Dual‐Ion‐Diffusion Induced Degradation in Lead‐Free Cs2AgBiBr6 Double Perovskite Solar Cells

Lead‐free double perovskite Cs2AgBiBr6 has attracted increasing research interest in addressing the toxicity and stability challenges confronted by lead halide perovskites. While most of the studies on this Cs2AgBiBr6 material have been focusing on photovoltaic performance and potential applications, its long‐term stability and degradation mechanism are well under‐explored. Herein, high‐quality Cs2AgBiBr6 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 Cs2AgBiBr6 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 Cs2AgBiBr6 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. A dual‐ion‐migration phenomenon and its underlying possible mechanism are reported for the lead‐free double perovskite Cs2AgBiBr6, where the diffusive behavior of both Ag and Br contribute significantly to the degradation of the perovskite thin‐film and long‐term operational stability of the Cs2AgBiBr6 solar cells.