| Cu-based solar cells are those using Cu-based absorbers such as Cu(In,Ga)Se2,CuInS2, Cu(In,Ga)(S,Se)2, and CuZnSnS2. The traditional Cu-based solar cellscontain a CdS buffer layer between the absorber layer and the window layers. Due tothe toxicity of Cd and the relatively lower band gap of CdS that is not enough forshort wavelength photons to get through, the Cd-free buffer layers were widelystudied. Among those Cd-free buffer layers, the bivalent zinc compounds containingS or O are the most promising substitutes for the CdS buffer, because they arenon-toxic, cheap, and with wide band width. In addition, the CIGS solar cells with ahigh efficiency usually use Chemical Bath Deposition (CBD) to make their bufferlayers. However, the CBD is not matching with the vacuum dry methods throughwhich other layers of the cells fabricate. In the thesis we will use vacuum-basedmethods to deposite ZnS and Zn(O, S)films.In this thesis, ZnS thin films are deposited on glass substrates by e-beamevaporation and Zn(O, S) thin films are deposited by magnetron co-sputtering. Thestructural, optical and electrical properties of those films were studied by XRD, AFM,Raman spectrumeter, UV-Vis-NIR spectrophotometer and Hall measurement system.The relationship between substrate temperature and the properties of ZnS thin filmswere analyzed; the relationship between critical parameters (substrate temperature,sputtering pressure, sputtering power) and the properties of Zn(O, S) thin films wereinvestigated systematically, Optimized parameters of the sputtering process for betterperformance of Zn(O, S) thin films were obtained.The dissertation’s main results are as follows:(1) The ZnS thin films deposited by e-beam evaporation exhibit polycrystallinesphalerite structure (β-ZnS) regardless of the substrate temperature. With anincreasing substrate temperature, the films show a higher degree of (111)preferred orientation and larger grain size. However, the trend turns to theopposite when the temperature is over200℃. The film deposited at200℃shows the best crystallinity. All the films have a low absorption of visible lightthat is less than10%. Due to the quantum size effect, the band gaps of the ZnSfilms are higher than that of ZnS powder.(2) When the pulse power used for sputtering ZnO target is greater than the rf powerused for sputtering ZnS target, the Zn(O, S) thin films deposited by Magnetronco-sputtering with two targets are polycrystalline nanostructured thin films witha hexagonal wurtzite structure and with a preferred c-axis growth. The Ramanspectra shows that the thin films are mixed crystals with two modes. The optical absorption of the films is lower than5%in visible spectral region. Moreover, theelectrical properties of the films were investigated with sputtering processconditions.(3) Optimizations of the substrate temperature, the sputtering power, and thesputtering pressure all contribute to the improvement of the structuralcharacteristics, surface morphology, and the optical and electrical properties ofthe Zn(O, S) polycrystalline thin films with mixed crystals. The Zn(O, S) thinfilms with appropriate properties can be deposited as a buffer layer to matchdifferent Cu-based solar cell absorber layers. Moreover, the sputtering powershould be as small as possible to prevent splash damage to the absorber.(4) Optimized sputtering parameters are obtained. The substrate temperature is200℃, the sputtering pressure is0.1~0.5Pa, the pulse power for sputtering ZnO is60W, and the rf sputtering power for depositing ZnS is20W. |
PDF Full Text Request