Theoretical Study On Electronic Property And Photovoltaic Performance Of Lindqvist-and Keggin-type Polyoxometalates-Based Dyes

Polyoxometalates (POMs) are made of transition metals, which are linked by oxygen bridges. Because of its unique advantages such as structural diversity and physical and chemical properties, POMs have good applications in materials science, catalytic chemistry, electromagnetic materials as well as biological medicine and other fields. In addition, POMs are good electron acceptors, and have lower transition energy, so they have potential application in the dye sensitized solar cells (DSSCs). Looking for the appropriate sensitizers, and then improving the photoelectric conversion efficiency of DSSCs is the core issue of DSSCs research. Quantum chemistry calculation methods are used to study the relationship between the structure and properties of the compounds. It is important to understand the properties of this kind of compounds, and to guide the experimental synthesis, as well as to expand the application. In recent years, density functional theory (DFT) and computer technology have been continuously developed and updated, which is the prerequisite for designing the excellent POM-based dyes.In this paper, DFT and time dependent DFT (TDDFT) methods have been used to explore the electronic properties, geometrical structures, spectral and photovoltaic properties based on the derivatives of Keggin-and Lindqvist-type polyoxomolybdates. Through the analysis of relationship between spectral properties, electronic properties and geometric structure of POMs-based derivatives, the microscopic nature of POMs-based derivatives as the DSSCs photosensitizer were clarified. The main research work includes the following three aspects:1. A series of different transition metal-substituted Keggin-type phosphotungstate dyes were designed. The electronic properties, absorption spectra and performance parameters of dyes were systematically investigated by using DFT and TDDFT methods. The results reveal that the absorption spectra of systems [PW11O39MCH2COOH]5-/4- (M= Ru?(2), Ir?(3), Os? (4), Co? (5)) are red-shifted compared to the experimental system [PW11O39RhCH2COOH]5-(1). Especially, the spectra of systems 4 (M=Os?) has wide and strong absorption in visible region. And it has high electron injection efficiency and light harvesting efficiency, which may have important application in DSSCs.2. DFT and TDDFT calculations have been employed to investigate the light harvesting efficiency (LHE), dye regeneration efficiency (DRE), charge recombination efficiency (CRE), holes injecting efficiency (HJE) and reorganization energy (Ereorg) of POM-based dyes with electron donating/withdrawing groups. The results show that the maximum absorptions of dyes with electron donating groups (systems 1-3) are red-shifted comparing with those containing the electron withdrawing groups (systems 4 and 5). Balances all parameters, the performance of system 1 (-NH2) is better than others. Therefore, the introduction of-NH2 into POM-based dyes may improve the performance of dye in DSSCs.3. A series of hexamolybdate-based p-type DSSCs containing double D-?-A chains were studied by using DFT and TDDFT methods. The energy levels, absorption spectra, electronic transition character and photovoltaic parameters of dyes 1-6 were systematically evaluated. The highest occupied molecular orbital (HOMO) energies of all dyes are more negative than the VB of NiO and the lowest unoccupied molecular orbital (LUMO) energies are more positive than the I2/I3- redox level, which are in favor of the hole injection and dye regeneration. Dye 5 has three strong and broad absorption bands in 300-750 nm due to the introduction of thiophene. To a great extent, the introduction of thiophene is helpful to improve the light absorption intensity of the systems. The hole injection energy, dye regeneration energy and charge recombination energy of dye 5 are relatively small, which are advantageous to the hole injection and dye regeneration. The introduction of thiophene (dye 5) is the most effective strategy to improve the absorption and to incease the efficiency of dye in DSSC. So, the dye 5 is expected to have excellent performance for DSSCs.