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.
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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