Selenium Migration and Local Structures in Cu‐Doped CdSeTe Solar Cells after Aging

Abstract Selenium grading plays a critical role in state‐of‐the‐art Cadmium Telluride photovoltaic cells by enhancing long‐wavelength absorption and extending minority carrier lifetimes —key to enabling the current performance record of 23.08%. However, very little is understood about selenium motion. In this study, a comprehensive, multimodal, and multiscale approach is employed to investigate Se migration and local structural changes in copper (Cu)‐doped CdSeTe solar cells subjected to accelerated stress. X‐ray fluorescence (XRF) microscopy shows unexpected levels of Se diffusion after 500 h under heat (75°C) and light (0.8 suns, 80 mW / cm 2 ), suggesting the coexistence of fast and slow diffusion channels even at low temperatures, with unexpectedly low activation energies (<0.85 eV). X‐ray Absorption Near Edge Structure (XANES) analysis indicates a preferential migration of Se atoms to anionic lattice sites and a reduction in Se‐Cl co‐passivation at Te‐terminated dislocation cores. These findings point to a reconfiguration of Se local environments and highlight the potential role of extended structural defects in enabling Se transport at low temperatures. Additionally, XANES results suggest that the presence of metallic Cu across the absorber layer may contribute to back‐contact degradation and reduced hole density in both fresh and aged devices.