Synthesis And Photoelectrochemcal Propetries Of Ⅱ-Ⅲ Semiconductor/ZnO Nano Structure Composite Films

With the increasing demand for energy, the limited fossil fuel reserves and theincreasingly serious environmental pollution, searching for reproducible resources isbecoming an imperious task. Solar energy is an ideal new energy to meet the targetbecause of its abundant, clean and inexhaustible characteristics. Solar cell is animportant application for solar energy. Typically, quantum dot sensitized solar cells(QDSCs) has been identified as a promising solar cell due to its simple preparationprocess and inexpensive cost. The theoretical solar-to-electric energy conversionefficiency of QDSCs is relatively considerable. However, up to now, the energyconversion efficiency of QDSCs is still inferior to the traditional solar cell.QDSCs are made up of a semiconductor photoanode and a platinum cathode in anelectrolyte solution, so we can improve the energy conversion efficiency of QDSCsthrough improve the performance of different component. As we known, photoanodeis the vital part which can produce the photon-generated carriers, also can collectelectrons and send them to the external circuit. As a result, the photoanodes with goodperformance of photoresponse, charge separation, and electron transference are ofimportance for the excellent conversion efficiency for the DSSC.Zinc oxide (ZnO) is an important wide band gap semiconductor material, and thevalue is3.37eV. ZnO has been demonstrated as a promising candidate for photoanodedue to its appropriate energy band position and good corrosion resistance in aqueoussolution. ZnO also have amazing electronic transmission characteristics, good morphology control, simple preparation process and low cost. In this thesis, we willfocus on the preparation of the ZnO films with different morphology, and realized thequantum dots (QDs) sensitization through a successive ionic layer adsorption andreaction (SILAR) technique. The photoelectrochemical properties of obtainedZnO/Ⅱ-Ⅵ semiconductor composite film are investigated. The main contents are asfollows:(1) Al-doped ZnO (AZO) nanorod array film was grown directly on FTOsubstrates throuth a simple hydrothermal method. The content of Al element in ZnOnanorod was controlled by altering the concentration of Al3+in the original solution.PL spectra reveal that Al doping has augmented defect concentration of the ZnO. Inaddition, the optical absorption spectra of AZO nanorod array film show that a smallblue shift is observed compared to the undoped ZnO. The blue shift was due to theFermi level moving into the conduction band with the increase of the carrierconcentration, according to the theory of the Burstein–Moss effect.(2) CdSe QDs sensitized ZnO/CdSe and AZO/CdSe composite films wereobtained through SILAR method, and their photoelectrochemical properties wereinvestigated. With light illumination, the ZnO NR electrode has a photocurrentdensity of0.34mA/cm2at0V versus Ag/AgCl electrode. The photocurrent density ofAZO electrode is0.33mA/cm2, which is smaller than pure ZnO. We ascribe it to thefact that Al doping induced lattice defects may turn to be recombination centers ofphotogenerated charge carriers and then lower electron-hole separation efficiency.Both of CdSe nanoparticle sensitized samples show a significant enhancement inphotocurrent, ZnO/CdSe has a photocurrent density of2.48mA/cm2and that ofAZO/CdSe is4.28mA/cm2. This can be attributed to that the deposition of CdSenanoparticles extends the photoresponse of ZnO electrode in the visible light region,and that the heterojunction between ZnO and CdSe is benefit to the separation ofphotogenerated electron-hole pairs. In addition, the AZO/CdSe electrode shows alarger photocurrent density, nearly twice of ZnO/CdSe electrode. We attribute theimprovement to the fact that Al doping augments more active centers and thenincreases the amount of CdSe grown on AZO. Larger amount of CdSe of course leads to significantly enhanced optical absorption, which contributes to boost the amount ofelectron-hole pairs, providing a great quantity of carriers.(3) ZnO nanosheet (ZnONS) array films with a large percentage of high-energy{0001} surfaces were prepared on FTO substrates for the first time by using simplehydrothermal method. The influences of the amount of sodium citrate and reactiontime on the morphology and structure of ZnO nanosheet array films have been studiedin the experiment. Studies have shown that sodium citrate plays a crucial role in theformation of ZnONS array films. By carefully investigating their optical andphotoelectrochemical properties, we found that the optimum growth condition forZnO NS are0.05g sodium citrate and12h reaction time.(4) CdS and CdSe QDs were deposited on the surface of the ZnONS array filmsby using the SILAR method, and their photoelectrochemical properties wereinvestigated carefully. With increasing the number of SILAR cycles, thephotoelectrochemical properties of composite films were first improved and thendecreased. The optimal photocurrent densities of ZnONS/CdS(9c) andZnONS/CdSe(7c) photoanodes are2.12mA/cm2and3.1mA/cm2at0V versus SCE,and the value of co-sensitized ZnONS/CdS(9c)/CdSe(7c) photoanode is4.42mA/cm2.The improved photoelectrochemical performance of co-sensitized ZnONS/CdS/CdSephotoanode can be attributed to that the existence of narrow energy band gapsemiconductors greatly broadened the spectral response scope. The typically type-IIcascade structure also favors the separation and transmission of photogeneratedelectron-hole pairs.