**3. Development of perovskite-structured photovoltaic materials**

Perovskite-structured photovoltaic materials can be divided into four groups according to the demand of the research objectives. The first group, organolead halide perovskite, is used to improve the power conversion efficiency up to its theoretical limit ~31%. The second group is lead-free perovskite. The purpose of the lead-free perovskite-structured material is to find an alternative to replace the toxic lead. The third group is lead-reduced perovskite-structured material. Owing to the unstable performance of lead-free perovskite, partial substitution of lead can not only reduce the lead content but also retain the power conversion efficiency. The last group, two-dimensional (2D) perovskite-structured material, with the presence of hydrophobic alkyl ammonium, exhibits good moisture resistance. In this section, we provide a report of the four types of perovskite-structured photovoltaic materials and their relevant applications.
