituent materials, as well as interface instabilities, is of crucial importance for commercialization. Semiconductor oxides (SO) constitute a fundamental part of highly efficient photovoltaic technologies such as PSCs. Electron transport semiconductor oxides, like TiO₂, are characterized by an oxygen vacancy (O_vac)-mediated conductivity caused by a deviation in stoichiometry, the presence of impurities, or both. In oxygen-containing atmospheres, and especially under UV light, holes generated at the nonstoichiometric oxide surface react with the oxygen adsorbed at an O_vac increasing charge recombination and degradation of the solar cell. Different methods have been employed to passivate or eliminate these O_vac. For example, the application of organic interfacial modifiers with anchoring groups specifically selected to bond with oxides, or the application of less reactive SnO₂ which results in less hygroscopicity, fewer O_vac at its surface, and less UV-damage. Another possibility is the application of a coating of secondary oxides, like Al₂O_3, applied to supress surface defects, avoid interfacial recombination, and enhance device stability. A less-explored option is the application of complex oxides with singular properties, such as ferroelectric, multiferroic, magnetic, etc. In this talk, we report our most recent studies on the application of classic oxides (binary, doped, nanostructured) and complex oxide compounds (ternary, ferroelectric, etc.) as transport layers in Halide Perovskite Solar Cells. We will discuss their effect on the long-term stability of complete solar cell devices.   

Since the emergence of perovskite photovoltaics in 2009 the scientific community has witnessed steep progress of the technology in terms of solar to electric power conversion efficiency 25.5% accompanied by advancement in addressing stability issues and developing a deeper understanding of fundamental structure-processing-property relationships.

The technology transfer from lab to fab has proven challenging, however merely a decade after discovery the commercialization of organic-inorganic metal halide perovskites in terms of 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. Thus, a cost-effective, reliable fabrication process capable of delivering highly efficient and durable large-area perovskite modules is vital for success of the technology. 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 commercial utility. Our perovskite solar cell solutions offer high power density at low light intensity conditions and at the same time exhibit long lifetime to guarantee autonomous operation for IoT devices far beyond typical battery lifetime. It has been a long but exciting way from the first laboratory samples to the first commercial perovskite solar cell production line. The first applications of hybrid perovskite solar cells will include powering IoT devices and many more will follow soon.

Perovskite solar cells are still improving their high performance in J-V characteristics in various compositions of perovskites including all inorganic materials. The most important element in the J-V performance is open-circuit voltage (Voc) which reflects suppression of charge recombination losses at the interfaces. We achieved high levels of Voc for all inorganic CsPbI₂Br (bandgap 1.9eV) with 1.42 V ¹ and for mixed cation Cs-FA-MAPb(I,Br)₃ perovskite cells (bandgap 1.51eV) with 1.19V.² The CsPbI₂Br device was fabricated by using amorphous SnO_x layer that passivate the perovskite and meso-SnO₂ interface, and enables Voc exceeding 1.1V even under week indoor illumination (200 lx) with power conversion efficiency (PCE) >34%. The mixed cation perovskite cells, working with conversion efficiency >22%, shows Voc close to its SQ limit (ca.1.21V) as a result of interfacial modification with phenyethylamine bromide as a dipole-inducing layer. ² These works show that the successful passivation of the junction interfaces is essential for improving Voc toward application of perovskite photovoltaic cells to circumstances of large light intensity variation. Technologies based on interfacial molecular engineering are expected to improve the device performance of lightweight flexible plastic devices. We employed an organic peroxide, artemisinin (water-insoluble anti-malarial drug), to modify the interface of SnO₂ ETL and multi-cation perovskite (Rb-Cs-FA-MA)Pb(I,Br)₃).³ This redox-active molecule is considered to interreact with the oxide surface and lead cation. The artemisinin-based passivation was applied to the SnO₂-perovskite interface of a plastic film flexible device. The device showed decreased hygroscopicity due to the effect of artemisinin, leading to an increase in shelf life. Its effects of enhancing Voc improved the device efficiency up to 21%. Organic passivators should be chemically and thermally stable non-ionic materials and inactive against their diffusion. Our challenges to design durable passivation materials for Voc improvement are underway.

Ion migration is a well-known problem in perovskite materials. It causes baseline drift, lowers imaging resolution, accelerated decomposition and device performance degradation. In particular in X-ray detectors, the effect of ion migration is more obvious under working bias. The first principals study reveals that the 0D structure perovskite would show effectively reduced ion migration between neighbouring unit cells compared with the popular 2D and 3D perovskites. A nucleation-controlled strategy is developed to grow superior inch-sized high-quality 0D-structured lead-free (CH3NH3)3Bi2I9 perovskite single crystals (MA3Bi2I9 PSCs) with significantly lower ion migration, much reduced dark current and better environmental stability compared to other perovskite materials, enabling us to design and fabricate a new type of 0D-structured lead-free perovskite X-ray detector. It is found that the X-ray detectors show surprisingly high sensitivity, 15 times more than that of the state-of-the-art commercial α-Se detectors, with very low detection limit that is desired for medical diagnostics, material inspection, etc. Furthermore, their response time is as short as 0.98 ms, the shortest among all X-ray detectors reported in literature, which may allow us to develop an X-ray screening system with reduced X-ray dose and improved resolution.

The emergence of metal halide perovskites has revolutionized the field of emerging photovoltaics. Typically, the active layers of perovskite solar cells are deposited either from solution, or alternatively by thermal evaporation. In this talk, I will describe how the two methods can be combined to fabricate highly efficient all-inorganic CsPbI3 perovskite solar cells. Specifically, half of the active layer is deposited by solution processing, following by thermally evaporating the second half. While devices fabricated by each method separately show a reasonable performance of 13-15%, the combination of the two methods leads to perovskite solar cells with improved open-circuit voltage, short-circuit current and fill factor, leading to a maximum photovoltaic performance >20%. Moreover, devices fabricated by this hybrid approach exhibit a significantly reduced hysteresis. We show that the improvement in performance and decrease in hysteresis are associated with a reduced density of defects in the active layers fabricated by the hybrid approach, thus demonstrating its high potential for the fabrication of efficient perovskite solar cells.

Hybrid organic-inorganic perovskites are highly promising absorber materials for third generation photovoltaic technologies. These materials have low production costs and enable solar cell efficiencies comparable to the best crystalline silicon PV cells [1]. The main obstacle for their widespread practical implementation is the low operational stability defined by both external and internal factors. External factors such as the action of oxygen and moisture from the atmosphere can be suppressed by encapsulating the photoactive layer. Intrinsic factors causing degradation of the materials include thermal and photochemical aging effects. In particular, the light-induced decomposition of MAPbI₃ leads to the formation of metallic lead and molecular iodine due to photolysis of PbI₂ [2]. 
Although the degradation mechanisms are usually considered as a bulk property, they are more likely to occur at the scale of individual grains and between the boundaries. Along with this, infrared scattering scanning near-field microscopy (IR s-SNOM) techniques are being actively developed to visualize the cation dynamics in lead halide perovskites [3]. Since changes in the concentration and localization of organic cations have a strong influence on the material optoelectronic characteristics and the solar cells performance, the IR s-SNOM provides great opportunities for experimental investigation of the photochemical degradation processes in these materials with nanoscale spatial resolution. Herein, we applied IR s-SNOM technique for the first time to follow the nanoscale dynamics of the light-induced decomposition of MAPbI₃ films under well-controlled anoxic conditions inside the glove box.
The obtained results revealed that the light-induced aging of the MAPbI₃ films has a spatially heterogeneous character. The perovskite decomposition is started at the grain boundaries and is accompanied by the formation of core-shell structures, where the perovskite grains are covered with a “skin” of PbI₂, which is the MAPbI₃ main aging product. It is shown that lead iodide does not decompose further into metallic lead and molecular iodine under the used experimental conditions (low temperature, LED light). On the contrary, PbI₂ recrystallization with the formation of big flat crystallites is observed. 
To summarize, we demonstrated a high potential of using IR s-SNOM as a method to study the photodegradation of perovskite films with nanoscale local spatial resolution. The application of this technique in combination with other complementary methods represents a promising approach for identifying the most stable absorber material compositions.

Long-term operational stability is a prerequisite for the commercialization of perovskite solar cells. Inorganic perovskite solar cells exhibit a high thermal stability and efficiencies of 20.8 %, which corresponds to 72 % of the Shockley-Queisser limit for the band gap of 1.72 eV [1]. However, the reported PCE is still lower than for organic-inorganic perovskite solar cells, mainly due to lower open-circuit voltages (V_OC). The V_OC of the record inorganic perovskite solar cell with CsPbI_3-xBr_x composition and 1.72 eV band gap is 1.23 V, while organic-inorganic cells of the same band gap reach a V_OC of 1.31 V [2].

This study investigates state-of-the-art inorganic CsPbI₂Br perovskite solar cells to reveal the efficiency potential and the losses induced by each layer. This is achieved by measuring intensity-dependent photoluminescence (PL) of each layer stack. This technique enables us to construct potential JV curves, revealing not only the potential V_OC, but also the potential fill factor [3]. Based on realistic assumptions of the photocurrent, we can therefore calculate the efficiency potential of the solar cell or stack which could be reached by avoiding any transport losses.

For the present study, CsPbI₂Br solar cells in n-i-p and p-i-n configuration are fabricated using an air-annealing procedure specifically developed for this composition. The CsPbI₂Br films have an efficiency potential of 21.6 % when deposited on glass, 20.7 % on MeO-2PACz and 19.9 % on mesoporous TiO₂, suggesting a very low defect density in the CsPbI₂Br perovskite.

In the n-i-p CsPbI₂Br solar cells, losses in QFLS are caused in almost equal parts by the electron transport layer (ETL) and hole transport layer (HTL). In the p-i-n solar cells, the main part of the losses is caused by the ETL. Importantly, both configurations show a strong mismatch between the quasi Fermi level splitting (QFLS) in the perovskite layer and the measured V_OC at the contacts. The inorganic p-i-n solar cell has a QFLS-e∙V_OC mismatch of 170 meV. The organic-inorganic reference solar cell with the same band gap shows a QFLS- e∙V_OC mismatch of only 10 meV. Using ultraviolet photoelectron spectroscopy (UPS) measurements, we examine possible reasons for this striking difference and discuss potential solutions.

Overall, this study helps to identify the most promising layer stacks for inorganic n-i-p and p-i-n perovskite solar cells and reveals the contribution of each interface to the voltage and fill factor losses.

APbX₃ lead perovskites, where A is either an organic (methylammonium MA⁺ and formamidinium FA⁺) or an inorganic (Cs⁺) species, are a highly promising photovoltaic materials, due to the exceptionally high power conversion efficiency (> 25%) demonstrated recently in the case of mixed-cation and mixed-halide perovskites. However, the poor intrinsic stability of complex lead halides remains a major hindrance in the commercialization of this emerging photovoltaic technology. Experimentally it is known, and has been demonstrated by our group that bromide-containing mixed halide perovskites have much lower photostability when compared to the equivalent iodide-based materials. The light-induced photochemical aging produces metallic lead as one of the final decomposition products in the case of all the experimentally studied complex lead halides (MA_0.15FA_0.85PbI_2.55Br_0.45, Cs_0.1MA_0.15FA_0.75PbI_2.55Br_0.45, Cs_0.15FA_0.85PbI_2.55Br_0.45, MA_0.15FA_0.85PbI₃, Cs_0.1MA_0.15FA_0.75PbI₃ )  except for Cs_0.15FA_0.85PbI₃, which demonstrated outstanding stability under white light exposure. To check the effect of the halide substitution our theoretical calculations compared FAPbI₃ with the bromide-containing mixed-halide perovskite FAPbBr_0.45I_2.55 and indicated that hole-coupling drives the formation of interstitial-vacancy halide pair defects to become more thermodynamically favorable, thus leading to the accelerated degradation of the halide-mixed perovskites. It is notable that oxidation of this type is possible also without illumination; however, our calculations demonstrate that illumination greatly promotes it. Our calculations also showed the relative insensitivity of the halide vacancy towards the composition of the perovskite.

Perovskite solar cells (PSCs) have achieved tremendous progress in terms of power conversion efficiencies (PCE) during the last decade which features a high commercialization potential of this PV technology. To achieve high PCE, significant efforts have been made to design and investigate new hole-transport layer (HTL) materials. Along with the spiro-OMeTAD, polytriarylamines (PTAAs) represent one of the most commonly used families of HTL materials [1]. However, even the most promising HTL materials delivering high PCEs will not bring PSCs towards commercialization if their high cost is not taken into consideration [2]. Unfortunately, both spiro-OMeTAD and PTAA are prohibitively expensive materials with the current price of ~500 and ~2000 $/g respectively [3].

Spiro-OMeTAD is expensive due to its multistep synthesis that requires low temperature, sensitive and aggressive reagents, and costly sublimation purification. The high cost of PTAA is driven by both complexity of the utilized synthetic methods, which often require several synthetic steps, and the high cost of metal-based catalysts, including noble metal complexes. For example, Pd-catalyzed Suzuki reaction has been actively utilized as the method of polycondensation of triarylamines with halogen and boron-based functional groups [4]. Unstable and pricey Ni(COD)₂ has also been utilized for the polymerization of brominated triarylamines. Although the oxidative polymerization of arylamines has been actively utilized for the synthesis of materials for electroluminescent devices [5], the application of this approach for the synthesis of HTL materials for perovskite solar cells remains still unexplored.

Herein, we report a straightforward synthesis of a series of polytriarylamines using an efficient and cheap FeCl₃-assisted oxidative polymerization. This simple yet efficient synthetic approach allowed us to obtain a series of polytriarylamines. These polymers outperformed commercial PTAA, which was utilized as a reference, when used as HTL materials in MAPbI₃-based n-i-p perovskite solar cells. In particular, reproducible PCEs of >18% were reached for ITO|SnO₂|PCBA|MAPbI₃|HTL|VO_x|Ag device configuration.

This work was supported by Russian Science Foundation (project 19-73-30020).

Metal-halide perovskites have recently emerged as one of the most promising low-cost promising materials for photovoltaic and light-emitting technologies, due to their excellent optoelectronic properties and fabrication versatility. Since the advent of the first perovskite solar cells (PSCs) in 2009, their power conversion efficiency (PCE) has now reached 25.6% [1], for active areas smaller than 1 cm². Moreover, their operational stability is also constantly improving [2-3]. The interest in transferring the existing technology into large-area perovskite modules using industrial compatible techniques, necessary for industrial development is exploding.

Lately, we have demonstrated highly efficient, large area, planar PSCs where the MAPbI₃ perovskite layer has been deposited by thermal co-evaporation of PbI₂ and MAI. The high-quality co-evaporated perovskite thin films are uniform over large areas showing low surface roughness, and a long carrier lifetime. The high-quality perovskite thin films together with vacuum processed charge transport layers PSCs with PCE above 20% in both n.i.p [4, 5] and p.i.n [6] configurations. The co-evaporated MAPbI₃ guarantee an impressive thermal and environmental stability maintaining over ≈95% and ≈80% of their initial PCE after 1000 and 3600 h respectively under continuous thermal aging at 85 °C without encapsulation. TE_MAPbI₃ PSCs demonstrate remarkable structural robustness, absence of pinholes, or significant variation in grain sizes, and intact interfaces with the HTM, upon prolonged thermal aging.

The first co-evaporated mini-modules achieved record PCEs of 18.13% and 18.4% for active areas of 20 cm² [7] and 6.4 cm² [8].

Moreover, looking forward to tandem integration and building-integrated photovoltaics we have also developed coloured semi-transparent PSCs and mini-modules for all the range of colours realized.

These results represent a significant step towards the development of high-quality large-area perovskite solar cells and mini-modules, one of the main requirements for the commercialization of the technology.

Mixed metal-halide perovskite solar cells (PSC) are advancing in power conversion efficiency (PCE) faster than any other photovoltaic technologies, reaching over 25% [1]. However, the reported record efficiencies are obtained with lab-scale devices. Promoting this technology to the market level with efficient and stable devices manufactured with cost-effective, industrially scalable processes is the persisting challenge. Here we report on bifacial PIN perovskite photovoltaic modules processed on glass,  passing the 1000 hour damp-heat test by retaining 94% of its initial PCE. The same device stack was transferred to a flexible substrate. This resulted in the first flexible, bifacial PIN modules with a state of the art efficiency and passing the 1000 hour damp-heat test by retaining 100% of its initial PCE.

Figure 1: Image of the flexible, semi-transparent bifacial PIN perovskite module with monolithic series laser interconnection.

REFERENCES:

[1] https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20200203

Halide perovskite solar cells are on the brink of commercialisation with exciting efficiency trajectories for both single junction and tandem configurations. Critical to their commercial success is demonstration of long-term stability, yet our fundamental understanding of failure modes is still lacking. Here, I will show our recent work investigating how nanoscale heterogeneity plays a key role in performance and stability [1]. I will show how nanoscale phase impurities, even at trace levels, lead to carrier traps [2] and critical photo-instabilities in devices. Finally, I will show how phase engineering will be essential to remove these problematic inclusions, ultimately leading to extremely phase and operational stable halide perovskite absorbers and devices.  

Deterministic GHz-rate single photon sources at room-temperature would be essential components for various quantum applications. However, both the slow intrinsic decay rate and the omnidirectional emission of typical quantum emitters are two obstacles towards achieving such a goal which are hard to overcome simultaneously. We solve this challenge by a hybrid approach, using a complex monolithic photonic resonator constructed of a gold nanocone responsible for the rate enhancement, enclosed by a circular Bragg antenna for emission directionality [1,2]. A repeatable process accurately binds colloidal quantum dots to the tip of the antenna-embedded nanocone. As a result we achieve simultaneous 20-fold emission rate enhancement and record-high directionality leading to an increase in the observed brightness by a factor as large as 450 (80) into an NA = 0.22 (0.5). We project that these miniaturised on-chip devices can reach photon rates approaching 1.4´10⁸ single photons/second thus enabling ultrafast light-matter interfaces for quantum technologies at ambient conditions.

We also demostrate a significant progress towards a practical plug-and-play single photon source using giant colloidal quantum dots that show blinkingless emission which is stable over hours, on an antenna device which allows back optical pumping of the quantum dots and a front directional single photon emission with high single photon and an almost unity colllection efficiency [3,4].

Keywords: Single photon sources, quantum dots, quantum optics, quantum information, plasmonics, colloidal nanocrystals

References:

[1] Hamza Abudayyeh and Ronen Rapaport, Quantum Sci. Technol. 2 034004 (2017)

[2] Abudayyeh et al., ACS Nano 15, 11, 17384 (2021)

[3] Abudayyeh, et al.,  APL Photonics 6, 036109 (2021)

[4] Abudayyeh, et al., ACS Photonics 6, 446 (2019)

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) quantum dots (QDs) are subject of intense research due to their exceptional properties as both classical¹ and quantum light sources.^2-4 Here, we report a comprehensive investigation of the room temperature single QD optical properties. The results reveal the origin of the QD homogeneous PL linewidths, and the peculiar size-dependent exciton photoluminescence line broadening and the exciton and multi-excitons recombination dynamics. Experimental results are corroborated by ab-initio molecular dynamics. 

Such findings guide the further design of robust single photon sources operating at room temperature. 

References

[1] Akkerman et al., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 17, 394–405 (2018).

[2] Becker et al., Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189–193 (2018).

[3] Rainò et al., Superfluorescence from lead halide perovskite quantum dot superlattices. Nature 563, 671–675 (2018).

[4] Utzat et al., Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science 363, 1068–1072 (2019).

Multiply-excited states in semiconductor nanocrystals feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photo-luminescence provides a natural probe to investigate these states, room temperature single-particle spectroscopy of their emission has so far proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. In this work, we perform heralded spectroscopy of single quantum dots by incorporating the rapidly developing technology of single-photon avalanche diode arrays in a spectrometer setup. This allows us to directly observe the biexciton-exciton emission cascade and to measure the biexciton binding energy of single nanocrystals at room temperature, even though it is well below the scale of thermal broadening of the transitions due to finite temperature and that of spectral diffusion, the shift of the transition energy due to fluctuating electric fields. Single-particle heralded spectroscopy enables us to identify correlations of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential, which are masked in ensemble measurements, and to overcome artifacts due to inhomogeneous broadening [1]. Time-resolved spectrometry, as demonstrated here, has the potential of greatly extending our understanding of charge carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.

Periodic arrays of nanoholes are patterned by electron beam lithography (EBL) upon a 170 µm thick glass coverslip, after the deposition of a sputter coated Indium Tin Oxide (ITO) and a spin coated polymethyl methacrylate (PMMA) electronic resist thin films. Colloidal semiconductor giant core/shell CdSe/CdS nanocrystals, shielded by an additional silica layer, are then trapped with remarkable efficiency into the PMMA holes by a capillary assembly method based on a finely tuned drop casting process. The resulting planar solid state arrays of semiconductor quantum dots demonstrate single photon emission properties, studied in detail by photoluminesce and second order photon correlation measurements.

Multiply-excited states in semiconductor nanocrystals (NCs) exhibit intriguing physics and are key in nanocrystal-based quantum technologies. While simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets (NPLs) its probability can be tuned according to size and shape. In particular, multiple excitons strongly bound in just one dimension are free to re-arrange in the lateral plane, reducing the probability for multi-body collisions. In this work, we analyze multi-exciton dynamics in two-dimensional CdSe/CdS core/shell NPLs of various sizes through the measurement of second-, third-, and fourth-order photon correlations.¹ Thus, for the first time, we could directly probe the dynamics of the two, three, and four exciton states at the single NC level. Remarkably, although higher orders of correlation vary substantially among a NPLs sample, they strongly correlate with the value of second-order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. This deviation suggests that many-body contributions are present already at the level of tri-excitons.

In addition, multi-exciton interactions can be visually expressed in the emission patterns of different excitonic states. Type-II ZnSe/CdS dot-in-rod NCs, presenting a dipole radiation pattern due to anisotropy, can be used to demonstrate this phenomenon. A defocused imaging setup coupled to a novel single-photon avalanche detector (SPAD) array containing 23 pixels makes it possible to directly observe and analyze the out-of-focus dipole emission patterns of single excitons and bi-excitons in a single NC, allowing a deeper understanding of the underlying multi-exciton dynamics in semiconductor NCs.

Colloidal Quantum Dot (CQD) Molecules represents a new class of artificial molecules where the electron wavefunction in two neighboring CQD artificial atoms hybridizes within the connected nanocrystals. Like naturally occurring molecules, the electronic coupling strength is expected to manifest widely varied properties in CQD molecules, altering not only single-particle characteristics but also complex many-body interactions in CQDs. Herein, we discuss in detail the signatures of single CQD molecules at different coupling strengths, as reflected on their emitted photon statistics at room temperature. The neck diameter in CdSe/CdS core/shell CQD homodimer molecules allowed to tune the hybridization of the electron wavefunction at a fixed center-to-center distance. At small neck diameter, the electronic between two CQDs are weak, which increases progressively with neck filling.  We show that the photophysical properties of CQD molecules at weak coupling regime are governed by strong localization of electron wavefunction at individual CQD atoms, whereas in the strong coupling regime, due to facile delocalization the CQD molecules resemble the properties of single artificial atoms. A radiative multi-exciton configuration is preferred in the photo-excited weakly coupled molecules mostly governed by dipole-interactions, leading to strong photon bunching. In the strong coupling regime, electron tunneling activates a unique Auger process leading to single-photon emission via dissociation of biexciton, from a system consisting of two CQDs. This study validates the analogy of CQD molecules with the bonding nature of naturally occurring molecules that exhibit distinct ionic and covalent types of bonding in different coupling regimes, manifesting different properties. A detailed spectroscopic signatures following the time-tagged-time-resolved data will be discussed. The demonstration of multitude opto-electronic configuration tuning the extent of hybridization brands the CQD molecules as novel building blocks for many applications such as in low-threshold lasing and single-photon emitter in quantum applications.

The spectrum of multiexciton emission from colloidal quantum dots at room temperature is important for their use in high-power applications, but an in-depth characterization has not been possible until now. We present and apply a novel spectroscopic method to quantify the biexciton linewidth and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs. In our method, we select photons emitted by a biexciton emission cascade and reconstruct their spectrum. We find a biexciton linewidth of 86 meV on the single-QD scale, similar to that of the exciton. Variations in the biexciton energy are correlated with but are more narrowly distributed than variations in the exciton energy. Using a simple quantum-mechanical model, we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

Recent and nearly simultaneous developments in the fields of nanocrystal semiconductors, nanoplasmonics and nanophotonics may conspire to address deficiencies of available single-photon sources (SPSs). Namely, SPSs may now be accessible that operate in the telecommunications bands at room temperature, that are bright, and that can be easily integrated into quantum-advantaged communications, networking and sensing technologies. I will discuss our efforts to understand, design and synthesize heterostructured colloidal semiconductor quantum dots (QDs) that can meet the rigorous requirements for single-photons-on-demand in the infrared (IR) and at room-temperature.^1-4 In general, QD emitters, especially those that emit in the infrared, are slow to cycle photons. In order to dramatically enhance the rate of spontaneous emission, such emitters can be combined with nanoscale plasmonic materials or antennas. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the IR. Among semiconducting plasmonic NCs, spinel metal oxides (sp-MOs) are an emerging material with distinct advantages in accessing the telecommunications bands in the IR and affording useful environmental stability. Here, I will discuss the plasmonic properties of several sp-MO NCs, known previously only for their magnetic functionality, and demonstrate their ability to modify the light-emission properties of telecom-emitting QDs, achieving Purcell enhancement factors up to ~50-fold for telecom-emitting QDs in simple plasmonic-spacer-emitter sandwich structures or significantly higher radiative rate enhancement aided by more sophisticated plasmonic nanoantenna architectures. Finally, in collaborative work,⁵ we have demonstrated prospects for on-chip integration and addressing losses in brightness associated with the omnidirectionality of free-standing emitters. In particular, using a scanning-probe method we directly place QDs into hybrid metal-dielectric antenna that cause their photons to emit in a highly focused, directional stream, dramatically enhancing collection efficiency and utility as a SPS. Taken together, these advances in nanomaterial chemistry, nanointegration and hybrid-material design are showing the way to practical utilization of nano-enabled SPSs for quantum applications.

Luminescent colloidal CdSe quantum dots (QDs) provide an opportunity to cover a wide range of emission wavelengths across the visible spectrum. Their high-temperature synthesis procedures offer crystalline materials with precise dimensions and well-defined optoelectronic properties. However, the core-only CdSe QDs show low photoluminescence (PL) quantum efficiency (QE) due to inefficient surface passivation by organic ligands, leading to charge trapping. Core/shell CdSe/CdS nanostructures with wurzite CdSe cores are known to improve the PL QE and long-term stability. A small lattice mismatch of 4% between the crystal structures of CdSe and CdS offers to grow a thick or giant CdS shell. Due to the electron delocalization in CdSe/CdS giant shell nanocrystals (NCs), the PL lifetime can be tuned from a few nanoseconds to microseconds. In this project, we studied a straight forward approach to synthesize high quality CdSe/CdS NCs with near-unity PL QE. Design of experiments (DOE), a statistics supported approach, is used to define the influence of reaction parameters in order to achieve a better synthesis procedure. Thirteen experiments were performed according to the DOE and monitored at 13 respective points for each experiment. PL QE and CdS shell thickness were analyzed with respect to all the variables used in DOE. PL QE up to 97 % has been achieved. Positive and negative effects were analyzed and optimal condition for the factors has been discussed.

Keywords  Colloidal quantum dots, core/shell nanostructures, near unity PL QY, Design of experiments

The low-temperature colloidal production of II-VI nanoplatelet heterostructures has stimulated the interest of researchers due to the possible uses of these materials in various optoelectronic devices. Here, we report a  room-temperature coating by CdS or ZnS dots of pre-prepared CdSe nanoplatelets. The dot coating process made use of a synthesis developed for the formation of free-standing CdS and ZnS species, involving injection of a metal precursor into reactive sulfur-amine solutions at room temperature. CdSe structured nanoplatelets with 1.75 nm thickness were used as the core constituent. The structural properties were investigated using Fourier transform infrared spectroscopy and advanced electron microscopy, while the elemental mapping was verified using high-angle annular dark field transmission electron microscopy. The results showed a dots-on-plate structure, resembling an intermediate configuration between core/crown or core/shell heterostructures. A thorough study of optical properties demonstrated a dramatic spectral shift of the absorption edge to lower energies, regardless of the uniformity of the coating layer, maintaining spectral stability over two months while stored at ambient conditions. Other optical measurements included examination of temperature-dependent photoluminescence and transient photoluminescence decay, from room temperature down to ~ 4 K. These measurements revealed excitons, trions, and trapped carrier recombination emission with variable relative intensities following the temperature change. The dots-on-plate structures studied displayed a short photoluminescence decay (< nanosecond), compatible with that of core/shell (crown) structures prepared at high temperatures. As a result, the presented techniques are alternate pathways that eliminate the requirement for excessive temperatures and/or multi-step processes, instead providing rapid, inexpensive, and scalable procedures with practical advantages.

This talk will address the influence of doping on the optical and magneto-optical properties of lead halide perovskites, revealing possibilities of their implementation in various spin-based applications. The Ni²⁺ ions were embedded into cesium lead halide-perovskite via a combined cation/anion exchange process. This study uses a thorough investigation of Ni²⁺ doping effects on the magneto-optical properties of CsPb(Br_xCl_1-x)₃ nanocubes, monitored by extensive spectroscopic methods such as steady-state, transient and magneto photoluminescence (PL) spectroscopy which recorded at various temperatures. The PL decay curve showed evidence for charge carrier trapping. The magneto-PL measurements revealed the occurrence of multiple recombination events with different degrees of circular polarization (DCP), with an apparent contrasting behavior in the undoped and doped samples. These observations unfold energy splitting of the bright triplet states (m_s = ±1 components) and the existence of emission processes related to delayed fluorescence by carrier localization at trapping sites (also confirmed by ODMR). The study reveals the role of the dopant in stiffening the entire crystal to the degree that enabled resolving the identification of carriers' localization sites.

Layer-by-layer (LBL) assembly is an appealing approach for the processing of nanoparticles and nanocolloids into thin films suitable for various applications¹. One of the major advantages of this method is its simplicity: the process requires neither sophisticated hardware nor high-purity components. This simplicity is combined with the high quality of the resulting coatings, whose thicknesses can be controlled at the nanometer level. In the field of light-emitting diodes (LEDs), thickness of the emissive material film is very important to assure the copious recombination of the opposite charges injected with resulting high photoemission². In addition, for Quantum Dot-based LEDs it’s crucial to have a good conductivity within the thin layer of emissive nanocrystals to facilitate the carrier mobility². Therefore, to merge these two important requirements, we developed a ligand exchange process on CsPbX₃ perovskite nanocrystal films which enables Layer-by-layer deposition.

We replaced, in solid state, oleate and oleylamine ligands (long insulant chains generally used for the synthesis of CsPbX₃ nanocrystals), with Didodecyldimethylammonium Bromide, Ammonium Thiocyanate and other ligands that enhance the photoluminescence quantum yield and improve the performances of LEDs. The exchange has been carried out through a spin-coating technique, using solvents with strategic polarity to avoid NCs dissolution or damaging. Maintaining the quick and simple method of spin-coating, we deposited an increasing number of layers and, through AFM measurements, we noticed considerable thickening of material.

The easy handling of this twofold process makes it very attractive for a laboratory-to-industry scale-up transition and the potential applicability to many nanoscale materials opens the door to a large variety of layer permutations.

Transition metal dichalcogenide (TMD) nanosheets have become an intensively investigated topic in the field of 2D nanomaterials, due to their semiconductor nature, the direct band gap transition and the broken inversion symmetry going from bulk to monolayer. These properties makes TMDs suitable for different technological applications such as photovoltaics, valleytronics, or hydrogen evolution reactions (HER), or transistors. Among them, MoX₂ (X = S, Se) are only direct-gap semiconductors when their thickness is reduced to a monolayer, hence an important effort is devoted to obtain single layer TMDs. Colloidal synthesis of TMDs has been developed in recent years as it provides a cost-efficient and scalable way to produce few-layer TMDs having homogenous size and thickness, yet obtaining a monolayer has proved challenging. Here we present a general method for the colloidal synthesis of mono- and few-layer MoX₂ (X = S, Se) using elemental chalcogenide and metal chloride as precursors. Using a synthesis with slow injection of the MoCl₅ precursor under nitrogen atmosphere, and optimizing the synthesis parameters with a Design Of Experiments (DOE) approach, we obtained a monolayer MoX₂ sample with the required semiconducting (2H) phase, a band gap of 1.96 eV for 2H-MoS₂ and 1.67 eV for 2H-MoSe₂, respectively, both displaying fluorescence at cryogenic and elevated temperatures. A correlation between the blue shifted absorption spectrum and the spectral difference between the Raman modes was established and confirmed that a single-layer thickness was obtained.

The recent advances in the colloidal synthesis of strongly emitting lead halide perovskite nanocrystals (NCs) with precise size and composition control open new possibilities for the fabrication of optoelectronic devices. Yet, the physics of the band-edge exciton fine structure of perovskites is still a subject of discussion [1], despite its central impact on the optical properties of these materials and therefore on their potential use in various applications, especially as quantum light sources. At cryogenic temperatures, we show that single cesium lead halide (CsPbX3) perovskite NCs display bright photoluminescence even though their band-edge exciton fine structure presents a dark ground singlet state. We introduce resonant PL excitation of the excitonic sublevels to investigate the indistinguishability character of the photons emitted by these NCs. Importantly, we demonstrate that the presence of a long-lived ground exciton state favors the formation of biexcitons and thus the emission of pairs of correlated photons. We show that this is a general behavior that can be observed for perovskite NCs as well as for conventional CdSe quantum dots. This property makes single CsPbI3 NCs versatile bright, quantum light sources with photon statistics that can be tuned from bunching to antibunching, using magnetic coupling or thermal mixing between dark and bright exciton states [2].

Finally, I’ll describe a recent study [3] of the photophysical properties of copper indium sulfide quantum dots which are emerging as promising alternatives to cadmium- and lead-based chalcogenides systems in various applications and for which the nature of the emission pathways remains a subject of debate.

Nanometric semi-conductor colloidal nanocrystals, like CdSe/CdS ones, are excellent single photon sources. Their emission is stable and bright, with a spectral bandwidth of the order of Dl=20-30nm. In specific conditions of high excitation or high confinement, their emission is dramatically changed. They lose their single photon source quality, the dynamic of the emission is accelerated.  the spectrum becomes very large and can reach a few 100nm. Such condition can be achieved either by exciting at high power the emitters or by coupling them to antennas [1], increasing optical confinement.

By in situ optical lithography using  high order Laguerre Gaussian mode, we deterministically positioned a single colloidal CdSe/CdS emitter inside a plasmonic patch antenna. The single nanocrystal embedded in a thin dielectric medium, is sandwiched between a thick gold layer and thin gold patch. We achieve inside plasmonic antennas a high interaction between the emitters and the confined field excited inside the antenna. We observe large modification of emission for quantum inside antennas illustrated by directive emission, high brightness and increased efficiency. We evidence a dramatic increase of absorption cross-section of emitters inside these antennas. At higher excitation, multi excitation recombination becomes radiative and the emission intensity becomes non linear.

[1] Amit Raj Dhawan, Cherif Belacel, Juan U. Esparza-Villa, Michel Nasilowski, Zhiming Wang, Catherine Schwob, Jean-Paul Hugonin, Laurent Coolen, Benoît Dubertret, Pascale Senellart, Agnès Maître, Extreme multiexciton emission from deterministically assembled single emitter subwavelength plasmonic patch antennas, Light: science and application, 9, 33 (2020)

[2] A. R. Dhawan, M. Nasilowski, Z. Wang, B. Dubertret, A. Maitre , sunder review, ^,https://hal.archives-ouvertes.fr/hal-03376851, Efficient single-emitter plasmonic patch antenna fabrication by deterministic in situ optical lithography using spatially modulated light (under review)

II-VI semiconductors have been known to favour three-dimensional crystal structures. It is therefore a surprise that they form two-dimensional platelets at the few monolayers limit. Considering nanoplatelets of CdSe, we examine both the bulk limit where a zinc-blende crystal structure is favoured as well as the few monolayers limit considering nanoplatelets with varying thicknesses, within density functional theory based calculations. In agreement with experiment, we find that the formation energy is less for the nanoplatelets for few layers than for the bulk structure, indicating that it is energetically more favourable for them to form. Different surface terminations for the nanoplatelets as well as different atoms at the surface are explored in order to understand the energetics. Here again, we find the Cd-terminated (100) surface facets to be most stable. The possible energetics allowing the nanoplatets to form as well as their electronic structure will be discussed.

The anion and cation exchanges in nanocrystals are common synthetic strategies to tune their optoelectronic properties. The process of ion exchanges often leads to the formation of heterostructures inside the nanocrystals - that may control their stability and other properties. A fundamental understanding of ion exchange processes and ion exchange induced heterostructures is necessary.

In this talk, we will be discussing the anion exchange in CsPbX₃ nanocrystals and how it can lead to the formation of internal heterostructures. It is generally believed that the anion-exchanged nanocrystals possess a homogeneously alloyed composition. We have employed variable energy hard X-ray photoelectron spectroscopy to determine the internal heterostructure of anion-exchanged nanocrystals - which has a gradient-alloyed heterostructure. We will also discuss the mechanism of the anion exchange in pre-synthesized CsPbX₃ nanocrystals.

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) 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 in order to provide an overview of the current status of the quest for Cd- and Pb-free HNCs and 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-basedmulticomponent heteronanorods with diameters in the quantum confinement regime.

Benefiting from their large compositional space, direct bandgap nature, and outstanding structural stability,^[1–4] vacancy-ordered layered double perovskites with formula Cs₄M(II)M(III)₂X₁₂ (M(II): Cu²⁺, Mn²⁺; M(III): Bi³⁺, Sb³⁺, In³⁺; X: Cl^-, Br^-, I^-), have recently attracted increasing attention as substitutive materials of lead-based halide perovskites for potential commercial optoelectronic applications. The layered double perovskite structure comprises one layer of [M(II)X₆]^4- octahedra inserted in between two layers of [M(III)X₆]^3- octahedra. As a representative, Cs₄CuSb₂Cl₁₂ has been successfully synthesized both in the form of single-crystalline powder^[2] and NCs,^[3,5] exhibiting a narrow direct bandgap (1.0–1.8 eV) and impressive stability, which however still suffers from the high toxicity of Sb element and the absence of emission at room temperature. To overcome these drawbacks, Cs₄CuIn₂Cl₁₂, with toxic Sb³⁺ substituted by relatively non-toxic In³⁺, could be a promising candidate to fulfill the requirement of compositional engineering and optical tunability particularly in the UV range. Yet, to date, the synthesis of Cs₄CuIn₂Cl₁₂ layered double perovskites has not yet been reported for neither bulk film nor NCs.

Herein, we report the first-ever colloidal synthesis of lead-free Cs₄CuIn₂Cl₁₂ layered double perovskite NCs using a modified hot-injection method.^[5,6] The synthetic details are described in the Supporting Information (SI). While a standard hot-injection reaction in moisture-free environment resulted in Cs₄CuIn₂Cl₁₂ with a very low photoluminescence quantum yield (PLQY) of 0.12%, we found that the handling of synthesis precursors in the presence of moisture (RH~40%) enhances PLQY by more than one order of magnitude up to 1.70 %. Water-assisted in-situ synthesis has been recognized as an effective strategy to tune the optical properties and stability for both lead-based^[7,8] and lead-free halide perovskite NCs^[9,10] via the controlling of NCs size, shape, and crystallinity. Nevertheless, there is still a lack of deep understanding of how water influences NCs growth regime and corresponding PL property, especially for lead-free double perovskite NCs. The introduction of moisture in the precursor (namely “wet” precursor conditions) of Cs₄CuIn₂Cl₁₂ NCs (w-Cs₄CuIn₂Cl₁₂) induces the morphological transformation from 3D nanocubes (NCus) to 2D nanoplatelets (NPLs), driven by the ionized H₃O⁺ and OH^- from the water content as additional capping ligands. The ultrafast transient absorption studies suggest a strengthened self-trapped exciton (STE) effect for w-Cs₄CuIn₂Cl₁₂ NCs compared to the NCs synthesized in “dry” conditions (d-Cs₄CuIn₂Cl₁₂ NCs), resulting in the conversion of dark transitions into radiative transitions, which directly contributes to the PLQY.

Investigating halide perovskites’ optoelectronics properties for stimuli-responsive materials (SRMs) opens new avenues in smart chromic windows, switchable optoelectronics, and information display applications. Here, we carried out an in-depth study of the structural transformation of FAn+2PbnBr3n+2 (2≤ n ≤∞) layered halide perovskite using excess organic cation (FA/Pb>1), strain, optical and thermal input. Initially as FA/Pb ratio increases, three-dimensional (3D, FA/Pb=1, n=∞) (FAPbBr3) perovskite transform to two-dimensional (2D, FA/Pb=1, n=2) (FA2PbBr4) layered halide perovskite and is aligned in <110> orientation. The intermediate n phases (2≤ n ≤∞) in FAn+2PbnBr3n+2 system stabilize in metastable states and are accessible via strain, optical or thermal input. These results were supported by X-ray diffraction, solid-state nuclear magnetic response (ssNMR), in-situ temperature-dependent Raman spectroscopy, optical spectroscopy, and ab initio simulation techniques. Furthermore, upon photo-decomposition of FABr, the <110> oriented 2D perovskite transforms to 3D perovskite.  Therefore, a metal oxide mesoporous substrate has been introduced to prevent FA decomposition and impart reversibility (2D to 3D and vice versa), which facilitate as a reservoir for FA cations. Shuttling of FABr between the reservoir and the HP film under light and moisture ensure reversible chromism.  We realised that the transformation in the FAn+2PbnBr3n+2 structure is governed by the H-bonds strength of the intermediate n phase (lower n phases to higher n phases or vice versa). Additionally, the remnant organic solvent in the film (observed using proton transfer reaction-mass spectroscopy (PTR-MS)) is crucial to realize structural transformation and, therefore, the reversible chromism. Finally, we relate photo-induced reconfigurability with photo-adaptable intelligent sensors. The realization of structural configurability using just one cation is not only employable in solar cells and LEDs but simultaneously can be utilized in artificially intelligent sensors by optimizing their operating parameters according to the ambient environment.

Ternary I-III-VI₂ NCs, such as CuInS₂ and AgInS₂ are receiving attention as heavy-metals-free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio-imaging. The origin of the optical properties of NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes-shifted and long-lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviors in defect-free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low-bandgap NCs would be accompanied by increased re-absorption losses. In this work, we performed spectroscopic studies on stoichiometric CuInS₂ and AgInS₂ NCs, and in the process it was revealed for the first time, the spectroscopic signatures predicted for the free band-edge exciton, thus supporting the VB-structure model. At very low temperatures, we also observed dark-state emission from these NCs likely originating from enhanced electron-hole spin interaction. The impact of the observed optical behaviors on LSCs was evaluated by Monte Carlo ray-tracing simulations. Based on the emerging device design guidelines, optical-grade large-area (30×30 cm²) LSCs with optical power efficiency (OPE) as high as 6.8% were fabricated, corresponding to the highest value reported to date for large-area devices.

While initial theories on quantum confinement in colloidal quantum dots (QDs) led to analytical band-gap/size relations or sizing functions, numerical methods proofed more accurate to describe size quantization. However, for lack of reliable sizing functions, researchers fit experimental band-gap/size datasets using models with redundant, physically meaningless parameters that break down upon extrapolation. Here, we propose a new sizing function based on a proportional correction for non-parabolic bands. Using known bulk semiconductor parameters, we accurately predict size quantization for group IV, III-V, II-VI, IV-VI and metal halide perovskite semiconductors, including straightforward adaptations for negative-gap semiconductors and non-spherical QDs. Refinement with respect to experimental data is possible using the Bohr diameter as a fitting parameter, by which we show a statistically relevant difference in band-gap/size relation for wurtzite and zinc blende CdSe. The general sizing function proposed here unifies QD size calibration, and enable researchers to assess bulk semiconductor parameters and predict size quantization in unexplored materials

Two-dimensional nanoplatelets (NPLs) of II-VI semiconductors exhibit the narrowest optical features among nanocrystals. Despite the great maturity of cadmium based NPLs, their optical properties are limited by the band gap of the bulk material. To push toward infra-red wavelength, mercury-based NPLs are good candidates as they keep narrow optical properties^[1].

To obtain such material, only cation exchange is yet available. However, this procedure leads to the creation of structure defects and to the degradation of the optical properties. In this context, we developed an optimized procedure for cation exchange and rationalized the role of the solvent, the temperature and the ligands during this process. This optimized procedure has then been applied to Cd-based alloys of CdSe_xTe_1-x to obtain HgSe_xTe_1-x NPLs of 3MLs with photoluminescence for every composition. Using UV-visible and X-ray spectroscopy, the evolution of the band alignment according to the composition has been established revealing a crossover from n- to p-type behavior as the Te content increases in HgSe_xTe_1-x NPLs. In parallel, this trend has also been observed in time-resolved photoemission measurements.

Quasi-Type II CdSe@CdS dot-in-rod nanorods (NRs) exhibit high absorption coefficients (~10⁷ M^-1 cm^-1) in the UV/Vis and efficient charge-separation following photoexcitation, with the photogenerated hole localizing to the CdSe core and the electron delocalized over the entire structure. These properties render them attractive materials as photosensitizers in photocatalytic reactions such as light-driven water splitting, especially when coupled to a reaction centre such as metal nanoparticles.¹ On the other hand, some drawbacks of these structures have to be considered when designing a catalytic system. For instance, molecular functionalization of NRs is limited to specific anchoring groups such as thiols and amines.² Additionally, the catalytic efficiency is limited by slow (> 100 ps) hole removal, whereas the electron transfer steps are usually very fast (<10s of ps). Last, these structures are prone to photo-oxidation, which limits their long-term usage.

Recently, we demonstrated a photocatalytically able system consisting of polydopamine (PDA) coated NRs.³ The PDA-coating tried to counteract the above-mentioned drawbacks of NRs: it served as a scaffold for a molecular catalyst and acted as a charge-mediator between NRs and catalyst, potentially improving long-term stability. However, the underlying charge transfer processes have not been fully resolved yet.

Here, the photoinduced charge transfer processes between NRs and PDA are investigated. First, as a model system, the interaction between water-soluble NRs and molecular dopamine is investigated at different pH values and dopamine concentrations using steady-state and time-resolved absorption and photoluminescence spectroscopies. Insights gained from these quenching experiments serve as a basis for explaining the charge transfer processes in PDA-coated nanorods. Last, implications of these fundamental processes on photocatalysis are considered.

Chemical transformations reactions, such as the cation exchange reaction, have been one of the most exciting means of studying and post-synthetically modifying nanocrystals. These reactions can create atomic arrangements that are impossible to reach in bulk materials due to kinetic limitations, and these reactions are believed to enhance diffusion beyond bulk-derived limits. In this talk I’ll first discuss cation exchange induced accelerated diffusion and then cation exchange to form a dual-interfaced heteroepitaxy.

The phenomenology of solid-state transformations and diffusion at short length scales remains poorly understood but is increasingly important for nanostructured devices that utilize “nano properties”. Using in-situ synchrotron x‑ray diffraction (XRD), we directly interrogate the structure and reaction kinetics of lead sulfide (PbS) nanocrystals transforming into cadmium sulfide (CdS) through cation exchange. The epitaxial relationship of zincblende CdS to rocksalt PbS breaks the overall symmetry of the core-shell nanocrystal without requiring the loss of unit cell symmetry, leading to anomalous peak shifts in the diffraction pattern. The magnitude of the interdiffusion coefficient, D̃, is larger by four orders of magnitude or more compared to the slowest diffusing species in our system (self-diffusion of Cd in CdS). This surprising result suggests interdiffusion is enhanced in nanocrystals. These results illustrate that the distinction between chemical diffusion in a potential gradient and diffusion at thermodynamic equilibrium has not been fully appreciated.

Additionally, through a cation exchange reaction on copper sulfide nanoparticles we have created dual interface Cu_2-xS-ZnS heterostructures, with a metastable Cu_2-xS layer. The copper sulfide phase region can be tuned to form two-dimensional (2D), single atomic layers (<1 nm). As the nanoparticles transform, we observe a solid-solid phase transformation of the copper sulfide phase from the initial low-copper phase Cu_1.8S into a higher copper phase djurleite (Cu_1.94S), but as the epitaxial strain increases a second phase transformation back to roxbyite Cu_1.8S occurs to minimize strain energy. This work demonstrates novel routes to metastable phases through strain stabilization. The copper sulfide can be etched with phosphines in oxidizing conditions. Importantly, this etching reaction is capable of removing Cu_2-xS from Cu_2-xS-ZnS epitaxial heterostructures with perfect selectivity, that is, the phosphines completely remove the Cu_2-xS without disturbing the ZnS. The etching reaction is preceded by abstraction of sulfur from the particles, destabilizing the Cu_1.81S roxbyite phase. 

Heterojunction nanocrystals (HNCs) consisting of two or more semiconductor components contain junctions at the interface of the constituent blocks. Unlike the electronic property tuning by changing the size or shape of mono-component nanocrystal, HNCs rely on controlling energy states via band-offset engineering at the material interface. We demonstrate the usefulness of band-offsets by designing component size modulated type-II coupled quantum dots (CQDs-HNCs) composed of a fixed sized ZnSe quantum dots and size-tuned CdS quantum dots. The interface of single CQDs-HNCs is probed by using scanning tunneling microscopy (STM) at the local scale. Furthermore, we demonstrate fabrication of a differentiator using a single Cu₂S-CdS HNC. The differentiator is capable for conversion of variety of waveforms demonstrating the possibility of fabrication of single HNC electronic component. 

Colloidal semiconductor Quantum Dots (CQDs) that contain hundreds to thousands of atoms manifest size dependent tunable properties and have reached an exquisite level of control, leading to their technological applications in optoelectronics and bioimaging. Considering them as artificial atoms, CQD molecules connected with molecular linkers such as DNA strands were studied. Yet, in these structures the presence of a high potential barrier limits coupling between the quantum states of neighboring CQDs limiting their full potential as coupled systems. Although coupled quantum dots were prepared by means of molecular beam epitaxy (MBE), their typical dimensions limit coupling to small energy scales suitable to low temperature operation. Furthermore, while MBE grown structures are inherently buried within a host semiconductor, CQDs are free in solution and accessible for wet-chemical manipulations.

Herein, we introduce a facile and powerful strategy for programmed synthesis of coupled CQDs molecules with precise control over the composition and size of the barrier in between the artificial atoms to allow for tuning the electronic coupling characteristics and their optical properties.¹ Our approach entails fusing two CdSe/CdS core/shell CQDs via constrained oriented attachment.² This yields a dimer with a tailored barrier dictated by the shell composition, thickness and fusion reaction conditions. The fusion reaction also enables tuning the neck barrier width.³ The possible nanocrystal facets in which such fusion takes place are analysed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling is revealed by ensemble and single particle spectroscopic signatures, in agreement with quantum mechanical simulations.⁴ Cryogenic single nanocrystal spectroscopy accompanied by the simulations, unravels the complete tracking of fluorescence and charging events in such coupled colloidal quantum dot molecules.

This sets the stage for nanocrystals chemistry to yield a diverse selection of coupled CQD molecules utilizing the rich collection of ubiquitous artificial atom core/shell CQD building blocks. Such CQD molecules are of direct relevance for numerous applications including in displays, sensing, biological tagging and emerging quantum technologies.

Lead halide perovskite nanocrystals (NCs) have drawn tremendous research interest in the past five years fueled by their outstanding optical and electronic properties.[1, 2] In particular CsPbBr₃ NCs exhibit excellent photoluminescence with a very narrow peak and high quantum yield. They can be conveniently synthesized in high yield by means of hot-injection methods.[3-5]  However, due to their ionic nature, perovskite NCs are unstable in polar solvents, and when dispersed in water they quickly lose their luminescence properties and chemical integrity.

            Many efforts have been undertaken to enhance aqueous stability, encompassing changes in stoichiometry (synthesis under Pb-poor conditions)[6] and encapsulation with appropriate organic compounds or oxides (e.g., block copolymers,[7] SiO₂[8]). On the other hand, the growth of an inorganic shell, as generally applied in the case of conventional nanocrystals/quantum dots, has turned out to be challenging: upon prolonged heating of perovskite NCs, uncontrolled ripening leads to a variety of sizes and shapes. Furthermore, classic shell materials crystallize in different structures and exhibit large lattice mismatches.

            Here, we present novel approaches for the growth of metal sulfide and metal oxide shells on CsPbBr₃ nanocrystals, keeping a low size distribution and high fluorescence quantum yield. In the case of metal sulfide shells, aqueous phase transfer of the core/shell NCs could be achieved while maintaining the photoluminescence in water for extended periods. The improved stability of the core/shell perovskite NCs paves the way for their use in various applications such as single-photon emitters and photocatalysis.

Semi-conducting nanoplatelets (NPL) exhibit outstanding optical properties but contain toxic heavy metals such as cadmium, lead or mercury. Indium sulfide is believed to be a credible alternative for semi-conductor based applications. We report the synthesis of ultrathin indium sulfide In₂S₃ nanoplatelets which display a giant aspect ratio using a simple and fast solvothermal method. We show that these NPL display a thickness controlled at the atomic level below the nanometer, a width of around 9 nm and a length which can reach several micrometers. We determined the atomic composition of the inorganic core by Rutherford backscattering spectrometry (RBS) and measured by X-ray photoelectron spectrometry (XPS) an oleylamine surface coverage of 2.3 ligands per nm². X-ray diffraction experiments and simulations as well as high-resolution dark-field STEM point towards a trigonal crystallographic structure (gamma phase). Depending on the dispersion solvent, these long ribbon-like nanoplatelets can form well dispersed colloids or bundles in which they stack face to face. Their large aspect ratio induces the formation of gels. Their lateral dimensions can be tuned by the amount of water present in the reaction media: anhydrous synthesis conditions lead to hexagonal nanoplates while controlled addition of water induces a symmetry break yielding long NPL with a rectangular

Transition Metal Dichalcogenide (TMDC) monolayers exhibit a direct band gap in their semiconducting crystal phase and have great potential for future photonics through spin- and valleytronics and their ultrafast response to external stimuli, which is essential for fast optoelectronic components [1, 2]. The materials are typically obtained by exfoliation and chemical vapor deposition methods that yield samples of high quality for fundamental research but lack potential scalability. Colloidal wet-chemistry represents a challenging yet promising approach to synthesize TMDCs from precursors in a solution-processed and flexible manner. We show how fundamental properties of MoS₂ nanosheets including the layer count, the lateral dimension and the crystal phase is controlled. The initially forming a metallic crystal phase, attractive for catalytic hydrogen evolution, is kinetically preferred and transforms into the thermodynamically stable semiconducting crystal phase throughout the reaction. In order to show that the colloidal approach is able to compete with conventional manufacturing methods we investigate photoluminescence of ultrathin colloidal WS₂ nanosheets synthesized with WCl₆ and elemental sulfur in oleic acid and oleylamine at 320 °C, finding photoemission for the first time [3]. We observe mono- as well as multilayer photoluminescence exhibiting comparable characteristics to exfoliated TMDC monolayers and underpinning the high quality of colloidal WS₂ nanosheets. In addition, the observed monolayer emission has a narrow linewidth, which qualifies colloidal WS₂ as a potential single photon emitter complementing epitaxially grown WS₂ quantum emitters [4]. Our results render colloidal WS₂ as straightforwardly synthesized and highly promising 2D semiconductors with optical properties competitive with conventionally fabricated ultrathin WS₂.

Among semiconductor colloidal nanocrystals, 2D nanoplatelets (NPLs) are geometrically seen as well-defined flexible substrates for the self-assembly of molecules. In the presence of stress brought by surface stabilizers, helical structures are formed according to the parameters of the initial material ^[1,2].

Here, we demonstrate the control of the NPLs helices radii through the organic ligands, described as an anchoring group and an aliphatic chain of a given length. A perfect control in surface chemistry allows the tuning of the morphological feature of these nanohelices. Nonetheless, their optical properties are well-preserved upon surface modification. Structural studies done on these anisotropic nanoparticles unveil a preferential orientation effect on the  resulting X-ray scattering patterns.

A mechanical model accounting for the misfit strain between the inorganic core and the surface ligands, enables to predict the nanohelices radii. The model treats the substrate layer, anchoring group and aliphatic chain contributions individually and demonstrates good agreement for all studied homo- and hetero-structure NPLs. It reveals ultimately that the self-assembly of organic ligands is equivalent to a layer, of Young modulus in lateral compression estimated close to 0.9 GPa. Furthermore, the chirality of the nanohelices shown in this work can be tuned by the ligands anchoring group and inverted from one population to another.