Mechanics of Wafers and Cells
Fraunhofer Center for Silicon Photovoltaics CSP
In solar cell manufacturing, different mechanical loads are applied on wafers and solar cells. The loads are caused by process steps on the one hand, as well as by the handling of wafers and cells on the other hand. In order to avoid breakage, especially during the ongoing reduction of thicknesses, the wafers and solar cells have to withstand these loads. Furthermore, defects in the wafer or solar cell must be considered, which are induced, reduced or changed during the manufacturing process.
Following topics are focused within the team »Mechanics of Wafers and Cells« for a comprehensive understanding of failure and breakage mechanisms:
- failure behavior and strength of wafers and solar cells
- mechanical properties of sawing wires (elastic and plastic)
- microstructural analysis for sawing wires, wafers and solar cells (defects, microstructure, topography, roughness, geometry, etc.)
- damage evaluation due to handling of wafers and cells (quantification of the damage potential of handling steps and equipment)
- analysis of abrasion behavior within silicon
Therefore the following methods are used:
- mechanical experiments (3-point-bending or 4-point-bending, ring-ring, ball-ring, ball-on-3-balls, dynamic drop tests, etc.) e.g. to determine strength and the damage potential of wafers and cells
- static and dynamic simulations by use of Finite-Element.-Method (FEM) for calculation of fracture stresses and stress/load analysis of processes
- use of statistical methods for strength analysis and prediction of breakage rates or failure probability (e.g. based on Weibull distribution)
- microstructure analysis with optical microscope, infrared microscope, high resolution scanning electron microscope (SEM)
- failure analysis by use of fractography (identification of failure origin, crack propagation and failure mechanism)
- analysis of residual stresses in silicon using photoelasticity
Thus, single manufacturing processes or the full cell line until the final solar cell can be evaluated and optimized in terms of mechanical loads, damage, and breakage rate. This allows to identify the cause of damage and decrease breakage rates. Thus, the production of mechanically reliable solar cells can be ensured.
On this page:
Mechanical Modeling of Multicrystalline Silicon Wafers Considering Microstructural Properties
During cell production, breakage of wafers leads to high economic losses, which turns the reduction of breakage rates into one of the primary research objectives in cell production industry. Comparing the strength of monocrystalline and multicrystalline wafers (mc-Si), previous research, done by Fraunhofer CSP, showed that multicrystalline wafers have lower mechanical strengths than monocrystalline ones (Figure 1).
The lower strength of mc-Si wafers is related to the difference in microstructure, such as different defect structure, effects of grain boundaries or stress concentrations due to different crystal grain orientations. However, in most of the models for strength characterization, the wafers’ microstructural properties, such as grain shapes and orientations, are neglected. Thus, there is only a minor theoretical understanding of the dominating strength limiting factors in mc-Si wafers.
The researchers at the Fraunhofer CSP used the Finite-Element-Method (FEM) to create a three-dimensional mechanical model of a full multicrystalline wafer, including grain shapes as well as crystal grain orientations in order to study the theoretical impact of grain structure. The model calculates stress distributions and the risk of breakage based on statistical strength data (Weibull parameters). Since it is technologically difficult to determine the crystal orientation of hundreds of grains in a whole wafer experimentally, grain orientations were assumed to be random and studied using the Monte-Carlo-Method. In total, three different random grain distributions were used to generate randomly orientated grains for three different grain size distributions each. For the simulation, the wafers were loaded by a uniaxial tensile load, whereas the stress in the wafers had to be distributed according the crystal structure. In order to achieve representative results, 7200 simulations were performed. To evaluate the influence of the grain structure, the failure probabilities for each wafer were calculated, using the statistical Weibull parameters of bending experiments, considering the size effect of strength.
The comparison of the mean failure probabilities from the Monte-Carlo simulations shows that multicrystalline silicon wafers generally have higher failure probabilities than monocrystalline wafers for the same load conditions, strongly influenced by the grain orientation distribution as well as the grain size (Figure 2). This effect is primarily caused by the interaction of the grains. It could be shown that the increase of fracture probability is due to stress concentrations, which result from different crystal grain orientation and grain structure. The grain boundaries contain more severe stresses than the grains themselves. The investigations also suggest that grain shapes have a higher influence on local stresses than grain sizes.
Wafer Strength: Healing of Defects by Thermal Treatment
The amount of crystalline silicon photovoltaic is still dominating the solar energy market. Due to expensive manufacture conditions nearly 30% of the total costs of a solar module have to be spend for production of as-cut wafers. Therefore, wafer breakage needs to be avoided in order to save costs. But defects in the wafers due to the sawing process are still present and can finally also lead to breakage. It is known that several process steps in cell processing show a positive effect to the mechanical behavior of silicon.
That means wafer and cell breakage can be reduced. For example, the wafer strength is increased by chemical etching processes. Surface damages are removed and the wafers are able to withstand higher loads during the manufacturing process and in a solar module. A similar increase of wafer strength could also be done by thermal treatment of as-cut wafers.
The Fraunhofer CSP has patented a solution to increase the mechanical strength of as-cut and textured wafers. By applying a heating procedure the fracture strength of silicon wafers can be improved. This annealing effect can be observed and quantified in mechanical testing. Figure 1 shows the setup of a four line bending test that was used to determine the change in mechanical behavior of the wafers. Fracture stresses are analyzed by using the Weibull distribution with the parameters of characteristic fracture stress and Weibull modulus, which represent the wafer strength and scattering, respectively. The results show a significant correlation between the fracture stress and the temperature of the annealing step (Figure 2). A first significant increase of fracture strength was already observed at 200°C. The maximum of wafer strength was determined at a temperature of 600°C. In that case the wafer strength was improved by 27% in comparison to untreated wafers. Beyond 600°C up to 1000°C the characteristic fracture stress decreases. In contrast the Weibull modulus, which represents the scattering, does not change and is still in a typical range for as-cut silicon wafers. Further mechanical and microstructural analysis showed a thermal oxidation of the crack tip and crack surfaces, which explains the strengthening effect by thermal treatment. Hence the measured fracture toughness at single cracks of annealed samples was also increased, which is equivalent to higher fracture strength. Furthermore it could be shown, that a thermal surface treatment, e.g. by laser processes, is sufficient to achieve the annealing effect of increasing wafer strength, which can reduce the risk of wafer breakage in manufacturing.
Directional Strength of AI-BSF Solar Cells with Continuous Busbars
The strength of full Al-BSF solar cells with continuous busbars strongly was tested systematically in 4-point bending tests. The strength or breakage stress depends on the tested surface (back or sunny side) and the direction of loading related to the busbars.
On the sunny side the strength is independent of the loading direction or the presence of a busbar inside the tested area in 4-point bending. On the back side the strength is significant lower when the busbar is loaded parallel compared to the load across to the busbar. If there is no busbar inside the tested area, the strength is even higher. This difference in strength is caused by different defect types and intrinsic stresses in silicon in the metallization regions while the back side metallization with overlapping Al and Ag metallization shows the strongest influence on strength.
In other cell layouts the strength behavior can change due to differences in the metallization concept, e.g. different busbar/pad layouts or laser processing. With these detailed strength characterizations of solar cells the breakage risk can be estimated for manufacturing and PV module lifetime.
- non-contact thickness and topography measurement of wafers
- universal testing machine (mechanical properties), - for wafers and cells (3-point and 4-point bending test, ring-on-ring test, ball-on-ring test), - for sawing wires (pull test: static, dynamic and cycle tests)
- dynamic edge loading with wafers and cells by use of a drop tester
- experimental setup for scratch tests to analyze the abrasion in silicon
- micro indentation tests for determination of mechanical and fracture mechanical parameters
- pressure foils for loading analysis on wafer/cell level
- photoelasticity measurement set-up for investigations of residual stress in silicon on block and wafer level
- workstations for numerical simulations (Finite-Element-Method) of mechanical, thermomechanical, fracture mechanical and statistical analyses