The Mechanism of Universal Green Antisolvent for Intermediate Phase Controlled High-Efficiency Formamidinium-Based Perovskite Solar Cells

Antisolvent-assisted crystallization is adopted widely in the preparation of double-cation [e.g., cesium/formamidinium (Cs/FA), formamidinium/methylammonium (FA/MA)] and triple-cation (e.g., Cs/FA/MA) FA-based perovskite solar cells (PSCs) to enhance their power conversion efficiencies (PCEs) and device stabilities. Although many antisolvents have been applied to treat various perovskites with different compositions, their crystallization mechanisms have remained unclear. In this study, we investigate the effects of a series of green antisolvents—namely the ethers diethyl ether, anisole, diisopropyl ether (DIE), and dibutyl ether—on the crystallization of perovskites. We found that the formation of the intermediate phase was heavily determined by the antisolvent’s polarity. Indeed, through judicious control of the antisolvent’s polarity, it was possible to form a pure intermediate phase, without a PbI2 nor perovskite phase. Upon thermal annealing, the crystallization of perovskites was improved. Understanding the mechanism of formation of the intermediate phase led us to identify DIE as a green antisolvent with universal perovskite compatibility, achieving champion PCEs of 20.05%, 20.15%, 21.26%, for Cs/FA, FA/MA, Cs/FA/MA, respectively. The PCE of large-area (1 cm2) PSCs reached 18.51%. Furthermore, the repeatability of these champion PCEs was greatly improved. The work contributes to the understanding of antisolvent’s polarity as an important factor affecting the formation of a pure intermediate phase of FA-based perovskite, which also offers the greener approach for the production of PSCs.