A Spiro‐Configured Self‐Assembled Molecule as Hole Transport Material for Organic Solar Cells Featuring High‐Efficiency and Universality

Abstract Self‐assembled molecules (SAMs) have emerged as promising alternatives to conventional hole transport layers (HTLs) in organic solar cells (OSCs), owing to their ability to finely tune interfacial energetics and improve charge selectivity. In this work, a spiro‐configured SAM molecule, 4PA‐SAcF, designed as a high‐performance HTL is reported for OSCs. Compared to its non‐spiro analog 4PA‐DMAc, 4PA‐SAcF exhibits a larger dipole moment, deeper HOMO level, and enhanced electrical conductivity. More importantly, its spiro backbone facilitates stronger intermolecular interactions and ordered molecular packing, as confirmed by single‐crystal X‐ray diffraction. These features result in compact and uniform interfacial layers with reduced defect density and improved hole extraction. As a result, PM6:Y6‐based OSCs employing 4PA‐SAcF delivered a power conversion efficiency (PCE) of 19.52%, which is among the highest values reported for this material combination. Furthermore, 4PA‐SAcF demonstrates versatility as a HTL for improved photovoltaic performance across various non‐fullerene systems, with a PCE of 19.90% acquired in D18:L8‐BO system and 20.37% achieved in a quaternary system. These results confirm the broad applicability and high performance of 4PA‐SAcF as a versatile interfacial material. This study highlights the potential of spiro‐configured organic semiconductors as next‐generation SAM‐based HTLs and provides a rational molecular design strategy for advancing high‐efficiency OSCs.