Preparation And Research On The Photovoltaic Properties Of Modified Photoelectrodes Based On Tungsten Polyoxometalate

Dye-sensitized solar cells(DSSC)have been developed since 1991.As the most promising technology,the photovoltaic devices are well developed,however,there are some issues pertaining to dye-sensitized solar cells.TiO2 photoanodes based on the DSSC can only absorb ultraviolet light(λ < 387 nm),which makes a mismatch of the absorption spectrum of the solar cells with the solar irradiation spectrum.The wide bandgap of TiO2 causes insufficient electron injection driving force,which limits the increase of photocurrent density.The traditional ruthenium dye N719 has a weak light absorption in the visible region,consequently reducing the power conversion efficiency.Traditional Pt counter electrode(CE)is an ecpensive noble metal and scarce in nature.The high production cost of Pt counter electrode has hampered the further development of DSSC.Therefore,it is very important to prepare new composite materials for photoanodes,improving the catalytic performance and stability of the counter electrode.In this thesis,the polyoxometalate materials with excellent properties are introduced into the photoelectrodes of dye-sensitized solar cells in order to improve the performance of DSSCs.Three kinds of polyoxometalate compounds were synthesized and their photoelectrochemical properties were studied.Their adaptability for application in the traditional TiO2 photoanode DSSC system is investigated about the energy level match,response to the solar irradiation spectrum and the catalytic activity of DSSC.Moreover,the mechanism of improved efficiency is analyzed in terms of energy level structure,electron lifetime and stability of the DSSC.The results indicated that polyoxometalates modified TiO2 photoanodes extend the light absorption range up to the visible region,which plays a dual role of spectral compensation and spectral efficiency.The enhanced efficiency can be attributed to the reduced band gap of TiO2,improved efficiency of electron injection from N719 dye into TiO2 conduction band,photogenerated charge separation process.The polyoxometalate composite Pt counter electrode is an alternative and high-efficiency photocathode in DSSC.SiW11Ni and SiW11 Cu were prepared by a simple solvothermal method.The possibility of application to the dye-sensitized solar cells was evaluated by spectroscopic electrochemical and UV-visible near-infrared technologies.The roal of polyoxometalates in DSSC photoanode was studied by means of current-voltage curve,surface photovoltage voltage,electrochemical impedance spectroscopy,open-circuit voltage decay,and incident photon to current conversion efficiency.The results show that the two kinds of polyoxometalates can reduce the band gap width,enhance the driving force of electron injection,expand the absorption efficiency of solar spectrum,and improve the photocurrent density.Finally,it is found that the conversion efficiency of DSSC with polyoxometalate modified TiO2 photoelectrode was improved by 22% with 31% photocurrent gain.Introducing the SiW11 Ni into the photocathode of DSSC reduces the amount of Pt in noble metal in the counter electrode.The catalytic properties of the SiW11Ni/Pt CE were analyzed by electrochemical impedance spectroscopy,cyclic voltammetry curve and Tafel polarization curve.The results indicated superior catalytic activity for SiW11 Ni composite Pt counter electrode,higher cycling rate of I-/I3-in the electrode electrolyte and accelerated the regeneration rate of the dye.Compared to the DSSC with a pristine Pt photocathode,an improvement of 32% and 28% in the photocurrent and efficiency were obtained respectively.We then applied the SiW11 Cu into two photoelectrodes of the DSSC simultaneously,and optimized the cell structure and tested the performance of the DSSC.The results showed that SiW11 Cu plays a dual role in DSSC,which reduced the production cost of the cells(in terms of grams,the cost of SiW11 Cu is about 1/50 th than that of Pt).The power conversion efficiency improved by 16% and the short-circuit current density increased by 34%.Using a hydrothermal synthesis route,we synthesized Na3.5Co4[Bi2Co2W19.75O70(H2O)6]·39.5H2O(BiWCo)polyoxometalate which shows better light absorption performance and can be used as a photosensitizer in DSSC.The absorption range of the solar cells in the visible region was further enhanced up to 800 nm.The BiWCo compound increased the number of electrons injected into TiO2,inhibited the recombination of electrons,and improved the electron injection ratio.Compared with the blank cell,the conversion efficiency of BiWCo/TiO2 photoanode cell was increased from 6.38% to 8.16%,and the short-circuit current density was increased by 32%.