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

Abstract

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.