Recent Results for All-Dry-Processed CdTe/CdS Solar Cells
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1165-M02-08
Recent Results for All-Dry-Processed CdTe/CdS Solar Cells Ramesh G. Dhere, Joel N. Duenow, Anna Duda, Stephen Glynn, Jian Li, Wyatt K. Metzger, Helio Moutinho, and Timothy A. Gessert National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
ABSTRACT Several wet-processing steps are used in fabricating high-efficiency CdTe/CdS solar cells. These steps can hinder in-line processing; thus, developing an all-dry processing option is attractive for a manufacturing-friendly process. In this study, we systematically modified the baseline process used in our laboratory to replace CdS deposited by chemical-bath deposition (CBD) with sputter-deposited CdS and Cu-doped graphite paste back-contact with Cu-doped ZnTe deposited by radio-frequency sputtering. In addition to CdTe deposited by close-spaced sublimation, we also used conventionally evaporated CdTe. The results show that replacing only CBD CdS with oxygenated CdS deposited by sputtering produces devices with performance comparable to baseline devices if the front bilayer SnO2 is replaced by a Cd2SnO4/ZnSnO alloy. Replacing the graphite paste back-contact with sputter-deposited Cu-doped ZnTe resulted in device performance comparable to baseline devices. Incorporating both dry processing steps gave performance comparable to the devices with sputtered CdS with a SnO2 front contact. We used capacitance-voltage and minority-carrier lifetime measurements to analyze the factors affecting device performance and we present the results here. INTRODUCTION The evolution of CdTe/CdS solar cells over the last 30 years has involved several developments that have improved device performance. Several wet-processing steps are used in fabricating high-efficiency CdTe/CdS solar cells. The chemical-bath deposition (CBD) process is ideal for conformal coverage of rough surfaces. Thus, introducing CBD CdS allowed the thinning of the CdS layer without shorting between the front transparent conducting oxide (TCO) and CdTe layer, which can considerably impact device performance. In addition, oxygen incorporated in CBD CdS was found to limit interdiffusion at the CdS/CdTe interface [1]. Both of these factors have helped to improve device performance, and incorporating CBD CdS have resulted in the highest-efficiency devices due to the significant increase in short-circuit current [2]. The ohmic contact to CdTe is one of the most problematic areas due to low doping in the CdTe layer and the high work function of CdTe. Most contacts to CdTe contain Cu. The baseline process at the National Renewable Energy Laboratory (NREL) uses a nitric/phosphoric acid etch to create a Te-rich surface, followed by the application of graphite paste containing Cu and Hg to form an ohmic contact. Although these wet processes improve device performance, such steps can hinder the development of in-line processing. Thus, developing an all-dry processing option is attractive for manufacturing process development. Wu et al. [3] at NREL have developed sputtered oxygenated CdS as an alternative to
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