Mechanics of Wafers and Cells

Fraunhofer Center for Silicon Photovoltaics CSP

© Fraunhofer 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.

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Figure 1: Ball-on-Ring and 4-Point-Bending tests show higher characteristic fracture stresses of monocrystalline silicon wafers, compared to multicrystalline ones.

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).

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Figure 1: Multicrystalline wafer during four line bending test.

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.

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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.

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  • 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

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