Molecular Design And Theoretical Study Of Organic Dye Sensitizer

Organic dyes have the great application potential on the sensitizer of dye-sensitized solar cells(DSSCs). In this work, the quantum chemistry calculations were used to design and study D-π-A and D-A-π-A type organic sensitizers. Density functional theory(DFT) and time-dependent DFT(TD-DFT) calculations were carried out to demonstrate the effects of sensitizers on light-harvesting efficiency in an attempt to shed light on the structure-property relations. The designed dyes were expected to own much better properties than the known ones as the sensitizers of DSSC. Some interesting and significant results of the manuscript are summarized as follows:1. Theoretical study of acene-bridged dyes.The effects of acene units and different substitution positions of electron donors on the optoelectronic properties of the acene-modified dyes have been investigated using density functional theory(DFT) and time-dependent density functional theory(TDDFT) methods. The donor sites have a significant influence on the absorption wavelength mainly because of different molecular orbital(MO) contributions of the highest occupied molecular orbital(HOMO) on the bridging acene units, and the increasing MO contribution would lead to the red shift in the absorption spectra. Meanwhile, the donor is located close to the center of the π-conjugated bridge, and the absorption spectra are extended. Then the results obtained from the hexacene-based dyes confirm the conclusions2. Theoretical study of D-A-π-A type organic sensitizersTo obtain efficient sensitizer, a series of D-A-π-A metal-free dyes have been designed based on one of the best sensitizers WS-9 by modifying auxiliary acceptor and characterized theoretically. The results illustrate that introduction of auxiliary heterocycle acceptor is revealed to very narrow band gap(HOMO-LUMO), leading to an obvious red-shifted broad near-infrared absorption band in the range of 750-1950 nm compared to WS-9(536 nm). The critical parameters in close connection with the short-circuit current density(Jsc), open circuit voltage(Voc), including singlet excited state lifetime(τ), total dipole moments(μnormal), the conduction band of edge of the semiconductor substrate(ΔECB), and regeneration driving forces(ΔGreg) are superior to those of WS-9. Therefore, these novel sensitizers would be a promising candidate for improving the performance of the DSSCs.3. Theoretical study of Benzothiadiazole derivative as anchorWe report the characteristics of benzobisthiadiazole analogues with different heteroatom substitution as electron anchoring groups in DSSCs for the first time. In order to provide a systematic analysis of the designed anchoring groups, the widely used cyanoacrylic acid anchor was used as the reference. Theoretical calculations show that the newly designed anchors may display a decent level of light absorption. More importantly, the ultrafast electron injection was observed from the dyes SPN and SPS into the TiO2 surface.About 90% of the electron injection occurs in the first 15 fs, then completed after t~100 fs, and the pathway of electron injection is direct, leading to very efficient transfer of the wavepacket through the TiO2 semiconductor. Therefore, the performance of the SPN and SPS anchors are equivalent and even superior to the cyanoacrylic acid for promising anchoring groups.