High-Efficiency CdTe Polycrystalline Thin-Film Solar Cells with an Ultra-Thin Cu x Te Transparent Back-Contact
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High-Efficiency CdTe Polycrystalline Thin-Film Solar Cells with an Ultra-Thin CuxTe Transparent Back-Contact X. Wu, J. Zhou, A. Duda, J. C. Keane, T.A. Gessert, Y. Yan, and R. Noufi National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401 ABSTRACT To fabricate a high-efficiency polycrystalline thin-film tandem cell, the most critical work is to make a high-efficiency top cell (>15%) with high bandgap (Eg=1.5-1.8 eV) and high transmission (T>70%) in the near-infrared (NIR) wavelength region. The CdTe cell is one of the candidates for the top cell, because CdTe state-of-the-art single-junction devices with efficiencies of more than 16% are available, although its bandgap (1.48 eV) is slightly lower for a top cell in a dual-junction device. In this paper, we focus on the development of an ultra-thin, low-bandgap CuxTe transparent back-contact to produce high-efficiency CdTe cells with high NIR transmission. We have achieved an NREL-confirmed 13.9%-efficient CdTe transparent solar cell with an infrared transmission of ~50% and a CdTe/CIS polycrystalline mechanically stacked thin-film tandem cell with an NREL-confirmed efficiency of 15.3%. INTRODUCTION To move thin-film solar-electric technology forward in the next 10 years, the DOE/NREL High Performance Photovoltaic Project (HiPerf PV) has selected polycrystalline tandem thinfilm solar cells to advance the state-of-the-art efficiency to 25%. In the polycrystalline thin-film tandem cell project [1], the most critical issue is to make a high-efficiency top cell (>15%) with high bandgap (Eg=1.5-1.8 eV) and high transmittance (T>70%) in the near-infrared (NIR) wavelength region, because the top cell contributes more than two-thirds of the power to a twojunction device. Several high-bandgap alloys based on I-III-VI and II-VI compounds, such as CuGaSe2 (CGS), Cu(InGa)(SeS)2 (CIGSS), and Cd1-xZnxTe (CZT), are being investigated. The 10.9%-efficient CIGSS cell (1.50 eV) [1, 2], 10.2%-efficient CGS cell (1.64 eV) [3], and 11.5%efficient Cd0.95Zn0.05Te cell (~1.53 eV) [4] have been reported. CdTe is also a candidate for the top cell, because CdTe efficiencies of more than 16% have been demonstrated [5], although its bandgap (1.48 eV) is lower than an ideal top cell in a current-matched dual-junction device [6]. However, it is not an issue in a mechanically stacked four-terminal thin-film tandem cell. We have published several papers that discuss how to make high-efficiency CdTe cells using a modified CdTe device structure [5,7,8]. In this paper, we focus on how to use an ultra-thin, low-bandgap CuxTe transparent back-contact to achieve high transmission in the NIR region while maintaining high efficiency. TRANSPARENT ULTRA-THIN CuxTe BACK-CONTACT Although the top cell contributes more than two-thirds of the power of a two-junction tandem device, it is critical to ensure high sub-bandgap transmittance to the bottom cell to maximize its contribution to the total power output. To qualify the CdTe cell as a top cell, we must improve the NIR transparency
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