Theory Design of Two Polymer Donors for Organic Heterojunction Solar Cells

Abstract

A highly efficient organic solar cell needs a copolymer as donor possessing low-band gap, high open-circuit voltage (Voc), good charge transfer at donor-acceptor (D-A) interface, and good transport properties in polymers. As the first step towards this goal, we constructed two polymers (PBnDT-HTAZ and PBnDT-6CNTAZ) incorporating benzodithiophene (BnDT) as the electron-rich units and either 1H-benzo(d)(1,2,3)triazole (HTAZ) or 1H-benzo(d)(1,2,3)triazole-6-carbonitrile (6CNTAZ) as the electron-deficient unit. The designed PBnDT-6CNTAZ was introduced a cyano in the electron-deficient unit. PBnDT-HTAZ and PBnDT-6CNTAZ as donors and (6,6)-phenyl C61-butyric acid methyl ester (PC60BM) as an ac- ceptor in hetero-organic solar cell, as well as the D-A complexes are investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) in this work. The electronic properties, optical absorption properties, intramolecular and intermolecular charge transfer, exciton binding energy of two polymers and corresponding D-A com- plexes are discussed. By means of Marcus theory, the exciton-dissociation, charge-recombination rate of D-A complex at the D-A interface, and the hole-transport rate of polymer donors are studied. According to the computational results, it indicates that the two polymers have strong and wide absorption peaks in visible region, as well as strong intramolecular charge trans- fer and intermolecular charge transfer for corresponding D-A complex. The designed PBnDT-6CNTAZ possesses lower the energy of the highest occupied molecular orbital (HOMO) so that PBnDT-6CNTAZ exhibits larger open-circuit voltage and better ability of antioxidant in comparison with PBnDT-HTAZ. The two polymers corresponding D-A complexes have small exciton binding energy. The PBnDT-6CNTAZ has good ability of charge transfer and larger hole-transport rate in compari- son to PBnDT-HTAZ. It is thus clear that incorporating cyano in polymer (PBnDT-6CNTAZ) have an important impact on performance of organic solar cell. Therefore, we can infer that PBnDT-6CNTAZ may be a potential donor material of bulk heterojunction solar cells. The present calculated results demonstrated a first attempt of providing a theoretical model for the introduction of electron-deficient groups in copolymer donors. These results may provide a structural guideline for optimiz- ing chemical construction of copolymer donors to improve the performance of bulk heterojunction solar cells. Our computa- tional models and conclusions can be applied to design and predict other new polymer donors. Keywords organic solar cell; donor-acceptor complex; benzotriazole; charge transfer