Conjugated Thiophene Linker Design of the Self‐Assembled Monolayer Toward Highly Efficient and Stable Organic Solar Cells

ABSTRACT Achieving high power conversion efficiency (PCE) and operational stability remains a critical challenge of organic solar cells (OSCs). While the 2PACz self‐assembled monolayer (SAM) offers suppressed parasitic absorption and facilitated charge collection over the routinely‐used PEDOT:PSS hole‐transport layer (HTL), their operational stability is often limited by the vulnerable chemical structure under high‐energy photon illumination. In this work, we design and synthesize two SAMs, PhPACz and ThPACz, whose flexible alkyl linker between phosphonic acid and carbazole of 2PACz is substituted by rigid phenyl or thiophene to manipulate the HOMO and LUMO orbitals and further mediate the electron density distribution of SAM, thereby promoting charge delocalization at the interface and enhancing photochemical stability. The ThPACz SAM delivers suppressed oxygen defects on the ITO surface and largely enhanced the bond dissociation energy at the vulnerable C‐N bond (from 72.2 kcal/mol in 2PACz to 97.3 kcal/mol in ThPACz), translating to a champion efficiency of 20.5% in D18:L8‐BO binary OSCs, alongside a significantly extended T 80 lifetime (over 7 times) under either visible or 365 nm UV light, outperforming the 2PACz based benchmark. This study underscores the critical role of conjugated linkers in designing high‐performance, stable SAMs for next‐generation OSCs.