Tuning the electron donating ability in the triphenylamine-based D-π-A architecture for highly efficient dye-sensitized solar cells

A new series of triphenylamine-based organic dye sensitizers, namely TPA1-TPA6 with introduced auxiliary donor groups including carbazole and diphenylamine moieties to triphenylamine core, and with inserted fluorene moiety between triphenylamine and auxiliary donor, was designed and theoretically investigated. The starburst triphenylamine dyes acting as electron donor groups capped with mono- and di-substituted auxiliary donors coded as D-D-π-A and 2D-D-π-A respectively, were studied for the propose of comparison with only one triphenylamine moiety as donor, namely TPA, in the D-π-A system. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted to investigate the molecular structures, frontier molecular orbitals, absorption spectra as well as dye-(TiO2)38 complexes of these dyes as implemented in the Gaussian 09 program. Among three dye architectures, the results suggested that the 2D-D-π-A system showed the largest absorption range. We found that different types of auxiliary donors provided different light-harvesting ability; the diphenylamine auxiliary donor can improve properties of red-shifted absorption region. In addition, the optimized geometries showed that adding of diphenylamine auxiliary donor provided smaller external dihedral angles (EDA) leading to wider absorption range with strong charge-transfer character compared to other dyes. Our results are suggested to be the possible reasons for the enhancement of conversion efficiency in dye-sensitized solar cells.