Enhancing the Performance of Perovskite Solar Cells with CPMIMBF4 Ionic Liquid Additives

Perovskite solar cells (PSCs) have attracted widespread attention due to their excellent photovoltaic performance. The two-step solution method for perovskite films offers ease of operation and reproducibility compared to the one-step solution method. However, the crystallization process of perovskite is difficult to control, resulting in low-quality perovskite films. Herein, we introduce an ionic liquid additive of 1-cyanopropyle-3-methylimidazolium tetrafluoroborate (CPMIMBF4) into a PbI2 precursor to improve the device performance. The CPMIMBF4 additive serves as a bridge between PbI2 and dimethyl sulfoxide (DMSO), effectively inhibiting the rapid nucleation rate of PbI2 and promoting the formation of a PbI2 film with high crystallinity, which increases the ability of PbI2 to react with organic ammonium salt solutions. By using the CPMIMBF4 additive, we obtained the optimal (001)-oriented perovskite film, enhanced the quality of the perovskite film, and reduced the trap density of the devices. And the imidazole cations (CPMIM+) remain on the surface of the perovskite, forming a hydrophobic protective layer and passivating uncoordinated Pb2+. At the same time, the BF4- anions fill the vacancy left by I-. Ultimately, we achieved a power conversion efficiency (PCE) of 24.25% for PSCs on TiO2 substrates. This study provides a perspective for the application of multifunctional ionic liquids in PSCs.