Fabrication and Characterization of Cd 1-x Mg x Te Thin Films and Their Application in Solar Cells
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1012-Y02-02
Fabrication and Characterization of Cd1-xMgxTe Thin Films and Their Application in Solar Cells Ramesh Dhere1, Kannan Ramanathan2, John Scharf1, David Young1, Bobby To1, Anna Duda1, Helio Moutinho1, and Rommel Noufi1 1 5200, NREL, 1617 Cole Blvd, Golden, CO, 80401 2 Miasole, 2590 Walsh Avenue, Santa Clara, CA, 95051 ABSTRACT We present our work on developing Cd1-xMgxTe (CdMgTe) polycrystalline thin films for application in tandem solar cells. CdMgTe thin films were fabricated by co-evaporation of CdTe and Mg at a substrate temperature of 400˚C. The spatial separation of the two sources resulted in a compositional gradient, which allowed the deposition of a wide range of compositions from fewer runs. Stable films for compositions up to x=0.73 were fabricated. The structural analysis showed that the as-deposited films have a sphalerite structure and are preferentially oriented in the direction, and the lattice constant varies linearly with composition. Optical analysis of the samples shows that the optical bandgap varies linearly with composition. The optical absorption coefficient of the alloy films for the entire composition range (up to x=0.73) is over 6x104 cm-1 for energies above the bandgap, indicating a direct-gap semiconductor. We have fabricated solar cells using these films with the following structure: glass / SnO2 / CdS / CdMgTe / back contact. Using the same processing conditions used for CdTe solar cells, the device efficiency was less than 2%. With modified contact processing, we were able to improve efficiencies to over 5%. Post-deposition chloride heat-treatment in oxygen ambient resulted in partial dissociation of the CdMgTe alloy and loss of Mg. With modified chloride processing, we were able to reduce alloy decomposition and further improve the efficiencies. We have obtained devices with open-circuit voltages of 845 mV and efficiencies of 8% for CdMgTe devices with a bandgap in the range of 1.6ñ1.62 eV, which are the highest reported values for CdMgTe alloys. INTRODUCTION Thin-film solar cells based on CdTe have reached efficiencies of 16.5% in laboratory devices [1]. Module efficiencies, however, lag considerably at present, but are expected to progress with improved manufacturing processes. For the next generation of products, further improvement is possible by boosting the efficiency of these devices. Developing tandem solar cells for moreefficient use of the solar spectrum is one avenue to boost performance. Two-junction tandem solar cells require a top cell with a bandgap in the range of 1.6ñ1.8 eV. Ternary alloys of CdTe are of great interest for this application because of the success of CdTe solar cells. Considerable work has been conducted on Cd1-xZnxTe (CZT) polycrystalline thin films, but the development of CZT solar cells is hampered by the high affinity of Zn to oxygen and chlorine, which are routinely used in fabricating high-efficiency CdTe solar cells [2]. CdMgTe offers several unique advantages, such as a large change in bandgap for the same composition as compared to C
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