The photovoltaic effect was discovered by Edmund Becquerel in 1839, but was not fully understood until the 1950s. In suitable materials, photons of light excite electrons out of their resting state. If the electrons and "holes" generated by this excitation are separated by an internal electrical field, then a current is produced. Because of the wide spectral range of sunlight, the maximum efficiency of a PV cell is about 28% - though higher efficiencies can be achieved by a "cascade" of complementary cells.
The main PV technologies currently in use or under development are:
Crystalline silicon: This technology, currently the workhorse of the PV industry, relies on the cutting of silicon ingots into wafers. Commercial products using monocrystalline cells achieve efficiencies of 12-18%, but are relatively costly and intolerant of impurities. Multicrystalline cells have efficiencies of 10-16%.
Thin film silicon: Thin film amorphous silicon cells, such as those manufactured in the UK by Intersolar, can be made at relatively low cost. The cells' efficiencies are low, and degrade rapidly to around 4-6%. However, Canon Japan and the US-based Solarex Corporation and United Solar System Corporation are understood to have developed amorphous silicon cells with higher efficiencies and minimal degradation.
Thin film crystalline silicon offers much higher efficiencies - but to date has been handicapped by the need to use substrates capable of withstanding the high-temperature manufacturing process.
Thin film cadmium telluride (CdTe): Manufacture of these cells by electroplating or vacuum deposition allows them to be mass-produced at low cost. BP Solar's Appollo cell, which will be made at the company's new facility in California, operates at an efficiency of 8-9%. However, concerns remain over the toxicity of cadmium releases during production and end-of-life disposal (see other box).
Copper indium diselenide (CIS): High efficiency levels of 14-17% have been demonstrated for this thin film technology. However, the cells are complex and difficult to manufacture, and are not in significant commercial use.
Titanium dioxide (Gratzel cell): This technology, developed by the Swiss Federal Institute of Technology, is based on the sensitisation of fine films of crystalline materials. Titanium dioxide, a plentiful and cheap raw material, is particularly suitable. The technology, which is not yet commercially available, is expected to offer efficiencies of up to 10% at low cost. A particularly attractive feature is that, because the cells absorb infra-red rather than visible light, they can be used in window applications.