NOVELTY - Conjugated polymer containing 10 or more repeating units of substituted fused nitrogen containing heterocyclic compounds (I) are new.    USE - The conjugated polymer is useful in optical, electronic, or optoelectronic device, where the device is organic FET, organic light-emitting transistor, and organic photovoltaic device, and in the composition (all claimed).    ADVANTAGE - The composition exhibits a peak optical absorption spectrum red shift of at least 100 nm when the composition is cooled at 120 degrees C to room temperature (claimed). The conjugated polymers: have desirable properties as semiconducting materials, are cheap and easy to synthesize (especially by methods suitable for mass production), and can exhibit good solubility and great solution processability, and that enable highly efficient organic photovoltaics; enable highly efficient thick-film (300 nm) organic photovoltaics, via the formation of a polymer/fullerene morphology containing highly crystalline.    DETAILED DESCRIPTION - Conjugated polymer containing 10 or more repeating units of substituted fused nitrogen containing heterocyclic compounds of formula (I) are new.    X = F or Cl;    Y1 = S or N-R1;    R1 = 1-20C straight-chain or branched alkyl;    Z = S or Se;    R = straight-chain, branched or cyclic alkyl with 2-40C, in which at least one of non-adjacent C are optionally replaced by O, S, C(O), C(O), O, O-C(O), O-C(O)-O- , -CRO=CROO- or -CC- and in which at least one H are optionally replaced by halo or CN, or aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl or heteroaryloxycarbonyl having 4-30 ring atoms that is optionally substituted by at least one non-aromatic groups; and    Ar1 = arylene, or heteroarylene (both optionally substituted), where Ar1 does not contain any polycyclic group.    INDEPENDENT CLAIMS are also included for:    (1) composition comprising the conjugated polymer dissolved or dispersed in a liquid medium; and    (2) organic photovoltaic device comprising n-type semiconductor material adjacent to the conjugated polymer, and the power conversion efficiency is at least 8%, preferably 10%.