Efficient bulk heterojunction devices based on phenylenevinylene small molecule and perylene-pyrene bisimide

We report the fabrication and characterization of photovoltaic devices using a bulk heterojunction (BHJ) photoactive layer consisting of a small molecule (T), which contains a central p-phenylenevinylene unit, intermediate thiophene moieties, and terminal cyano-vinylene 4-nitrophenyls as the donor and a perylene-pyrene bisimide (PPI) as the acceptor. The difference in the LUMO levels (0.5 eV) of these materials is sufficient for the photoinduced charge transfer in the bulk heterojunction active layer. The optimum blend ratio (by weight) between T and PPI is 1 : 3.5 with a power conversion efficiency (PCE) of about 1.87%, beyond that the PCE starts to decrease. The incorporation of a thin ZnO layer between the organic BHJ layer and the top Al electrode increases the PCE to 2.46%, which is attributed to the enhanced light absorption due to the optical interference between incident light and reflected light from the Al electrode. It is also attributed to the improved electron transport in the device, since the conduction band of ZnO closely matches the work function of the Al electrode. The PCE is further increased to 3.17% when the device with the ZnO layer is annealed at a temperature of 100 degrees C for 5 min. This PCE is among the highest values reported to date for solar cells using solution processable small molecules.