Different chain length of conjugated polymers based on 4,6-dithien-2-ylthieno [3,4-c] [1,2,5]-thiadiazole (DTTTD) have been extended to investigate the structural, electronic and optical properties of (DTTTD)N donors in both vacuum and solvent environments using density functional theory (DFT) and time dependent DFT (TDDFT) calculations. The calculated results reveal that the bond length alternation (BLA), the HOMO LUMO energy gap (E-g), Fermi energy level position (E-F) and optical band gap (E-opt) of corresponding donors with N >= 5 (where N = 5 is the effective conjugated length (ECL) of DTITD-based extended polymers) become size-consistent. The moderately-polar solvent environment cause of more stabilizing HOMO and LUMO energy levels, lower transition energy (TE), larger transition dipole moment (mu(ij)) and higher oscillator strength (f) of DITTO investigated polymers. Therefore, the red-shifted in absorption spectra is occurred with respect to the vacuum state. In addition, the external electric field (F-ext) smoothly affects optical properties of investigated donors. Based on our calculations, it was found that the trimer of DTTTD-based extended polymers is the most suitable donor to design the highperformance organic solar cells (OSCs) possessing a narrow HOMO-LUMO energy gap, highest exciton life time, broad and intense absorption spectra that cover the solar spectrum leads to maximum light harvesting efficiency (LHE), higher charge transfer (CT) ability and high open-circuit voltage (V-oc). On the other hand, the value of Voc of larger extended polymers decreased due to the HOMO upward shifted. (C) 2017 Elsevier Ltd. All rights reserved.