Types of solar cell

Microchips and transistors are universally fabricated using slices cut from perfect crystals of silicon. These crystals are carefully grown under controlled conditions and are expensive to produce.

Solar cells can be made with single crystal silicon too. Indeed, cells using this material have provided the best performance of any silicon solar cells, with solar-to-electrical conversion efficiencies of up to 24%. Long-term performance of single crystal silicon cells is good too, but the cost of the single crystal material remains relatively high.

In an attempt to bring fabrication costs down, many manufacturers have experimented with alternatives to single crystal silicon. One of the most widely used is a form called polycrystalline silicon; this is silicon made up of lots of tiny individual crystals instead of one large crystal. Such material is much cheaper to produce but has proved less efficient than the single crystal material. However it does produce reliable and stable cells, at a significantly lower cost. Efficiencies of over 18% have been achieved.

Cheaper still is a completely non-crystalline form of silicon called amorphous silicon. This was initially found to suffer from a serious degradation problem when exposed to light, an effect which reduced efficiencies by 20-40%. With extensive redesign, it has now proved possible to circumvent the most serious aspect of this problem. The amorphous cell does still suffer degradation of around 20% but its output eventually stabilises. Cell efficiencies of around 13% can now be achieved (after degradation has taken place).

The cheaper and simpler amorphous silicon fabrication process has allowed some more complex solar cell designs to be developed. For example, some amorphous cells have been fabricated as three cells one on top of the other, designed to absorb first blue, then green and finally red light. This three cell design offers the potential for higher efficiency than a single cell absorbing the whole spectrum.

All silicon solar cells require extremely pure silicon. The manufacture of pure silicon is both expensive and energy intensive. The traditional method of production requires 90 kWh of electricity for each kilogram of silicon. Newer methods have been able to reduce this to 15 kWh/kg. This still means that a silicon solar cell takes 2 years to generate the energy needed to make it.5 This compares with around 5 months for a solar thermal power plant.6

Silicon-based solar cells dominate the market today. Single crystal and polycrystalline silicon cells remain the most popular, accounting together

Table 13.1 Solar cell production, 2GG3, by region

Production (MW)

Japan 364

Europe 193

USA 1G3

Rest of the World 84

Total 744

Source: Renewable Energy World.15

for 89% of production in 2003. Of that, polycrystalline cells accounted for 62%.7 Amorphous silicon adds a further 3%. These materials are likely to remain dominant for several years. There are alternatives to silicon. Most promising of these are cells fabricated from cadmium telluride or from copper indium selenide. Manufacture of solar cells based on these materials has begun, with a total production of 7MW in 2003.8

Fifty years of experience with silicon solar cells means that long-term performance can now be assessed with accuracy. Modern silicon solar cells sold in panel form for installation on roofs come with a 25-year warranty. Longer lifetimes still should eventually be plausible. The long-term performance of newer materials has yet to be established.

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