Homologous Self-Assembled Oligomers as Dual Anode/Cathode Interface Layers for Efficient Organic Photovoltaics

Engineered interfacial layers that tune electrode energetics, suppress recombination, and enforce carrier selectivity are critical for high-efficiency organic photovoltaics (OPVs), yet current anode and cathode interface counterparts rely on distinct chemistries, increasing synthesis complexity and hindering scale-up. Here, we report a versatile molecular platform that generates both anode and cathode interface layers (AILs and CILs) from the homologous donor-acceptor-donor (carbazole-benzothiadiazole-carbazole) oligomer. Using a concise three-step route, we synthesized phosphonate-functionalized CIL (2DPeBcz) and phosphonic-acid-functionalized AIL (2DPaBcz) materials linked by a short, two-carbon alkyl spacer, which promotes dense molecular packing. Consequently, the self-assembled monolayer 2DPaBcz increases the ITO electrode work function and passivates oxygen vacancies, while its analogue 2DPeBcz based on phosphonate forms a robust interfacial dipole on Ag. Incorporating these layers in D18:BTP-eC9 devices achieved a power conversion efficiency of 19.3% and improved storage, light-soaking, and thermal stability. Electrical characterization further validated reduced trap density and balanced carrier mobility, highlighting the advantages of this versatile structural approach for scalable, high-performance OPV interfaces. Overall, this work presents a unified synthetic strategy for AIL and CIL to be derived from a single molecular backbone, simplifying material preparation while maintaining competitive OPV performance and improved device stability.