Reconstruction of the (EMIm)MA_1-Pb[(BF₄)I_1-]₃ Interlayer for Efficient and Stable Perovskite Solar Cells

Defective grain boundaries (GBs) and surface trap states are detrimental to the efficiency and stability of perovskite solar cells (PSCs). In this research, ionic liquid (IL) is used to control the defect states at the perovskite surface and GBs. The newly formed (EMIm)MA_1-Pb[(BF₄)I_1-]₃ interlayer promotes secondary grain growth to diminish GBs; besides, EMIM⁺ and BF₄^- fill the vacancies of MA⁺ and I^- and also passivate undercoordinated Pb²⁺ trap states. The newly formed interface largely reduces the nonradiative recombination, thus enhancing the solar-cell performance to 19.0% (AM 1.5, 1 sun) with higher photovoltage and fill factor than the control device. Due to the hydrophobicity of the (EMIm)MA_1-Pb[(BF₄)I_1-]₃ interlayer, the unencapsulated device stability in 30 days is much better than the control device under relative humidity (RH) = 20%. This work highlights IL-induced secondary grain growth and a defect passivation method for efficient and stable PSCs.