Synthesis Of Ag/TiO₂、CdS Micro-nanostructured Films And Their Application In Solar Cells

Over the years, it has been being the goal and interest of the solar cells to cutdown the cost and improve efficiency, and make them more competitive thantraditional fossil fuels. Now, the application of novel nanometer materials isconsidered to be the most hopeful path to prepare the next generation solar cells withhigh efficiency and low cost. Studies have shown that one-dimensional nanostructureswith good morphology, high surface area and excellent charge transport propertieshave a huge potential application in photovoltaic solar cells. Thus, in this thesis, wewill focus on the synthesis of one-dimensional semiconductor nanomaterials, and theirapplication in the dye-sensitized solar cells and heterojunction solar cells, and someworthy results are presented.(1) Oriented coaxial Ag/TiO2nanowire arrays have been successfully grown onFTO substrates by solvothermal route with AgNO3, butyl titanate and ethylene glycolas precursors. The influences of the amount of AgNO3on the morphology andstructure of content of the Ag/TiO2nanowire have been studied in the experiment.Studies have shown that AgNO3played an important role in the silver core diameterand TiO2shell thickness. After filling the interspaces between the nanowires withTiO2nanoparticles, the composites were used as the photoanodes in dye-sensitizedsolar cells. For comparison with the traditional pure mesoporous TiO2dye-sensitizedsolar cells, when the amount of AgNO3in precursors is1.25mmol, a22.1mA·cm-2short-circuit current density,0.69V open-circuit voltage and9.8%energy conversion efficiency under one sun illumination has been achieved for the Ag/TiO2dye-sensitized solar cells, which is significantly better than the pure mesoporous TiO2solar cells. This result is because the Ag core in the Ag/TiO2photoanodes serves aselectron transport shortcut, and the TiO2nanoparticles provided the high specificsurface area, which will improve the charge collection efficiency in the dye-sensitizedsolar cells.(2) A compact and uniform CdS nano-crystalline (NC) thin film with no pinholeswas deposited onto the well-cleaned FTO substrate by chemical bath depositionmethod. Then the oriented and high-density CdS nanorod (NR) arrays weresuccessfully grown on the NC thin film by facile hydrothermal method. It was foundthat with CBD time increasing, the NR density increases but the NR length decreases,and the optical absorption and photoelectrochemical properties of the CdS films havethe tendency of increasing first and then decreasing. When the chemical bathdeposition and hydrothermal time were30min and14h respectively, the CdS NRfilm demonstrated superior optical absorption and photoelectrochemical properties,and approached a1.80mA·cm-2short-circuit current density and1.27V open-circuitvoltage. At the mean time, the CdS composite films showed superiorphotoelectrochemical property to the pure CdS NC and NR films, which was becausein the composite films the surface area was high, the electron transport rate was fastand the FTO substrate was encapsulated by a layer of compact CdS NC, whichprotected the electrons from reacting with the oxidized ions and improved the chargecollection efficiency.(3) A compact and uniform CdTe film was deposited onto the CdS nanorod arrayfilm by magnetron sputtering method. The CdTe film only grew at the top of the CdSNRs, and not filled in the spaces among the NRs, because the density of CdS NRs wastoo high, and in the magnetron sputtering process the substrate temperature was toolow and sputtering power was too high, thus reducing the heterojunction area and thenumber of electrons in the solar cells. After being CdCl2heat treatment, CdTe filmshowed recrystallization, grain size increased, and the crystallization of the film wasenhanced. However, there were many holes in the CdTe film, which were not conducive to the charge separation and transmission in the film. For the optoelectronictest, the short-circuit current density, open-circuit voltage, fill factor, energyconversion efficiency and the highest IPCE value of the CdS/CdTe heterojunctionsolar cell were4.62mA·cm-2,0.56V,0.35,0.91%and64%. Compared to thetraditional CdS/CdTe planar heterojunction solar cells, the photovoltaic performanceof our solar cell was not high, because the three-dimensional (3D) heterojunction solarcells were not yet formed in this experiment.(4) A compact and uniform PbS nano-crystalline thin film with no pinholes wasdeposited onto the CdS nanorod array film by chemical bath deposition method. Thesolution process represented a promising technique for the PbS film filled the narrowspaces among the CdS NRs to form CdSNR/PbS3D heterojunction film. Moreover,compared to the traditional CdSNC/PbS planar heterojunction film, the CdSNR/PbS3D heterojunction film possessed of high photoabsorption coefficient throughout thevisible and near-infrared region of the incident light, which led to the enhancedphotovoltaic performance with a short-circuit current density of10.87mA·cm-2andan overall energy-conversion efficiency of1.01%. The observed improvement ismostly attributed to the three-dimensional heterostructure enabling increasedheterojunction area, improved charge carrier collection and enhanced opticalabsorption ability.