Tin halide perovskites for efficient lead-free solar cells

Lead halide perovskites revolutionized photovoltaics thanks to their explosive growth in efficiency in just few years, faster than any other photovoltaic technology. Nevertheless, their long-term stability is the main obstacle to their commercialization and the safeness of the lead content of the cell is another insidious issue that is being underestimated by the scientific community. Some studies are currently suggesting that the bioavailability of the lead in the mixed organic-inorganic perovskites is higher than that of the lead commonly present in the soil. For this reason, it is important to explore and develop a safer alternative to lead halide perovskites, relying on the extremely vast knowledge accumulated on these compounds. Tin is the obvious choice for this task due to its similarity to lead but it has a lower bioavailability. Moreover, the bandgap of tin halide perovskites could ensure a better exploitation of the solar spectrum, leading to a higher Shockley–Queisser limit. At the current state of the art, tin-perovskites are still very far from the expected efficiencies. This is related to the same process that makes it so valuable for health. Tin is readily oxidized from Sn2+ to Sn4+ when exposed to air, forming very stable and inert compounds, that are very slowly uptaken by plants. This oxidation is very difficult to contrast and prevents the formation of stoichiometric perovskite. Many different approaches could be adopted to solve this issue and, in this chapter, we will give a comprehensive picture of this new and promising field. The chapter will be divided into three sections. In the first we will introduce ASnX3, illustrating the main features of these compounds in respect to lead counterparts. In the second part we will describe the most recent progress in the performances of ASnX3 devices. Finally, in the third section we will present the most promising approaches for the stabilization of such systems that could lead to stable and efficient ASnX3 devices.