p-CdTe/n-CdS Photovoltaic Cells in the Substrate Configuration

In this thesis, p-CdTe/n-CdS solar cells in the substrate configuration have been studied. The focus is on device fabrication, performance optimization, and the development of methods for forming ohmic contacts to p-CdTe. In contrast to the conventional superstrate configuration, in which glass is necessarily used as the supporting substrate, p-CdTe/n-CdS solar cells in the substrate configuration can be fabricated on top of a thin, flexible and non-transparent metal sheet. The metal sheet serves as both the supporting substrate and as the electrode. Utilizing thin metal sheets as the substrate enables roll-to-roll production, which offers pathways to low-cost manufacturing. It also provides new opportunities to explore interface engineering in optimizing device performance.;p-CdTe/n-CdS solar cells in both the superstrate and the substrate configurations were fabricated. Powder mixtures of copper telluride (Cu2Te) and tellurium (Te) in different weight ratios were prepared as sputtering targets to generate a copper containing back contact buffer between p-CdTe and the molybdenum electrode. The best efficiency of 11.2 % with much reduced rollover effect in the J-V characteristics was produced with 50 nm of Cu0.16Te in the superstrate configuration. A total of ~22 % degradation in device efficiency occurred after undergoing 227 days of a light soaking test. Devices in the substrate configuration with different thicknesses of Cu0.16Te as the back contact buffer were also fabricated. It was found that the application of a vapor cadmium chloride (CdCl2) treatment was detrimental to device performance in contrast to the devices with substrate configuration. Air annealing was found to produce functional substrate devices, but only of low efficiencies, ~3.3 %.;A high-work function molybdenum oxide (MoOx) thin film deposited by thermal evaporation was utilized as a copper-free back contact buffer for devices in the substrate configuration. MoOx with a thickness between 140~190 nm was found to generate a low-energy barrier contact with p-CdTe and produce a device efficiency of ~9 % with much reduced rollover effect in the J-V characteristics. Exposure of MoO x to air, known to reduce the work function was found to have little effect on the device performance. With MoOx as the back contact buffer, the device stability was evaluated by thermal annealing up to 150 °C under nitrogen flow. A total of 20 % loss in device efficiency was observed in 78 hr with little degradation in short-circuit current and 10 % losses in both the open-circuit voltage and the fill factor. The MoO x is shown to be an effective and chemically stable back contact buffer for the devices in the substrate configuration.;Preparation of CdTe films by close-space sublimation (CSS) with low substrate temperatures below 400 °C was conducted under pure oxygen or argon. CdTe films prepared under pure oxygen have a higher density of nucleation sites and a strong preferred orientations in the (111) direction. Little grain growth and recrystallization were observed after the CdCl2 treatment. In contrast, CdTe films prepared under pure argon have flat and smooth grains with random orientations. Further grain growth and recrystallization were observed with the CdCl2 treatment. p-CdTe/ n-CdS solar cells in both the substrate and superstrate configurations with MoOx as the back contact buffer were fabricated. Device efficiencies of ~3 % and ~7 % were achieved for the substrate configuration using pure oxygen and pure argon, respectively. The morphology of the CdTe films prepared under pure argon is beneficial to the device performance with significant improvement in the short-circuit current, the open-circuit voltage and the fill factor. However, the intrinsic stresses in CdTe films prepared with substrate temperatures below 400 °C resulted in film delamination during the CdCl 2 treatment regardless of the preparation ambient. Devices in the superstrate configuration were also fabricated under pure argon and ~3.2 % efficiency was achieved. The low efficiency was attributed to large-grain CdTe films and a decrease in shunting resistance. Spectral responses of the devices in the superstrate configuration indicated that CdTe films prepared under pure argon are n-type.