Soldering of solar cell strings is a critical step in the production of photovoltaic modules. During the soldering process
significant mechanical stresses are induced in the stringed cell assembly. Since silicon has a much smaller coefficient of
thermal expansion than copper it is compressed by the copper-ribbon during the cooling phase. The resulting stresses can
cause micro-cracks in the silicon cell, which are a major reason for cell breakage within the production line. Furthermore
those stresses may lead to a delayed failure of the solder interconnections or cell cracking in the field. Therefore ribbon
manufacturers try to create very soft ribbon material, which tends to be rather plastically deformed than generating
stresses such that the silicon is prevented from damage. Nevertheless, the general tendency of using thinner wafers in cell
production and the projected step towards the usage of lead-free solders increase the mechanical requirements on the cell
interconnectors and make systematic scientific investigations inescapable.
The purpose of this work is to analyze the micro-structure of ribbon in detail and to correlate it with its mechanical
material behavior. An electron backscatter diffraction method was used to evaluate grain sizes and orientations in various
annealing steps of the ribbon. These results were compared to their mechanical properties, achieved by conventional
mechanical testing. As a result of these investigations the annealing process of the ribbon was optimized on laboratory
scale to achieve highly adjusted material properties. Finally the benefit was verified by numerical simulation of the
soldering process.
In recent investigations using various analysis methods it has been shown that mechanical or thermal loading of PV
modules leads to mechanical stress in the module parts and especially in the encapsulated solar cells. Cracks in
crystalline solar cells are a characteristic defect that is caused by mechanical stress. They can lead to efficiency losses
and lifetime reduction of the modules.
This paper presents two experiments for systematic investigation of crack initiation and crack growth under thermal and
mechanical loading using electroluminescence. For this purpose PV modules and laminated test specimens on smaller
scales were produced including different cell types and module layouts. They were exposed to thermal cycling and to
mechanical loading derived from the international standard IEC 61215.
Cracks were observed mainly at the beginning and the end of the busbars and along the busbars. The cracks were
analyzed and evaluated statistically. The experimental results are compared to results from numerical simulations to
understand the reasons for the crack initiation and the observed crack growth and to allow module design optimization to
reduce the mechanical stress.
Photovoltaic modules (PV modules) are supposed to have a lifetime of more than 20 years under various environmental
conditions like temperature changes, mechanical loads, etc. Common outdoor exposure may influence efficiency and
lifetime which necessitates assessment of PV module performance and detection of output deficits. For this purpose
reliable and nondestructive testing methods are desirable.
Commercially available PV modules were tested by different analysis methods. The PV module's electrical properties
were investigated by thermography and electroluminescence measurements. The combination of these two techniques is
well-suited to detect many cell and module defects. A crystalline module showed significant cell breakage after
temperature cycle test. To observe the mechanisms of this specific defect type laminated test specimens on smaller scales
were produced and analyzed over production process and during temperature cycles derived from the international
standards IEC 61215 and IEC 61646. The defect study on small scales allows conclusions about the defect's influence on
larger PV modules. Further methods capable for mechanical characterization like Laser Doppler vibrometry, surface
geometry scan and digital image correlation are presented briefly. The combination of the methods mentioned above
allows a very precise assessment of the mechanical and electrical capability which is essential for reliability and lifetime
concepts.
KEYWORDS: Glasses, Solar cells, Finite element methods, Temperature metrology, Silicon, Cameras, Crystals, Digital image correlation, Data modeling, 3D modeling
Solar module lifetime is limited by the fatigue behavior of its cell interconnectors: the copper-ribbons. Every change in
temperature induces thermo-mechanical stresses in the module components due to their thermo-mechanical mismatch.
The purpose of this work is to quantify this load on the copper-ribbons between the individual cells of a cell string during
a thermal cycling test by measuring cell displacement using digital image correlation and to compare the results to finite
element analysis (FEM). Furthermore with help of FEM the influences of different materials were investigated, allowing
material and layout optimizations with respect to copper-ribbon loading.
Waferbonding techniques are frequently used for MEMS/MOEMS fabrication. In this paper, the potential application and methodical limitations of different strength testing approaches including tensile testing and double-cantilever- beam testing for wafer-bonded components are investigated. Special attention is given to the influence of the interfacial atomic bonding strength, the role of interface voids and notches caused by chemical or physical etching steps prior to bonding on the fracture limit. A particular aim of the paper is to discuss the potential of the Micro Chevron-Test for the assessment of the wafer bonding process with particular respect to the quality control during MEMS fabrication. In addition, the methods can also be applied to investigate the lifetime and fatigue properties of wafer- bonded samples exposed to constant or cyclic stresses.
The fracture toughness of micromachined polycrystalline silicon samples, pre-cracked with an indenter or notched using a focused ion beam (FIB) machine, were tested using either bending or tensile loading. Fracture mechanics approaches were applied to determine the fracture toughness from these results. For the pre-cracked specimens tested by tensile loading, a fracture toughness value of KI,crit equals 0.86 MP(root)a derived. The FIB notched specimens had higher fracture toughness values, probably due to the influence of the notch tip radius and the FIB process. In addition, fatigue investigations of un-notched tensile specimens were performed using tensile cyclic loading with frequencies of 50, 200 and 1000 Hz. A reduction in the tensile strength from 1.10 GPa to 0.75 GPa after 108 cycles was detected while no influence of the test frequency on the fatigue behavior was observed.
In this paper, the testing principles and different application examples of the Micro-Chevron-Test (MC) are discussed. The chevron pattern required for testing can be fabricated either by wet or reactive ion etching. It is shown that the test has a higher accuracy than common tensile or bend strength tests, allowing also the determination of fracture mechanic parameters, such as fracture toughness. In addition one can characterize the spatial strength distributions for the bonded wafer in order to determine the sources of production yield problems. Furthermore, the sample size can be reduced to the typical size of micro electro mechanical systems (MEMS) devices allowing the MC sample fabrication to be integrated into the MEMS fabrication process. Therefore, the test can be applied as an effective, reliable and precise tool for wafer bond process development and for quality control during the fabrication of micromechanical devices.
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