Abstract In this work, light-emitting and photovoltaic properties of new low band-gap copolymers (PFO-DBSe) based on 9,9-dioctylfluorene and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole (DBSe) were investigated. The peak positions of photoluminescence and electroluminescence (EL) of the copolymers reach 758 and 727 nm, respectively, and locate in near-infrared light region. At a current of ∼5 mA, EL emission with peak position of 701 nm and a maximum external quantum efficiency of 1.62% was achieved from EL device with poly(9,9-dioctylfluorene) and the copolymer with DBSe content of 5% as the blend-type emitter, by utilizing of a configuration of ITO/PEDOT/PVK/emitter/CsF/Al. This is a high efficiency for deep red EL emission. The copolymer with DBSe content of 35% possesses good solar light spectral coverage according to its absorption spectrum, and a bulk-heterojunction photovoltaic cell with the copolymer as the electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor displayed an energy conversion efficiency of 0.91% under an AM1.5 solar simulator at 100 mW/cm2.