Perturbation of Copper Substitutional Defect Concentrations in CdS/CdTe Heterojunction Solar Cell Devices
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Perturbation of Copper Substitutional Defect Concentrations in CdS/CdTe Heterojunction Solar Cell Devices D. Albin, R. Dhere, X. Wu, T. Gessert, M.J. Romero, Y. Yan, and S. Asher National Renewable Energy Laboratory Golden, CO 80401, USA
ABSTRACT The efficacy of implementing terrestrial-based photovoltaics is dictated by trade-offs in device performance, cost, and reliability. Presently, the highest efficiency polycrystalline CdS/CdTe superstrate solar cells utilize back contacts containing copper as an intentional dopant. Accelerated stress data correlates copper diffusion from this contact with performance degradation. Degradation at the device level exhibits two characteristic modes that are influenced by CdTe surface treatments prior to contacting. Rapid degradation associated with a rapidly decreasing open-circuit voltage can occur in cases where processing favors stoichiometric CdTe surfaces. Slower degradation associated with roll-over is typified by treatments favoring the presence of Te at the back contact. The chemical composition and extent of Te-rich contact interfaces is revealed by transmission electron microscopy. Deep-level transient spectroscopy of NP etched and non-etched devices show Te-rich conditions are necessary for the detection of deep-acceptor CuCd defect levels at (Ev +0.28 to 0.34 eV). Low keV cathodoluminescence measurements show that these defects can be found localized at the back surface of CdS/CdTe devices. INTRODUCTION Owing to its nearly ideal bandgap, high absorption coefficient, and ease of film fabrication, polycrystalline CdTe is a promising candidate for low-cost, thin-film solar cells. Small-area CdS/CdTe cells with efficiencies of 15%-16% have been made by several research groups [1,2,3]. All use a standard device structure consisting of CdS/CdTe layers deposited on a transparent-conductor coated glass substrate. All structures also achieve high performance by utilizing a carbon or graphite-dag paste contact applied as a back contact to the top CdTe layer. In many (if not all) of these cases, Cu-containing dopants are mixed with the carbon layer to improve the ohmic nature of the contact. The exact mechanism by which this is achieved is somewhat unclear though several theories exist: 1) improved tunneling due to increased doping, 2) better band alignment due to fermi level adjustment, and 3) the formation of interfacial telluride layers [4]. Though beneficial to the initial device performance, Cu dopants introduce long-term device stability issues as measured through elevated temperature under illumination stress testing. A comparison of graphite-dag-based back contacts, with and without Cu constituents, substantiates this [5]. Finally, it has also been shown that precontact surface treatments (nitricphosphoric acid and Br in MeOH) impact both the initial device performance and the relative stability of the contact [6]. In particular, it was the presence of elemental Te at the CdTe/back contact interface that determined the ultimate stability of the device.
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