Photoconductivity and photovoltaic solar cells

If radiation of a suitable wavelength falls on a semiconductor it will excite electrons across the band gap. The most obvious effect of this is that the conductivity of the material increases. The magnitude of the effect is roughly proportional to the light intensity. This effect, called the photoconductive effect, has been used in light meters, exposure meters and automatic shutters in cameras and many other devices. In practice, a dc voltage is applied to the ends of a semiconductor. On illumination the resistance of the semiconductor falls, which provides a means of measurement. 

Solar cells must have a large area, to collect as much sunlight as possible. Some systems use reflec- 

Recently, much effort has been put into the cons-tmction of solar cells with use of polymers. These have the great advantages of low weight and flexibility. However, efficiencies are not yet adequate for commercial purposes. 

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