A One-Step Ionic Liquid Interface-to-Bulk Modification for Stable Carbon-Based CsPbI₃ Perovskite Solar Cells with Efficiency Over 15%

Hole transport layer-free, carbon-based, all-inorganic CsPbI₃ perovskite solar cells (PSCs) have exhibited great potential in photovoltaic applications owing to their low cost and excellent thermal stability. However, the low power conversion efficiency (PCE) hampers its development, mainly due to the existence of defects inside the CsPbI₃ film or at TiO₂ electron transport layer/CsPbI₃ interface. Herein, these issues were addressed through a facile TiO₂ post-treatment strategy using 1-butyl-3-methylimidazole hexafluorophosphate (BMIMPF₆) ionic liquid. First, BMIMPF₆ can passivate TiO₂/CsPbI₃ interface defects by forming strong bond between the electron-rich N atoms and uncoordinated ions. Second, BMIMPF₆-modified TiO₂ shows reduced hydrophilicity, inducing decreased heterogeneous nucleation and is favorable for obtaining high-quality CsPbI₃ film. Thirdly, the non-volatile BMIMPF₆ can diffuse to the perovskite film surface during annealing, further passivating defects located at perovskite grain boundaries and surface. Based on this one-step ionic liquid interface-to-bulk modification, the modified device achieves a champion PCE of 15.09%, which is 14% higher than the control device (13.27%). In addition, the modified device also shows enhanced long-term stability, which remains 96% of initial PCE after 30 days storage in dry air. The work demonstrates the superiority of multifunctional ionic liquid applied to all-inorganic carbon-based PSCs, providing a guidance for its commercialization.