Electronic and photophysical properties of the bend D–T–A–T–D derivatives for small-molecule organic photovoltaic (SM-OPV) solar cells: a DFT and TD-DFT investigation

Abstract

A series of D–T–A–T–D derivatives (D, electron-donating moiety; T, π-conjugated linker; A, electron-acceptor moiety) with seven electron donor moieties and various electron abilities are designed to investigate the influence of the donor on photophysical properties for small-molecule organic photovoltaic solar cells. The 4,8-dimethoxybenzodithiophene (D1), triphenyldsramine (D2), 4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline (D3), 9,9-dimethyl-9H-fluorene (D4), 9-methyl-9H-carbazole (D5), 4-methyl-4H-dithieno-pyrrole (D6), and 4,4-dimethyl-4H-cyclopenta-dithiophene (D7) are adopted as the electron donor moiety. The BDTC (buta-1,3-diene-1,1,4,4-tetracarbonitrile) is used for the A moiety, and the thiophene (T) is used for the π-conjugated linker. The optimized structure of D–T–A–T–D derivatives exhibits the bend molecular conformation due to the steric effect within the A moiety. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of these derivatives are dependent on the electron donating ability of D, which influences the open-circuit voltage and driving force. Reorganization energy suggests that these derivatives are good hole-transporting type materials. Projected density of state analysis demonstrates that in the HOMO, the electron density distribution is delocalized on the terminal D and T moieties, while in the LUMO, the electron density distribution is localized mainly on the A moiety. The maximum absorption peak, which has relatively high light harvesting efficiency, is due to the π to π* transition and can be tuned by the electron-donating ability and the resonance energy of the D moiety. The bend D6–T–A–T–D6/D7–T–A–T–D7 derivatives with D moiety of 4-methyl-4H-dithieno-pyrrole (D6) and 4,4-dimethyl-4H-cyclopenta-dithiophene (D7) are good candidates as electron donor materials for SM-OPV.