Recent Advances in Tin-Based Perovskite Solar Cells

Since 2009, organic-inorganic halide perovskites have been widely studied in the field of optoelectric materials due to their unique optical and electrical properties. Pb-based halide perovskite solar cells (PSCs), in particular, currently have a record efficiency of 25.2%, thus showing strong potential in commercialization. However, the market prospects of PSCs have been hampered by the toxicity of lead-based materials. Therefore the seeking of less toxic and environmentally friendly elements that can replace Pb is of great interest. Tin-based perovskites are the most promising choice at present due to its similar electronic configuration as Pb, and can even have more superior semiconductor properties. As a rising star of lead-free perovskite solar cells, tin-based PSCs have drawn much attention and made promising progress during the past few years. However, it is still challenging to obtain efficient and stable tin-based PSCs because of the low defects formation energy and the oxidation of bivalent tin. Among all Pb-free perovskite materials that show photovoltaic performance, formamidinium tin tri-iodide (FASnI3) based PSCs are the most promising because of the suitable band gap, low exciton bind energy, and high carrier mobility. The main drawbacks of tin-based perovskite material are its instability because of the easy oxidation of Sn2+ into Sn4+ and high dark current which arises from high p-type carrier concentration. The latter originates from the low formation energy of Sn vacancies. Many strategies have been developed to overcome these problems and promote the performance of tin-based PSCs. On one type of pursuit to avoid the oxidation of Sn2+, reduction additives (e.g., SnF2, pyrazine, hydrazine vapor, hydroxybenzene sulfonic acid or its salt, and Tr-conjugated polymer) and solvent-free processing have been introduced and shown to be effective up to a point. In another type, Cs or Br alloying and construction of low-dimensional structures in tin-based perovskite have also been shown to be promising. In this review, the optical and electrical properties of tin-based perovskite are systematically discussed. And then, the film fabrication methods and different device architectures of tin-based PSCs are summarized. Finally, the current challenges and a future outlook for tin-based PSCs are discussed.