Theoretical Studies On The Dye Regeneration Mechanism In Dye-sensitized Solar Cells

Dye-sensitized solar cell (DSSC) is a new generation of sustainable photovoltaic devices. Due to its high incident photo-to-current efficiency (EPCE), thermal stability, simple production process, and low cost, dye-sensitized solar cell have attracted the scientist’s attention. Based on above unique advantages, DSSC promise to be the alternatives of traditional silicon cell and widely used in mechanical, bionic, military and other field. Most of the emphases of the DSSC researches are to enhance the incident photo-to-current efficiency by designing new dye sensitizer and optimizing the device preparation. Cell lifetime, however, is indispensable in valuing the cell performance. Dye regeneration is a key process to influence the cell lifetime and dye regenerate efficiency. This study focuses on the dye regeneration of dye sensitizer and halogen redox couple. Based on the theoretical calculation, the interaction behavior of dyes and redox couple before and after dye regeneration was confirmed and the intermediates in the dye regeneration were obtained.This paper is divided into five chapters.In chapter 1, we summarized of the basic structure and the working principles of DSSCs. In chapter 2, we have an overview of theoretical methods. Then, we showed the specific research about dye regeneration in the following chapter. The main contents are described as below:1. We report a DFT study of interaction between N3 dye and iodide ion under the influence of different deprotonation situations. There are two kinds of interaction mechanisms:Ru position and Car position. There were several stable intermedium forms in different deprotonation degree. The stability of these intermedium forms would be perturbed gradually while the number of eliminated protons increased. In the initial period of absorption and injection the dye will not attacked by the iodide ion. Additionally, dye with more carboxyls will reduce the activity of redox reaction. The iodide ion attacking made the charge contribute on N3 molecule more negative, nevertheless the N3I" still has ability to attract another iodide ion and the attacking of iodide ion will also influence the electronic transition and absorption properties.2. In order to explore the mechanism of dye regeneration, the intermediate forms of this reaction were searched by first principle calculations in this study. The possible intermediate forms were obtained by analyzed the reactivity of four organic dyes with different donors and two electrolyte ions (I" and Br"). Then comparing their structures, interaction energy and reaction free energy, most appropriate intermediate forms were screened out. We found Br-/Br3-could be a good redox couple if the redox potential matched with the energy gap of dye. In particular, steric-hindrance was likely the most influential factor in determining the intermediate forms.3. We report a theoretical study to ascertain dye regeneration in dye sensitized solar cells (DSSCs) with the TiO2-dye-electrolyte model. Oxidized L2 dye, three electrolyte main components and (TiO2)9 cluster has been selected in this model. The calculated results show that the combination capability of electrolyte to L2+@Ti02 is according to the order of I-> I3-> I2. Furthermore, oxidized dye together with electrolyte can get good performance of absorption in visible light region. Specifically, I" may accelerate the electronic transition and injection by participating in the positive charge transfer.I2 may lead to the opposite transition and I3- ions seem to be a mixed result of both effects. The charge transfer pathway during dye regeneration process has been found.