NOVELTY - Organic photovoltaic device (A11) comprises an anode, cathode and a photoactive layer in between anode and cathode. The photoactive layer comprises a semiconducting compound, which is a polymer comprising an electron acceptor unit and a second unit. The electron acceptor unit is substituted fused heterocyclic compounds (I). The second unit comprises a terthiophene.    USE - Used as organic photovoltaic device.    ADVANTAGE - (A11) has a power conversion efficiency of at least 5%, preferably at least 9%. (A12) has power conversion efficiency of at least 5%, preferably at least 8% (claimed). (A11) or (A12) utilizes the polymer, which exhibits optimized optical absorption, good charge transport characteristics and chemical stability in ambient conditions, low temperature processability, large solubility in common solvents and processing versatility, provides photoactive layer with high performance in ambient conditions, i.e. demonstrates at least one of low band gap, high fill factor, high open circuit voltage and high power conversion efficiency and provides photovoltaic device with optimized light absorption/charge separation, light emitting device with charge transport/recombination/light emission and field effect device with high carrier mobility and/or good current modulation characteristics.    DETAILED DESCRIPTION - Organic photovoltaic device comprises an anode, cathode and a photoactive layer in between anode and cathode. The photoactive layer comprises a semiconducting compound, which is a polymer comprising an electron acceptor unit and a second unit. The electron acceptor unit is substituted fused heterocyclic compounds of formula (I). The second unit comprises a terthiophene.    a = pi -conjugated moiety comprising at least one aryl or heteroaryl; and    R1 = 1-40C alkyl, 2-40C alkenyl or 1-40C haloalkyl.    An INDEPENDENT CLAIM is also included for an organic photovoltaic device (A12) comprising an anode, cathode and a photoactive layer in between the anode and the cathode, where the photoactive layer comprises an electron donor compound and electron acceptor compound, where the electron donor compound is a polymer comprising electron acceptor unit and electron donor unit, and the electron acceptor unit is substituted dithieno(3,2-c;2',3'-e)azepine-4,6-dione moiety of formula (a2) and the electron donor unit is an oligothiophene.