Yuhuan Meng

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.

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.

Bismuth triiodide is a lead-free direct wide-bandgap solution-processable semiconductor that could be an alternative to lead-based perovskites in tandem or multijunction solar cells. However, the power conversion efficiency of single-junction BiI₃ solar cells remains low. Here, we determine the main loss mechanisms of BiI₃ solar cells in both n-i-p and p-i-n architectures. Overall, p-i-n devices have higher power conversion efficiency than that of n-i-p. It is found that n-i-p devices have higher (and significant) non-radiative recombination at the interface of the BiI₃/transport layer, resulting in a lower open-circuit voltage than p-i-n devices. Moreover, the high series resistance (>70 Ω cm²) and a low average electron–hole diffusion length (∼60 nm) contributes to the low short-circuit current density (2) and fill factor (3 can increase the diffusion length of BiI₃ to improve the performance of BiI₃ solar cells. Solar cells with the configuration ITO/NiOx/Li:BiI₃/PC₆₁BM/bis-C₆₀/LiF/Ag obtain a power conversion efficiency of 1.3% under AM 1.5 G illumination. The deep understanding of the main loss mechanisms of this work paves the way for future optimization of BiI₃ solar cells.

A new approach to stable, low resistance inexpensive printed flexible conductive inks is proposed. Silver ink has been studied extensively and commercialized for applications in printed electronics due to the inherent high conductivity and stability of silver, even in particulate-based percolation networks processed at temperatures compatible with low cost polymer films such as polyethylene terephthalate (PET). Recent interest in flexible and even stretchable circuits, however, has presented new challenges for particle-based inks as mechanical strains can result in the opening of critical particle-to-particle contacts. Here we report a facile, low cost method for the single step synthesis of stable, printable nanoscale Ag₂O-decorated Ag flake inks which can be converted to highly conductive Ag films at 150°C curing temperature without the use of limited shelf life organometallics or low metal loading metal nanoparticles to modify the interface between silver flakes. Analysis indicates that decoration of Ag flakes with Ag₂O NPs improves the conductivity and flexibility of printed silver films by forming bridging interconnections between Ag flakes after low temperature reduction of the Ag₂O NPs. In this work, printed nano-decorated silver conductors with starting oxide to metal weight ratios of 5:95 exhibited lateral resistivities lower than 1.5×10^-5 Ω cm, which was 35% less than films derived from undecorated Ag flake inks of the same total Ag loading and binder system. This resistivity difference increased to 45% after cyclic bend testing showing increased resilience to repeated flexing for the nano-decorated inks.