Introduction

The photovoltaic effect is the ability of certain materials, particularly semi-conductors, to transform
luminous energy into electricity. Since last five years, the need of photovoltaic cells is growing.
Photovoltaic cells or solar cells are made from a crystalline or amorphous material, either a solid block or
thin film, which represents 40 to 60% of the production cost of solar cells. Three types of materials can be used :

  • Monocrystalline silicon cells (the most common)
  • Polycrystalline silicon cells (more limited performance)
  • Amorphous silicon deposited as a thin film on glass substrates (the latest)

Wafers are produced by slicing ingots of mono- or polycrystalline silicon after grinding, gluing, cutting separating, cleaning and measuring steps.

A key step : slicing the ingot

post34The slicing of the silicon ingots is carried out using a multi-wire system (figure 2) comprising a thin steel wire wound between two reels (figure 3). The carrier liquid is generally based on Poly Ethylene Glycol (PEG), oil or glycol based substances. However, the trend is to reduce the viscosity of the carrier
liquids.

Particles, such as silicon carbide (SiC) are dispersed inside the carried liquid as abrasive.

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Interest in controling the particle size

Control particle size distribution in slicing application permits to :

Reduce energy consumption

Conserve the ability of the particles to remain in suspension

Reduce the quantity of SiC in the suspension

Facilitate the recycling of the suspension

Increase the slicing speed by using particles with a monomodal, mono-dispersed particle size distribution and generally centred around 10 μm (figure 3).

Reusing the abrasive suspension, comprising the abrasive and the carrier liquid

Particle size analysis in liquid mode

post35An example of silicon carbide particle size analysis is presented in figure 5. Analysis conditions and special diameters are also reported. This sample is monomodal and centred to 11μm.

  • Carried liquid : Water
  • Dispersing agent : PEG
  • Ultrasounds : 120s during dispersion
  • Mathematical model : Mie n=2.64 – 0i

Special diameters of the particle size distribution

  • D10 = 7.12 μm
  • D50 = 11.37 μm
  • D90 = 18.16 μm

Particle shape analysis

The shape of the particles is an important criterion to be measured in this application. Many theoretical
models permit to correlate the abrasion efficiency with the particle shape.

In real cases, morphological analysis by optical microscopy is well adapted. Interaction between particles
are weak and a fine particle size distribution enhance the sample dispersion (figure 7).

Specific shape parameters can be used in this application such as :

Circularity ratio frm-5

Proportional to the ratio of the area over the perimeter squared.

In the case presented, CR= 0.74. This parameter highlights the effect of the perimeter in relation to the surface area.

Convexity frm-6

Ratio between the convex and real perimeters.

In the case presented, Convexity = 0.89.
This parameter enables the surface effects to be quantified.

Sphericity frm-7

Ratio of the radiuses of the inscribed and circumscribed circles.
In the case presented, Sphericity = 0.48.
This parameter enables the deviation from circularity to be quantified.

Conclusion

Abrasives are used in solar industry to prepare wafer. Carbide and diamond dispersed in liquid media are usually needed in this application for their hardness.

During the slicing process the particle size is decreasing and particles are rounder. Control the particle size in the carrier liquid permits to avoid the presence of fine particles.

Particle size and shape analysis of abrasives in solar industry is needed to improve slicing process and quantify the abrasive efficiency.

 

References

[1] 6th National Phovoltaic Symposium, SIG Genève, (2005)

[2] Photovoltaic materials, past, present, future

Solar Energy Materials and Solar Cells 62, 1-9, (2000)