DFT and TD-DFT investigations of metal-free dye sensitizers for solar cells: Effects of electron donors and π-conjugated linker

In this study, we have designed a series of metal-free organic D-π-A (electron donor-π-conjugated-acceptor) dyes employed in dye-sensitized solar cells (DSSCs). The optimized structures and photo-physical properties of these dyes have been investigated by using the density functional theory (DFT/B3LYP/6-31G(d)) method. These dyes consist of electron-donor (triphenylamine, coumarine, fluorene, etc) and -acceptor/anchoring (cyanoacrylic), connected by the π-conjugated linker as an electron spacer. In particular, both electron-donor and π-conjugated linkers are important and make impact on the performance of the dyes in the DSSCs. Computational analysis have indicated that a dye with stronger electron-donating group enhances the HOMO energy as compared to a weaker electron-donating group. The time-dependent density functional theory (TD-DFT) method has also been performed to calculate the electronic absorption spectra of these dyes. Two major absorption peaks have been obtained for metal-free organic dyes in the visible region. One is assigned to the intra-molecular charge transfer (ICT) band at 590-770 nm and the other absorption peak is associated with the π → π∗ transition of the entire molecule. The π-conjugated linkers with electron-withdrawing substituents have been shown to generate a slightly blue-shifted absorption band as compared to those without any substituent. The Natural Bond Orbital (NBO) analysis for organic dyes has also been revealed that the origin of charge-transfer arises from electron-donating group to electron-withdrawing moiety. The projected density of state (PDOS) analysis for these dyes depicts that the electron density of HOMO is located at the electron-donating group and it has also been extended to the π-conjugated linker. The electron density of LUMO is concentrated at the π-conjugated linker and at the electron-withdrawing moiety. The computed results suggest that the intermolecular charge-transfer mechanism is operative in these D-π-A dyes when applied to DSSC.