A rigid planar low band gap polymer PTTDPP-DT-DTT for heterojunction solar cell: a study of density functional theory

A low band gap polymer, PTTDPP-DT-DTT, was studied using density functional theory and time-dependent density functional theory to obtain geometric structures, frontier molecular orbitals, ionization potentials, electron affinities, optical absorption and charge transfer properties in the application of solar cells. Marcus theory was used to calculate the charge separation and recombination rates of the donor and acceptor (D/A) interfaces [(PTTDPP-DT-DTT)(2)/PC61BM and (PTTDPP-DT-DTT)(2)/PC71BM]. It was found that with the increase of the conjugated unit of the polymer, the reorganization energies of holes and electrons are both decreased, indicating that the ability for charge transport is enhanced. For D/A interfaces, (PTTDPP-DT-DTT)(2)/PC61BM and (PTTDPP-DT-DTT)(2)/PC71BM have appropriate exciton binding energies, and the rate of charge separation is much faster than the rate of charge recombination. The performance of (PTTDPP-DT-DTT)(2)/PC61BM is better than that of (PTTDPP-DT-DTT)(2)/PC71BM in the charge transfer process. Simulation of polymer/fullerene solar cells provides a deep understanding of the relationship between structure and performance.