Testing of Solar Cells
Tremendously high quantities, long life expectancy, and very few to no testing guidelines— this is the current situation in the photovoltaics industry. Testing is essential for companies who want to remain players in this fiercely competitive market and achieve the promised performance data and life expectancy while offering the lowest prices. Solar cells can survive the tremendous drop in prices in recent years only by improved quality, cost-effective production and better safety.
Solar cells are exposed to environmental extremes on a daily basis. Rain, hail, storms, and large fluctuations in temperature should not impair the functionality of the product. They must be able to withstand the weight of snow, ice, and installation to achieve a desired life expectancy of 40 years without damage. These expectations are high for both electrical and mechanical characteristics. However, test standards for mechanical requirements for components do not exist. Apart from internal individual contract provisions between suppliers and manufacturers, standardized quality assurance guidelines for solar cells do not exist. The lack of standards and individual customer needs means customized test equipment and test are needed. This is where materials testing experts come into play.
General Test Methods/Standards
General Test Methods/Standards
Shipbuilding and marine structures: toughened safety glass for rectangular windows and side scuttles; punch method of non-destructive strength testing (ISO 614:1989-09)
Glass in building: determining flexural strength of glass—Part 5: Double-ring flexure-test on flat specimens with small test surface areas (ISO/DIS 1288-5:2007); German version EN ISO 1288-5
DIN EN 1465
Adhesives: determination of tensile lap-shear strength of rigid-to-rigid bonded assemblies (ISO 4587:1979, modified)
DIN EN 1464
Adhesives: determination of peel resistance of high-strength adhesive bonds—floating roller method (ISO 4578:1990, modified). German version EN 1464:1994 (DIN EN 1464:1995-01) replacement for DIN 53289:1979-09
Glass in building: determining flexural strength of glass—Part 3: Testing specimens supported at two points (four-point bending) (ISO/DIS 1288-3:2007); German version EN ISO 1288-3:2007
IEC 61215 and EN 61215
IEC 61215 and EN 61215 describe a wide variety of qualification tests, based on potential aging influences, for artificial loading of materials used in PV modules. The following individual loading groups are identified:
Sunlight including UV
Climate (cold, head, humidity, changes in climate)
Mechanical loading (hail, wind, suction and pressure, snow)
The materials are considered to have passed the tests if no major visible damage is apparent and the performance output and insulation properties have not changed or have only changed negligibly compared to the position at the start of the test. The test certificate to IEC 61215 has established itself in the past few years as a quality mark for crystalline PV modules and is nowadays required by most national and international funding authorities.
IEC/EN 61215 and IEC/EN 61646
IEC or EN 61215 for thick-film modules and IEC or EN 61646 for thin-film modules (Crystalline Silicon Terrestrial Photovoltaic Modules—Design Qualification and Type Approval).
The IEC 61215 test is very rigorous, as solar modules produced by the company must withstand up to 25 years outdoors in almost all geographical regions and environmental conditions found through the world. Provided no faults arise during the process, the IEC test takes over 5 months in total. In order to satisfy this rigorous standard, a batch of test modules must pass an environmental test protocol involving UV radiation, extreme temperatures, including prolonged exposure to high temperatures and humidity, mechanical loading and torsion, and tests for resistance to hail and stone impact. In parallel to the environmental tests the modules are tested overall for their electrical performance, insulation and any potentially critical areas or locations are identified.
EN 1288-2, October 2007
Glass in building: determining flexural strength of glass—Part 2: Double-ring flexure-test on flat specimens with large test surface areas (ISO/DIS 1288-2:2007); German version EN ISO 1288-2:2007
IEC 61646 and EN 61646
IEC 61646 and EN 61646 describe a wide variety of qualification tests, based on potential aging influences, for artificial loading of materials used in thin-film modules. The following individual loading groups are identified:
- Sunlight, including UV
- Climate (cold, head, humidity, changes in climate)
- Mechanical loading (hail, wind, suction and pressure, snow)
The materials are considered to have passed the tests if no major visible damage is apparent and the performance output and insulation properties have not changed or have only changed negligibly compared to the position at the start of the test. The test certificate to IEC 61646 has established itself in the past few years as a quality mark for crystalline PV modules and is nowadays required by most national and international funding authorities.
Examples of Testing Solutions
To understand how destructive material testing is used in solar cell and module manufacturing, examples of production control for the individual stages of production of thick-film cells testing applications are introduced.
Solar cells or photovoltaic cells are electrical components that convert the radiant energy contained in light directly into electrical energy.
- Thick-film solar cells are made from monocrystalline or polycrystalline silicon. They are widely used in Central Europe because of their high efficiency (over 20%). Additionally, they are characterized by a high degree of efficiency per surface area unit. For example, approximately 8 sq. m. of roof area is needed to produce an output of 1 Kwp. The amount of material and energy required to manufacture these modules is relatively high, therefore quality assurance is essential.
- Thin solar cells exist in a number of variations regarding substrate and vapor-deposited materials: amorphous or micro-crystalline silicon (a-Si, μ-Si), gallium arsenide (GaAs), cadmium tellluride (CdTe), or copper-indium-(callium)-sulfur-selenium compounds (CIGS). Thin-film cells differ from solar cells based on crystalline silicon wafers primarily in their production process and in the film thickness of the materials used.
- Organic solar cells (made of plastics with semiconductor properties)
Testing requirements may vary considerably according to the type of solar cell, as the various technologies in use can in some instances lead to fundamentally different designs.
Peel test on Tedlar film
Plastic EVA (ethylene vinyl acetate) or cast resin films are laminated on both sides to form waterproof corrosion protection, while a Tedlar® film or a glass sheet on the rear provides additional protection for the module. The strength of the bonds is tested by means of a 90° peel test, in which the Tedlar® film is clamped in screw grips and pulled off the glass sheet. A single-column testing machine is suitable for this test as it allows glass sheets of various sizes to be tested without difficulty. This test is carried out during production to check that machine parameters are set correctly. It is also used in goods inwards checks and during requalifying tests after expiry of the use-by date. To meet the standards, the Tedlar® film must withstand peel forces up to 250 N.
Pull-out test between junction box and connecting cable
Various safety tests are recommended for the completed module, including strength testing the frame structure, determining the press-in and extraction forces of the corner brackets, pull-out tests on electrical connections and tensile, compression and flexure tests on the mounting system for attaching the module. Cable pull-out tests on the junction box can be performed using a 2-column testing machine from Zwick’s Allround series. The junction box is retained in special specimen holders,while the connecting cable is held in screw grips and pulled out of the box using forces up to 5 kN.
Punch test on toughened safety glass to ISO 614 Form A and B
In the puncture test a 25x25 mm glass sheet is positioned in a double ring flexure test kit (consisting of support and load ring and stop pins for fixing the specimen). A die is then used to load the specimen up to break. The metal container prevents glass splinters from entering the machine drive. This test is carried out as per standard using a 2-column testing machine with a maximum force of 50 kN.
ProLine with 4-point flexure test on structural glass to EN 1288-3
- Either a static puncture test to EN 1288-5 or a 4-point flexure test to EN 1288-3 can be used to test the safety of the glass sheet employed.
- A ZwickRoell AllroundLine testing machine is used for both tests, which requires the incorporation of safety devices to prevent damage or injury due to flying glass splinters.
- This testing system has been specially assembled for use in determining the flexural strength of flat glass according to EN 1288-3 and meets all requirements of the standard. All anvils and bending rollers can be rotated freely and are to the required dimensions and separations.
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