Chain length and torsional dependence of exciton binding energies in P3HT and PTB7 conjugated polymers: A first-principles study

Exciton binding energies (Eb) in promising organic photovoltaic (OPV) materials, specifically in poly(3-hexylthiophene) (P3HT) and thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7) are investigated using density functional theory (DFT) and time-dependent DFT methods at the molecular level. A systematic investigation of the dependence of Eb on backbone torsion and chain length in P3HT and PTB7 is carried out by calculating the fundamental gap (Egfund), HOMO-LUMO gap (Eghl) and optical gap (Egopt) of these systems. We found that PTB7 has lower Egfund,Eghl,Egopt and Eb that are less-dependent on torsional angle compared to P3HT. These findings indicate that excitons are easily dissociated to produce charge carriers in PTB7 than in P3HT and hence supports recent experimental findings that PTB7-based OPV devices with improved active layer morphology have much higher power conversion efficiency than the most investigated P3HT-based OPV devices.