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 °C for 5 min. This PCE is among the highest values reported to date for solar cells using solution processable small molecules.

Two novel conjugated low band gap small molecules (SMs), M1 and M2, containing benzobisthiadiazole and thienothiadiazole central units, respectively, were synthesized. Both SMs carried terminal cyanovinylene 4-nitrophenyl at both sides which were connected to the central unit with a thiophene ring. The long-wavelength absorption band was located at 591–643 nm and the optical band gap was 1.62–1.63 eV, which is lower than that of P3HT. These two SMs were investigated as electron donor materials along with PCBM or F as electron acceptors for fabrication of bulk heterojunction (BHJ) organic photovoltaic devices. F is a modified PCBM which has been previously synthesized and used as an electron acceptor for poly(3-hexylthiophene) (P3HT). The power conversion efficiency (PCE) for M1:PCBM, M1:F, M2:PCBM and M2:F was 1.05%, 2.02%, 1.23% and 2.72%, respectively. The higher PCE for the devices with M2 as the electron donor has been related to the improved hole mobility for M2. However, the improved PCE for the devices with F as the electron acceptor has been attributed to the more intense absorption of F in the visible region than that of PCBM and also to the higher open circuit voltage resulting from the higher LUMO level of F. We have also fabricated devices with M2:F cast film from mixed solvents. The PCE for the BHJ devices with the as cast and thermally annealed M2:F (mixed solvents) is 3.34% and 3.65%, respectively.