Synthesis,Structure And Characterization Of Polynuclear Copper Substituted Sandwich-type Polyoxometalates And Its Application In Photoelectrodes Of Dye-sensitized Solar Cells

The sharp consumption of fossil energy and the environmental problems caused by it have led to an increasing demand for energy in the development of today society.To this end,we need to develop and use new sustainable energy to replace fossil energy.Solar energy is the cleanest and most sustainable energy source that is most easily available and widely used.Dye-sensitized solar cells（DSSC）can convert solar energy into electricity,which has many advantages as one of the most promising third-generation photovoltaic device.For example,it has lower manufacturing costs and convenient manufacturing methods.These advantages make DSSC have great potential to replace traditional silicon-based photovoltaic devices,but DSSC have yet to be studied in large-scale commercial applications.DSSC is mainly composed of photoanode,counter electrode and electrolyte.The photoanode is based on semiconductors（such as TiO₂,ZnO,etc.）,which mainly plays the role of transporting electrons and adsorbing dyes,and affects the photovoltage and photocurrent.But the electronic reorganization existing between the dyes and TiO₂ will reduce the DSSC performance,and the ultraviolet absorption range of TiO₂ is narrow.In order to improve the photoelectric conversion efficiency of DSSC,it is necessary to improve the photoanode.And polyoxometalates（POM）have very special redox properties,which can retain the original structure after the multi-electron reversible redox process.The introduction of transition metals allows the absorption spectrum of the sandwich-type POM to extend from a single ultraviolet light to visible light,even the entire spectrum.And the sandwich-type POM can serve as an electron acceptor to capture photo-generated electrons in the semiconductor conduction band,which can inhibit electron recombination to a certain extent.Such properties make the application of polyacids in photoanodes very promising.Doping the sandwich-type POM into photoanodes to suppress electron recombination is an important method to reduce photoelectron losses.In this paper,we synthesized two new organic-inorganic hybrid sandwich POM as electron acceptors to apply in photoanode,which effectively improved the photovoltaic performance of DSSC.In addition,we choose a pure inorganic sandwich-type polyacid with strong absorption in the visible region,and compound this POM with TiO₂ to improve the light absorption of TiO₂ and electron transport effect,thereby improving the photoelectric performance.The specific work is as follows:1.Two organic-inorganic hybrid sandwich tungstates was hydrothermally synthesized,Hydrothermal synthesis of two organic-inorganic hybrid sandwich tungstates,compound 1 is H₆[Cu（im）₄][Na₂Cu₃（H₂O）₃Cu（H₂O）₂(SbW₉O₃₃)₂]·36H₂O（1）（im=imidazole）,which is a quad-core sandwich polyoxotungstate,compound 2 is H₅[Cu(im4)][Na（H₂O）₂Cu₅（H₂O）₅(AsW₉O₃₃)₂]·31H₂O（2）,which is a five-core sandwich polyoxotungstate.The properties were characterized by infrared（IR）,powder X-ray diffraction,solid diffuse reflectance spectroscopy,thermogravimetry,and cyclic voltammetry.The[Cu（im）₄]²⁺cation and the sandwich POMs unit of two compounds are alternately connected to form a one-dimensional chain structure.We study compound 1 application in DSSC.The energy level calculated by the cyclic voltammetry curve is-4.89 eV,shows that the energy level of compound 1 matches the conduction band of TiO₂.Compound 1 can be used as an electron acceptor in a photoanode,which can accelerate the movement of electrons.At the same time,the addition of POMs increases the absorption of light energy by TiO₂,which can improve the photoelectric performance of DSSC.Adding compound 1 to TiO₂ as a photoanode of DSSC has a significant effect on the improvement of the photoelectric conversion efficiency of DSSC.The photoelectric conversion efficiency of DSSC is increased from 6.08%to 7.71%,which is 26.8%higher than that of pure P25.2.Apply pure inorganic tungstate Na₉[Na₃（H₂O）₆Cu₃（H₂O）₃(SbW₉O₃₃)₂]·40H₂O（Cu3Na3）in photoanode to improve the photoelectric conversion efficiency of DSSC.The properties were characterized by infrared（IR）,powder X-ray diffraction,solid diffuse reflectance spectroscopy,thermogravimetry,and cyclic voltammetry.The LUMO energy level calculated from the cyclic voltammetry curve is-5.02 eV,shows that the LUMO energy level of the compound Cu3Na3 is lower than that of TiO₂,indicating that the compound Cu3Na3 can be used as an electron acceptor in photoanodes.The solid diffuse reflectance spectrum shows that the compound Cu3Na3 has a wide ultraviolet absorption spectrum range（200-900 nm）.Using sol-gel method to compound it with TiO₂ can extend the absorption range of TiO₂ to the visible region and improve the light absorption of TiO₂.The compound Cu3Na3 has a significant effect on the efficiency of DSSC after being applied to the photoanode.The compound Cu3Na3 improves the efficiency of DSSC from 6.08%to 6.91%,and the photoelectric conversion efficiency is improved by 13.66%compared to pure P25.