Reductions in the cost of solar cells commonly use thin-films instead of crystalline wafers, which can avoid the use of heavy and fragile glass plates as module encasings. This provides a lower thermal budget for the production of the base material and may permit simple manufacturing processes such as liquid coating or spray coating rather than the usual semiconductor fabrication steps of low pressure chemical vapour deposition (CVD) or physical vapour deposition (e.g. sputtering). Potentially the greatest simplicity is offered by organic polymer films whose development for light emitting diodes has already reached commercial production: their commercial use in solar cells requires further development to increase the relatively low efficiency and sensitivity to air and moisture (see organic solar cell page for more details).
Another advantage of a thin-film technology is the facility to use a large area substrate which in addition may be flexible. Thin-film cells based on quaternary compounds of copper indium gallium selenium (CIGS) or binary II-VI compounds (e.g. CdTe) offer efficiencies of 15-20% but require high temperature processing that restricts the choice of a flexible substrate material to metal foils or polyimide. Thin-films based on amorphous silicon (actually a-Si:H) give lower efficiency (~10%) but their low temperature synthesis enables a wider choice of plastic substrate. Woven polyester textile is an appealing substrate as it is available in a wide variety of forms and is used in everyday applications as well as highly sophisticated technical applications. At Heriot-Watt we use polyester fabric as the substrate for amorphous and nanocrystalline thin-film silicon cells: this development has been supported by Power Textiles Limited through Scottish Government funding. We have optimised the plasma-enhanced CVD synthesis of thin-film silicon for application to polyester: intriguingly, the nanocrystalline form of this material appears from Raman spectroscopy to have the wurtzite hexagonal structure rather than the usual cubic form of Si.
