Preparation and characterization of monolithic HgCdTe/CdTe tandem cells
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Preparation and characterization of monolithic HgCdTe/CdTe tandem cells S. L. Wang*, J. Drayton, V. Parikh, A. Vasko, A. Gupta, and A. D. Compaan Department of Physics & Astronomy, University of Toledo, Toledo, OH, 43606, USA ABSTRACT A prototype monolithic HgCdTe/CdTe superstrate tandem cell has been fabricated by RF sputtering, comprising a CdTe/CdS top cell, a ZnTe:N/ZnO:Al interconnect junction and a HgCdTe/CdS bottom cell. The Hg1-xCdxTe film as the bottom absorption layer was deposited by RF sputtering with 70% or 85% Cd content in the Hg1-xCdxTe magnetron target. Hg1-xCdxTe films with band gap from 0.98 eV to 1.45 eV were obtained by controlling the deposition temperature. CdCl2 thermal treatments were used to improve the Hg1-xCdxTe film electrical properties. A nitrogen-doped ZnTe film combined with an aluminium (Al) doped ZnO film formed a good interconnect junction. Results of Voc = 0.99 V and Jsc = 2.1 mA /cm2 were obtained in the best such tandem cell at one sun (AM1.5). INTRODUCTION 25% conversion efficiency is a goal for polycrystalline thin films in the high performance PV program of the US Department of Energy [1, 2]. The predicted achievable efficiency at AM1.5 is 28.2% for top and bottom cell band gaps of 1.72 eV and 1.14 eV, respectively. As a singlejunction solar cell material, the best sputtered CdTe cell is 14% [3] and has reached 16.5% for close-spaced sublimation [4]. Large-scale manufacturing of low-cost CdTe cells has been achieved [5], therefore, CdTe-based alloys such as CdZnTe or CdMnTe, which can be adjusted to 1.74 eV band gap, are attractive candidates for top cell materials [6, 7]. In addition to these IIVI alloys with wide band gaps, alloys of CdTe with HgTe form possible narrow-band-gap bottom cell candidates. Hg1-xCdxTe is flexible enough to tailor the band gap from -0.15 eV to 1.5 eV. Although polycrystalline CdZnTe and CdMnTe can fit the optical requirement for such tandem cells, their poor electronic properties remain obstacles to tandem cell applications. Compared to CdZnTe and CdMnTe, the electronic properties of HgCdTe can be easily controlled by proper annealing. A sputtered, single heterojunction HgCdTe/CdS cell showed typically Voc ~ 0.39 V and Jsc ~ 13 mA/cm2. This has encouraged us to fabricate HgCdTe/CdTe tandem cell as a first step toward approaching a high efficiency, all II-VI tandem cell. In this paper, we investigate processing of a HgCdTe/CdTe two-terminal tandem cell, particularly studying the characteristics of RF sputtered Hg1-xCdxTe films and HgCdTe/CdS single junctions. EXPERIMENTAL DETAILS We have chosen to maintain a separate deposition system for the Hg-based alloys and also a separate system for sputtering of the ZnTe/ZnO layers. Therefore the whole tandem structure was deposited in three separate RF sputtering systems. The deposition sequence started with a cleaned SnO2:F-coated TEC-15 glass substrate loaded into a dual magnetron sputtering system for CdS and CdTe deposition; then vapor CdCl2 treatment was carried out in dry air at 390 oC.
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