Spatial Configuration Regulation of Dicyanide Molecules to Enhance the Comprehensive Performance of Carbon-Based Hole-Transport-Layer-Free CsPbI ₂ Br Solar Cells

Carbon-based hole-transport-layer-free CsPbI2Br solar cells have garnered significant attention due to their good stability and suitability as top cells in tandem devices. However, CsPbI2Br cells are prone to forming numerous defects, such as uncoordinated Pb2+ at grain boundaries during annealing, resulting in power conversion efficiency (PCE) of the corresponding photovoltaic devices far below their theoretical limit. To passivate defects in CsPbI2Br, molecular additives with different spatial conformations, including malononitrile (MDN), 1,3-dicyanobenzene (1,3-DCB), and 2,7-dicyanonaphthalene (2,7-DCN), are investigated. Theoretical calculations and experimental characterizations jointly demonstrate that, compared to alkyl-chain molecular frameworks, aromatic rings preferentially induce electron donation due to their π−π conjugation effect with terminal cyano groups (−CN). The π−π conjugation effect is more pronounced in naphthalene rings, enabling stronger passivation of uncoordinated Pb2+. Furthermore, the spatial distance between the two −CN in 1,3-DCB and 2,7-DCN molecules achieves an exquisite geometric matching with adjacent uncoordinated Pb2+ in the CsPbI2Br lattice. Thus, through the dual synergistic effects of electron donation and geometric matching, 2,7-DCN promotes the formation of a high-quality film with excellent crystallinity and low surface roughness. The champion PCE of the optimized device is enhanced from 11.80% for the control device to 15.87%, accompanied by a significant improvement in long-term stability.