Transient Dipole Strategy Boosts Highly Oriented Self-Assembled Monolayers for Organic Solar Cells Approaching 21% Efficiency

Abstract

Self-assembled monolayers (SAMs) based on carbazole with minimal parasitic absorption, such as the most widely used [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz), dominate the high-performance hole transport layer (HTL) for conventional organic solar cells (OSCs). However, the small dipole moment of the 2PACz molecules results in weak molecular dipole-dipole interactions, leading to disordered dipole orientation and restricting work function modulation, which causes serious interfacial energy loss. Here, we grafted thiophene groups at both ends of the carbazole in 2PACz to obtain an SAM material (Th-Cz), which formed a transient resonance structure during thermal annealing, resulting in a twice-enlarged dipole moment. This strengthened molecular dipole-dipole interactions, facilitating ordered arrangement and dipole orientation of the Th-Cz film, contributing to a higher work function, which enhanced hole extraction and suppressed energy losses at the SAM/active layer interface. Additionally, van der Waals interactions between Th-Cz and the donors enabled the donor crystallizing before the acceptor, and this phenomenon is different from the cocrystallization observed in 2PACz-based active layers. This manipulation of crystallization dynamics favors vertical phase separation with a donor-rich phase at the bottom of active layers, leading to balanced charge-carrier mobilities. The resultant OSCs based on PM6:Y6 and D18-Cl:N3:AT-β2O with Th-Cz as HTL achieved power conversion efficiencies (PCEs) of 19.34% and 20.91% (certified 20.67%), respectively, setting a record PCE for the PM6:Y6-based OSCs and achieving the highest certified PCE for single-junction OSCs to date. Notably, Th-Cz also demonstrated exceptional compatibility with flexible OSCs, delivering a record PCE of 19.63%.