Theoretical Studies On Charge Transfer Characteristics And Absorption Spectra Of Porphyrin Modified Lindqvist-type Polyoxometalate Derivatives

Energy crisis also make the environmental pollution problem more and more serious and we are in urgent need of searching clean and safe energy resources and technologies.Thus,the intensive research has focused on effectively harnessing solar energy by using photovoltaic cells.Particularly,dye-sensitized solar cells(DSSCs)have attracted continuous attention because of their advantages,such as low cost,high efficiency,and simplified production processes.As a light harvesting reagent in DSSCs,the performance of dye sensitizer has a direct impact on the photoelectric conversion efficiency and light harvesting efficiency.Polyoxometalates(POMs),as one kind of significant anionic metal-oxide clusters,have become a subject of general interest not only for their excellent electronic properties and strong electron-withdrawing ability,but also for extensively employed as inorganic building blocks for the construction of organic-inorganic hybrid materials.These hybrids have potential applications in DSSCs served as sensitizers.In this paper,POM-based organic-inorganic hybrids were designed.Density functional theory(DFT)and Time-dependent DFT(TDDFT)calculations on electronic structures,absorption spectra and charge transfer(CT)character of designed hybrids were performed in order to explore their applications in DSSC.The work is mainly divided into the following three parts:1.Based on porphyrin derivative(system 1),Lindqvist-,Keggin-,Anderson-type POMs organic-inorganic hybrids(system 2-4)were designed aiming to investigate the charge transfer character and screen the high performance p-type sensitizers.The DFT and TDDFT calculations show that the Lindqvist-and Keggin-type POMs affect the lowest unoccupied molecular orbital energy levels,while the Anderson-type POM does not contribute to the frontier molecular orbitals.Furthermore,the absorption spectra of Lindqvist-type POM porphyrin derivative(system 2)exhibits strong and broad absorption in the visible region and is red shifted about 100 nm in comparison with system 1,indicating that it will be a promising dye sensitizer.2.Derived from an excellent sensitizer,Zn-porphyrin bearing phenylethynyl substituent,a series of Zn-porphyrin-polyoxometalate hybrids with different ?-linkers were designed as sensitizers in p-type dye-sensitized solar cells.UV-vis spectra and CT characters of studied hybrids were investigated by DFT and TDDFT methods to reveal the influence of ?-linkers on sensitizers.The long ?-linkers and high delocalization of systems 2-5 lead to the red shift and broadened absorption peak compared with system 1.CT analysis on the amount of transferred electrons(qCT),CT distance(dCT)and t index assessing the charge separation extent indicate that system 3 reaches the maximum CT and spatial charge separation.Meanwhile,its spectrum well overlaps with the solar spectra.Therefore,system 3 would be a promising ptype sensitizer.3.A series of donor-acceptor-?-bridge-acceptor(D-A-?-A)type hybrids were designed by introducing Lindqvist-type POM into synthesized dyes aiming at revealing the impact of POMs on the performance of dye.The spectroscopic properties and CT characters were studied using DFT and TDDFT methods.Compared with the synthesized dyes,the designed dyes show larger responsive range of spectra.CT of the designed dyes are all from donor side to POM and-COOH,suggesting strong electronic coupling with TiO2.Further analysis on CT parameters indicates that the introduction of POM results in stronger,longer,and tighter CT process.Among them,WS-51-POM possesses a wide range of spectrum overlaping best with solar spectra as well as excellent CT ability.These designed dyes can be promising candidates for DSSCs.