Theoretical study on novel double donor-based dyes used in high efficient dye-sensitized solar cells: The application of TDDFT study to the electron injection process

The geometries and electronic structures of organic dye sensitizers, CCT1A, CCT2A, CCT3A, CCT1PA, and CCT2PA comprising double-donor groups, pi-spacer, and acceptor group forming D-D-pi-A system, were studied using DFT and TDDFT. The calculated results have shown that TDDFT calculation using a newly-designed functional which takes into long-range interaction, CAM-B3LYP, was reasonably capable of predicting the excitation energies and the absorption spectra of the molecules. The adsorption of these dyes on the TiO2 anatase (101) surface and the electron injection mechanism were also investigated using a dye-(TiO2)(38) cluster model, employing PBE and TD-CAM-B3LYP calculations, respectively. The adsorption energy (E-ads) of CCTnA (n = 1-3) was calculated to be -15.26, -18.93, and -20.12 kcal/mol respectively, indicating strong adsorption of dye to a TiO2 surface by carboxylate groups. These calculated results suggested that the CCT3A is a promising candidate for highly efficient DSSCs. It was shown that the electron injection mechanism occurs by direct charge-transfer transition in a dye-TiO2 interacting system, resulted in the stronger electronic coupling strengths of the anchoring group of the dyes and the TiO2 surface which corresponded to higher observed J(SC) as expected in CCT3A dye. Through a combined theoretical and experimental investigation we have shown that the trend of charge-injection efficiency in dye-sensitized solar cells constituted from dyes is determined by the adsorption energy of dye-(TiO2)(38) complexes. (C) 2012 Elsevier B.V. All rights reserved.