Theoretical Studies On The Molecular Design And The Effect Of Local Electric Field In Dye-sensitized Solar Cells

The energy and environmental crisis have been stimulating therapid design and development of renewable energy sources overthe past few decades. Many newly emerging solar-energy-basedrenewable energy sources are extensively investigated to relievethe pressure of the shortage of fossil fuels in the near feature.Because of the high conversion efficiency and the low cost, dye-sensitized solarcells(DSSCs) have attracted extensive attentions in recent years.In this thesis, a series of Ru(II) and metal-porphyrin dye sensitizers are designed and investigated by the quantum chemical method. Their geometrical structures,electronic structures, and other properties are systemically and detailedly discussed.In addition, the local electric field affects the performanceof dye-sensitized solar cells have been investigated.We hope that our calculations could facilitatefuture experimental, and provide an effective strategy to understand the functionality of DSSCs in detail.This thesiswas divided into five chapters.In charter 1, we made a summary of the basic structure and the fundamental principles of the DSSCs, illustrations about the main factors that can affect efficiency and performance of DSSCs devices, as well as the strategies had been proposed to enhance the photoelectric conversion efficiency. Moreover, in chapter 2, we have an overview of theoretical methods including the basic theory of quantum mechanics and themainquantum chemistry calculation method. Lastly, we showed the specific research about the DSSC system in the following Chapter. The main contents are described as below.1. Ruthenium(II) complexes, as the dye sensitizer in the solar cell system, has attracted great interest. In the present study, based on the ruthenium(II) complex N749, new sensitizers have been designed theoretically to increase the stability and the efficiency of dye-sensitized solar cell(DSSC).By investigating the ground state geometries, electronic structures, and spectroscopic properties with density functional theory(DFT) and time-dependent DFT(TDDFT), the orbital components and absorption transition have been obtained. The effect of tri-pyrrin ligand in the designed new sensitizerscan be demonstrated from our results. The results show that the absorption spectrums are systematically broadened and red-shifted with the increase sizes of the pyrrole ligands. The important unoccupied orbitals referred to chargetransfer are mainly from di/tri-pyrrinderivative groups.Consequently, the chargetransfer to the di/tri-pyrrin derivative groups has been strengthened. According to our study, the di/tri-pyrrinderivative ligand is more efficient thanthe NCS- ligand in absorbing visible light. The calculation results also indicate that the electronic structures of the N749 derived sensitizers are significantly influenced bythe different substituted positions of the thienyl groups on di/tri-pyrrin ligands.Thus, the efficiency of DSSCs would be different.Our research predicted that the Ru(II) complexes containing 5,10-(2-thienyl)-4,6,9,11-tri-pyrrin ligand may enhance the visible light absorption of DSSC.This is in accordance with the corresponding experiment. These results are expected to assist the molecular design for new dyes in future DSSCs.2. The dye sensitizers play an important role in dye-sensitized solar cell(DSSC). Owing to the synthetic challenge and cost of precious metal-complex dyes, increasing researcheshavebeen focused on the organic moleculedyes, porphyrin and light metal porphyrins dyes. In this paper, three natural porphyrin derivatives as dyeswith Ti O2 nanoparticulate model are studied theoretically usingdensity functional theory(DFT) approaches to explore their spectroscopic properties and application future in DSSC. The detailed orbital componentsand absorption transitionsof thsesporphyrin derivatives are analyzed from the calculated results. Key parameters of the short-circuit current density(Jsc)including light harvesting efficiency(LHE), electron injection driving force(ΔGinject) and nonlinear optical properties(NLO) were discussed. In addition, the calculated values of opencircuit photovoltage(Voc) for these dyes were also presented.The tetrapyrrole macrocycle of porphyrin with central metals Mg or Zn can enrich the absorption strength greatly. Our researchreveals that the Zn-porphyrin sensitizer can be used as potential sensitizer for DSSCs due to its best electronic and optical properties and good photovoltaic parameters.This study is expected to understand natural dye sensitizers andassist the molecular design of new dyes for the further DSSC improvement.3.The local electric field formed between dye sensitizers and semiconductor interface is one of key factors to determine the overall performance of dye-sensitized solar cells(DSSCs). Herein, a strategy has been proposed to explore the influence of the local electric field on the functionality of DSSCs of YD2-O-C8 dye via calculating the relevant properties in various electric field strengths. The YD2-O-C8 dye has been systemically studied with density functional theory(DFT) and time-dependent DFT(TD-DFT) for its electronic structure and optical properties in tetrahydrofuran(THF) solution. The absorption spectra are gradually narrowing and blue-shifting while increasing the electric field strength. Two key parameters of the light harvesting efficiency(LHE) and the Ti O2 conduction band shift(?Ecb) have been examined for the YD2-O-C8 sensitized Ti O2 system. It is found that it is of great importance to reduce the charge accumulation on the Ti O2 film, which lowers the electric field strength and shows the best performance of DSSCs. This study is expected to deepen our understanding of the function of local electric field and the operational principles of the DSSCs for further optimization.