Theortical Design Of New Dye Molecules To Increase Conversion Efficiency Of Dye-sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attracted considerable attention in recentyears as they offer the high conversion and easy processing of renewable energysources in low cost. Although significant progress has been made since their discovery,at present, DSSCs based on ruthenium sensitizers have reached power conversionefficiencies of about only12%. Recently, more attention has been focused in theapplication of metal-free organic/natural dyes as alternatives to the Ru complexes dueto their environmental friendliness. Even though the performance of DSSCs based onmetal-free dyes is slightly lower than or near to that of the ruthenium complexes, itcan be considered as a new generation of sensitizers. In recent years, considerableprogress has been made in the performance of DSSCs by experimental as well astheoretical methods, resulting in organic DSSCs that are reported to have anefficiency up to9.8%. There is still much room for molecular engineering to generatehigher conversion efficiencies. In order to increase the efficiency of DSSCs, it isimportant to search suitable dyes. Advancements in quantum chemical methods, inparticular the development of density functional theory (DFT) and time-dependentDFT (TDDFT), coupled with the nearly exponential growth of computer power, haveopened the door for evaluating and designing organic sensitizers with a reasonablelevel accuracy. In this master’s thesis, we designed some organic dyes for dye-sensitized solarcells in theoretical approach. There are four chapters in this paper. In Chapter one, wemade a summary of the solar cells and a simple introduction of DSSCs. In Charter2,an overview of theoretical methods including molecular orbital theory, configurationinteraction theory, perturbation theory, density functional theory, electronic excitedstate theory, the choice of basis set, absorption spectrum. In Charter3and4, wedesigned some organic dyes and the main contents of research are as follows:1. Based on dye V5, we designed three metal-free organic dyes named DPPB,DPPS, and DPPO in theoretical approach. We screened these dyes from the followingaspects, geometries, electronic structures of ground and excited states as well asdipole moments. The calculated results show that the quasi-planar π conjugationbetween bridge and acceptor in DPPO may speed up the charge transfer from thedonor to acceptor and therefore enhance the efficiency of this solar cell system. Thedesigned dyes have smaller HOMO-LUMO energy gap values, and the absorptionbands are broadened and shifted to longer wavelengths compared toV5sensitizer.With the consideration of conduction band shift of semiconductor, these theoreticallydesigned dyes would result in larger Voccompared with V5. We hope the currentexploration can offer insight and understand for designing high efficiency dyes forDSSCs.2. Using density functional (DFT) and time-dependent density functional(TDDFT) methods, we designed three porphyrin dyes with different acceptors, suchas carboxylic acid, cyanoacrylic acid, and2-cyano-N-hydroxyacrylamide based onYD2-o-C8. Compared to the so far best sensitizer (YD2-o-C8), these design dyeshave small highest occupied orbital to lowest unoccupied orbital (HOMO-LUMO)band gaps, wide absorptions with large oscillator strength of Q bands. The designedDye2and Dye3are better in electronic coupling with TiO2than YD2-o-C8, whileDye1is similar to YD2-o-C8. Moreover, electron injection is much faster for Dye2and Dye3than for YD2-o-C8. All the features show that our designed dyes, especially Dye2and Dye3, would have better performance than the existing dye YD2-o-C8. Inaddition, our results point out that the2-cyano-N-hydroxyacrylamide is a newpromising acceptor. This study is expected to assist the molecular design of newefficient dyes for dye-sensitized solar cells (DSSCs).