Hugh W. Hillhouse

In situ absorbance measurements reveal two photooxidation pathways in FA _0.8 Cs _0.2 Pb(I _0.83 Br _0.17 ) ₃ thin films. Kinetic modeling explains the dual role of water, with a posited hydrate formation pathway retarding degradation at high temperature.

The kinetics of light-induced Pb ⁰ formation in mixed-cation, mixed-halide perovskite films are studied using sub-bandgap absorption measurements. Oxygen is shown to prevent Pb ⁰ formation and modulate the optoelectronic performance decline.

Halide perovskites are promising photovoltaic (PV) materials with the potential to lower the cost of electricity and greatly expand the penetration of PV if they can demonstrate long-term stability under illumination in the presence of moisture and oxygen. The solar cell service lifetime, as quantified by T₈₀ (the time required for the power conversion efficiency to drop to 80% of its starting value), for utility, commercial, or residential PV systems needs to be several decades in order to yield low-cost electricity, and thus it is not practical to directly measure it. It would be useful if T₈₀ could be predicted from the initial dynamics of a solar cell's performance, but until now no models have been developed to do so. In this work, we report the development of machine learning models to predict T₈₀ of ITO/NiO_x/CH₃NH₃PbI₃/C₆₀/BCP/Ag solar cells operating at maximum power point under 1-sun equivalent photon flux in air at varying temperatures and relative humidities. Efficiency losses are driven by short-circuit current and fill factor, indicating that photochemical reactions with O₂ and H₂O are a major contributor to degradation. Spatial patterns evident from in situ dark field optical microscopy also suggest that the electric field gradient at device edges plays a significant role in perovskite decomposition. Models are trained using a menu of features from three distinct categories: (i) measurements of the initial rates of change of device parameters, (ii) ambient conditions during operation (temperature & partial pressure of H₂O), and (iii) features based on underlying physics and chemistry. We show that a theory-based physiochemical feature derived from a model of the chemical reaction kinetics of the rate of degradation of CH₃NH₃PbI₃ is particularly valuable for prediction and was selected as the most dominant feature in the best performing models. With a dataset consisting of 45 degradation experiments with T₈₀ values ranging over a factor of almost 30, the model predicts T₈₀ with an average accuracy of about 40% on samples not used in training. This hybrid ML approach should be effective when applied to other compositions, device architectures, and advanced packaging schemes.

Continuous Flash Sublimation (CFS) is a vapor deposition technique that shows commercially relevant deposition rates, is amenable to continuous operation, and exceeds previously reported efficiencies for the same material class.

Understanding the chemical reactions that halide perovskite semiconductors undergo in the presence of moisture, oxygen, and light are essential to the commercial development of perovskite solar cells. The presentation will summarize our results and conclusions from the in-situ measurement optical transmittance, light scattering, quantitative spatially resolved photoluminescence, and photoconductivity (from which an ambipolar diffusion length is calculated) during the decomposition of MAPbI3 and two representative low and high bandgap perovskite compositions: (FA0.75,Cs0.25)(Pb0.50,Sn0.50)I3 and (FA0.80,Cs0.20)Pb(I0.83,Br0.17)3 in the presence of moisture, oxygen, and light (individually and combinations thereof). We report the quantitative chemical decomposition rate constants for based on optical absorbance above and below the perovskite bandgap as a function of the environmental species present. These data are used to derive a rate law for each material. These first-of-their-kind measurements reveal the dominant degradation pathways in each of these three archetypal perovskites. Briefly, for MAPbI3 we discovered a new chemical reaction pathway referred to as a water-accelerated photo-oxidation (WPO) pathway, which is the dominant chemical degradation pathway when light, oxygen, and humidity are present (and much faster than other dry photooxidation pathways) [1]. For the low bandgap (FA0.75,Cs0.25)(Pb0.50,Sn0.50)I3 kinetic measurements that a simple oxidation pathway is the most dominant [2]. More recent results from monitoring the decomposition dynamics of (FA0.80,Cs0.20)Pb(I0.83,Br0.17)3 reveal there are two dominant pathways, depending on the presence of oxygen and moisture, which lead to distinct degradation products. These latest results are from a forthcoming manuscript on this composition. [1] Siegler, Dunlap-Shohl, Meng, Yang, Kau, Sunkari, Tsai, Armstrong, Chen, Beck, Meila & Hillhouse, "Water-Accelerated Photooxidation of CH3NH3PbI3 Perovskite," J. Am. Chem. Soc. 2022, 144, 12, 5552-5561. [2] Meng, Sunkari, Meila & Hillhouse, "Chemical Reaction Kinetics of the Decomposition of Low Bandgap Tin-Lead Halide Perovskite Films and the Effect on the Ambipolar Diffusion Length," ACS Energy Lett. 2023, Accepted.

Understanding the degradation mechanism and quantifying the degradation rate of hybrid organic-inorganic halide perovskite (HP) materials are essential for achieving the commercialization of HP solar cells. Here, we focus on evaluating the reaction products and kinetics of the degradation of mixed Sn-Pb halide perovskite (FA0.75Cs0.25Pb0.5Sn0.5I3) thin films in response to oxygen, moisture, and illumination using in-situ measurements of optical transmittance, photoluminescence, X-ray diffraction, and UV-Vis-NIR spectroscopy. We determined that the decomposition occurs by a dry oxidation pathway (1x10-9 mol/(m2s) at 25 °C in air) and a water-accelerated oxidation pathway (3x10-9 mol/(m2s) at 25 °C in 50% RH air). In 50% RH air, the water-accelerated oxidation dominates at low temperatures, but the dry oxidation pathway (which has a higher activation energy) dominates at temperatures greater than 40 °C. A kinetic rate expression for the decomposition is determined using 32 degradations at different environmental conditions. The subsequent collection of an additional 33 degradation runs was used to evaluate the accuracy and robustness of the model with a mean test error of 18%. Further, we developed a predictive model of the decay of the diffusion length (predicting the time required for the diffusion length to decrease to 80% of its initial value) with a prediction test error of 24%. Out of 59 possible features, the chemical decomposition rate expression was selected as the most dominant feature in all 4-feature models with low prediction test error. The results highlight the importance and utility of quantitative measurements of perovskite degradation.

Halide perovskites have the potential to disrupt the photovoltaics market based on their high performance and low cost. However, the decomposition of perovskites under moisture, oxygen, and light raises concerns about service lifetime, especially because degradation mechanisms and the corresponding rate laws that fit the observed data have thus far eluded researchers. Here, we report a water-accelerated photooxidation mechanism dominating the degradation kinetics of archetypal perovskite CH₃NH₃PbI₃ in air under >1% relative humidity at 25 °C. From this mechanism, we develop a kinetic model that quantitatively predicts the degradation rate as a function of temperature, ambient O₂ and H₂O levels, and illumination. Because water is a possible product of dry photooxidation, these results highlight the need for encapsulation schemes that rigorously block oxygen ingress, as product water may accumulate beneath the encapsulant and initiate the more rapid water-accelerated photooxidative decomposition.