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Common Types Of Solar Cells.

What are the Common Types Of Solar Cells?

Over 90% of the solar cells in common use today are manufactured from crystalline silicon (Si). This is despite the fact that silicon is a relatively poor light absorbing material.

It does however produce very stable cells with efficiency in the 11-16% range, approaching two-thirds of the theoretical maximum. Manufacturers can also draw on the huge fund of knowledge built up by the micro-electronics industry's use of silicon over the past 30-40 years.

There are three types of silicon PV cell: monocrystalline, multi or polycrystalline (both crystalline forms can be referred to as c-Si) and amorphous. Monocrystalline, produced by slicing wafers from high purity single crystal rods or boules of silicon is the most efficient of the commonly used type of solar panels for sale.

It is also the most expensive. Rather more cost-efficient to produce are polycrystalline cells, formed from molten silicon cast into blocks and subsequently sawn into plates. Defects occur during solidification and this affects efficiency. Both types have their electrical contacts either screen printed on or laser etched to minimise reductions in efficiency.

Crystalline silicon cells produce approximately 0.5v so production modules usually contain 36 cells connected in series. This should provide enough output to charge a 12 volt deep-cycle battery. The modules are protected from the elements by being laminated and hermetically sealed beneath toughened, high light-transmission glass. Manufacturers often offer these modules with 25 year warranties, a testament to the high stability of these cells.

Least efficient is the amorphous (a-Si) thin film type whereby a super-thin film of silicon, around one micron thick (human hair is 50-100 microns thick) is deposited on a substrate layer of glass or stainless steel.

This process is typically no more than 8% efficient, and can be found in low power applications such as watches and calculators. However as it is also relatively cheap to produce, it lends itself well to large area applications and is often used in building facades.

Some a-Si cells suffer significant degradation in power output, between 15-35%, when exposed to sunlight. Reducing the thickness of the silicon layer to increase the strength of the electrical field would further compromise efficiency as light absorption would be reduced.

To overcome this, cells containing two or three layers of a-Si have been developed. These cells involve quite complex production processes and although more stable, efficiency isn't increased.

Amorphous or thin film cells make up around 10% of the current PV market. Alternatives to amorphous silicon include copper indium (gallium) diselenide (CIGS or CIS) and cadmium telluride (CdTe), both in polycrystalline form. Research is ongoing into microcrystalline silicon, which has the potential to combine the higher efficiency of c-Si cells with the large area deposition capabilities of a-Si technology.

A further interesting line of research is that centred on electrochemical PV cells, or Grätzel cells, whereby the active component is a liquid dyed to improve light absorption. Laboratory efficiency is currently around 7% but with their use of cheap materials and inherent low production costs, they may well have a useful role.