Theoretical Insight into Organic Dyes Incorporating Triphenylamine-Based Donors and Binary π-Conjugated Bridges for Dye-Sensitized Solar Cells

The design of light-absorbent sensitizers with sustainable and environment-friendly material is one of the key issues for the future development of dye-sensitized solar cells (DSSCs). In this work, a series of organic sensitizers incorporating alkoxy-substituted triphenylamine (tpa) donors and binary pi-conjugated bridges were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Molecular geometry, electronic structure, and optical absorption spectra are analyzed in the gas phase, chloroform, and dimethylformamide (DMF) solutions. Our results show that properly choosing the hetero aromatic atoms and/or adding one more alkoxy-substituted tpa group can finely adjust the molecular orbital energy. The solvent effect renders the HOMO-LUMO gaps of the tpa-based sensitizers decrease in the sequence of DMF solution < chloroform solution < gas phase. The absorption spectra are assigned to the ligand-to-ligand charge transfer (LLCT) characteristics via transitions mainly from tpa, 3,4-ethylenedioxythiophene (edot), and alkyl-substituted dithienosilole (dts) groups to edot, dts, and cyanoacrylic acid groups. The binary pi-conjugated bridges play different roles in balancing the electron transfer and recombination for the different tpa-based sensitizers. The protonation/deprotonation effect has great effect on the HOMO-LUMO gaps and thus has great influence on the bands at the long wavelength region, but little influence on the bands at the short wavelength region.