Fraunhofer Center for Silicon Photovoltaics CSP
Fraunhofer Center for Silicon Photovoltaics CSP

Material Characterization

Module with ultrasonic transducers
© Fraunhofer CSP
Module with ultrasonic transducers.

A characterization of the physical properties of materials is an important condition for the reliability assessment, age determination and the quality assurance of PV module components. Apart from standard test methods (e.g. tensile and bending tests, peel tests) the Fraunhofer CSP also offers special customized test procedures, such as the strength assessment of encapsulated solar cells and an in-laminate fatigue strength evaluation of solar cell interconnectors. These tests may always constitute the basis for model building and further simulations. In addition, individual tests can be carried out and developed on request of our customers. In combination with age tests, we are able to retrace the alteration of material properties.

Services

  • Determination of material characteristics (e.g. Young’s modulus, Poisson’s ratio, yield stress)
  • Assessment of the strength of solar glass according to DIN 1288
  • Strength assessment and crack examination of encapsulated solar cells
  • Non-destructive characterization of elastic and microstructural properties solar module components with ultrasound
  • Assessment of the fatigue strength of solar cell interconnectors
  • Measurement of the thermal profile of polymer materials as well as material combinations by means of differential heat flow calorimetry (DSC), rotation rheometer and thermomechanical analysis (TMA)
  • Determination of the degree of cross-linking after lamination by applying Soxhlet extraction
  • Polymer analytics (YI, UV-Vis, FTIR, DSC, DMA)

Examples

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electroluminescence shots module cracks
© Fraunhofer CSP
Step-by-step electroluminescence shots provide information about the cracks in the solar module resulting from a certain mechanical load.

Investigation of Cracks in Encapsulated Solar Cells

Cracks in solar cells are a common problem in everyday practice. They affect the module performance and - in the worst case - can lead to short circuits. In order to investigate these cracks, at Fraunhofer CSP we developed a method, exposing the solar cells in the module laminate to mechanical stresses under defined conditions. For this purpose, we combined a universal testing machine with a 4-point bending device with an electroluminescent camera (EL camera) and programmed an automatic test sequence. While the load is continuously increased, an EL pick-up is generated for each load step, so that at the end of the test, cracks can be exactly assigned to the load at which they were created. With the help of a finite element simulation, a fracture stress can be assigned to each crack development. As a result, a so-called in-laminate strength can be calculated. It is thus possible to analyze future module designs with novel solar cells, glasses or encapsulation materials under defined laboratory conditions and to predict the likelihood of cracking under a certain mechanical load.

double-ring method
© Fraunhofer CSP
The mechanical strength of glass can be checked using the double-ring method on plate-shaped samples with large test areas (DIN EN ISO 1288-2).

Investigation of Optical Quality and Mechanical Reliability of Solar Glass

As a supporting element, glass has a decisive influence on the efficiency of photovoltaic modules. Their reliability is significantly determined by the strength of the glass. In addition, there is a need for research on the extent to which the optical properties are influenced by the manufacturing processes during PV module production.

Module with ultrasonic transducers
© Fraunhofer CSP
Module with ultrasonic transducers.

Characterization of Elastic and Microstructural Properties of Solar Module Components with Ultrasound

The properties of its individual components are crucial for the reliability of a solar module. In order to be able to measure and monitor the mechanical and microstructural properties of various solar module components during production, we use novel ultrasonic characterization methods. For example, it was possible to determine the effective modulus of elasticity, the Poisson's number, the flow stress as well as the mean grain size and orientation of solar cell connectors without destruction.

fatigue behavior solar cell connectors
© Fraunhofer CSP
On the basis of specific experiments, the fatigue behavior of solar cell connectors can be simulated under varying mechanical and thermomechanical loads.

Connector Fatigue

The fatigue of solar cell connectors is a decisive defect of solar modules. A connector failure affects the module's series resistance, resulting in power loss, locally elevated temperatures, and safety issues during operation (electric arc). A load on the connectors takes place when the cells move relative to one another due to temperature changes or mechanical module loads. By means of specific experiments, we examine the constitutive (stress-strain behavior) and fatigue behavior of the connector materials. We use the material data obtained in finite element models to calculate the lifetime (cycles to break) in the specific PV module under arbitrarily changing mechanical and thermomechanical loads.

Conductive Adhesive as Solder Alternative

Conductive adhesives are a promising alternative to standard soldering processes. The advantages are the lower process temperatures (<150 ° C), relatively simple process technology with the possibility of automation and environmental friendliness, since no lead and flux are used.
At the Fraunhofer CSP, we are able to characterize all relevant properties: The processing properties are determined by means of thermoanalytical methods (DSC, TGA, DMA, TMA). Structures for the determination of volume resistance and contact resistance can be produced and measured as required. We evaluate the mechanical properties of the adhesive joints by means of standard peel tests as well as special test sets which also include electrical characteristic values.