Ag-excess in situ elemental reaction strategy yields defect-suppressed Cu2AgBiI6 thin films for lead-free photovoltaics

Inorganic Cu 2 AgBiI 6 (CABI) solar cells represent promising lead-free alternatives but are currently hindered by limited device performance arising from high defect densities. The fundamental origin lies in the solubility mismatch among precursors—specifically, the near-insolubility of AgI in standard processing solvents, which leads to severe Ag + depletion during crystallization. This stoichiometric imbalance generates abundant cation vacancies that act as non-radiative recombination centers. To circumvent this limitation, we introduce an in situ elemental reaction strategy employing controlled Ag excess. This method elevates the steady-state Ag + activity during crystallization, effectively suppressing cation vacancy formation and reducing deep-level trap density. Consequently, photovoltaic devices based on an FTO/c-TiO 2 /m-TiO 2 /Cu 2 AgBiI 6 @AgI/Carbon architecture achieve a power conversion efficiency of 1.09%, nearly three times that of the control device (0.39%). This work demonstrates that defect chemistry engineering through controlled Ag excess is an effective route to optimizing Cu-Ag-Bi-I film quality for eco-friendly photovoltaics.