How SnF₂ Impacts the Material Properties of Lead-Free Tin Perovskites

Lead-based halide perovskites (APbX₃) are fascinating optoelectronic materials. Because of toxicity issues of Pb, Sn-based halide perovskites are studied, although less so, as an alternative. Adding SnF₂ often improves the properties of Sn halide perovskite-based devices. This effect is usually ascribed to suppression of Sn²⁺ → Sn⁴⁺ oxidation and/or decreased Sn vacancy concentration. These effects will change the doping, sometimes in opposite directions. Here we review the effect of addition of SnF₂ during the formation of ASnX₃ layers as observed by different groups, both to the properties of the layers themselves and to photovoltaic cells made from these layers. SnF₂ can affect many different properties of the ASnX₃ perovskites, including film morphology, doping, control over formation of unwanted crystal phases, material stability to various factors, and energy level positions. It also improves (in general) the performance of photovoltaic cells made with these layers. Besides focusing on all these issues, we also describe possible doping scenarios for the perovskites, including some that do not appear to have been considered before and conclude that the doping mechanism depends strongly on whether the oxidation of Sn²⁺ to Sn⁴⁺ occurs during the materials preparation or after the film is formed, and if oxygen is involved.