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 holetransporting 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 MAS-nI(3-y)Br(y) and MASnCl(3-z)Br(z) in the stoichiometric proportions of their precursors, confirmed by XRD. A device with MASnIBr(1.8)Cl(0.2) (SnCl2= 10%) showed the best photovoltaic performance: J(SC)= 14.0 mAcm(-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(1.8)Cl(0.2) cell. Transient PL decays showed the intrinsic problem of tinbased perovskites with average lifetimes less than 100 ps.