Dual Role of Self-Assembled Monolayers: Interfacial Dipoles and Charge Tunneling in High-Efficiency Inverted Perovskite Solar Cells

High efficiencies of inverted perovskite solar cells that use self-assembled monolayers (SAMs) are closely tied to unresolved interfacial mechanisms. This study decouples the dual functions of carbazole-based SAMs featuring tailored alkyl chains. Kelvin probe force microscopy confirms that an interfacial dipole field, enhanced by longer chains, suppresses recombination and increases open-circuit voltage ( V OC ). In parallel, conductive atomic force microscopy shows that the alkyl chain itself serves as a tunneling barrier. Longer chains impede hole extraction, resulting in saturated tunneling current and a consequent decline in the maximum power point voltage ( V max ). This inherent trade-off defines the dual roles of SAMs: enhancing V OC through the dipole effect while mediating charge transfer via tunneling. An optimal balance is achieved with a four-carbon alkyl chain (4PACz), which combines efficient tunneling with strong passivation. Our findings provide a new design principle that co-optimization of dipole strength and tunneling probability is essential for developing high-performance SAMs.