Studies On Hierarchical Structured Photoanode And Platinum-free Counter Electrode Materials In Dye-sensitized Solar Cells

In dye-sensitized solar cell(DSSC),the photoanode acts as a supporter for dye adsorption and electrons separation and transmission;the counter electrode can collect electrons from the external circuit and catalyze the reduction reaction of electrolyte.The compositions and structures of photoanodes can affect the device performances,in which the TiO2 is the most commonly used photoanode material,and its conductive band receives photogenerated electrons from the dye and transfers them to the external circuit.However,the electron mobility is slow in porous TiO2 nanoparticles films,which increase the recombination probability of photogenerated electron-hole pairs.Through designing the composition,structure and morphology of the photoanodes,the light utilization efficiency and electron transport rate of the photoanodes can be improved,and the recombination can be reduced.On the other hand,as a commonly used catalytic material for counter electrodes(CEs),platinum(Pt)always increases the costs of DSSC devices.At the same time,Pt can be dissolved in the I3-/I-electrolyte to form by-product of Pt I4,which will lead to the loss or deactivation of Pt,causing a sharp decline in device performances.Aiming at the problems of low photoelectric conversion efficiency and high cost of catalytic electrodes in DSSC,this thesis has completed the following research work:(1)The microspheres assembled by ZnIn2S4 nanosheets are synthesized by hydrothermal method.And the new colloid is prepared by mixing ZnIn2S4 with TiO2 slurry.The photoanode with TiO2/ZnIn2S4 heterojunction structure(ZIS-2)is prepared by traditional blade-coating method.X-ray powder diffraction,energy dispersive spectra and X-ray photoelectron spectrum are used to characterize the components and compositions of the photoanode,indicating that TiO2/ZnIn2S4 heterojunction photoanode is successfully prepared.Field emission scanning electron microscope(FESEM)images show that TiO2 nanoparticles are uniformly dispersed on the surface of ZnIn2S4 nanosheets.UV-vis absorption spectra and cyclic voltammetry test results show that TiO2 and ZnIn2S4 have matching energy levels,which are conducive to form the heterojunction.Since ZnIn2S4 can absorb visible light,the generation and introduction of heterojunction can also improve the utilization efficiency of DSSC for visible light.TiO2/ZnIn2S4 heterojunction photoanode(ZIS-2)can accelerate the separation of photogenerated electron-hole pairs in photoanode.ZIS-2 photoanode is applied to DSSC,obtaining an efficiency of 8.09%,which is higher than the device based on alone TiO2(7.07%)and ZnIn2S4(0.09%)photoanodes.(2)The composite TiO2/Ag nanoparticles(NPAs),TiO2/Ag nanoplates(NPLs),and TiO2/(AgNPLs@SiO2)photoanodes are prepared by spin-coating the silver nanoparticles(AgNPAs),silver nanoplates(AgNPLs)and silica coated silver nanoplates(AgNPLs@SiO2)onto the surface of TiO2 photoanode,respectively.The microstructure of TiO2/(AgNPLs@SiO2)is characterized by TEM,FESEM and XPS.The UV-vis spectra show that AgNPLs produces a strong plasma resonance absorption peak in the near infrared region,which is beneficial to improve the adsorption and utilization efficiency for red light.Meanwhile,the enhanced near field is conductive to the dye adsorption on porous electrode.More importantly,SiO2 shell can protect AgNPLs from corrosion,prolonging the lifetime of electrodes.The results of electrochemical characterizations show that plasma resonance effect of AgNPLs@SiO2 can improve the separation efficiency of photogenerated electron-hole pairs.Therefore,DSSC based on TiO2/(AgNPLs@SiO2)photoanode gives an efficiency of 8.72% under AM 1.5 G simulated light irradiation and 1.56% under near-infrared light irradiation,respectively,illustrating the extended light use range.(3)The Pt-free catalytic CEs based on the polypyrrole(PPy)are constructed by combining the spin coating and cyclic voltammetry methods.In other words,the PPy/MWCNT composite electrode is obtained by cyclic voltammetry electrodepositing PPy on the conductive glass substrates coated with multi-wall carbon nanotubes(MWCNT).The composite catalytic electrode materials are tested and characterized by Fourier transform infrared spectra(FTIR),UV-vis absorption spectra and SEM.The results show that the carboxyl group on the surface of MWCNT could be doped into PPy as anion,and the PPy is tightly wrapped on the surface of MWCNT due to the existing ?-? interactions.The electrochemical test in the electrolyte of DSSC show that the presence of MWCNT can improve the dispersion of PPy,leading to the higher conductivity and larger specific surface area of PPy/MWCNT than pure PPy prepared under the same deposition condition.Because large specific surface area is conducive to expose more active site,and high conductivity is advantageous to the electron migration.Therefore,the DSSC with PPy/MWCNT CE gives a photoelectric conversion efficiency of 7.15%(accounting for 92.14% of Pt-based DSSC),higher than that of the alone MWCNT(1.72%)and PPy(5.72%)CEs.(4)In order to expose the catalytic active site of conductive polyaniline(PANI)CEs,the PANI nanoribbons CEs are prepared by using electrospun V2O5 nanofibers(V2O5 NFs)as template and oxidant.The effects of composition of electrospinning solution(ammonium metavanadate and polyvinyl alcohol)and electrospinning conditions on the structure of V2O5 NFs are studied.The effects of aniline concentration and reaction time and post-treatment on the structure of the generated V2O5-PANI NFs and PANI NRs electrodes are studied.The results of sample characterization show that the prepared PANI NRs possess typical spectra characteristics of PANI,indicating that the difference of oxidants would not affect the main structure of the PANI.SEM and TEM tests show that the prepared PANI NRs CEs presents ultra-thin and flexible serrated nanoribbons structure.The network structure composed of these serrated nanoribbons is not only beneficial to tight contact between electrolyte and electrode materials and ion diffusion,but also the serrated structures are conducive to exposing more active sites.The photoelectric conversion efficiency of DSSC assembled with PANI-NRs CE reaches 97.44% of Pt-based DSSC.The results show that the catalytic activity of electrode materials can be improved by designing the microstructure.(5)The transition metal sulfides are expected to be a new type of high-efficiency CEs materials for DSSC due to their high catalytic activity for I3-reduction.The Co In2S4 electrode with nanosheet array structures is prepared by solvent-thermal method on FTO conductive glass substrate.The influences of raw materials and solvent composition on electrode compositions and microstructures are studied systematically.The results show that indium sulfide acts as the template and reactant for forming the continuous nanosheet network.With the increase of water content in the solvent,the contents of metal hydroxide increase,and the interconnected nanosheet structures become into the mixed structures of nano octahedron and nanosheets.The main reason is the difference in hydroxyl ions concentration in solvents with different content of water,which leads to the differences in concentration product.Therefore,the ethanol solution containing trace amount of water is beneficial to form the complex ternary sulfide with continuous nanosheet network structures.Considering that the iron group elements have similar ionic radius,electronic structure and chemical properties,the interconnected ternary MIn2S4(M=Fe,Co and Ni)nanosheet array electrode are prepared by the optimal method.XRD,EDS,SEM and isothermal nitrogen adsorption/desorption are used to characterize the composition and morphology of ternary MIn2S4(M=Fe,Co and Ni)electrodes.The results show that in MIn2S4(M=Fe,Co and Ni)CEs,the Co In2S4 electrode material has the most matching energy level with the redox pairs in electrolytes,which is conducive to the electrons transfer from electrode to electrolyte;It has high specific surface area and abundant pore structure,which is helpful to expose more active sites,so it has the best catalytic activity.DSSC based on Co In2S4 CEs yields a photoelectric conversion efficiency of 8.83%,superior to that of Pt-based DSSC(8.19%).