| Abstract | This thesis deals with the design and theoretical investigation of a new series of organic dye sensitizers to develop newly the high power conversion efficiency. The starburst triphenylamine dyes were introduced by using carbazole and diphenylamine moieties acting as auxiliary donor groups. The starburst triphenylamine dyes acting as electron donor groups capped with mono-and di-substituted auxiliary donors coded as D-D--A and 2D-D--A, respectively, were studied for the propose of comparison with only one triphenylamine moiety as donor in the D--A system. Among these architectures, the results suggested that the 2D-D--A system showed the largest absorption range. We found that different typed of auxiliary donors provided different light-harvesting ability; the diphenylamine auxiliary donor can improve properties of the better light harvesting ability. 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.
A series of newly designed triphenylamine-based sensitizer incorporating a benzo-thiadiazole (BTD) unit as an additional electron-withdrawing group in a specific donor-acceptor--acceptor (D-A--A) architecture has been investigated. We found that different positions of the BTD unit provided significantly different responses for light absorption. Among these, it was established that the further the BTD unit is away from the donor part, the broader the absorption spectra, which is an observation that can be applied to improve light-harvesting ability. However, when the BTD unit is connected to the anchoring group a faster, unfavorable charge recombination takes place; therefore, a thiophene unit was inserted between these two acceptors, providing red-shifted absorption spectra as well as blocking unfavorable charge recombination.
We performed a theoretical investigation on a series of organic dyes incorporating an anthracene moiety between a carbazole donor group and a cyanoacrylic acid acceptor, in which a triple bond (TB)-modified moiety acts as a -conjugated linker. We found that optimized anthracene structures lay almost perpendicular to the plane of the adjacent substituents. The introduction of a modified TB moiety significantly decreases the dihedral angle and results in a planar structure, which extends the length of the -conjugated system to provide a broader absorption spectrum. Introduction of a TB moiety into the dye structure facilitates electron transfer from the donor and acceptor. The TB-modified dye structure has a significant effect on electron injection from the dye sensitizer to the TiO2 surface.
The effect of different electron donors and conjugate bridges on the structural optical, and electron transfer properties of new designed dyes were studied. The different electron donor of fluorene, carbazole, and phenothiazine showed different structural conformation. Fluorene and carbazole acting as electron donors provided the planar conformation, while phenothiazine acting as electron donor provided the butterfly conformation was found. However, these two different conformations are not significant effect on energy level, intramolecular charge transfer property and optical property. For the effect of conjugated-bridge, the LUMO energy level can be significantly decreased when increased the thiophene units. In addition, the red shift of absorption spectra was found when the conjugated bridge was extended, Our results are suggested to be the possible reasons for enhancement of conversion efficiency in dye-sensitized solar cells. We hope our work could provide a theoretical guidance for the future research of dye-sensitized solar cells.
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