Collaborative Research and Development by EpiSolar and NREL of Processes and Materials for Flexible CdS/CdTe Superstrate Devices: Cooperative Research and Development Final Report, CRADA Number CRD-14-550 PDF Download
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The objective of this work is to collaborate with EpiSolar to develop and test processes that are consistent with the goals and milestones of an NREL FPace1 (Foundational Program to Advance Cell Efficiency) project entitled 'High-Temperature, Roll-to-Roll (RTR) CdTe Superstrate Devices Using Flexible Glass.' The primary milestone for this CRADA relates to demonstration of a 15% efficient laboratory device.
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The objective of this work is to collaborate with EpiSolar to develop and test processes that are consistent with the goals and milestones of an NREL FPace1 (Foundational Program to Advance Cell Efficiency) project entitled 'High-Temperature, Roll-to-Roll (RTR) CdTe Superstrate Devices Using Flexible Glass.' The primary milestone for this CRADA relates to demonstration of a 15% efficient laboratory device.
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Research performed at NREL to produce CdS/CdTe devices on St. Gobain coated-glass material to establish a baseline CdS/CdTe device process and determine baseline device performance parameters on St. Gobain material. Performance of these baseline devices compared to similar devices produced by applying the established baseline CdS/CdTe process on alternative St. Gobain coated-glass materials.
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Pages : 4
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Research performed at NREL to produce CdS/CdTe devices on St. Gobain coated-glass material to establish a baseline CdS/CdTe device process and determine baseline device performance parameters on St. Gobain material. Performance of these baseline devices compared to similar devices produced by applying the established baseline CdS/CdTe process on alternative St. Gobain coated-glass materials.
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This CRADA involves analyses on a variety of CdTe and CdTe-PV related materials, test structures, and solar cells produced at FSLR or NREL. Single-crystal (sx) material will be primarily MBE grown and provided by FSLR. Joint activities are focused on establishing the appropriate quality metrics as a basis for high performing sx devices and to study the dopability of the material. Polycrystalline (px) materials will be provided by NREL, FSLR, or fabricated jointly. Px activities are focused on the characterization of performance limiting mechanisms including bulk defects, grain boundaries, and the hetero-interface. A combination of growth, characterization, and theory will be applied. Analysis will include techniques that are well established in CdTe research at NREL (e.g., LIV/DIV/CV, AS/DLTS, LTPL, TRPL) as well as techniques that have not been widely applied to the PV part of CdTe technology (e.g., PCD, CL, uPL, SE).
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NREL study may provide future guidance in improving CdS/CdTe photovoltaic device performance. The majority of minority carrier lifetime (MCL) studies performed on CdS/CdTe photovoltaic (PV) devices have correlated device performance primarily with the fast decay observed in time-resolved photoluminescence (TRPL) measurements (t1). This decay is believed to be associated primarily with recombination in depletion width (W{sub D}), and therefore should be a good indicator of device quality if carrier generation occurs primarily within WD. However, although previous studies have shown that t1 can be a good indicator of broad device quality, it does not correlate as well with small changes in device performance and/or with differences observed between superstrate and substrate devices. Researchers at the National Renewable Energy Laboratory (NREL) have shown that in this case, the parameter t2 (from the longer-term decay of TRPL) may not only provide a better correlation with device open-circuit voltage (V{sub OC}) for superstrate devices but may also provide guidance for inter-comparison with alternative device designs (e.g., substrate devices). It is also suggested that previous studies may yield added value if a larger number of TRPL parameters (i.e., t1, t2, and respective amplitudes) are re-examined as a function of device performance. The parameter t2 may not only provide a better correlation with device VOC for superstrate devices but may also provide guidance for inter-comparison with alternative device designs (e.g., substrate devices).
Author: Deidra Ranel Hodges
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ABSTRACT: Cadmium telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal band gap of 1.45 eV, its high optical absorption coefficient and availability of various device fabrication methods. Superstrate CdTe solar cells fabricated on glass have to-date exhibited efficiencies of 16.5%. Work on substrate devices has been limited due to difficulties associated with the formation of an ohmic back contact with CdTe. The most promising approach used to-date is based on the use of an interlayer between the CdTe and a metal electrode, an approach that is believed to yield a pseudo-ohmic contact. This research investigates the use of ZnTe and Sb2Te3 as the interlayer, in the development of efficient back contacts. Excellent adhesion and minimum stress are also required of a CdTe thin film solar cell device on a flexible stainless steel (SS) foil substrate. Foil substrate curvature, flaking, delamination and adhesion as a result of compressive strain due to the coefficient of thermal expansion (CTE) mismatch between the flexible SS foil substrate and the solar cell films have been studied. A potential problem with the use of a SS foil as the substrate is the diffusion of iron (Fe), chromium (Cr) and other elemental impurities into the layers of the solar cell device structure during high temperature processing. A diffusion barrier limiting the out diffusion of these substrate elements is being investigated in this study. Silicon nitride (Si3N4) films deposited on SS foils are being investigated as the barrier layer, to reduce or inhibit the diffusion of substrate impurities into the solar cell. Thin film CdTe solar cells have been fabricated and characterized by AFM, XRD, SEM, ASTM D3359-08 tape test, current-voltage (I-V) and spectral measurements. My individual contributions to this work include the Molybdenum (Mo) development, the adhesion studies, the silicon nitride (Si3N4) barrier studies, and EDS and SEM lines measurements and analysis of substrate out-diffused impurities. The rest of my colleagues focused on the development of CdTe, CdS, ZnTe, the CdCl2 heat treatment, and other back contact interlayer materials.
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NREL and NanoSpray will cooperate to leverage NREL's existing solar cell processing facilities at NREL's Atmospheric Processing Platform to determine optimal processing method using 6" by 6" CdTe modules.
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NREL has developed advanced processes for CdTe solar cells, but because of the temperature limitations of conventional soda lime glass, many of these processes have not been transferred to manufacturing. Corning is developing high temperature substrate glasses that are believed to be manufacturable and will lead to lower $/watt modules costs. The purpose of this CRADA is to evaluate these glassesin the advanced NREL processes. In addition, the CRADA seeks to develop manufacturable processes for transparent conductive oxide layers based on cadmium stannate.
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NREL scientists have worked with Washington State University and the University of Tennessee to improve the maximum voltage available from CdTe solar cells. Changes in dopants, stoichiometry, interface design, and defect chemistry improved the CdTe conductivity and carrier lifetime by orders of magnitude, thus enabling CdTe solar cells with open-circuit voltages exceeding 1 volt for the first time. Values of current density and fill factor for CdTe solar cells are already at high levels, but sub-par voltages has been a barrier to improved efficiencies. With voltages pushed beyond 1 volt, CdTe cells have a path to produce electricity at costs less than fossil fuels.
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The main focus of the work at NREL was on the development of Cu-doped ZnTe contacts to CdTe solar cells in the substrate configuration. The work performed under the CRADA utilized the substrate device structure used at NREL previously. All fabrication was performed at NREL. We worked on the development of Cu-doped ZnTe as well as variety of other contacts such as Sb-doped ZnTe, CuxTe, and MoSe2.We were able to optimize the contacts to improve device parameters. The improvement was obtained primarily through increasing the open-circuit voltage, to values as high as 760 mV, leading to device efficiencies of 7%.
