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 (similar to 1.8 eV), and a very long conjugation length (similar to 147 angstrom) 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.