The effect of conjugated spacer on novel carbazole derivatives for dye-sensitized solar cells: Density functional theory/time-dependent density functional theory study

The ground-state structure and frontier molecular orbital of D-p-A organic dyes, CFT1A, CFT2A, and CFT1PA were theoretically investigated using density functional theory (DFT) on B3LYP functional with 6-31G(d,p) basis set. The vertical excitation energies and absorption spectra were obtained using time-dependent DFT (TD-DFT). The adsorptions of these dyes on TiO2 anatase (101) were carried out by using a 38[TiO2] cluster model using PerdewBurkeErnzerhof functional with the double numerical basis set with polarization (DNP). The results showed that the introduction of thiophenethiophene unit (TT) as conjugated spacer in CFT2A could affect the performance of intramolecular charge transfer significantly due to the inter-ring torsion of TT being decreased compared with phenylenephenylene (PP) spacer of CFP2A in the researhcers' previous report. It was also found that increasing the number of p-conjugated unit gradually enhanced charge separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of these dyes, leading to a high-efficiency photocurrent generation. The HOMOLUMO energy gaps were calculated to be 2.51, 2.37, and 2.50 eV for CFT1A, CFT2A, and CFT1PA respectively. Moreover, the calculated adsorption energies of these dyes on TiO2 cluster were similar to 14 kcal/mol, implying that these dyes strongly bind to TiO2 surface. Furthermore, the electronic HOMO and LUMO shapes of all dyeTiO2 complexes exhibited injection mechanism of electron via intermolecular charge-transfer transition. (c) 2012 Wiley Periodicals, Inc.