We carry out a computational study of optical properties of two novel 5,6-difluorobenzo{[}c{]}{[}1,2,5{]}-thiadiazole-based polymers, PFBT-T12TT and PFBT-T20TT, aimed at elucidating the remarkably higher potential for applications in polymer solar cells exhibited by the latter as compared to former. While the two molecules differ only in terms of the long alkyl side-chains, their geometries optimized via Density Functional Theory (DFT) based calculations at B3LYP/6-31G(d) level reveal differences in internal coordinates (dihedral angles), which are important in tuning the electronic structure and optical properties. By employing molecular dynamics (MD) simulations in combination with DFT techniques, electronic structure is obtained at room temperature. The subsequently calculated energies of the highest occupied molecular orbitals (HOMOs) are found to be in reasonable agreement with the available experimental data. More importantly, on one hand, the calculated HOMO-LUMO (lowest unoccupied MO) gap is found to be nearly similar for both the molecules accounting for nearly identical short-circuit current observed in polymer solar cell devices based on them. On the other hand, PFBT-T20TT is found to exhibit appreciably more delocalization of electronic density corresponding to the HOMO level than that in PFBT-T12TT arising from conformational distortions and pointing towards improved charge carrier mobility in the former. To take into account the influence of phonons, we have also adopted the multi-mode Brownian Oscillator (MBO) model to calculate the absorption spectra, which yields excellent fitting of the experimental measurements in both the solution and the thin-film states. The parameters of the MBO model extracted from the fitting procedure indicate a strong exciton-phonon coupling in PFBT-T12TT as compared to PFBT-T20TT, consistent with the trends revealed via the DFT-MD results.