  Two new carbazole-based organic dye sensitizers are designed and investigated
in silico. These dyes are designed through chemical modifications of the
conjugated bridge of a reference organic sensitizer known as Carbz-PAHTDDT (S9)
dye. The aim of designing these dyes was to reduce the energy gap between their
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) and to red-shift their absorption response compared to those of
the reference S9 dye sensitizer. This reference dye has a reported promising
efficiency when coupled with ferrocene-based electrolyte composition. To
investigate geometric and electronic structure, density functional theory (DFT)
and time-dependent DFT (TD-DFT) calculations were conducted on the new dyes as
well as the reference dye. The present study indicated that the long-range
correction to the theoretical model in the TD-DFT simulation is important to
produce accurate absorption wavelengths.The theoretical studies have shown a
reduced HOMO-LUMO gap and red-shifted absorption spectra for both of the new
candidate dyes. Such findings suggest that the new dyes are promising and
suitable for optoelectronic applications.
