Geometrical parameters, electronic structures, optoelectronic and photovoltaic properties of new copolymers poly(Ethylcarbazole-Terphenyl) (PECz-Ter) containing Ethylcarbazole as a donor group and Terphenyl rings as electron acceptors, have been investigated through Density Functional Theory, has been theoretically studied using the density functional theory (DFT) with the hybrid B3LYP exchange correlation function and the split valence 6- 31G (d,p) basis set. The theoretical results including the geometries optimization of (PECz-Ter)n was carried out, showing a reduction in the band gap when going from n=1 to the n=5 monomer. Structural parameters, optoelectronic and photovoltaic properties have been analyzed and discussed in terms of conjugative pathway between the electron-donating and electron-accepting moieties. Optical properties of the studied oligomers of PECz-Ter (n=1-5) characterized by the bathochromic effect and the decreases value of the wavelength λabs with increasing the chain length from n=1 to n=5 monomer. From these results, the correlation structure-properties are better understood. Furthermore, the copolymers PECz-Ter, which is blended with fullerene derivative [6,6]-phenyl- C61-butyric acid methyl ester (PCBM) and derivatives, seems to be a good active layer in bulk heterojunction solar cells in the case of the studied oligomers of (PECz-Ter) blended with C60 or C70. The subject of this work is to design novel materials for organic solar cells, and help to understand the structure–properties relationship of these new systems.