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 (XBr, 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.