The Construction And Photoelectric Properties Of A New CeO₂-based Dye-sensitized Solar Cell

The energy crisis has become an important problem for human survival,and solar cells are regarded as one of the most potential solutions.As a new generation solar cell,dye-sensitized solar cells(DSSCs)have wide application prospect,which is due to the advantages of low cost,simple process,high efficiency and good stability.However,the following problems still exist in the current research of DSSCs,which restricts the improvement of its photoelectric conversion efficiency and the large-scale application:(1)the combination of the light electrons and the holes caused by the defect of the TiO2 film is the main cause of the dark current;(2)because the absorption layer can only absorb the visible light of wavelength 300-800nm range,the infrared light cannot be used effectively,which limits the further improvement of DSSCs efficiency;(3)when the precious metal Pt is used as the counter electrode,its catalytic effect is susceptible to environmental factors,reacting with iodide electrolyte to produce PtI4 and losing of catalytic activity.The problem further restricts the large-scale production and the application of DSSCs.In this dissertation,the novel dye-sensitized solar cells were designed with composite photoanode constructed of rare earth ions doped CeO2 and its composite materials with different morphology as assistant layer.The matching of energy level between CeO2 and TiO2 forms an electronic pathway to reduce the recombination of the optical electrons and holes.The conductivity of the CeO2 is improved by forming a complex with SnO2,which speeds up the transmission of electrons in the thin film.Using the intrinsic upconversion luminescence of rare earth ions,the near-infrared light in the sunlight is fully utilized to broaden the absorption spectrum,in order to improve the efficiency of photoelectric conversion.In addition,on the basis of theoretical calculation,hollow hierarchical structures Coo.ssSe and Te doped CoTe nanowires with exposed(102)facets are designed and synthesized.The catalytic capability for the I3-reduction reaction was studied.As a free-platinum counter electrode,it is used in DSSCs fabricated with CeO2-based composite photoanode to solve the problem of poisoning and inactivation of electrode materials,and to reduce the cost of the battery,which provides a theoretical basis for the practical application of dye sensitized solar cells.The main research works in this dissertation are divided into the following five parts:1.DSSCs fabricated of P25(CeO2:Yb,Er nanotubes)composite photoanode based on energy level matching principleYb,Er doped CeO2 nanotubes(CeO2:Yb,Er NTs)were synthesized by hydrothermal method with Ag nanowires as templates.And the composite photoanode was prepared with CeO2:Yb,Er NTs and TiO2 photoanode.The structure and components were analyzed by scanning electron microscope,transmission electron microscope,X-ray diffraction instrument and X-rays photoelectron spectrometer.The photoelectric conversion performance was studied by photoelectric test,and the electron transfer of the interface was studied by electrochemical impedance method.The energy band of CeO2 is analyzed by density functional theory,and the optimized structure CeO2 has a band gap of 1.96 eV.Through UV-vis spectral analysis,the band gap of CeO2:Yb,Er NTs is 2.8 eV,and the conduction band and valence band of CeO2:Yb,Er NTs is lower than that of TiO2.The energy level matching at the interface between the two substances provided an electronic pathway for the transmission of the light-born electrons and holes,which is propitious to reduce the charge recombination,and improve the DSSCs photoelectric conversion efficiency.2.DSSCs fabricated of high electron mobility P25(SnO2/CeO2:Yb,Er hollow nanosphere)composite photoanodeThrough the first principle calculation,the analysis of CeO2 energy band and state of density shows that it has poor conductivity and low electron migration rate.In order to improve the electron transfer ability of the material,SnO2 with high electron migration rate is introduced to construct the composite material with CeO2:Yb,Er.SiO2@CeO2:Yb,Er was prepared by using SiO2 as the template,and then SiO2 decomposition was carried out simultaneously with the SnO2 formation in the hydrothermal process.SnO2/CeO2:Yb,Er hollow nanosphere(HNSs)was obtained.In order to verify the universality of the method for preparing hollow SnO2 shell,Fe2O3@SnO2 nanoboxes are prepared by the same method using core-shell structure Fe2O3@SiO2 as template.Then the Fe3O4@SnO2@CN nanoboxes synthesized by coating carbon shell with adjustable nanometer gap show good cycling stability when it used as anode material in lithium ion battery.The core-shell structure nanoboxes should be prepared by the same method,which is proved that the preparation of SnO2 nanoshell is universal.The photoelectric test results show that the photoelectric conversion efficiency of DSSCs is improved by the DSSCs of the composite photoanode formed by SnO2/CeO2:Yb,Er and P25.Through the open-circuit voltage decay,the life change of the photogenerated electron in the film is studied,and the assistant effect of SnO2/CeO2:Yb,Er is obviously increased the life change.3.CeO2:Yb,Er@SiO2@Ag upconversion composite nanofibers as an assistant layer enhanced near-infrared harvesting for DSSCsYb and Er are usually used as sensitizer and activator in the up-conversion nanomaterials,and the up-conversion luminescence property is available in the appropriate crystal field.The CeO2:Yb,Er nanofibers were prepared by electrospinning method,which was coated by SiO2 and treated with high temperature,so that CeO2:Yb,Er@SiO2 nanofibers had high conversion luminescence property.Finally,the Ag nanoparticles were loaded on the surface of CeO2:Yb,Er@SiO2,and the fluorescence luminescence was enhanced by the surface plasmon resonance effect.Compared with the DSSCs prepared by the blank TiO2 photoanode,the photoelectric test results show that the DSSCs fabricated with CeO2:Yb,Er@SiO2@Ag composite nanofibers as assistant materials exhibited a photoelectric conversion efficiency 8.17%,an increase of 25.9%.The photoelectric conversion efficiency of about 0.14%was obtained under 980 nm laser as illumination light source.In addition,the absorption of the IPCE curve in 800?1000 nm near-infrared light shows that the introduction of the up-conversion nanomaterial can broaden the DSSCs absorption range and improve the utilization rate of DSSCs for sunlight.4.Construction of CeO2-based composite photoanode DSSCs with hollow hierarchical structure Co0.85Se as free-platinum counter electrodeOn the basis of the previous three chapters,DSSCs forming with the CeO2-based composite photoanode and free-platinum counter electrode are constructed to solve the problem of deactivation of the electrode due to toxic catalytic performance,and to reduce the cost of the cells.By using density functional theory to calculate the reaction energy barrier of I3-reduction reaction catalyzed by electrode material,the comparison results show that the C0.85Se can be used as an ideal counter electrode material in DSSCs,which has a small reaction barrier.According to the theoretical calculation results,the Co0.85Se(Hollow hierarchical structure,HHSs Co0.85Se-M)was synthesized by the methanol-water reaction system,which has the irregular hollow graded structure.Co0.85Se with different morphologies and crystallinity were obtained by changing the types of alcohols in the reaction system,such as ethanol,N-propanol and N-butanol.From nitrogen desorption analysis,Co0.85Se has a large specific surface area and allows more active sites exposing to the electrolyte,which is beneficial to the catalytic reaction.The photoelectric test results show that DSSCs fabricated with the high crystalline HHS Co0.85Se-M synthesized by methanol-water reaction system as counter electrode have higher photoelectric conversion efficiency than that of Pt.Cyclic voltammetry,electrochemical impedance and Tafel polarization curve test show that HHS Co0,85Se-M is comparable to the catalytic performance of precious metal Pt for I3-reduction.The photoelectronic conversion efficiency of DSSCs forming with the CeO2-based composite photoanode and HHS Co0.85Se-M as counter electrode is further improved,which provides a theoretical basis for the design of a new type of DSSCs.5.Construction of CeO2-based composite photoanode DSSCs with CoTe/Te nanowires as free-platinum counter electrodeBased on density functional theory,the I3-reduction reaction under CoTe catalyzing is studied theoretically.The results show that with CoTe(102)as the active crystal surface,it has a smaller reaction energy barrier than Pt,suggesting that the CoTe has the catalytic activity comparable to the precious metal Pt.A one-step hydrothermal method was used to synthesize the CoTe/Te composite nanowires of surface exposed(102)facets.The catalytic activity for I3-reduction was studied by cyclic voltammetry,electrochemical impedance and Tafel polarization curve.The photoelectronic experimental results show that the Te doped CoTe nanowires with exposed(102)facets show a high catalytic activity.The DSSCs forming with the CeO2-based composite photoanode and CoTe/Te nanowires as counter electrode show a good photoelectric conversion performance and provide a theoretical basis for the practical application of low-cost DSSCs.