Symposium EN05—Emerging Energy and Materials Sciences in Halide Perovskites

The fabrication of stable and efficient halide perovskite solar cells under ambient conditions is a formidable challenge due to the sensitivity of halide perovskites to moisture, oxygen, and thermal stress effects ^[1–4]. In order to simplify the synthesis processes and consequently bring down the production cost, there is an imperative interest to develop a method for fabricating perovskite solar cells with long term stability under ambient conditions.
This work presents a method for synthesizing efficient and stable mixed-cation halide perovskites by tuning their physical and microstructural properties entirely under ambient conditions. We highlight the essential contribution of lead thiocyanate additives and solvent treatment in the fabrication of perovskite thin films and the very promising performance and stability of their solar cells made using this method ^[4,5]. The developed approach leads to a significant improvement of both the morphology and the crystallinity of the films. As a result, the optimized solar cells without encapsulation retained ~80% of their initial performance after ten months of storage, resulting in a lifetime T₈₀ of approximately 5000 hours (i.e., ~7 months). Our findings suggest that the efficiency and stability of the perovskite solar cells are highly dependent on the processing conditions, thin film morphology, trap density, and carrier mobility in the device before and after storage. The ability to develop these perovskite solar cells entirely under ambient conditions coupled with their high performance paves the way towards the industrial production of low-cost, efficient, and long-term stable solar cells.


References
[1] N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel, Science 2017, 358, 768.
[2] L. K. Ono, Y. Qi, S. (Frank) Liu, Joule 2018, 2, 1961.
[3] D. Bryant, N. Aristidou, S. Pont, I. Sanchez-Molina, T. Chotchunangatchaval, S. Wheeler, J. R. Durrant, S. A. Haque, Energy Environ. Sci. 2016, 9, 1655.
[4] I. M. Asuo, D. Gedamu, N. Y. Doumon, I. Ka, A. Pignolet, S. G. Cloutier, R. Nechache, Mater. Adv. 2020, 1, 1866.
[5] I. M. Asuo, S. Bouzidi, I. Ka, F. Rosei, A. Pignolet, R. Nechache, S. G. Cloutier, Energy Technol. 2021, 9, DOI: 10.1002/ente.202000791.

Perovskites now play a pivotal role as an emerging class of semiconductors after their first introduction in solar cells in 2009, but their commercialization has been hindered so far by the toxicity of lead and by their instability. For this reason, in the last few years, part of the perovskite community has focused on the search for alternative elements to lead that are not a threat to the environment and that can produce a stable perovskite structure. Vacancy-ordered halide perovskites, with general formula A₂B⁺⁴X₆, are one of the alternative structures that have been designed to replace Pb²⁺ with non-toxic tetravalent cations. Among these, cesium titanium(IV) bromide perovskite is an environmentally friendly material that has attracted attention in the last three years for its application in solar cells with power conversion efficiencies reaching 3.3% [1]. However, a solution method suitable for the preparation of thin films has not yet been developed. We present here the first colloidal synthesis of pure and mixed bromide–iodide cesium titanium perovskite nanocrystals (NCs), carried out using the hot-injection method [2]. With this method, we were able to synthesize stable solutions of mixed bromide/iodide Cs₂TiBr_6-xI_x NCs that can be easily deposited as thin films. In our approach, titanium and cesium organic salts are dissolved in octadecene with oleic acid and a zwitterionic ligand and react at 140 °C with a mixture of trimethylsilyl bromide and trimethylsilyl iodide. The resulting perovskite NCs show the expected cubic structure, with lattice parameter increasing with the higher amount of iodide in the structure, and exhibit bandgap tunability from 2.3 eV to 1.2 eV going from a pure bromide to a pure iodide perovskite. Moreover, we have observed that the method used to quench the reaction plays a fundamental role in determining the shape of the NCs. Finally, we measured that the Cs₂TiBr₆ NC films are stable for 1 hour at 100 °C and after 4 hours of UV light irradiation in a nitrogen atmosphere. However, they decompose quickly in air. Their fast decomposition in air motivated us to look for strategies to stabilize them. Since cesium tin(IV) halide perovskites have the same crystal structure as cesium titanium(IV) halide perovskites and are stable in air, we decided to replace partially or fully the titanium with tin, using the same synthetic method developed for pure titanium vacancy-ordered perovskites. A series of new materials, namely mixed titanium-tin Cs₂Ti_1-xSn_xI₆ vacancy-ordered perovskite nanocrystals, was synthesized and characterized. The lattice parameter slightly increases from a pure titanium to a pure tin perovskite, going from 11.5 Å to 11.7 Å, and the bandgap increases up to about 1.55 eV when replacing all the titanium with tin.

[1] Chen, M., Ju, M. G., Carl, A. D., Zong, Y., Grimm, R. L., Gu, J., ... & Padture, N. P., Joule, 2018, 2(3), 558-570.
[2] Liga, S. M., and Konstantatos, G., J. Mater. Chem. C, 2021, DOI: 10.1039/d1tc01732b.

Electronic defect passivation has been used as a strategy to achieve high performance perovskite solar cells. This strategy relies on interlayers that hinder carrier transfer through the bulk and across interfaces. I will discuss how different crystallographic phases of the halide perovskite affect charge carrier transport in the bulk. Charge carrier transport across interfaces will be further studied to understand whether crystalline structures or amorphous phases are able to efficiently allow transport out the device. Synchrotron-based characterization techniques, such as grazing incidence x-ray spectroscopy and x-ray fluorescence will be used to understand the structural and chemical composition of the films, whereas intensity-modulated photocurrent spectroscopy will be used to understand transport processes in the devices. As a result of these contributions, we aim at further understanding the role of interfacial layers and texture on carrier transport for efficient optoelectronic devices.

Halide perovskites are a class of semiconductor materials holding great promise for various energy conversion applications such as photovoltaics, light-emitting diodes and even electromechanical devices. Their advantages including the ease of processing, bandgap tunability, phase modification and defect tolerance are largely due to the soft and dynamical ionic lattice of halide perovskite structure. In particular, all-inorganic halide perovskites, with better thermal stability and a rich diagram of chemical/phase transformations, provide ideal platforms for the fundamental understanding of their structural dynamics as well as studying the emerging properties. A facile anion exchange chemistry is one of the examples of their dynamical lattice behaviors, which has been utilized in tuning the electronic bandgaps while the host perovskite structure still remains. In this presentation, I will first focus on a quantitative study of the anion exchange kinetics by directly visualizing the transformation process on single-particle level with spatial, temporal and spectroscopic information, enabled by the confocal photoluminescence imaging microscopy and a design of vapor-phase chemical treatment. We identified the two-step transformation for CsPbBr₃ to CsPbI₃ as: 1) initial surface reaction; 2) subsequent lattice anion diffusion. The quantitative imaging of anion exchange also revealed a diffusion-limited reaction mechanism. Another example of structural dynamics is the phase transition in CsPbI₃ between a metastable perovskite phase and a non-perovskite phase that is reversible and responsive to external stimulus such as temperature, moisture and light. The optical microscopic imaging method is also applied to study a moisture-induced phase transition kinetics where the nucleation and the propagation steps can be decoupled on individual single crystals.

I will later focus on discussing a different structural phase transition in CsGeX₃ (X=Cl, Br, I) where Ge (II) cation displacement leads to a rhombohedral distortion that introduces an inversion symmetry breaking and the emergence of ferroelectricity. Through a combination of ab initio theory and experiments from structural characterizations to multiscale material behaviors, we identified the role of lone pair stereochemical activity of Ge (II) in determining the ferroelectric origin and found out their spontaneous polarizations. Furthermore, characteristic ferroelectric domain patterns are imaged with both piezo-response force microscopy and nonlinear optical microscopic method. We anticipate that our discovery of this new semiconducting ferroelectric system will expand the understanding of physical phenomena related to ferroelectrics and introduce new possibilities to halide perovskites as well.

Since the late 1970s, operando electrochemical approaches have allowed a direct connection to the defect chemistry in semiconductor materials and the quantification of their energy levels with unprecedented limits of detection. However, the applicability of these solution-based electrochemical methods for defect quantification in metal halide perovskites has been rather limited and restricted to a few non-solvents.
Here, we demonstrate a straightforward three-electrode configuration with a "peel and stick" electrolyte for the electrochemical and spectroscopic investigation of metal halide perovskites. Our ability to effectively characterize defect energetics and densities is illustrated by controlling hole and electron injection in methylammonium lead triiodide and triple cation perovskite films using a systematic modulation of the potential. Introducing redox active probes facilitates operando mapping of the electronic structure, including probing the density of states, elucidation of surface defect reactivity, and assessment of defect passivation strategies, as electron transfer events represent charge injection and charge extraction events in operational optoelectronic platforms. Additional advantages include the ability to make chemical-electronic-physical connections, translate from macro-to nanometer length scales, and study the effect of photon flux, heat, humidity, and/or electrical bias. Finally, we illustrate the electrolyte film is optically transparent to visible and X-ray photons for simultaneous characterization of band gap and physical structure, and can be easily removed for characterization of the near-surface composition and energetics using photoelectron spectroscopies.
Overall, we establish an electrochemical approach that overcomes challenges in characterizing defects in printable electronic materials and provides opportunities for the operando characterization of thin film perovskites, organic semiconductors, quantum dots, conventional semiconductor materials, material blends and device stacks, where the removable solid electrolyte functions as the "top contact". These advanced electrochemical characterization platforms are crucial towards the search for low cost, improved and stable (opto)electronic materials to realize industry-scalable technologies in the field of photovoltaics, charge storage systems, photoelectrochemical cells, etc.

Materials combining semiconductor functionalities with spin control are desired for the advancement of quantum technologies. Here, we study the magneto-optical properties of novel paramagnetic Ruddlesden-Popper hybrid perovskites Mn:(PEA)₂PbI₄ (PEA = phenethylammonium) and report magnetically brightened excitonic luminescence with strong circular polarization from the interaction with isolated Mn²⁺ ions. Using a combination of superconducting quantum interference device (SQUID) magnetometry, magneto-absorption and transient optical spectroscopy, we find that a dark exciton population is brightened by state mixing with the bright excitons in the presence of a magnetic field. Unexpectedly, the circular polarization of the dark exciton luminescence follows the Brillouin-shaped magnetization with a saturation polarization of 13% at 4 K and 6 T. From high-field transient magneto-luminescence we attribute our observations to spin-dependent exciton dynamics at early times after excitation, with first indications for a Mn-mediated spin-flip process. Our findings demonstrate manganese doping as a powerful approach to control excitonic spin physics in Ruddlesden-Popper perovskites, which will stimulate research on this highly tuneable material platform with promise for tailored interactions between magnetic moments and excitonic states.

In recent years, two-dimensional metal-halide perovskites (2D MHPs) have emerged as a promising class of due to their desirable characteristics, with bright and color pure emission at room temperature stemming from their high excitonic binding energy. The study of excitonic properties in 2D MHPs is not only relevant for light emitting and photovoltaic applications [1], but has garnered interest also for the unique physics of exciton-polarons in these materials, where the coexistence of multiple bright and dark states has been revealed [2-4]. Optical and electronic properties of lead-based 2D MHPs are conventionally tuned by changing halides or organic cations, but the effect of substituting the metal cation is also crucial to the development of more environmentally friendly alternatives based on tin [5-6].
Here we investigate the excitonic energetic landscape of the prototypical 2D MHP phenethylammonium tin iodide ((PEA)₂SnI₄), and compare it with its lead counterpart to clarify the role of the metal cation [7], demonstrating that the role of the organic-inorganic interactions can also be effectively manipulated by the metal cation, especially moving from the heavier lead to lighter tin. We elaborate on the origin of multiple excitonic resonances uniquely observed in the linear absorption spectrum of (PEA)₂SnI₄ at energies about 200-300 meV above the primary exciton. We investigate their origin through calculations based on density functional theory and many-body perturbation theory, which suggest that the excitonic series at these higher energies are composed of electronic transitions from a lower lying valence band. Importantly, the valence band splitting does not stem from changes in spin-orbit coupling, but it is driven by the octahedral conformations that follow small variations in the organic-inorganic interactions within the crystal lattice.
We then discuss transient absorption spectroscopy results upon resonant and non-resonant exciton generation conditions to investigate the interaction between the multiple excitonic resonances: we experimentally show that the presence of the higher energy states results in a relatively slow nanosecond component in the formation dynamics of the primary exciton, in addition to the ultrafast phonon-driven hot carrier thermalization. While the presence of such slow relaxation channel for the excitons might be beneficial to many optoelectronic applications, our work suggests its possible control via systematic design of the organic cation. Moreover, our observations indicate that spin-orbit coupling does not play a primary role in the intricate yet crucial change as in the excitonic characteristics imparted by the tin substitution.

[1] Nat. Nanotechnol. 15, 969–985 (2020)
[2] Nat. Mater. 18, 349–356 (2019).
[3] J. Mater. Chem. C, 2020, 8, 10889-10896
[4] J. Phys. Chem. Lett. 2020, 11, 9, 3173–3184
[5] Nature 369, 467–469 (1994)
[6] Adv. Mater. 2019, 31, 1803230
[7] 10.26434/chemrxiv.14330018.v1

Understanding the atomic-scale crystallographic properties of photovoltaic semiconductor materials such as silicon, GaAs, and CdTe, has been essential in their development from interesting materials to large-scale energy conversion industries. However, studying photoactive hybrid perovskites by transmission electron microscopy (TEM) has proved particularly challenging due to the large electron energies typically employed in these studies.[1] In particular, the very close structural relationship between a number of crystallographic orientations of the pristine perovskite and lead iodide has resulted in severe ambiguity in the interpretation of EM-derived information, severely impeding the advance of atomic resolution understanding of the materials.

Here, we successfully image the archetypal CH(NH₂)₂PbI₃ (FAPbI₃) and CH₃NH₃PbI₃ (MAPbI₃) hybrid perovskites in their thin-film form with atomic resolution using a carefully developed protocol of low-dose STEM.[2] Our images enable a wide range of previously undescribed phenomena to be observed, including a remarkably highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI₂ interfaces, with a striking absence of long-range disorder in the crystal. These findings explain why inter-grain interfaces are not necessarily detrimental to perovskite solar cell performance, in contrast to what is commonly observed for other polycrystalline semiconductors. Additionally, we observe aligned point defects and dislocations that we identify to be climb-dissociated, and confirm the room-temperature phase of CH(NH₂)₂PbI₃ to be cubic. We further demonstrate that degradation of the perovskite under electron irradiation leads to an initial loss of CH(NH₂)²⁺ ions, leaving behind a partially unoccupied, but structurally intact, perovskite lattice, explaining the unusual regenerative properties of partly degraded perovskite films. Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

[1] Adv. Mater. 2018, 30, 1800629
[2] Science 370, eabb5940 (2020)

Halide perovskite materials exhibit promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25.5 % in single-junction devices and 29.5% in tandem devices. Formamadinium (FA) lead iodide (FAPbI₃) and FA-rich perovskites are preferred for photovoltaic applications, but their widespread adoption is hindered by the rapid degradation of the desirable corner-sharing cubic (3C) phase to an undesirable wide bandgap, face-sharing, hexagonal (2H) phase under ambient conditions. Alloying FA with small amounts of Cs⁺ and methylammonium (MA) on the A site cation of the ABX₃ perovskite structure has proven a promising strategy for stabilizing photoactive perovskite phases. For example, photovoltaic devices fabricated with compositions such as Cs_0.05FA_0.78MA_0.17Pb(I_0.83Br_0.17)₃ (triple cation) perovskites have achieved high device efficiencies with greatly enhanced reproducibility and ambient stability ¹. Recently, there has also been renewed interest in methods to stabilize pure FAPbI₃ through strategies such as the incorporation of MA via treatment with methylammonium thiocyanate vapour ², addition of formamidinium formate ³ or methylammonium formate ⁴.
The mechanism of improved stability obtained from these approaches is generally explained as either originating from a tuning of the Goldschmidt tolerance factor towards the perfect cubic perovskite structure via cation mixing in the case of Triple Cation perovskites ^1,5, by templating growth of the corner-sharing cubic structure in stabilized-FAPbI₃ perovskites^2,4, or by reducing intrinsic defect density ³. A key tenet in all these explanations is that the final photoactive perovskite material, regardless of stabilization approach, is a cubic perovskite structure and that this is the structure that should be pursued for optimal stability and performance. Here, we reveal that contrary to conventional wisdom, this is not the case. In this talk using scanning electron diffraction (SED) we reveal the nanoscale structural origins of improved material stability in (Cs_0.05FA_0.78MA_0.17)Pb(I_0.83Br_0.17)₃ and stabilized thin films of a- FAPbI₃, two of the most promising candidates for commercial PV applications. Further, we will describe how these nanoscale stability mechanisms are related to our recent reports of performance limiting trap clusters in high-performing perovskite films⁶. Together, our talk will help answer questions such as “What are the nanoscale origins of instability in perovskite materials and devices?”, “how important is phase purity for performance?”

1. Saliba, M. et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science 9, 1989–1997 (2016).
2. Lu, H. et al. Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells. Science 370, (2020).
3. Jeong, J. et al. Pseudo-halide anion engineering for α-FAPbI 3 perovskite solar cells. Nature 592, 381–385 (2021).
4. Hui, W. et al. Stabilizing black-phase formamidinium perovskite formation at room temperature and high humidity. Science 371, 1359–1364 (2021).
5. Kim, J. Y., Lee, J.-W., Jung, H. S., Shin, H. & Park, N.-G. High-Efficiency Perovskite Solar Cells. Chem. Rev.120, 7867–7918 (2020).
6. Doherty, T. A. S. et al. Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites. Nature 580, 360–366 (2020).

Power conversion efficiency of organic-inorganic metal halide ABX₃ (A: methylammonium-MA, formamidinium-FA, cesium-Cs; B: lead-Pb, tin-Sn; X: iodine-I, bromine-Br, chlorine-Cl) perovskite-based solar cells highly depends upon the structural and optoelectronic quality of the absorber layer. For polycrystalline perovskite films, higher material quality generally implies larger grain size, continuity of grains with reduced grain boundaries, increased carrier lifetimes and diffusion lengths, and reduced recombination, thereby increasing both the current and voltage generation in solar cells. Variation in the anion-cation composition affects the bandgap energies, absorption coefficient, and material structural quality such as crystal structure, phase stability, and grain size. In semiconductors like these perovskites, a wider exponentially decaying tailing effect in the absorption coefficient spectra below the bandgaps is indicative of the presence of lattice vibrations, defects, and impurities. The Urbach energy is the width of this exponentially decaying tail, and its value is reported for more than 60 unique perovskite compositions with bandgaps ranging from 1.2 – 2.3 eV. Urbach energies for pristine perovskite films are measured using the combination of photothermal deflection spectroscopy and spectroscopic ellipsometry. In mixed Sn + Pb perovskites, films with < 50% Sn increase the Urbach energy, whereas intermediate or higher Sn contents lower it. Doping small amounts of Cl and Br (< 30%) with I slightly decreases the Urbach energy, whereas Cl or Br content > 30% increases it. More importantly, a correlation between the Urbach energy of perovskite absorber and corresponding device performance, in terms of the open-circuit voltage (V_OC) deficit here, has been studied. V_OC deficit is defined as the difference between the absorber bandgap normalized to the electron charge and the measured device V_OC, and largely refers to the existence of imperfections in the absorber layer. Most perovskite solar cells fabricated with low Urbach energy absorbers also exhibit low V_OC deficit and vice versa. However, those with Cs-based inorganic or mixed Sn + Pb perovskites with very high Sn content show a large V_OC deficit even though the corresponding pristine absorber layers show low Urbach energy. In these cases, the absorber layers inside these solar cells degrade or transition to non-perovskite phases due to atmospheric exposure, and no longer reflect the pristine film conditions at which their Urbach energies were measured.

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called “hollow” perovskites. We can take advantage of their defect tolerance to engineer a new family of 3D highly defective “hollow” iodide and bromide perovskites with general formula (FA)_1-x(en)_x(Pb)_1-0.7x(X)_3-0.4x (FA = formamidinium, en = ethylenediammonium, x = 0-0.44, X=Br, I). The corresponding materials were characterized by a battery of techniques, including single crystal X-ray diffraction (XRD), high resolution powder X-ray diffraction (PXRD), and solid state nuclear magnetic resonance (NMR) measurements. Pair distribution function (PDF) analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure, validating further that the corresponding materials are lead deficient and that en inclusion has the same structural effect as in the case of the “hollow” iodide analogs. By manipulating the number of defects, we finely tuned the optical properties of the pristine FAPbBr₃, by blue shifting the band gap from 2.20 eV to 2.60 eV for the 42% en sample. A most unexpected outcome was the fact that above 40% en incorporation the material exhibits strong broad light emission with 1% photoluminescence quantum yield that is maintained after exposure in air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite and is a clear demonstration that defect engineering can be utilized in our favour to add unusual, exotic optoelectronic properties to this versatile class of materials.

Surface passivation is an effective way to boost the efficiency and stability of perovskite devices. However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. In this talk, I will present a novel multi-functional semiconducting organic ammonium cationic interface modifier inserted between the light-harvesting perovskite film and the hole transporting layer. We show that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non-radiative recombination. Together with improved energy level alignment and stabilized interface in device, we demonstrate a triple-cation mixed-halide medium-bandgap perovskite solar cell with an excellent power conversion efficiency of 22.06% (improved from 19.94%), and the suppressed ion migration and halide phase segregation, which lead to a long-term operational stability over 1000 hours. Using a similar concept, we achieved high performance red LEDs with external quantum efficiency over 20%. Our strategy provides a new practical method of interface engineering in perovskite solar cells and LEDs towards improved efficiency and stability.

As the stability and efficiency of perovskite solar cells improve, efforts toward developing a manufacturing process have expanded. One promising candidate for high-speed perovskite producition is vapor transport deposition (VTD). A vairant of this process is used by First Solar LLC for the deposition of CdTe solar modules and has assisted CdTe in becoming the market leader in thin film photovoltaics. While VTD presents a viable fabriation route for single-step or two-step perovskite deposition, an understanding of the processing fundamentals is required to design functional equipment. This information is acquired from the precursor technology: close space vapor transport (CSVT).
Previously, we presented a two-step, all-vapor, CSVT deposition process to fabricate methylammonium lead iodide (MAPbI₃) thin films.¹ The two steps of this process are 1) the deposition of a PbI₂ thin film, followed by 2) a reaction in methylammonium iodide (MAI) vapor.¹ While the PbI₂ deposition process is well-understood,^2,3 the MAI reaction remains uncharacterized. Our preliminary work found that the consumption of PbI₂ to form MAPbI₃ could be approximated using a first-order reaction rate law, though deviations between model and experiment were notable.¹ Here, we expand upon our understanding of the MAI reaction process using controlled experimentation and mathematical modeling.
Experiments were carried out by depositing thick films of PbI₂ (600-800 nm) onto C₆₀/ITO/glass substrates. Isothermal reactions in MAI vapor were then carried out at a fixed pressure of 9 Torr for three temperatures: 110°C, 120°C, and 130°C. Reaction times were varied at each condition, and replicates were carried out to assess process reproducibility. This allowed the intermediate reaction states to be studied using variable-angle glancing incidence x-ray diffraction (GIXRD) and cross-sectional scanning electron microscopy (SEM).
GIXRD measurements were carried out on reacted samples from an incident angle of 0.3° to 3.0° in 0.1° increments and were used to determine the structure of the sample at different x-ray penetration depths. These scans showed that the principal tetragonal MAPbI₃ peak (110)/(002) at 2θ = 14.0° was present at every incident angle, while the principal PbI₂ (001) peak at 2θ = 12.6° appeared at different angles of incidence depending on the reaction temperature and time. The onset angle was measured for each sample and used to calculate an x-ray penetration depth to determine the MAPbI₃ film thickness.
Cross-sectional SEM measurements were performed on each partially-reacted sample to determine the structure as a function of film thickness. It was observed that a distinct layered structure formed during the reaction. There was good agreement between the calculated x-ray penetration depth through MAPbI₃ and the thickness of the top layer, which confirmed the layers were MAPbI₃ on top of unreacted PbI₂.
The MAI reaction was found to be consistent with a non-catalytic, heterogeneous, solid-fluid reaction model.⁴ A mathematical model will be developed based on these experimental data, and a reaction rate law will be presented at the conference. Using the three reaction temperatures, the temperature dependence of the reaction will also be presented and will account for the concentration of MAI taking part in the reaction⁵ and the temperature dependence of the Thiele modulus.⁴
The information to be presented at the conference will serve as a critical stepping-stone for the development of high-throughput, two-step perovskite vapor processes.

References
1. A. Harding et al. RSC Adv. 2020, 10, 16125-16131.
2. A. Kuba et al. 2020 47th IEEE Photovoltaic Specialists Conference (PVSC), 1203-1206.
3. A. Harding et al. 2021 48th IEEE Photovoltaic Specialists Conference (PVSC).
4. C. Y. Wen. Ind. Eng. Chem. Res. 60, 34-54. 1968.
5. A. Dualeh et al. Chem. Mater. 2014, 26, 6160-6164.

2D hybrid lead halide perovskites are light-harvesting semiconductors with tailorable structural and photophysical features that provide a vast compositional space to engineer new and environmental stable materials for optoelectronic applications. Herein, we actuate a comprehensive exploration of the structure-property relationships in several, newly reported series of 2D halide perovskites with thick layers (n≥2) in bulk crystal and film form. Firstly, thermochemical evaluation of the representative 2D structure types of Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) perovskites is presented based on calorimetric measurements that reveal that the enthalpy of formation for the studied RP perovskites is negative while for the DJ perovskites is positive.¹ Film stability tests demonstrate consistent observations with the thermochemical findings, where RP lead iodide perovskites are both thermodynamically and environmentally stable candidates for optoelectronic applications. Synthesis and characterization of the second known example of a n=6 perovskite compound: (PA)₂MA₅Pb₆I₁₉ (where PA= pentylammonium) is reported with a favorable enthalpy of formation.¹ Secondly, the methodical tailoring of the 2D perovskite structure’s composition (organic and inorganic component), unveils trends in the comparison of thick-layered lead iodide and lead bromide perovskites, for the acquisition of halide perovskites with enhanced stability.² New members of n=2 lead bromide perovskites that belong to the Ruddlesden-Popper structure type are presented with formula: (C_mH_2m+1NH₃)₂(CH₃NH₃)Pb₂Br₇ (m = 6−8) incorporating long chain alkyl-monoammonium cations (C_mH_2m+1NH₃) of hexylammonium (m=6), heptylammonium (m=7) and octylammonium (m=8). 2D thick-layer lead bromide perovskites reveals pronounced differences with their lead iodide congeners, in terms of lattice match between the inorganic [PbX₆]^4- perovskite layer and organic layer, carrier lifetime and film stability behavior.² Lastly, we examine the crystal chemistry of thick-layer perovskites consisting of organic molecules with bifunctional groups and investigate thoroughly the excited-state dynamics by transient absorption (TA) spectroscopy. Our work highlights the importance of the optimal synthetic design and engineering of environmentally robust, 2D lead halide perovskite materials for next-generation optoelectronic devices.


<!--[if supportFields]><spanstyle='mso-element:field-begin'></span><spanstyle='mso-spacerun:yes'> </span>ADDIN EN.REFLIST <span style='mso-element:field-separator'></span><![endif]-->1. Vasileiadou, E. S.; Wang, B.; Spanopoulos, I.; Hadar, I.; Navrotsky, A.; Kanatzidis, M. G., Insight on the Stability of Thick Layers in 2D Ruddlesden–Popper and Dion–Jacobson Lead Iodide Perovskites. J. Am. Chem. Soc. 2021, 143 (6), 2523-2536.
2. Eugenia S. Vasileiadou, I. H., Mikaël Kepenekian, Jacky Even, Qing Tu, Christos Malliakas, Daniel Friedrich, Ioannis Spanopoulos, Justin M. Hoffman, Vinayak P. Dravid, and Mercouri G. Kanatzidis, Shedding Light on the Stability and Structure−Property Relationships of Two-Dimensional Hybrid Lead Bromide Perovskites. Chem. Mater. 2021, 10.1021/acs.chemmater.1c01129.
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The use of lead-based perovskites in photovoltaic applications has demonstrated to be a successful alternative for future energy generation, but still lead toxicity is a concern and a limitation in some countries. Lead-free double perovskites have attracted attention of the research community due to the ability of tuning their bandgap, structure and stability by selectively choosing the combination of metals. Cs₂AgBiBr₆ presents great ambient stability, a relatively easy synthesis route, and consists of cheap and abundant materials. However, the large and indirect band gap of this composition hinders its potential as an efficient light-harvesting material. Doping is a promising strategy to improve the optical and electronic properties of this material, even though only few works studied in details the chemistry and physics involved in Cs₂AgBiBr₆ doping. In this work, we work on some modifications of Cs₂AgBiBr₆ single crystals with various cations and realize structural investigations combined with photophysical tests to elucidate the mechanism involved in the doping. The understanding of doping allows the comprehension of Cs₂AgBiBr₆ chemistry, opening opportunities to obtain other promising double perovskites.

Halide perovskites with desirable stability, electronic band structure, and optical absorption are sought for solar cells, electronic devices, infrared sensors and quantum computing. Compositional manipulation via alloying at cation or anion sites, or via incorporation of point defects and impurities, can help tune their properties. In this work, we develop AI-based frameworks for the on-demand prediction and multi-objective optimization of the phase stability, band gap, optical absorption spectra, photovoltaic efficiency, and defect formation energies for a chemical space of ABX₃ halide perovskites with several choices for A, B and X, mixing allowed at each site, and several possible crystal structures. These frameworks are powered by high-throughput density functional theory (DFT) computations, dimensionality reduction techniques applied upon unique encodings of the atom-composition-structure (ACS) information, and rigorous training of advanced neural network (NN)-based predictive and optimization models. In the current work, we restrict the atom choices to the following: A Î {FA, MA, Cs, Rb, K}, B Î {Pb, Sn, Ge, Ba, Sr, Ca}, and X Î {I, Br, Cl}; we also note that the design frameworks can easily be extended to other atom choices with the infusion of fresh data.

DFT-based geometry optimization and property estimation is performed for hundreds of perovskite alloys simulated using special quasi-random structures (SQS), with new data points sequentially selected using Bayesian active learning approaches to comprehensively traverse the compositional chemical space and iteratively improve prediction accuracies. NN regression models trained upon the DFT data enable the simultaneous prediction of all properties of any ACS combination in the perovskite chemical space, with a special focus on high entropy alloys containing fractions of 5 or 6 elements at the A or B sites. We find that with all end-point and selected mixed compositions included in the training dataset, the predictive models are successfully able to reproduce known formation energy and band gap bowing behavior in perovskite alloy compositions. Further, multi-fidelity learning helps to bridge the gap between (high quantities of) low accuracy calculations and (lower quantities of) high-fidelity data, constituted of either accurate, expensive computations or experimental measurements collected from the published literature. The best predictive models are combined with two different multi-objective optimization techniques, namely, state-of-the-art genetic algorithms and variational autoencoders, to determine optimal high entropy alloy perovskite compositions with desired stability, optoelectronic, and defect properties. Finally, AI-based recommendations are synergistically coupled with targeted synthesis and characterization, leading to successful validation and discovery of novel halide perovskite compositions for improved performance in solar cells.

References
1. A. Mannodi-Kanakkithodi, J. S. Park, N. Jeon, D. H. Cao, D. J. Gosztola, A. B. F. Martinson, M. K. Y. Chan, "Comprehensive Computational Study of Partial Lead Substitution in Methylammonium Lead Bromide", Chemistry of Materials 31 (10), 3599–3612 (2019).
2. A. Mannodi-Kanakkithodi, M. Toriyama, F. G. Sen, M. Davis, R. F. Klie, M. K. Y. Chan, "Machine learned impurity level prediction in semiconductors: the example of Cd-based chalcogenides", npj Computational Materials 6, 39 (2020).
3. A. Mannodi-Kanakkithodi, J. S. Park, A. B. F. Martinson, M. K. Y. Chan, "Defect Energetics in Pseudo-Cubic Mixed Halide Lead Perovskites from First-Principles", Journal of Physical Chemistry C. 124, 31, 16729–16738 (2020).
4. A. Mannodi-Kanakkithodi, M. K. Y. Chan, "Computational Data-Driven Materials Discovery", Trends in Chemistry 3, 2, 79–82, (2021).
5. A. Mannodi-Kanakkithodi, R. E. Kumar, D. Fenning, M. K. Y. Chan, "Data-Driven Design of Novel Halide Perovskite Alloys", in preparation.

Hybrid perovskite solar cells (with chemical formula ABX₃) are of great interest due to the recently measured power conversion efficiency of greater than 25% (but theoretically, 33.7%). Perovskite structures are easily customisable, with a range of options for A, B and X. This enables us to both tune the electronic band gap and the stability by varying the composition. Two promising perovskites are the CH₃NH₃PbI₃ (MAPI) and CH(NH₂)₂PbBr₃ (FAPB) structures. By varying the ratio of FA and MA and doping with Br, we can potentially tune the band gap and effective masses (and hence electronic transport).

In order to compromise for a more desirable bandgap for stand-alone PV and better structural stability, the hybrid perovskite constituents are then strategically layered into superlattice form. We explore the bulk properties and interfacial engineering to improve the material’s electronic transport.

We present a theoretical investigation of the structural and electronic properties of MAPI(x)/FAPB(y) superlattice, performed using first–principles density functional theory and temperature dependent vibrational entropy correction. Our results show that dominating number of layers of FAPB over MAPI will decrease the band gap. We discuss why the formation energies of superlattice in hexagonal and cubic phases differ across temperature and contradicts from the ones of its ground-state constituents. The solutions will be suggested for experimental validation.

Perovskite solar cells saw a phenomenal development over the past years and for some time dwarfed all progress in the field of organic solar cells. Meanwhile, the introduction of non-fullerene acceptors unlocked skyrocketing efficiencies of up to >18% in organic solar cells recently, with an absorption spectrum extending well into the infrared.^[1] Therefore, new opportunities open up, to implement them for high efficiency tandem architectures. As perovskites and organic semiconductors share similar processing technologies they are attractive partners for the monolithic tandem integration. On top of that, low gap perovskite materials are almost exclusively based on Sn²⁺ containing structures, which still give rise to fundamental stability concerns, as the 2+ state is easily oxidized resulting in deterioration by parasitic self-doping of the active material.^[2]

Likewise, serious concerns currently exist about photo instability of most non-fullerene organic systems. Now, in surprising contrast to these almost paradigmatic concerns, we evidenced an outstanding operation stability of an organic single junction (~ 100% PCE remaining after >1000 h continuous operation) that is based on the PM6:Y6 active system (>17% efficiency), if excitons are exclusively generated on the acceptor material Y6 (which is the case in a tandem application). Hence, in stark contrast to perovskite-perovskite tandem approaches, the sub-cell limiting operation stability shifted from the low gap (Sn-) perovskite to the wide gap (1.85 eV) Br-rich perovskite cell. To this end we were also able to show, that the effect of detrimental halide segregation in high (50%) Br content perovskite solar cells strongly depends on the choice of hole transporting material, providing a possible pathway towards how those concerns might be further addressed in the future.

Despite of high promise, as of yet the efficiency of perovskite/organic tandem cells remain underwhelming at approximately 20% due to severe optical and electronic losses.^[3] By a combination of interface optimization at the wide-gap perovskite cell, a lossless and almost perfectly shunt resilient ALD grown indium oxide interconnect and the highly robust gas quenching technique utilizing n-methyl-2-pyrrolidon (NMP) as co-solvent, we are now able to demonstrate perovskite / organic tandems with efficiencies approaching 24%. As all three utilized sub processes for the building blocks are extremely reproducible, we were at the same time able to achieve an extremely narrow process variation.
Based on a realistic simulation, we foresee efficiencies of perovskite/organic tandem cells exceeding 31% in the future.

[1] Liu Q, et al. Science Bulletin 65, 272-275 (2020).
[2] Lin R, et al. Nature Energy 4, 864–873 (2019).
[3] Chen X, et al. Joule 4, 1594-1606 (2020).

Lattice-strain has been recognized as a critical attribute in determining electronic and optoelectronic properties of halide materials, on the basis of experiment and theoretical estimation. Herein, we propose a unique experimental approach to induce extra lattice-strain in flexible cesium lead bromide (CsPbBr₃) thin films by modulating the level of bending curvature of the plastic substrate to produce different magnitudes of tensile or compressive strain during the two-step solution deposition. The CsPbBr₃ thin films processed with the in situ strain of −0.83% to +0.83% were used to fabricate a self-powered photodetector of Ag:LiF/ Phenyl-C61-butyric acid methyl ester (PCBM)/CsPbBr₃/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/ITO/PEN structure. As a result, the maximum responsivity of 121.5 mAW⁻¹ and detectivity of 2.28x10¹⁰ cmHz^0.5W^-1 were achieved at a light intensity of 0.47 mWcm⁻² for the maximum tensile strain of +0.83% without applying a bias, which were the increases by ~61 and ~55%, respectively, relative to the unstrained reference case. These improvements were attributed to the weakened orbital coupling between Pb 6s and Br 4p with longer Pb−Br bonds in the case of tensile strain, causing the down-shifts in the valence band minimum and conduction band minimum, with the promoted drift of photogenerated carriers.

All-inorganic perovskites are particularly attractive for optoelectronic applications such as light emitting diodes and lasers, due to their excitonic properties and benefits of thermal and environmental stability in comparison to some hybrid analogues.[1] Especially for lasers all-inorganic perovskites, such as CsPbCl₃, with emission in the blue/violet wavelength region are desirable. Unfortunately, as of yet, the solution-based preparation of CsPbCl₃ is strongly complicated by the poor concomitant solubility of the two precursor salts PbCl₂ and CsCl.

Recent reports have shown the conversion of CsPbBr₃ by exposure to hydrochloric acid (HCl) as a chlorine source.[2] While the resulting CsPbCl₃ layers showed some blue emission, the HCl simultaneously caused significant etching of the exposed perovskite layers.

Here, we propose a non-corrosive and highly controllable halide exchange process for CsPbBr₃ via pulsed exposure to metal halide precursors (e.g. SnCl₄ or TiCl₄) inside an Atomic Layer Deposition (ALD) reactor. The successive conversion from CsPbBr₃ to CsPb(Br_1-xCl_x)₃ (up to x = 1) with an increasing number of ALD pulses is confirmed by optical absorption and emission spectroscopy, X-ray diffraction (XRD), as well as Rutherford-Backscattering Spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS). Grazing-incidence wide-angle X-ray scattering (GIWAXS) evidenced that the layers are homogeneously converted. We demonstrate that the converted films do not show any signs of etching due to the conversion process. We furthermore demonstrate that our concept can be likewise applied for CsPbBr₃ layers that have been imprinted by thermal nanoimprint, which is particularly important with regard to photonic nanostructures. In first orienting studies, optically pumped amplified spontaneous emission (ASE) at λ = 426 nm with a threshold of 1.2 mJ/cm² at room temperature could be achieved in these converted layers. This is a significant step towards all-inorganic perovskite lasers emitting in the violet/blue spectral region.

[1] Pourdavoud N., et al., Adv. Mater. 2019, 31, 1903717, DOI: 10.1002/adma.201903717
[2] J. M. Pina, et al., Adv. Mater. 2021, 33, 2006697, DOI: 10.1002/adma.202006697

Light-soaking, photo-damage and self-healing are intriguing phenomena widely reported in halide perovskites. Yet, their origin is still not understood even if they strongly influence the stability of the material. We discussed recently which could be the chemical origin of various phenomena connected to these effects [1] revealing how the stability of the interstitial Br defects (associated with photodamage) in MAPbBr₃, FAPbBr₃, CsPbBr₃ is inversely related to the kinetic of self-healing of photodamage after intense illumination.
In this talk, I will extend the description of the effects of intense illumination to MAPbI₃ showing how this material can also self-heal from photodamage with kinetics in the order of (several) minutes. I will reveal the presence of multiple (chemical) pathways activated by intense illumination some of which cause an increase of the photoluminescence (light soaking) and some of which decreasing it (photodamage). All of these are, at least partially, reversible and proceed from what can be called the “high entropy” of the halide perovskites.
I will also show how water, even in conditions that do not cause the degradation of the material, strongly influences all the mentioned processes actually “protecting” MAPbI₃ from photodamage but also substantially impeding light-soaking. This causes the measurements performed over a short time in an inert atmosphere to provide better results because of transient light-soaking effects which, eventually, disappear over time in both humid and inert atmospheres.
The talk will conclude with a panoramic over the literature of the reported effect trying to identify any other critical experimental condition inducing variations on the photodamage and light soaking effect or the kinetics and extent of self-healing. I will also provide additional information on the activation energies involved in the mentioned processes as obtained studying the temperature dependence of these in MAPbBr₃, FAPbBr₃ and CsPbBr₃

Halide perovskite semiconductors are emerging candidates for new age Photovoltaics with high power conversion efficiency in single-junction cell and tandem geometry.¹ Researcher are exploring the structural, photo-physics, and optoelectronic properties of these semiconductors.^{<span style="font-size:10.8333px">2,3</span>} In this quest, we investigate the different layers of cells from the electron transport layer to the oxide protection layer, the active layer, and the hole transport layer in terms of thickness, molar concentration of precursors, doping concentration for optimal performance. The photoluminescence study of ETL-perovskite reveals that electrons extracted efficiently without quenching due to the high conductivity of tin oxide and better interface alignment with the perovskite layer. A significant change of near about 21 ‰ between the highest and lowest power conversion efficiency was observed by a simple variation of process parameters.

Keywords: Process Engineering; Halide Perovskite; Planer Structure; Photoconversion Efficiency

Reference:
[1] I. Mora-Seró, M. Saliba, Y. Zhou, Towards the Next Decade for Perovskite Solar Cells, Solar RRL, 4, 2, 1900563, 2020.
[2] K. Zhu, M. Yang, Y. Zhou, N. P. Padture, Organo-metal halide perovskites films and methods of making the same, US Patent, 10,910,569, 2021.
[3] S. Singh, Laxmi, and D. Kabra, Defects in halide perovskite semiconductors impact on photo-physics and solar cell performance, J. Phys. D: Appl. Phys. 53, 503003, 2020

The commercialization of perovskite solar cells (PSC) for next-generation photovoltaic technology hinges on improvements to their long-term stability. Several environmental stressors (light, humidity, temperature, bias, and oxygen) are known to trigger degradation in PSC, which limits device lifetimes to far below those of state-of-the-art Si cells. For effective commercialization, the large parameter space of perovskite compositions and synthesis conditions must be traversed to design optimal devices. Such devices must also undergo lengthy stability testing to prove their viability. With this development process in mind, taking a standard trial-and-error materials research approach to PSC design, fabrication, and stability testing becomes untenably time consuming while demanding significant human labor costs. The field of machine learning (ML) presents an alternate strategy that can contribute to a new research paradigm: one with greater pace, rationality, and reproducibility than trial-and-error methods. Here, we first present ML as a toolkit to accelerate PSC commercialization and demonstrate a general framework for projects to follow. We establish a roadmap to guide researchers through all steps of ML model development, which include collecting the initial training data, data cleaning and pre-processing, feature engineering, model selection, and model optimization and testing.

Next, we showcase this framework in action by using ML to forecast the optical performance of perovskite thin films under dynamic environmental conditions. We track the photoluminescence (PL) signal from the films over time while controlling the relative humidity (rH) and temperature in a sealed, nitrogen-filled chamber, then use ML to predict the PL response for each film composition using rH and temperature data alone. For this time-series prediction task, we implement recurrent neural networks (RNN), a class of neural networks with a historical state or “memory.” The network forecasts dynamic PL responses using the environmental state (rH and temp) at a specific time point along with the historical state, which represents physical changes that have occurred within the film. Our optimized model generates PL forecasts over 4 hours for methylammonium lead triiodide (MAPbI₃) and triple-cation (formamidinium-methylammonium-cesium) perovskite thin films with normalized root mean square error (NRMSE) of 18% and 14%, respectively. Our network can be driven by weather forecast data, predicting future PL output with high time resolution (one point every 15 seconds) and providing insight into the optical response and stability of perovskite materials under various weather conditions. This work illustrates the potential of ML for PSC development through the example of time-series prediction of optical properties. We envision extensions of this concept to other perovskite compositional families and to other combinations of environmental stressors, including full devices under standard operating conditions and International Summit on Organic Photovoltaic Stability (ISOS) stability testing protocols.

Building upon the success of lead halide perovskites, lead-free silver−bismuth semiconductors have become increasingly popular materials for optoelectronic applications. Of these materials, perhaps the most prominent is Cs₂AgBiBr₆, which has demonstrated a better thermodynamic stability than MAPbI₃ but whose photovoltaic power conversion efficiency lags behind [1]. One reason for this is the strong electron-phonon coupling in Cs₂AgBiBr₆, which sets a fundamental limit on the charge-carrier mobilities and hence critically affects device performance. Understanding the nature of the charge-lattice coupling in Cs₂AgBiBr₆, and its influence on charge-carrier dynamics, is thus crucial for the optimization of these materials in a variety of optoelectronic applications and to further the understanding of a wide range of related silver− bismuth−halide semiconducting materials. Of these, one particularly promising contender is Cu₂AgBiI₆, which was first reported earlier this year [2] and is notable for its direct bandgap.

In this study, I report on the evolution of photoexcited charge carriers in Cs₂AgBiBr₆ [3] and Cu₂AgBiI₆ [4]. In both materials, I observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of a few picoseconds to an intrinsic small polaronic state. I attribute this to the strong charge-lattice coupling in these bismuth-silver materials, while the temperature-independence of the self-trapping is indicative of low electronic dimensionality due to the electronic isolation of the distinct Ag⁺ and Bi³⁺ sites. Through a combination of temperature-dependent photoluminescence and absorption spectroscopy, I identify the electronic states occupied before and after localization, directly tracing the localization of the charge carriers with time and interpreting its influence on the charge-carrier mobility in terms of a quantitative model. Since the small polaron motion is temperature-activated, I find that their mobility still exceeds 1 cm² V⁻¹s⁻¹ in both materials, leaving open the prospect of their application in efficient photovoltaic devices.

[1] Longo, G.; Mahesh, S.; Buizza, L. R. V.; Wright, A. D.; Ramadan, A. J.; Abdi-Jalebi, M.; Nayak, P. K.; Herz, L. M.; Snaith, H. J. Understanding the Performance-Limiting Factors of Cs₂AgBiBr₆ Double-Perovskite Solar Cells. ACS Energy Lett. 2020, 5, 2200–2207.
[2] Sansom, H. C.; Longo, G.; Wright, A. D.; Buizza, L. R. V; Mahesh, S.; Wenger, B.; Zanella, M.; Abdi-jalebi, M.; Pitcher, M. J.; Dyer, M. S.; Manning, T. D.; Friend, R. H.; Herz, L. M.; Snaith, H. J.; Claridge, J. B.; Rosseinsky, M. J. Highly Absorbing Lead-Free Semiconductor Cu₂AgBiI₆ for Photovoltaic Applications from the Quaternary CuI−AgI-BiI₃ Phase Space. J. Am. Chem. Soc. 2021, 143, 3983–3992.
[3] Wright, A. D.; Buizza, L. R. V; Savill, K. J.; Longo, G.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Ultrafast Excited-State Localization in Cs₂AgBiBr₆ Double Perovskite. J. Phys. Chem. Lett. 2021, 12, 3352–3360.
[4] Buizza, L. R. V; Wright, A. D.; Longo, G.; Sansom, H. C.; Xia, C. Q.; Rosseinsky, M. J.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Charge-Carrier Mobility and Localization in Semiconducting Cu₂AgBiI₆ for Photovoltaic Applications. ACS Energy Lett. 2021, 6, 1729–1739.

This paper examines the effects of grain size on the mechanical properties of hybrid organic-inorganic perovskites (HOIPs). Nanoindentation measurements and statistical deconvolution techniques are used to study the Young’s moduli and hardness or strengths of perovskite layers in HOIPs. The size dependence of indentation hardness is discussed within the context of Mechanism-based Strain Gradient (MSG) plasticity theories. These are also used to obtain information on the underlying geometrically necessary and statistically stored dislocation densities. The grain size effects associated with the different annealing temperatures (between 80 and 170^oC) are also characterized using Hall-Petch approaches. The implications of the results are then discussed for the microstructural design of robust perovskite solar cells on rigid and flexible substrates.

Layered metal halide perovskites are attractive semiconductors for a range of different optoelectronic applications owing to their large structural versatility and rich photophysics. As is the case for the more conventional 3D perovskites, thin films of these materials can be deposited directly from solution, thereby holding the promise of procuring flexible and cost-effective films through large-scale fabrication techniques. However, such solution-based deposition techniques often induce large degrees of heterogeneity due to poorly controlled crystallization. While much attention is focused on the optimization of thin film microstructure and its relation to device performance in the case of 3D perovskites, the microstructure of layered perovskite thin films remains markedly underexplored.

In this talk, I will present a detailed characterization of the optical properties and microstructure of the archetypal 2D perovskite phenylethylammonium lead iodide ((PEA)₂PbI₄). Using a combination of structural characterization techniques, we demonstrate that thin films with large grains and strong texture are readily obtained through a simple and scalable blade-coating process. Moreover, we highlight the large impact that the stoichiometry of the precursor solution has on the crystallinity, morphology, and optical properties of the resulting thin films.

All in all, we illustrate the large potential for scalable fabrication of 2D perovskite films and underline some of the major hurdles to be overcome for the further development of devices based on large area layered perovskite film.

Thermal management in LEDs and lasers is important for both efficiency and lifetime. Thermal management is especially challenging in perovskite-based devices [1], due to their exceptional thermal properties, such as ultra-low thermal conductivity λ, low thermal diffusivity a, and small volumetric heat capacity c_vol. at room-temperature [2]. These thermal properties strongly depend on temperature and structural phase transitions, which in some perovskites even occur at moderate temperatures. In this context it should be noted that for inorganic cesium-based perovskites there are only a few reports on the thermal properties at their respective phase transitions [3].

Here the thermal properties of perovskites are studied using a Scanning Thermal Microscope, which takes advantage of the 3ω-method in the frequency domain without the need of elaborate sample preparation. Furthermore, it allows the simultaneous acquisition of the thermal properties λ(T), a(T), and c_vol(T) [4]. The thermal properties of nano crystalline and large crystalline CsPbBr₃ perovskite thin films were examined from room-temperature to 150°C, in which orthorhombic to tetragonal to cubic phase transitions occur [5]. The thermal properties of large crystalline films with layer thicknesses 110 nm, 150 nm, and 200 nm were investigated.

It could be confirmed from the λ(T), a(T), and c_vol.(T) characteristics that CsPbBr₃ thin films with small crystallites (200nm) show a second order phase transition at 88°C from orthorhombic to tetragonal and a first order transition at 132°C from tetragonal to cubic, as described in earlier works [6]. In stark contrast, for thin films with large crystals (5-10µm), instead of abrupt changes in l(T) at the tetragonal to cubic phase transition temperature, a continuous increase was found. The onset of the increase of the thermal conductivity shifts slightly to lower temperatures with decreasing the layer thickness from 200 nm to 110 nm. In addition, we also find that the thermal diffusivity a(T) significantly decreases already in the tetragonal phase in comparison to the orthorhombic phase. On the other hand, only the thin films with a thickness of 110 nm a typical signature of latent heat (pointing to a first order transition) is found in the transition from tetragonal to the cubic phase, basically associated with the freedom of rotational modes of PbBr₆ octahedra around the three principal axes, while c_vol(T) of the thicker films (150 nm and 200 nm) indicate a 2^nd order phase transition. It should be noted that the 1^st order phase transition of the 110 nm thick CsPbBr₃ perovskite film was found at a lower temperature 115°C than that reported for polycrystalline films and large single crystals at 132°C.

These temperature dependent thermal properties of large crystalline CsPbBr₃ perovskite thin films are similar to that of a material that exhibits phase transitions causing the material to contract upon increasing the temperature due to either a redistribution or ordering effects. This remarkable behavior at phase transitions requires particular attention because it will significantly influence the thermal management in perovskite-based LEDs and lasers.

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5. Zhang M. et al., CrystEngComm,2017,19,6797
6. Hirotsu S. et al., J. Phys. Soc. Jpn., 1974, 37, 1393–1398

Within the challenging race for alternative energy sources, hybrid metal halide perovskite solar cells (PSCs) have undergone unprecedented progress with efficiencies reaching now 25.5% [1-3]. These remarkable performances result from the exceptional optoelectrical properties of hybrid perovskite materials. Coupled with their potential for low fabrication cost, perovskite solar cell technology is very promising. However, since hybrid halide perovskites have a highly ionic character, they can decompose under external stresses such as moisture, solvents and heating cycles [4-6]. Reducing environmental stresses imposed by moisture or oxygen for example, in order to improve the long-term stability of perovskite solar cells, is critical to the deployment of this technology.

In 2017, Grancini et al [7] published a structure proven to be stable for more than 10,000 hrs, measured uned controlled standard conditions, by engineering an ultra-stable 2D/3D (HOOC(CH₂)₄NH₃)₂PbI₄/CH₃NH₃PbI₃ perovskite junction. This structure is based on a fully printable architecture made of three mesoporous layers in which the perovskite is embed.

In this communication, we investigate the effects of changing the perovskite composition in this architecture by adding and/or replacing iodine with bromine into its chemical composition to tune device, increasing the band gap of the material [8], opening the gate to potential application in water splitting (V_OC > 1.23 V at pH = 0 [9]). After characterization of their photovoltaic properties, the cells, reaching V_OC ~1.3 V are studied by SEM coupled to EDX, XRD, Raman, absorption and photoluminescence spectroscopies.


1. Green, M. A., Ho-Baillie, A. & Snaith, H. J. The emergence of perovskite solar cells. Nature Photonics 8, 506–514 (2014).
2. Green, M. A. & Ho-Baillie, A. Perovskite Solar Cells: The Birth of a New Era in Photovoltaics. ACS Energy Lett. 2, 822–830 (2017).
3. Best Research Cell Efficiencies. NREL https://www.nrel.gov/pv/cell-efficiency.html (2021).
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5. Leijtens, T. et al. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 5, 1500963 (2015).
6. Rong, Y., Liu, L., Mei, A., Li, X. & Han, H. Beyond Efficiency: the Challenge of Stability in Mesoscopic Perovskite Solar Cells. Adv. Energy Mater. 5, 1501066 (2015).
7. Grancini, G. et al. One-Year stable perovskite solar cells by 2D/3D interface engineering. Nature Communications 8, 15684 (2017).
8. Yu, W. et al. Diversity of band gap and photoluminescence properties of lead halide perovskite: A halogen-dependent spectroscopic study. Chemical Physics Letters 699, 93–98 (2018).
9. Walter, M. G. et al. Solar Water Splitting Cells. Chem. Rev. 110, 6446–6473 (2010).

In optoelectronic devices, the function and performance depends crucially on the proper alignment of the energy level landscape throughout the device, allowing for efficient charge transport across the various interfaces. For applications containing halide perovskites as active layer it turned out that such interfaces can show rather complex behavior. Not only the energetic alignment of transport levels play a role - more importantly, the perovskite composition and formation can be significantly influenced by chemical reactions taking place at these interfaces. In order to prepare highly efficient and stable devices, it is crucial to understand and control these interactions.
In this talk, I will summarize our work on a variety of metal-oxides such as SnO₂, MoO₃, ZnO, TiO₂ etc. where we use photoelectron spectroscopy to analyze which components are responsible for the strong interface chemistry. For this, we investigated a variety of different perovskites (i.e. organic vs. inorganic ones, I vs. Br, etc.) as well as the individual perovskite precursors. We find that the reactivity strongly depends on the individual material combination and that different metal oxides show fundamentally different reaction/degradation pathways. Furthermore, we found that by changing the defect density of the surface we can affect the degree of thin film degradation significantly.
Overall, I will show how photoelectron spectroscopy measurements can help to probe and understand the processes going on at these various bottom contact materials which should ultimately help to improve the stability of perovskite related devices.

Despite the outstanding rapid evolution of Perovskite based solar cells efficiencies, the main drawbacks of this technology remain their intrinsic and extrinsic stabilities. Researches nowadays are focusing on enhancing the device architecture where interfaces properties play a crucial role on the final performances and durability. In particular, it is crucial to understand the physical and chemical phenomena occurring at those interfaces, site of defects accumulation and carrier recombination, band mis-alignment or even chemical drift.
This approach is particularly challenging on perovskite solar cells because of the alternating of organic / inorganic layers, requiring a fine optimization of the analysis conditions. The present study addresses the question of the reliability of the chemical information when profiling perovskite layers by X-ray Photoelectron Spectroscopy (XPS) and Glow Discharge Optical Emission Spectroscopy (GD-OES).
XPS and GD-OES are first performed separately on a half-cell constituted of glass/FTO/c-TiO₂/m-TiO₂/triple cation Perovskites (Cs_0.05(MA_0.14, FA_0.86)_0.95 Pb (I_0.84, Br_0.16)₃). The high sensibility of perovskite layers requires to first evaluate the stability of those layers during the transfer from the glove box where they are elaborated to the spectrometers and during analyses themselves (X-Ray, UHV and ion bombardment) to find optimal operating conditions. Especially, the perturbation generated by ion beam and plasma etching has to be accurately studied to infer possible artefacts likely leading to misinterpretation and to determine the best parameters for the abrasion sequences.
In the case of GD-OES, not only the Radio Frequency power and the plasma gas pressure are changed, but also the nature of this gas (Ar, Ar/O, Ne). Concerning XPS, both monoatomic (Ar⁺) and cluster (Ar_n⁺) projectiles are tested, varying their energies and the cluster size. Finally, GD-OES and XPS show similar trends for the composition profiles. However, we observe that all the conditions employed for XPS profiling lead to a systematic reduction of lead as well as the degradation of the organic part and iodine loss, more or less pronounced. This raises the question of the bombardment effect on the perovskite surface, influencing the integrity of the collected chemical information. Craters obtained with the ion gun of the XPS spectrometer have been accurately characterized to have a better understanding on the bombardment impact on the perovskite surface and also beyond. First, AFM (Atomic Force Microscopy) and SEM (Secondary Electron Imaging) clearly evidence a remarkable change in the surface morphology between a bombarded surface by Ar⁺ and Ar_n⁺, and thus a proof that analyzed surfaces during XPS sequential sputtering differ. Secondly, the perturbed depth is evaluated in the case of Ar⁺ by means of PL (Photoluminescence) and optical measurements. Obtained spectra inside craters resulting from different sputtering duration (i.e. different residual perovskite thicknesses) show no peak shift nor phase segregation. This demonstrates that there is no cumulative effect of sputtering nor drastic chemical modifications.
Finally, a strategy for studying the complete device by an innovative coupling between GD-OES and XPS, very complementary, is discussed. The GD-OES, semi-quantitative technique, assures a fast depth profiling to quickly reach specific areas of interest, which makes it interesting to perform profiling on complete cells and quickly reach the perovskite layer. On the other hand, XPS allows to precisely probe the composition of the extreme surface and to have access to the atomic composition and the chemical environments. This strategy has already been satisfactorily implemented on CIGS, Si and III-V materials. Results of this coupling will be presented and discussed in the case of a perovskite absorber, showing the potential as well as the remaining obstacles that still need to be addressed.

Hybrid perovskites are being commercialized using roll-to-roll processing and are attractive for flexible optoelectronics. This raises questions about how bending impacts the structure and stability of thin films. Here, we examine how mobile interfaces respond to bending in MAPbI3, the prototypical halide perovskite. MAPbI3 is a ferroelastic, which means that it forms domains that are contained within the features seen via SEM. We will first review distinctions between ferroelastics and ferroelectrics, and explain conditions necessary for ferroelasticity: briefly, ferroelastics have spontaneous strain, while ferroelectrics have spontaneous electric polarization. Ferroelastic domains have identical crystal structure and different crystallographic orientations, and are separated by interfaces called twin walls. Bending MAPbI3 moves the twin walls, which changes the proportions of the ferroelastic domains. We mapped the stresses needed to move the twin walls onto the stress-strain curve of MAPbI3. Bending MAPbI3 films outwards causes faster degradation to the undesired PbI2 phase: this degradation is correlated with nucleation of new ferroelastic domains that form to accommodate the applied strain, and consequently, formation of new twin walls. The enhanced degradation is attributed to twin walls containing higher concentrations of point defects, which is a well-known phenomenon in oxide ferroelastics. To prevent twin wall movement or nucleation, and thus reduce degradation, one must keep mechanical or thermal stresses below 50 MPa. The roles played by twin walls in influencing ion migration, carrier trapping, and degradation are discussed. [1]

[1] Kennard, R. M., Dahlman, C. J., DeCrescent, R. A., Schuller, J. A., Mukherjee, K., Seshadri, R., & Chabinyc, M. L. Ferroelastic Hysteresis in Thin Films of Methylammonium Lead Iodide. Chem. Mater., 2020, 33, 298-309.

One of the main challenges for perovskite solar cells is the lack of stability in hot and/or humid environments. In this research, we intend to improve the thermal stability of inverted(p-i-n) structured perovskite solar cell devices by depositing alumina (Al₂O₃) as a passivation layer at charge transporting layer (CTL)/absorbing layer interfaces using atomic layer deposition (ALD) technique. To do this, we use fully solution processed Poly[bis(4-phenyl) (2,5,6-trimethylphenyl) amine (PTAA) hole transport layer (HTL) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM)/Bathocuproine (BCP) electron transport layer (ETL). For the absorber perovskite material, we use a triple cation and mixed halide material with composition Cs_0.05(FA_0.83MA_0.17)_0.95PbI_0.83Br_0.17 and 1.62 eV bandgap. We fabricated devices with and without alumina at the HTL/perovskite, perovskite/ETL and both CTL/perovskite interfaces. To investigate how the alumina affects the thermal stability, compare the device performance of the samples with the passivation layer at various interface as a function of time on a 55 °C hotplate. To avoid other potential degradation pathways, the samples are heated inside an N2-filled glovebox. The performance was tracked by measuring the open circuit stability of the samples for 960 hours. The device with alumina at the HTL/absorber layer maintained 75% of the initial value, while the sample with the alumina at the absorber/ETL maintained 56% of the initial value. Both of these were higher than the control sample, which held 44% of the initial value. These results clearly indicate that incorporating alumina at the interfaces of the inverted perovskite solar cell can increase the thermal stability of the devices. In addition to the open circuit stability tests, we completed maximum power point (MPP) tracking on the devices held at 50 °C in a moisture-free, N₂ environment for 85 hours. The MPP tracking data also shows the improved stability of the devices with alumina compared to the control device. These measurements clearly exhibit the improved stability of the alumina passivated inverted perovskite solar cell devices in comparison with the non-passivated inverted structured devices.

Organic inorganic hybrid halide perovskites have emerged as an innovative material with excellent optoelectronic properties, in bulk or as single crystals with low defect density with specific morphologies [1]–[3]. Perovskite solar cells (PSCs) have become a trending technology in photovoltaic research due to a rapid increase in efficiency in recent years. [4].
However, they show a degradation of their performance under operational conditions (light, bias, environmental stress, etc.). To increase their long-term stability is one of the biggest challenges for market applications. The presence of strong internal electric fields, the existence of ferroelectric domains, or the diffusion of ions/defects are suggested as possible causes for the degradation processes. Several authors suggest the existence of native vacancy defects in these materials and attribute to those defects a major role in the control of the optoelectronic properties, such as hysteresis in the photo-induced current-voltage curve, as well as in device degradation and lifetime. [5], [6]
Among various degradation mechanism in PSCs, it’s important to understand the mechanism in both bulk perovskite and at the interfaces between perovskite layer and transport layers.
Electrochemical impedance spectroscopy (EIS) is a powerful technique to examine the charge carrier dynamics in perovskite solar cells. It gives insight about internal electrical processes in PSCs and distinguishes between bulk and interfacial processes [7]–[9]. Each physical parameter can be extracted in the form of resistance (R), capacitance (C) and Warburg capacitance (W) using an equivalent electrical circuit model. In addition, dielectric modulus is applied for studying the microscopic mechanism of charge transport, contribution of grains (crystals) and grain boundaries and recombination dynamics [10].
In this study, we focus on the degradation of inverted planar structure perovskite solar cells through impedance and modulus spectroscopy. Two samples with the same structure are prepared and stored in air and under dark for 30 days. EIS is measured periodically, on the first sample, without J-V measurement to prevent EIS results from being affected by electrical field that occurs during J-V measurements, while J-V characteristic is measured periodically on the second sample. From combined impedance and modulus data, we confirmed that grains and grain boundaries can be distinguished. The results of modulus spectroscopy imply that the grain size decreases and grain boundaries increases which accelerates ionic accumulation and electronic polarization at interfaces. The results of J-V measurement confirms this hypothesis. Furthermore, we prepared two types of PSCs with electron transport layer based on wet- and dry- processes in order to investigate the effect of solvent on degradation mechanism.

[1] N. G. Park et al., MRS Bull., 43, 7, 527–533 (2018).
[2] M. A. Green et al., Nat. Photonics, 8, 7, 506–514 (2014).
[3] J. Chen et al., J. Mater. Chem. C, 4, 1, 11–27 (2015).
[4] “NREL efficiency chart.” [Online]. Available: https://www.nrel.gov/pv/cell-efficiency.html.
[5] F. El-Mellouhi et al., 9, 18, 2648–2655 (2016).
[6] Y. Wang et al., Science, 365, 6454, 687–691 (2019).
[7] A. Guerrero et al., J. Phys. Chem. C, 120, 15, 8023–8032 (2016).
[8] Feng et al., ACS Appl. Energy Mater., 3, 8, 8017–8025 (2020).
[9] Fabregat-Santiago et al., ACS Energy Lett., 2, 9, 2007–2013 (2017).
[10] T. Zangina et al., Results Phys., 6, 719–725 (2016).

Most commercial devices, including photovoltaics (PVs), have gone through a familiar research and development trajectory — increasing performance, upscaling, improving stability, and enhancing mechanical reliability — before making it to the marketplace successfully. In this context, perovskite solar cells (PSCs) are likely to be no exception, but little attention has been paid to the latter issue of mechanical reliability. In fact, enhancing the mechanical reliability of PSCs is particularly important and challenging because the low formation energies of MHPs that makes them easy to solution-process renders them inherently poor in mechanical properties: they are compliant (low Young’s modulus), soft (low hardness), and brittle (low toughness). To address this perhaps final hurdle in the path towards PSCs commercialization, several rationally designed microstructural- and interfacial-tailoring approaches are used. These include grain-coarsening, grain-boundary functionalization, and interfacial engineering. Most importantly, these approaches are designed such that they not only enhance the PSCs mechanical reliability but also increase performance and improve stability. The scientific rationales for these approaches are discussed, together with the presentation of the current results.

During the last few years research community has been interested in studying hybrid (organic-inorganic) metals halide Perovskite materials, a brilliant semiconductor as an active layer in new generation photovoltaics, due to its large absorption coefficient, excellent charge mobilities, and the capability of tuning the bandgap. An additional value to this technology is the possibility to be fabricated by low temperature (below 100°C) and solution-processable techniques, lowering the actual production cost of its Silicon counterpart. Perovskite also widens its potential application as from standard Perovskite Solar Farm to Tandem thin film application, Building Integrated Photovoltaics (BIPV), Building Applied Photovoltaics (BAPV), e-Mobility and Internet of Things (IoT). Perovskite solar technology has become a trending topic in the last decade, reaching promising efficiencies up to 25.5%. In the first decade from its photovoltaic application, researchers mainly focused on obtaining high results instead of caring about stability measurements over time using standard stress test conditions, such as thermal and light soaking stressing tests. Only in 2020, an interlaboratory effort among more than 50 researchers agreed on standardizing stability measurements for Perovskite Solar Cells, recognizing its importance to address the research in this topic in order to grab the attention of the investors, stakeholders and the rest of the population, with the final aim of lowering the CO₂ consumption as stated in the Sustainable Development Goals of the United Nations by 2030. For this reason, we conducted our work taking into account the integrated lifetime energy yield (LEY), a fundamental parameter to check the performances of a device over time. We started by studying a standard triple cation perovskite, with the presence of Methylammonium (MA), Formamidinium (FA) and Cesium (Cs) as cations, Iodide (I) and Bromide (Br) as anions, in a combination to form a narrow bandgap active material (~1.55eV) suitable for single-junction device. We then move towards wider bandgap materials (~1.63eV) more suitable for tandem application with the scope of finding a common strategy to build a robust device stable over time irrespective of the perovskite used. In order to improve the intrinsic stability of the perovskite layer, we focused on the addition of perovskite precursors first, such as the use of PbI₂ excess, the addition of FACl (both materials known for increasing the device performances), and the removal of MA cation, the most volatile material which easily degrades and it is mainly known for accelerating intrinsic degradation. We finally combine all these engineering approaches and have arrived to a stage of inserting ionic liquids, such as 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4), and alkylamine ligands, such as Oleylamine (Oam), studying their effects at different trace amounts separately and together. Our work reveals that the excess of PbI₂, more than 4%, and the presence of FACl shows lower LEY with respect to their references, whereas the removal of MA in the perovskite crystal indeed improves LEY values, also with the presence of BMIM-BF4 and OAm. As a final remark of our study, we upscaled the best combination of additives in the perovskite film, giving us the better LEY, for the fabrication of high efficient Perovskite Solar Module (PSM), showing exciting ISOS-T1 and ISOS-L1 test results.

Recent progress in photovoltaic materials with high radiative efficiency, such as halide perovskites, raises a set of principle questions about optimal photonic management, limiting efficiencies of photovoltaic (PV) converters with advanced photonic management, and design of such converters. Here we propose a practical design of the single cell converter with nonreciprocal photonic management, generalize the finite time thermodynamics to electron distributions with the photon-induced chemical potential, calculate the limiting efficiency of the nonreciprocal converter with a single junction cell, and evaluate efficiency of such converters with available GaAs and halide perovskite cells.
Advanced photonic nanostructures with broken time-reversal symmetries demonstrate a strong potential to revolutionize numerous applications ranging from optical quantum-information technologies to solar energy conversion. Nonreciprocal optical structures can provide high absorptivity, α(ω, n) ≈ 1, together with near-zero emissivity, e(ω, n) ≈ 0, in some narrow absorption cone with the axis direction n and high emissivity, e(ω, n′) ≈ 1, together with near-zero absorptivity, α(ω, n′) ≈ 0, in the strongly different direction n′. Due to separation of emission and absorption, nonreciprocal optical structures can drastically reduce emission from a PV converter without reduction in the light absorption. In our design, we split the absorption cone into two parts. The first part will be used for absorption of solar light. The second part of the cone is employed for re-absorption of the emission by the same PV cell. Alternatively, we can employ the design with high quality mirror doom, which reflects the emitted light back to the cell. In this design, the nonreciprocal high absorption - low emission electrodynamics should be realized just in the narrow angle directed to the narrow hole for incoming solar light.
By using of cells with large number of junctions, we can also suppress the losses related to the relaxation of photocarriers and approach the Carnot efficiency with ideal PV material with negligible nonradiative recombination. Operating of the proposed PV converter with multi-junction cell at the Carnot limit requires an infinite number of external recycling processes. So, any small dissipation in the non- reciprocal photonic system significantly affects this limiting solution. This result provides straightforward resolution of the thermodynamics paradox related to optical diode.
We generalize the finite time thermodynamics to nonequilibrium electron distribution functions specific for PV conversion and derive the limiting efficiency of a single-junction non-reciprocal PV converter. Nonreciprocal photonic management of PV cells with high radiative efficiency may substantially increase the photovoltaic performance. We evaluate performance of the nonreciprocal converter with high quality GaAs and halide perovskite solar cells. Our results show that the nonreciprocal management increases the conversion efficiency of these cells by ~ 5%.
For solar light conversion, the nonreciprocal high absorption-low emission should be realized just in very narrow angle range (9.3 ×10−3 radians) and the narrow photon energy range of 5 - 7 kB T above the bandgap. The light emitted from the cell may be reflected back by high quality mirror doom. Development of such PV converters may be expected in the nearest future.

Despite promising initial efficiencies of metal halide perovskite solar cells, their operational stability is hindered by degradation which limits commercial viability. Mechanical strain has recently been identified as a significant factor influencing perovskite degradation. Specifically, tensile strain in perovskites has been shown to accelerate undesirable PbI₂ formation (1, 2). Therefore, it is crucial to identify and reduce sources of residual tensile strain to enhance the stability of metal halide perovskites. Thermal annealing promotes crystallization and removes excess solvent from solution processed perovskites. High temperature crystallite attachment combined with a coefficient of thermal expansion (CTE) mismatch between the perovskite and rigid substrate, such as glass, results in in-plane biaxial tensile stress (2).

Despite the demonstrated origin of residual tensile strain and its influence on perovskite stability only a few studies have addressed potential solutions (3,4). Here, we evaluate several strategies to reduce or avoid tensile strain on their ability to enhance stability of perovskites without sacrificing optimal initial optoelectronic properties and efficiencies. First, room temperature crystallization and mechanical attachment of perovskite crystallites to the substrate avoids tensile stress from the CTE mismatch during the cooldown from the elevated anneal temperature. In fact, perovskites attached to the substrate at room temperature are under compression at operating conditions (i.e. 65C) which is hypothesized to enhance stability (1, 2). The film formation kinetics can be influenced by several factors to induce room temperature perovskite attachment. For example, certain antisolvents and quench methods rapidly remove excess solvent, decomplex solvate phases, and induce a chain reaction perovskite crystallization at room temperature. Furthermore, extended times between the quench and subsequent anneal allows for adequate time for perovskite crystallites to form and mechanically attach to the substrate.

Using room temperature perovskite attachment to avoid tensile strain from the CTE mismatch has several advantages over alternative strain mitigation strategies. Using substrates with CTEs matched with perovskites or manipulating strain post crystallization introduce additional costs and can sacrifice optimal photovoltaic performance. We present a solution to reduce tensile strain in metal halide perovskites: room temperature attachment using typical processing methods.

1. J. Zhao, Y. Deng, H. Wei, X. Zheng, Z. Yu, Y. Shao, J.E. Shield, J. Huang, Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells. Sci. Adv., 3, (2017)
2. N. Rolston, K.A. Bush, A.D. Printz, A. Gold-Parker, Y. Ding, M.F. Toney, M.D. McGehee, R.H. Dauskardt, Engineering Stress in Perovskite Solar Cells to Improve Stability. Adv. Ener. Mats. 8, (2018)
3. D-J Xue, Y. Hou, S-C Liu, M. Wei, B. Chen, Z. Huang, Z. Li, B. Sun, A.H. Proppe, Y. Dong, M.I. Saidaminov, S.O. Kelly, J-S Hu, E.H. Sargent, Regulating strain in perovskite thin films through charge-transport layers, Nat. Comms., 11, (2020)
4. H. Wang, C. Zhu, L. Liu, S. Ma, P. Liu, J. Wu, C. Shi, Q. Du, Y. Hao, S. Xiang, H. Chen, P. Chen, Y. Bai, H. Zhou, Y. Li, Q. Chen, Interfacial Residual Stress Relaxation in Perovskite Solar Cells with Improved Stability. Adv. Mats., 31, (2019)

Bandgap-tunability of metal halide perovskites endows them with a wide selection for assembling all-perovskite tandem solar cells to break the efficiency limit for single-junction solar cells. FAPbI₃ showing outstanding optical and electronic properties has been exclusively employed to pursue record efficiencies for single-junction perovskite solar cells, yet it has not been exploited in the fabrication of all-perovskite tandem solar cells. Here, to pair with high-quality FAPbI₃, all-inorganic CsPbI_xBr_3-x with superior thermal stability is introduced as the top subcell. By finely tuning the bandgap of CsPbI_xBr_3-x, we achieved an excellent current match of the two subcells. Moreover, a self-assembled polyelectrolyte was used as the surface modifier for NiO_x to reduce the non-radiative recombination at the CsPbI_xBr_3-x/NiO_x interface, which contributes to improved hole extraction and overall device performance. More importantly, this new tandem combination based on Pb-perovskites significantly advances the operational stability, one such a tandem cell delivered an initial efficiency of >23% and remained 80% of the efficiency after >1000 hours operation under continuous one sun (AM 1.5G) illumination at 65 °C. Our strategy provides an alternative pathway for realizing practical application of all-perovskite tandem solar cells.

Methylammonium lead triiodide (MAPbI₃) perovskite is one of the most studied perovskites, and a power conversion efficiency (PCE) over 20% has been demonstrated for single junction solar cells. However, MAPbI₃ perovskite is not an ideal option to fabricate high performance and stable single junction perovskite solar cells mainly due to its intrinsic moisture and thermal instability. Formamidinium (FA) as another organic cation has been applied as a replacement for MA to solve the instability issues. However, FAPbI₃ suffers from phase instability under room temperature. It spontaneously transfers from a black photoactive α-phase to a yellow photoinactive δ-phase at room temperature in a few days. To solve the phase instability problem, alkaline cation such as Cs⁺ and Rb⁺ cations are introduced to partially replace the large FA cation to adjust the Goldschmidt tolerance factor. We investigate the phase and device stability of FAPbI₃ perovskites by introducing both Cs⁺ and Rb⁺ cations in compared to Cs⁺-only with the perovskite thin films fabricated via one-step plus anti-solvent solution process. We also investigate the stability of phase and device made by spin-coating plus solid-state ligand exchange of FAPbI₃ colloid quantum dots. We find that even with incomplete incorporation, Rb⁺ cation can significantly improve the device performance. We reveal the defect-mediated cation and anion migration under electric field using cross-sectional secondary electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry, and identify that Rb⁺ is more vulnerable compared to Cs⁺. By simply mixing the precursor solution before spin coating, we significantly reduce the defect states in both types of perovskites and improve the device stability against an electric field. The modified precursor solution provides the devices with Rb_0.05Cs_0.1FA_0.85PbI₃ and Cs_0.15FA_0.85PbI₃ active layers that retain 68% and 92% of their initial PCE, respectively, over 30 days under N₂ protection. Surprisingly, solar cells made with spin-coating plus solid-state ligand exchange of FAPbI₃ quantum dots display long-time stability with no device performance degradation over 30 days under N₂ protection. We conduct systematically structural, chemical, electronic, and photophysical studies to reveal that cations and anion, used in solid-state ligand exchange, can selectively passivate surface defects and suppress ion migration. The rationally controlled ligand exchange of colloid perovskite quantum dots could open a new avenue to manipulate the chemical stability of halide perovskites.

Whether methylammonium is beneficial or detrimental, necessary or superfluous for high-performing wide-bandgap perovskite compositions suitable for tandem photovoltaics has remained a contested enigma. Borrowing from advances in single-junction perovskite design, mixed cation and halide chemistries have shown improvements to both the open-circuit voltage and stability of top-cell devices, but also suffer from compositional complexity and the risk of phase separation under the wide range of operating conditions a PV cell experiences. Here, we investigate the role of methylammonium (MA) on the formation, halide incorporation, optoelectronic properties, and photovoltaic performance of perovskite thin films designed for perovskite-silicon tandems devices, exploring the >1.6 eV triple-halide compositional space (A-site: Cs/MA/FA).

We use synchrotron X-ray microscopy combined with correlative mapping of optoelectronic performance to assess the role of MA-incorporation on the microscopic chemical distribution and device performance. We see little distinction in performance between MA and MA-free triple halide perovskite compositions. In contrast, whether MA is present or not, Cl incorporation to create triple halide compositions shows distinct benefits, and we gain insights into the limits of Cl incorporation before phase segregation manifests and how MA modulates those limits. Our results suggests that MA is not critical to the high-quality wide bandgap, chlorine-incorporated perovskites and that its removal may facilitate the development of more stable perovskite solar cells.

For perovskite solar cells (PSCs) to be commercially viable, the slow and energy-insufficient thermal annealing step must be eliminated. Among the photo-irradiation methods proposed to replace thermal annealing, photonic curing is the fastest conversion method. Photonic curing delivers short (20 µs to 100 ms) but intense light pulses from a broadband (200-1500 nm) xenon flash lamp, making it the only method to convert perovskite under 20 ms. This processing time can be extrapolated to a roll-to-roll web speed of 40 m/min based on laboratory processing conditions.¹ However, most reported PSCs made by photonic curing under 1 second have inferior performance (~10% PCE). Although SEM images show dense and pinhole-free perovskite films, AFM images indicate secondary wavy features of 500 nm-wide ridge and 80 nm-deep trenches on photonically cured perovskite films, the existence of which correlates with poor device performance. We suggest that this morphology feature is produced by volatile solvent evaporation during the fast photonic curing process. Two approaches have been made to remedy this issue: (1) adding CH₂I₂ as the third solvent in the conventional DMF-DMSO system and (2) applying a controlled air-blowing step before photonic curing to remove excess solvent further. Combining these two approaches produces photonically-cured perovskite films with a comparable film roughness and device performance. Alkyl halide additives have been reported to enhance PSC performance by modulated solvent-solute interactions and C-X (X = Cl, Br, and I) cleavage. Photonic curing can cleave CH₂I₂, producing disassociated iodide ions to replenish iodine loss induced by photonic curing, which is confirmed by EDX. As a co-solvent, the high boiling point of CH₂I₂ can also make the solvent less volatile, reducing surface roughness in photonically cured perovskite films. Additionally, photonically-cured perovskite films have longer PL lifetimes and a higher recombination resistance compared to thermally-annealed counterparts. As a result, we demonstrate that photonic curing is a suitable method to replace thermal annealing in high-throughput PSC fabrication.

(1) Xu, W.; Daunis, T. B.; Piper, R. T.; Hsu, J. W. P. Effects of Photonic Curing Processing Conditions on MAPbI 3 Film Properties and Solar Cell Performance . ACS Appl. Energy Mater. 2020, 3 (9), 8636–8645. https://doi.org/10.1021/acsaem.0c01243.

Recently, Colloidal all inorganic lead halide perovskite quantum dots (PQDs) have generated much interest and activity, due to the great promise they offer for use in optoelectronic applications. However, their poor colloidal stability, which can be traced to the ionic nature of the crystal cores and the high desorption rates of the native ligands, has impeded progress in fundamental research and slowed integration of such materials in devices. Developing novel surface ligands with strong coordination to passivate surface traps and enhance fluorescence, while improving colloidal stability is needed.
Here, we detail the synthesis of a new family of multifunctional polymer ligands and their use for surface engineering of the CsPbBr₃ nanocrystals. The ligand synthesis relies on the nucleophilic addition reaction between poly(isobutylene-alt-maleic anhydride), PIMA, and distinct nucleophiles. The modular ligand stoichiometrically displays multiple ammonium and/or imidazolium salt groups for coordination onto the PQD surfaces, along with several alky chains with different length to promote affinity to various organic solvents. The results show that the PQDs coated with those polysalts exhibit great colloidal stability and significantly enhanced photoluminescence. The coating also allows the resulting colloids to be dispersed a wide range of solvent conditions, including ethanol and methanol, while maintaining strong fluorescence and preserving the nanocrystal integrity after ligand substitution.

Highly efficient organic-inorganic perovskite solar cells (PSCs) consisting of multiple layers of functional materials are needed to be mechanically stable for the development of perovskite solar cells that can be deposited on rigid or flexible perovskite solar cells. In this study, we explore the possible effects of interfacial robustness on the performance of organic-inorganic PSCs. The interfacial fracture energies are determined for the interfaces in model perovskite solar cell structures. These are obtained using Brazil disk specimens that are used to study the mode-mixity dependence of the interfacial fracture toughness in bi-material interfaces. The underlying crack/microstructure interactions are investigated along with the crack-tip shielding mechanisms associated with crack growth under monotonic loading. These are elucidated via scanning electron microscopy and energy-dispersive X-ray spectroscopy studies of the crack profiles and fracture surfaces. These reveal the kinking in-and-out of interfaces that can occur between layers in PSCs. The observed crack-tip shielding models are also modeled with zone shielding fracture mechanics models. The measured interfacial toughening/fracture toughness levels are then correlated with the performance characteristics (power conversion efficiencies, short circuit currents, and open circuit voltages) of PSCs with perovskite layers that are produced using spin coating and vapor deposition techniques. The implications of the results are then used to rank the robustness of interfaces in multilayered PSCs.

A promising application of machine learning in materials science is service lifetime forecasting¹ of halide perovskites, a potentially disruptive photovoltaic technology limited by degradation in ambient environments due to the action of heat, light, moisture, and oxygen. It is imperative for prospective manufacturers to know how long their products are likely to survive to provide warranties and develop sound business plans. In prior work, we have developed predictive machine learning models that can accurately forecast the service lifetime of the archetypal halide perovskite, methylammonium lead iodide (MAPbI₃), in a variety of different environmental conditions, using data available from a unique characterization tool that simultaneously measures transmittance, photoluminescence, and photoconductivity in situ. Early modeling efforts indicated that the material decomposition rate, detected by using the optical transmittance of perovskite films, is strongly predictive of decay in the perovskite’s carrier diffusion length. The importance of environmental factors on halide perovskite degradation is well, if incompletely, understood. However, the degree to which sample-to-sample variation can affect lifetime forecasts has not been studied. It is well-known that the manner and environment in which a perovskite film is prepared can have large impacts on its photovoltaic performance, but these links are often opaque and may involve factors that are hard or impossible to measure. To understand how best to handle this inherent uncertainty, we employ a dataset consisting of over 60 perovskite degradation runs in which environmental conditions are held constant, and identify features that capture the influence of this variation on service lifetime. We observe that T80-LD, the time it takes for the diffusion length to decay to 80% of its starting value, can vary by over an order of magnitude in the same ambient conditions, making the ability to capture this variation extremely important for forecasting. In assigning decomposition rate-related features high importance, machine learning algorithms that forecast T80-L_D thus uncover key links between the extent of a perovskite film’s degradation and the evolution of its electronic properties: T80-LD possesses a strong inverse relation to the material decomposition rate, as calculated from in-situ transmittance measurements. This relationship is quite similar to the T80-LD-decomposition rate relationship observed over a dataset in which environmental conditions are deliberately varied, showing that these relatively simple models can nevertheless uncover the general patterns underlying degradation. We have recently developed a first-principles kinetic model to predict the initial degradation rate of MAPbI₃ from the ambient environmental conditions,² and we explore the performance of hybrid models that incorporate both a priori information about the expected degradation rate in a given environment using physics-inspired features derived from the kinetic model, and in situ optical and optoelectronic measurements made on each specific sample in the dataset. This approach maximizes the predictive potential of the models by encoding both contextual and sample-specific information.
References:
(1) Stoddard, R. J.; Dunlap-Shohl, W. A.; Qiao, H.; Meng, Y.; Kau, W. F.; Hillhouse, H. W. Forecasting the Decay of Hybrid Perovskite Performance Using Optical Transmittance or Reflected Dark-Field Imaging. ACS Energy Lett. 2020, 5 (3), 946–954. https://doi.org/10.1021/acsenergylett.0c00164.
(2) Siegler, T. D.; Dunlap-Shohl, W. A.; Meng, Y.; Kau, W. F.; Sunkari, P. P.; Tsai, C.-E.; Armstrong, Z. J.; Chen, Y.-C.; Beck, D. A. C.; Meila, M.; Hillhouse, H. W. Water-Accelerated Photo-Oxidation of CH3NH3PbI3 Perovskite: Mechanism, Rate Orders, and Rate Constants. ChemRxiv 2021. https://doi.org/10.33774/chemrxiv-2021-30ggh.

The mixed-conducting nature of halide perovskites has a variety of impacts on their optoelectronic properties and device performance. Current-voltage hysteresis, reverse-bias breakdown, photo-brightening, carrier-induced degradation, and thermal stability are all examples of behaviors that have been linked to the appreciable ionic conductivity in these materials. However, despite the significance of ionic defects on perovskite performance, there remain many open questions as to the number of ionic defects in perovskite semiconductors and their effects on device performance. In this work I will present recent advances in our understanding of one phenomenon linked to ionic defects in perovskite semiconductors: carrier-induced degradation. Using correlated structure, optoelectronic property, and transport measurements we monitor the chemistry and performance of MAPbI_xBr_(1-x)3 perovskites over extended illumination and document both reversible and irreversible changes in performance in halide perovskites. Using complimentary XRD, photothermal deflection spectroscopy, photoluminescence, and mixed-conductivity measurements we document changes in crystal structure, bandgap, and Urbach energy, and connect those changes to variations in ionic conductivity and optoelectronic performance. In the case of mixed halides, we see a non-reversible increase in bromine concentration and corresponding increase in the photoluminescence stability with prolonged illumination, suggesting that the loss of iodine leaves behind a higher-quality and more photostable film. Overall, these advances shed light on the relationship between perovskite chemistry, ionic defect density, and photostability in this increasingly important class of semiconductors and suggest pathways towards increased device performance and stability.

Stability of perovskite solar cells has advanced significantly by removing extrinsic degradation factors such as moisture and oxygen through encapsulation techniques. However, encapsulated devices often show irreversible degradation during photo-stability test. In this contribution, we investigated photo-aging mechanism by observing changes in intrinsic defects excluding extrinsic degradation factors. We fabricated perovskite solar cells with similar power conversion efficiencies of 21% but different Pb to I ratio at interface, which exhibited completely different photo-aging trend. We investigated the shift in their intrinsic properties during photo-aging in nitrogen-filled chamber. Evolutions in opto-electronic properties and elemental constituents during photo-aging were traced by various measurement, including photoluminescence spectra, UV-visible absorption spectra, X-ray fluorescence measurements, and time-of-flight secondary-ion mass spectroscopy. Relatively Pb-rich interface presented halide sublimation and strain relaxation, implying formation of new defects states near grain boundaries and interfaces. We investigated changes in Fermi level and intrinsic defect properties and energetic distribution by X-ray photoelectron spectroscopy measurements, space charge limited current measurement, and thermal admittance spectroscopy. Formation of donor-type deep-traps were shown in the perovskite after irreversible degradation.[1] We suggest balanced stoichiometry at the interface and passivation of Pb-I related defects are crucial for enhancing photo-stability.[1] We studied interfacial defect passivation using novel fluorinated organic spacers by synthesizing fluorine groups with different anchoring positions on organic spacers and compared their properties. We report an enhanced efficiency by effective defect passivation retaining over 90% of their initial power conversion efficiency during 500 hours of continuous 1-Sun illumination and outstanding hydrophobic nature without encapsulation.

[1] S. H. Cho et al., Advanced Energy Materials, 11.17 (2021): 2100555.

The triple-cation mixed halide perovskite Cs_0.05(MA_0.17FA_0.83) _0.95Pb (I_0.83Br_0.17)₃ is one of the most promising photon absorbers that is being considered for emerging photovoltaic applications. This is due to its outstanding optoelectronic properties. However, the limited thermal stability of mixed halide perovskites is still a major challenge that may limit their applications over a wide range of temperatures that may be encountered in service. Hence, in this work, we study the temperature-dependence of the degradation of solution processed triple-cation mixed halide perovskite solar cells. These are subjected to temperatures that simulate potential service conditions. Our results show that the reductions in device power conversion efficiency originate from decreasing charge mobility and the formation of charge recombination sites at the ETL/perovskite layer interface. This is shown (using scanning electron microscopy) to be associated with the thermal-induced delamination of the layered structure. The underlying degradation of the solar cell characteristics is also shown to be associated with the formation of defects during the controlled heating of mixed halide perovskite cells. The implications of the results are then discussed for the development of perovskite solar cells with improved thermal stability.

Hybrid perovskites (HPs) have created intense excitement in the chemistry and materials community for a variety of reasons, not the least of which is their remarkable capacity for facile tuning of optoelectronic properties. Specifically, regarding the layered Ruddlesden-Popper perovskites this is accomplished by tuning the stoichiometric ratio of the small intercalating ion (e.g. methylammonium) to the larger organic spacer cation (usually an alkyl ammonium such as n-butylammonium). The inclusion of the bulkier alkylammonium cation has the effect of breaking up the regular packing of the lead halide octahedra, creating “slabs” of the traditional perovskite structure, separated from each other by the dielectric environment created by the layers of the bulky spacer cations. The size of these slabs is dictated by the stoichiometry between the two varieties of cations, and synthesis of pure single crystals of these various layered perovskites has been accomplished via solution-grown methods. Attempts to create a phase-pure thin film, containing only slabs of a certain thickness have yielded mixed results. The deliquescent properties of HPs (the uptake of vapor phase amine molecules by HPs leading to the liquefaction of the HP) have been thought of as a way to improve crystallinity of HP thin films. We show herein that this deliquescent behavior of HPs can be used as a way to exert control over the stoichiometry of HP thin films by carefully controlling the vapor-phase stoichiometry of an amine atmosphere during deliquescence. This approach may even be a way to realize the elusive goal of creating a phase-pure Ruddlesden-Popper thin film, something that has not been accomplished as yet through conventional solution processing methods.

Lead halide perovskites have attracted enormous interest from both the scientific and industrial communities due to their unprecedented success in optoelectronic devices. Particularly impressive is their performance as active layers in solar cells, where certified power conversion efficiencies (PCE) surpassing 25% have been demonstrated. Despite this tremendous success, several key challenges prevent their large-scale deployment and require further research. Among these challenges, the presence of hysteresis in perovskite solar cells has triggered significant efforts in developing strategies for its suppression. The fundamental origins of hysteresis, however, remain under debate. Moreover, the interplay between the presence of specific ionic defects in the perovskite layer and the appearance of hysteresis remains largely unexplored.
Focusing on methylammonium lead triiodide (MAPbI₃) as a model system, we purposefully introduce defects into the perovskite layer by fractionally modifying the solution precursor stoichiometry.[1] We find that only overstoichiometric devices – those with excess of iodine – show significant hysteresis, as well as a greatly reduced short-circuit current (J_SC), thus resulting in low PCEs. While exposure to O₂ under illumination is typically associated with causing the degradation of perovskite thin films,[2] surprisingly, for the overstoichiometric perovskite layers such exposure leads to the elimination of hysteresis and the recovery of the short-current density to the expected value for high-performance perovskite solar cells.
To investigate the origin of this effect, we characterized the composition of the overstoichiometric films before and after exposure to O₂ under illumination. As expected, we find that the I/Pb ratio in the overstoichiometric films, measured by X-ray photoemission spectroscopy, is rather high (4.08±0.15) due to the excess iodine introduced during processing. After the exposure to oxygen, this ratio is reduced to 3.86±0.19, suggesting that a photooxidative process leds to the expulsion of excess iodine from the surface of the perovskite film. 2D photoluminescence mapping in the fluence and frequency domains [3] revealed that the overstoichiometric samples exhibit a large density of traps, which, upon photooxidation, are eliminated from the film, bringing it to the same electronic quality as a perfectly stoichiometric MAPbI₃ film. A similar, albeit less pronounced effect, was also observed for overstoichiometric triple cation and MA-free perovskite solar cells, suggesting that photooxidative healing can be applied to various perovskite compositions.

References:
[1] P. Fassl et al, Energy Environ. Sci. 11, 3380 (2018).
[2] Q. Sun et al, Adv. Energy Mater. 7 (20), 1700977 (2017).
[3] A. Kiligaridis et al, Nat. Commun. 12, 3329 (2021).

Methylammonium lead iodide has become a holy-grail of perovskite material since its inception owing to the facile fabrication (e.g., one/two-step film process, Inverse temperature crystallization) and versatile semiconductor applications (e.g., Light Emitting Diode, photovoltaic) with its superior absorption coefficient, charge mobility, and diffusion length properties. Despite extensive studies of MAPbI3 in optoelectronic application, it possesses a significant challenge that may prove counter-productive as in seemingly inconsistent results of TDPL (Temperature-Dependent Photoluminescence) properties, particularly during phase transition. Unification explanation is essential since TDPL properties have emerged as standard to elucidate the optoelectronic's defect, charge carrier dynamics, and device performance.

In this study, we investigate three archetypical MAPbI3 crystals (Single-crystal, Polycrystalline, Highly oriented Crystal) representing different crystal states based on optoelectronic device variation. We illustrated considerable numbers of diffraction peak trends over the temperature range and observed onset phase transition from certain orientations of crystal states and the co-existence phase of orthorhombic-tetragonal phase of MAPbI3 at PC and HOC using TD-XRD (Temperature Dependent X-ray Diffractometer). We calculated the FWHM value based on the Gaussian model to illustrate the peak broadening trend that occurred strongly on PC and HOC states throughout the temperature range as evident of inter-domain stress by domain boundaries. We showed stress accumulation on MAPI SC and PC with (TDRS) Temperature-Dependent Raman Spectroscopy. Accessing low wavenumber of octahedral twist (SC) and C-N torsion (PC), we realized abrupt lattice rearrangement and smooth transition Raman vibration between SC and PC during temperature change.
In conclusion, we found that domain-size and density of domain boundaries from MAPbI3 crystal states determine the degree of residual stress/strain for thermal lattice expansion and reconstruction lattice to define the phase transition of crystal. We observed that the phase transition characteristics agree with TDPL properties as the origin of MAPbI3 distinctive characteristics as optoelectronic devices. We consider crystal states of being an essential factor to control in order to obtain consistent device properties.

The widely used Methylammonium (MA+) and Formamidinium (FA+) cations are relatively acidic compounds, with the potential to dissociate in the halide perovskites, giving a proton (H+) and the corresponding ammine. Despite the fact that they were considered not to do so, I will bring new evidence that the dissociation does happen and that H+ can migrate, associate with eventual water molecules internalized in the perovskite, be absorbed by basic hole transporting layers and other related phenomena. I will also comment on the chemistry of H+ in solution and show how it influences the final state of a perovskite layer after the coating.

As it happens in general, in the perovskites the chemistry of protons is reversible and is characterized by chemical equilibria, which are modified by a large number of factors. I will provide indications on how temperature, illumination intensity, electronic doping of the perovskite material and the presence of other molecules pushes the acid-base equilibrium towards the dissociation or the association. The complexity of these equilibria is at the base of a series of phenomena that have been observed in halide perovskites like fast ion-migration, light-soaking and reversible damage repair (self-healing).

In this talk, I will provide proof of the migration of protons in the perovskites as obtained by exchange of H+ with deutons (D+) and discuss their diffusion coefficient in halide perovskite single crystals. Through optical microscopy (confocal, hyperspectral, fluorescence lifetime imaging spectroscopy), I will also provide proof on how the proton chemistry influences the photoluminescence of the perovskites showing how proton deficient perovskites have better optoelectronic properties. In particular, I will analyze how the positive effects of methylamine treatment in the perovskites can be related to proton chemistry and how light might locally originate methylamine causing the effect of light soaking in the perovskites. I will also show that H+ acts as an oxidizer when the perovskite is put into contact with metals and its influence should be considered when depositing contacts directly on the perovskite material.

I will also analyze the chemistry of protons in the perovskite precursor solutions showing how their presence influences which particular lead complexes are actually in solution during crystallization. I will show how the electrochemical potential of the precursor solution (and therefore of the perovskite material) is partially determined by the presence of protons with consequences on the quality and quantity of defects in the perovskite materials.

To conclude, the aim of this talk is to provide the listener a wide view on proton chemistry in halide perovskites providing the theoretical tools to understand the influences that proton chemistry has on the optoelectronic properties and the stability of the perovskites. This talk will be rich in chemical knowledge but it will be adapted to an audience working on halide perovskite devices with more engineering or physics background.

Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (＞23%) and low-cost fabrication. Directions to further improve these solar cells include strategies to enhance their stability and their efficiency by modifying either the perovskite absorber layer or the electron/hole transport layer. For example, the transparent electron transport layer (ETL) can be an important tuning knob influencing the charge extraction, [1] light harvesting, [2] and stability [3] in these solar cells, or the use of up-conversion nanoparticles to get better performance in the near IR part of the visible spectrum. [4] Here we present two strategies based on nanostructuration, first a fundamental study of upconversion fluorescence enhancement effects near Au nanodisks by scanning near-field optical microscopy and second the effects of a nanocolumnar TiO2 layer on the performance and the stability of Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite solar cells. For the first case, the enhancement and localization of light near the metallic structures are directly visualized by using a single Er/Yb-codoped fluorescent nanocrystal glued at the end of a sharp scanning tip. [5] For the second we find that, compared to devices with planar TiO2 ETLs, the TiO2 nanocolumns can significantly enhance the power conversion efficiency of the perovskite solar cells by 17 % and prolong their shelf life. By analyzing the optical properties, solar cells characteristics, as well as transport/recombination properties by impedance spectroscopy, we observed light-trapping and reduced carrier recombination in solar cells associated with the use of TiO2 nanocolumn arrays. [6]

References

[1] S.S. Mali, et al., Chemistry of Materials 27, 1541 (2015).
[2] C. Liu, et al., Journal of Materials Chemistry A 5, 15970 (2017).
[3] M. Salado, et al., Nano Energy 35, 215 (2017)
[4] M. Bauch et al., Plasmonics 9, 781 (2014)
[5] L. Aigouy, et al., Nanoscale 11, 10365 (2019)
[6] Z. Hu, et al., ACS Appl. Mater. Interfaces 12, 5979 (2020)

Lead-based halide perovskites have emerged as a most prominent candidate for emerging opto-electronic applications. In this talk we will briefly showcase recent efforts towards designing new Pb-free semiconductors that are alternatives to traditional halide perovskites, for which computational approaches from first-principles have been extensively successful and revealed a series of new compounds within the so-called halide double perovskites family and vacancy ordered perovskites. Among these, Cs₂BiAgBr₆ has the narrower indirect band gap of 1.9 eV, [1] and Cs₂InAgCl₆ is the only direct band gap semiconductor, yet with a large gap of 3.3 eV. [2] All of them exhibit low carrier effective masses and consequently, are prominent candidates for a range of opto-electronic applications such as photovoltaics, light-emitting devices, sensors, and photo-catalysts. [3] We will specifically outline the computational ab initio design strategy that led to the synthesis of these compounds, and particularly focus on the insights we can get from first-principles calculations in order to facilitate the synthesis, improve their opto-electronic properties and the in-silico identification of compounds with properties that are similar to the lead-halide perovskites. The newly developed concept of analogs will lead us to identify a new oxide double perovskite semiconductor, Ba₂AgIO₆, which exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs₂AgInCl₆, but with a band gap in the visible range at 1.9 eV. [4] In the final part of our talk, we will employ this strategy to explore the phase space of vacancy-ordered double perovskite and discuss the case of Zr-based compounds as stable alternates to Cs2TiX6 with X=Br,I, which exhibit lighter charge carrier effective masses.[5]

[1] G. Volonakis et al., Journal Physical Chemistry Letters 7 1254 (2016)
[2] G. Volonakis et al., Journal Physical Chemistry Letters 8 772 (2017)
[3] G. Volonakis et al., Journal Physical Chemistry Letters, 8 3917 (2017)
[4] G. Volonakis et al., Journal Physical Chemistry Letters, 10 1722 (2019)
[5] B. Cucco et al., Applied Physics Letters (under review 2021)

Mixed lead-tin (Pb-Sn) perovskites have excelled as leading candidates for low band gap absorber materials demonstrating efficiencies up to 21% for single junctions^1,2and 24.8% for all-perovskite tandem solar cells³. Although various ionic species and ratios can be combined to form low band gap perovskites, the propensity of Sn²⁺ to oxidize comprises the Achilles’ heel of all of these compositions resulting in undesirable p-type doping of the material and poor optoelectronic quality^4–7. A deep understanding of the intrinsic degradation mechanisms is imperative in order to successfully integrate these materials into perovskite multi-junctions and even though the deleterious effects of Sn oxidation have been widely explored, the composition’s influence on degradation mechanisms has captured less attention.

Herein, the impact of the intrinsic stability of various low band gap compositions is investigated by monitoring changes in their structural and optoelectronic properties during either thermal or humidity stressing over time. As expected, the vulnerability of the material under humidity exposure scales with increased Sn content, while the degradation is governed by the A-cation choice under thermal stressing and absence of oxygen, with the compositions including volatile species proving to be more susceptible. Interestingly, even under humidity exposure different degradation response was observed for compositions with the same Sn content and different A-cation combination, indicating that ambient stability depends not only in Sn content but on A-site as well. Moreover, practical characterization methods which best capture this degradation at the time scales meaningful for device operation are identified. Our side-by-side degradation assessment highlights how materials composition can alter the degradation footprint and particularly how ion choice influences the speed and the mechanism of degradation, which can inform compositional selection for the practical development of all-perovskite tandems.

¹ J. Tong, Z. Song, D.H. Kim, X. Chen, C. Chen, A.F. Palmstrom, P.F. Ndione, M.O. Reese, S.P. Dunfield, O.G. Reid, J. Liu, F. Zhang, S.P. Harvey, Z. Li, S.T. Christensen, G. Teeter, D. Zhao, M.M. Al-Jassim, M.F.A.M. van Hest, M.C. Beard, S.E. Shaheen, J.J. Berry, Y. Yan, and K. Zhu, Science 364, 475 (2019).
² J. Werner, T. Moot, T.A. Gossett, I.E. Gould, A.F. Palmstrom, E.J. Wolf, C.C. Boyd, M.F.A.M. van Hest, J.M. Luther, J.J. Berry, and M.D. McGehee, ACS Energy Lett. 5, 1215 (2020).
³ R. Lin and H. Tan, Nature Energy (2019).
⁴ D. Meggiolaro, D. Ricciarelli, A.A. Alasmari, F.A.S. Alasmary, and F. De Angelis, J. Phys. Chem. Lett. 11, 3546 (2020).
⁵ F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, and M.G. Kanatzidis, Nature Photonics (2014).
⁶ N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A.-A. Haghighirad, A. Sadhanala, G.E. Eperon, S.K. Pathak, M.B. Johnston, A. Petrozza, L.M. Herz, and H.J. Snaith, Energy & Environmental Science 7, 3061 (2014).
⁷ M.T. Klug, R.L. Milot, J.B. Patel, T. Green, H.C. Sansom, M.D. Farrar, A.J. Ramadan, S. Martani, Z. Wang, B. Wenger, J.M. Ball, L. Langshaw, A. Petrozza, M.B. Johnston, L.M. Herz, and H.J. Snaith, Energy Environ. Sci. 13, 1776 (2020).
⁸ L.E. Mundt, J. Tong, A.F. Palmstrom, S.P. Dunfield, K. Zhu, J.J. Berry, L.T. Schelhas, and E.L. Ratcliff, ACS Energy Lett. 5, 3344 (2020).
⁹ T. Leijtens, R. Prasanna, A. Gold-Parker, M.F. Toney, and M.D. McGehee, ACS Energy Letters 2, 2159 (2017).
¹⁰ T. Leijtens, R. Prasanna, K.A. Bush, G.E. Eperon, J.A. Raiford, A. Gold-Parker, E.J. Wolf, S.A. Swifter, C.C. Boyd, H.-P. Wang, M.F. Toney, S.F. Bent, and M.D. McGehee, Sustainable Energy Fuels 2, 2450 (2018).

A-site cation management in metal halide perovskites (MHPs) has shown to appreciably impact the stability and optoelectrical properties of the material. There has been notable investigation on MHPs at the nanoscale using atomic force microscopy (AFM) – based techniques. [1, 2] The incorporation of a small cation such as Rb⁺ cation to the A-site of the traditional perovskite has considerably alleviated shortcomings in device performance. Though these advancements are well-realized at the macroscopic level, a comprehensive study on the electrical characteristics at the nanoscale remains incomplete. At the macroscopic level, quad-cation (Cs_0.07Rb_0.03FA_0.76MA_0.14Pb(I_0.85Br_0.15)₃) solar cells show reduced J-V hysteresis (90%) in comparison to dual-cation (Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃) devices. We resolve their electrical performance through photovoltage [3] and photocurrent with nanoscale spatial resolution using Kelvin probe force microscopy (KPFM) and photoconductive-AFM (pc-AFM), respectively. At the nanoscale, our dark KPFM results indicate upward band bending at grain boundaries (GBs) for both chemical compositions. We note increased voltage homogeneity (34%) in quad-cation perovskite largely due to improved halide homogeneity by Rb⁺ cations. We observe a 55% faster post-illumination residual voltage decay in quad-cation perovskite. For the pc-AFM experiments, a Schottky contact is formed between the metal probe and the perovskite, which leads to the collection of holes at the probe end. From the upward band bending indicated by the KPFM results, we expect to see the presence of holes at GBs due to an electron barrier formation. We find that the photocurrent maps for quad-cation perovskite are in agreement with the expected outcome with most GBs reading a photocurrent value higher than their adjacent grains. Surprisingly, the majority of the GBs in dual-cation perovskites display photocurrent response comparable to dark current values. We conclude defect-assisted recombination at GBs in dual-cation perovskite and defect-passivation at GBs in quad-cation perovskite due to the addition of Rb⁺ cations. Further, we uncover that the defect-passivated GBs in quad-cation perovskite form a highly conductive GB network that enhances the overall photocurrent response of the material by ~50%. In addition, the photocurrent is independent of topography for both chemical compositions. Our analytical approach, which uses the combined knowledge of band bending and photocurrent response at GBs from KPFM and pc-AFM, respectively, can be used as a universal method to probe the presence and passivation of defects at GBs for any optoelectronic material. Our results underscore the beneficious characteristics, such as improved efficiency, reduced J-V hysteresis, and increased uniformity in photovoltage/current response present in quad-cation perovskite, imparted by the addition of Rb⁺ cation.[4]
[1] J.M. Howard, R. Lahoti, M. S. Leite. Adv. Energy Mater. 10, 201903161 (2019) – Invited Review
[2] E. M. Tennyson, C. Gong, M. S. Leite. ACS Energy Letters 2, 2761 (2017) - Invited Review
[3] E. M. Tennyson et al. ACS Nano 13, 1538 (2019).
[4] R. Lahoti, et al. (2021), in preparation.

Organic-inorganic halide perovskites (OIHPs) has emerged as the most promising light-absorber materials in the photovoltaic community due to their near-ideal bandgaps. However, the low formation energies of OIHPs render them unstable. While significant progress has been made in improving the stability of OIHPs, perovskite solar cells (PSCs) will also need to be mechanically reliable if they are to service satisfactorily for decades. In this context, we study the fracture behavior of PSCs by measuring their cohesion energies (Gc) using double cantilever beam method and report a novel approach to strategically enhance the interfacial adhesion and performance of PSCs using self-assembled monolayers (SAMs), where we find that the perovskite solar cell stability is closely intertwined with its mechanical reliability. This work points to a new route for designing mechanically robust PSCs with long-term durability.

Planar perovskite solar cells(PSCs) using low temperature process is a promising candidate for high throughput production such as roll-to-roll process using a flexible substrate. However, the performance of the planar PSCs is still lower than that of the mesoscopic PSCs using high temperature process. Here, we report a new concept on low temperature processed porous planar electron transport layer (ETL) inspired by mesoporous structure for improving the performance of flexible devices. The structurally and energetically designed porous planar ETL has induced the formation of high quality perovskite and a preferred band alignment, resulting in the improved charge collection efficiency in device. Through the porous planar ETL, we have achieved an efficiency of 20.7% with a certified efficiency of 19.9 % on a flexible substrate, which is the highest power conversion efficiency ever reported. In addition, for the first time, we succeed to fabricate a large area flexible module with porous planar ETL, demonstrating an efficiency of 15.5% and 12.9% on 100 cm² and 225 cm², respectively. We believe that this strategy shad a new light on the overcoming a limit of flexible devices.

Core-shell refractory plasmonic nanoparticles are used as excellent nanoantennas to improve the efficiency of lead-free perovskite solar cells (PSCs). SiO₂ is used as the shell coating due to its high refractive index and low extinction coefficient, enabling the control over the sunlight directivity. An optoelectronic model is developed using 3D finite element method (FEM) as implemented in COMSOL Multiphysics to calculate the optical and electrical parameters of plain and ZrN/SiO₂-modified PSCs. For a fair comparison, ZrN-decorated PSCs are also simulated. While the decoration with ZrN nanoparticles boosts the power conversion efficiency (PCE) of the PSC from 12.9% to 17%, the use of ZrN/SiO₂ core/shell nanoparticles shows an unprecedented enhancement in the PCE to reach 20%. The enhancement in the PCE is discussed in details.

The fast development of metal halide perovskite solar cells over the last years has reached a level where application in commercial devices is becoming very realistic. This requires scalable deposition techniques like vacuum deposition. It is widely known that during the processing of perovskite, many factors influence the quality of the absorber layer. For solution processed perovskite layers, the research on this topic is growing, but in the case vacuum deposition, the influencing factors are still not fully known. One important factor is the quality of the vacuum in the deposition chamber. Because of material changes of the precursors, residual water, and the vacuum system power, the background pressure changes from deposition to deposition, or even over the course of the processing of a single absorber layer. Due to the different deposition properties of the organic and inorganic precursor salts, the materials are influenced differently. To ensure high reproducibility and high film quality, an increase in the control over all processing parameters is important. In this work, we investigate the influence of the background pressure during the deposition process on important factors like the material tooling and with it the stoichiometry, the crystal structure and finally the device performance. By varying the background pressure about ~ 2 orders of magnitude (~2*10^-6mbar à 1*10^-4mbar) a bandgap change of the produced perovskite of 60 meV was observed. Besides the stoichiometry the layer thickness is strongly influenced by the background pressure. This results in a decrease in device performance from 15.6 % to 12.6 %.

Hybrid halide perovskites have many exciting optoelectronic properties that make them suitable for photovoltaic and lighting applications. Unlike conventional inorganic semiconductors, including silicon, cadmium telluride, and gallium arsenide, hybrid halide perovskites posess many dynamical degrees of freedom. These dynamics underly many of their optoelectonic properties, suggesting a new paradigm of dynamics-based structure property relationships. Neutron scattering and spectroscopies are ideal tools to better understand the dynamics of these materials. Here, we present several case studies that demonstrate how neutron scattering enriches our understanding of the phase behavior and functional properties of hybrid halide perovskites. We propose that these insights provide targets for engineering perovskite functional properties.

Inorganic halide perovskites are attractive for achieving the wide bandgap optimal for a high-efficiency perovskite-perovskite tandem based on today’s Pb-Sn low bandgap compositions. However, they have suffered from lower photoluminescent quantum yield relative to hybrid compositions and phase instability. To improve upon metastable CsPbI3, we explore triple-halide alloying of minor amounts of Br and Cl with I. In agreement with previous reports for hybrid analogues, we observe a chlorine solubility limit in the majority iodine-bromine all-inorganic perovskite lattice. Past this solubility limit we observe the perovskite forming a split phase of iodine-bromine-rich and bromine-chlorine-rich clusters. Interestingly, these dual-phase thin-films show superior performance, which hints at possible synergistic effects of this chemical heterogeneity. We leverage multi-modal synchrotron microscopy and correlative spectroscopic micro-photoluminescence (µPL) on all-inorganic triple halide perovskites CsPbX3 (X-site: I/Br/Cl) films to elucidate mechanisms for superior performance in the face of phase segregation. The results suggest that a greater focus on harnessing the flexibility of the inorganic perovskite material system holds promise to retrace the outstanding performance and stability gains made in hybrid analogues.

Thin-film perovskite solar cells (PSCs) now rival established Si photovoltaics inefficiencies and could be less expensive with fewer resources required for processing. A major drawback of the leading PSCs is the presence of Pb in their structure, raising concerns about toxicity. Cs2TiBr6 is one leading candidate among Pb-free perovskites. It has a suitable bandgap to be the top absorber in tandem-junction photovoltaics at ~1.8 eV, and Cs2TiBr6-based PSCs have demonstrated efficiencies up to 3.3%. However, a complete view of the optical properties, carrier dynamics, and environmental stability remains incomplete.

Herein, we use time-resolved photoluminescence spectroscopy to investigate optical emission and carrier lifetime in both large-grain and thin-film Cs2TiBr6 under different environmental conditions. Cs2TiBr6 demonstrates high stability in an N2 with photoexcited radiative lifetimes on the order of 0.5 ns, sufficient for photovoltaic consideration. We investigate the origins of several prominent emission features and their dependence on sample morphology as well as on environmental conditions.

Halide perovskites have emerged as high-performance semiconductors for efficient optoelectronic devices, not least because of their bandgap tunability using mixtures of different halide ions. This makes them particularly attractive as candidates for tandem solar cells or spectrally tailored lighting.
We demonstrate that spatially varying energetic disorder in the electronic states of such mixed-halide films causes local charge accumulation, creating photodoped regions, which unearths a strategy for efficient energy conversion in solar cells and light-emitting diodes at low charge densities. We combine temperature-dependent photoluminescence microscopy with computational modelling to quantify the impact of local bandgap variations from disordered halide distributions on the global photoluminescence yield in mixed-halide perovskite films. We find that fabrication temperature, surface energy, and charge recombination constants are key for describing local bandgap variations and charge carrier funneling processes that control the photoluminescence quantum efficiency. We report that further luminescence efficiency gains are possible through tailored bandgap modulation, even for materials that have already demonstrated high luminescence yields.

Sulfides and selenides in the perovskite and related crystal structures - chalcogenide perovskites - are an exciting family of semiconductors for solar energy conversion and optoelectronics [1]. Here, we explore materials in the Ba-Zr-S system to determine their potential for thin-film photovoltaics (PV).
We demonstrate making BaZrS₃ thin films by gas-source molecular beam epitaxy (MBE) [2]. BaZrS₃ forms in the orthorhombic distorted-perovskite structure with corner-sharing ZrS₆ octahedra (space group Pnma, no. 62), and has a direct band gap of E_g = 1.85 eV. We present a wealth of data reporting film processing and properties, including reflection high-energy electron diffraction (RHEED), atomic-force microscopy (AFM), X-ray reflectivity and diffraction (XRR and XRD), scanning transmission electron microscopy (STEM), spectroscopic ellipsometry, Hall transport, and photoconductivity. Our single-step MBE growth process produces films that are atomically smooth, single phase, chemically homogeneous, and oxygen free. The films are brightly colored even at 20 nm thick, due to the strong optical absorption typical of chalcogenide perovskites. Films grow via two, competing epitaxial growth modes: (1) buffered epitaxy, with a self-assembled interface layer that relieves the epitaxial strain, and (2) direct epitaxy, with rotated-cube-on-cube growth that accommodates the large lattice constant mismatch between the oxide and the sulfide. The propensity for these two modes can be tuned by adjusting the H2S gas delivery rate. We further show results making thin films of BaZr(S,Se)₃ alloys (achieved by co-flowing H₂S and H₂Se), which provide a path towards tuning the direct band gap in the range 1.3 – 1.9 eV.

We also report on time-resolved photoluminescence (TRPL) and photoconductivity (TRPC) studies of excited-state transport in BaZrS₃ and the related, Ruddlesden-Popper phase Ba₃Zr₂S₇ (E_g = 1.25 eV). We model the data using semiconductor physics simulations, to enable more direct determination of parameters key for photovoltaic performance (e.g. surface recombination velocity, Shockley-Read-Hall lifetime, diffusivity) than is possible with empirical data modeling. We find that chalcogenide perovskites have SRH lifetime exceeding 100 ns and ambipolar diffusion length on the order of 10 m. If time allows, we will present further results of advanced characterization including impedance spectroscopy and X-ray absorption spectroscopy.

Our high-quality thin-film synthesis and excited-state transport measurements suggest a bright future for chalcogenide perovskites thin-film PV. We will end by briefly summarizing the state of the field, and highlighting key challenges on the road to device demonstrations and commercialization.

[1] Jaramillo, R. & Ravichandran, J. In praise and in search of highly-polarizable semiconductors: Technological promise and discovery strategies. APL Materials 7, 100902 (2019).
[2] Sadeghi, I., Ye, K., Xu, M., LeBeau, J. M. & Jaramillo, R. Making BaZrS3 chalcogenide perovskite thin films by molecular beam epitaxy. arXiv:2105.10258 [cond-mat] (2021). at <http://arxiv.org/abs/2105.10258>

Hand in hand with the outstanding progress of perovskite photovoltaics, reports of very high values of external quantum efficiency (EQE) appeared. Values of EQE up to 98 % were reported, typically peaked in the range between 400 and 500 nm. Even considering a very high internal quantum efficiencies of the perovskite close to 100%,^[1] this observation strikes as a contradiction at a first glance, as the transmittance of ITO/glass substrates typically does not exceed 90% in this wavelength range. Therefore reports of high EQE numbers (EQE is one of the standard characterization procedures for solar cells to exclude or correct spectral mismatch of light sources used for solar cell fabrication^[2]) frequently give rise to doubts or concerns about potentially erroneous EQE measurements.
Here, we discuss and explain the root cause of the high EQE by a combination of experimental data and optical simulations. We show, how the transmittance into the active material is significantly governed by its refractive index, enabling very high portions of light being coupled into the high refractive perovskite material at the given wavelength region. At the same time transmission into a low refractive material like air, gives rise to a significant reflectance.^[3]
These effects can be nicely seen in optical simulation, experimentally acquired EQE spectra, and optical absorption/reflection measurements. Finally we are able to unravel, that the maximum of the transmittance spectrum is mainly shifted by the thickness of the underlying transparent conductive oxide (TCO) layer. As an example, for our wide gap FA_0.8Cs_0.2Pb(I_0.5Br_0.5)₃ cells (1.85 eV) we found that an increasing ITO thickness would even increase the maximal reachable current and therefore being beneficial in a twofold purpose: reducing the sheet resistance of the bottom electrode while simultaneously increasing the in-coupling of light into the active layer.

[1] Q. Lin et al. Nat. Photonics 2015, 9, 106.
[2] J. A. Christians et al. J. Phys. Chem. Lett. 2015, 6, 852.
[3] K.O. Brinkmann et al. Solar RRL (in review)

The number of publications on perovskite solar cells (PSC) continues to grow exponentially. Although the efficiency of PSC is exceeded 25.5%, not every research laboratory can reproduce this result or even pass the border of 20%. Unfortunately, it is not always clear which dominating mechanism is responsible for the performance drop. Here, we develop a simple method of light intensity analysis of JV parameters allowing the understanding what are the mechanisms appearing in the solar cell and limiting device performance. The developed method is supported by the drift-diffusion model and aimed to help in the explanation of the parasitic losses from the interface or bulk recombination, series, or shunt resistance in the perovskite solar cell. This method can help not only point on the dominating of bulk or interface recombination in the devices but also determine which interface is more defective. The detailed and stepwise guidance for such a type of light intensity analysis of JV parameters is provided. The proposed method and the conclusions of this study are supported by a series of case studies, showing the effectiveness of the proposed method on real examples.

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 perovksite compositions are found to be stable against high temperatures of up to 85 C, the situation is more complex in working devices, as a stable device requires both - a stable semiconducotr layer and stable interfaces. A p-tyüer interface consisting of polymeric mulitlayers with variable dopign is introduced as a robust anode interface. By integrating the most stable perovskite into this structure, we achieve stable devices without encapsulation that attain nearly 100% of their initial efficiency and 98% of their peak efficiency, after 1000 hrs of continuous mpp operation at temperatures of 60-65 degrees Celsius. 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.

The champion efficiency of perovskite solar cells (PSCs) is over than 25%, and the external quantum efficiency of perovskite light-emitting diodes (PeLEDs) in green and red is over than 20%, which is great potential for practical application. In this report, I will talk about our recent progresses in PSCs and PeLEDs. 1) By rational design the composition of perovskite, we got over than 25% efficiency while using nearly pure FAPbI₃, 2) For the perovskite light-emitting diodes with blue emission, we modulated the hole transport layer to reduce the injection barrier, and got over than 16% external quantum efficiency of PeLEDs in blue.

Metal halide perovskite materials are regarded as highly promising materials for high performance optoelectronics due to excellent optical absorption, long carrier diffusion length, tunable composition, etc., thus have triggered broad attention for applications including solar cells, photodetectors and light-emitting diodes. Conventionally, solution processes have been utilized to fabricate perovskite materials. In our work, in order to achieve high material quality and device scalability, we have developed chemical vapor deposition process to grow ordered high density perovskite nanowire (NW) arrays in nanoengineering templates with materials including MAPbI₃, MASnI₃, CsPbI₃ and FAPbI₃. The geometrical factors of NWs, namely, periodicity, diameter and length can be precisely nanoengineered. Intriguingly, we have discovered that the chemically and mechanically robust template can effectively protect perovskite NWs against water and oxygen invasion thus the material chemical stability and phase stability are significantly better than planar perovskite films. To explore device applications of perovskite NW arrays, they have been fabricated into novel optoelectronic devices such as photodetectors and bionic eye. Meanwhile, we have also fabricated high efficiency thin film Pero LEDs on self-organized nanophotonic templates and achieved much improved external quantum efficiency (EQE) as compared to planar control device. These results will be demonstrated in this presentation. And they suggest that in addition to material optimization, device structure innovation, particularly utilization of various types of nanostructures can lead to high performance optoelectronics.

Although perovskite solar cells (PSC’s) with efficiencies of 25.5% [1] have been reported, the best-performing perovskite solar cells rely primarily on lab-scale solution-processed techniques that typically yield small-scale (~0.1 cm²) devices. Besides scaling up of these devices, demonstration of stability of these devices are also essential. [2-5]

Deposition of perovskite absorber layers by thermal co-evaporation have gathered considerable interest for silicon-perovskite tandem solar cells. The main advantages for thermal evaporation include potential compatibility with silicon solar cell processing, conformal coatings, control of deposited layer thicknesses, and possibility of multi-layer processing. We have recently demonstrated highly eficient, large area, planar PSCs with uniform MAPbI₃ perovskite active layer deposited by thermal co-evaporation of PbI₂ and MAI. The high-quality co-evaporated perovskite thin films are pinhole-free and uniform over several centimetres, showing large grain sizes, low surface roughness, and a long carrier lifetime. The high-quality perovskite thin films translates to small area PSCs (0.16 cm²) with PCE above 20% and high reproducibility. Large area PSCs with area up to 4 cm² did not show a significant drop in the PCE. Furthermore, the first thermally evaporated mini-modules with an active area larger than 20 cm² achieved the record PCE of 18.13%. [6]

Thermal stability is a critical criterion for assessing the long-term stability of perovskite solar cells. Thermally evaporated perovskites also deliver remarkable thermal stability compared with solution processed solar cells. The improved stability may be attributed to the perovskite growth process leading to a compact and almost strain-free films. This work represents an important step towards the development of high quality, stable, and reproducible large-area perovskite solar cells and mini-modules, the main requirements for the industrialization of this technology.

Metal halide perovskites (MHPs) have demonstrated huge potential to build a rich library of materials for a new disruptive optoelectronic technology. The main strength comes from the possibility of easily tune the semiconductor bandgap in order to integrate it in devices with different functionalities – in principle. In reality, this cannot be achieved yet. In fact, while defect tolerance is claimed for MHPs with a bandgap of about 1.6 eV, the model system object of intense investigations, MHPs with lower and higher bandgaps are far from this claim. They show various forms of instabilities which are mainly driven by a strong defect activity.

Here I will discuss our studies on the nature of defects and their photo-chemistry as a function of the semiconductor bandgap and chemical composition in order to identify their role in defining the semiconductor electronic properties and stability.

The hybrid metal halide perovskite (MHP) family exhibits large structural fluctuations which are believed to contribute to, or perhaps cause, their remarkable optoelectronic properties. Additionally, experimental studies including Raman spectroscopy and X-ray total scattering demonstrate that these fluctuations cause local deviations from the average bulk structure in the three-dimensional MHPs, such as CH₃NH₃PbBr₃ (herein MAPB). [1–3] MAPB has three well defined crystal structures; orthorhombic, tetragonal, and cubic, with phase transitions identified at 141 K and 235 K. An additional incommensurate phase bridges the orthogonal and tetragonal phases. These phase transitions proceed as an unlocking of the organic cation and rotational modes of the PbBr₆ octahedra. Several studies have characterized the dynamics of the organic cation, the PbBr₆ octahedra, and their interactions, however the dynamics of the Pb-Br bond have received less attention. [4–7]
We investigate the temperature dependence of correlated motions and anharmonicity of the Pb-Br bond with a combined extended X-ray absorption fine structure (EXAFS) and single crystal X-ray diffraction (XRD) study of MAPB. EXAFS provides a snapshot of the local environment around the probe atom, rather than the time- and space-averaged structure obtained from XRD. Fitting the EXAFS spectra to a cumulant expansion reveals that the Pb-Br bond exhibits anharmonicity even in the low-temperature orthorhombic phase. We also examine the correlated motion of the Pb and Br atoms by comparing the mean squared displacement obtained from XRD with the mean squared relative displacement obtained from EXAFS. [8,9] In this picture, we find that Pb and Br motions are strongly correlated along the bond direction and somewhat correlated perpendicular to the bond direction. Interestingly, the effective Pb-Br spring constant and correlation coefficients increase through the orthorhombic-tetragonal transition, a highly unusual effect likely resulting from a charge rearrangement on the Pb atom which increases the Pb-Br orbital overlap. Finally, we compare the various asymmetric Pb-Br pair distribution functions which can arise from an anharmonic potential. An in-depth understanding of anharmonicity and the phase transitions will allow designs of structurally robust MHP devices.
NJW and MFT gratefully acknowledge full support from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the U.S. Department of Energy through contract number DE-AC36-08G028308.

References:
[1] A. N. Beecher, et al. ACS Energy Lett. 1, 880 (2016).
[2] O. Yaffe, et al. Phys. Rev. Lett. 118, 136001 (2017).
[3] R. Comin, et al. Phys. Rev. B 94, 094301 (2016).
[4] Y. Guo, et al. Phys. Rev. Materials 1, 042401 (2017).
[5] I. P. Swainson, et al. Phys. Rev. B 92, 100303 (2015).
[6] A. C. Ferreira, et al. Phys. Rev. Lett. 121, 085502 (2018).
[7] G. Laurita, et al. Chemical Science 8, 5628 (2017).
[8] C. H. Booth, et al. Phys. Rev. B 52, R15745 (1995).
[9] W. R. Busing and H. A. Levy, Acta Cryst 17, 2 (1964).

Chemical doping of lead halide perovskites remains challenging. One major issue is that commonly considered dopants (such as bismuth, substituting on the lead site) have high ionization energies, making them ineffective. Our recent density functional theory (DFT) calculations indicate that the deep donor behavior of bismuth (along with antimony) is due to their p orbitals giving rise to mid-gap states. Furthermore, the s orbitals of gallium dopants lie deep in the CsPbBr₃ band gap, also leading to deep defect behavior. In contrast, the group 3 elements scandium and yttrium do not give rise to such mid-gap states, and are thus shallow donors when incorporating on the Pb site. These donors are stable relative to other configurations and have modest formation energies. Analogous behavior is found in FAPbI₃ and CsPbCl₃, indicating that n-type conductivity could be achieved in lead halide perovskites with Y and Sc dopants [1].
A second issue is the potential for native defects (such as vacancies and interstitials) to compensate intentional dopants. Via a thorough first-principles inventory of native defects and hydrogen impurities, we determine the stability and electronic behavior of potential compensating species. We find that a number of relevant species exhibit deep levels (most notably the bromine interstitial and hydrogen interstitial) challenging the idea that “defect tolerance” in these materials is due to the shallow character of native defects [2]. Guided by the theoretical identification of these defects with DFT, growth efforts can take steps to mitigate trap-assisted non-radiative recombination to boost efficiencies of lead halide perovskites and aid in efforts at achieving controllable electrical conductivity.

[1] Chem. Mater. 33, 6200 (2021).
[2] J. Mater. Chem. A 9, 7491 (2021).

This work was supported by the ONR/NRL 6.1 Basic Research Program.

Using JV scan speeds from 0.01 to 4000 V/sec one can find that seemingly all lead halide perovskite solar cells show considerable photocurrent JV hysteresis somewhere between the 10 ms and 10 second time scales. The JV hysteresis generally shows up as a large change in fill factor (FF) and energy efficiency, although smaller hysteresis can be found in the dark current as well. Mobile ions (vacancies) seem to be able to explain most of the JV hysteresis, although there is still argument about the maximum mobility of the ions in relation to the fastest observed hysteresis.
In many perovskite cells, the fast JV hysteresis coexists with a reversible day/night cycle in the short circuit current (Jsc) and FF. This day night cycle varies strongly between cell types and fabrications, ranging from degradation in the dark and recovery in the light, to the complete opposite. I will discuss new measurements of the amount of excess charge stored during an 8 hour illumination cycle at the maximum power point (MPP), at short circuit, and at the MPP voltage in the dark. After an 8 hour illumination cycle, we find that measurable current continues to flow out of the cell for up to 10 hours after the light and voltage are removed. The amounts of stored charge can be large (e.g. up to 1% of the Pb ions in the absorber) without causing irreversible damage to the film. In some cells the amount of charge stored for a given illumination time is linearly related to the loss of efficiency over that time. I will discuss some experiments on the chemical nature and physical location of this charge, but, sadly, will not give a definitive answer.

Effects of humidity on the optoelectronic properties and degradation mechanisms of ABX₃ Halide Perovskites, HaPs, where A= Methylammonium (MA), Formamidinium (FA), or Cesium (Cs), B= Pb, X=Chloride (Cl), Bromide (Br), or Iodide (I), have been investigated thoroughly. However, no studies exist on how humidity influences their structural and mechanical integrity, which is critical for their successful integration into future technologies. The out-of-plane mechanical properties of HaPs are known to depend strongly on the Pb-X bond, but the effect of humidity remains ill-explored. This presents a difficulty in engineering devices, containing these materials, that are meant to function over a range of ambient conditions, as well as casting doubt on the generality of fundamental behavior, measured under uncontrolled humidity conditions.
Single crystals of five different HaP, MAPbX₃ (X = Cl, Br, and I) and APbBr₃ (A = Cs and FA) were grown and used to investigate the role that the different A and X ions play for humidity-dependence of mechanical properties. A two-pronged approach employing both instrumented nanoindentation (INI), and atomic force microscopy (AFM)-based nanomechanical measurements were used to measure in situ changes in elastic and plastic deformation under different humidity conditions. AFM measurements included both elastic modulus determination using the contact resonance technique and hardness measurements, using the AFM to both apply load to sample and to image the indentation profile in-situ using the same diamond tip. The INI was done at slightly larger loads and inferred the indentation area by standard calibration techniques and not from the imprint image. By combining analysis between AFM and INI methods, we are able to verify the significance of these results from two independent techniques which are based on similar but distinct measurement and analysis procedures.
Our results reveal that for MAPbCl₃, MAPbBr₃, and FAPbBr₃ the elastic modulus (E) increases by 4-10% while the hardness (H) decreases by ~25% when RH (relative humidity) increases from 10% to 60% and that these changes are reversible. E is a measure of the elastic response and H is related to the plastic response. CsPbBr₃ and MAPbI₃ show negligible humidity dependence within experimental uncertainty, with the latter showing differences between shallow and deep indentations. We propose an explanation for these observations incorporating the interplay between composition, lattice structure, and absorbed H₂O.

Combining the advantageous electrical and optical properties of semiconductors with magnetic characteristics gives access to extraordinary phenomena and applications. Fully inorganic dilute magnetic semiconductors (DMS) have been known for decades, which show diverse functionalities like control of magnetism by electrical fields. Typically, the material class of DMS is obtained by introducing a substantial number of magnetic ions to an otherwise non-magnetic host semiconductor.
In this work we investigate the magneto-optical properties of the heavily doped hybrid semiconductors MAPbBr₃ (CH₃NH₃PbBr₃) with circularly polarized broadband femtosecond transient absorption (CTA), from which we find strong interactions between the excited charge carriers’ angular momentum and the dopants’ spin.
Due to their outstanding optoelectronic properties and high defect tolerance, organo-metal halide perovskites form an ideal system for efficient magnetic doping. We prepared a selection of magnetic perovskites using simple solution processing techniques by doping of the host MAPbBr₃ perovskite with three different magnetic ions Mn²⁺, Co²⁺, Ni²⁺. We investigated the fundamental optical, structural and magnetic properties of the perovskites with absorption and photoluminescence spectroscopy, XRD and SQUID measurements. Doping of the host lattice leads in all cases to structural changes and an increase or decrease of the bandgap of several 10 meV for Co²⁺ and Mn²⁺/Ni²⁺, respectively. We observe paramagnetic properties for all dopants and different doping concentrations.
To reveal the impact of doping on excitation dynamics, we employ the technique of CTA at cryogenic temperatures down to 4 K and magnetic fields up to 700 mT to gain a better understanding of possible ultrafast processes and coupling mechanisms between the angular momentum of photoexcited charge carriers and the d-electron spin of the dopants. Measuring CTA with different combinations of right- and left-handed circularly polarized pump (λ_ex = 515 nm) and probe (super-continuum white light) at variable temperatures and magnetic fields with an exceptional spectral resolution of 0.1 nm enabled us to precisely compare the energetic shifts and ultrafast dynamics of the charge carriers. For all investigated samples we observed an energy splitting of the CTA spectra for two different circular polarizations of the pump pulse even at zero magnetic field, which is substantially larger for the doped samples. Both, lattice deformations and remanent magnetism due to high doping concentrations could lead to an energetically preferred spin alignment of the photoexcited charge carriers mediated via spin-orbit coupling. We plan to employ transient circularly polarized photoluminescence spectroscopy (CPL) to show if this spin polarization is long-lived and if it will lead to a significant degree of circularly polarized PL, which would be of high interest for future light emitting diode applications.

The majority of halide perovskite solar cells are thin film devices.^[1] This is not surprising if we look at the simplicity of making devices based on stacks of multiple thin films as opposed to making single crystal solar cells. However, single crystals seem to have some fundamental advantages relative to thin films, because their lack of grain boundaries apparently leads to defect densities that are orders of magnitude lower in single crystals as opposed to thin films.^[2] The typically stated data in support of this claim is based mostly on two methods, namely the use of the so-called trap-filled limit in the current-voltage curve of single-carrier devices^[3] and the use of capacitance-voltage measurements or related techniques^[4,5] to determine defect densities via the width of space charge regions. Both methods have in common that they measure quantities that are affected by the charge density of a defect and not by the defect density itself. An important consequence is that bulk charge densities have to compete with surface charge densities of electrodes to be visible in a measurement. Furthermore, for a charge density to be visible in any measurement, the charge density has to be high enough that its associated Debye length is at least on the order of the film thickness (or shorter). Otherwise, the charge density will be unable to affect the electrostatic potential over the available distance and in consequence will be unable to visibly affect the current or capacitance measured in a device. From these simple electrostatic arguments, one can conclude that charge densities have to exceed a certain threshold value that depends on the permittivity ε and thickness d of the sample. Actually, both arguments noted above lead to a detection limit that could be expressed by noting that the density of charged defects has to exceed a minimum value N_min ∝ ε/d² that inversely depends on d². By doing a meta-analysis of literature data, we note that nearly no reported defect density of a thin film done by a charge-detecting method exceeds threshold criteria for both methods that are derived analytically and verified numerically. Only few of the single crystal data exceed the threshold, suggesting that defect densities might be much lower than originally thought in many cases and further suggesting that there is no reliable trend that would support the presence of superior electronic properties in single crystals. Furthermore, we use numerical simulations to identify possible reasons that may lead to the possible misinterpretation of data as being representative of a certain defect density. We find that in particular, small variations of injection barrier heights can lead to apparent trap-filled current regimes that may cause literature data to follow the detectability limit. Finally, we also do a meta-analysis of charge carrier lifetimes and non-radiative voltage losses that does not show any indication of a trend favoring single crystals which one would expect if there was a strong trend in defect densities.

[1] National Renewable Energy Laboratory Best Research-Cell Efficiencies https://www.nrel.gov/pv/assets/pdfs/cell-pv-eff-emergingpv.202001042.pdf (NREL,2021).
[2] C. Ran, J. Xu, W. Gao, C. Huang, S. Dou, Chemical Society Reviews 2018, 47, 4581.
[3] M. A. Lampert, Reports on Progress in Physics 1964, 27, 329.
[4] Z. Ni, C. Bao, Y. Liu, Q. Jiang, W.-Q. Wu, S. Chen, X. Dai, B. Chen, B. Hartweg, Z. Yu, Z. Holman, J. Huang, Science 2020, 367, 1352.
[5] Heath, J.; Zabierowski, P., Capacitance Spectroscopy of Thin-Film Solar Cells. In Advanced Characterization Techniques for Thin Film Solar Cells, Abou-Ras, D.; Kirchartz, T.; Rau, U., Eds. Wiley-Vch: Weinheim, 2011; pp 81-105.

Organic-inorganic metal halide ABX₃ (A: methylammonium-MA, formamidinium-FA, cesium-Cs; B: lead-Pb, tin-Sn; X: iodine-I, bromine-Br, chlorine-Cl) perovskite-based photovoltaics (PV) are of great interest in the field because they can be manufactured in a low-cost solution process, have desirable opto-electronic properties, and can produce high efficiency devices. When silver (Ag) is used as a back contact metal in perovskite solar cells, it can diffuse into the underlying charge transport and absorber layers of the device leading to optical and electronical losses. Mitigating these losses is essential to developing higher efficiency devices. Perovskite solar cells consisting of nominally soda-lime glass / tin doped indium oxide (ITO) /PEDOT:PSS / MA_0.3 FA_0.7Sn_0.5Pb_0.5 I₃/ fullerene(C₆₀) / bathocuproine (BCP) with Ag and copper (Cu) metal back contacts are examined by spectroscopic ellipsometry over the photon energy range from 0.74 to 4.0 eV. The measured ellipsometric spectra are analyzed using a parametric model based upon the complex optical properties and thicknesses for each component layer as well as interfaces between materials. In these devices, it is observed that the metals used as back contact may diffuse into and mix with the electron transport and hole blocking layers, BCP and C₆₀, to form a physically mixed layer consisting of metal, BCP, and C₆₀. To further validate these results and to identify the optical and electronic losses in the device, external quantum efficiency (EQE) simulations are compared to experimental EQE. The physically mixed layer of metal, C₆₀, and BCP near the back contact can substantially absorb back reflected photons from the metal interface, thereby reducing the potential current generation in the perovskite absorber. The presence of this physically mixed layer shows an optical loss of up to 1 mA/cm² in terms of short circuit current density. Film stacks of glass/C₆₀/BCP or SnO₂/Ag or Cu are also measured using spectroscopy ellipsometry to study if SnO₂ can prevent diffusion of the back contact metal and prevent optical losses. The optical and electronic losses are quantified from these spectroscopic ellipsometry and EQE measurements for cells with Ag and Cu metal contacts with and without the presence of SnO₂. The film stacks containing SnO₂ shows that it can prevent diffusion of the back contact metal, and EQE simulations with SnO₂ show recovery of some of this optical loss arising from the formation of the physically mixed metal, BCP, and C₆₀.

Lead-based halide perovskite with B-site doping is an effective strategy to enhance its optoelectronic devices. Herein, we study the B-site doping of MAPbBr₃ thin films with copper (Cu²⁺). Developing doped perovskites, MA(Pb:Cu)Br₃, thin films show darkness in their color from yellowish orange color for pristine MAPbBr₃ thin film to dark brown color for Cu-doped perovskite thin film, enlarged average grain boundaries area from 0.042 μm² for pristine MAPbBr₃ to 0.71 μm² for 40% Cu-doped MAPbBr₃, and lowering in the optical bandgap from 2.32 eV for pristine MAPbBr₃ to 1.85 eV for 50% Cu-doped MAPbBr₃. The Cu ions incorporation in the perovskite crystal lattice is confirmed by the X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) measurements. Surprisingly, Cu-rich new phase was observed via EDS measurement at 50% Cu-doped perovskites. This new phase was further confirmed with the appearance of a new XRD diffraction peak which neither related to crystal structure peaks of MAPbBr₃ nor initial precursor. The absorption band edge of 50% Cu-doped MAPbBr₃ is developed to the near-infrared range. The conductivity of MAPbBr₃ increases over three orders of magnitude upon doping. More interestingly, photocurrent measurements of doped perovskites show the generation of band carriers upon excitation at the near-infrared region. This work provides an insight into the importance of doping over phase modulation and tailoring the optoelectronic and electronic properties of halide perovskites for a wide range of efficient optoelectronic devices.

Perovskite solar cells (PSCs) have recently reached over 25% in power conversion efficiency and are emerging as a promising competitor to silicon technology. The key challenge that remains in further developing of the PSCs is their scaling towards commercial fabrication. Fully printed PSCs are still a major goal for the solar industry. As perovskites are inherently temperature sensitive, a key requirement for printing of these devices are low-temperature processed layers that can form the individual components of a solar cell device. Hole transporting layers (HTLs) among the entire structure are the most problematic ones, where the conventional spiro-MeOTAD, an organic molecule, is too expensive and unstable for commercial applications. Contrarily, inorganic nanomaterials can overcome the limitations of organic HTLs. Nickel oxide (NiO) has been widely used as an HTL in PSCs, however, the processing temperatures of such films is typically >250 °C to achieve sufficient crystallinity. The only way to omit high-temperature post-deposition annealing of solution-processed NiO HTLs is by spin-coating dispersion already containing metal oxide particles. However, for highly efficient perovskite solar cells additional interface engineering also needs to be studied to facilitate the charge transfer. Interestingly, ligand engineering approaches have been comprehensively investigated on a wide range of nanoparticles, yet these research outcomes are not sufficiently applied in perovskite devices. Our motivation was to fabricate NiO nanoparticle-based HTL inks for low temperature deposition, dedicated for perovskite solar cells. To achieve this, we have synthesized fine NiO nanoparticles via heat-up approach. The as-obtained NiO NPs are dispersible in nonpolar solvents, due to oleate (OA) ligands present at their surface. However, these long alkyl chain ligands are not favourable for efficient charge transfer. For that reason, our second aim was to apply ligand modification procedures to improve the hole transfer within the device after removal of long alkyl ligands, and also, to investigate how the introduced ligands impact the charge extraction at the perovskite/HTL interface.
Two different ligand exchange routes were applied, and for the very first time, deposited as NiO HTLs. In the first procedure, a Meerwein’s salt Me₃OBF₄, was used where the resulting NiO nanoparticles are stabilized by BF₄^- ligands. The second procedure involved 4-hydroxybenzoic acid (HBA).To assess the performance of the synthesized and modified NiO HTLs we have fabricated PSCs of the inverted architecture. Before the deposition the NiO inks were processed through microfluidic mixer system for improved film morphology. All of the HTL films were annealed at a low temperature <150 °C to remove the residual solvent. Such processing parameters are in line with printing regime, especially on flexible substrates, which is an additional advantage of the developed materials. The fabricated devices exhibited stabilized PCEs of 17.9% and 17.1% for the ligand-exchange NiO BF₄ and NiO HBA HTLs, respectively, which were superior in comparison with control devices harnessing the native surface chemistry (OA) and a postprocessing oxygen-plasma treatment step (PCE of 16.1%). Furthermore, while all NiO HTLs exhibited good photostability factors, a significant enhancement was observed for the ligand-exchange NiO HTLs, which, for NiO-BF4, showed a 0.2% reduction in efficiency over a 300 h testing period compared with 0.9% for the NiO-OA-based PSCs. These findings showcase the importance of ligand engineering of NiO NPs for achieving efficient hole-transporting materials and its prospects for developing low-temperature-processable and high-stability PSCs. Future work will consider the printability of these dispersions to achieve fully printed flexible inverted PSCs.

Despite the remarkable performance of lead perovskite (Pb perovskite) solar cells, many researchers are looking for an alternative to Pb perovskite to overcome its fatal problems, such as instability and Pb toxicity. Antimony chalcoiodides (SbChI, Ch = S, Se) are recently considered as potential absorbers due to their low-toxicity and photovoltaic properties [1].
In this presentation, we introduce a versatile solution-based method for the fabrication of antimony chalcoiodide films. The method consists of two steps: deposition of antimony chalcogenides Sb₂Ch₃ (step I) and its conversion to antimony chalcoiodides SbChI (step II). Antimony chalcogendies were fabricated by SbCl₃-thiourea/selenourea based solution approach. Conversion to SbChI was performed by spin coating SbI₃ solution on Sb₂Ch₃ and subsequent heating. Pure phase Sb(S,Se)I films with high crystallinity were obtained at low-temperature of 150–200°C. The absorption, structure, morphology, and electronic structure were controlled by tuning S/Se molar ratio and annealing temperature. In addition, Bi-alloyed SbChI films were fabricated using BiI₃ instead of SbI₃ in step II. Our results demonstrate that two-step solution method is applicable to the fabrication of various chalcoiodides.

REFERENCES
[1] Y. C. Choi and K.-W. Jung., Nanomaterials, 2020, 10, 2284.

Lead halide perovskites have recently emerged as one of the most promising future candidates for replacing silicon-based photovoltaics. Their unique optoelectronic properties, such as tunable and direct bandgap and high absorbance coefficient, in combination with a low-cost fabrication process, make them promising candidates for the use in next-generation and tandem solar cells. Although their photocurrents are reaching the theoretical maximum, the devices’ open-circuit voltage can be improved significantly. Thus, an operando characterization method that can map the contributions to the device’s photovoltage is called for.
With the progress in device performance, surface and SRH recombination are suppressed, making radiative recombination more dominant. In this process, electron-hole pairs recombine to generate photons that can be either reabsorbed elsewhere within the device or be emitted through photoluminescence. Since the photoluminescence quantum yield (PLQY) and the harvestable voltage are tightly coupled, mapping the contribution to photoluminescence within the device can inform on the contribution to the buildup of photovoltage within it.
The spatial external luminescence efficiency (SELE) is the probability for a photon absorbed at a specific point in the device to contribute to the measured photoluminescence. The SELE can be extracted by combining wavelength-dependent PLQY measurements with detailed optical modeling. Hence, the SELE maps the contribution of different regions to the device photovoltage. In this contribution we apply SELE extraction to perovskite solar cells. This spatial information can shed light upon different loss mechanisms, quantify surface recombination at the interfaces between the different layers in the device, and discriminate between radiative and non-radiative recombination processes at the surface and in the bulk of the device.

While the power conversion efficiencies of perovskite solar cells have exceeded 24.2% in less than a decade of extensive research, the material still suffer from instability due to moisture and high temperatures, hindering scalability and commercialisation. In this work, we have synthesized the CsPbBr₃, which is known to have poor controlled microstructures. The perovskite was a 3D hybrid organic-inorganic material, which was then treated further with different amounts of PEABr concentrations (10%, 20%, 40%, 60%, 80% and 100%) to produce a 2D materials of the form PEA₂Cs_n-1Pb_nBr_3n+1. A gradual PL shift from 540 nm (CsPbBr₃) to 507 nm (100% PEABr) can be observed, indicating a shift of the distribution of average crystal size to smaller values as the PEABr ratio increases, which in turn indicates the formation of a 2D perovskite. An efficiency exceeding 15% can now be achieved through additive–controlled nanostructure tailoring for 2D perovskites.

Perovskite solar cells are an important photovoltaic technology with high efficiencies exceeding 20% due to their optimal band gap, large absorption coefficient, and high charge mobilities. One of these challenges is the understanding and control of their defect structures because perovskite compounds are relatively soft ionic crystals and ions are migrating in the crystals relatively low activation energy.
Several research groups have reported on the effects of proton irradiation on perovskite solar cells on perovskite compounds and solar cell characteristics. It has been reported that hardness and self-healing associated with defect formation in perovskite compounds by proton irradiation.
In this study, we have investigated the effect of low energy ion irradiation to both all inorganic and organic-inorganic mixed halide perovskite films on the structure, morophology, optical, electronic properties and photovoltaic properties systematically. Several kinds of halogen ion beam irradiations on the halide perovskite thin films were systematically investigated by using a tandem type ion accelerator with changing acceleration voltages and irradiation time. Detailed experimental results will be reported on the conference.

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Stability is one of the significant barriers in commercializing halide perovskite, HaP, solar cells, PSCs. Metal oxide (MO) selective contacts have already been shown to improve the stability of PSCs substantially. However, especially the surfaces of, and thus interfaces with MOs are reactive, as they readily adsorb water and or O₂ and form hydroxides when exposed to moist air. Adsorption of hydroxide can change the oxidation states of the MO, forming trap states at the interface with the HaPs and increase the MO reactivity and causing HaP degradation. Furthermore, the oxygen in the hydroxide groups themselves is electro-negative and can react with the methylamine in the HaP compounds, forming PbI₂. We demonstrate RF sputtering of nickel oxide (NiO_x) as a hole transport layer, followed by an ultra-thin nickel nitride passivation layer, which becomes a buffer layer at the interface between the NiO_x and the HaP, deposited on the passivated NiO_x. The thin, amorphous nickel nitride layer suppresses interfacial HaP degradation, as shown by the increased stability of PSCs with a nickel nitride layer. Using RF sputtering to deposit inorganic passivation layers is an innovative step towards a scalable procedure of stable HaP based solar cells.

Thermal stability of organic−inorganic hybrid perovskites (OIHPs) remains as one of the critical challenges against the stable operation of perovskite solar cells (PSCs) in direct sunlight with elevated temperatures. Here, we show that the addition of a polystyrene-co-polyacrylonitrile (SAN) copolymer can significantly enhance thermal stability of OIHPs and improve the stability of the corresponding PSCs by suppressing the migration of organic cations in OHIP. The methylammonium lead iodide (MAPI) with SAN incorporated within the perovskite layer featured a superior thermal stability compared to pure MAPI without SAN, only displaying an average of 5−15% decrease in PCE even after continuous thermal aging for 24 h at 100 °C. The secondary ion mass spectrometry revealed that the thermal degradation of the pure MAPI was largely associated with MA+ out-migration. Conducting atomic force microscopy analysis further indicated that the incorporated SAN led to a suppression of ionic currents present at the grain boundaries of the perovskite film, which was understood by high immiscibility between SAN and MA+ components as confirmed by the experimentally estimated Flory−Huggins parameter between them. This study newly identifies a promising potential of using polymer grain encapsulation for enhancing thermal stability of OIHPs and their solar cell performance by suppressing the outdiffusion of cationic organic components.

Lead halide perovskites attract growing attention because they are promising for applications to solar cells, light-emitting diodes, and various optoelectronic devices. In recent years, we have demonstrated their unique optical properties, such as long excited-state lifetime and high photoluminescence quantum efficiency [1-7], which attribute to the excellent performance of the perovskite-based devices. In contrast, the carrier transport characteristics of this class of materials are poor compared with those of conventional semiconductors; the reported carrier mobility is below 100 cm²/Vs at room temperature, which is a two-order magnitude smaller than that of GaAs. The origin of such a low carrier mobility has been discussed in terms of polarons due to large electron-phonon interactions. Moreover, it has been pointed out that polarons are also involved in the photo-accelerated ion migration and photodegradation. Therefore, it is essential to gain a thorough understanding of the nature of polarons in halide perovskites. However, the polaron formation mechanisms and its impact on the optoelectronic properties are still under active debate.
Determination of carrier masses is the direct way to discuss the polaron effect. The magneto-reflectance measurement is one of the most powerful tools to estimate carrier mass through the observation of Landau levels and high-order exciton Rydberg states. We recently performed high-sensitivity measurements using circular dichroism of reflectance at low temperatures (1.5 K) under relatively weak magnetic fields (< 7T), where the cyclotron energy is sufficiently small compared with the LO phonon energy [8]. We successfully observed the higher-order Rydberg exciton states and Landau levels in CH₃NH₃PbX₃ [X = I, Br, and Cl], which enabled us to determine the reduced mass and the exciton binding energy accurately. For CH₃NH₃PbBr₃ and CH₃NH₃PbI₃, we estimated heavier reduced mass compared with that reported in the previous strong-field experiments, which evidences the polaron mass enhancement. The degree of mass enhancement agrees well with the prediction by the Fröhlich large polaron model. This means that the carrier mass is enhanced only by 50 % even in the case of coupling constant α = 3 (strong coupling regime), while α < 3 holds for lead halide perovskites. Therefore, the large polarons might not be the origin of the small carrier mobility.
To confirm this idea, we performed highly sensitive carrier transport measurements using an AC photo-Hall technique [9]. This technique enables us to estimate carrier mobilities even in highly insulating materials. We find that both electron and hole mobilities are significantly enhanced by photo-doping and exceed 300 cm²/Vs, which are comparable to the electron Hall mobilities of conventional inorganic semiconductors.
In the presentation, we will show the detailed experimental results and discuss the impact of polarons on optoelectronic properties of halide perovskites. We will also discuss the possible device application utilizing the strong electron-phonon interactions of lead halide perovskites.
Part of this work was supported by JST-CREST (JPMJCR16N3), JSPS KAKENHI (JP 19K03683, JP19H05465, JP20K03798), Canon foundation, and Chiba Iodine Resource Innovation Center.

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Resistive RAMs(Re-RAMs) have come to the fore as a promising candidate for next generation information and data storage because of their strikingly fast speed, prolonged data retention ability and high storage density. In a first, we developed an electrochemical metallization based Re-RAM with three-dimensionally integrated single-crystalline perovskite (methyl ammonium lead iodide) quantum wire (PQW) array rooted in a nano-engineered porous alumina membrane (PAM), as the switching medium. The PQWs grown by a vapor-solid-solid-reaction (VSSR) process was sandwiched between top silver (Ag) and bottom aluminium (Al) or gold (Au) contacts respectively. The PQW based Re-RAM demonstrated ultra-fast switching speed of 100 ps which is a record for perovskite Re-RAMs and also among the fastest for all types of Re-RAMs reported. The ultra-fast switching speed was ascribed to the engendered ionic (Ag⁺) and electronic mobility and subsequent accelerated filament formation within the body of the monocrystalline PQWs. The PQW Re-RAM demonstrated a high ON/OFF ratio of 10⁷, estimated data retention > 2 years and record high (for all perovskite Re-RAMs reported) cyclic endurance of 6×10⁶ cycles. The excellent retention and endurance performance was attributed to the material and electrical stability instilled by the electrically insulating PAM to the PQWs. Compared to Re-RAMs based on perovskite nanowires of larger diameter (around 250 nm), the PQWs with diameter around 10 nm, portrayed a much lower proclivity of high resistance state (HRS) degradation to low resistance state (LRS) upon multiple cycles of operation owing to the relatively less number of remnant filamentary Ag paths formed due to prolonged cyclic operations. This rendered the PQW devices to be more electrically robust and power efficient for longer cycle operations. Also, utilizing the ultra-high density (2×10¹¹/cm²) of the PQWs, a 14 nm lateral dimension ultra-small memory cell was constructed. Coupled with multi-bit storage, an effective device footprint area of 76.5 nm² was achieved for single bit storage. As a concept proof, an 8×8 Re-RAM crossbar array device was fabricated which demonstrated temporally robust alphabetic data storage, with a unique metal-semiconductor-insulator-metal (MSIM) architecture to alleviate the sneaky path problem. The MSIM scheme based on quasi-self-selecting elements, exhibited a strong potential for unhindered scalability in the future, a problem often faced with integrating external selecting diodes and transistors. The PQW Re-RAMs also responded to light stimuli, demonstrating optical programmability of the LRSs. These intriguing characteristics, in conjunction, present an attractive potency of PQW Re-RAMs as a promising technology for multifunctional non-volatile memories with broad applications in future storage and advanced computing systems.

Halide perovskites have been at the forefront of scientific and technological interest for a decade, triggered by the discovery of their noteworthy performance in solar cells, light sources, X/γ-ray detectors, and display devices. Furthermore, the potential for exploiting spin-properties of the halide perovskites for applications in spin-electronic and spin-optical devices has emerged, based on the materials' spin-orbit coupling, Rasbha/Dresselhaus effects, optically-induced spin polarization, and relatively long spin coherence and lifetime, compared to classic semiconductors. The current work presents optical experimental evidence for an intriguing phenomenon, related to the anisotropy of Rashba/Dresselhaus effects. The study explored different luminescence processes in a single bulk crystal of MAPbBr₃, using magneto-photoluminescence spectroscopy, monitored along several different crystallographic directions. The photoluminescence spectra were dominated by dual exciton emission peaks, each exhibiting a highly non-linear response to a magnetic field with an anisotropy from -B₀ to +B₀, a strong dependence on the axis of observations (parallel or perpendicular to a crystallographic symmetry axis) and on the detected spectral region. A theoretical model, implementing a dominant anisotropy in Rashba/Dresselhaus terms, anisotropy in the Landé g-factors, and a contribution from an Overhauser effect- the hyperfine interaction, corroborated the experimental results. Even though the Rashba/Dresselhaus effects vary from location to location on the crystal, they show reproducible features such as MPL dips at low magnetic fields, and facet-dependent MPL trends at high magnetic fields. Our work shows that these are consequences of a multi-component Rashba/Dresselhaus effect and g-factor anisotropy. The Rashba/Dresselhaus couplings likely arise from different nano-crystallinities within the bulk of halide perovskites, such as grain boundaries.

Opto-electronic and mechanical properties of Pb Halide Perovskite (HaP) thin films, as well as ways to contact them, will be influenced strongly by morphology, and, for several physical properties, by crystallographic orientation. Thus, a method to grow smooth, well-oriented thin films in a pre-selected orientation in a scalable and potentially high-throughput way, is of interest.
We report that highly oriented, smooth HaP films, especially of methylammonium lead- bromide (MAPbBr₃) and -iodide (MAPbI₃) can be deposited by CW laser ablation. HaPs were time and again shown to be different from conventional (inorganic) semiconductors like Si, GaAs, CdTe, CIGS, both in terms of which preparation methods give high quality material and in their physical properties. While pulsed laser-based synthesis / deposition methods have been reported, our finding that a low power continuous laser (orders of magnitude less than the equivalent power used in pulsed laser, and significantly below that of flash deposition/annealing), can yield highly oriented, relatively smooth thin films on FTO (~ 20 nm rms) and Si (~3 nm rms) substrates of MAPbBr₃ (001) and MAPbI₃ (110), using their single crystals as targets, adds to the HaP surprises. The quality and homogeneity of the thin films were checked with XRD, AFM and Photoluminescence. The deposition is solvent-free, at room temperature and works in vacuum levels comparable to those used in laboratory physical vapor deposition (PVD). Further properties of the materials, and timer-permitting, of devices will be described.

Recently, inorganic perovskite quantum dots (PQDs) have attracted a lot of attention for optoelectronic and photovoltaics applications because of tunability and stability. The case of CsPbI₃ deserves special attention for solar cells and LEDs devices. The inclusion of Cs and the small size of PQDs improve stability, however absorption band is blue-shifted that represent a drawback for solar cells design and LEDs. Therefore, bigger PQDs to redshift the absorption band with better stability and high photoluminescence quantum yield (PLQY) are desired. However, bigger nanocrystals are unstable and transform into yellow phase non-perovskite that is inactive under photoexcitaiton. Here in this work, we report the synthesis of CsPbI₃ QDs with a PLQY ~100% at 690 nm and FWHM~31nm with a reduction lower than 10% of efficiency after 12months, synthetized by hot injection at 170^oC. It is analyzed in detail the effect of precursors and temperature on the synthesis process and evaluates the spectroscopic properties such as decay time, band gap and emission. PQDs were tested to design an LED and solar cells devices showing excellent performance, and then offering good opportunities for non-linear photonics and photoelectronic devices.

Halide perovskite (HaPs) solar cells have shown outstanding progress in performance in recent years, exceeding power conversion efficiency of 25%, thus promising to be a strong competitor for the current silicon solar cell technology. Limits to this trend originate from non-radiative recombination of electrons and holes as the dominant loss processes in photovoltaics. This phenomenon originating from defect states which arise imperfection of the crystalline quality in the bulk, and particularly at the interfaces of photovoltaic devices. Remarkably, the quality of the perovskite film as the light-harvesting materials has been improved to such a high level with accordingly long carrier diffusion lengths that recombination in the bulk of perovskite layers does not significantly limit the functionality of the device. Therefore, study of the defects states at the surface and at the interfaces in the layer stack has advanced to the center of attention. [1] Here, we focus on one of these interfaces and discuss the chemical properties and energy level alignment at the interface between HaPs absorber and carbon nanotube charge transport layers.
Single-walled carbon nanotubes (SWCNT) with their unique optoelectronic properties, show a strong potential to be integrated in various architecture of perovskite solar cells.[2] SWCNTs with high charge carrier mobility improve carrier collection which in turn, enhance HaPs solar cell performance. In addition, their high optical transmission in the visible range makes them an excellent candidate for charge transport layer in HaPs solar cells. Tunable electronic properties, via changing the design of the tubes and the doping characteristics, make them suitable for both electron and hole transport layers.
In the present study, we report on the electronic and chemical properties and the energy level alignment at the interface between HaPs materials, MAPbI₃ and MAPbBr₃, and SWCNTs overlayers. The SWCNT layers were grown by ultrasonic spray deposition at various thicknesses on top of the perovskite absorber. We then used synchrotron-based hard x-ray photoemission spectroscopy (HAXPES) to investigate the electronic characteristics and compositional gradient of the two systems at 2 keV and 6 keV excitation energy. HAXPES measurement provides similar features of the target layer as soft X-ray photoemission spectroscopy (XPS). However, the higher kinetic energy of the emitted electrons in HAXPES increases the information depth and therefore allows us to selectively probe the immediate interface and sub-surface region, respectively.
Our results show generally different surface and interface electronic and chemical characteristics between MAPbI₃ and MAPbBr₃ before and after SWCNTs film growth. We observe selective core level peak shifts which we attribute to the formation of new species at the interface. Furthermore, the compositional study shows the distribution of elements is different between buried and topmost perovskite layer for MAPbI₃ and MAPbBr₃ upon deposition of the SWCNT, respectively. These gradients result in deficiency and/or surplus of the perovskite components that in turn affect the landscape of interface defects and consequently the device performance.


1. Schulz, Philip, David Cahen, and Antoine Kahn. "Halide perovskites: is it all about the interfaces?." Chemical reviews 119.5 (2019): 3349-3417.
2. Habisreutinger, Severin N., Robin J. Nicholas, and Henry J. Snaith. "Carbon nanotubes in perovskite solar cells." Advanced Energy Materials 7.10 (2017): 1601839.

Methylammonium lead iodide (MAPbI₃) perovskite has shown high defect tolerance. This is well known that defects in the perovskite absorber layer hinder the power conversion efficiency (PCE) of perovskite solar cells in approaching the Schockley-Queisser limit. Understanding the effect of defects in the performance of the perovskite solar cells is a very interesting area and needs further exploration.
Different types of intrinsic defects (vacancies, substitutions, and interstitials) are possible in MAPbI_3. Sources of defects in the perovskite layer can be from impurities. Even grain boundaries are considered as sources for high defect densities in solution-processed polycrystalline MAPbI₃ perovskite films. The quality of the films depends on factors like the purity of the precursors, solvents, process parameters, crystal growth, etc. The defect levels of both point and structural defects can be the contribution of breaking and addition of bonds i.e., wrong bonds or dangling bonds.
Cuprous iodide (CuI) has attracted extensive attention in p-i-n structured perovskite solar cells as a hole transport layer because of its high transmittance, optimal band matching, and ease of processibility. CuI has been used as the passivating material to improve the film quality by reducing defects. Cu²⁺ is expected to passivate the defects because of its small size and also prevent iodine from migrating from its place. I^- ion may help to overcome iodine deficiency caused due to ion migration. Iodine defects are much easier to create because of their low formation energy.
Passivation of defects and imperfections is expected to cause a reduction in ion migration to achieve improved efficiency and stability. J-V performance of the devices was analysed under dark and light conditions. There is no change in the crystal structure as confirmed from the XRD analysis. High intensity of PL peaks has been obtained in the passivated film which is related to more band-to-band recombination i.e., less number of traps in the film. Since the charge carrier density is different for passivated and non-passivated perovskite film, a difference in the capacitive behaviour of the films is expected. The capacitive behaviour is related to charge transport and charge accumulation. Hence analysis of the capacitive behaviour of the devices under the dark and light has been carried out to understand the change in polarization behaviour because of the addition of passivation material.

Perovskite solar cells (PSCs) have quickly risen in efficiency since their initial fabrication in 2009, with the current record power conversion efficiency (PCE) being over 25%. Quickly approaching the Shockley—Queisser limit of 33% for single-junction solar cells, focus on stability and scalability has become crucial. While solution deposition of the perovskite absorber layer by spin-coating has resulted in the highest efficiency devices, vapor deposition methods have shown promise and may help resolve both the stability and scalability problems that PSCs are currently facing. Atomic layer deposition (ALD), as a vapor deposition process, provides advantages such as fine thickness control, improved conformity and uniformity, and the ability to be incorporated into existing industrial processes such as roll-to-roll manufacturing. The stoichiometry of the perovskite may also be easily tuned through manipulation of the precursor doses. This work focuses on depositing perovskite through a two-step ALD/ molecular layer deposition (MLD) hybrid process, with ALD referring to the inorganic aspects of the process and MLD incorporating organic materials. The first step, which we have recently achieved, is to deposit a conformal lead iodide film which may be converted to methylammonium lead triiodide (MAPbI₃) through exposure to methylammonium iodide (MAI). For the lead precursor, Pb(tmhd)₂ was chosen due to its adequate vapor pressure at relatively low source temperatures (less than 200°C) and higher reactivity when compared to other commonly utilized lead precursors such as Pb(acac)₂. HI (aq) was chosen as the iodine precursor due to its high vapor pressure and reactivity. The precursors involved are easily accessible through commercial means and do not require any additional processing before use. The second step, which we are currently developing, involves the introduction of a third precursor within every cycle of the previous recipe, which would allow for an effective ALD/MLD process for MAPbI₃. Once we are able to deposit highly uniform perovskite thin films by ALD/MLD, this will allow us the ability to deposit 2D perovskite passivation layers which will help improve stability by decreasing defects at the perovskite/charge transport layer interface. This will increase moisture and thermal stability in the PSCs while also being a scalable process.

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.

[1] A. Sutanto, P. Caprioglio, N. Drigo, Y. J. Hofstetter, I. Garcia-Benito, V. I. E. Queloz, D. Neher, M. K. Nazeeruddin, M. Stolterfoht, Y. Vaynzof, G. Grancini “2D/3D Perovskite Engineering Eliminates Interfacial Recombination Losses in Hybrid Perovskite Solar Cells”, Chem DOI:10.1016/j.chempr.2021.04.002 (2021)

I acknowledge the HY-NANO project that has received funding from the European Research Council (ERC) Starting Grant 2018 under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 802862).

Metal-halide perovskites are one of the most promising active materials for photovoltaic and light-emitting technologies, due to their excellent optoelectronic properties and fabrication versatility. Within a decade, perovskite solar cells (PSCs) have achieved record power conversion efficiency (PCE) of 25.6% [1], for active areas smaller than 1 cm², and a constantly improving operational stability [2-5]. This rapid progress has triggered the interest in transferring the existing technology from small area devices into large-area perovskite modules, necessary for industrial development.

Recently 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 of the thin films translates PSCs with PCE above 20% in both n.i.p [6, 7] and p.i.n [8] configurations, and impressive thermal and environmental long-term stability [9] even without encapsulation. Furthermore, the first co-evaporated mini-modules achieved record PCEs of 18.13% and 18.4% for active areas of 20 cm² [6] and 6.4 cm² [10]
At the same time, looking forward to tandem integration and building-integrated photovoltaics we have also developed colored semi-transparent PSCs and mini-modules with transparency above 75% in near-infrared for all the range of colors 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.

References:

1. NREL. Best Research-Cell Efficiency Chart; U.S. Department of Energy; https://www.nrel.gov/pv/cell-efficiency.htm.
2. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050.
3. N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel, Science 2017, 358, 768.
4. S. Yang, S. Chen, E. Mosconi, Y. Fang, X. Xiao, C. Wang, Y. Zhou, Z. Yu, J. Zhao, Y. Gao, Science 2019, 365, 473.
5. Y. Wang, T. Wu, J. Barbaud, W. Kong, D. Cui, H. Chen, X. Yang, L. Han, Science 2019 365, 687.
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
7. L Li, HA Dewi, W Hao, N Mathews, S Mhaisalkar, A Bruno, Coatings, 2022 10(12), 1163
8. L 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.
9. HA Dewi, L Li, W Hao, N Mathews, S Mhaisalkar, A Bruno, Adv. Funct. Mater. 2021 2100557,
10. L Li, HA Dewi, W Hao, L Jia Haur, N Mathews, S Mhaisalkar, A Bruno, Solar RRL, 2020, 4, 2000473

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 Sn⁴⁺ to Sn²⁺ 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 Sn²⁺ 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-cm²-area all-perovskite tandem cells, and in-lab PCEs of 25.6% and 21.4% for 0.049 and 12 cm² devices, respectively. The encapsulated tandem devices retain 88% of their initial performance following 500 hours of operation under one-sun illumination in ambient conditions.

Perovskite photovoltaic technology has advanced substantially, with the present record efficiency for single-junction devices reaching > 25%. One of the most promising strategies for commercializing these devices is to apply a perovskite top cell in tandem with a Si bottom cell to reach ultrahigh efficiency beyond the Shockley-Queisser limit for single-junction devices. The ideal band gap for the tandem configuration is ~1.67 to 1.75 eV for the top cell and 1.12 eV for the bottom cell, which, fortuitously, is the Si band gap. The band gap of perovskites can be tuned by (partial) replacement of iodine anions with bromine or chlorine. However, the replacement of I with Br by more than 20%, which is necessary to enlarge the band gap to ~1.7 eV, leads to stability issues under illumination through phase separation that forms I-rich and Br-rich structures. One approach to stabilize the perovskite is to create a two-dimensional (2D) phase in which sheets of [PbX₆]^2- octahedra are separated by an excess number of long-chain (or aromatic) molecules that act as a passivation agent. Common long-chain or aromatic molecule-based 2D additives include n- butylammonium iodide (n-BAI) and phenethylammonium iodide (PEAI). Most of the recent studies have focused on the cation components of the 2D additives rather than focusing on the anions. We developed a 2D-3D mixed wide band gap (1.68 eV) perovskite using a mixture of thiocyanate (SCN) with the more conventional choice, iodine. Through a careful application of atomic resolution transmission electron microscopy, we demonstrated that electrical and charge transport properties as well as the physical location of 2D passivation layers can be controlled with anion engineering of the 2D additives. Moreover, we can use this approach to extend light stability and to improve device performance. For a perovskite device, we achieved a PCE of 20.7% that retained > 80% of its initial efficiency after 1000 hours of continuous illumination in working conditions. For a monolithic 2T perovskite/Si tandem solar cells, the champion 2T tandem device achieved a PCE of 26.7%. If time permits, I will also discuss our recent progress in perovskite/Cu(In,Ga)Se₂ tandem solar cells.

One of the challenges of perovskite solar cells is its instability. 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 were the first to exceed the strict requirements of International Electrotechnical Commission standards for thermal cycling damp heat and humidity freeze. Such a major breakthrough represents an important step towards commercial viability.

Planar or semi-planar type perovskite photovoltaic (PV) cells have been developed with various carrier transport materials in normal and inverse structures. Planar structures could be cost-efficient in device manufacture compared with mesoporous ones because of simplified layered structures in which deposition of a metal oxide electron transport layer (ETL) is replaced with a hole-blocking thin surface layer covering the electrode substrate. Successful modification of the electrode surface with organic or inorganic molecular layers as hole-blocking nanosheet materials leads to enhancement of efficiency and stability of the perovskite devices. Hysteresis in current-voltage (J-V) characteristics is also suppressed as a result of well-balanced electron/hole transports being accompanied by an increase in open-circuit voltage (Voc). As an inorganic nanosheet, thin films (<10 nm) of amorphous metal oxides such as TiOx and SnOx are effective in creating a void-less compact interlayer to connect ETL and perovskite. In fabrication of CsPbI₂Br-based semi-planar photovoltaic cells, we modified the surface of SnO₂ ETL with an amorphous SnOx (<5 nm) as an interlayer for junction of CsPbI₂Br layer. Using a dopant-free polymer hole-transport layer (HTL), the CsPbI₂Br device yielded a power conversion efficiency of 17% with a high Voc reaching 1.42V.¹ A reference device without the SnOx interlayer gave a poor J-V performance with a large hysteresis and a low Voc (<1.2V), indicating the essential role of the interlayer. Another example of inorganic nanosheet is MXene, a 2D layered material comprising Ti₃C₂. We treated a colloidal solution of MXene by oxidation to form nanosheets of HO-Ti₃C₂T_x, which show semiconductivity suitable for ETL. Thin layer (<20 nm) of the oxidized MXene works as a good ETL in a planar device with efficiency over 18%.² Efficiency reaches more than 20% when HO-Ti₃C₂T_x is hybridized with SnO₂, which chemically interacts HO-Ti₃C₂T_x via oxygen bonding. Being mechanically and thermally robust, MXenes are promising nanomaterials to modify the function of heterojunction interfaces. Organic small molecules also serve as modulators to passivate the defects at the junction interface. Among various molecules, we found an organic peroxide, artemisinin (water-insoluble anti-malarial drug), works to modify the interface of SnO₂ ETL and multi-cation perovskite (Rb_0.05Cs_0.05(FA₀.₈₃MA_0.17)_0.9Pb(I_0.95Br_0.05)₃).³ This redox-active molecule is considered to interreact with the oxide surface and lead cation in passivation process. Artemisinin-based passivation was applied to the SnO₂-perovskite interface of a plastic film flexible PV 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 21.1% with Voc of 1.13V; the efficiency is the highest value ever achieved for plastic perovskite PV device. All of the above inorganic and organic passivators are chemically and thermally stable non-ionic materials and inactive against their diffusion. Such properties will guarantee structural stability of ultra-thin hole-blocking interlayers involved in defect passivation.

References:
1. Z. Guo, A. K. Jena, I. Takei, G. M. Kim, M. A. Kamarudin, Y. Sanehira, A. Ishii, Y. Numata, S. Hayase, and T. Miyasaka, J. Am. Chem. Soc. 2020, 142, 21, 9725–9734.
2. L. Yang, D. Kan, C. Dall'Agnese, Y. Dall'Agnese, B. Wang, A. K. Jena, Y. Wei, G. Chen, X. -F. Wang, Y. Gogotsi, and T. Miyasaka, J. Mater. Chem. A, 2021, 9, 5016-5025
3. L. Yang, Q. Xiong, Y. Li, P. Gao, B. Xu, H. Lin, X. Li and T. Miyasaka, J. Mater. Chem. A, 2021, 9, 1574-1582.

Sn-Pb perovskites have shown their potentials in wide-bandgap photovoltaics and near-infrared photodetection applications which could become alternatives to traditional silicon and inorganic devices. To achieve efficient devices, high-quality and thick Sn-Pb perovskite films featuring well-packed, smooth, pinhole/void-free are highly desirable. Besides, understanding the crystallization kinetics and tuning the crystallization are fundamentally important to reach the Sn-Pb perovskite films, and have been limitedly explored. Herein, we propose an approach of double-side crystallization tuning through low-temperature space-restricted annealing in MA-free Sn-Pb perovskite films. By simultaneously retarding the crystallization in the top of precursor films and promoting the crystal growth of the bottom of precursor films, we achieve high-quality and block-like 1 μm thick Sn-Pb perovskite films with improved crystallinity and reduced trap density [1]. The high-quality Sn-Pb perovskite can also be fabricated on metal/ silicon substrates for promoting direct integration with CMOS electronics and realizing an efficient photodiode imaging array [2]. Meanwhile, we develop the approach of in-situ tin(II) inorganic complex anti-solvent process for particularly tuning the perovskite nucleation and crystal growing process. Interestingly, we uniquely form the quasi core-shell structure of the Pb-Sn perovskite-tin(II) complex as well as heterojunction perovskite structure at the same time for suppressing the nonradiative recombination, extending carrier lifetime, and enhancing the carrier extraction. As a result, the mixed Pb-Sn low-bandgap perovskite device achieves a high power conversion efficiency up to 19.03% with a fill factor over 0.8, which is among the highest fill factor in high-performance Pb-Sn perovskite solar cells. Remarkably, the device fail time under continuous light illumination is extended by over 18.5 folds reaching 560 h [3]. To realize the integration of Sn-Pb perovskite for tandem cell applications, we also develop a new thermionic emission-based interconnecting layer (ICL) structure with the enhanced solvent-resistance feature. Fundamentally, the thermionic emission plays a critical role in the electron transport process in the ICL, which is confirmed through both experimental and theoretical studies [4]. The works not only demonstrate the solid potential of Sn-Pb perovskite in solar cells and photodetectors but also promoting all perovskite tandem solar cells for low-cost photovoltaics.
[1] H. Liu, H.L. Zhu, W.C.H. Choy, et al, "Double-Side Crystallization Tuning to Achieve Over 1 μm Thick and Well-aligned Block-like Narrow-bandgap Perovskites for High-efficiency Near-infrared Photodetectors", Adv. Funct. Mat. in press.
[2] H.L. Zhu, W.C.H. Choy, et al, ACS Nano, 13, 11800, 2019.
[3] C. Li, W.C.H. Choy, et al, Adv. Energy Mater., 10, 1903013, 2020.
[4] C. Li, W.C.H. Choy, et al, Adv. Energy Mater., 8, 1801954, 2018.

Two-dimensional (2D) material are playing an important role in perovskite photovoltaics. Halide perovskite and 2D materials, including 2D perovskites, can be combined to enhance efficiency and stability of solar cells. Recently, a new perspective class of 2D materials - MXenes demonstrated a beneficial result for interface engineering in perovskite solar cells (PSCs) [1,2]. In general, the MXenes are metal carbides/nitrides with single or multilayer 2D structures and have general formula of Mn+1XnTx (n = 1, 2, 3), where M represents an early transition metal, X is a carbon and/or nitrogen atom and T are surface functionalization groups (–F, –O, –OH). In this talk I will present the progresses made in the use of MXenes and other 2D materials such as Graphene, MoS2 to improve the performance and the stability of perovskite solar cells. %With a thorough multiscale experimental investigation, we point out that 2D materials can tune interfaces properties, reduce ion migration and modify the work-function of the perovskite absorber and charge transporting layers, all aspects that directly impact on the final efficiency and the stability under accelerated stress tests.
The use of 2D materials allows us to reach more than 26% efficiency in a tandem graphene-perovskite/silicon cell [3] and ≈20% on 10 cm² module, and improve the stability for light soaking stress beyond 2000 hours out of glove-box. By using 2D materials we realize a solar farm with 0.5 sqm panels obtained with single junction graphene-perovskite sub-modules with efficiency up to 16% (on a substrate area of more than 100cm2) and panel efficiency exceeding 10%

References:
[1] Agresti, A., Pazniak, A., Pescetelli, S. et al. Titanium-carbide MXenes for work function and interface engineering in perovskite solar cells. Nat. Mater. 18, 1228–1234 (2019). https://doi.org/10.1038/s41563-019-0478-1
[2] D. Saranin et al. Transition metal carbides (MXenes) for efficient NiO-based inverted perovskite solar cells, Nano Energy 82, 105771, (2021)
[3] E. Lamanna et al, Mechanically Stacked, Two-Terminal Graphene-Based Perovskite/Silicon Tandem Solar Cell with Efficiency over 26%, Joule 4, 865 (2020)

The perovskite structure is proving to be a versatile platform for creating photoactive compounds for application in solar cells, achieving excellent power conversion efficiencies, long carrier diffusion lengths and low recombination rates. However, a number of challenges remain, including current density-voltage hysteresis and structural degradation in air and light.

Many parameters impact on device properties from grain structure to the integrity of the interface between the transport and photoactive layers. Here we investigate the local crystal structure and intragrain defect structure of popular photoactive perovskite compounds and correlate these observations with device performance and properties.

We develop and apply specific strategies for pristine structure analysis using transmission electron microscopy (TEM). This includes the preparation of TEM foils using processes that mimic the preparation of the device photoadsorber (and avoids the use of ion beams) and the subsequent examination in the TEM using a total electron dose well below measured damage thresholds [1,2].

We investigate the crystal structure and associated defects in several photoactive perovskites: the mixed organic cation perovskite MA_xFA_1−xPbI₃ (with methylammonium, MA= CH₃NH₃⁺ and formadinium, FA=NH₂CH=NH₂⁺ and x=0, 0.15, 0.5, 0.8, 0.9 and 1) [3, 4], the more structurally stable pure inorganic CsPbI_(1+x)Br_(2-x) [5] and the mixed cation FA_xCs_1−xPbI₃.

We reveal different types of point and planar defects induced by different elemental compositions and stoichiometries and measure the corresponding changes in device performance, including charge carrier lifetime, open-circuit voltage-deficit, and current-voltage hysteresis. We identify which structural defects are benign and which may be detrimental to device performance, suggesting further avenues for improving device properties and performance.

[1] Rothmann MU, Li W, Zhu Y, Liu A, Ku, Bach U, Etheridge J, Cheng YB; Advanced Materials 1800629 (2018) and manuscript in preparation (2021).
[2] Rothmann MU, Li W, Etheridge J, Cheng YB Advanced Energy Materials 7 23 (2017).
[3] Rothmann MU, Li W, Zhu Y, Bach U, Spiccia L, Etheridge J, Cheng YB Nature Communications 8 14547 (2017).
[4] Li W, Rothmann MU, Zhu Y, Chen W, Yuan Y, Choo YY, Yang C, Wen X, Bach U, Cheng YB, Etheridge J Nature Energy 6 624–632 (2021).
[5] Li W, Rothmann MU, Liu A, Wang Z, Zhang Y, Pascoe AR, Lu J, Jiang L, Chen Y, Huang F, Peng Y, Bao Q, Etheridge J, Bach U, Cheng YB Advanced Energy Materials 7 20 (2017).

Perovskite materials have attracted increasing attention in different application fields thanks to their composition diversity and extraordinary variety of electronic properties. Perovskite solar devices are nowadays the fastest advancing photovoltaic technology, but a large-scale application is restrained by open challenges that still delay their market entry.

How can the design of functional materials, and in particular perovskite materials, be accelerated using supercomputers?
In this talk I will discuss how to combine first-principles calculations and machine learning to speed up the discovery of new materials.

Characterizing the electronic properties of crystalline bulk materials can however be insufficient to evaluate their suitability for applications. Interfaces are in fact at the heart of electronic devices: the ability to shape potential gradients at interfaces opens up the possibility to manipulate electrons and develop new functionalities. The design of interfaces, as well as the deep understanding and control of their properties, is still beyond the current state of the art. I will discuss some recent progresses in this direction.

The presentation will review recent experimental and theoretical results for 2D multilayered halide perovskites, dedicated to photovoltaic or light emission applications. The complex interplay between structural and optoelectronic properties will be emphasized, including the influence of light soaking, the ultrafast relaxation of the lattice induced by hot carriers and the localization of carriers at the layer edges.

Hybrid perovskites are emerging as highly efficient materials for optoelectronic applications, however, their operational lifetime has remained a limiting factor for their continued progress. In thin-film perovskite light-emitting devices, ionic redistribution may distort the perovskite crystal structure, lowering conductivity and light emission due to the formation of vacancies and other traps. Our strategy for enhanced lifetimes involves producing differentiated ion motion within a device with a rational materials blend. The materials selectively move additive ions while restricting the transport of perovskite ions. To accomplish differentiated ion transport with optimal thin-film morphology, we combine a perovskite, a polyelectrolyte such as poly(ethylene oxide), and a salt additive such as LiPF₆. The added mobile Li⁺ and PF₆⁻ ions redistribute more favorably than the intrinsic ionic species and largely preserve the inherent structure of the perovskite film. At 0.5 wt% LiPF₆, CsPbBr₃ devices exhibit 100 h operation at high brightness in excess of 800 cd/m² under constant current driving, achieving a maximum luminance of 3260 cd/m². This performance extrapolates to a 6700 h luminance half-life from 100 cd/m², a 5.6-fold improvement over devices with no LiPF₆. For quantum dot blended devices, a 45 h half-life was obtained at 3640 cd/m², extrapolating to over 37000 h at 100 cd/m². We rationalize these findings through optical spectroscopy, impedance spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction.

Identification of electronic processes at charge-selective interfaces is crucial not only for the further development of high efficiency solar cells based on metal halide perovskite (HaP) absorbers but also for other materials systems in photovoltaic research, photocatalysis etc. Transient surface photovoltage (SPV) measurements provide information about dynamics of charge separation and relaxation across interfaces from the nanosecond to the second range. Passivation of different hole-selective ITO / SAM / HaP contacts (SAM: self-assembled monolayer, here: carbazole-based molecules with a phosphonic acid linker group) is studied by transient SPV, compared with radiative bulk recombination (measured by transient photoluminescence) and correlated with the performance of solar cells. It is shown that transient SPV and transient photoluminescence provide complementary information on charge transfer kinetics and trapping/de-trapping mechanisms. Hole transfer rates to the ITO and densities of traps at ITO / SAM interfaces were obtained by fitting SPV transients with a minimalistic kinetic simulation model. The hole transfer rates and the density of interface traps depended strongly on the SAM`s structure, and densities of interface traps as low as 10⁹ cm^-2, in par with highly passivated c-Si surfaces, were reached for Me-4PACz, which was previously used in record perovskite/silicon tandem solar cells. The extracted hole transfer rates and densities of interface traps correlated well with corresponding fill factors and open-circuit voltages of high efficiency solar cells based on HaP absorbers. Our results show that, aside tuning of barriers for charge transfer, SAMs can efficiently passivate defect states at interfaces.

Halide perovskite photovoltaics (PV) are known to have a wide range of chemical compositions. The perovskite lattice is quite tunable and able to accommodate various ions of different sizes. To date, halide perovskite solar cells (PSC) yield the highest efficiency and stability when absorbers comprised of a mixture of ions are utilized. However, some compositions such as FAPbI₃ (FA = formamidinium) or the fully inorganic composition CsPbI₃ exhibit non-ideal geometric conditions, often measured using the Goldschmidt tolerance factor. When ions are too large or too small for the lattice non-perovskite δ-phases that are detrimental for PV applications are formed. Mixed compositions allow for a fine tuning of the geometric conditions and enables the formation of photoactive perovskite phases.^[1] Nevertheless, chemical heterogeneity in the film can initiate phase segregation of the material into pure phases, which as noted prefer their non-perovskite δ-phases, and consequently lead to degradation of the device performance.

Spatially resolved analyses have repeatedly revealed microscopic heterogeneity in halide perovskite thin films, including compositional heterogeneity. Evidence has been found that decomposition during operation could be triggered/facilitated by local compositional variation from the averaged mixed phase.^[2] However, characterizing the chemical heterogeneity on small length scales has proven challenging. Recently work by Barrier et al demonstrated an X-ray scattering based methods for probing this nanoscale heterogeneity in halide perovskites.^[3]
In this talk, we use these methods reported by Barrier et al. to probe nanoscale heterogeneity in PSC absorber layers and couple these results to device level stability studies to understand the role heterogeneity plays in device performance. This work relies on the hypothesis that the cubic to tetragonal phase transition width is a measure of the chemical heterogeneity in mixed halide perovskites. To measure the phase transition width from the tetragonal to the cubic phase, temperature-dependent in situ wide angle X-ray scattering is utilized. Barrier et al. report that the transition is abrupt for pure MAPbI₃ (MA = methyl ammonium), however, the transition is stretched over a larger temperature range for the mixed compositions measured in their study.

To build on this work we prepare a set of FA_0.83Cs_0.17PbI₃ perovskite thin films, choosing a variation of annealing conditions. The variation in annealing time and temperature can impact the compositional heterogeneity as measured using in-situ X-ray scattering. We couple these results to device level stability studies and ex-situ scattering studies of the degraded devices to understand how heterogeneity impacts the operational stability of these materials. In this talk, we will provide a summary of the approach used to characterize the heterogeneity in these films and comment on the loss in device performance and subsequent phase segregation induced by nanoscale heterogeneity.

References:
[1] Z. Li, M. Yang, J. S. Park, S. H. Wei, J. J. Berry, K. Zhu, Chem. Mater. 2016, 28, 284.
[2] L. T. Schelhas, Z. Li, J. A. Christians, A. Goyal, P. Kairys, S. P. Harvey, D. H. Kim, K. H. Stone, J. M. Luther, K. Zhu, V. Stevanovic, J. J. Berry, Energy Environ. Sci. 2019, 12, 1341.
[3] J. Barrier, A. Gold-Parker, R. E. Beal, E. Wolf, L. Waquier, R. Szostak, A. F. Nogueira, M. D. McGehee, M. F. Toney, submitted n.d.

Understanding the chemical reactions that hybrid organic-inorganic halide perovskite (HP) semiconductors undergo in the presence of moisture, oxygen, and light are essential to the commercial development of HP solar cells and optoelectronics. Here we use optical absorbance (105 degradations were conducted over 41 unique environmental conditions) to quantify the kinetics of methylammonium lead iodide (MAPbI₃) degradation in response to combinations of moisture, oxygen, and illumination over a range of temperatures. We discover and report a new chemical reaction pathway referred to as a water-accelerated photo-oxidation (WPO) pathway [1], which is the dominant chemical degradation pathway when light, oxygen, and humidity are present. At 25 °C in 50% RH air under 1 Sun illumination, the water-accelerated photooxidation proceeds with a rate of 2 x 10^-7 mol/m²s with an activation energy of E_A=0.07 eV. Commonly reported pathways based on: (i) dry photo-oxidative degradation (DPO, degradation in oxygen under illumination but without water as a reactant), (ii) humidity-driven hydrate formation (degradation with humidity with and without illumination but without oxygen), (iii) blue light exposure, and (iv) thermally driven degradation (without humidity and oxygen) are demonstrated to have rates that are respectively 50, 200, 1000, and >1000 times slower than the water-accelerated photooxidation processes. The activation energy of the DPO process is much higher at 0.62 eV, and at higher temperatures (85°C) the rate of the DPO process is roughly equivalent to the WPO process. Given the much lower activation energy and much faster rate of the WPO process, the overall rate of degradation is very sensitive to small amounts of water. Water thus behaves like catalyst. Since it is consumed in one step and later re-generated, we refer to it as an accelerant. The WPO process has significant implications for perovskite stability and encapsulation, which will be discussed. A detailed kinetic model is developed for both the WPO and DPO processes. The rate determining step for WPO is proton abstraction from water by superoxide radical that creates several different reactive oxygen species, which then react with the perovskite. However, the rate determining step for the DPO process is proton abstraction from methylammonium by superoxide radical. A detailed rate law for each process is derived from this mechanism, and the entire dataset is fit to extract rate constants for DPO and WPO processes. The presentation will focus on the kinetics of perovskite degradation and will include experiments on the broader (FA, MA, Cs)Pb(I,Br)3 space as well as discuss how chemistry inspired features (like the rate of chemical reaction) can be used in machine learning models for perovskite degradation.

[1] Siegler, T.D., Dunlap-Shohl, W.A., Meng, Y., Kau, W.F., Sunkari, P.P., Tsai, C., Armstrong, Z.J., Chen, Y., Beck, D.A.C., Meila, M., Hillhouse, H.W., "Water-Accelerated Photo-oxidation of CH3NH3PbI3 Perovskite: Mechanism, rate orders, and rate constants," ChemRXiv 2021, preprint online June 21 at https://doi.org/10.33774/chemrxiv-2021-30ggh

[2] Stoddard, R.J., Dunlap-Shohl, W.A., Qiao, H., Meng, Y., Kau, W.F., Hillhouse, H.W. “Forecasting the Decay of Hybrid Perovskite Performance Using Optical Transmittance or Reflected Dark-Field Imaging,” ACS Energy Lett. 2020, 5, 3, 946-954

One of the main goals for perovskite materials is to approach ideal efficiency limits in solar cells or LEDs: being the radiative decay of optical excitations unavoidable, this can be done by reducing contributions from traps and defects. Despite all the advances in device performances, the bimolecular radiative recombination rate in metal halide perovskites is still not clearly established: coefficients derived under different assumptions can take very different values, and a lively debate is going on trying to conjugate their exceptionally long excitation lifetimes with large optical absorption coefficients. Here we provide a calibrated radiometric measurement of the instantaneous photoluminescence flux emitted by thin-films of hybrid perovskites with controlled carrier density under pulsed excitation. In all the analysed samples we uncover a radiative recombination rate much lower than the bimolecular recombination coefficient, revealing the presence of non-radiative bimolecular recombination rate that can be attributed to a population of dark states, possibly large polarons. Our findings suggest that to minimize unwanted decay channels for optical excitations one should also take account of non-radiative contributions to the bimolecular decay, that is due to extrinsic effects, like trapping and defects, and can be substantially reduced with materials optimization.

In the last decade, power conversion efficiencies (PCE) of thin-film perovskite solar cells (PSCs) have rapidly grown from 3.8% to 25%. As most of PSCs are already achieving efficient light absorption and charge collection, enhancing electroluminescence (EL) is the key for them to approach the Shockley-Queisser limit. While the classical optics restricts the EL quantum efficiency (ELQE) of thin-film optoelectronics to 1/2n², which is 7.4% for perovskites (n=2.6), photon recycling (PR) and scattering can give a chance to overcome it by extracting photons trapped in waveguide and substrate modes.
Here, we investigate the role of PR and scattering in a PSC with passivation of octylammonium (OA) bromide, having PCE of 24%. The device exhibits the high ELQE of 13.7% at 1-sun equivalent excitation, exceeding the classical limit. The primary evidence for PR is given by the self-excitation experiment, where EL of one pixel induces photocurrents of nearby pixels through the reabsorption of photons trapped in substrate mode. Spectral analysis of the spatially resolved photoluminescence and electroluminescence prove that both PR and scattering contribute to the external radiation of PSCs.
Moreover, by resolving the previous optical issues and establishing a new optical model, we quantify various optical parameters in PSCs. At open-circuit condition, 80% of out-coupled photons are shown to have undergone at least one PR or scattering event. Consequently, PR and scattering increase the ELQE of our PSC by a factor of 4.5 (3.1% à 13.7%), reducing the non-radiative voltage loss (V_nr) from 89 mV to 50 mV, and increasing PCE from 23.3% to 24.0%. We also discuss future strategies for further improvement in current state-of-the-art PSCs.

Despite the demonstrated high efficiency and low-cost prospect for perovskite solar cells, great concerns of lead toxicity and instability remain for the commercialization of this technology. Here, we report an encapsulation strategy for perovskite modules based on lead-adsorbing ionogel, which effectively prevents lead leakage even if the modules are completely damaged and help to withstand long-term stability tests. The ionogel can absorb lead up to 30% of its mass because of the lead absorbing polymer backbone and phosphate ionic liquid. The elastic ionogel layers are integrated on both metal electrodes and on top of front glass window of perovskite modules, which significantly enhances their impact resistance in simulated hail tests, and can hold the shattered broken glass. The self-healable ionogel can physically prevent the permeation of water into the perovskite layer by blocking the cracks of broken glass, and absorbs lead that might otherwise leak. With the thermal and mechanical compatibility of ionogel and commercial encapsulant, the encapsulated solar cells pass the Damp Heat and Thermal Cycling accelerated stability tests according to IEC 61215 standard. The encapsulated perovskite mini-modules with ionogel show an aperture efficiency of 18.5%. The ionogel encapsulation also reduces lead leakage to undetectable level (<1 part per billion) after the hail-damaged module is soaked in water for 24 hours. Even under extreme scenario of being rolled over by a car followed by water-soaking for 45 days, the ionogel encapsulation reduces lead leakage by three orders of magnitude. This work provides a new strategy to address lead leakage and multiple-facet device stability simultaneously for perovskite modules.

Creating functionality in electronic devices is often achieved by modulating carrier concentrations with doping. In photovoltaics, optimization of doping concentrations can be an important tool to reduce recombination losses e.g. in crystalline Si solar cells. In emerging photovoltaic technologies such as the halide perovskites, comparably little work is done so far on intentionally controlling carrier densities in order to maximize efficiency. Some recent publications have discussed the effect of doping on solar cell efficiency both experimentally and theoretically using similar arguments and concepts as in classical Si photovoltaics. A fairly unconventional concept was recently presented by Feldmann et. al , who proposed that photo−doping by small bandgap variations leads to increased external photoluminescence (PL) quantum efficiency Q_e^lum which might then lead to the increase in open-circuit voltage and potentially in photovoltaic efficiency. While it is an interesting thought that stoichiometric and energetic disorder could be beneficial for efficiency, the concept is in complete opposition to earlier findings in other photovoltaic communities working on multinary compounds (e.g. Cu(In,Ga)Se₂) where extensive theoretical work on the negative effect of band gap inhomogeneities was developed about 15 years ago.Hence, one motivation for the current study is the reconciliation of these opposing findings that have important consequences on how to optimize devices and how to explain performance gains or losses.
Here, we provide a theoretical basis for the findings of Feldmann et al. and explain the phenomenon of photodoping as well as its relation to conventional doping. We also aim to develop models to perform a critical assessment of the role of doping on PL quantum efficiency and photovoltaic device performance. To tackle the different questions, we use two approaches. First, we develop a simple analytical model to study the effect of small lateral band gap variations and conclude that it cannot possibly be the source of photodoping and thus the substantially increased Q_e^lum observed by Feldmann et.al. We also find that such lateral fluctuations in the bandgap of the material can have adverse effects on the device performance. We then explore the relation between doping and photodoping and describe the consequences of these two forms of modulating carrier densities on luminescence, charge transport and photovoltaic efficiency using numerical device modelling. Furthermore, we highlight how the effects of doping on device performance depend on the choice of asymmetric defect capture coefficients as well as mobility of the charge transport layers.

The all-inorganic halide perovskite compounds are currently attracting interest for photovoltaic, radiation detection, and thermoelectric applications. They exhibit long lifetimes of photoexcited carriers beneficial for photovoltaics, and ultralow thermal conductivity of interest for thermoelectrics. Among the simplest members of this family, CsPbBr3 undergoes a typical sequence of phase transitions upon cooling, from cubic to tetragonal at ~400K and from tetragonal to orthorhombic at ~360K, associated with zone-boundary lattice instabilities. We have conducted inelastic neutron scattering (INS) of phonons on single-crystals of CsPbBr3, to probe anharmonic effects around lattice instabilities, and the origin of the ultralow thermal conductivity. Heavily overdamped Br-dominated acoustic modes were found with INS along the edges of the Brillouin zone in the tetragonal and cubic phases, forming a continuum of quasielastic scattering, which we associate to the strongly anharmonic potential energy surface against octahedral tilts. Further, we performed first-principles simulations of the anharmonic phonon self-energy that reproduce the observed overdamping. These large atomic fluctuations exhibit two-dimensional correlations of Br octahedra rotations in real-space and impact optoelectronic properties by modulating the Pb-Br derived electronic states at the edges of the bandgap. In addition, we show how other compositions of metal halide perovskites exhibit similar diffuse rods, including the double perovskite Cs2AgBiBr6 and CsSnBr3 or CsPbCl3, which indicates that anharmonic halide octahedral tilts are a common feature of the structure and its dynamics for this family of materials.
[1] T. Lanigan-Atkins*, X. He*, M. J. Krogstad, D. M. Pajerowski, D. L. Abernathy, Guangyong N. M. N. Xu, Zhijun Xu, D.-Y. Chung, M. G. Kanatzidis, S. Rosenkranz, R. Osborn and O. Delaire, "Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3", Nature Materials (2021).
This work was funded by the U.S. DOE, Office of Science, Basic Energy Sciences, MSE Division.

Adjusting A-site cation composition in metal halide perovskites (MHPs) is an important approach to modulate the static lattice strain and hence enhance the efficiency and stability of MHPs solar cells. However, an understanding of how A-site cation affects the lattice dynamic during the operation of MHPs devices is missing. In this work, we investigate the role of A-site cation in the lattice strain of MHPs upon light illumination and heating—two inevitable factors during the operation of photovoltaic devices. We reveal that A-site cation plays an important role in light-induced strain in MHPs. In particular, formamidinium (FA) cation is different compared to the other cations studied in this work, where FA-based MHP does not show light-induced strain but the other cations based MHPs show a light-induced strain. We also discovered that the MA distribution is strongly coupled with the strain distribution, and MA heterogeneity cab lead to a local light-induced strain heterogeneity under illumination. We further performed density functional theory (DFT) calculations to look at the photoexcitation induced lattice change in the atomic scale and explore the underlying mechanism. We found that hydrogen bonding interactions and ionization energy of A-site cation are potential reasons responsible for the light-induced lattice strain in MHPs. These findings offer an approach to modulate the light-induced strain in MHPs, which is critical for further improving the efficiency and stability of MHP photovoltaics.

Acknowledgments: This work is conducted at the Center for Nanophase Materials Sciences, a US Department of Energy Office of Science User Facility. We also acknowledge support from StART, a UTK-ORNL science alliance program.

Organic-inorganic halide perovskite solar cells (PSCs) have been considered a viable
member of next generation photovoltaics, which can address the scalability changes with a low-cost solution process. The energy lose in the hybrid system has been approved to be principally caused by defects/traps at grain boundaries and surfaces as well as point defects such as interstitial defects or vacancies in the perovskite compounds crystal lattice. It has been approved that halide ions migrating into charge transporting layers will be detrimental to their transport properties, especially under solar cell working conditions. Inspired by the quasi-2D-perovskite which shows a great device maintenance in photovoltaic performance, here, the hexagonal boron nitride (h-BN) was incorporated into the perovskite absorber as a cationic diffusion barrier. This low-cost, lower dimensional material can work as a growth template which leads to a a larger grain size. On the other hand, the integration of h-BN enables a better device stability as the result from the efficient suppression on the ion-migration induced degradation. The H-BN/ perovskites has an initial PCE of 19.7% that shows no degradation exhibit in over 72 h, whereas the PCE of pristine MAPI devices diminishes by more than 30% within the first 24 h.

Lead halide perovskite semiconductors MAPbX₃ (MA = CH₃NH₃, X = I, Br, and Cl) are attracting much attention as a new class of materials for optoelectrical device applications because of their outstanding optical properties. Recently, unique nonlinear optical responses such as large optical modulation [1], efficient higher-order harmonic generation [2–4], and strong spin-orbit coupling inducing Autler–Townes effect [5] were found in MAPbX₃ perovskites. The precise determination of nonlinear optical coefficients is important to understand their fundamental material properties and the origin of their large nonlinear optical responses.
Because of strong light scattering in their grain structures and surfaces, it is difficult to determine intrinsic nonlinear optical coefficients for polycrystalline and nanocrystals. To exclude these influences, we fabricated high-quality single crystal MAPbX₃ (X = Cl and Br) samples and clarified their third-order nonlinear optical coefficients for the wide wavelength range. The obtained spectra near the band gap can be quantitatively explained by a two-band model including the exciton effect and 4×4 Kane model. Based on this model, we successfully determined the Kane energy (23 eV) and exciton reduced mass (0.20 m₀), and found that the nonlinear optical coefficient is enhanced about twice by the exciton effect in MAPbCl₃ single crystal [6]. In addition, we studied the two-photon absorption spectra of MAPbBr₃ single crystal with circularly and linearly polarized excitation light [7]. We observed a dip structure in the linear-circular dichroism spectrum of two-photon absorption, which corresponds to the spin-orbit splitting energy according to the 8×8 Kane model. From the experimental results and a theoretical analysis of the two-photon absorption spectra, we successfully determined the spin-orbit splitting energy (0.8 eV), exciton binding energy (28 meV), and reduced exciton mass (0.089 m₀), which characterize the electronic structure of MAPbBr₃. Our results provide a direct experimental reference on the electronic structure of halide perovskites.
Part of this work was supported by JSPS KAKENHI (JP19H05465 and JP20K03798) and CREST, JST (JPMJCR16N3).
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[3] Y. Sanari et al., Phys. Rev. B 102, 041125(R) (2020).
[4] K. Nakagawa et al., Phys. Rev. Materials 5, 016001 (2021).
[5] G. Yumoto et al., Nat. Commun. 12, 3026 (2021).
[6] K. Ohara et al., Phys. Rev. Materials 3, 111601(R) (2019).
[7] K. Ohara et al., Phys. Rev. B 103, L041201 (2021).

Perovskite solar cells (PSCs) have become one of the most promising types of next-generation solar cell ever since 2009 due to their excellent photovoltaic performance. Despite of decent photo conversion efficiency (PCE), PSCs with a Spiro-OMeTAD hole transport layer (HTL) have been shown to have poor device stability and noticeable current-voltage (J-V) hysteresis PV performance.
In our study, we used PTAA as a substitute for Spiro-OMeTAD and compared the PV parameters along with the device stability of both PSCs. To compare the parameters between doped PTAA and Spiro-OMeTAD PSCs, we used the regular n-i-p device with the structure of FTO/c-TiO2/m-TiO2/MAPbI3/HTL/Au. Three different HTLs were prepared: pristine PTAA, Li-TFSI doped PTAA and Spiro-OMeTAD. From the cross-section SEM results, we found the Spiro-OMeTAD layer had a thickness of around 180 nm, while the thickness of the PTAA layer with and without Li-TFSI were all around 50 nm. The data for the current density-voltage (J-V) curves of the PSCs were collected using a PV test station. The MAPbI3 (MAPI) PSC with Spiro-OMeTAD showed typical PCE of 17.3% in reverse scan (RS) and 14.2% in forward scan (FS), while the MAPI PSC using doped PTAA as the HTL had typical PCE values of 16.6% in RS and 14.7% in FS. Although the Spiro-OMeTAD PSC showed a slightly higher PCE in RS, the sample showed a greater hysteresis in comparison to the PTAA one. Moreover, the continuous maximum power point (MPP) test in an ambient environment (humidity ~ 60%) with a Xenon solar lamp on non-encapsulated PSCs showed that the PSC using PTAA HTL presented a slightly more stable current output compared to the Spiro-OMeTAD counterpart. Using contact angle measurements, we found the typical water contact angles of 69.8° and 88.2° for Spiro-OMeTAD and PTAA layers, respectively, indicating that the PTAA was more hydrophobic than the Spiro-OMeTAD. We suggest that the higher hydrophobicity of the PTAA could contribute to better PSC stability when exposed to moisture with moderate high temperatures.

Currently, the energy conversion efficiency(PCE) of perovskite solar cells (PSCs) has reached 25.5% and there is a great deal of interest in the practical application of perovskite solar cells (PSCs). Almost all reported PSCs use polycrystalline films as photoactive layers. However, since it contains many grain boundaries, it has been pointed out about the possibility of deterioration through the grain boundaries. Recently, solar cells using a perovskite single crystal thin film as a photoactive layer have been reported to improve durability and solar cell characteristics because they have a longer carrier diffusion length, a lower trap density, a higher light absorption coefficient, and a lower ion diffusion length. However, the world record for PCE has been achieved with polycrystalline thin films. It is important to develop manufacturing technology for precisely structurally controlled perovskite single crystal thin films. In this study, several kinds of lead halide perovskite single crystal thin films were prepared under various conditions by a space-limited reverse temperature crystallization method, and their structures and optoelectric properties were evaluated. .. We will report on the detailed experimental results.

Spectral conversion aims to tailor the incident solar spectrum such that it is better suited for particular photovoltaic absorber material; here an organometal halide perovskite semiconductor. In our perovskite solar cells with double cation perovskite absorber (bandgap 1.57eV), 41% of the incident spectrum is wasted by sub-bandgap losses. With the target to reduce these losses in the future, we research up-conversion (UC) crystal BaF₂:Yb³⁺, Er³⁺ for the annihilation of two low-energy photons to form a high-energy photon. When tested under terrestrial sunlight representing one sun above the perovskite’s bandgap and sub-bandgap illumination at 980 nm (via a laser), the BaF₂ crystal emits usable upconverted photons in the spectral range of 520 to 700 nm. In our bifacial PSC with the UC crystal beneath, these upconverted photons contribute to 0.38 mA/cm² enhancement in short-circuit current density at a laser intensity equivalent to 120 suns. We demonstrate that UC scales non-linearly with incident intensity, with an 880 suns equivalent intensity resulting in 2.09 mA/cm² enhancement in current. Our study demonstrates that UC is a potentially viable process to extend the response of PSC to a wider spectral range.

As greenhouse gas emissions rise and induce the complications associated with climate change, solar cells, devices which convert sunlight into electricity through the photovoltaic effect, are regarded as a promising solution to the issue. The use of the organic and inorganic hybrid perovskite (OIHPs) as the photoactive semiconductors for new generation of solar cells has been widely investigated due to panchromatic light absorption, high charge mobility, and low exciton binding energy, etc, which contributes to high performance perovskite solar cells (PSCs) with high photo conversion efficiency (PCE) surpassing other types of thin film solar cells and comparable to III-IV and II-IV types of solar cell devices. Among different compositions of OIHP, methylammonium lead iodide (MAPbI₃ or MAPI) is one of the most widely studied OIHP due to its overall simple processing and good moisture endurance. On the other hand, the formamidinium lead iodide perovskite (FAPbI₃ or FAPI) becomes more popular in the community due to its optimal bandgap, broader light absorption, as well as better thermal stability, etc, as compared with the MAPI perovskite. However, preparation of a high quality FAPI perovskite films with minimized surface and structural defects are more difficult than MA based perovskite, due to less favorable lattice parameters after replacing with FA cations. Previous studies have indicated that addition of methylammonium chloride (MACl) into the FAPbI₃ system can help improve the crystallinity and morphology of FAPbI₃ films. Thus, the objective of this study was to study the effects of various concentration of MACl (15, 30, and 45 mol%) addition for the fabrication of high quality FAPbI₃ films
Specifically, the FAPI with 30% MACl yielded optimal results with large and compact grains, while FAPI doped with 15% MACl and 45% MACl yielded imperfections such as rough surface with pinholes. Further analysis revealed that optimal FAPI-MACl (30%) formed grains with average size around 820 nm, more than double that of the grains on MAPI perovskite, which has an average value around 370 nm. In addition, the analysis of photovoltaic performance was also conducted. The typical reverse and forward J-V scans of the PSC based on pristine-FAPI revealed PCEs of 12.3% and 8.1% respectively. In contrast, the typical reverse and forward scans of the FAPI with 30% MACl revealed higher PCE of 16.5% and 16.4% respectively. The improved PCE with negligible hysteresis of the FA-based PSC indicates the improvement in crystallinity and quality of the perovskite films with the optimal MACl concentration. The XRD analysis further indicated that: 1) pure FAPI exhibited a dominant photo-inactive δ-phase, and 2) the addition of MACl, particularly at concentrations larger than 30%, promoted the photo-active α-phase and reduced the photo-inactive δ-phase. Lastly, UV-vis absorption spectra revealed that FAPI perovskites with or without MACl all have broader light absorption, with the onset wavelength of around 780 nm, as compared with 750 nm of MAPI perovskites. In conclusion, our results confirm that the addition of MACl as the processing additive, particularly at a concentration of 30%, produced good quality FAPbI₃ films, leading to high performance PSCs with decent PCE and low hysteresis.

Next-generation solar energy harvesting technologies rely on the discovery, refinement, and implementation of new organic photovoltaic materials with high quantum efficiency, excellent stability, and reliability. In recent years, organic bulk heterojunction (BHJ) solar cells based on non-fullerene acceptors (NFAs) have emerged as potential candidates due to rapidly improving power conversion efficiencies, now exceeding 18% in single-junction cells. Improved performance of recent NFAs is attributed to wider solar spectral absorption, efficient exciton dissociation, and strong π-π interactions, leading to excellent charge transport and collection. Despite these innovations in synthetic chemistry, there remain significant losses in all steps of the photoconversion process from exciton dissociation within the BHJ to charge collection at the contacts. Especially, bimolecular recombination losses are known to dominate in BHJ cells due to non-ideal band energetics, disordered morphology of donor and acceptor domains, and poor interfaces and contacts. Electric charge- and photocurrent-based spectroscopies using impedance spectroscopy provide a direct probe to reveal these relatively slow processes not accessible to conventional ultrafast spectroscopies. Furthermore, the non-invasive nature of this measurement can quantify performance-limiting dynamical processes in fabricated devices operating at the maximum power point. In this presentation, an impedance and photocurrent spectroscopy method will be discussed to quantify bimolecular recombination rates in NFA-based organic solar cells.^1-5 Two emerging NFA-polymer BHJ systems, indacenodithienothiophene-, and Y6-based NFAs, will be used to demonstrate the effectiveness of the methodology to assess differences in cells resulting from different molecule structure and processing conditions such as thermal annealing. Extracted variation in device parameters is correlated with improved morphology and packing motifs. Overall, this in-situ analysis allows a more realistic device model that can distinguish the competing effects on functionalization in NFA molecules such as π-extension and fluorination. In this manner, the new understanding gained in these systems provides a rational strategy for further optimization of high-performance and stable organic solar cells.
References:
¹ Journal of the American Chemical Society 142, 14532 (2020).
² Adv. Eng. Mater. 10, 2000635 (2020).
³ ACS Eng. Lett. 5, 1780 (2020).
⁴ ACS Appl. Mater. Inter. 11, 14166 (2019).
⁵ Adv. Funct. Mater. 27, 1703805 (2017).

Halide perovskites are a very promising class of material for optoelectronics. Employing inexpensive and scalable synthesis methods, earth-abundant semiconductors that exhibit desirable optical properties, including unity quantum yield, can be achieved. The composition of both the cation and anion (halide) sublattices of perovskite crystals can be manipulated over a wide range to engineer their light absorption and emission behavior. Halide ion phase separation is a well-known barrier for the application of mixed-halide perovskites in solar cells and LEDs, whereby the presence of large populations of photo-generated or electrically-injected carriers causes undesirable changes of the local band gap by demixing of initially homogeneous mixed halide ion compositions. Previous reports have established that illumination with above the band gap energy causes the phase separation of mixed iodide-bromide perovskites into iodide- and bromide-rich domains and is reversible in the dark, where evidence of structural changes in mixed-halide alloys has been observed by X-ray diffraction. The phase separation mechanism appears to involve a self-reinforcing demixing of Br- and I- ions driven by reducing the potential energies of photogenerated carriers that trap in regions of smaller bandgap. Subwavelength imaging has been performed to visualize the process in single crystalline and poly-crystalline organic perovskites. However, many aspects of the phase separation are not well understood, such as factors that influence the length scale, kinetics of the phase separation, and whether the mechanism involves spinodal decomposition or nucleation and growth.
We investigate the mechanism of phase separation in mixed-halide CsPbI_xBr_3-x perovskite crystals driven by photocarrier generation and spatial redistribution during absorption of band gap light. We visualize the phase separation process and its dynamics at the nanoscale using Cryo-TEM-CL, where formation of iodide-rich regions is detected as locally red-shifted emission. An extracted length scale for the phase separation is ~ 200 to 500 nm depending upon the degree of phase separation. Using photoluminescence spectroscopy, ellipsometry, and x-ray diffraction, we demonstrate that the temporal dynamics of this phase separation can be modulated by controlling both 1) the intensity of illumination, which, when increased, promotes faster phase separation; and 2) the temperature, which, when increased has the opposite effect. Correlating the optical property modulation with structural changes via in-situ experiments reveals that both the amplitude and the length scale of this fully-reversible phase separation increase as a function of time. Combined with phase field modeling that accounts for the coupling between the electronic and chemical effects, this points to a diffusional transformation mechanism, similar to spinodal decomposition, combined with coarsening of the iodide-rich domains at longer time-scales.

Developing compact, efficient, and sustainable power sources are of the urgent demand for the next generation of self-powered micro/nano devices for IoT (Internet of Things). One of the most prospective routes is to harness environmental energy by piezoelectric nanogenerator (PNG), as it is compact, flexible and has high electricity generation capabilities. In this aspect, by dispersing highly-piezoelectric nanoparticles (NPs) in a flexible polymer, creating a composite film is an attractive, yet easier approach in terms of PNG fabrication scalability, and device performance; as it can be synergistically benefitted from the incorporated highly piezoelectric NPs & flexible host polymer matrix. The quest for that, we rationally designed a novel perovskite/ PVDF composite piezoelectric film, which simultaneously benefits from a self-assembled highly ordered porous structure of PVDF, and new hybrid halide perovskite-formamidinium lead bromine iodine (FAPbBr2I). Enhanced mechanical strain by the self-assembled highly porous PVDF, and reduced film conductivity by the FAPbBr2I (higher relative permittivity~13) , generates a strain-induced piezo potential of 85 V (peak-to-peak), and current of up to 30 µA (peak-to-peak) (138 g (gram) of applied load). Benefiting from the above, a high output peak power density of ~ 18 mW/cm³ (while~ 3 mW/cm³ at 7 MΩ load resistance) was obtained for the self-powered integrated wireless electronics node (SIWEN) and then extended it to the real application by realizing wireless structural health monitoring.
In order to synthesize the hybrid perovskite-polymer composite piezoelectric film, low temperature and fully solution-based growth processes were adopted. The FAPbBr2I precursor solution was prepared via dissolving FAI (formamidinium iodide ≥ 99%, Sigma-Aldrich) and PbBr2 (lead (II) bromide ≥ 98% , Sigma-Aldrich) of equal molar ratio (0.5:0.5) in N, N DMF (N, N-dimethylformamide ≥ 99%, Sigma-Aldrich) solvent, followed by stirring at 60 °C for 12 hours. The PVDF solution was dissolved in the N, N DMF with constant stirring at 50 °C for 24 h. The final concentrations of FAPbBr2I and PVDF in the N, N DMF were kept at 20 wt. % and 10 wt. %, respectively. Intentionally, the concentration of PVDF in the final solution was diluted by perovskite precursor and the thickness of preformed films was controlled to less than 50µm. The aim is to decrease the inner vapor pressure during the film growth and to form the highly ordered porous structure. The optimized 20 wt. % FAPbBr₂I –PVDF solution was drop-casted on a glass substrate and stored about 1 hour for the degassing process (ambient condition). In order to obtain the composite films, an annealing process at 1200C for 2-3 hours was performed (self-assembled highly ordered porosity was confirmed by SEM-EDS mapping, and the crystallinity by the X-ray diffraction peaks). After peeling off, to align the dipoles, a high voltage electrical poling of the film (thickness ~30 µm) was performed with an electric field of (50-120 V/ µm) for 2-3 hours (β- phase was confirmed by FTIR spectrum analysis). Finally, the film was sandwiched between two copper/polyester substrates by using a thermal pressing technique for the integration of PNG.
The generated output voltage and current of perovskite-polymer PNG (P-PNG) were measured at 30 Hz frequency by applying a periodic mechanical vibration from an electrodynamic shaker (ET-126-I from Labworks Inc.) and placing a 138 g (gram) metal block (stainless steel) on top of the device. The high piezo potential 85V (peak-to-peak), and current of 30 µA (peak-to-peak), was recorded by a commercial oscilloscope. Later, to assess a real-time application, we designed a self-powered integrated wireless electronics node (SIWEN) enabled by the PNG, that can communicate with personal electronics

Halide perovskites (HAPs) have attracted immense interest in the optoelectronic community over the past decade. While the power conversion efficiency of perovskite solar cells has rapidly increased during this time, fundamental material properties, such as the true arrangement of the valence bands and dispersion relation of halide perovskites or defects at/close to their surfaces are still under debate.
Here, we report on photoelectron spectroscopy investigations on a multi cation multi halide perovskite composition, which has been used for recently reported highest silicon/perovskite tandem solar cell efficiencies.^[1]
Conventionally, He-I UPS is applied to study the valence band maximum, required for understanding the electronic formation of semiconductor interfaces. He UPS suffers from a rather high noise floor and the need for correction of He-satellite lines. As we show in our study, the characteristically low density of states close to the valence band maximum^[2,3] can only be revealed by very high sensitivity PES techniques. To this end, we apply a “classical” but rarely used technique: near-UV photoemission spectroscopy in constant final mode (CFSYS) and can directly trace the density of states over a very high dynamic range of seven orders of magnitude. This is four orders of magnitude better compared to conventional He-I UPS. Additionally, with the low photon energy, the information depth is strongly enhanced to up to 10 nm and thus, the gained information is significantly less distorted by changes of material composition in the first monolayers or surface contaminations, as compared to He-I UPS.
Using a newly developed model^[4] to describe the valence band edge and exponential band tail, we can distinguish between two valence band maxima, at two different positions in k-space, in one angle-integrated spectrum. Furthermore, it becomes apparent, that the widely applied method to determine the valence band maximum (by an extrapolation of the leading valence band edge on a semi-logarithmic scale) strongly depends on the noise level and can thus not provide reliable results. Importantly, we also show that there is a fundamental caveat to such extrapolation approaches: The highest valence band maximum at the R-point of the Brillouin zone overlaps with a distinct defect band and can thus only be located by combining the analysis of experimental data with DFT calculations.
The low detection limit of CFSYS further reveals occupied gap states, i.e. defect states that will influence recombination and thereby device performance. By rescaling our data to published DFT calculations^[5], we can quantify these depending on the damage by illumination (UV, X-Rays) or storing condition (vacuum, short air exposure). To our knowledge, this is the first time that such defects are not only observed, but also quantified from photoelectron spectroscopy data. These unprecedented results show the decomposition of HAP films and creation of defect states at a sensitivity level, which can hardly be reached by any other PES technique.
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[5] Y. Zhou et al., J. Phys. Chem. C 2017, 121, 1455.

Lead halide perovskites have recently attracted attention as high-quality semiconductors with excellent optical properties, promising for various device applications. In contrast, the reported carrier mobilities in lead halide perovskites are relatively low compared to other high-quality inorganic semiconductors. The low mobilities have been discussed in terms of polarons, which enhance the carrier mass through electron-phonon coupling and reduce the carrier mobility. However, the impact of polarons on carrier transport of lead halide perovskite still remains unclear. To reveal the physical origin of the low carrier mobility, we need to perform the reliable Hall measurement in high-quality single crystals and evaluate their intrinsic mobility. Nevertheless, lead halide perovskite materials have very high resistivities; therefore, it is difficult to measure the Hall mobility of carriers by the conventional method.
In this study, we performed an AC Hall measurement of a CH₃NH₃PbBr₃ bulk crystal at room temperature under light irradiation [1]. We illuminated the sample by a green laser (λ=555 nm) to generate photocarriers uniformly. We can estimate the mobilities of holes and electrons independently from conductivities and Hall coefficients under different photoexcitation intensities based on two-carrier analysis [1]. The conductivities were measured by van der Pauw method. We evaluated the resistivities from the slope of I-V profiles to eliminate the photovoltaic effect. To obtain the Hall voltage in a low-current regime, we performed the Hall measurement under ac-magnetic field (0.4 T / 0.1 Hz) by a lock-in technique. We prepared undoped single crystals of CH₃NH₃PbBr₃ by the anti-solvent crystallization method [2]. The crystals were thinned to 50 µm in order to increase the current density and generate the photocarrier more uniformly. We fabricated Au/rubrene electrodes at the four corners of the sample. The intermediate layer of rubrene between CH₃NH₃PbBr₃ and Au reduces the contact resistance.
The observed carrier mobilities were enhanced drastically with increasing photocarrier density and show the square-root dependence on photocarrier density [3]. We consider that the dependence of photocarrier density is attributable to the screening of the charged dislocations by photocarriers [4]. We obtained the hole and electron mobilities of 322 cm²/Vs and 318 cm²/Vs at the maximum photocarrier density, and these mobilities would further increase with carrier density. The mobilities obtained from this research are much higher than those reported in previous studies. The result implies that polarons do not have a large impact on the carrier mobilities, which is consistent with our recent magnetoreflectance study [5]. Rather, we consider that the scattering by defects is more important in room-temperature carrier transport in lead halide perovskite. Therefore, it is significant to reduce defects, which could help deliver the full potential of this material.
Part of this work was supported by JST-CREST (JPMJCR16N3), JSPS KAKENHI (JP19K03683), Canon foundation, and Chiba Iodine Resource Innovation Center.

[1] O. Gunawan, et al., Nature 575, 151-155 (2019).
[2] D. Shi, et al., Sience 347, 519-522 (2015).
[3] T. Kimura, et al., Appl. Phys. Express 14, 041009 (2021).
[4] H. M. Ng, et al., Appl. Phys. Lett. 73, 821 (1998).
[5] Y. Yamada, et al., Phys. Rev. Lett. 126, 237041 (2021).

To maximize the output power of PNGs, the piezoelectric charge (d₃₃) and voltage (g₃₃) coefficients of the piezoelectric materials are both important, which will determine the output current and voltage respectively. However, in the past decades, it is difficult to combine them in one material for piezoelectric energy harvesting, such as piezoelectric nanogenerators (PNGs). In this work, we firstly report a combined large d₃₃ of 137 pCN^-1 and giant g₃₃ of 980 ×10^-3 VmN^-1 in a novel vacancy-ordered double perovskite of TMCM₂SnCl₆ (TMCM is trimethyl chloromethyl ammonium). The piezoelectric origin is demonstrated as the halogen-bonding mediated synergistic movements of atom displacement in inorganic [SnCl₆]⁻² octahedrons and dipole reorientation of organic molecular TMCM via a combined density functional theory (DFT) and experimental study. The piezoelectric composite films are grown via dispersing TMCM₂SnCl₆ into the polydimethylsiloxane (TMCM₂SnCl₆@PDMS). The final PNGs exhibit the highest outstanding outputs with an open-circuit voltage of 149 V (peak-to-peak), short-circuit current density of 1.54 μAcm^-2 (peak-to-peak), and total power of 26 μW under periodically vertical compression (4.8 N). Moreover, the PNGs are used as the power source of self-powered wire-less electrics (SPWLE) and enable communication successful. Our studies not only innovatively open a door for a new green piezoelectric perovskite design but also extend its further application in energy harvesting and its SPWLE.

Among the emerging photovoltaic technologies, hybrid perovskite solar cells (PSCs) provide the highest energy conversion efficiencies years (Phaometvarithorn, Chuangchote, Kumnorkaew, & Wootthikanokkhan, 2018; Tang & Yan, 2021) however, it is necessary to promote the transition from laboratory-scale to large-area manufacturing ensuring high throughput and comprising the analysis of perovskite film formation and the scalable deposition techniques, aiming to improve the perovskite thin-film stability and homogeneity (Burkitt et al., 2020; Burkitt, Searle, A.Worsley, & Watson, 2018; Patidar, Burkitt, Hooper, Richards, & Watson, 2020).

Slot-Die Coating technique has been extensively applied in industry, where a great effort is directed to stabilize the coating process (Miller, 2012; O. J. Romero, Suszynski, Scriven, & Carvalho, 2004; Oldrich Joel Romero, Scriven, & Carvalho, 2006; X. Ding, J. Liu, 2016). In this direction, a prior condition is achieving a uniform volume distribution along the slot-die head and related to the crystallization process for uniform film formation, which to the best of our knowledge this has not been studied for perovskite inks. In this paper we propose to explore the nature of the flow patterns inside the slot-die head and the influence of this study in the fabrication of hybrid perovskites films by slot-die processing.

We included an analysis of the volume uniformity inside a lab-scale slot-die head, as a function of the head angle and inlet flow, designed for the fabrication of perovskite solar cells, is accomplished. We identify issues such as poor filling of the shim apertures and a non-fully developed laminar flow inside the slot-die head for the common lab processing conditions, as the main causes of non-uniform volumetric flow and the result is nonhomogeneous films and crystallization defects of the perovskite. These results contribute to pave the way towards highly reproducible and scalable perovskite solar devices by slot-die coating.

Organic-inorganic hybrid halide perovskite solar cells (PSCs) have been extensively researched and achieved impressive power conversion efficiency (PCE) of 25.5% at laboratory scale. To date, several metal oxides, such as ZnO, TiO₂, and SnO₂ are used as electron transport layer (ETL) for the fabrication of efficient PSCs. Particularly, ZnO showed favorable energy level, smooth surface coverage and most importantly low-cost deposition capabilities are highly desired for efficient photovoltaic performances and their future commercialization and market adoption. However, the application of ZnO ETL to PSCs remains limited by drawbacks such as low conductivity and large amounts defects, which inevitably lead to carrier recombination. Notable efforts have been made by using doping, nanocomposites or interface engineering strategies to resolve these drawbacks to enhance the performances of PSCs. Tungsten (W) is one of the candidates for doping ZnO due to the similar ionic radius of Zn²⁺ (0.074 nm) and W⁶⁺ (0.064 nm). In this study, we introduced W-doped ZnO synthesized using Aloe Vera leaf extract as the ETL and investigated their influences in PSCs performance. Our results demonstrated a smooth and dense surface morphology of W-doped ZnO deposited on ITO-patterned substrate. Higher crystallinity was observed for Cs_0.1FA_0.9PbI₃ perovskite layer formed on W-doped ZnO because the as-prepared W-doped ZnO material started with better crystallinity. Ultimately, PSCs with W-doped ZnO ETL exhibited PCEs as high as 16.2%, which is superior to those of PSCs with pristine ZnO as the ETL (12.9%). The integrated J_sc values of the IPCE spectra are 20.4 and 22.5 mAcm⁻² for the pristine ZnO and W-doped ZnO based PSCs, respectively. These values are coherent with the J_sc values extracted from the J–V curves. Hence, the present study provides a significant advance to the design of photovoltaic devices with respect to charge transport and electron-hole recombination.

Despite over a decade of research on metal halide perovskites (MHPs) in the context of photovoltaic applications, understanding the nature of electronic and ionic processes associated with current-voltage (I-V) hysteretic behavior has been limited. In this study, we explore the hysteretic behavior in (FAPbI₃)_0.85(MAPbBr₃)_0.15 perovskite devices with lateral Cr and Au electrodes by applying first order reversal curve (FORC) bias waveform in I-V, Kelvin probe force microscopy (KPFM) measurements, and in-situ chemical imaging by time-resolved time-of-flight secondary ion mass spectrometry (tr-ToF-SIMS).
Previous studies suggested that I-V hysteresis in MHP devices is associated with various interfacial phenomena such as space charge layers, resulting from ion migration^1,2, interfacial charge accumulation and recombination³, and charge trapping and de-trapping at the interface⁴. Particularly, ion migration has a critical role in the hysteresis and transient phenomena in MHPs devices.^4,5 Many intriguing properties in MHPs are highly associated with the interplay between electronic charge carriers and multiple mobile ion species. The ion migration is in fact one of the most acute issues that perovskites community faces, due to the key role it plays on device stability and performance. Although, double cations and double halides perovskites have been attracted much attention due to their high efficiencies and remarkable long-term stability in photovoltaics. More complex interplay between electronic and ionic charge carries is expected in the double cations and double halides perovskites due to the presence of multiple mobile ions.
Here, we strategically utilize the combination of time- and voltage-dependent I-V and KPFM, with in-situ chemical imaging by tr-ToF-SIMS measurements to identify spatial localization and nature of the involved species. To study the time dynamics, we use a bias waveform referred as FORC that allows separating bias-induced chemical effects in dark and under illumination. The FORC waveform is also applied to the KPFM measurements to clarify spatial- and time-resolved contact potential difference (CPD) in the devices. The time- and bias-dependent ion distribution is explored using tr-ToF-SIMS to directly investigate the chemical changes in operando devices. This approach supported by machine learning analysis provides the correlation between ion dynamics and the hysteresis phenomenon in I-V and CPD-V curves. Some of the key findings in this study include: i) We discovered that Br^- ions accumulate at the interface between (FAPbI₃)_0.85(MAPbBr₃)_0.15 perovskites and drain electrode in both on- and off-fields. ii) The low mobility of MA⁺ ions result in transient behavior and contribute to the hysteretic phenomena in the I-V and KPFM results. iii) It was shown that Pb²⁺ ions can be reduced at the interfaces due to electrochemical reactions with the electrode in the presence of charge injection and photogenerated charge carriers. iv) The ion migration can be fully or partially reversible while the electrochemical reaction at the interface can result in non-reversible and permanent damage to the device performance and stability.
Overall, our results clearly show the important role of interfacial phenomena and highlight that appropriate interface engineering is critical for highly efficient and stable MHPs-based devices. Our study suggests that lower concentration of Br^- and MA⁺ ions can result in higher interfacial stability and less hysteretic behaviors in double cations and double halides perovskite devices.

This article is already submitted and currently under review.

References:
1 Mosconi, E. & De Angelis, F. ACS Energy Letters 1, 182 (2016).
2 Tress, W. The Journal of Physical Chemistry Letters 8, 3106-3114 (2017).
3 Rong, Y. et al. Energy & Environmental Science 10 (2017).
4 Snaith, H. J. et al. The Journal of Physical Chemistry Letters 5, 1511 (2014).
5 Tress, W. et al. Energy & Environmental Science 8, 995 (2015).

Solution-processed halide perovskite solar cells (PSCs) have attracted tremendous attention due to their skyrocketing power conversion efficiency (PCE) from 3.8% to 25.5% in recent years. The rapidly rising efficiency is mainly attributed to the improved perovskite film quality, which is usually achieved through precursor engineering, compositional regulation, and optimization of the film fabrication process. However, most developments of these methods are empirical, and few efforts have been devoted to the fundamental studies of the perovskite film formation mechanism. In this work, the underlying perovskite crystallization process from the initial precursor solution to the final polycrystalline film was investigated and unveiled with the aid of synchrotron-based in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. Specifically, the detailed crystallization pathways of mixed perovskites (MA_0.17FA_0.83Pb(I_0.83Br_0.17)₃) during the one-step spin-coating process were monitored via state-of-the-art in situ GIWAXS. Three crystallization stages were successfully identified: the precursor solution (stage I), hexagonal δ-phase (2H) (stage II), and complex phases (stage III) including the intermediate phase, hexagonal 4H, 6H polycrystals, and perovskite α-phase. In the light of in situ GIWAXS results, we revealed the existence of an “annealing window” for the first time. The as-cast film should be timely annealed within stage II (annealing window) before the formation of complex phases, in order to obtain a high-quality perovskite film. Intriguingly, the incorporating of Cs⁺ can significantly extend the annealing window by inhibiting the formation of complex phases, thereby improving the device performance and reproducibility. This work provides insightful guidelines for manipulating the perovskite crystallization pathways towards higher performance. Furthermore, in situ GIWAXS was employed to monitor the perovskite crystallization kinetics during the two-step spin-coating process, including a first step of depositing the PbI₂ layer and a second step of converting PbI₂ into perovskites with organic salts (e.g. MAI, FAI). It was found that incorporating Cs⁺ in the first step induced an alternative pathway from δ-CsPbI₃ to perovskite α-phase, which is energetically more favorable than the conventional pathways from PbI₂. However, the new phase transition caused an imbalance between the perovskite nucleation and crystal growth, resulting in pinholes in the film. Fortunately, introducing GA⁺ in the second step can promote crystal growth and eliminate pinholes, boosting efficiencies by over 23%. This work demonstrates an unprecedented advantage of the two-step process over the one-step process, allowing precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth processes.

Reference:
1) M. Qin, K. Tse, T.-K Lau, Y. Li, C.-J. Su, G. Yang, J. Chen, J. Zhu, U-S. Jeng, G. Li, H. Chen, and X. Lu*, Manipulating the Mixed-Perovskite Crystallization Pathway Unveiled by In Situ GIWAXS, Adv. Mater. 2019, 31, 1901284.
2) M. Qin, H. Xue, H. Zhang, H. Hu, K. Liu, Y. Li, Z. Qin, J. Ma, H. Zhu, K. Yan, G. Fang, G. Li, U-S. Jeng, G. Brocks, S. Tao*, and X. Lu*, Precise Control of Perovskite Crystallization Kinetics via Sequential A-site Doping, Adv. Mater., 2020, 32, 2004630.

Lead halide perovskite (HaP) materials of the form ABX₃ have attracted substantial global research interest for over a decade and commercial solar cell and light-emitting diode (LED) applications are starting to appear on the market [1]. The A-site cation (A-cation) can be an organic or inorganic cation (methylammonium, formamidinium, cesium), the B-site cation is lead in the most well studied HaPs, and the X-site cation can be iodide, bromide, chloride. In spite of the recent research interest and a history of basic research dating back to as early as the 1970's, there are fundamental questions to be answered. One concerns the optoelectronic function of the A-cation [2]. Solar cell and mechanical nano-indentation studies suggest the A-cation does not play an optoelectronic role, while time-resolved photoluminescence studies suggest it does.

Here we report our findings that are relevant for unraveling this scientific mystery, utilizing the first application of core level spectroscopy to single crystals of HaPs. In the first part of my talk, I will present a comparative study of three archetypal lead bromide perovskite (APbBr₃) compounds using Br K-edge high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) and simulated XAS spectra. We find a nearly linear correlation between relative Goldschmidt tolerance factor, a widely used structural descriptor that is affected by the size of the A-site cation, and the energetic width of the conduction band [3]. These findings are potentially relevant for understanding the mechanism of slow hot electron cooling in HaPs. Using Br K X-ray emission spectroscopy (XES), we find relative differences in lead-bromide bond ionicity. These results are potentially relevant for understanding the origin of A-cation-related trends in device-relevant energy levels, which are crucial for designing HaP device interfaces. In the second part of my talk, I will present a study which utilizes N K-edge Resonant Photoelectron Spectroscopy (RPES) to search for electronic coupling between the organic and inorganic sub-lattices of methylammonium lead tri-iodide (MAPbI₃), the prototypical hybrid organic-inorganic HaP. We find that the occupied states are weakly coupled, but find substantial coupling in the unoccupied states via the observation of normal Auger decay upon resonant excitation [4]. I will also talk briefly about findings from a complementary computational-oriented study [5]. Our studies challenge a notion based on previous studies that the A-cation is electronically inert.

References
[1] https://www.perovskite-info.com/companies-list/perovskite-application-developers
[2] Egger, D. A. et al. Adv. Mater. 2018, 30 (20), 1800691. https://doi.org/10.1002/adma.201800691.
[3] Man, G. J. et al. A-Site Cation Influence on the Conduction Band of Lead Bromide Perovskites. In Manuscript 2021.
[4] Man, G. J. et al. Electronic Coupling between the Organic and Inorganic Sub-Lattices of a Hybrid Organic-Inorganic Perovskite Single Crystal. arXiv 2020. http://arxiv.org/abs/2011.02016
[5] Sterling, C. M. et al. J. Phys. Chem. C 2021, 125 (15), 8360–8368. https://doi.org/10.1021/acs.jpcc.1c02017.

Codoping chloride double perovskites of general formula Cs₂Na_xAg_1-xInCl₆ with Bi³⁺ and Ln³⁺ (Ln = Yb,Er) ions is attracting increasing interest owing to the potential of this class of materials as broad-band phosphors across the visible and near infrared (NIR) spectrum [1-4]. Beside the application in phosphors, it is interesting to assess the suitability of these materials as NIR gain and laser media when lanthanide concentration is raised up towards the level of 100% at.
Here, we explore the optical performance of a number of Ln-based chloride double perovskites with special emphasis on Ln optical cross-sections, excited-state lifetimes and radiative efficiencies. We compare double perovskites’ optical gain figures of merit with those of a conventional Ln-doped crystal, that is, yttrium aluminium garnet (YAG).
All chloride double perovskites were synthesized as polycrystalline powders by solvent evaporation of acidic solutions of precursor salts.
Cs₂Na_0.8Ag_0.2InCl₆ doped with Yb³⁺ and Er³⁺ at ~1% at. level displayed unprecedentedly long excited-state lifetimes of ~3 ms (Yb) and ~21 ms (Er), from which radiative efficiencies for the emissions at ~1030 nm (Yb) and 1530 nm (Er) were estimated to be ~60% and ~80%, respectively. In Cs₂NaYb_0.9Bi_0.1InCl₆ and Cs₂NaYb_0.9Bi_0.1InCl₆, where Ln concentration is ~3×10²¹ at/cm³ (equivalent to YAG doping level of ~20% at.), Ln emission lifetimes exhibited only a minor decrease to ~2.5 ms (Yb) and ~12 ms (Er), demonstrating that Ln concentration quenching processes are relatively ineffective in these materials.
Absorption and emission cross-section spectra, inferred from measurements of diffuse absorptance, allowed to estimate optical gain coefficients and, consequently, derive all-optical figures of merit for end-pumped lasing. These estimates were also done for reference laser crystals, Yb:YAG and Er:YAG [5]. Owing to the largely suppressed concentration quenching, chloride double perovskites effectively emerge as interesting materials for NIR solid-state lasers.

References
1 - J. Luo, et al., Nature (2018), 563, 541
2 - G. Zhang, et al., Dalton Trans. (2020), 49, 15231
3 - H. Arfin, et al., Angew. Chem. Int. Ed. (2020), 59, 11307
4 - S. Yang, et al., J. Phys. Chem. C (2021), 125, 10431
5 - F. Liu, et al., in preparation.

Hybrid organic inorganic perovskites are optoelectronic materials with tunable chemical and electronic structures. Incorporating chiral organic molecules into perovskite networks also attracts great attention due to their potential optical communication applications. Nevertheless, most reported chiral perovskite materials possess high toxic Pb, which potentially limits their practical applications. Here, we introduce chiral organic molecules R/S-1-cyclohexylethylamine (R/S-CHEA) into lead-free bismuth based materials with perovskite motif, and grow color tunable chiral lead-free (R/S-CHEA)₄Bi₂Br_xI_10-x (x = 0 to 10) crystals and thin films. We perform single crystal XRD measurement and DFT calculation to identify their crystal structure and electronic band structure. We further investigate their optical properties. We find the absorption peak position is shifted from 386 nm to 493 nm, while Circular Dichroism (CD) signals can be turned widely from 366 nm to 535 nm, by changing the ratios of bromide and iodide anions in (R/S-CHEA)₄Bi₂Br_xI_10-x. Specifically, we employ steady-state and time-resolved PL spectra measurements on (R-CHEA)₄Bi₂Br₁₀ film, which has a broad emission peak located at around 520 nm and shorter lifetime compared with that of Cs₂AgBiBr₆. Besides, temperature and fluence dependent time-resolved PL spectra are also performed to investigate the charge carrier dynamics in (R-CHEA)₄Bi₂Br₁₀. Our demonstration of color tunable chiral lead-free perovskites gives the possibility to discover high performance and nontoxic optical spintronic materials.

Halide perovskites have lured enormous attention due their unique optoelectronic properties. However, the complex stoichiometry of stable halide perovskite compounds motivates the search for volatility- and solubility-insensitive deposition methods.
Pulsed Laser Deposition (PLD) is a versatile method capable of multi-element ablation off a solid target irrespective of its chemical complexity, allowing single-source deposition of complex halide perovskite compositions [1–3]. This relaxes concerns of thin film formation such as incompatible solubility issues with solution process or different elemental volatilities in the case of co-evaporation [4]. Here we use PLD to trigger the structural instability of CsPbI₃ at room temperature by partial replacement of Pb with Bi or that of I with Br. The link between the PLD parameters and the film crystallinity and morphology was investigated proving compact, smooth and highly crystalline black orthorhombic CsPbI₃. The stoichiometric control was implemented by the precursors molar concentration in the PLD target. The correlation between different levels of Bi concentration and the crystallinity, film stoichiometry, and optoelectronic structure of CsPbI₃ was monitored using X-ray diffractometry (XRD), X-ray florescence (XRF), photoluminescence and photothermal deflection spectroscopy (PDS) analyses. The incorporation of Bi in the structure of CsPbI₃ resulted in improved stability of the film up to 154 hours. In a similar fashion, introducing Br to CsPbI₃ resulted in films stable over 100 days.
The stabilization of halide perovskites through adding chemical species during PLD unlocks its grave potential in growing optimal halide perovskite films and optoelectronic devices. In addition, the solvent-free nature of PLD, nearly stoichiometric transfer, scalability, and control over the chemical composition and thickness, make this technique particularly interesting for composition screening of dopants and multicompounds, as well as to fabricate and study multi-layered solar cells such as all-perovskite tandem solar cells.

[1] U. Bansode, R. Naphade, O. Game, S. Agarkar, and S. Ogale, Hybrid Perovskite Films by a New Variant of Pulsed Excimer Laser Deposition: A Roomerature Dry Process, J. Phys. Chem. C 119, 9177 (2015).
[2] S. Cheng, Q. Chang, Z. Wang, L. Xiao, E. E. M. Chia, and H. Sun, Observation of Net Stimulated Emission in CsPbBr ₃ Thin Films Prepared by Pulsed Laser Deposition, Adv. Opt. Mater. 2100564 (2021).
[3] V. M. Kiyek, Y. A. Birkhölzer, Y. Smirnov, M. Ledinsky, Z. Remes, J. Momand, B. J. Kooi, G. Koster, G. Rijnders, and M. Morales-Masis, Single-Source, Solvent-Free, Room Temperature Deposition of Black γ-CsSnI3 Films, Adv. Mater. Interfaces 2000162, 1 (2020).
[4] T. Soto-Montero, W. Soltanpoor, and M. Morales-Masis, Pressing Challenges of Halide Perovskite Thin Film Growth, APL Materials.

The progress of tin-based halide perovskite solar cells (Sn-HaPSCs) is obstructed by their poor stability arising from tin oxidation. Herein, we introduced the phenethylammonium thiocyanate (PEASCN) as a pseudohalide functional additive into FASnI₃ perovskite film to improve the optoelectronic properties. This approach is found to be effective for the suppression of Sn-oxidation and the formation of compact and larger grain film with a higher degree of crystallinity. The device with PEASCN additive improved the device efficiency from 4.52% (for pristine Sn-HaP) to 9.65% with superior device stability. The device analysis revealed that the PEASCN additive has improved the optoelectronic properties coupled with a built-in potential and suppression of bulk and interface defects in the Sn-HaPSC. This work corroborates that the incorporation of pseudohalide-based functional additive in the FASnI₃ is propitious for better film formation, passivation of detrimental surface chemistry, and defects at interface and bulk.

Hybrid organic-inorganic perovskite (HOIP) semiconductors based on metal halide frameworks offer unprecedented opportunity to tailor structural and materials properties using the full flexibility afforded by the associated inorganic and organic components,^1,2 and such tunability points towards wide-ranging potential for applications including solar cells, light-emitting devices, detectors, transistors and advanced computing devices. In this talk we will focus on the role that organic cation chirality can have on HOIP structure and properties including, for example, the impact on chiroptical and electrical transport (e.g., circularly polarized absorption and chirality-induced spin selectivity³), electronic structure (e.g., spin splitting of the conduction band⁴) and thermal properties (e.g., accessing a low-melting temperature and glassy state⁵). The above examples point to the great potential of using chirality for enabling and coupling control over light, charge and spin within the important HOIP semiconductor family.

References:
[1] B. Saparov, D. B. Mitzi, Chem. Rev. 116, 4558 (2016).
[2] D. B. Mitzi, Prog. Inorg. Chem. 48, 1 (1999).
[3] H. Lu, C. Xiao, R. Song, T. Li, A. E. Maughan, A. Levin, R. Brunecky, J. J. Berry, D. B. Mitzi, V. Blum, M. C. Beard, J. Amer. Chem. Soc. 142, 13030 (2020).
[4] M. K. Jana, R. Song, H. Liu, D. R. Khanal, S. M. Janke, R. Zhao, C. Liu, Z. V. Vardeny, V. Blum, D. B. Mitzi, Nature Commun. 11, 4699 (2020).
[5] A. Singh, M. K. Jana, D. B. Mitzi, Adv. Mater. 33, 2005868 (2021).

Recently, solution processed hybrid perovskites show intriguing electronic and optoelectronic properties applicable in various device application, and the developing of high quality materials with improved homogeneity and less defects is crucial. In this talk, I mainly focused on the rational design and fabricate perovskite films, and the construction of efficient and stable photovoltaic devices. Two related aspects are discussed: 1) Develop liquid medium annealing process that leads to the films with high crystallinity, less defects, desired stoichiometry, and overall film homogeneity, to achieve high performance photovoltaics with certified efficiency of 24.5%; 2) Propose methods such as "redox ion pair" and "multiple non-covalent bond synergistic effect" to suppress defect pairs in a sustainable way, which greatly improved the long-term stability of perovskite solar cells for over 2000 h under the stressors, e.g. light, heat, and electricity. Corresponding degradation mechanism of hybrid perovskite films and devices under operational conditions are revealed on the molecular and atomic scales.

Metal halide perovskites have shown promising optoelectronic properties suitable for light-emitting applications. The development of perovskite light-emitting diodes (PeLEDs) has progressed rapidly over the past several years, and molecular additives have played an important role in these advances. By rational design of these molecular additives, we can significantly enhance the interaction with defects sites and minimize non-radiative recombination losses. In addition, we also demonstrate that the molecular additives can manipulate the perovskite crystallization and help to enhance the radiative recombination. As such, we have been able to demonstrate perovskite LEDs with high external quantum efficiencies over 20%. We find that our devices can also work efficiently in an emitting/detector switchable mode, with tens-megahertz speed for both functions. We further demonstrate the potential of the dual-functional diode for biomedicine diagnosis applications (as a monolithic heart pulse sensor) and for inter- and intra-chip bidirectional optical communications.

After several years of rapid development of scalable printing of perovskites thin films, the obtainable film morphologies have been well optimized with desired smoothness, compactness, and full coverage on substrates, which are all indispensable for making a good solar cell. Now the next step is scalable defect controlling for further promoting the solar cell efficiency and stability. Two important issues arise:
(1) What types of defects are detrimental to perovskite solar cells? How do they impact efficiency or stability?
(2) How can we eliminate such defects with scalable, industry-compatible process?
This talk will discuss our recent efforts in answering these two questions. We start with formamidinium-cesium perovskite, which are known to be thermally more stable than methylamine perovskite. However, the former is found to be less photostable. We studied the light-induced degradation mechanism and found that it is the generation of iodine interstitials that capture photo-generated holes and degrade the solar cell photocurrent. We investigated the mechanism of iodine interstitial generation process and invented a defect compensation strategy by choosing a slightly AX (A is formamidinium or cesium cation and X is iodine) excessive composition to effectively suppress the process. Eventually, we improved the large area perovskite solar module's photostability with the highly-industry-compatible defect compensation strategy.

Pb- based halide perovskite, HaP, single-junction solar cells have improved dramatically to over 25% for small laboratory cells. Considering the toxicity of lead ions, substituting Pb with other non-toxic elements in the perovskite structure has become one of the most significant challenges associated with these materials. In this work, we replace Pb with Sn + Ge in an all-inorganic HaP for improved stability, focussing on CsSn_xGe_1-xBr₃. We do so by using a new deposition technique for thin film formation. The choice of Br as the halide is influenced by the fact that we have (much) smaller divalent cations on the B site. Naturally, doing so leads to a larger bandgap than the corresponding iodide, with enhanced environmental stability, already aided by having inorganic Cs instead of organic cations. X-Ray Diffraction results show that CsSn_xGe_1-xBr₃ forms a homogeneous solid solution, and up to 70% Ge, maintains the cubic structure of pure CsSnBr₃ with a shift to smaller lattice spacings with increasing Ge concentration. The optical bandgap is found to be tunable from 1.7 eV to 2.3 eV, from pure Sn to pure Ge, respectively. In N₂ atmosphere, the stability of the mixed Sn-Ge HaP is improved compared to that of the pure Sn one.

Wide bandgap mixed-halide perovskites as the top absorbers in tandem solar cells typically exhibit a larger voltage deficit in contrast to their corresponding tri-iodide-based perovskites. With this work we are tackling the challenge of directly correlating the multi-stage synthesis of bromide-containing perovskites with optoelectronic properties and consequently device performance. Our results show that the incorporation of bromide occurs via a halide homogenization process where the perovskite composition transitions from an initial bromide-rich phase to the final target stoichiometry. In situ photoluminescence measurements during synthesis reveal that bromide inclusion alters the formation dynamics. By combining experiments with first-principle calculations we suggest that the formation of FAMACsPb(I_1-xBr_x)₃ starts with the nucleation of a bromide-rich phase from solution during supersaturation followed by a retarded growth stage. The retarded growth stage is associated with a halide homogenization process that brings the composition of crystallized material to the target stoichiometry. This homogenization process not only changes the formation dynamics of mixed-halide perovskites but also promotes defect formation, which consequently leads to increased non-radiative recombination losses in the final perovskite film and thus, could explain the larger voltage deficit.

Lead-free halide perovskites have gained interest in competition to the lead-based counterparts which are affected by toxicity and instability over moisture. Among them, double perovskites have been developed to alternatively substitute the divalent cations Pb2+ in single perovskite with a combination of a monovalent and trivalent cation, forming structure as A₂BB’X₆.
Here we report the highly stable Cs₂Na_1-xAg_xIn_1-yBi_yCl₆ compounds, with Silver (x) and Bismuth (y) fractions ranging from 10% to 1 ppm, that exhibit efficient and stable broadband, white-light emission ideally suited for lighting applications.
Double perovskites were synthesized in small crystals by a super-saturation precipitation method followed by filtration and their photoluminescence quantum yield (PLQY) was measured. The highest PLQY was found to be very close to 100% and the optical properties of the crystals were studied by a combination of time resolved photoluminescence and differential transmission, in order to establish the nature of the light-emitting excited states. The measurements confirmed that emission is produced by self-trapped exciton states; the two key elements to achieve such outstanding emission properties are Bi, whose 6p states enhance the lifetime for electrons in the conduction band, and Ag, with 4d states at the top of the valence band allow efficient radiative recombination of excitons. A systematic study as a function of Bi and Ag content was employed to reveal the diffusion length for optical excitations, localization processes as well as the intrinsic limits for the excited state lifetime.
The results provide guidance for rational optimization of such compounds in view of the use as phosphors and active materials for LEDs and displays.

Semiconductor surfaces play an important role in the behavior of their electronic devices. The vulnerable lattice of metal-halide perovskite renders a fragile surface, where defect states can easily form. Very recently, most of the highly efficient perovskite optoelectronic devices have been reported to employ effective surface passivation strategies, further confirming the significance of surface states in regulating their device performance. Here, we present methodical studies towards understanding the surface states in perovskite thin films utilizing a strategy of molecular “defect indicator” that is able to interact with specific surface defects. In formamidinium (FA)-methylammonium (MA) mixed cation perovskite thin films, a nonuniform distribution of cations is uncovered with FA cations being close to the top and MA close to the bottom of the film, leading to unique surface defect energetics. Antisite FA_I was found to become a dominant deep trap on the surface of this system.

Lead halide perovskites have undergone rapid developments in recent years to become well established semiconductors for future optoelectronic applications. These advancements are achieved through the understanding of the solution and surface chemistry of perovskites. A selection of functional molecules lead to better control of the film formation dynamics and passivation of surface and bulk defects. These molecules are capable to form coordinate bonds, known as Lewis acid/base interactions, with perovskites. Especially Lewis bases are in the centre of attention, because they have a great impact on the performance: they promote long term stability, heal surface defects and improve the morphology of the films. However, some solvents (e.g. water) which passivate perovskite defects can cause degradation upon longer exposure. This indicates a more complex interaction which needs to be explored.
Photoluminescence (PL) is a simple and effective tool that allows us to track interactions between perovskite and reactive molecules in a controlled environment. We have evaluated by in-situ PL measurements a variety of solvents that are commonly used in perovskite processing to understand photobrightening and photodegradation responses regarding the interactions between the functional groups and the surface. Establishing the mechanism behind these interactions will provide a better understanding on how the stability of perovskites can be improved.

Electroluminescence (EL) mapping technique is a non-contact, non-destructive, fast, and simple testing method that spatially resolves the inhomogeneity in the electrical and optical properties of solar cells and detects micro-cracks or pinholes. It is an essential diagnostic tool for the quality control of commercial solar cells and modules. However, it is less commonly used for characterizing small-sized thin-film solar cells fabricated in laboratories.

Here, we assemble a fast EL imaging system with a high spatial resolution of ~1 μm/pixel to investigate the inhomogeneity and decomposition processing in a variety of perovskite solar cells (PSCs) with different absorber layer compositions and device configurations. We observe that most high-efficiency (>22%) PSCs show uniform and bright EL signals, while defective cells exhibit weak and spatially non-uniform EL intensity. Under different aging conditions (e.g., electric field, thermal stress, humid air, light soaking, etc.), PSCs with different absorber compositions exhibit different degradation behaviors. To explore the degradation mechanism in detail, we quantitatively compare the spatially-resolved and time-resolved EL images of FA/Cs-based PSCs with different Cs contents ranging from 0 to 40% aged in humid air with controlled humidity. Interestingly, we find that the perovskites with a high Cs content (>20%) exhibit more severe phase segregation and faster degradation in humid air, while the perovskites with too little Cs (< 10%) are phase-unstable and lose EL signal rapidly. The findings shed light on compositional engineering to enable uniform, efficient, and stable PSCs.

Metal halide perovskite light-emitting diodes (PeLEDs) show great potentials to be the next-generation lighting technology, with external quantum efficiencies (EQEs) exceeding 20% for infrared, red and green LEDs. However, the efficiencies of blue and white devices severely lag behind. To improve the performance of blue PeLEDs, we employed an integrated strategy combining dimensional engineering of perovskite film and recombination zone modulation in the LED device to obtain an EQE up to 5 %.^[1] While further incorporating the strategy of interfacial engineering, highly efficient blue PeLEDs with EQEs over 10% have been successfully realized in our group, establishing an excellent platform for white-light emission. In our latest work, we demonstrated efficient white PeLEDs by optically coupling a blue PeLED with a red emitting perovskite nanocrystal layer in an advanced device structure, which allows to extract the trapped optical modes (waveguide and SPP modes) of blue photons in the device to the red perovskite layer via near-field effects. As a result, a white PeLEDs with EQE over 12% is achieved, which represents the state-of-the-art performance for white PeLEDs.^[2]

References:
[1] Z. Li, Z. Chen, H.-L. Yip et al. Nat. Commun. 2019, 10, 1027
[2] Z. Chen, Z. Li, H.-L. Yip et al. Joule 2021, 5, 456

Metal halide perovskites have attracted tremendous attentions as new generation functional materials for applications in a variety of optoelectronic devices, such as solar cells and light emitting diodes (LEDs), owing to their exceptional optical and electronic properties, as well as low temperature solution processability. While remarkable progresses have been achieved for highly efficient perovskite solar cells (PSCs) and LEDs (PeLEDs) during the last decade, numerous issues and challenges remain, including stability, lead toxicity, large scale manufacturing, etc. To achieve high efficiency and stability simultaneously, surface passivation of perovskite thin films and nanocrystals by organic/inorganic species is one of the most effective approaches, which could not only reduce surface defects, but also protect the surface from atmospheric moisture. In this talk, I will discuss our recent efforts on the development and study of various types of surface passivation agents for halide perovskite thin films and nanocrystals, with which highly efficient and stable PSCs and PeLEDs are achieved. For instance, a low-cost industrial organic pigment, quinacridone (QA), has been investigated as surface passivation agent for PSCs by solution processing of a soluble QA derivative followed by thermal annealing to convert it into insoluble QA. Passivation with strong interactions between QA and metal halides, together with the hydrophobicity of QA coating, enabled highly efficient PSCs with remarkable stability.

The efficiencies of all-perovskite tandem devices are improving quickly. However, their complex interconnection layer (ICL) structures – with typically four or more layers deposited by different processes – limit their prospects for applications. Here, we report an ICL in all-perovskite tandem cells consisting merely of a fullerene layer and a SnO_2-x (0<x<1) layer. The C₆₀ layer is unintentionally n-doped by iodine ions from the perovskite and thus acts as an effective electron collecting layer. The SnO_2-x layer, formed by the incomplete oxidization of tin (x=1.76), has ambipolar carrier transport property enabled by the presence of a large density of Sn²⁺. The C₆₀/SnO_1.76 ICL forms Ohmic contacts with both wide and narrow bandgap perovskite subcells with low contact resistivity. The ICL boosts the efficiencies of small-area tandem cells (5.9 mm²) and large-area tandem cells (1.15 cm²) to 24.4% and 22.2%, respectively. The tandem cells remain 94% of its initial efficiency after continues 1-sun illumination for 1,000 hours. The efficnecy of all-perovskite tandem solar cell is further improved to beyond 25% by increasing the carrier diffussion length of Sn-Pn NBG perovskite to ~2um via doping modification.

In contrast to the traditional inorganic semiconductors quantum wells, solution-processed hybrid organic–inorganic perovskites (HOIP) quantum wells, as emergent new materials, can be a vital player in optoelectronics such as solar cells, LEDs, transistors, and photodetectors. The molecular engineering of organic semiconductors, combing with the composition engineering of monolayer perovskites offer the tunability of electrical and optical property.
In this report, we investigate the carrier transport dynamics of the (4Tm)2PbI₄ and (4Tm)2SnI₄, where 4Tm is a π-conjugated oligothiophene ligand of 2-(3″′,4′-dimethyl-[2,2′:5′,2″:5″,2″′-quaterthiophen]-5-yl)ethan-1-ammonium. By using the state-of-the-art ultrafast photocurrent spectroscopy with a sub-20 ps time resolution, the (4Tm)2PbI₄ and (4Tm)2SnI₄ HOIP quantum wells demonstrate > 10 cm²/Vs carrier mobility, and the transport dynamics can be described by a band-like transport mechanism with a rapid trapping time of 60 ps after photoexcitation. Our work demonstrate the promising applications of HOIP quantum well in high-speed photodetection and optical quantum communication.

The development of novel hole transporting materials (HTMs) which can improve the stability of lead-halide perovskite solar cells (PSCs) operating in ambient conditions is a valuable strategy to address the future commercialization of this ultra-cheap photovoltaic technology. Among other HTMs, we have identified poly(3-hexylthiophene) (P3HT), a highly soluble and easy-to-process from solution semiconducting polymer, to be a good compromise for ensuring decent efficiencies to PSCs at relatively low-costs in comparison with the benchmark HTM based on the Spiro-OMeTAD small molecule. Spiro-OMeTAD indeed requires doping with LiTFSI and tert-butyl pyridine additives to be sufficiently active for hole transport and, in this regard, we have recently demonstrated an up-to-now unknown mechanism of de-doping induced by the latter additive, that we will discuss in this contribution.[1] We then looked for other possible nanomaterials to couple with P3HT for the development of even more efficient HTMs and stable PSCs. We will thus discuss here the outcomes of our most recent contribution, [2] in which we suggest that a CuSCN nanoplateletes/P3HT composite, combining hole extraction and transport properties with water oxidation activity, transforms incoming water molecules and triggers the in situ p-doping of the conjugated polymer, improving transport of photocharges. Insertion of the nanocomposite into a lead perovskite solar cell with a direct photovoltaic architecture causes stable device performance for 28 days in high-moisture conditions. Our findings demonstrate that the engineering of a hole extraction layer with water-splitting additives could be a viable strategy to reduce the impact of moisture in perovskite devices. References [1] F. Lamberti et al. Chem 2019, 5, 1; [2] M. Kim et al., T. Gatti, F. Lamberti Commun. Mater. 2021, 2, 6