Br-Related Local Structural Dynamics Associated with Reversible Photovoltaic Evolution in Mixed-Halide Perovskite Solar Cells Revealed via Br K-Edge X-ray Absorption Spectroscopy

Abstract

The microscopic origin of photovoltaic performance evolution in mixed-halide perovskite solar cells (PSCs) remains unclear because direct observation of element-specific local structural evolution has been limited. In this study, the relationship between photovoltaic performance and local halide structural evolution was investigated via Br K-edge and I L3-edge X-ray absorption spectroscopy. Br K-edge spectral features exhibited systematic changes during repeated measurement/storage cycles that correlated strongly with normalized photovoltaic performance. Composition-dependent analysis revealed that the correlation between Br-related spectral evolution and photovoltaic performance was the strongest at an intermediate Br content (35% Br), whereas no comparable composition dependence was observed in the I L3-edge response. A state-transition model is hypothesized to describe the relationship between local Br structural evolution and photovoltaic performance evolution. These results demonstrate that photovoltaic performance evolution is more closely associated with Br-related local structural evolution than with I-related spectral changes, providing critical insights into the microscopic processes associated with photovoltaic performance evolution in mixed-halide PSCs.