Fused Terminal Group p‐Type SAMs for Organic Photovoltaics: Alkyl Chain Length Effects

Fused aromatic self‐assembled monolayers (SAMs) have emerged as effective p‐type contacts for organic solar cells, but the role of alkyl linker length in such bulky systems remains unclear. Here, two phosphonic acid SAMs bearing an identical fused phenanthro[1,10,9,8‐cdefg]carbazole (PCz) terminal group, PCz‐C2PA and PCz‐C4PA, were synthesized and implemented as hole‐selective layers in PM6:PY‐DT bulk‐heterojunction devices with an ITO/SAM/PM6:PY‐DT/PDINN/Ag architecture. The two SAMs exhibit nearly identical frontier energy levels and similar work‐function shifts on ITO, yet lead to distinct device characteristics: the PCz‐C4PA‐based cells achieve a power conversion efficiency of 11.40% compared to 9.14% for PCz‐C2PA, with comparable short‐circuit current densities but a substantially higher open‐circuit voltage and fill factor. Analysis of voltage losses and light intensity dependence indicates that the longer C4 linker reduces both nonradiative and trap‐assisted recombination, while transient measurements and morphology studies point to faster charge extraction, longer carrier lifetimes, and a smoother active‐layer interface for PCz‐C4PA. These findings demonstrate that, even for SAMs with the same bulky fused‐aromatic terminal group, the alkyl linker length is a critical handle for tuning interfacial recombination and charge transport, offering a straightforward molecular design guideline for p‐type SAMs in high‐performance organic photovoltaics.