Fabrication and characterization of CuInSe₂/CdS/ZnO thin film solar cells

M.Sc.I-III-VI2 compound semiconductors are important photovoltaic (PV) materials with optical and electrical properties that can be tuned for optimum device performance. Recent studies indicated that the efficiencies (1) > 18%) of CuInSe2/CdS/ZnO thin film devices are in good agreement with that of standard silicon cells. In this study, CuInSe 2 absorber films with excellent material properties were produced by relatively simple and reproducible two-stage growth techniques. In these approaches, metallic precursors (Cu/InSe, InSe/Cu, Cu/InSe/Cu and InSe/Cu/InSe) were deposited by thermal evaporation from specially designed graphite heaters at temperatures around 200°C. In the second stage of the process, the alloys were exposed to elemental Se vapour or H2Se/Ar gas. A systematic study was conducted in order to determine optimum growth parameters for the different deposition processes. Optimum material properties (homogeneous and dense films with a high degree of compositional uniformity) were obtained when InSe/Cu/InSe precursors were selenized in elemental Se vapour or H2Se/Ar gas. Comparative studies also indicated that the reaction kinetics is enhanced when H2Se/Ar is used as chalcogen source. Fully selenized films were obtained at temperatures as low as 450°C in a H2Se/Ar atmosphere, compared to temperatures of 600°C in the case of Se vapour. The optical and electrical properties of the absorber layers were accurately controlled by small variations in the bulk composition of the films. A standard CdS/ZnO window layer technology was also developed in our laboratories and preliminary solar cell devices were fabricated and evaluated

Optimization of quaternary and pentenary chalcopyrite for applications in thin film solar cells

Ph.D.One of the solutions to the high cost of solar modules is the development of thin film solar cell technologies, which enable material saving, few processing steps, good stability in outdoor testing, high conversion efficiency and flexibility for large area coatings. Polycrystalline CuInSe2 (CIS) thin films and related quaternary and pentenary compounds such as Cu(In,Ga)Se2 (CIGS) and Cu(In,Ga)(Se,S)2 (CIGSS) are the most promising thin film candidates to fulfil the requirements of economically viable solar modules. Presently CIS, CIGS and CIGSS thin film solar cells are prepared mostly by two – stage deposition processes, where Cu-In-Ga alloys are deposited, followed by selenization and/or sulfurization using H2Se/Ar and/or H2S/Ar gases, Se and/or S vapours. Key problems related to this approach are (1) the widely reported compositional change and loss of material during the annealing and selenization stages, and (2) the formation of a graded film structure with most of the Ga residing at the back of the film, due to the difference in the reaction rates between the binary selenides. The present study aims to develop CIGS quaternary and CIGSS pentenary thin film absorbers which are substantially homogeneous and single phase. In order to achieve this aim different deposition processes were developed. This included thermal evaporation of pulverized compound materials from a single crucible with and without subsequent reaction of the precursors in Se vapour or H2Se/Ar atmosphere. Alternatively, controlled partial selenization/sulfurization of the Cu-In-Ga magnetron sputtered precursor films under controlled conditions of reaction time, temperature and gas phase concentration were applied to produce CIGSS films. The latter approach allowed homogeneous incorporation of Ga and S species into CIS compound material, and with that a corresponding increase of band gap of the material in the active region of the solar cell. CIGS quaternary and CIGSS pentenary based solar cells were completed by depositing a CdS buffer layer of around 50 nm thickness, high resistivity ZnO and low resistivity Al – doped ZnO with thicknesses of about 50 nm and 0.5 μm respectively. I-V measurements on fabricated solar cells, under standard A.M. 1.5 conditions, demonstrated good solar cell device quality with efficiencies of about 10 % and 15% respectively