Theoretical Study On The Properties Of Co-based Redox Mediators And The Mechanism Of Dye Regeneration For Dye-sensitized Solar Cells

With the growing consumption of fossil fuels and the accompanied problems of climate and environment, the development of cost-effective, environmentally-friendly and renewable energy technologies has become one of the most pressing challenges that the industrialized society is currently facing. As an emerging alternative to conventional photovoltaic technologies, dye-sensitized solar cells(DSCs) exhibit a range of interesting features, such as their comparable efficiency, inexpensive fabrication process, less polluting and good flexibility, and it has become a promising, and active, area of research. To achieve higher efficiency of DSCs, a large number of efforts have been devoted to the improvements of various processes in this complex system, among which the exploration of the dye regeneration mechanism is always one of the most significant aspects. A deep understanding of the elementary electron transfer process is vital to develop rational strategies for cell optimization. In this thesis, the regeneration mechanism of ruthenium(II) dye and triphenylamine-based organic dyes, as well as the redox properties of cobalt redox mediator are investigated by using the calculated methods based on density functional theory.(1) Density functional theory(DFT) calculations were carried out to explore the effects of alchemically modifying the polypyridine ligands and design efficient cobalt-based redox mediators for dye-sensitized solar cells. Firstly, we computed the oxidation potential of some classic cobalt mediators using various DFT methods, and discovered that among the employed DFT methods the hybrid functional B3 LYP with LANL2 DZ basis set provides the most accurate results for cobalt mediator. Next, using the validated method, a set of new cobalt mediators was designed. Our results show that the redox properties of cobalt mediators can be well tuned by altering the number and position of nitrogen atoms on the ligand ring. Adding oxygen atoms on the ligand ring will evidently increase the oxidation potential, which might be unfavorable for the dye regeneration. The designed good cobalt mediators possess similar oxidation potential and reorganization energy to the current high-efficiency redox mediators, thus are promising to be used in prospective DSCs.(2) The regeneration processes of triphenylamine(TPA)-based organic dyes with cobalt redox mediator in dye-sensitized solar cells have been investigated using density functional theory combined with the Marcus theory of electron transfer. The focus of this study is upon the impacts of the extension and the rigidification of π-linker on the light-harvesting ability of dye and the dye regeneration process. Our results show that with the extension or rigidification of the oligothiophene conjugation linker the absorption spectra of TPA dyes exhibit observable red-shift in the maximum absorbance that favors light-harvesting, while the electron transfer rates for dye regeneration decrease in some degrees due to the increased activation free energies and the reduced electronic coupling energies which hampers the dye regeneration. Importantly, the undesirable influences on dye regeneration by extending the linker moiety are more significant than that by the way of rigidification. Thus, the rigidification is a better choice than the extension of the conjugated moiety for the design of D-π-A type dyes based on the properties of light-harvesting and the kinetics of dye regeneration.(3) The regeneration mechanism of ruthenium(II) dyes by cobalt redox mediators in dye-sensitized solar cells has been investigated using density functional theory combined with the Marcus theory of electron transfer. Special attentions have been paid on the influences of dye protonation and dye-cobalt mediator interactive configuration on the dye regeneration process. Our results show that the regeneration reaction rates gradually increase with the growth of the dye protonation degree. By comparing the natures of protonated and deprotonated states, we find that design of charge-neutral dye is a better choice to obtain fast dye regeneration and suppress the undesired recombination reaction of the injected photoelectrons. Furthermore, the dye-cobalt mediator associated configuration is found to influence the regeneration reaction rates remarkably by varying the electronic coupling energy. The electronic coupling energy largely depends on the frontier orbitals distribution of the reactants, so the matching of the electronic configuration of the dyes and the cobalt mediators should be judiciously conceived.