| In the recent years, the renewable energy industry represented by solarphotovoltaic is entering the human energy structure, and will become the inevitabletrend of future energy development. Crystalline silicon cells still occupy the dominantposition in the solar cell market currently, however, the factors such as high costs inbattery and compent, and low photoelectric conversion efficiency are the majorobstacles in their large-scale promotion. In addition, the heat producted by theabsorbtion of high-energy ultraviolet photons will affect the stability and service lifeof the solar cells. Using a spectral conversion technique to translate a high energy UVphotons into two or more lower energy photons that can also be absorbed by thesolar cell, is one of the effective methods to expand the range of spectral response ofcrystalline solar cells. This method can not only improve the spectral response ofultraviolet photons, but also reduce the impact on the stability of solar cells caused byabsorbing ultraviolet photons.For the purpose of getting down-conversion materials applied to solar cells, weused hydrothermal method to synthesize YVO4matrix material, YVO4: Eu3+luminescent powders and YVO4: Eu3+, Yb3+down-conversion phosphor powders inthis article. Furthermore, the samples synthesized under different conditions werecharacterized and analyzed in their microstructure and optical properties. The mainconclusions were drawn as follows:(1) The macroscopic stress in the lattice of YVO4obviously changes with theelevated HRT. Moreover, a smaller average grain size resultes in a larger microscopicdistrotion, which is due to the enhanced surface effect. The phonon vibration in theYVO4crystal lattice originates mainly from the internal vibrations in thevanadium-oxygen tetrahedron, in addition to the Y-O and O-H vibrations. YVO4crystal has low phonon energy compared with other common down-conversion matrixmaterials, so it is proper as a matrix material for down-conversion system. Thebandgap of approximately3.8eV for YVO4leads to good absorption properties in theultraviolet region. The intrinsic emission of YVO4crystal is annihilated, and a broadband emission of VO43-in visible region is observed at room temperature.(2) The phenomenon of a certain agglomeration of small particles caused bysmall size effect will be improved by adding proper surfactants during the experiment. In this thesis, I have successfully synthesized YVO4particles with regularmorphologies such as spherical, square nanoplates and cuboid microrods when addingNa3cit, AO or Na2EDTA as surfactants, respectively. When using Na3cit as surfactant,and control the molar ratio of cit3-:Y3+as2:1, the effectiveness of controlling on themorphology of YVO4particles affected by cit3-groups gradually reduces, while thelight-emitting intensity of the samples gradually increases, with the increase in pHvalue. A high concentration of H+ions in an acidic solution leads to excessiveadsorption of cit3-groups on the YVO4phosphor powder, which is benefit for thesynthesis of spherical YVO4particles, but annihilates the superior luminescentproperty of YVO4phosphor powder, only shows the weak photoluminescence band ofcit3-groups. An annealing method was used in this article to remove the cit3-groupsfrom the phosphor surface. Gradually desorption of Cit3-from phosphor surfaceoccurs in the form of CO2and gaseous H2O molecules as the annealing temperatureincreases, resulting into the rough particle surface and the formation of holes. Hightemperature annealing process is good for the desorption of Cit3-groups and theimprovement of the YVO4crystallization.(3) Eu3+and Yb3+exist in YVO4crystal with the form of substitutional doping.With the increase in Eu3+concentration (1%,3%,5%,7%), the luminescence intensityof YVO4:Eu3+phosphor powder will first increase, and then decrease, reaching itsmaximum at5%. While in spectrum of YVO4:Eu3+,Yb3+phosphor powder, only theemission of Eu3+is observed, no information of Yb3+is seen.(4) The luminescence of YVO4phosphor powder is attributed to energytransition inside in VO43-groups. When Eu3+is single-doped, the YVO4matrixmaterial first absorb the ultraviolet photons and then passes most of the energy to Eu3+ions, and cause Eu3+radiative transition and the emission of VO3-4is relatively weakcompared with Eu3+. The luminescence mechanism of YVO4:Eu3+,Yb3+can beexplained as follows: Eu3+ions excited to high energy states partly transfer its energyto Yb3+ions through a phonon assisted cross-relaxation process or a cooperativeenergy transfer process, and partly radiative transition by itself. |
PDF Full Text Request