Formation of Stable Tin Perovskites Co-crystallized with Three Halides for Carbon-Based Mesoscopic Lead-Free Perovskite Solar Cells.

We synthesized and characterized methylammonium (MA) mixed tri-halide tin perovskites (MASnIBr_{2− x}Cl_x) for carbon-based mesoscopic solar cells free of lead and hole-transporting layers. Varied SnCl₂/SnBr₂ ratios yielded tin perovskites with three halides (I, Br, and Cl) co-crystallized inside the tin-perovskite. When the SnCl₂ proportion was ≥50 % ( x≥1), phase separation occurred to give MASnI_{3− y}Br _y and MASnCl_{3− z}Br _z in the stoichiometric proportions of their precursors, confirmed by XRD. A device with MASnIBr_1.8Cl_0.2 (SnCl₂=10 %) showed the best photovoltaic performance: J_SC=14.0 mA cm⁻², V_OC=380 mV, FF=0.573, and PCE=3.1 %, and long-term stability. Electrochemical impedance spectra (EIS) show superior charge recombination and dielectric relaxation properties for the MASnIBr_1.8Cl_0.2 cell. Transient PL decays showed the intrinsic problem of tin-based perovskites with average lifetimes less than 100 ps. [ABSTRACT FROM AUTHOR]