Abstract: Light soaking (LS) is widely employed to optimize CdTe-based solar cells, yet its microscopic origin remains controversial due to the entangled roles of illumination and thermal activation. Here, we establish a decoupled experimental strategy that independently disentangles light exposure and heating, revealing their fundamentally distinct and competing effects on defect dynamics in CdSeTe solar cells. Illumination alone induces a backward drift of Cu^-Cd acceptors under the light-enhanced built-in field, reducing apparent hole density and suppressing open-circuit voltage. In contrast, thermal activation promotes Cu redistribution toward the front junction and eliminates V_Cd^2- deep recombination centers, partially restoring device performance. Remarkably, when illumination and heating are simultaneously applied, a synergistic process emerges, combining enhanced carrier density, suppressed deep-level traps, and the restoration of selenium-induced passivation. This cooperative defect reconfiguration simultaneously elevates open-circuit voltage and fill factor, yielding a champion efficiency of 19.5 %. Our results establish a physical framework for light soaking in CdSeTe photovoltaics and highlight defect-passivation coupling as a key lever for post-fabrication performance optimization.

Key words: CdSeTe solar cells, Light soaking, Cu migration, Se passivation