Amorphous silicon cells

Less efficient but cheaper to produce are silicon-based cells which do not have a crystalline atomic structure. Hydrogenated amorphous silicon (a-Si:H) comprising a thin film 0.5 ^m thick form the basis of highly light-absorbent cells, hence the description 'thin film solar cell'. They are produced by atomic deposition over a large area and consequently much more economical to produce than crystalline silicon cells which involve slicing up slabs of crystalline silicon grown by a slow crystallization process.

Figure 6.1 Crystalline silicon photovoltaic cell

The efficiency of this technology has been improved by stacking cells which capture light at different wavelengths. Different alloys of silicon capture the blue, green and red/infrared parts of the spectrum. A triple junction terrestrial concentrator solar cell has been produced by Spectrolab Inc of the USA which has achieved an efficiency in the laboratory of 34%. The variations in the silicon are:

• Top: amorphous silicon alloy (a-Si alloy) - blue

• Bottom: a-Si + 40-50% germanium - red/infrared.

Light passes through all three layers and is reflected back by a silver/zinc oxide reflector. The cells are able to withstand highly concentrated sunlight and using a concentrator reduces the number of cells needed to produce a given amount of electricity.

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