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Technical Report

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National Renewable Energy Laboratory

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This program has involved investigations of the stability of CdTe and copper-indium-gallium-diselenide (CIGS) solar cells under damp heat conditions and effects of barrier coatings. CdTe and CIGS cells were subjected to environmental conditions characterized by 60ºC and 90% relative humidity (60/90), and to some extent 85ºC and 85 % relative humidity (85/85). Barrier coating technology developed at PNNL for OLEDs was used to investigate approaches to encapsulation of the two types of thin film solar cells. Moisture barriers consisting of multi-layer coatings involving alternating layers of polymer and aluminum oxide were deposited directly onto the front surface of CIGS cells and the rear surface of CdTe devices, with the front surface defined by the light receiving side of the cell. Most of the studies were conducted with directly deposited barrier coatings. The CIGS cells were provided by Shell Solar, Industries (SSI) in the form of 10 cm x 10 cm and 5 cm x 5 cm mini-circuits, and by the Institute for Energy Conversion (IEC) in the form of small laboratory cells grown on a 2 in x 2 in glass substrate. The SSI devices were actually CIGSS cells since they contained sulfur as well as selenium, but they are referred to as CIGS cells in this report. CdTe cells were provided by Dr. Sampath at Colorado State University (CSU). Current voltage characteristics of cells were taken periodically when under damp heat stress. Photoluminescence studies were also utilized for characterizing the effect of damp heat stress. An approach to encapsulation by direct deposition was determined for each type of cell that allowed the device to survive 1000 hours in a 60/90 damp heat environment. SSI cells were also tested under 85/85 conditions. The efficiencies of SSI thin-film CIGS cells with directly deposited barrier coatings subjected to 85/85 stress were found to degrade to 60% of their beginning-of-life value. The decline in efficiency versus time was independent of the number dyads (polymer/alumina pairs) used to form the moisture barrier. The result suggests that moisture ingress was either occurring at the edges, or a directly deposited multi-layer barrier is inadequate for the SSI circuits under the 85/85 stress condition. IEC cells were encapsulated by directly deposited, five dyad coatings. One device survived 2000 hours in 85ºC/85%RH without any degradation in efficiency. More recently, very good results were achieved with laminated SoloPower cells using PNNL coated PET. These reslts are briefly discussed. CdTe cells were tested with and without encapsulation. Uncapsulated CdTe degraded in an environment of 60ºC/90%RH, but were found to be quite stable when encapsulated with PNNL barrier coatings.


Barrier coatings; Copper-indium-gallium-diselenide (CIGS) solar cells; Damp heat; Photovoltaics; Solar energy


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NREL Report No. SR-520-47582