Theoretical Studies On Functionalized Organic D-π-a Dyesused For Dye Sensitized Solar Cells

Solar cell, as the clean and pollution-free renewable energy, has received extensive attention of the world due to the energy crisis and the increasingly severe environmental pollution. On the one hand, the high cost of the traditional crystalline silicon solar cells limits its large-scale application. On the other hand, dye sensitized solar cell(DSSC) become a promising alternative because of the relative high efficiency, low production cost, flexibility, easy fabrication. However, the photoelectric conversion efficiency still lags behind the traditional inorganic silicon solar cell. Thus, in order to accelerate the process of commercialization, we still need to further improve the photoelectric conversion efficiency of DSSC. As an important part of DSSC, the performance of sensitizers directly determines the efficiency of the device. Therefore, it is important to reveal the essential relationship between sensitizers’ structure and properties.In this thesis, Density Functional Theory(DFT) and Time-Dependent Density Functional Theory(TDDFT) were applied to investigate the functionalized organic D-π-A sensitizers used for DSSC. The important parameters affecting the efficiency of the DSSC, including geometries, electronic structures and photo-physical and chemical properties were taken into consideration to explain the experimental phenomena and predict the performance of the theoretically designed organic sensitizers and construct the relationship between the structure and properties of the sensitizer. We also hope this work could provide some theoretical guidelines for the development of high efficient DSSC. The main research content of this thesis includes the following four parts:1. Considering that tiny structure difference of sensitizers could induce significant efficiency difference of the DSSC, DFT and TDDFT calculations have been performed on the factors influencing the performance of the device, including light harvesting efficiency, electron injection driving force, the vertical dipole moment of the dyes upon adsorption onto Ti O2 and the number of the electrons transferred from the dye to the semiconductor after photoexcitation. As compared with the available experimental results, the reliability of the theoretical methods has been proved. At the same time, we found that the bithiophene moiety(4) instead of thiophene moiety(1) in the π spacer could extend the conjugated degree, and red shift the absorption spectrum. Thus the enhanced light harvesting ability are beneficial to the improvement of the short-circuit current density despite the similar driving force. We also found 4 with bithophene moiety has increased vertical dipole moment and similar electron transfer number, which is beneficial to the increase of the open-circuit voltage. Therefore, we expect 4 is a promising sensitizer with high performance.2. DFT and TDDFT calculations have been performed to investigate the conjugation order effects on the performance of the organic D-π-A dyes. There organic sensitizers with just different conjugation order were under study. We found that this modification has negligible effects on the light harvesting ability, electron injection driving force and electron numbers transferred from the dyes to the semiconductor. However, the vertical dipole moment and the charge recombination process were significant affected. And the calculation result is in good agreement with experimental data, further verified the reliability of the theoretical parameters applied to evaluate the performance of dye. This work also demonstrated that the π spacer sequence is very important to design and synthesize efficient organic sensitizers. And besides the commonly considered light harvesting ability, electron injection ability, the relation between dye molecular structure and charge recombination process should also be considered.3. In order to further improve the open circuit voltage of DSSC, based on the experimentally synthesized organic dye C219, we designed another three organic dyes with different electron donors. The electron donor effects on the performance of the DSSC especially the open circuit voltage were investigated through DFT and TDDFT methods. The calculation results show that 2 with indoine donor can cause the greatest degree of conduction band energy shifts, while others have similar shift. The electron density in the conduction band related with the electron injection and charge recombination was decreased in the order of 3 > 2 > 1 > C219, implying that as compared with C219, the designed dyes should have larger open circuit voltage potential. At the same time, we also found that 3 with coumarin donor should show larger short circuit current due to the enhanced light harvesting ability with red shift absorption spectrum and larger electron injection rate. Considering the factors affecting the performance of the cell, we could get the conclusion that 3-based solar cells may show higher performance than that of C219-based cells.4. Considering that the organic dye with the new anchor group 2-(1,1-dicyanomethylene)(DCRD) shows higher performance than the dye with the commonly used anchor group cyanoacrylic acid(CA), we performed a DFT and TDDFT calculation to shed light on the strengths and weaknesses of DCRD. After establishing the most stable absorption mode of DCRD on the(Ti O2)48(101), the interaction of the sensitizer, semiconductor and the electrolyte were investigated. We found that dye with DCRD could red shift the absorption spectrum and thus enhance the light harvesting ability through enhancing the matching degree between the absorption spectrum and the solar photo flux spectrum, which is beneficial for the improvement of the short circuit current. However, the S atom in DCRD could accelerate the charge recombination process through the dye-I2 interaction, which could lower the electron density in the conduction band and thus reduce the open circuit voltage. Therefore, as compared with CA, DCRD group is more suitable for the iodine free based DSSC. Moreover, we found that if the S atom in DCRD could be replace by O atom, the interaction energies between the DCRD and I2 could be significantly reduced and thus charge recombination process could be inhibited to some extent.