Theoretical Studies On Electron-withdrawing Groups Modification And Regeneration Mechanism Of D-?-A Dyes For Dye Sensitized Solar Cells

As the energy shortage and environmental disaster being more and more serious, the desire for safe, clean, pollution-free and renewable energy sources is more and more intense. Solar energy, as the perfect energy, has received extensive attention of the world. They want to get the higher effective utilization rate of energy through photoelectric conversion device. Dye-sensitized solar cells ?DSSCs? have attracted widespread attention since repored in 1991. Compared with the traditional silicon based solar cells, DSSCs own many advantages, such as the low cost, easy fabrication, environment friendly. Especially, the organic DSSCs have got more and more attention because of its abundant material, clean and non-toxic feature. At present, the efficiency of organic DSSC has reached to 12.5%. As the important part of DSSC, the performance of dyes directly determines the efficiency of the cell. Therefore, it is important to investigate thoroughly the relationship between the structure of dye and properties, and quantum chemistry is the effective method which is used in our paper.This paper mainly includes three parts of work:1. We performed this work in order to rationalize the negative effect of introducing different electron-withdrawing groups in phenylene-based ?-spacers on the performance of dye-sensitized solar cells based on five organic dyes consisting of triphenylamine as donor and cyanoacrylic acid as acceptor. Density functional theory ?DFT? and time-dependent DFT calculations on UV-absorption spectrum, the electron injection driving force, the shift of the conduction band energy level and the interaction energy of dye-I₂ and so on, which are associated with the performance of cell, were carried out. The results reveal that charge recombination between injected electrons and iodine as well as the electron injection driving force limit the open-circuit photovoltage and the short-circuit current density, respectively, eventually lead to the reduce of conversion efficiency. Therefore, in designing and developing more efficient dyes in future, the interaction between dyes and I₂ and the electron injection efficiency should be taken into account as the critical factors for the efficiency of DSSC, except for considering the absorption spectrum.2. Based on the high-efficient dye C275, seven dyes 1-7 with different electron-withdrawing groups have been designed. The geometries, electronic structures, absorption spectraum, electron injection life, charge recombination and some other factors associated with the properties of dyes have been investigated by DFT and TDDFT calculations, to give a deeper understanding of J_sc and V_oc. In order to further improve the power conversion efficiency of DSSCs, the theoretical maximum short circuit current density (J_sc^max) was calculated to evaluate the light harvesting ability. And the electron injection lifetime ??? was used to theoretically investigate the electron injection efficiency. Furthermore, the distance ?r? between the semiconductor surface and the dye cation hole was used to estimate the charge recombination process. The theoretical results reveal that dye 7 exhibits superior performance compared with C275, which could be the most promising candidate for organic dye in DSSCs.3. The regeneration of dye sensitizer molecules by iodide is a key process to determine the cell lifetime and dye regenerate efficiency in dye-sensitized solar cells. In order to explore the mechanism of dye regeneration, three possible kinds of dye regeneration reaction processes for two organic dyes by iodide have been investigated theoretically. The objective of this study is to interpret the significantly different regeneration rate constant between the organic dyes in the change from oxygen to sulfur in two triphenylamine donor unit. The corresponding surface electrostatic potential, the interaction energy and the Gibbs free energy differences ??G? of every reaction steps are calculated to reveal the different effect between iodide and O/S-containing organic dyes. The data suggest the direct reaction between ?dye?⁺ and I₂^- is the most favorite regeneration mechanism due to the lowest ?G barrier among three possible processes. Furthermore, the S-substitution in donor unit further decreases ?G barrier and positively impacts the regeneration rate.