Theoretical Studies On The Modification Of Dyes And Influence Of Semiconductor Doping In Dye-sensitized Solar Cells

A dye-sensitized solar cell is mainly composed of a semiconductor photoanode, a photosensitizer, an electrolyte and a counter electrode, which belongs to the group of thin film solar cells. Dye molecules absorb sunlight, capture photons, separate excitons and generate the injecting electrons; the semiconductor accepts injected electrons, transports electrons to FTO and electrons arrive at the counter electrode through the external circuit. The photovoltaic performance of dye-sensitized solar cells is largely determined by the electric properties of dye molecules and semiconductor. In this paper, we study LJBs organic dyes with different fluoro-substitutions and doped TiO₂ in detail for improving the photoelectric conversion efficiency. The theoretical simulations would provide valuable guidelines for designing more efficient dye-sensitised solar cells.The paper is divided into six chapters. The chapter 1 contains the history of the solar cell and developments of dye-sensitized solar cells. We expound the working principle of solar cells and develop the approach to improve the short-circuit current density and open-circuit voltage. The chapter 2 includes quantum mechanical principles and different modern quantum chemical calculation methods. From chapter 3 to chapter 6, we display the theoretical studies of fluoro-substituted organic dyes and doped TiO₂ in dye-sensitized solar cells based on DFT caculations. The research contents are divided into four parts and described as follows: 1. Theoretical studies of fluorine substituent effect on organic photo-sensitizers in dye sensitized solar cellsBased on titanium dioxide model and LJBs sensitizers（triphenylamine as the donor, 3, 4-ethylene dioxy thiophene plus various functionalized phenylenes as the π-spacer, and cyanoacrylic acid as the anchoring group） with different connection types on TiO₂ substrate, the reasonable dye-TiO₂ connection has been located. The results show that the dissociative adsorption of LJBs is more thermodynamically favorable than the neutral molecule bound configurations. LJBs adsorb onto TiO₂ via interaction between 3d orbital of surface Ti atom and 2p orbital of N/O atoms in the acceptors. This result is confirmed in both neutral and dissociation forms of LJBs molecules. According to our calculations, adding a meta- fluorine substituent to the phenyl group of cyanoacrylic acid（LJB-Fm） may damage the planarity and conjugation. Consequently, the light harvesting efficiency decreased, and that is particularly unfavorable for the DSSCs application. The Ortho F-substituted dye（LJB-Fo）, however, exhibited enhanced light absorption and more efficient intra-molecular charge transport. The bigger Jsc, Voc values of LJB-Fo system predicts its superior DSSCs performance. Additionally, LJBs anchored on TiO₂ surface via group-COO（LJB-H and LJB-Fo） lead to an indirect mechanism for electron injection. While LJB-Fm preferred to direct electron injection mechanism due to the strong orbital-coupling between sensitizer and the TiO₂ substrate. 2. Enhancing electron injection in dye-sensitized solar cell by adopting W⁶⁺ doped TiO₂ nanowires: A theoretical studyAs a donor type dopant in titanium dioxide, W⁶⁺ is found to move the conduction band edge（CB） of TiO₂ downward and influence the electron injection process in dyesensitized solar cells. In order to investigate the electron injection capability and efficiency optimization of DSSCs via W⁶⁺ doping, the geometry and electronic properties both of free and adsorbed TiO₂ nanowires are investigated in detail based on extensive density functional theory calculations. A four-layer（TiO₂）12 nanowire with twelve possible doping sites is set up and the effect of W⁶⁺ in different positions are analysed in detail. The results indicate that, in the W⁶⁺ doped（TiO₂）12 systems, Ti-OW（OW stands for the Oxygen atom which connected with the W atom） bonds are longer than the corresponding Ti-O bonds in（TiO₂）12. The CB edge is influenced by the doping position significantly. CB energy level moves upward gradually with W⁶⁺ moving deep in（TiO₂）12. For all adsorbed catechol/（TiO₂）12 systems with W⁶⁺ dopant, the Lowest Unoccupied Molecular Orbital maps are distributed over the layer where W⁶⁺ was doped. The W⁶⁺ doping position plays crucial importance in the electron injection and electron transport process in DSSCs. Therefore, different W⁶⁺ doping in TiO₂ would be a feasible strategy to control and improve semiconductor materials for more electron injection efficient DSSCs. 3. Theoretical studies of heteroatom-doping in TiO₂ to enhance the electron injection in dye-sensitized solar cellsThe density functional calculations have been explored to analyze the structural, electronic and charge transfer properties of the doped TiO₂ substrate and catechol-TiO₂ interfaces for dye-sensitized solar cells. The results demonstrate that dopant W⁶⁺ moves the CB（conduction band） edge downward and introduces 5d-unoccupied orbitals located in the CB bottom of the TiO₂. On the other hand, dopant Zn2+ shifts the CB edge upward with an insertion of 3d-occupied orbitals into the VB（valence band）. In catechol-TiO₂ systems, W⁶⁺ enlarges the energy difference between the LUMO of catechol and LUMO of TiO₂, which enlarges the driving force for electron injection in turn and results in an increased short circuit current（Jsc）. Our modeling injection dynamics and quantitative bader analysis of the interfacial charge transfer have revealed that the catechol-TiO₂ doped with W⁶⁺provides faster and more electron injection for dye sensitized solar cells（DSSCs）. While the Zn2+ doped system exhibits lower electron efficiency due to the minimized energy difference between the catechol LUMO and TiO₂ CB. 4. Theoretical Studies of the adsorption of Eosin Y on TiO₂ and the electron injection in dye-sensitized solar cellThe density functional calculations have been employed to analyze the structural, electronic, and charge properties of the Eosin Y sensitized different interfaces for dyesensitized solar cells. The obtained results indicate that Eosin Y sensitized TiO₂ with configuration H shows a higher total energy above 0.619 eV than configuration B; while Eosin Y adsorbs on TiO₂ with configuration B has larger adsorption energy than configuration H. Therefore, the configuration B would be more favorable in DSSCs applications. The electron-injection dynamics and quantitative analysis of charge transfer have been simulated and computed, respectively, and the results reveal configuration B would provide more injected electrons and faster electron injection process.