A Solution-Processed Cathode Interfacial Layer Facilitates Efficient Energy Level Alignment in Organic Photovoltaics

We synthesized and used small-molecule cathode interfacial materials in organic photovoltaic devices (OPVs). Two small-molecule interfacial materials with phosphine oxide and benzoimidazole groups induced strong interface dipoles and chelated with the metal electrodes. The molecules can be dissolved in various organic solvents for processing. We used isopropanol (an environmentally benign solvent) to dissolve the small-molecule materials and spin-coated solutions onto photoactive layers. The materials formed ohmic contacts between the electron acceptor PC71BM and the Al cathodes and facilitated efficient charge carrier extraction at the electrodes of OPVs. Analyses of the material and device properties of the two interfacial materials and the photovoltaic performances of OPVs with the materials revealed that the materials were promising cathode interfacial materials for photovoltaic applications. The interfacial material featuring two benzoimidazole groups optimally formed smooth interfacial layers affording better energy level alignment between the electron acceptors and the cathode