Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode

We report the synthesis and characterization of alloyed Sn–Pb methylammonium mixed-halide perovskites (CH 3 NH 3 Sn y Pb 1– y I 3– x Cl x ) to extend light harvesting toward the near-infrared region for carbon-based mesoscopic solar cells free of organic hole-transport layers. The proportions of Sn in perovskites are well-controlled by mixing tin chloride (SnCl 2 ) and lead iodide (PbI 2 ) in varied stoichiometric ratios ( y = 0–1). SnCl 2 plays a key role in modifying the lattice structure of the perovskite, showing anomalous optical and optoelectronic properties; upon increasing the concentration of SnCl 2, the variation of the band gap and band energy differed from those of the SnI 2 precursor. The CH 3 NH 3 Sn y Pb 1– y I 3– x Cl x devices showed enhanced photovoltaic performance upon increasing the proportion of SnCl 2 until y = 0.75, consistent with the corresponding potential energy levels. The photovoltaic performance was further improved upon adding 30 mol % tin fluoride (SnF 2 ) with device configuration FTO/TiO 2 /Al 2 O 3 /NiO/C, producing the best power conversion efficiency, 5.13%, with great reproducibility and intrinsic stability.