Theoretical Studies On The Application Of Zinc Porphyrin And Triphenylamine-based Organic Dyes In Dye-sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are the simulation of natural photosynthesis principle,by using dye molecules to absorb sunlight and thus convert light energy into electrical energy,which are the third generation of sustainable photovoltaic devices, with the followingadvantagements including high photoelectric conversion efficiency, thermal stability, simpleproduction process, and low cost. DSSCs have widely attracted the scientists’ attention for thelast twenty years. Dye sensitizers in DSSCs are responsible for collecting photons andexcitons separation, and their spectral absorption and orbital energy levels are the key factorsthat can determine the photoelectric conversion efficiency of the DSSCs devices. Thus, todevelop dyes with high-performance is an important research direction of DSSCs. In this study,density functional theory (DFT) method has been used to design and improve a series of zincporphyrins and triphenylamine-based donor-conjugation-acceptor (D-π-A) type organic dyemolecules. By changing the π-conjugated bridge with thiophene and the introduction ofadditional electron-withdrawing heterocyclic group (for instance diketopyrrolopyrrole andbenzothiazole, et al.), as well as different acceptor/anchor groups, the molecular propertiescould be modified. We hope the theoretical simulations can help to design more efficient dyesensitizers and provide theoretical support for DSSCs study.This paper is divided into five chapters. In charter1, we made a summary of the basicstructure and the working principles of the DSSCs, illustrations about the main factors that canaffect efficiency and performance of DSSCs devices, as well as the strategies toexperimentally and theoretically design organic dyes. In chapter2, we have an overview oftheoretical methods including the basic theory of quantum mechanics and the quantumchemistry calculation method main. Finally, we showed the specific research about the DSSCsystem in the following Chapter. The main contents are described as below.1. The density functional theory (DFT) and time-dependent DFT (TD-DFT) approacheshave been applied to obtain the optimized geometries, electronic structures, molecular orbitalsand absorption spectra of a series of meso-substituted zinc porphyrin analogues with phenyl and thiophene groups as the π bridging unit and cyanoacrylic acid as the acceptor unit. Theresults showed that the introduction of thiophene group increases the orbital splitting andchanges the absorption spectra properties significantly. It is indicated that when there is onlyone thiophene group included in the π bridge, the oscillator strength of B absorption band ismuch stronger. The increasing length of thiophene chain just changes the scope of specificabsorption enhancement. The effect of attaching an additional electron-donating groupdiphenylamine instead of phenyl to the porphyrin core also has been shown. It is found that thediphenylamine group reduces the band gap, and leads to facile intramolecular charge transferfrom diphenylamine and porphyrin ring unit to acceptor unit. These kinds of zinc porphyrinanalogues have the LUMO energy close to the conduction band of TiO2and more red-shiftedabsorption spectrum compared with phenyl substituted analogues.2. A series of organic donor-π-acceptor dyes with difference in π spacer have beendesigned and investigated theoretically as sensitizers for application in dye-sensitized solarcells. Density functional theory and time-dependent density functional theory calculationsreveal how the additional electron-withdrawing diketopyrrolopyrrole unit and π spacer orderinfluence the physical properties of the dyes, including spectral response, light harvestingefficiency, and electron injection rate. The results show that auxiliary acceptor leads to thegreatly red-shifted of the charge-transfer absorption band. Meanwhile, the number and sortorder of thiophene groups can significantly tune the orbital energy levels and change theelectronic transition processes. The thiophene groups locating between triphenylamine anddiketopyrrolopyrrole units would conduce to the light absorption and electron injectionefficiency. The results also suggest that the donor-acceptor-π-acceptor type dyes have indirectelectron injection mode compared with the direct mode in the dyes without containingdiketopyrrolopyrrole.3. D-A-π-A dyes have exhibited several excellent advantages including optimized energylevels, distinct improvement of photovoltaic performance and stability. By modulating theauxiliary acceptor and acceptor unit, the efficiency of D-A-π-A dyes based dye-sensitized solarcells can be further improved. Based on density functional theory methods, sixteendimethoxyl-substituted triphenylamine based D-A-π-A dyes composed of different acceptorand auxiliary acceptor groups were designed, and other four referenced homologous D-π-Adyes were also involved to compare in this work. The properties of all the dyes, includingintramolecular charge transfer, lighting harvesting efficiency, kinetic of electron injection, andvertical dipole moment, have been investigated theoretically to screen out the dyes whichwould produce high efficiency. Then the interaction between the selected dyes and the electron acceptor in electrolyte was discussed to reveal the interfacial charge recombination process. Incomparison with other dyes, TNA4would be a promising candidate for its better performanceon key parameters and achieving a balance between competing factors. Compared withcyanoacrylic acid,2-(1,1-dicyanomethylene) rhodanine group can serve as an excellentacceptor for future DSSCs applications.