SOLAR TECHNOLOGY READER

4.2 Solar Cells

Crystalline Silicon Cells

The great majority of solar pv is currently made from crystalline silicon cells. These can be either poly-crystalline - where the silicon is made up of numerous individual crystals, or mono-crystalline silicon - which are cut from a huge single crystal.

how solar cells are made

The process by which a single crystal of silicon is grown is called the Czochralski Process. The crystal is pulled from a molten crucible of liquid silicon by dipping in a single 'seed' crystal and then slowly pulling away from the liquid surface while rotating at the same time. By carefully controlling the speed of withdrawal and the temperature gradient in the crucible a solidified single crystal pillar shape with the same atomic orientation as the seed is created.

If this cylindrical crystal were sliced to produce silicon wafers, they would be round discs which couldn't be efficiently packed side by side into a solar panel - there would be gaps between the cells and this would leave parts of the solar panel surface inactive, reducing overall efficiency. So the cylinder is first cut along its length on four sides to make its shape closer to a square in cross-section.

The more that is sliced off, the closer the cell becomes to a perfect square shape, and the more working area you can squeeze into your monocrystalline PV panel. The less that is removed, the less material is wasted and the cheaper are the cells to manufacture. The compromise that most manufacturers have reached is to make a shape that was a square with rounded corners (pseudo-square).

Polycrystalline silicon wafers are made by melting the silicon feed stock - which can include the trimmings from cutting and slicing monocrystalline silicon. The molten silicon is poured it into a cube shaped mould and allowed to cool and solidify. The resulting block of silicon is sliced into pillars that are square in cross section and these are in turn sliced into perfectly square cells.

Once sawn into thin slices the silicon wafers are taken through various processes:

  • Acid or alkali etch to remove saw marks
  • Diffusion of phosphorous into one surface to create the n-type semiconductor layer
  • Surface texturing to improve light absorbtion
  • Vapour deposition of surface layers such as an antireflective coating
  • Screen printing of metal paste to form electrodes
  • Baking to cross-link the electrode to the semiconductor
  • Efficiency testing and batching

Historically, the differences between mono and poly silicon were:

  • Poly cells were square and mono had rounded corners and a characteristic diamond pattern between cells
  • The crystals in poly cells were visible and caught the light giving a patterned effect on the cell, whereas mono cells were matt
  • Mono cells were on average higher power because the grain boundaries betwen crystals in poly cells impeded electron movement and reduced cell efficiency

However, recently the differences between the two cell types have started to narrow. Reprocessing of trimmings has allowed mono cell manufacturers to economically cut the cells closer to a perfect square. New surface treatments and improvements to processing have reduced the patterned effect on poly cells. Nowadays the main difference between the two types is that poly cells tend to have a slightly bluer colour and mono panels have a more uniform black colour. The power outputs of poly and mono solar panels overlap greatly, with only the highest power mono panels exceeding poly cell panels.

Thin Film Solar Cells

Thin film solar cells are made by depositing thin layers of photovoltaic materials onto a substrate, which could be glass or may be a flexible plastic sheet. Deposition methods vary from printing to vapour deposition, depending on the materials that are being used. Proponents of thin film solar have argued that the intrinsically lower material consumption of the approach and its greater suitability for mass production would lead to significantly lower costs compared to crystalline silicon on a per-watt basis. However rapid progress and economies of scale in crystalline silicon solar has meant that these advantages have never appeared in practice and thin film solar commands only 7% of the global market currently.

The principal thin film technologies are:

  • CIGS - Copper indium gallium selenide
  • Amorphous Silicon
  • CdTe - Cadmium Telluride

Niche applications for which thin film solar is uniquely suited are flexible solar panels (which can be rolled out and adhered to a roof) and transparent solar cells (windows that also generate electricity).

Organic Solar Cells

Certain types of polymer can be synthesised that will conduct electricity and exhibit the photovoltaic effect. Currently the technology is yet to be commercialised due to the low efficiency of the materials that have so far been developed and their short life, but the advantages of polymers in cost, manufacturing flexibility and materials properties mean that a great deal of research is going into this field.

Perovskite Solar Cells

A perovskite is any material with the same type of crystal structure as calcium titanium oxide (CaTiO3). Although the materials have been known for years, their first use as a photovoltaic material was in 2009, and rapid progress has been made in cell efficiency in the laboratory with reported values rising from 4% in 2010 to 20.1% in 2014. The low cost materials can be deposited by simple processes such as spin coating or gravure printing. Perovskite materials have the potential to be the basis of a whole new type of solar cell or to work in tandem with silicon solar cells as a layer that increses overall cell efficiency.

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