Tin oxide stability effects{emdash}their identification, dependence on processing and impacts on CdTe/CdS solar cell performance
- National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401 (United States)
High efficiency polycrystalline thin film CdTe solar cells involve the growth of CdTe films on CdS/SnO{sub 2}/glass substrates. The CdS layer in such a structure is commonly reported to benefit from a brief hydrogen anneal prior to the deposition of the CdTe film. In this paper, we show that the SnO{sub 2} layer can be susceptible to reduction in H{sub 2} and that the degree of susceptibility is dependent on the type of SnO{sub 2} used. Chemical vapor deposited (CVD) SnO{sub 2}/glass substrates (Solarex Corp.) show the most resistance to reduction while room-temperature sputtered SnO{sub 2} films show the least resistance. When annealed under reducing conditions, Sn from the SnO{sub 2} reacts with S-containing impurities and oxygen in as-grown chemical bath deposited (CBD) CdS films to form SnS. Cd-containing impurities are more volatile resulting in a loss of Cd relative to S in films annealed in H{sub 2}. These films appear dark due to the presence of SnS, a grayish-black impurity, in the CdS and possibly SnO in the SnO{sub 2}. In normal CSS CdTe deposition processes where H{sub 2} annealing is followed by further heating to deposition temperatures in either He or He:O{sub 2} ambient, S loss occurs at temperatures exceeding the H{sub 2} anneal. If oxygen is absent, CdS films undergo loss of both Sn and S due to evaporation of the SnS. When O{sub 2} is present, SnS converts to SnO{sub 2} allowing for only the evaporation of sulfur. In this fashion, Sn levels on the CdS surface immediately prior to the deposition of CdTe, can be affected not only by the temperature of the H{sub 2} anneal, but also by the oxygen present during the CdTe deposition step. Modifications to the CdS/CdTe device fabrication process including the use of more stable tin oxide layers (CVD-grown) and lower temperature H{sub 2} anneals yield devices with higher open circuit voltage, fill-factors, and total-area efficiencies. (Abstract Truncated)
- OSTI ID:
- 552867
- Report Number(s):
- CONF-961178-; ISSN 0094-243X; TRN: 9722M0063
- Journal Information:
- AIP Conference Proceedings, Vol. 394, Issue 1; Conference: National Renewable Energy Laboratory (NREL)/Sandia National Laboratories (SNL) photovoltaics program review meeting, Lakewood, CO (United States), 18-22 Nov 1996; Other Information: PBD: Feb 1997
- Country of Publication:
- United States
- Language:
- English
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