Photothermal stability of encapsulated Si solar cells and encapsulation materials upon accelerated exposures

We have conducted a series of accelerated exposure test (AET) studies for various samples of crystalline-Si (c-Si) and amorphous-Si (a-Si) cells that were encapsulated with different superstrates, pottants, and substrates. Transmittance, fluorescence, color indices, impedance spectroscopy, laser optical beam induced current (OBIC), and light and dark current-voltage (I-V) measurements were used to characterize the samples. Nonuniform browning patterns of ethylene vinyl acetate (EVA) pottants were observed for glass/EVA/glass-encapsulated c-Si cell samples under solar simulator exposures at elevated temperatures. The EVA discolored gradually into a yellow-brown color when exposed under similar to 7.5 ultraviolet (UV) suns at a black panel temperature (BPT) of similar to 85 degrees C, and very rapidly into a dark brown color when exposed under similar to 9.0 UV suns at a BPT of similar to 145 degrees C. The latter conditions also caused extensive delamination of a polyolefin-based thermoplastic pottant laminated between two borosilicate plates. No visible color change of EVA could be observed for the c-Si cell samples heated in an oven at 85 degrees C or exposed under similar to 1.2 UV suns at BPT = 60-65 degrees C or 80-85 degrees C. Under similar to 1.2 UV suns exposure at a BPT of 60-65 degrees C, the thin layers of EVA or silicone in the lightweight c-Si cell samples, which were laminated in a polymer/solar cell/polymer configuration with Tedlar or Tefzel films, did not discolor because of photobleaching reactions. In comparison, the polyester or nylon superstrate films turned yellow on laminates of polymer/a-Si/polymer minimodules, and significant delamination was observed for the polyester/EVA layers. For all c-Si cell samples tested, irregular changes in the I-V parameters were observed that could not be attributed only to transmittance changes of the superstrate/pottant layers. Under UV-transmitting polymer superstrate films, current/efficiency losses were greater for the c-Si and a-Si cells that were laminated with silicone-type adhesives than with EVA or polyethylene pottants. (C) 2000 Elsevier Science B.V. All rights reserved.