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

Abstract

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_1.8 Cl_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_1.8 Cl_0.2 cell. Transient PL decays showed the intrinsic problem of tin-based perovskites with average lifetimes less than 100 ps.

Keywords: carbon electrodes; halides; perovskites; solar cells; tin.