Synergistic Electron-Deficient Surface Engineering: A Key Factor in Dictating Electron Carrier Extraction for Perovskite Photovoltaics

Work function modulation of transparent conductive oxides via self-assembled monolayers (SAMs) facilitates efficient hole or electron extraction in optoelectronic devices. However, recent SAMs for perovskite solar cells (PSCs) diverge from traditional interfacial dipole orientation design principles, instead leveraging electron-rich and electron-deficient surface modifications. In light of these discrepancies, this study systematically analyses electron-deficient materials of varying strength, revealing the dominance of surface modifications over interfacial dipole orientation. Specifically, modulating the electron-withdrawing strength by replacing the carboxylic acid group (Bpy-COOH) with a cyanoacrylic acid moiety (Bpy-CAA) in dual-functional bipyridine-based electron-selective molecular layers (ESMLs) enhances adsorption, electron extraction, and passivation in n-i-p PSCs. Consequently, Bpy-CAA devices achieve 23.98% efficiency, surpassing Bpy-COOH-based devices (23.20%), and maintain an impressive 21.63% efficiency in 1 cm² cells, the highest reported for 1 cm² n-i-p PSCs utilizing organic ESMLs. A remarkable efficiency of 26.00% is achieved by integrating Bpy-CAA as an interfacial layer into SnO₂/ESML/perovskite contacts while adapting this architecture into four-terminal perovskite/silicon tandem solar cells (4T-P/STSCs) yields an impressive efficiency of 30.83%, ranking among the highest reported efficiencies for 4T-P/STSCs. Overall, this work demonstrates that the electronic nature of the molecule is more decisive than dipole orientation for efficient electron extraction, and tailoring the dual-functional ESMLs effectively facilitated the development of efficient single-junction PSCs and 4T-P/STSCs.