Self‐Assembled Molecules Based on a Spiro‐Core and Conjugated Linkers Enable 21% Efficiency in Organic Solar Cells

Conventional carbazole-derived self-assembled monolayers (SAMs) are widely utilized as hole-transport layers in organic solar cells (OSCs). However, the planar structure of the carbazole core, combined with the conformational flexibility and insulating nature of the alkyl linkers, often results in inadequate SAM coverage, loose molecular packing, and inefficient charge transport, thereby limiting device performance and operational stability. In this work, we develop a novel SAM molecule, SAF-Ph-PA, by employing a sterically bulky spiro core as the backbone and replacing the conventional flexible alkyl linker with a rigid phenylene group. Compared to its alkyl-linked counterpart SAF-PA, this newly designed SAM exhibits denser and more ordered molecular packing, as well as enhanced hole extraction and charge transfer efficiencies. In OSCs based on the PM6:BTP-eC9 system, SAF-Ph-PA achieves an excellent power conversion efficiency (PCE) of 20.3%, representing one of the highest reported values for this binary system. Furthermore, SAF-Ph-PA demonstrates outstanding performance in devices incorporating a PM6:FTCC-Br:BTP-eC9 blend, achieving a peak PCE of 21.0% (certified 20.68%). These findings highlight the critical importance of a synergistic design strategy that integrates both the head group and the linker in SAM engineering, paving the way toward further efficiency enhancements in OSCs.