Enlarging crystal grains with ionic liquid to enhance the performance of perovskite solar cells

The grain boundaries of perovskite polycrystalline are defect region that not only provides carrier recombination sites but also introduces device degradation pathways. Efforts to enlarging the grain size of perovskite film and reducing its grain boundary are crucial for highly efficient and stable PSCs. Here, we applied an ionic liquid molecule BMIMBF4 to passivate the surface of the perovskite film. We find that the ionic liquid BMIMBF4 helps to reduce the activation energy for grain boundary migration and facilitate the grain growth, leading to significantly enlarged grain in the perovskite film. The reduction of grain boundary regions suppresses the non-radiative recombination from the boundary defects, causing for longer carrier lifetime in the perovskite film. The improved quality of the perovskite film results in an enhancement of the PCE to 20.4%. Under continuous illumination for 500 h, the PSCs remains 84% of its initial efficiency.