Dual‐Ion‐Diffusion Induced Degradation in Lead‐Free Cs 2 AgBiBr 6 Double 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.