Examination of Na-Doped Mo Sputtering for CIGS Devices (CRADA Final Report)

Technical Report · Sun Jan 01 04:00:00 EST 2012

DOI:https://doi.org/10.2172/1033030· OSTI ID:1033030

Repins, Ingrid ^[1]

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1. National Renewable Energy Laboratory (NREL), Golden, CO (United States)

Na is an important ingredient in Cu(In,Ga)(Se,S)₂ (CIGS) photovoltaic devices, due to its ability to increase carrier density and passivate surfaces, grain boundaries, and Se vacancies. In the highest efficiency devices, Na is supplied to the growing CIGS film by diffusion through the Mo back contact from the sodalime glass substrate. This arrangement is problematic in that the Na content of the film is only indirectly controlled: deposition times, deposition temperatures, Mo morphology, and glass composition are coupled with the Na diffusion. Results in the literature show that the Mo, not the CIGS, is the primary barrier to Na diffusion. Alternatively, using a Na-free substrate or diffusion barrier, Na may be introduced by a NaF precursor deposited on top of the Mo. However, this forces a trade-off between carrier density and adhesion. If the NaF is not entirely consumed, the CIGS layer peels away from the Mo, encouraging the use of less NaF even though the best device properties tend to result from the thickest possible NaF layers. Furthermore, although early studies show that gross control of Na is sufficient to produce 12-14% small devices, the industry has reached this level and is ready to progress to higher performance. The sensitivity of NREL device efficiency (15 to 20%) to Mo morphology suggests that fine control of Na content is necessary for very high performance. This work has investigated the use of Na doped Mo (“MONA”) sputtering targets for use in preparing CIGS devices. The Mo:Na material is doped to about 3% Na by weight, implying that a 40 nm layer on top of the standard Mo contact validation of their product. contains sufficient Na to dope a 2.5 µm CIGS film. The ability to control Na doping independent of both CIGS processing conditions and adhesion is an important gain for industry and research. Manufacturers gain a route to increased manufacturability and performance, while NREL researchers gain a tightened performance distribution of devices and increased process flexibility. Our immediate partner in this work, the Climax Molybdenum Technology Center, gains validation of their product.

• Research Organization: National Renewable Energy Laboratory (NREL), Golden, CO (United States); Climax Molybdenum, Phoenix, AZ (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office
• DOE Contract Number: AC36-08GO28308
• OSTI ID: 1033030
• Report Number(s): NREL/TP--7A10-53586; CRD-10-375
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
36 MATERIALS SCIENCE
ADHESION
CIGS layer
CRADA
DISTRIBUTION
FLEXIBILITY
MANUFACTURERS
MOLYBDENUM
Na diffusion
PERFORMANCE
PROCESSING
SPUTTERING
TARGETS
VALIDATION