T.J. McMahon

Electrochemical corrosion of SnO2:F transparent conducting layers in thin-film photovoltaic modules

We report on a degradation mechanism in thin-film photovoltaic (PV) modules activated by damp heat and voltages similar in magnitude to those generated by PV modules in power generation systems. This mechanism, which appears to be an electrochemical process involving the soda-lime glass superstrate with its conductive SnO2:F layer, can be greatly accelerated by subjecting modules to elevated temperatures and humidity, both of which increase the leakage currents between the frame and the active PV layers. Water vapor can affect the module damage in two ways: (1) by enhancing leakage currents, and (2) by entering through the module edges, it appears to promote the chemical reaction responsible for the SnO2 corrosion. Damage has been found to occur in both a-Si and CdTe modules.

Performance of single-junction a-Si modules under varying conditions in the field

We report on the actual performance of large-area photovoltaic (PV) modules deployed outdoors, which are representative of single-junction amorphous silicon (a-Si) thin-film technology. They are part of the performance and energy-ratings testbed (PERT) at the Outdoor Test Facility (OTF) and are under continuous measurement and load conditions, as executed by a data acquisition system (DAS). The goals are to analyze and compare energy production and performance for various PV technologies deployed in actual field conditions and to validate the PV energy-ratings methodology being developed for modules. For single-junction a-Si, we found the effective efficiency is very similar to that measured at standard conditions.

Ethylene-Vinyl Acetate Potential Problems for Photovoltaic Packaging

Photovoltaic (PV) devices are typically encapsulated using ethylene-vinyl acetate (EVA) to provide mechanical support, optical coupling, electrical isolation, and protection against environmental exposure. Under exposure to atmospheric water and/or ultraviolet radiation, EVA will decompose to produce acetic acid, lowering the pH and increasing the surface corrosion rates of embedded devices. Even though acetic acid is produced at a very slow rate, it may not take much to catalyze reactions that lead to rapid module deterioration. Another consideration is that the glass transition of EVA, as measured using dynamic mechanical analysis, begins at temperatures of about -15 degC. Temperatures lower than this can be reached for extended periods of time in some climates. Because of increased moduli below the glass transition temperature, a module may be more vulnerable to damage if a mechanical load is applied by snow or wind at low temperatures. Modules using EVA should not be rated for use at such low temperatures without additional low-temperature mechanical testing beyond the scope of UL1703

Testing of packaging materials for improved PV module reliability

A number of candidate alternative encapsulant and soft backsheet materials have been evaluated in terms of their suitability for photovoltaic (PV) module packaging applications. Relevant properties, including interfacial adhesion and moisture transport, have been measured as a function of damp-heat (85/spl deg/C / 85% relative humidity) exposure. Based on these tests, promising new encapsulants with improved properties have been identified. Backsheets prepared by industry and at NREL have been found to provide varying levels of moisture ingress protection. To achieve significantly improved products, further development of these candidates is ongoing. The relative effectiveness of various packaging strategies to protect PV devices has also been investigated.

Adhesion and Thin-Film Module Reliability

Among the infrequently measured but essential properties for thin-film (T-F) module reliability are the interlayer adhesion and cohesion within a layer. These can be cell contact layers to glass, contact layers to the semiconductor, encapsulant to cell, glass, or backsheet, etc. We use an Instron mechanical testing unit to measure peel strengths at 90deg or 180deg and, in some cases, a scratch and tape pull test to evaluate inter-cell layer adhesion strengths. We present peel strength data for test specimens laminated from the three T-F technologies, before and after damp heat, and in one instance at elevated temperatures. On laminated T-F cell samples, failure can occur uniformly at any one of the many interfaces, or non-uniformly across the peel area at more than one interface. Some peel strengths are Lt1 N/mm. This is far below the normal ethylene vinyl acetate/glass interface values of >10 N/mm. We measure a wide range of adhesion strengths and suggest that adhesion measured under higher temperature and relative humidity conditions is more relevant for module reliability

Accelerated and environmental module stress testing at NREL

This paper presents an overview of the Module Testing and Technology Validation task at the National Renewable Energy Laboratory. The extensive module testing capabilities at the Outdoor Test Facility are outlined, emphasizing the test facilities, equipment, and analytical services available. Highlights and results of several recent testing efforts are then presented, followed by a list of the external programs supported by the task. The paper concludes with a brief description of the new testing programs that are planned for the near future.