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 (MASnIBr2−xClx) 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 MASnI3−yBry and MASnCl3−zBrz in the stoichiometric proportions of their precursors, confirmed by XRD. A device with MASnIBr1.8Cl0.2 (SnCl2=10 %) showed the best photovoltaic performance: JSC=14.0 mA cm−2, VOC=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 MASnIBr1.8Cl0.2 cell. Transient PL decays showed the intrinsic problem of tin‐based perovskites with average lifetimes less than 100 ps. Tin in: Stable tin perovskites co‐crystalized with three different halide elements (I, Br, and Cl) were produced by the reaction of methylammonium (MA) iodide with SnCl2/SnBr2 mixtures at ratios equal to or less than 25/75. The best device made of a carbon‐based mesoscopic electrode and MASnIBr1.8Cl0.2 exhibited a PCE of 3.1 %.