Optical and electronic properties of para-functionalized triphenylamine-based dyes: a theoretical study

Molecular engineering of dyes has become a popular and most successful approach towards improvement of photovoltaic power conversion efficiency of dye-sensitized solar cells (DSSCs). We report the geometrical, optical, and electronic properties for para-substituted triphenylamine (TPA)-based dyes with D-π-π-A architecture. Results were realized through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. We used B3LYP/6–31 + G(d,p) and CAM-B3LYP/6–31 + G(d,p) level of theory for DFT and TD-DFT, respectively. Six electron-donating (ED) and electron-withdrawing (EW) groups were symmetrically grafted to the para-direction of the phenyl rings. Two anchoring groups namely: cyanoacrylic acid (CA) and hydantoin (HY) were used. Excellent relationships between electronic energies and the Hammett constants ( σ p ) have been reported. The results show that variation of both anchoring groups and substituents significantly affect the absorption of the dyes; maximum absorption for CA dyes was found ranging between 514–571 nm and 470–503 nm for ED and EW groups, respectively, while for HY dyes demonstrated maximum absorption between 502–537 nm and 480–496 nm for ED and EW, respectively. A linear correlation between σ p and λ max with R 2 > 0.97 was obtained. In addition, the mapping of the HOMO and LUMO energies suggests the intramolecular charge transfer and a strong electronic coupling between dye and semiconductor. Our theoretical calculations show that electron-donating substituents enhance the optoelectronic properties of the dyes. Analysis of chemical descriptors suggests that dyes containing alternative anchoring group HY substituted with –NH 2 and –N(CH 3 ) 2 may demonstrate improved performance of DSSCs.