Development of CdCl 2 Activation to Minimize Zn Loss from Sputtered Cd 1-x Zn x Te Thin Films for Use in Tandem Solar Ce
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.521
Development of CdCl2 Activation to Minimize Zn Loss from Sputtered Cd1-xZnxTe Thin Films for Use in Tandem Solar Cells Fadhil K. Alfadhili, Geethika K. Liyanage, Adam B. Phillips*, and Michael J. Heben Wright Center for Photovoltaics Innovation and Commercialization, School for Solar and Advanced Renewable Energy, Department of Physics and Astronomy, University of Toledo, Toledo, OH, USA 43606
ABSTRACT Increasing the band gap of cadmium telluride (CdTe) from 1.48 eV to > 2 eV can be achieved by alloying CdTe with ZnTe. Like CdTe, the alloyed films are expected to allow for low cost production, suggesting that Cd1-xZnxTe could be an ideal top cell for mass produced tandem devices. However, the CdCl2 activation of the alloyed films results in a significant loss of Zn, thereby reducing the bandgap. In this study, we demonstrate a novel CdCl 2 activation method that does not result in significant Zn loss. By performing the activation step in a closed, inert environment we are able to avoid oxidation of the Zn in the Cd 1-xZnxTe film; furthermore, by including sacrificial Zn in the container, an overpressure of ZnCl2 forms limiting the amount of ZnCl2 formed in the film. Both x-ray diffraction, optical measurements, and Auger spectroscopy show that the CdCl2 treatment with no flowing gas minimizes the loss of Zn from the CZT alloy.
INTRODUCTION Over the past five years, progress in CdTe solar cells have resulted in a record efficiency, 22.1% [1], approaching that of the Schockley-Queissar limit [2], As the device performance has improved, the cost of manufacturing CdTe modules has decreased, leading to an average production cost of $0.50 /W P [3], a number that makes power production from CdTe modules competitive with power generated using traditional sources. The next step for improving the efficiency of thin film solar cells beyond 25% is the development of tandem devices [4]. Modelling work done by Coutts et al. [5] shows that a 15% top cell with 1.72 eV bandgap is required to achieve a 25% two terminal polycrystalline tandem. If the low-cost production methods used for CdTe solar cells can be adopted for a polycrystalline top cell, low cost tandems may be possible.
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Cd1-xZnxTe (CZT) is a strong candidate for the top cell in polycrystalline tandems [4]. It is made by alloying CdTe and ZnTe; therefore the device fabrication is expected to follow that of CdTe devices. The bandgap of CZT ranges from 1.48 eV to 2.23 eV and can be controlled by varying the Zn concentration [6]. While reported CZT device efficiency is below 15% [7,8], improving the band alignment is expected to result in increased values [9]. In addition to low efficiency, the other challenge with CZT is that Zn reacts during CdCl2 activa
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