Polycrystalline Thin-Film Cadmium Telluride Solar Cells Fabricated by Electrodeposition; Final Technical Report, 20 March 1995-15 June 1998

Abstract

EXECUTIVE SUMMARY ObjectivesOne of the objectives of this project is to develop improved processes for the fabrication of high-efficiency CdTe/CdS polycrystalline thin film 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. The other objective was to study cell degradation, using stability tests under stress conditions and analysis of possible degradation mechanisms. Technical ApproachOur 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 the CdTe electrodeposition process, the effect of pre-treatment of CdS substrates, the post-deposition annealing of CdTe, and back contact formation using Cudoped ZnTe. Systematic investigations of these processing steps have led to better understanding and improved device performances of the CdTe-based solar cells.Stability tests under enhanced temperature showed that cell efficiency degradation is due mainly to an increase in "series resistance". Therefore the degradation processes studies were focused mostly on revealing and explaining the changes that occur in the back-contact region and in the bulk CdTe due to the ZnTe:Cu back contact presence. These changes were studied by means of detailed analyses of the cell I-V characteristics, the C-V profile in the CdTe, and compositional changes in the back-contact region. When discussing possible mechanisms of degradation, the compensation phenomena and charged impurities electromigration were taken into account. Major Results and ConclusionsCdS thin films prepared by chemical bath deposition. Film growth and structure.The structural properties of the films 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 can explain 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.Electrodeposited CdS Thin Films and their application.Electrodeposition is a non-vacuum technique and is compatible with our CdTe deposition process. It offers excellent control over the properties of the thin films through the influence of deposition potential, bath temperature, pH, and composition of reactants. In 2 this project preliminary studies were performed on the electrodeposition of CdS and its application in fabricating CdTe/CdS solar cells. The influence of deposition conditions on the film deposition rate, composition and morphology was studied. CdTe/CdS cells using electrodeposited and physical vapor deposited CdTe on electrodeposited CdS were prepared and ch...

Structural and optical properties of CdTe thin film: A detailed investigation using optical absorption, XRD, and Raman spectroscopies

“…The measured values of these parameters are listed in Table . The lattice constant of CdTe bulk material derived in this work (Table ) is close to the ideal value, which is 0.6481 nm .…”

“…, the data was fitted to a Lorentzian profile and the spectrum in was investigated by a different model described in Ref. , while did not use the fitting at all and because of this they could not estimate the peak width and grain size.…”