Pseudo-Halide Perovskite Solar Cells

Perovskite solar cells (PSCs) have achieved certified power conversion efficiency (PCE) over 25%. Though their high PCE can be achieved by optimizing absorber layer and device interfaces, the intrinsic instability of perovskite materials is still a key issue to be resolved. Mixed-halide perovskites using multiple halogen constituents have been proved to improve robustness; however, the anion at the X site in the ABX(3) formula is not limited to halogens. Other negative monovalent ions with similar properties to halogens, such as pseudo-halogens, have the opportunity to form perovskites with ABX(3) stoichiometry. Recently, thiocyanates and formates have been utilized to synthesize stable perovskite materials. This review presents the evolution of pseudo-halide perovskite solar cells in the past few years. The intrinsic properties, their effects on crystal structure, and bandgap engineering of the pseudo-halide perovskites are summarized. Various thiocyanate compounds applied in the fabrication of perovskite solar cells are discussed. The fabrication process, film formation mechanism, and crystallinity of pseudo-halide perovskites are elucidated to understand their effects on the photovoltaic performance and device stability. Other applications of pseudo-halide perovskites are summarized in the final section. Lastly, this review concludes with suggestions and outlooks for further research directions.