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Photovoltaic cells using molecular dyes, semiconductor quantum dots or perovskite pigments
as light harvesters have emerged as credible contenders to conventional devices. Dye sensitized
solar cells (DSCs) use a three-dimensional nanostructured junction for photovoltaic electricity
production and reach currently a power conversion efficiency (PCE) of over 15 % in full
sunlight. They possess unique practical advantages in particular highly effective electricity
production from ambient light, ease of manufacturing, flexibility and transparency, bifacial
light harvesting, and aesthetic appeal, which have fostered industrial production and
commercial applications. DSCs served as a launch pad for perovskite solar cells (PSCs) which
are presently being intensively investigated as one of the most promising future PV
technologies, the PCE of solution processed laboratory cells having currently reached 25.7%.
Present research focusses on their scale up to as well as on ascertaining their long-term
operational stability. My lecture will cover our most recent findings in these revolutionary
photovoltaic domains

Halide perovskite optoelectronic devices are generating excitement for new light emission and photovoltaic applications. The performance of these devices are approaching or even exceeding lab based record efficencies of competitor technologies. However, operational stability remains an issue and devices drop in performance over time, yet these degradation processes are poorly understood. Here, I will present results showing capabilities to image the local performance losses in LEDs and solar cells over time under operation conditions. Through combinations of luminescence microscopy techniques, we extract key performance parameters including quasi-fermi-level splitting, local spectral changes and local current-voltage maps. We correlate these properties with local chemical and structural changes to elucidate mechanistic understanding of the degradation pathways. We draw generalised conclusions about device instabilities, in turn allowing judicious design of devices to better mitigate these effects. These studies demonstrate powerful platforms for elucidating device behaviour and the steps required to push towards long term stability.

Perovskite photophysics in the past decade has been uprooted and revisited from the grounds several times. Initially, Metal halide perovskites were considered as excitonic semiconductors, due to their pronounced excitonic absorption and narrow-band efficient optical emission. Then ultrafast spectroscopy studies have revealed bimolecular recombination dynamics, proving that excitons are dissociated into opposite charge carriers, with beneficial effects for charge separation in solar cells. Perovskite photophysics seemed to have been rationalized: free carriers are favored over exciton by Saha equilibrium, radiative recombination is bimolecular and is the inverse process of optical absorption.

Yet several issues with such a picture started appearing, again from ultrafast spectroscopy measurements. I will review how radiometric time-resolved photoluminescence reveals that the radiative recombination rate is much lower than what expected from the absorption rate. Furthermore, the combination of transient absorption and time resolved photoluminescence in a tandem setup demonstrates that free carriers are majority even at low temperature and in 2D perovskites, when Saha equilibrium predicts instead that bound excitons should prevail.

An extensive debate on the exciton binding energy has ensued, resulting in the realization that the exciton binding energy measured in absorption, when perovskites are in their ground state, is significantly different from the binding energy after excitons have been created.

Clearly, a sound description of photophysics of halide perovskites needs additional ingredients describe the dissociation of excitons in the excited state. Phonon coherences, ultrafast electron diffraction, XAS and XRD are among the techniques that have evidenced a significant distortion of the perovskite lattice upon optical absorption, a phenomenon also known as the formation of a new quasiparticle, the polaron.

I will present a picture of perovskite photophysics that applies to both 3D and 2D materials and consists in the creation of excitons upon optical absorption, their spontaneous dissociation into opposite charge polarons and, finally optical emission by bimolecular recombination of polarons into excitons.

Mixed Tin/Lead (Sn/Pb) perovskites have the potential to achieve higher performances in single junction solar cells than Pb-based compounds. Sn/Pb based devices are generally fabricated in the p-i-n structure and frequently PEDOT: PSS is utilized as hole transport layer, even if there are many doubts on a possible detrimental role of this conductive polymer. Here, we propose the use of [2-(9H-Carbazol-9-yl)ethyl]phosphonic acid (2PACz) and the functionalized [2-(3, 6-dibromo-9H-carbazol-9-yl) ethyl] phosphonic acid (Br-2PACz) version, as substitutes for PEDOT: PSS. By using Cs0.25FA0.75Sn0.5Pb0.5I3 as active layer, we obtained champion efficiency as high as 19.51% on Br-2PACz, while 18.44% and 16.33% efficiency was obtained using as hole transport layers 2PACz and PEDOT: PSS, respectively. It is interesting to note, that the implemented monolayers enhance both the shelf lifetime of the device as well as the operational stability. Br-2PACz- based devices maintain 80% of the initial efficiency under continuous illumination for 230h, while PEDOT: PSS- solar cells drop to 79% after only 72 hours. Furthermore, Br-2PACz solar cells maintain 80% of the initial efficiency when stored for 176 days (4224h) in N2 atmosphere. Several factors seem to determine these improvements in samples using SAMs HTL. The carbazole-based molecules are able to form a self-assembled monolayer which show minimal parasitic absorption and low charge carrier recombination when compared to PEDOT: PSS films, additionally, the perovskite layer deposited on SAMs presented higher crystallinity, with reduced pinhole density and larger grains. The defect density for the perovskite films is also reduced when deposited on Br-2PACz or 2PACz compared to PEDOT: PSS. Finally, the problem of the wettability of these SAMs will be discussed and a new double layer facilitating the film deposition will be introduced.

Nuclear Magnetic Resonance (NMR) spectroscopy has become a useful technique for the study of both the physico-chemical and the structural properties of perovskite precursors.^[1] Thanks to the possibility to deliver a thorough characterization at a molecular level, this analytical technique can provide information about the interactions between the bulk perovskite precursors and the additives, as well as about the nature of the perovskite phases and the geometry in the crystalline phase. On the other hand, it can help in better understanding the degradation phenomena affecting the precursors in solution in dependence of solvent, concentration, and ageing conditions adopted.

After an overview of the tools that the technique offers, both for solution and solid-state characterization, we will describe the results obtained by applying solution NMR for: i) improving the morphology of thin films via exploitation of polymeric templates or organic gelators, ii) boosting the precursors solubility for achieving more efficient nucleation processes, and iii) detecting the occurrence of degradation phenomena that can affect the film performances.^[2],[3],[4] The NMR results, combined with other analytical techniques exploited for perovskite-film characterization, allowed relating the characteristics in solution with the physico-chemical properties of the films and the performances of photovoltaic devices.

Over the past few years, Organic-inorganic halide perovskites (OIHPs) have demonstrated remarkable improvements in their optoelectronic performance. The use of mixed-cations and mixed-halides in solar cell fabrication has enabled bandgap tunability for multi-junction solar cells. Nevertheless, a systematic evaluation of the microstructural behavior of wide bandgap (>1.7eV) mixed perovskites is still necessary due to the photo-instability that is dependent on composition.

This study focuses on the microstructural inhomogeneity in (FAPbI3)x(MAPbBr3)1-x, which exhibits a bandgap range from 1.55eV to 2.3eV. Advanced scanning probe microscopy techniques are employed to investigate this behavior. Contact potential difference (CPD) maps are measured by Kelvin probe force microscopy (KPFM), revealing an increase of lower CPD grains that correspond to the flat polymorph abnormal grains in the topographical map as the concentration of MAPbBr3 is increased. Chemical component analysis, performed using helium ion microscopy with secondary ion mass spectrometry, reveals that these flat grains are rich in MA, Pb, and I. Spectral photoluminescence shows clear phase segregation dependence on composition as MAPbBr3 increases, which is responsible for the formation of abnormal grains.

Bias-dependent piezo-response force microscopy (PFM) measurements confirm that ions are vigorously migrated on the flat grains, resulting in hysteretic dynamics in the piezoresponse-electric bias (P-E) loop. Finally, light-assisted KPFM measurements reveal that the flat grains contribute to phase segregation. Through various microstructural characterizations, our results indicate that the abnormal grains, as defect sites, are detrimental to phase segregation and ion migration.

In summary, this study provides new insights into the microstructural behavior of wide bandgap mixed perovskites, revealing the role of abnormal grains in phase segregation and ion migration. These findings contribute to the development of more efficient and stable perovskite solar cells, by providing a better understanding of the underlying mechanisms of the material's behavior.

Ruddlesden Popper 2D layered hybrid perovskites (HP) are gaining growing importance in optoelectronics, for applications ranging from photovoltaics to lighting, especially due to their improved stability. Despite their success, the dynamics of photocarriers in these materials still need to be assessed. We exploited out technique of ultrafast optical spectroscopy, combining the measurement of differential transmission (DT) and time resolved photoluminescence (TRPL) in the same spot of the sample, to probe 2D HP single crystal (phenethylammonium (PEA)2PbI4). We compared the response of single crystals and thin films under different excitation regimes, under one-photon and two-photon excitation, both resonant and non-resonant [1]. The results that we found in the single crystal show a sharp drop of DT signal in the first picosecond, combined with the constant relation between DT^2 and TRPL decays, compatibly with what already reported in [2]. The phenomenology that we report here can be interpreted as a dissociation of excitons on a sub-ps timescale, bringing to an ultrafast formation of unbound carriers until a situation of equilibrium is reached, that governs the decay of both bright and dark species. The fact that the same behaviour was observed in thin films and single crystals can give an important clue about the real impact of edge states in 2D hybrid perovskites, suggesting that the dynamics in this material may be not governed by extrinsic effects. 

The fascinating optoelectronic properties of Hybrid Halide Perovskites have renewed the interest in the development of innovative photovoltaic devices alternative to Silicon-based technology. Reaching a straightforward solution-processability of perovskite films, combined with high reproducibility and stability to temperature, illumination and ambient (oxygen, moisture) over operational time via low-cost and large area fabrication technology, represents a highly desirable prospect to foster hybrid perovskite device uptake in a competitive high-tech market. A good control of the crystallization process, during the deposition, is mandatory to obtain high performing perovskite film [1]. Nowadays, the use of a non-solvent, as dripping in the last stage of spin-coating or by immersing the film into a bath after a coating deposition, is the most common method to obtain a high quality perovskite film, although strongly limits the up-scaling.

The main purpose of this work is the engineering of perovskite film formation with the aim to develop a general method for controlling the crystallization process to solve two of the major issues of hybrid halide perovskite materials: processability and stability.

The developed strategy foresees the use of biopolymer as template to assist the perovskite crystallization in a single coating step deposition, without the use of any additional dripping solvents. The interest in the exploration of biomaterials as additive as well as the choice of the less toxic solvents for the precursors solution represent a first fundamental step towards environmental friendly process. Among the biopolymers, polysaccharides were used as rheological modifier to tune the viscosity of perovskite precursor solutions developing stable inks suitable for more scalable printing technology. Moreover, the non-covalent interactions between adjacent chains confers superior flexibility moisture/thermodynamic stability to the perovskite final films, enabling the nanocomposite material to accommodate a strain, whilst maintaining transport properties suitable for devices, thus very attractive for flexible device application [2-5].

This work provides for a more in-depth study of the kinetic of crystallization process of the perovskite assisted by the biopolymer, with a glance at the deposition technique used, towards the fabrication of strong and stable high performing device.

Metal halide perovskites are promising candidates for next-generation photovoltaics due to their excellent optoelectronic properties, ease of fabrication, and low-cost processing. Wide bandgap perovskite materials, such as methylammonium lead tribromide (MAPbBr3), have been shown to exhibit higher stability against air and moisture which makes them a promising alternative to the more widely studied iodide-based compositions. However, these materials suffer from non-radiative losses and limited lifetime under operating conditions. Ion migration, an inherent characteristic of halide perovskites, is one of the factors that contribute to this limited lifetime. One approach to control ion migration is the host-guest (HG) complexation strategy, which employs various macrocyclic host molecules that selectively bind and deliver geometrically compatible guest molecules through noncovalent interactions. We investigated the use of DB21C7 (dibenzo-21-crown-[7]) in wide-bandgap MAPbBr3 hybrid perovskite solar cells. The employment of DB21C7 as an interfacial modifier boosted open-circuit voltage to 1.5 V, accompanied by improved operational stability and comparable solar cell performance. This approach offers a viable route to regulate the behavior of hybrid perovskite photovoltaics.

(This abstract is part of a Manuscript in preparation "Ferdowsi, P. et al. Host-guest complexation in wide bandgap perovskite solar cells.")

Metal-halide perovskites have attracted extensive interest due to the high efficiencies obtained in photovoltaic devices.[1] The main drawback of this technology is the presence of lead which is a well-known and dreaded environmental pollutant. Also at very low concentrations, it is the cause of many diseases in humans, animals and plants. Moreover, the ionic nature of Pb²⁺ is at the origin of a very efficient uptake rate from the ground to plants.[2] For this reason many different approaches have been studied to tackle this concern, and the substitution of lead with tin is probably the most promising.[3],[4] Tin is an element of the same group of lead and can take its place in the perovskite lattice. Compared to lead, tin, is much more sensible to oxidation and can easily be oxidized from Sn²⁺ to Sn⁴⁺.[5],[6] On one side this behaviour is at the origin of the lesser toxicity of tin respect to lead but on the other side it represents a severe limitation during the processing of the material, as the formation of Sn⁴⁺ reduces the photovoltaic device performances. DMSO, which is one of the most used solvents for tin perovskites processing, has been identified as a source of oxidation due to a redox side reaction. Here we show that 4-tert-Butylpyridine (tBP) is a promising substitute for DMSO as it can mimic and exceed its complexation ability to tin iodide.[7] As SnI₂ has stronger Lewis acidity than PbI₂ it reacts faster with the organic salts leading to rapid crystallization of perovskite crystals which leads to a poor morphology with many pinholes. The stronger interaction between SnI₂ and tBP slows the crystallization process leading to the formation of a smooth defect-free perovskite film. We show that, together with the better morphology, perovskite synthesised using tBP possess better electronic properties in terms of defect density and hole density. Tin perovskite prepared using our procedure can be used to prepare photovoltaic device with a maximum PCE of 8.3% in a reproducible manner. 

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. In a decade of intensive research it has been a huge improvement in the performance of these devices, however, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. Sn-based perovskite solar cells are the devices presenting the highest performance after Pb-based but significantly below them. In addition, Sn-based perovskite solar cells exhibit a long term stability lower than their Pb containing counterparts, making stability their main problem. In this talk, we highlight how the use of proper additives and light soaking for defect engineering can increase significantly the stability of formamidinium tin iodide (FASnI3) solar cells, and discuss about the different mechanism affecting this stability, beyond the oxidation of Sn2+, and how they can be countered. As a second topic of the talk, the use of halide perovskite nanocrystals for the preparation of reproducible perovskite LEDs (PeLEDs) will be analyzed as well as the fabrication of Pb-free LEDs by inkjet printing.

Hybrid perovskite solar cells (PSCs) are one of the most promising technologies for new-generation photovoltaics due to outstanding semiconductor properties and low-cost solution processing methods for the fabrication. Indeed, PSCs dominated the PV scientific research in the last decade, by developing efficient and stable devices, produced by employing scalable and low-cost printing techniques, easily embedded in roll2roll or sheet2sheet production lines. However, PSC technology still requires to demonstrate the transfer from lab to fab, pushing the scientific community in finding brilliant solution for drawing a feasible and reliable route toward its commercialization. Moreover, the impressive potentiality of perovskite technology has been already demonstrated to compete on equal footing with traditional inorganic PV or to work in synergy with established silicon technology in tandem cell configuration.[1] As a matter of fact, the astonishing power conversion efficiency recently achieved by small area perovskite/silicon tandem solar cells (PCE>32%) demonstrated the technology potentialities to be appealing for the PV market.[2] However, such technology should keep the promise to be easily manufactured by employing the exiting silicon cell production line and by minimizing the Levelized Cost of Electricity (LCOE). Thus, the synergetic development of large area perovskite devices fitting the standard silicon wafer dimensions and the optimization of perovskite/silicon tandem architectures can definitively open up new horizons for winning the commercialization challenges. In this scenario, the use of interface engineering based on bi-dimensional (2D) materials is here proposed as an efficient tool for trap passivation and energy level alignment in perovskite devices, by mitigating the performance losses induced by the scaling-up process.[3] In particular, the successful application of 2D materials, i.e., graphene,[4] functionalized MoS₂,[5] and MXenes [6,7] in perovskite solar modules (PSMs) allowed to achieve PCE overcoming 17% and 14.5% over 121 and 210 cm² substrate area respectively. Moreover, an ad-hoc lamination procedure employing low temperature cross linking EVA (at 80°C-85°C) allowed to fabricate several 0.5 m² panels, finally assembled in Crete Island, in the first worldwide fully operating 2D material-perovskite solar farm.[8] The 2D material engineered structure employed for the opaque perovskite modules composing the solar farm, has been further modified and optimized for realizing small (0.54 cm²) and large area semi-transparent modules (active area > 60 cm²) suitable for tandem application, in two-terminal (2T) mechanically stacked architecture.[9] Finally, here we propose a novel design for realizing efficient tandem perovskite/Si panel based on voltage-matched (VM) architecture. Following this approach, the tandem panel can be realized by using commercial M2 (15.7x15.7 cm²) silicon heterojunction (Si-HJT) cells provided by ENEL-3SUN company, while the perovskite solar modules can be independently optimized, realized and stacked atop the Si-HJT cells employing an ad-hoc developed lamination process. Among the advantages of the VM architecture, the much less sensitivity toward spectral variations allowed to employ bifacial Si-HJT bottom cell, gaining extra power output exploiting the radiation reflected by the ground (albedo).

In back-contact electrode (BCE) perovskite solar cells (PSCs) architecture, the anode, cathode and selective layers coexist in the bottom perovskite interface while the top surface remains exposed. This results in two main advantages; first, it eliminates the parasitic light-absorption losses that are inherent to conventional sandwich-architecture devices and, second, it enables the use of in-situ characterization of the perovskite layer with techniques that are limited by the opaque metal electrode or top selective layers. However, the fabrication methods for these unconventional architectures rely heavily on expensive photolithography, which limits scalability. We present an alternative cost-effective microfabrication technique in which the conventional photolithography process is replaced by microsphere lithography in which a close-packed polystyrene microsphere monolayer acts as the patterning mask for the honeycomb-shaped electrodes.^[1] Using microsphere lithography, we achieve highly efficient devices having a stabilized power conversion efficiency (PCE) of 8.6%, twice the reported value using photolithography. Microsphere lithography also enabled the fabrication of the largest back-contact PSC to date, with an active area of 0.75 cm² and a stabilized PCE of 2.44%. We will provide a comprehensive overview of the particularities of BCE on the device optoelectronic performance and suggest applications for in-situ/operando measurements.

In the last decade, halide perovskites have emerged as promising low-cost semiconductors thanks to their unique optical and electronic properties and tunable bandgap [1]. These optoelectronic features have driven exceptional performances of perovskite single-junction, multi-junction solar cells (SCs), light-emitting diodes (LEDs), X-ray detectors, and transistors^.[2] These exceptional performances have been obtained by fabricating perovskite materials via solution-based deposition methods, which pose challenges for industrial-scale processing.

In this talk, I will show why thermal evaporation is a promising perovskite fabrication technique to bring this technology closer to reliable and extended production, by relying on excellent size scalability, promising stability, fine composition/properties control, and surface adaptability [5]. The co-evaporated perovskite thin films are uniform over large areas with low surface roughness and a long carrier lifetime.

I will discuss the rationale behind designing highly efficient co-evaporated optoelectronics devices with remarkable structural stability and impressive thermal stability [4].  I will show how vapor deposition also allows fine composition control to tailor the optimization in specific device architectures [5]. A fundamental understanding of perovskite’s electronic and optical properties allows for overcoming the materials’ limitations when implemented in devices such as perovskite SCs, and mini-modules [6]. These results represent a significant step toward the scalability of the perovskite technology.

References

1. S. D. Stranks et al, Science 2013, 342, 341; G. Xing et al  Science 2013, 342, 344; D. P. McMeekin et al., Science 2016, 351, 151; V. D’Innocenzo et al, Journal of the American Chemical Society 2014, 136, 17730

2. NREL. Best Research-Cell Efficiency Chart; U.S. Department of Energy; https://www.nrel.gov/pv/cell-efficiency.htm., Y. Cho, H. Ri Jung, W. Jo, Nanoscale, 2022, 14, 9248

3. F. U. Kosasih, E. Erdenebileg, N. Mathews, S. G. Mhaisalkar, A. Bruno, Joule 2022, 6, 2692-2734, Ávila, C. Momblona, P. P. Boix, M. Sessolo, H. J. Bolink, Joule 2017, 1, 431; Y. Vaynzof, Adv. Energy Mater 2020, 10, 2003073.

4. HA Dewi, L Li, W Hao, N Mathews, S Mhaisalkar, A Bruno, Adv. Funct. Mater. 2021 2100557,

5. H.A. Dewi, L. Jia, E. Erdenebileg, H. Wang, Mi. De Bastiani, S.De Wolf, N Mathews, S Mhaisalkar, A Bruno, Sust. Energy & Fuels. 2022, 6, 2428; J Li, HA Dewi, W Hao, J Zhao, N Tiwari, N Yantara, T Malinauskas, V Getautis, T J. Savenije, N Mathews, S. Mhaisalkar, A Bruno, Adv. Funct. Mater. 2021.

6. J. Li, H. Wang, X. Y. Chin, H. A. Dewi, K. Vergeer, T. W. Goh, J. W. M. Lim, J. H. Lew, K. P. Loh, C. Soci, T. C. Sum, H. J. Bolink, N. Mathews, S. Mhaisalkar, A. Bruno, Joule 2020, 4, 1035;  L Li, HA Dewi, W Hao, L Jia Haur, N Mathews, S Mhaisalkar, A Bruno, Solar RRL, 2020, 4, 2000473

The field of halide perovskites has been growing rapidly during the last decade and new properties and applications are continuously being discovered. One of the them are the discovery of 2D perovskites. Compared with 3D perovskites, 2D ones exhibits novel defect physics and chiroptical activity and wider tunability in chemical compositions. Such properties can find a plethora of applications, such as quantum computing, light emitting diodes, sensing and spintronics. Despite extensive research have been done, the unusual defect chemistry/physics and the origin of the chiral effect is not yet fully understood. Here, we use a combination of computational tools, ranging from ab-initio electronic structure methods, e.g. DFT and Tight Binding (TB) models with reactive molecular dynamics, to study the relation of these properties to the structures, chemical compositions of the materials. Furthermore, the same sets of tools also provide us insights into the tunability of the lattice dynamics and nucleation and growth conditions of these class of materials. These results do not only contribute to the understanding of the involved chemistry and physics, but they also provide materials design strategy for specific applications.

As the front runner among emerging photovoltaic technologies, perovskite solar cells (PSCs) with certified power conversion efficiencies (PCEs) over 26% show great promise for scale-up and future commercialization due to relatively simple and low-cost solution processes. However, the disordered stoichiometric compositions at surfaces generate abundant defects in the solution-processed perovskite films, particularly at surfaces and grain boundaries. Such defects shorten the carrier lifetime and limit photovoltaic performance. Moreover, these defects are responsible for accelerated ion migration and the initial invasion of moisture or oxygen, ultimately causing device instability. The defects also hinder the scale-up of PSCs, thus restricting commercialization. Efficient and stable PSCs with a simple active layer are desirable for manufacturing. Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art PSCs. This strategy, however, suffers from the inevitable formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up.
Firstly, the inclusion of larger ammonium salts is demonstrated leading to a trade-off between improved stability and efficiency, which is attributed to the perovskite films containing a 2D component.[1] The addition of 0.3 mole percent of fluorinated lead salt into the three-dimensional methylammonium lead iodide perovskite enables low-temperature fabrication of simple inverted solar cells with a maximum PCE of 21.1%. The perovskite layer has no detectable 2D component at salt concentrations of up to 5 mole percent. The high concentration of fluorinated material found at the film-air interface provides greater hydrophobicity, increased size and orientation of the surface perovskite crystals, and unencapsulated devices with increased stability to high humidity.
Secondly, the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI₂) that were designed for tailored defect passivation was studied.[2] Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 hours). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm² was achieved.

 

S. Milita*1, G. Calabres1, C. Pipitone2 , A. Martorana2 , F. Giannici2 A. Guagliardi3 , N. Masciocchi4

1CNR-IMM, via Piero Gobetti, 40129 Bologna, Italy

2 Dipartimento di Fisica e Chimica “Emilio Segrè”, Università di Palermo, viale delle Scienze, 90128 Palermo, Italy

3CNR-IC & To.Sca.Lab, via Valleggio 11, 22100 Como, Italy

4 Dipartimento di Scienza e Alta Tecnologia & To.Sca.Lab., Università dell’Insubria, via Valleggio11, 22100 Como, Italy

*milita@bo.imm.cnr.it

Perovskites have recently established themselves in a large number of (opto)electronic devices far beyond solar cells, such as gas sensors, transistors, photodetectors and thermoelectrics. However, the low long-term stability of the vast majority of inorganic and hybrid 3D perovskites still represents the main obstacle preventing their commercialisation. One possible solution is low-dimensional perovskites which have been shown to exhibit higher stability and lower ion migration than their 3D counterparts. Additionally, charge confinement allows ZT to be empowered. Thermal conductivity is reduced by scattering of phonons in the low-dimensional lattice, and the Seebeck coefficient is increased and the thermopower reaches its highest value when energy levels are discrete. To ensure technological applications, low-cost deposition processes, such as spin-coating, must be optimized to obtain a) controlled film morphology (roughness, grain size, grain boundaries, defects), b) pure phases and c) orientation of the layered structures parallel to the substrate. We report on the structural study of thin films of two different 1D pseudo-perovskite materials, namely PRSH)PbX3 (X = Br, I) and (TMSO)PbI/Bi3. Combining complementary techniques, such as GIWAXS, Specular XRD, SEM, EDX and XPS, we could describe the crystalline phases, mosaicity and disorder in film systems and their dependence on the solvent used, the concentration of the precursors and the stoichiometry. Furthermore, the time, ultraviolet radiation and moisture stability of the thin films are reported and correlated with the deposition conditions.

In recent years, photoactive chiral materials are attracting considerable interest owing to relevant applications in optoelectronics as well as high resolute diagnostics [1]. A recent interesting class of luminescent chiral materials is represented by chiral hybrid perovskites [2], since they are showing prominent circularly polarized emissions without any need of expensive ferromagnets or extremely low temperatures [3-5]. Indeed, the chiral source impacts specific non-covalent interactions occurring within the chiral scaffold, which in turn affect the efficiency of the chiral emissions [6]. Modern multiscale modeling and simulations nowadays have an unprecedented level of accuracy, enabling an efficient chiral design of luminescent materials. We herein present recent theoretical contributions aiming at understanding the fundamentals of the chiral transfer generating in tin and lead chiral hybrid perovskites. The most impacting factors influencing their CD signals were explored through ab-initio molecular dynamics simulations and analysis of the density of electronic states (DOSs) showing that the relevant chiroptical features are associated to a chirality transfer event driven by a metal-ligand overlap of electronic levels.

Metal halide perovskite (MHPs) solar cells represent a promising newcomer in the front of emerging photovoltaic technologies to address the dramatic energy crisis and climate change that we are facing. The exceptional properties of MHPs derive by their hybrid organic-inorganic nature, which also allows a low-cost and straightforward fabrication process. Solar cells containing MHPs as absorbing layer have already achieved a power conversion efficiency of about 25,7 %, close to the efficiency of silicon-based devices. Nevertheless, a major limitation is related to the poor stability of the material when exposed to operative conditions, namely temperature, light and moisture, which still prevents the uptake of this technology. Therefore, to further improve the performances of these devices, many surface processes have been applied to solar cells interfaces, most of which include a solution-based methodology ¹. The aim of these treatments is not only to improve the efficiency of solar cells in terms of carrier concentration and transport properties, but also to improve device stability under working conditions. Among the different surface treatments exploitable, the use of plasma represents a solvent-free and non-invasive promising strategy to boost MHP solar cells performances. Plasma-deposited coatings on perovskite, as fluorocarbon polymers, have shown to improve material resistance to humidity and photoluminescence properties². We have explored low-pressure plasma as innovative treatment applied to Metylammonium Lead Iodide perovskite surface. Different gases were tested, i.e. Ar,  N2, H2 and O2, and an interesting improvement of perovskite photoluminescence was observed for the Ar and H2 plasma treated films. The photovoltaic devices including perovskite layers treated with Ar plasma showed increased PCE compared to the untreated solar cell, thanks to an efficient removal of the superficial organic component, that was revealed through X-ray photelectron spectroscopy (XPS). Importantly, XPS also allowed to rationalize the differences observed between the different gases, results corroborated by theoretical calculation of interfaces. Starting from this study, new plasma surface treatments, plasma-assisted deposition and encapsulation processes will be object of study of future research, to achieve a more complete understanding of the interfacial defects and charge carrier dynamics and to further minimize performance losses and instability issues.

Hybrid 3D methylammonium (MA) lead halide perovskites (LHP) MAPbI₃ (MAPI) possess excellent optical and photovoltaic (PV) performances, ^[1] but suffer of limited stability in common environmental and working conditions, being ^[2] prone to decomposition favored by heat and moisture. ^[2,3] Large efforts are nowadays spent in the direction of improving the resistance to degradation ^[4] of MAPI active layers in PV devices. Solar cells applications further require the strict control of electron/hole percolation paths and mobility, which are significantly affected by crystal orientation, grain boundaries and other morphological features. ^[5] Recent advances in the fields have been attained by doping these materials with 2D (pseudo)perovskites and/or purely organic additives. ^[6]

Beyond the intrinsic challenges of new hybrid formulations and their optimization, a compelling further aspect to be considered is the environmental and cost sustainability of these semiconductors that are required to combine good performances, environmental stability and green-chemistry approaches.

With this goal in mind, herein we present the characterization of MAPI-based 2D/3D novel formulations in the presence of organic additives, in the form of films from one-pot deposition from a green solvent (DMSO), as a feasible and safe approach toward efficient and sustainable LHP-based PV devices. The formulations rely on the mixed-cation FA_xMA_(1-x)PbI₃ 3D-perovskite (FA = formamidinium), doped with constant amounts of larger organic mono- and di-cations (BA = benzylammonium; BAMB = 1,4-bis-(ammonium-methyl)-benzene; PENDA = 1,5-n-pentane di-ammonium) and increasing amounts of cornstarch in the pristine solution. Films spin-coated on SiO₂/Si substrates are then treated under different mild annealing conditions. The larger organic cation is engaged in the formation of 2D or lower-dimensional perovskites that are expected to impart higher air-stability and impact crystal orientation, whereas cornstarch is useful to tune the rheological and mechanical properties and film homogeneity, and avoids the anti-solvent step. Differently said, addition of cornstarch could make the solution suitable for printable deposition technologies on extended and flexible substrates. _[6] In-situ and ex-situ GIWAXS experiments, performed at Elettra and at ESRF synchrotrons, shed light on the kinetics of crystallization and allow to investigate other distinct effects of dopants and additives on the films texturing. Results will be discussed and correlated to the measured PCE of the films in solar cells device.