Alkali Metal Ion-Regulated Lead-free, All-Inorganic Double Perovskites for HTM-free, Carbon-Based Solar Cells

Quaternary Cs2AgBiBr6 perovskites have been considered as a potential candidate to simultaneously resolve the lead toxicity and instability issues of unprecedented organic–inorganic hybrid halide perovskites. Unfortunately, the photovoltaic efficiency is still lower owing to the great challenge to make high-quality Cs2AgBiBr6 film with fewer defects. Herein, we demonstrate alkali metal ions including Li+, Na+, K+, and Rb+ as mediators to regulate the crystal lattice and film quality of Cs2AgBiBr6 perovskites. A less-pinhole perovskite film is obtained by precisely controlling the doping dosage and element species, significantly reducing the defects. When assembled into a hole-transporting material-free, carbon-electrode perovskite solar cell, a significantly enhanced efficiency of 2.57% compared to the undoped device with 1.77% efficiency has been achieved owing to the suppressed shunt current loss. Additionally, this device displays superior tolerance under high-temperature and air conditions without encapsulation, providing new opportunities to promote the future development of lead-free Cs2AgBiBr6 perovskites in the photoelectric field.