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

Abstract Hole transport layer‐free, carbon‐based, all‐inorganic CsPbI 3 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 CsPbI 3 film or at TiO 2 electron transport layer/CsPbI 3 interface. Herein, these issues were addressed through a facile TiO 2 post‐treatment strategy using 1‐butyl‐3‐methylimidazole hexafluorophosphate (BMIMPF 6 ) ionic liquid. First, BMIMPF 6 can passivate TiO 2 /CsPbI 3 interface defects by forming strong bond between the electron‐rich N atoms and uncoordinated ions. Second, BMIMPF 6 ‐modified TiO 2 shows reduced hydrophilicity, inducing decreased heterogeneous nucleation and is favorable for obtaining high‐quality CsPbI 3 film. Thirdly, the non‐volatile BMIMPF 6 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.