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-π-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 TiO(2) anatase (101) were carried out by using a 38[TiO(2)] cluster model using Perdew-Burke-Ernzerhof functional with the double numerical basis set with polarization (DNP). The results showed that the introduction of thiophene-thiophene unit (T-T) as conjugated spacer in CFT2A could affect the performance of intramolecular charge transfer significantly due to the inter-ring torsion of T-T being decreased compared with phenylene-phenylene (P-P) spacer of CFP2A in the researhcers' previous report. It was also found that increasing the number of π-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 HOMO-LUMO 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 TiO(2) cluster were ~14 kcal/mol, implying that these dyes strongly bind to TiO(2) surface. Furthermore, the electronic HOMO and LUMO shapes of all dye-TiO(2) complexes exhibited injection mechanism of electron via intermolecular charge-transfer transition.