Constructing an Interfacial Gradient Heterostructure Enables Efficient CsPbI₃ Perovskite Solar Cells and Printed Minimodules

Abstract Severe nonradiative recombination originating from interfacial defects together with the pervasive energy level mismatch at the interface remarkably limits the performance of CsPbI₃ perovskite solar cells (PSCs). These issues need to be addressed urgently for high‐performance cells and their applications. Herein, an interfacial gradient heterostructure based on low‐temperature post‐treatment of quaternary bromide salts for efficient CsPbI₃ PSCs with an impressive efficiency of 21.31% and an extraordinary fill factor of 0.854 is demonstrated. Further investigation reveals that Br⁻ ions diffuse into the perovskite films to heal undercoordinated Pb²⁺ and inhibit Pb cluster formation, thus suppressing nonradiative recombination in CsPbI₃. Meanwhile, a more compatible interfacial energy level alignment resulting from Br⁻ gradient distribution and organic cations surface termination is also achieved, hence promoting charge separation and collection. Consequently, the printed small‐size cell with an efficiency of 20.28% and 12 cm² printed CsPbI₃ minimodules with a record efficiency of 16.60% are also demonstrated. Moreover, the unencapsulated CsPbI₃ films and devices exhibit superior stability.