Multifunctional\nIonic Liquid Enables the Grain Coarsening\nand Defect Passivation of CsPbBr<sub>3</sub> Films to Enhance the\nPerformance of Perovskite Solar Cells

CsPbBr<sub>3</sub>-based perovskite solar cells (PSCs)\nhave attracted\nincreasing attention owing to their superhigh stability, ease of fabrication\nprocess, and compatibility with the simplified device structure. It\nis a golden rule to promote the efficiency of PSCs to approach their\ntheoretical efficiency limit through fabrication of CsPbBr<sub>3</sub> films with large grains and low defects. Herein, an ionic liquid\nof 1-ethyl-3-methylimidazoliumiodide (EMI) is introduced into a CsBr/H<sub>2</sub>O solution, and it is then spin-coated onto PbBr<sub>2</sub> films to fabricate high-quality CsPbBr<sub>3</sub> films. After\noptimizing the concentration of EMI in the CsBr/H<sub>2</sub>O solution,\nit can produce a significant grain coarsening effect, which is evidenced\nby the increase in mean grain size from 607 to 1070 nm under the assistance\nof EMI. On this foundation, EMI is also deposited onto the optimized\nCsPbBr<sub>3</sub> films so as to passivate the surface defects. After\noptimizing the grain size and the surface defects by EMI, it suppresses\nthe nonradiative recombination owing to the passivation effects of\nEMI, which is achieved through forming an interaction between the\nuncoordinated ions (Cs<sup>+</sup> and Pb<sup>2+</sup>) and EMI. As\na result, the power conversion efficiency of solar cells increases\nfrom 7.16% to 9.78% based on the simplified device structure of FTO/TiO<sub>2</sub>/CsPbBr<sub>3</sub>/Carbon, and it exhibits high stability\nin open air conditions. This work provides a feasible approach to\nimprove the quality of CsPbBr<sub>3</sub> films and their performance\nin solar cells, and it enriches the strategies to obtain highly efficient\nCsPbBr<sub>3</sub> solar cells.