703) 487-4650 e* c> Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. Executive Summary The objective of this project is to develop improved processes for the fabrication of CdTeKdS polycrystalline thin film solar cells. The technique we use for the formation of CdTe, electrodeposition, is a non-vacuum, low-cost

more »

... que that is attractive for economic, large-scale production. Technical Approach During the past year, our research and development efforts have focused on several steps that are critical to the fabrication of high-efficiency CdTe solar cells.These include the study of growth and properties of electrodeposition CdS, quantitative studies of CdTe-CdS interdiffusion using x-ray diffraction and photoluminescence, and back contact formation using Cu-doped ZnTe with an emphasis on low Cu concentrations. We have also started to explore the stability of our ZnTe-Cu contacted CdTe solar cells. Results Studies of the ~o w -properties of electrodeposited CdS thhflhm Uniform, high-quality CdS thin films are highly desirable for the fabrication of highefficiency CdTeKdS solar cells. Currently, chemical bath deposition is the most successful among all techniques. Other techniques, such as vacuum sublimation, have been used. However, because of a higher density of pinholes in the deposited films, the minimum CdS thickness required for obtaining high V, is much greater than for CBD CdS. Electrodeposition is a non-vacuum technique and is compatible with our CdTe deposition process. Moreover, it offers excellent control over the properties of the thin films through the influence of deposition potential, bath temperature, pH, and composition of reactants. We have investigated the electrodeposition of CdS and its application in fabricating CdTdCdS solar cells. The electrodeposition of CdS was done in a system that consisted of a glassy-carbon anode, a Ag/AgCl reference electrode, and a cathode (sample substrate). The experimental conditions we explored in this study were: pH from 2.0 to 3.0; temperatures of 80° and 9 K ; CdC1, concentration of 0.2 M; deposition potential from -550 to -600 mV vs. Ag/AgCl electrode; [Na&Q] concentration between 0.005 and 0.05 M. The electrodeposition rate of CdS was studied as a function of the solution temperature, sodium thiosulfate concentration, pH, and the acid used. The deposition rate increases with increase of the thiosulfate concentration and decrease of solution pH. Such a dependence can be understood based on the known disproportion of thiosulfate ions. With decreasing pH and increasing thiosulfate concentration, the disproportion rate increases, leading to increased deposition rate of CdS on the electrode. The high deposition rate observed at high solution temperature may be caused by both an increased reaction rate at the electrode and the increased thiosulfate disproportion rate. We also observed that the acid used to adjust the pH has a large impact on the deposition rate. The deposition is faster in a hydrochloric acid solution than in a sulfuric acid solution. The Faradaic efficiency of the electrodeposition process was calculated. The Faradaic efficiency is lower at low pH, caused presumably by the hydrogen evolution at low pH which contributes to the reduction current. In all 1 ... 111 iv vlll