Investigation of CdZnTe for Thin-Film Tandem Solar Cell Applications
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Investigation of CdZnTe for Thin-Film Tandem Solar Cell Applications
Ramesh Dhere, Tim Gessert, Jie Zhou, Sally Asher, Joel Pankow, and Helio Moutinho, National Renewable Energy Laboratory, Golden, CO 80401. ABSTRACT Modeling of two-junction tandem devices shows that for optimal device performance, the bandgap of the top cell should be around 1.6-1.8 eV. CdZnTe alloys can be tailored to yield bandgaps in the desired range. In this study, three approaches were used to fabricate these films. The CdTe and ZnTe films were deposited by close-spaced sublimation (CSS) and radiofrequency sputtering (RFS) techniques respectively. In the first approach, we used mixed powders of CdTe and ZnTe as the source for film deposition by CSS. Even for the ZnTe/CdTe (95:5 ratio) source material, the deposited films were entirely CdTe due to higher vapor pressure of CdTe. In the second approach, we used pre-alloyed CdZnTe powders (CERAC, Inc.) as the source. Due to the lower sticking coefficient of Zn, even for the source composition of 75% Zn, these films contained very low quantities of Zn (~5%). We tried unsuccessfully to increase the Zn content in the films by confining Zn vapor by enclosing the region between the source and substrate, reducing the substrate temperature to 400°C, and adjusting the source/substrate distance. Finally, we used thin-film couples consisting of 300-nm-thick CdTe deposited by CSS and 300-nm-thick ZnTe deposited by RFS; the samples were then heat-treated in cadmium chloride vapor. Compositional analysis of the samples showed extensive interdiffusion of Cd and Zn for the annealed samples. We will present the data on the various stack configurations of CdTe and ZnTe, the effect of different post-deposition anneals, the effect of oxygen on the interdiffusion and alloy formation and its possible correlation to the device performance degradation. INTRODUCTION Polycrystalline thin-film solar cells based on II-VI and I-III-VI compounds have been quite successful, with efficiencies of laboratory devices reaching 16.5% for CdTe [1] and 19.2% for CIGS [2] single-junction solar cells. For further improvement, one needs to look at the tandem device option. Device modeling of two-junction tandem devices shows that the optimal bandgap of the top cell should be around 1.6-1.8 eV [3]. Ternary alloy CdZnTe is a good candidate as it is a well-behaved system and is based on CdTe. In this paper, we use three key approaches that have been responsible for improved performance of CdTe devices: 1) CSS technique for the deposition of alloy layers because the best-performing CdTe devices have resulted from the films prepared by CSS at high deposition temperatures, 2)CdCl2 heat treatment, which is a crucial step for the fabrication of high efficiency CdTe devices, 3) We explored the use of oxygen during different stages on film properties because it is another essential component of high-efficiency CdTe device fabrication. One of the objectives of this work is to investigate the differences in CdTe and CdZnTe systems that co
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