The from 0.5 to 3 eV range is an important part of solar energy. Photons in this range can be used by special semiconductor devices for the production of electrical

energy. Photons of suitable energy generate charge carrier pairs in the semiconductor. Making use of built-in electrical space the charge carriers can be separated and may do electrical work. One of the most frequently used solar cell types can be manufactured from polycrystalline (Fig. 1.) and amorpous (Fig. 2.) silicon with pn-junction. The current — voltage characteristics of the junction is well known. The efficiency of the solar cell is given by the quotient the highest available electrical power and the power of incident solar rays. Typical efficiency for commercially available — e. g. in architecture — used silicon based solar cells is 10 to 15 percent. The

solar cell is characterized by another parameter called fill factor, which describes the form of the current-voltage curve, and it is between 0.7 and 0.8 for a well-made commercial cell. How can the efficiency of the solar cell be improved? There is a very important factor: the energy band gap.

Transport behaviour of the carrier in different materials is another important parameter. The carrier which is generated outside the space — charge region should the junction by diffusion before recombination. This is realized in silicon, in which the diffusion length of electrons in the p-region is around hundred microns. The efficiency can be improved with an antireflection layer on the surface.

Efficiency is influenced by the following parameters: band gap, absorption and diffusion length.