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.