Atomic and electronic structure of polymer organic semiconductors: P3HT, PQT, and PBTTT
First-principles pseudopotential density functional calculations were employed to investigate the atomic structure of PQT-C6, P3HT, and PBTTT-C6. The calculations show that a substantial rotation of the conjugated planes around the polymer axis is energetically favorable for all three polymer crystals. This rotation results in reduced overlap of molecular orbitals, and therefore increases the effective mass in the ƒà-ƒà stacking direction. The mobility was estimated within an acoustic deformation potential model. Similar values for the effective mass and mobility were obtained for P3HT and PBTTT-C6. Therefore, the higher mobility observed for PBTTT in comparison to P3HT could be a result of improved structural ordering rather than being an intrinsic property of crystalline regions of the polymer. Calculations indicate that substantial interdigitation of the alkyl side chains is energetically favorable for PBTTT.
Northrup, J. E. Atomic and electronic structure of polymer organic semiconductors: P3HT, PQT, and PBTTT. Physical Review B, 2007; 76: 245202.