Stability and Decomposition Kinetics of Alloyed Perovskite Semiconductors for Next-Generation Photovoltaics

Abstract

Organic-inorganic metal halide perovskite semiconductors have captured the attention of the global scientific community for the past decade plus due to their excellent optoelectronic characteristics and ease of fabrication. These characteristics make them promising for application in low-cost, high efficiency solar cells. Despite their many favorable characteristics, the inherent instability of halide perovskites remains a significant hurdle that presently inhibits their commercialization. Although much work has been done to gain insight into stability of the most promising perovskite compositions, a comprehensive understanding of the chemical reaction landscape that governs decomposition has not been fully elucidated. The work presented here is focused on measuring and modeling decomposition reactions and, where appropriate, linking them to declines in relevant physical properties. First, I study the decomposition of FA0.8Cs0.2Pb(I0.83Br0.17)3, a promising composition for application in perovskite-on-silicon tandem solar cells, under combined light and heat stress. These conditions represent the limiting case of a well encapsulated solar cell, and thus, such tests probe the most basic hurdles that a viable absorber layer material must pass for photovoltaic applications. Through combined structural and optical characterization, we find that films degrade rapidly under illumination in an inert N2 environment, with notable generation of reduced lead species. Using in situ measurements of ambipolar diffusion length and photoluminescence, I show that film optoelectronic degradation is altered when oxygen is present in the ambient at sufficient concentrations. I then measure the kinetics of reduced lead species formation with in situ sub bandgap absorption measurements and model the observed kinetics. Next, I extend the kinetic studies of FA0.8Cs0.2Pb(I0.83Br0.17)3 thin films to the photooxidation regime. Under ambient conditions in which the partial pressure of oxygen is >3 kPa, photooxidation decomposition dominates. In situ above bandgap absorbance measurements reveal two dominant modes of photooxidation: dry-photooxidation and water-accelerated photooxidation. Interestingly, the presence of water vapor in the ambient is found to increase the rate (posited and modeled to be via water-accelerated photooxidation) at lower temperatures but decreases the rate (posited and modeled to be the result of hydrate formation) at higher temperatures compared to dry conditions. I then derive a rate expression and fit relevant parameters for the three processes assumed to be at play: dry-photooxidation, water-accelerated photooxidation, and hydrate formation. The derived model fits the data well across a range of conditions (<15% median error). Finally, I consider the effects that varied bromide X-site concentration has on stability, an important lever for tuning the bandgap of halide perovskites. I find that, under combined light and heat stress, the rate of increase of sub bandgap absorption, an indicator of reduced lead species formation, is faster in compositions with greater X-site bromine concentration. This finding is correlated with in situ measurements of ambipolar diffusion length during light heat stress. I also test the stability of the films in the presence of I2 vapor, a known byproduct of light induced decomposition and photooxidation, finding that the inverse is true: higher bromide fractions impart greater stability. Collectively, the work included in this dissertation demonstrates significant progress toward the understanding of the full decomposition landscape of alloyed lead halide perovskites.