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.
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