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The development of highly stable and efficient wide-bandgap (WBG) perovskite solar cells (PSCs) based on bromine-iodine (Br–I) mixed-halide perovskite (with Br greater than 20%) is critical to create tandem solar cells. However, issues with Br–I phase segregation under solar cell operational conditions (such as light and heat) limit the device voltage and operational stability. This challenge is often exacerbated by the ready defect formation associated with the rapid crystallization of Br-rich perovskite chemistry with antisolvent processes. We combined the rapid Br crystallization with a gentle gas-quench method to prepare highly textured columnar 1.75–electron volt Br–I mixed WBG perovskite films with reduced defect density. With this approach, we obtained 1.75–electron volt WBG PSCs with greater than 20% power conversion efficiency, approximately 1.33-volt open-circuit voltage ( V oc ), and excellent operational stability (less than 5% degradation over 1100 hours of operation under 1.2 sun at 65°C). When further integrated with 1.25–electron volt narrow-bandgap PSC, we obtained a 27.1% efficient, all-perovskite, two-terminal tandem device with a high V oc of 2.2 volts.

The presence of unreacted lead iodide in organic-inorganic lead halide perovskite solar cells is widely correlated with an increase in power conversion efficiency. We investigate the mechanism for this increase by identifying the role of surfaces and interfaces present between methylammonium lead iodide perovskite films and excess lead iodide. We show how type I and II band alignments arising under different conditions result in either passivation of surface defects or hole injection. Through first-principles simulations of solid-solid interfaces, we find that lead iodide captures holes from methylammonium lead iodide and modulates the formation of defects in the perovskite, affecting recombination. Using surface-sensitive optical spectroscopy techniques, such as transient reflectance and time-resolved photoluminescence, we show how excess lead iodide affects the diffusion and surface recombination velocity of charge carriers in methylammonium lead iodide films. Our coupled experimental and theoretical results elucidate the role of excess lead iodide in perovskite solar cells., 4 figures, 1 table, 21 pages + 13 pages of Supporting Information

The photovoltaic parameters of triple cation perovskite [Cs

Perovskite solar cells (PSCs) with an inverted structure (often referred to as the p-i-n architecture) are attractive for future commercialization owing to their easily scalable fabrication, reliable operation and compatibility with a wide range of perovskite-based tandem device architecturessup1,2/sup. However, the power conversion efficiency (PCE) of p-i-n PSCs falls behind that of n-i-p (or normal) structure counterpartssup3-6/sup. This large performance gap could undermine efforts to adopt p-i-n architectures, despite their other advantages. Given the remarkable advances in perovskite bulk materials optimization over the past decade, interface engineering has become the most important strategy to push PSC performance to its limitsup7,8/sup. Here we report a reactive surface engineering approach based on a simple post-growth treatment of 3-(aminomethyl)pyridine (3-APy) on top of a perovskite thin film. First, the 3-APy molecule selectively reacts with surface formamidinium ions, reducing perovskite surface roughness and surface potential fluctuations associated with surface steps and terraces. Second, the reaction product on the perovskite surface decreases the formation energy of charged iodine vacancies, leading to effective n-type doping with a reduced work function in the surface region. With this reactive surface engineering, the resulting p-i-n PSCs obtained a PCE of over 25 per cent, along with retaining 87 per cent of the initial PCE after over 2,400 hours of 1-sun operation at about 55 degrees Celsius in air.

MAPbBr3 and MAPbI3 films cast onto glassslides and physically paired together undergo halide exchange to formmixed halide films. The change in halide composition in these two∼130 nm thick films occurs concurrently with Br– diffusing toward the MAPbI3 film and I– diffusing toward the MAPbBr3 film. The diffusion of thesehalide species, which is tracked through changes in the absorption,offers a direct measurement of thermally activated halide diffusionin perovskite films. The increase in the rate constant of halide diffusionwith increasing temperature (from 8.3 × 10–6 s–1 at 23 °C to 3.7 × 10–4 s–1 at 140 °C) follows an Arrhenius relationshipwith activation energy of 51 kJ/mol. The thermally activated halideexchange shows the challenges of employing layers of different metalhalide perovskites in stable tandem solar cells.

The control of grain size and surface properties is an important parameter in controlling the optoelectronic and photovoltaic properties of metal halide perovskites. When CsPbBr3 nanocrystal (∼10 nm in diameter) films were annealed at 100–125 °C, they grow in size to produce ∼400 nm diameter crystallites while transforming into bulk perovskite films. Characteristic changes in the optical properties were noted when such transformation occurred from nanocrystals into bulk. By tracking absorbance and emission spectra and morphological changes of CsPbBr3 films at different annealing times and temperature, we were able to establish the mechanism of particle growth. The presence of nanocrystals and larger crystals during the intermediate annealing steps and narrowing size distribution confirmed the Ostwald ripening mechanism for the crystal growth. The energy of activation of crystal growth as determined from the temperature dependent optical properties was estimated to be 75 kcal/mol.

Owing to its high hole conductivity and ease of preparation, CuI was among the first inorganic hole-transporting materials that were introduced early on in metal halide perovskite solar cells, but its full potential as a semiconductor material is still to be realized. We have now performed ultrafast spectroelectrochemical experiments on ITO/CuI electrodes to show the effect of applied bias on the excited-state dynamics in CuI. Under operating conditions, the recombination of excitons is dependent on the applied bias, and it can be accelerated by decreasing the potential from +0.6 to −0.1 V vs Ag/AgCl. Prebiasing experiments show the persistent and reversible “memory” effect of electrochemical bias on charge carrier lifetimes. The excitation of CuI in a CuI/CsPbBr3 film provides synergy between both CuI and CsPbBr3 in dictating the charge separation and recombination.

Record-breaking efficiency achieved with quantum dot solar cells made of perovskite nanocrystals demands understanding of the excited-state interactions between perovskite nanocrystals and metal oxide electron transport layers. The interfacial electron transfer between excited CsPbBr3 perovskite nanocrystals and metal oxides (TiO2, SnO2, and ZnO) was elucidated using transient absorption spectroscopy and found to occur with a rate constant in the range of 2–4 × 1010 s–1. In an inert atmosphere, back electron transfer helps to maintain the stability of the perovskite nanocrystals. However, the presence of oxygen introduces instability as it scavenges away transferred electrons from the electron-transporting metal oxide, leaving behind holes to accumulate at CsPbBr3 nanocrystals, which in turn induce anodic corrosion. X-ray photoelectron spectroscopy measurements have enabled us to identify PbO as the major photodegraded product. The importance of the surrounding atmosphere and the supporting metal oxide in...

The suppression of halide ion exchange between CsPbBr3 and CsPbI3 nanocrystals achieved through capping with PbSO4–oleate has enabled us to deposit different perovskite nanocrystals as aligned arrays on the electrode surfaces without intermixing of species. The electrophoretic deposition of PbSO4–oleate-capped CsPbX3 (X = Cl, Br, I) nanocrystals suspended in hexane solution on mesoscopic TiO2 films allows the design of controlled architecture with single or multiple layers of perovskite films. The hierarchy in the assembly of these nanocrystals is seen first through the linearly organized nanocrystals in hexane followed by the deposition of larger linear rods ∼500 nm in length. Since most of the photophysical properties of nanocrystals are retained in these aligned arrays, we can design films with tunable luminescence including white color. The electrophoretic deposition of layered films of perovskites in a controlled fashion opens up new ways to design tandem perovskite solar cells and tunable display de...

Electrochemistry, as an analytical tool is gaining its foothold in the characterization of organic metal lead halide per-ovskites, however electrosynthetic applications are yet to be explored fully. Electrochemical deposition of hole-transport layers with desired properties could be an attractive tool to assemble complex solar cell architectures. We have now successfully electropolymerized PEDOT (Poly(3,4-ethylenedioxythiophene)), a hole-transporting material on the surface of methyl-ammonium lead iodide perovskite in a controlled fashion and evaluated its performance in perovskite solar cells. The electrochemical post-treatment protocol is shown to be important to maximize the photo-voltaic performance of the solar cells. The proposed electrochemical deposition methodology expands the pool of tech-niques available for hole transporting layer deposition in perovskite solar cells.

The ease of halide ion exchange in metal halide nanocrystals offers an opportunity to utilize them in a layered or tandem fashion to achieve graded bandgap films. We have now successfully suppressed the halide ion exchange by capping CsPbBr3 and CsPbI3 nanocrystals with PbSO4–oleate to create a nanostructure assembly that inhibits the exchange of anions. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr3 or CsPbI3 for several days. Furthermore, the effect of PbSO4–oleate capping on the excited state dynamics has also been elucidated. The effectiveness of PbSO4–oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid toward the tandem design of perovskite light-harvesting assemblies.

The unique optoelectronic properties of lead halide perovskites have triggered a new wave of excitement in materials chemistry during the past five years. Electrochemistry, spectroelectrochemistry, and photoelectrochemistry could be viable tools both for analyzing the optoelectronic features of these materials and for assembling them into hybrid architectures (e.g., solar cells). At the same time, the instability of these materials limits the pool of solvents and electrolytes that can be employed in such experiments. The focus of our study is to establish a stability window for electrochemical tests for all-inorganic CsPbBr3 and hybrid organic–inorganic MAPbI3 perovskites. In addition, we aimed to understand the reduction and oxidation events that occur and to assess the damage done during these processes at extreme electrochemical conditions. In this vein, we demonstrated the chemical, structural, and morphological changes of the films in both reductive and oxidative environments. Taking all these results together as a whole, we propose a set of boundary conditions and protocols for how electrochemical experiments with lead halide perovskites should be carried out and interpreted. The presented results will contribute to the understanding of the electrochemical response of these materials and lead to a standardization of results in the literature so that comparisons can more easily be made.

Cesium lead halide perovskite films with a systematic change in the halide composition of CsPbBr3−xIx, in which iodide concentration varies from x = 0 to x = 3, provide a built-in gradient band structure. Such a gradient structure allows for the integrated capture of visible photons and directs them to the energetically low-lying iodide rich region. Annealing gradient halide perovskite films at temperatures ranging from 50 °C to 90 °C causes the films to homogenize into mixed halide perovskites. The movement of halide ions during the homogenization process was elucidated using UV-Visible absorbance and X-ray photoelectron spectroscopy. The halide ion movement in CsPbBr3−xIx gradient films was tracked via absorbance changes in the visible region of the spectrum that enabled us to measure the temperature dependent rate constant and energy of activation (74.5 kJ/mol) of halide ion homogenization. Excited state processes of both gradient and homogenized films probed through transient absorption spectroscopy showed the direct flow of charge carriers and charge recombination in both films.

Record-breaking efficiency achieved with quantum dot solar cells made of perovskite nanocrystals demands understanding of the excited-state interactions between perovskite nanocrystals and metal oxide electron transport layers. The interfacial electron transfer between excited CsPbBr

The charging of a mesoscopic TiO2 layer in a metal halide perovskite solar cell can influence the overall power conversion efficiency. By employing CsPbBr3 films deposited on a mesoscopic TiO2 film, we have succeeded in probing the influence of electrochemical bias on the charge carrier recombination process. The transient absorption spectroscopy experiments conducted at different applied potentials indicate a decrease in the charge carrier lifetimes of CsPbBr3 as we increase the potential from −0.6 to +0.6 V vs Ag/AgCl. The charge carrier lifetime increased upon reversing the applied bias, thus indicating the reversibility of the photoresponse to charging effects. The ultrafast spectroelectrochemical experiments described here offer a convenient approach to probe the charging effects in perovskite solar cells.

Semiconducting materials are essential building blocks of the majority of devices in the 21st century. The ability of semiconductors to generate charge carriers under band gap irradiation makes them suitable for a myriad of applications (e.g., solar cells, photocatalysts, photodetectors). In recent years the emergence of organic-inorganic lead halide perovskites revitalized these research fields. As a result, the efficiency of derived solar cells reached 22.3% in less than 10 years. To unravel the reasons behind their outstanding performance, it is vital to understand their optoelectronic properties. Spectroelectrochemical methods are suitable to determine fundamental optoelectronic properties (band edge and trap state energies) and electrochemical bias induced chemical changes in these materials. However, to probe charge carrier dynamics in these materials, it is essential to utilize ultrashort laser techniques, as these fast processes fall into the femto- or picosecond timescale. In my presentation the effect of applied bias on the charge carrier dynamics of semiconductor electrodes will be discussed. Ultrafast spectroelectrochemical experiments were carried out on FTO/TiO2/CsPbBr3 and ITO/CuI systems by coupling ultrafast transient absorption spectroscopy with electrochemical techniques.1 These two systems have practical relevance in perovskite based solar cells, and their excited state behavior under operating conditions was scrutinized. It was found that the excitonic features of both CsPbBr3 and CuI are responsive to the applied external bias, within their electrochemical stability window. The accumulation of electrons on the TiO2/CsPbBr3 or ITO/CuI interface have a pronounced effect on charge carrier lifetimes in these studied materials. The change in charge carrier lifetimes in both CsPbBr3 and CuI was completely reversible showing the dependence of excited state dynamics on the externally controlled charge carrier density. This validates the in situ electrochemical transient absorption measurement as a useful tool to probe the charge carrier injection process in different semiconductor systems. Scheidt, R. A., Samu, G. F., Janáky, C. & Kamat, P. V. Modulation of Charge Recombination in CsPbBr3 Perovskite Films with Electrochemical Bias. J. Am. Chem. Soc. 140, 86–89 (2018).

The charging of a mesoscopic TiO2 layer in a metal halide perovskite solar cell can influence the overall power conversion efficiency. By employing CsPbBr3 films deposited on a mesoscopic TiO2 film, we have succeeded in probing the influence of electrochemical bias on the charge carrier recombination process. The transient absorption spectroscopy experiments conducted at different applied potentials indicate a decrease in the charge carrier lifetimes of CsPbBr3 as we increase the potential from −0.6 to +0.6 V vs Ag/AgCl. The charge carrier lifetime increased upon reversing the applied bias, thus indicating the reversibility of the photoresponse to charging effects. The ultrafast spectroelectrochemical experiments described here offer a convenient approach to probe the charging effects in perovskite solar cells.

A thermal-shock strength-testing technique has been developed that uses a high-resolution, high-temperature infrared camera to capture a specimen's surface temperature distribution at fracture. Aluminum nitride (AlN) substrates are thermally shocked to fracture to demonstrate the technique. The surface temperature distribution for each test and AlN's thermal expansion are used as input in a finite-element model to determine the thermal-shock strength for each specimen. An uncensored thermal-shock strength Weibull distribution is then determined. The test and analysis algorithm show promise as a means to characterize thermal shock strength of ceramic materials.

Simplifying the manufacturing processes of renewable energy technologies is crucial to lowering the barriers to commercialization. In this context, to improve the manufacturability of perovskite solar cells (PSCs), we have developed a one-step solution-coating procedure in which the hole-selective contact and perovskite light absorber spontaneously form, resulting in efficient inverted PSCs. We observed that phosphonic or carboxylic acids, incorporated into perovskite precursor solutions, self-assemble on the indium tin oxide substrate during perovskite film processing. They form a robust self-assembled monolayer as an excellent hole-selective contact while the perovskite crystallizes. Our approach solves wettability issues and simplifies device fabrication, advancing the manufacturability of PSCs. Our PSC devices with positive-intrinsic-negative (p-i-n) geometry show a power conversion efficiency of 24.5% and retain >90% of their initial efficiency after 1,200 h of operating at the maximum power point under continuous illumination. The approach shows good generality as it is compatible with different self-assembled monolayer molecular systems, perovskites, solvents and processing methods., Improving the manufacturability of perovskite solar cells is key to their deployment. Zheng et al. report a one-step deposition of the hole-selective and absorber layers that addresses wettability issues and simplifies the fabrication process.