Multifunctional Ionic Liquid Enables the Grain Coarsening and Defect Passivation of CsPbBr₃ Films to Enhance the Performance of Perovskite Solar Cells

CsPbBr3-based perovskite solar cells (PSCs) have attracted increasing attention owing to their superhigh stability, ease of fabrication process, and compatibility with the simplified device structure. It is a golden rule to promote the efficiency of PSCs to approach their theoretical efficiency limit through fabrication of CsPbBr3 films with large grains and low defects. Herein, an ionic liquid of 1-ethyl-3-methylimidazoliumiodide (EMI) is introduced into a CsBr/H2O solution, and it is then spin-coated onto PbBr2 films to fabricate high-quality CsPbBr3 films. After optimizing the concentration of EMI in the CsBr/H2O solution, it can produce a significant grain coarsening effect, which is evidenced by the increase in mean grain size from 607 to 1070 nm under the assistance of EMI. On this foundation, EMI is also deposited onto the optimized CsPbBr3 films so as to passivate the surface defects. After optimizing the grain size and the surface defects by EMI, it suppresses the nonradiative recombination owing to the passivation effects of EMI, which is achieved through forming an interaction between the uncoordinated ions (Cs+ and Pb2+) and EMI. As a result, the power conversion efficiency of solar cells increases from 7.16% to 9.78% based on the simplified device structure of FTO/TiO2/CsPbBr3/Carbon, and it exhibits high stability in open air conditions. This work provides a feasible approach to improve the quality of CsPbBr3 films and their performance in solar cells, and it enriches the strategies to obtain highly efficient CsPbBr3 solar cells.