Kent Terwilliger

System voltage potential-induced degradation mechanisms in PV modules and methods for test

Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent. Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potential-induced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.

Optical Durability of Candidate Solar Reflectors

Concentrating solar power (CSP) technologies use large mirrors to collect sunlight to convert thermal energy to electricity. The viability of CSP systems requires the development of advanced reflector materials that are low in cost and maintain high specular reflectance for extended lifetimes under severe outdoor environments. The long-standing goals for a solar reflector are specular reflectance above 90% into a 4 mrad half-cone angle for at least 10 years outdoors with a cost of less than

Testing and analysis for lifetime prediction of crystalline silicon PV modules undergoing degradation by system voltage stress

13.8/m 2 (the 1992

Test-to-Failure of crystalline silicon modules

10.8/m 2 goal corrected for inflation to 2002 dollars) when manufactured in large volumes. Durability testing of a variety of candidate solar reflector materials at outdoor test sites and in laboratory accelerated weathering chambers is the main activity within the Advanced Materials task of the CSP Program at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Test results to date for several candidate solar reflector materials will be presented. These include the optical durability of thin glass, thick glass, aluminized reflectors, front-surface mirrors, and silvered polymer mirrors. The development, performance, and durability of these materials will be discussed. Based on accelerated exposure testing the glass, silvered polymer, and front-surface mirrors may meet the 10 year lifetime goals, but at this time because of significant process changes none of the commercially available solar reflectors and advanced solar reflectors have demonstrated the 10 year or more aggressive 20 year lifetime goal.

Stress induced degradation modes in CIGS mini-modules

Acceleration factors are calculated for crystalline silicon photovoltaic modules under system voltage stress by comparing the module power during degradation outdoors with that in accelerated testing at three temperatures and 85% relative humidity. A lognormal analysis is applied to the accelerated lifetime test data, considering failure at 80% of the initial module power. Activation energy of 0.73 eV for the rate of failure is determined for the chamber testing at constant relative humidity, and the probability of module failure at an arbitrary temperature is predicted. To obtain statistical data for multiple modules over the course of degradation in situ of the test chamber, dark I–V measurements are obtained and transformed using superposition, which is found to be well suited for rapid and quantitative evaluation of potential-induced degradation. It is determined that shunt resistance measurements alone do not represent the extent of power degradation. This is explained with a two-diode model analysis that shows an increasing second diode recombination current and ideality factor as the degradation in module power progresses. Failure modes of the modules stressed outdoors are examined and compared with those stressed in accelerated tests.

Ethylene-Vinyl Acetate Potential Problems for Photovoltaic Packaging

Accelerated lifetime testing of five crystalline silicon module designs was carried out according to the Terrestrial Photovoltaic Module Accelerated Test-to-Failure Protocol. This protocol compares the reliability of various module constructions on a quantitative basis. The modules under test are subdivided into three accelerated lifetime testing paths: 85°C/85% relative humidity with system bias, thermal cycling between −40°C and 85°C, and a path that alternates between damp heat and thermal cycling. The most severe stressor is damp heat with system bias applied to simulate the voltages that modules experience when connected in an array. Positive 600 V applied to the active layer with respect to the grounded module frame accelerates corrosion of the silver grid fingers and degrades the silicon nitride antireflective coating on the cells. Dark I–V curve fitting indicates increased series resistance and saturation current around the maximum power point; however, an improvement in junction recombination characteristics is obtained. Severe shunt paths and cell-metallization interface failures are seen developing in the silicon cells as determined by electroluminescence, thermal imaging, and I–V curves in the case of negative 600 V bias applied to the active layer. Ability to withstand electrolytic corrosion, moisture ingress, and ion drift under system voltage bias are differentiated according to module design. The results are discussed in light of relevance to field failures.

Testing of packaging materials for improved PV module reliability

The stability of monolithically integrated copper (indium, gallium) (selenium, sulfur) (CIGS) based thin film solar cells on glass were evaluated as a function of highly accelerated stress testing. Mini-modules exposed to high humidity (85°C and 85% RH) had a dominant failure mechanism involving increased resistance in the ZnO:Al transparent conducting oxide. Under Dry heat (85°C and 0% RH) performance loss was much slower and involved the weakening of diodes lowering Voc and loss of fill factor. These mini-modules were encapsulated using either ethylene vinyl-acetate (EVA) or a Silicone. It was found that encapsulation with EVA led to greater increases in series resistance. These experiments point to the importance of module packaging, transparent conducting oxide stability and cell integration in constructing durable CIGS modules.

New barrier coating materials for PV module backsheets

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

Acceleration factor determination for potential-induced degradation in crystalline silicon PV modules

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.

Elucidating PID Degradation Mechanisms and In Situ Dark I–V Monitoring for Modeling Degradation Rate in CdTe Thin-Film Modules

High moisture barrier high resistivity coatings on polyethylene terepthalate (PET) have been fabricated and characterized for use in PV module back sheet applications. These thin film barriers exhibit water vapor transmission rates (WVTR) as low as 0.1 g/m/sup 2/-day at 37.8/spl deg/C and have shown excellent adhesion (> 10 N/mm) to both ethylene vinyl acetate (EVA) and PET even after filtered xenon arc lamp UV exposure. The WVTR and adhesion values for this construction are compared to and shown to be superior to candidate polymeric backsheet materials.