Accelerated testing and failure of thin‐film PV modules

Abstract

Packaging-related PV module failure is distinguished from cell failure, and those failures specific to thin-film modules are reviewed. These are categorized according to the type of stress that produced them, e.g., temperature, voltage, moisture, current, and thermal cycling. An example is given that shows how to relate time under accelerated stress to time in use. Diagnostic tools for locating the affected area within a large-area module are pointed out along with the importance of interpretation of the visual appearance of the different damage mechanisms. INTRODUCTIONT he U.S. Department of Energy encourages and supports the photovoltaic (PV) industry in developing a module technology that will last 30 years in the field. 1 An additional goal is to identify a short-term means to certify that a module technology will, indeed, last 30 years. The achievement of the first goal for thin-film (TF) PV modules will be more difficult than for the single-and multi-crystalline Si modules that have been available for some time. There are at least four reasons for this: (1) TF cells have a lower threshold for corrosion and delamination; (2) several of the amorphous Si and CdTe TF technologies use the SnO 2 -coated soda-lime glass superstrate configuration requiring an edge-deleted module perimeter; (3) SnO 2 -coated glass under high cell-to-frame voltage conditions and humidity can cause delamination of the SnO 2 /glass interface; and (4) TF module configurations usually lack the extra isolation that is afforded by extra layers of laminating material (EVA or thermoplastic) found in non-TF modules.Regarding the second goal, arguments have been made that achievement of a test protocol to certify a 30-year lifetime is not possible. 2 Short of such a 30-year lifetime certification process, an approach for rank ordering and testing of failure modes has been developed; R. G. Ross 3 has put forth another method that rank orders mechanisms according to analysis of the 'life-cycle energy cost impact'. Rank ordering allows the PV industry to address the more pressing problems first, without the encumbrance of complicated lifetime-prediction methdologies. Recently, M. A. Quintana et al. have reported commonly observed degradation in field-aged PV modules. 4 The visual appearance of many failure mechanisms related to packaging can be very striking. Any one of the TF layers can be damaged from corrosion, electrolytic electromigration, diffusion, cracking, and delamination. Especially notable are the effects of glass corrosion. Each of the failure mechanisms that results in visual damage has its own appearance. Considerable progress can be made in understanding module-packaging failure if proper interpretation of the visual damage is provied by an astute observer. The following is a review of failure mechanisms and strategies special to the certification and analysis of TF module reliability. Much of this work was carried out at the Jet Propulsion Laboratory (JPL) in the 1980s under the direction of R. Ross; these references are vital as...

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