Polycrystalline thin film cadmium telluride solar cells fabricated by electrodeposition. Annual technical report, 20 March 1995--19 March 1996

Abstract

Executive Summary ObjectivesThe objective of this project is to develop improved processes for the fabrication of CdTe/CdS polycrystalline thin fllm solar cells.The technique we use. for the formation of CdTe, electrodeposition, is a non-vacuum, low-cost technique that is attractive for economic, large-scale production. Technical ApproachDuring the past year, our research and development efforts have focused on several steps that are most critical to the fabrication of high-efficiency CdTe solar cells. These include the optimization of CdTe electrodeposition process, the effect of pre-treatment of CdS substrates, the post-deposition annealing of CdTe, and back contact formation using Cu-doped ZnTe. Systematic investigations of these processing steps have lead to better understanding and improved device performances of the CdTe-based solar cells. ResultsStudies of the �rowth and properties ofCBD CdS tbjo films. The structural properties of the fllms and the growth mechanisms were studied by investigating CdS samples prepared at different deposition times. A duplex structure with an inner compact layer and an outer porous layer and three growth stages is consistent with our data. The measurement of refractive index as a function of deposition time provides a simple way to determine the transition point from compact layer growth to porous layer growth, which is important for the optimization of CdS growth conditions in photovoltaic applications. Careful XRD analyses allowed unambiguous determination of crystal properties.Effe ctofCdCl, treatmentofCdS films. We-investigated the effect of CdCl2 treatment of CdS films on the photovoltaic performance of CdTe solar cells. X -ray diffraction studies indicated that the diffusion of S into CdTe is qualitatively the same for CdTe/CdS films fabricated with both as deposited and CdCl2-treated CdS. A major difference was observed in the extent of Te diffusion into CdS for the two types of CdS films. Full conversion of CdS into CdS 1.yTey was observed for films prepared with as-deposited CdS, while the fo rmation of the ternary phase was below the detection limit for films prepared with CdCl2-treated CdS. Photoluminescence measurements confirmed this result. The difference in interdiffusion leads to differences in optical transmission of CdS films and spectral response of CdTe/CdS solar cells. An increase of 2.7 mA/cm 2 in short-circuit current density was observed as a result of improved spectral response in the wavelength range of 500-600 nm for the CdCl2-treated CdS. Optimization of CdTe and CdS film thicknesses. We have investigated the effect of CdTe and CdS thickness on the photovoltaic performance of the resulting cells. We found that an increase in the CdTe thickness from 2 J.lm to 3 . 5 J.lm lead to a consistent improvement in V oc by 40-60 m V. The shunt resistance of the cells was also increased. As a result, V oc greater than 800 m V and Rsh greater than 1000 n cm2 were regularly obtained. SEM measurements indicated that the average CdTe grain size after CdCb treatm...