Dual-Ion-Diffusion Induced Degradation in Lead-Free Cs2AgBiBr6Double Perovskite Solar Cells

Lead-free double perovskite Cs(2)AgBiBr(6)has attracted increasing research interest in addressing the toxicity and stability challenges confronted by lead halide perovskites. While most of the studies on this Cs(2)AgBiBr(6)material have been focusing on photovoltaic performance and potential applications, its long-term stability and degradation mechanism are well under-explored. Herein, high-quality Cs(2)AgBiBr(6)thin-films are developed for lead-free double perovskite solar cells with a decent efficiency of 1.91%. By exploring the ambient stability of these photovoltaic devices, it is found that the Cs(2)AgBiBr(6)exhibits a unique dual-ion-migration phenomenon, where Ag and Br ions gradually diffuse through the hole-transporting layer in the long-term operation. This phenomenon leads to the degradation of the Cs(2)AgBiBr(6)perovskite and subsequent device failure. Theoretical calculations indicate that low formation energies of the Ag and Br vacancies, and low diffusive energy barriers contribute to the dual-ion-migration effect. A possible mechanism involving a vacancy-mediated ion-migration is proposed to explain this phenomenon. These key findings are essential for halide double perovskites not only in providing a new knowledge base for further addressing the challenge of double perovskite stability, but also in extending their optoelectronic/electronic applications where mixed electronic, ionic and photonic properties may be desired.