TEM study of Locations of Cu in CdTe Solar Cells
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1012-Y04-09
TEM Study of Locations of Cu in CdTe Solar Cells Yanfa Yan, Kim Jones, Jie Zhou, Xuanzhi Wu, and Mowafak Al-Jassim NCPV, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401, USA ABSTRACT Using scanning transmission electron microscopy and nanoprobe X-ray energy-dispersive spectroscopy, we studied the locations of Cu in CdTe solar cells and test interfaces with intentionally introduced Cu sources. We found three primary locations of Cu: 1) back-contact region, 2) CdTe layer, and 3) CdTe/CdS/TCO junction areas. In the back-contact region, Cu diffused from back-contact can convert the Te-rich layer into Cu-Te compounds. In the CdTe layer, a higher concentration of Cu is found to distribute along grain boundaries, but not in twin boundaries and stacking faults. In the CdTe/CdS/TCO junction area, Cu is found uniformly in the CdS layer. However, significant segregation of Cu into CdS/TCO interfaces is also found. INTRODUCTION The general features of CdTe/CdS solarcells and having small area efficiencies of up to 16.5% are widely reported [1]. Usually, to achieve high-efficiency CdTe solar cells, a Cu-containing back-contact is needed. At NREL, NP etching and CuxTe-containing graphite paste, or a CuxTe thin layer are applied during the back-contact process [2]. However, it is known that Cu does not remain in the back-contact area, but diffuses into CdTe and even to the front junction, causing significant degradation [3,4]. Therefore, it is important to find the locations of Cu in CdTe solar cells. So far, Cu location has been investigated mainly by secondary-ion mass spectroscopy (SIMS) [5,6]. Cu accumulation in the CdS region has been reported. SIMS is a proven technique for sampling 1-D low dopant levels (
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