Study Of Fundamental Problems In CdTe Thin Film Solar Cell And Fabrication Of High Efficiency CdTe Solar Cells

CdTe is an Ⅱ-Ⅵ compound semiconductor. Because of its applicable energy gap and relatively high absorption coefficient, it attracted a lot of investigation interests as a solar cell semiconductor material. CdS/CdTe heterojunction thin film solar cells in which CdTe acts as an absorber layer is one of the most promising candidates for low-cost photovoltaic application. Small-area CdTe cells with an energy conversion efficiency as high as17.3%and commercial-scale modules with an efficiency of14%have been demonstrated recent years. Because of the limit of material’s original nature, the investigation and manufacture of CdTe thin film solar cells face a lot of challenges:high work function of P-type CdTe films, causes a schottky barrier between CdTe film and back contact which limits cells performance; low carrier concentration of CdTe films limits the voltage of devices; diffusion of impurity ions (like Cu ions) influences the stability of CdTe devices and modules; the lack of Te in earth crust limits the application of CdTe solar cell. According to the situation, we begin to carry out our researches.In Chapter Ⅰ, we first discussed the background of energy crisis and pointed out the ways out should be solar energy. Then we introduced the development of solar cells and the theories of photovoltaic effect. Lastly, the physical properties of CdTe and CdTe thin film solar cells.In Chapter Ⅱ, CdS films with a uniform surface and high transmittance were prepared by chemical bath deposition (CBD) technique. XRD measurements showed that these films had a hexagonal (002) preferred orientation. CdTe films with good crystal quality were prepared by a home-made close spaced sublimation system. CdTe films had a cubic (111) preferred orientation which could reduce lattice mismatch between CdS and CdTe films and was good for CdTe solar cells. We prepared CdTe solar cells devices with CBD CdS films and CSS CdTe films and obtained a photovoltaic conversion efficiency of2.04%.In Chapter III, we studied the heat treatment processes of both CdS and CdTe films. We heat treated CdS films with a dip-coated CdCl2layer, by which way we improved the crystal quality of CdS films, enhanced hexagonal (002) preferred orientation, reduced the oxidation of CdS surface and improved optical properties of CdS films. We also studied the relationship between CdS films heat treatment and CdTe devices performance, and prepared CdTe solar cells with an efficiency of11.6% by using CdCl2heat treated CdS films. We studied the oxidation of CdTe thin films in air coated with and without a CdCl2layer. It was found that the presence of a thin CdCl2coating layer on the CdTe film surface enhanced the oxidation of the CdTe surface. Near the film surface area, the film was composed of a low-melting eutectic mixture of CdTe and oxides of CdTe and Te. The formation of the low-melting mixture was assisted by the much lowered melting point of CdCl2, the presence of oxides, and the CdTe compound. This low-melting layer enhanced the oxygen reaction with the CdTe film. Then we studied the influence of CdTe heat treatment on CdTe devices, the results showed that CdTe solar cells heat treated at400℃had the highest efficiency. We also found that an interdiffusion between CdS and CdTe films happened when CdTe films were heat treated, which depleted some CdS films and reduced the interface state density. Lastly, we designed a close spaced CdCl2vapor heat treatment process, by which way we obtained a CdTe solar cell which had an efficiency as high as12.4%.In Chapter IV, we focused our investigation on the back contact of CdTe solar cells. We studied the influence of bromine methanol (BM) solution etching on CdTe films and devices properties. BM solution etching removed oxides which formed when CdTe films were heat treated and create a Te rich layer on CdTe films surface. We improved CdTe devices performance by optimizing the BM solution concentration and etching time. We investigated the copper doping properties in the back contact. By using an appropriate Cu thickness in the back contact, we successfully eliminated the schottky barrier and Roll Over of the current voltage curve. By optimizing etching and copper doping processes, we obtained CdTe solar cells which had an efficiency as high as13.2%. At the same time, we investigated some other back contact materials like:ZnTe:Cu, Sb2Te3and MoO3. They all showed good properties as a back contact in CdTe solar cells.In Chapter V, Cu/Au and Sb2Te3back contact CdTe solar cells with an absorber layer thickness was0.5,1,2.5and5μm was fabricated. Our investigations showed that the solar cell efficiency decrease due to the insufficient absorption of0.5μm-thick CdTe solar cells would be only about0.5%. An increased intensity of deep recombination states in the band gap, which was responsible for the reduced open-circuit voltage and fill factor for ultra-thin solar cells, was induced due to the not-well-developed polycrystalline CdTe microstructure and the CdS/CdTe heterojunction and the presence of Cu in the back contact. The experimental results presented in this study demonstrated that1-μm-thick absorber layer is thick enough to fabricate CdTe solar cell with a decent efficiency. An efficiency of7.9%was obtained for a1-μm-thick CdTe solar cell, and5.2%was obtained for a0.5-μm-thick CdTe solar cell.