14% CdS/CdTe Thin Film Cells with ZnO:Al TCO
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14% CdS/CdTe Thin Film Cells with ZnO:Al TCO Akhlesh Gupta and Alvin D. Compaan Department of Physics & Astronomy, University of Toledo, Toledo, OH, 43606, USA ABSTRACT An Al-doped ZnO front contact was successfully used for the first time for the fabrication of high efficiency CdS/CdTe thin-film solar cells. The ZnO:Al films were deposited on aluminosilicate glass by RF sputtering from a ZnO:Al2O3 target. The ZnO:Al film has ~95% average transmission in the visible spectrum with ~3 ohm/square sheet resistance. The CdS and CdTe thin films were also deposited by RF sputtering and devices were completed with a vapor CdCl2 treatment and evaporated Cu/Au back contacts. The highest processing temperature was 387 oC, reached during the vapor CdCl2 treatment. The devices were tested at NREL with efficiency of 14.0% which is a record for an all-sputtered CdS/CdTe solar cell. The ZnO-based cell had JSC of 23.6 mA/cm2 compared to 20.7 mA/cm2 for our recent NREL-tested 12.6% cell on a commercial soda-lime-glass/SnO2:F substrate. Other parameters of the 14% ZnO based cell are: FF = 73.25% and VOC = 814 mV. The improved performance is almost entirely due to higher current because of better optical and electrical properties of ZnO:Al TCO. We report also on relative stability between devices on SnO2:F and ZnO:Al TCO, under one-sun light soak at VOC. INTRODUCTION Fluorine-doped tin oxide (SnO2:F) coated glass has been used very widely as a front conducting and transparent contact for CdS/CdTe solar cells. Doped and conducting ZnO (mostly Al-doped ZnO or ZnO:Al), which has better electrical and optical properties, has rarely been used. The best efficiency reported for CdS/CdTe cell on a ZnO:Al TCO is ~9% [1] compared to ~16% [2] on SnO2:F TCO. The low efficiencies with ZnO:Al is believed to be due to its thermal instability at high growth/processing temperature (e.g., close-spaced-sublimation (CSS), vapor-transport-deposition (VTD) techniques) and its instability in high acidic or basic media (e.g., solution growth, electrodeposition techniques). The main aim of this work is to examine the effect of using ZnO:Al as a front contact TCO for the fabrication of high efficiency CdS/CdTe devices using magnetron RF sputtering, which is a low temperature deposition technique. A comparison is also reported here of the electrical and optical properties of SnO2:F and ZnO:Al TCOs, and their CdS/CdTe devices and stability performances. EXPERIMENTAL DETAILS ZnO:Al films were deposited using RF magnetron sputtering from a ceramic ZnO:Al2O3 (2%) target at 220 oC on 1 mm thick, ultrasonically alkaline-soap-cleaned aluminosilicate (ASG) or sodalime glass (SLG) slides. The depositions were carried out in pure argon gas at 48 W of RF power and 10-12 mTorr pressure. The deposition rate was ~ 2 Å/sec. The films were characterized for optical and electrical properties. The CdS/CdTe solar cells were fabricated by deposition of 130 nm thick CdS and 2.3 micron CdTe layers using RF magnetron sputtering at 250 oC on as-deposited ZnO:Al/ASG (1mm) or SnO
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