Abstract A new low-band gap small molecule (Se-SM), which contains dithyenyl-benzoselenadiazole as central unit and terminal cyanovinylene 4-nitrophenyls, was synthesized. In addition, a new phenylenevinylene copolymer (P) containing dithyenyl-dinitrobenzothiadiazole moieties was synthesized by Heck coupling. Se-SM showed broad absorption band with long-wavelength absorption maximum ( λ a,max) at ∼640 nm and optical band gap ( E g opt ) of 1.67 eV. Copolymer P had λ a,max around 420 nm and E g opt of 2.31 eV. The dark current–voltage characteristics and incident photon to current efficiency spectra of the devices based on copolymer P and Se-SM indicates that both materials behave as p-type organic semiconductors. The power conversion efficiency (PCE) of the photovoltaic devices based on these materials is low. However, the PCE of the devices fabricated with the blends of copolymer P or Se-SM with PCBM, was improved significantly. The increase is attributed to the formation of bulk heterojunction with increased interfacial area. The effect of the incorporation of Se-SM molecule on the photovoltaic properties of copolymer P:PCBM blend has been also investigated. The overall PCE of the copolymer P:PCBM:Se-SM device is approximately 2.24%, which is further enhanced up to 3.16%, when the thermally annealed blend is used. Therefore, we conclude that the Se-SM molecule increases the light harvesting property of the blend and also provides efficient path for holes and electron in copolymer and PCBM phases, respectively.