Ionic Liquid-Assisted Crystallization Strategy Enables Simultaneous Regulation of Microstructure and Trap States for...

Developing a feasible and effective crystallization approach to simultaneously amend microstructure and trap states in antimony sulfoselenide (Sb₂(S,Se)₃) absorber is extremely crucial and challenging for high-efficient solar cells. Herein, a regulation strategy is proposed to control crystallization process of Sb₂(S,Se)₃ using ionic liquids (ILs) consisted of halide (X) anions (Cl^-, Br^-, and I^-) and [BMIM]⁺ cations. In particular, the [BMIM]Br creates a liquid microenviroment on Sb₂(S,Se)₃ surface before decomposition, accelerating the mass transfer, which induces micron-size grains. Moreover, the [BMIM]Br can promote the [211]-oriented growth via stronger adsorption on (211) facets of Sb₂(S,Se)₃. Additionally, the inhibited S and Se loss results in a near stoichiometric composition of Sb₂(S,Se)₃ film, which greatly raises the hole concentration and optimizes the band alignment. Very important transformation from severe antisite defect Sb_S to slight vacancy defect V_Se2 remarkably suppresses the non-radiative recombination. As a result, with more effective carrier transport and collection, the [BMIM]Br-modulated device achieves a 10.89% efficiency and a 72.74% fill factor, which are separately one of the highest values for Sb₂(S,Se)₃ solar cells so far. This work shines a new light on breaking the bottleneck in the development of Sb₂(S,Se)₃ solar cells.