Size mismatch induces cation segregation in CsPbI₃: Forming energy level gradient and 3D/2D heterojunction promotes the efficiency of carbon-based perovskite solar cells to over 15%

The application of CsPbI₃ inorganic perovskite in carbon-based perovskite solar cells without hole transporter (C-PSCs) is expected to enhance device stability. However, due to the lower hole selectivity of carbon electrode than that of hole transporter, the efficiency of CsPbI₃ C-PSCs is significantly suppressed. Herein, size mismatch-induced cation segregation in CsPbI₃ is studied and employed to solve the problem. After treating CsPbI₃ with CsX (X[dbnd]Br, Cl and F) ethanol solutions, the larger size mismatch between X^- and I^- ions would induce the more obvious cation segregation. The moderate size mismatch between I^- and Cl^- ions allows a partial substitution of I^- ions with Cl^- ions in CsPbI₃ to form energy level gradient, and the CsCl residue on the surface tends to react with CsPbI₃ to generate 2D Cs₂PbI₂Cl₂ nanosheets, serving as an electron blocking layer. The synergistic effect of the energy level gradient and the electron blocking layer well improves hole selectivity at CsPbI₃/carbon interface to reduce carrier recombination loss. As a result, the C-PSCs based on the CsCl-treated CsPbI₃ achieve an efficiency of 15.23%, a record value for inorganic C-PSCs.