Insights into the role of D-A-π-A type pro-aromatic organic dyes with thieno[3,4-b]pyrazine as A acceptor group into dye-sensitized solar-cells. A TD-DFT/periodic DFT study

Abstract

Time-dependent density functional theory (TD-DFT)/periodic DFT calculations were performed to determine the role of pro-aromatic organic D-A-pi-A type dyes (the NL1-NL17 family) with Thieno[3,4-b]pyrazine (Tpy) as A acceptor group into dyesensitized solar-cells (DSSC). This work presents a discussion of the ground and excited states of these dyes along with the aromaticity analysis and the electron injection step using a dye@(TiO2)(72) model. The results suggest that the pro-aromatic behavior increases from the thiophene ring to the pyrazine when an acceptor p-bridge such as phenyl is used. This strong pro-aromaticity is also reflected in the electron injection step, studied using a 3x2 3 layer (TiO2)(72) slab model. The resulting adsorption energies (Delta E-ads and Delta G(ads)) and the electron injection (Delta G(inject)) in the stablest coordination mode, Bid_CN_COOH, indicate that the redox reaction (Dye* -> Dye(+) + e(-)) is stronger and more spon than the adsorption reaction (Dye(+) + TiO2 [+e(-)] -> Dye@TiO2) in the electron injection. In this way, the highest efficiency of NL6 and NL12 is a consequence of the more significant pro-aromatic characteristics and the more spontaneous redox process. Finally, these NL dyes are promising in the molecular engineering of D-A-pi-A metal-free types dyes.