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  • Polycrystalline Thin-Film Cadmium Telluride Solar Cells Fabricated by Electrodeposition

    Final Technical Report 20 March 1995 – 15 June 1998

    January 1999 • NREL/SR-520-26009

    J.U. Trefny, D. Mao, V. Kaydanov, T.R. Ohno, D.L. Williamson, R. Collins, and T.E. Furtak Department of Physics Colorado School of Mines Golden, Colorado

  • National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 NREL is a U.S. Department of Energy Laboratory Operated by Midwest Research Institute • Battelle • Bechtel

    Contract No. DE-AC36-98-GO10337

    January 1999 • NREL/SR-520-26009

    Polycrystalline Thin-Film Cadmium Telluride Solar Cells Fabricated by Electrodeposition

    Final Technical Report 20 March 1995 – 15 June 1998

    NREL technical monitor: B. von Roedern Prepared under Subcontract No. XAF-5-14142-11

    J.U. Trefny, D. Mao, V. Kaydanov, T.R. Ohno, D.L. Williamson, R. Collins, and T.E. Furtak Department of Physics Colorado School of Mines Golden, Colorado

  • This publication was reproduced from the best available copy Submitted by the subcontractor and received no editorial review at NREL

    NOTICE

    This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

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  • NOTICE

    This report was prepared as an account of work sponsored by the National Renewable Energy Laboratory, managed by Midwest Research Institute, in support of its Subcontract No. XAF-5-14142-11 with Colorado School of Mines. Neither the National Renewable Energy Laboratory, nor the Midwest Research Institute, nor Colorado School of Mines, nor any of their employees, nor any of their contractors, nor any of their subcontractors, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

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    EXECUTIVE SUMMARY

    Objectives

    One 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 Approach

    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 the 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 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 Conclusions

    CdS 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

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    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 characterized. Moderate cell efficiencies were obtained in this preliminary study. Modification of our standard post-deposition treatment led to a considerable increase in cell efficiency. Although it was still lower (8.7%) than for the CBD CdS, it became clear that there is room for the further improvement of the cell performance by means of post- deposition treatment optimization and also by selection of a proper TCO material for the front contact. The latter greatly influences the CdS film morphology and adhesion.

    CdTe - CdS interdiffusion. Post-deposition heat treatments of CdS and CdTe/CdS thin films with CdCl2

    coating are very important steps in our processing procedure leading to considerable improvement of the cell performance. Recrystallization and interdiffusion processes at the annealing with CdCl2 are much more pronounced in CBD CdS and electrodeposited CdTe (low-temperature deposition processes) than in "high temperature" deposited films (e.g., deposited by the close space sublimation method). Our cell efficiency increases considerably as a result of the CdS and CdS/CdTe post-deposition treatments and there are indications that there is room for the further improvement of the cell performance through post-deposition steps optimization. In this project we studied diffusion processes and also their influence on the electronic properties of the films. Because of the finite solubility of CdS and CdTe within each other, and interdiffusion between the CdS and CdTe layers, a CdTe-rich ternary phase (CdTe1-xSx) is formed in the CdTe layer. A CdS-rich ternary phase (CdS1-yTey) may also be formed in the CdS layer. The degree of these interdiffusion processes is affected by the CdCl2 treatment of CdS films and was quantitatively measured in our study by monitoring the change of line shape of XRD patterns. Photoluminescence measurements were also used to study S diffusion into the CdTe. The results of the studies were as follows.

    (1) While only a negligible portion of the CdS is converted to CdS1-yTey for the CdCl2-treated CdS, it is fully converted to the ternary phase when as- deposited CdS is used. The y value was estimated to be 4.4% in CdS1-yTey.

    (2) In CdTe annealed at temperature >4000C, two phases coexist: pure CdTe located at the central part of the grains, and a ternary phase CdTe1-xSx concentrated at the grain boundary region. The content of S in the ternary phase is defined by the solubility limit for the annealing temperature used. The total amount increases with annealing temperature. It was found that annealing at 4100C for 45 min (our standard procedure) leads to thinning of the CdS by 60-70 nm. That imposes a lower limit on the initial CdS film thickness.

    Electronic properties of CdS films treated under different conditions were also studied. It was found that annealing at 4500C for 50 min increases the bandgap by 60-70 mV bringing it close to the bulk CdS bandgap value. Conductivity of as-prepared films is very low and its temperature dependence is characterized by an activation energy of about 0.7 eV. The films reveal very high photoconductivity with characteristic times of decay

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    achieving hours. Both effects (the activation character of th

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