Synthesis And Properties Of Carbazole-Based Sensitizers For Dye Sensitized Solar Cells

Compared with inorganic silicon-based solar cells, dye-sensitized solar cells (DSSCs), due to their relatively simple structure, easy preparation and low cost, have become one of the most promising photovoltaic technologies. Sensitizers, the key components for DSSC, are divided into two categories:metal complex dyes and metal-free organic dyes. Metal-free organic dyes have received more and more attention due to structural diversity, easy preparation, low cost and high molar extinction coefficient in contrast to metal complex dyes. In addition, the optical and electrochemical properties of organic dyes can be tuned easily through structural modification. This dissertation focuses on the synthesis and properties of the novel organic dyes and is composed of the following two parts:1. Synthesis and properties of indolo[3,2,1-jk]carbazole-based dyesFive organic dyes (L-1-L-5) with typical D-π-A skeleton, with rigid planar indolo[3,2,1-jk]carbazole as the electron donor, cyanoacetic acid as the electron acceptor and different aromatic rings (thiophene, benzene) as the π-conjugated bridges, were synthesized. All of them were characterized by NMR (1H NMR,13C NMR) and mass spectra (MS). The UV-Vis absorption and fluorescence emission spectra properties of the dyes were investigated. The results showed that the dyes L-1-L-5exhibited strong absorptions in the300～500nm range and strong fluorescence range from400to600nm. When adsorbed on TiO2surface, the maximum absorption peaks of all the dyes showed significant redshifts, which would be beneficial to harvesting sunlight. In addition, the introduction of thiophene expanded π-conjugated system and the absorption spectra, which was helpful to increase the absorption ability of the dyes. The electrochemical tests showed that the HOMO energy levels of all the dyes were higher than the T/I3-redox potential. Therefore, the oxidation state of dyes can be effectively reduced by I-/I3-. While the LUMO energy levels of all the dyes were more negative than the TiO2conduction band energy level, which meant there was enough driving force to inject the electron of excited dye quickly into the TiO2conduction band. Molecular simulation results showed that intramolecular charges of all the dyes can be effectively separated under the condition of photo-excitation. The J-V curves showed that the dye L-2displayed the highest photoelectric conversion efficiency (3.21%), which is comparable to the conversion efficiency of N719(3.91%) under the same test conditions.2. Synthesis and properties of alkoxylated carbazole-based dyesTwo types of carbazole-based dyes (L-6～L-11and L-12～L-17) were synthesized by introducing alkoxy groups (-OCH3,-OC8H17-n) at3,6-or2,7-positions on carbazoles as electron donors, cyanoacetic acid as the electron acceptors and different aromatic rings (thiophene, benzene) as the π-conjugated bridges. All of them were characterized by NMR (1H NMR,13C NMR) and mass spectra (MS). The UV-Vis absorption and fluorescence emission spectra properties of these carbazole-based dyes were investigated. The results showed that the dyes L-6～L-11and L-12～L-17exhibited strong absorptions in the300～500nm range. The spectra properties of L-10and L-17with2,2’-bithiophene as π-conjugated bridge were superior to other dyes with similar structures. Therefore, L-10and L-17exhibited better optical absorption properties. Compared with the absorption spectra in solution, when adsorbed on TiO2surface, the maximum absorption peaks of the two types of dyes showed significant redshifts. We also found the spectral properties of3,6-dialkoxylated carbazole dyes were superior to those of2,7-dialkoxylated carbazole ones, which indicated that dyes L-6～L-11had stronger light-harvesting capability. The electrochemical properties showed that the properties of3,6-dialkoxylated carbazole dyes were better than those of2,7-dialkoxylated carbazole dyes. The HOMO energy levels of the two types of dyes were higher than the I-/I3-redox potential. Therefore, the oxidation state of dyes can be effectively reduced by I-/I3-. While the LUMO energy levels of all the dyes were more negative than the TiO2conduction band energy level, which meant there was enough driving force to inject the electron of excited state dye quickly into the TiO2conduction band. Molecular simulation results showed that under the condition of photo-excitation, intramolecular charges of the two types of dyes can be effectively separated. The J-V curves showed that the two types of dyes with long alkoxy chains could effectively inhibit recombination between the injection electron and electrolyte, hence improved the open circuit voltage. Comparing the photoelectric properties between the two types of dyes, we found the short-circuit current densities of3,6-dialkoxylated carbazole dyes were higher than those of2,7-dialkoxylated carbazole dyes with similar structures. The highest photoelectric conversion efficiency for3,6-dialkoxylated carbazole-based dyes was3.21%(L-9) and the lowest was1.62%(L-11). While for2,7-dialkoxylated carbazole dyes, the highest photoelectric conversion efficiency was only1.61%(L-13, L-16). We conclude that the photoelectric properties of3,6-dialkoxylated carbazole dyes were better than those of2,7-dialkoxylated carbazole ones.