Presentation Details
| Investigation of backsheet cracking in triple-layered photovoltaic backsheet inner layers Stefan Mitterhofer1, Zelin Li1, Song Syun Jhang1, Yu Wen Ho1, Ashlee R.Aiello2, Karissa L.Jensen2, Xiaohong Gu2, Michael D.Kempe3, William B.Hobbs4, Liang Ji5. 1Associate, National Institute of Standards and Technology, Gaithersburg, MD, USA.2National Institute of Standards and Technology, Gaithersburg, MD, USA.3National Laboratory of the Rockies, Golden, CO, USA.4Southern Company, Birmingham, AL, USA.5UL Solutions (retired), Northbrook, IL, USA |
Abstract
Inner layer cracking has been observed in the field in various polymeric backsheets with different layouts. However, the root causes of these cracks, their initiation, and their propagation have not been thoroughly investigated yet. This work presents such an investigation in two different backsheets. The first has a polyethylene terephthalate (PET) outer layer, a PET core layer, and an ethylene vinyl acetate (EVA) inner film consisting of three coextruded sublayers, where a pigmented layer is sandwiched between two non-pigmented ones. Samples are exposed to A3 conditions according to IEC TS 62788-7-2 for up to 3000 hours, or to the field in a desert and a subtropical climate for up to 3 years. The second backsheet consists of a fluoropolymer outer layer, then a PET layer, followed by a coextruded polypropylene (PP) consisting of three sublayers. Samples of this backsheet are extracted from fielded modules after roughly 8 years in the field in a moderate climate, after electrical insulation failures were detected. The samples are then measured with laser-scanning confocal microscopy, cross-sectional nano-indentation, and Raman spectroscopy. Young’s modulus of EVA increases over time, especially for the pigmented layer. Correspondingly, cracks start to appear within this layer along the transverse direction and propagate over time to the non-pigmented ones. Chemical degradation of EVA is indicated by increased fluorescence and decreased intensity in the aliphatic C-H region revealed by Raman spectroscopy. The PP inner layer exhibits a flaking crack pattern, limited to the area close to the frame edge of the module. The pigmented PP layer has a higher modulus than the non-pigmented layers and exhibits more severe cracking. The results indicate that the pigmented inner layers within the backsheets can be a weak point. More severe property changes are seen in these pigmented layers which serve as crack initiation sites.
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No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.