Nanostructures and Electronic Properties of a High-Efficiency Electron-Donating Polymer

The development of organic photovoltaic (OPV) solar cells has seeded a bright hope of achieving low-cost solar energy harvesting. Practical realization and successful commercialization require enhancing the efficiency of solar energy harvesting, which, in turn, relies on the core understanding of structure–property relationships in OPV materials. Here, we report the first large-scale density functional calculations of the nanoconformational and electronic properties of the thieno­[3,4-b]­thiophene-alt-benzodithiophene copolymer (PTB7), a high-efficiency OPV material. These first-principles results include the chain length dependence of the torsional potential, the nearest-neighbor torsional coupling, the band gap, and the electronic conjugation length. Importantly, PTB7 was found to have a torsional potential almost independent of chain length, very weak nearest-neighbor torsional coupling, a low band gap (∼1.8 eV), and a very long conjugation length (∼147 Å) compared to the other conjugated polymers like polythiophene and poly­(3-alkylthiophene). These results suggest that PTB7 can be an efficient electron donor for OPV devices.