Study On The Synthesis And Application Of Aromatic Amine Dyes Bearing Double Donor-Ï€-Acceptor Chains For Dye-sensitized Solar Cells

Solar cells acting as one of the most promising renewable energy technologies to transform sunlight into electricity have triggered global research on the development of new and clean energy sources. Dye-sensitized solar cells （DSSCs） have attracted considerable interest due to its simple preparation and low cost. It is a most effective way to develop DSSCs by using simple and varied structures, easy of design and synthesis, and inexpensive metal-free organic dyes instead of the noble ruthenium complexes. As a critical component in DSSCs of high efficiency, the sensitizer plays a vital role in the light harvesting efficiency providing electron injection into the conduction band of an oxide semiconductor （e.g TiO₂） upon light excitation. Then, the oxidized dye is regenerated by the electrolyte, such as an ionic liquid containing most frequently the iodide/triiodide couple as a redox system.In this thesis, twenty novel aromatic organic dyes containing six dyes bearing symmetry double donor-Ï€-acceptor （D-Ï€-A） chains（code as DC-S）, five dyes bearing asymmetry double D-Ï€-A chains containing threeÏ€-bond extended dyes bearing asymmetry double D-Ï€-A chains（code as DC-AS）, and six reference dyes bearing single D-Ï€-A chain（code as SC） have been engineered and synthesized as sensitizers for the application in DSSCs. All the dyes were based on carbazole, indole, diphenylamine, phenothiazine, and phenoxazine as donors and the structures of the dyes have been characterized by mass spectra （MS）, proton and carbon nuclear magnetic resonance, and elemental analysis technologies. The photophysical and electrochemical properties of the dyes were studied. DSSCs based on these dyes were constructed and detailed relationship between dye structures and solar cell performances has been investigated. At the same time, density functional theory calculations for the molecular structure and electron distribution of the dyes have been studied preliminarily.The significant differences in the photophysical and electrochemical properties of the sensitizers can be influenced by structure changes. The absorption spectra and electrochemical properties of DC-S were influenced slightly by different linker groups in the case of containing the same donors, and they were influenced by the electron donating property of donors in the case of containing the same linker group. For example, DC-S6 has a relatively broader absorption spectra and smaller E0-0 because the phenoxazine has stronger electron donor ability. The molar absorption coefficient of DC-S is twice to SC due to the twice D-Ï€-A units of the former, however, the absorption range of both is same. As for DC-AS, the absorption spectra of them are assigned to the overlap and complement of SC, and the stronger electron donating ability of the donor, the smaller E0-0 of them, for example, the following descending order of E0-0 is E0-0（DC-AS7）>E0-0（DC-AS5）>E0-0（DC-AS3）. As far as theÏ€-bond extended dyes bearing asymmetry double D-Ï€-A chains is concerned, the major absorption peaks of them show a slight blue shift compared to SC because of some form of molecular aggregates in the solution, but the range and intensity of absorption spectra are broader and stronger than the corresponding SC and the cosensitizers（mixtures by single D-Ï€-A dyes）.The photovoltaic performance of the cells was influenced further by the structure of the dyes. As for DC-S, the stronger electron donating properties of the dyes, the higher overall conversion efficiencies of the cells; and the efficiencies of the cells based on the dyes containing flexible linker groups are higher than those of the rigid linker groups. As for as DC-AS are concerned, in which a same donor in one D-Ï€-A chain of the dyes, the stronger electron donating properties of the donors in another D-Ï€-A chain, the higher efficiencies of the cells. The efficiency of the DC-AS4-sensitized cell is higher than the cell based on DC-AS3 due to the broader and stronger absorption and IPCE spectra for the former dye. The efficiency of DC-sensitized cell is higher than the cell based on corresponding SC due to more D-Ï€-A units absorbed on TiO₂ surface for DCThe results showed that the efficiency of the DSSCs based on DC-S6 was improved significantly when CDCA was added as co-adsorbent; however, the efficiency of the DSSCs based on SC-R2 was dropped under the same test conditions. The phenomenon may be ascribed to the enhanced bonding property on TiO₂ surface of DC-S6 initiated from its two anchor groups.The different dye-baths of same sensitizer for semiconductor sensitization have a crucial effect on the performance of the DSSCs due to the different absorption spectra different absorbed amount and different binding modes of anchored dyes on TiO₂ surface in various solvents. The results of the photovoltaic performances of the DSSCs revealed that the bigger polarity of solvents, the lower efficiency of the cells.The results of Density functional theory （DFT） calculations indicated that the HOMOâ†'LUMO excitation induced by light moved the electron density from the donor to the acceptor, thus allowing an efficient photoinduced electron transfer from the dye to the TiO₂ electrode.