Role of bifunctional additives towards highly efficient and stable tin perovskite solar cells

Tin halide perovskite (THP) materials are promising alternative candidates for lead-free hybrid perovskites for photovoltaic applications owing to their narrow bandgap (1.2–1.4 eV), high charge carrier mobilities, and low exciton binding energies. However, THP is highly oxidizable and always experiences uncontrolled rapid crystallization at room temperature, hampering the performance and stability of THP-based solar cells (THP-SCs) For quite some time, tin halides and tin powder have been used as tin (Sn2+) compensators to suppress Sn2+ oxidation in THP-SCs. However, their performance did not improve significantly owing to the phase segregation caused by the tin-based additives, which affected the morphology of the THP films. Recently, bifunctional additives (BAs) containing different functional groups have shown significant potential for alleviating Sn2+ oxidation and regulating rapid crystallization. This review highlights the role of BAs in obtaining highly efficient and stable THP-SCs, first by assessing the origins of Sn2+ oxidation and fast crystallization in THPs films, followed by a summary of the reports available on tin-based additives for THP-SCs. The need for BAs for THPs is then discussed, including their influence on Sn2+ oxidation, morphology, optoelectronic properties, and overall device stability. Finally, future perspectives are provided to further improve the efficiency and stability of THP-SCs via BA treatment.