what is photovoltaic?

Photovoltaics (abbreviated PV) is the most direct way to convert solar radiation into electricity and is based on the photovoltaic effect, which was first observed by Henri Becquerel [1] in 1839. It is quite generally defined as the emergence of an electric voltage between two electrodes attached to a solid or liquid system upon shining light onto this system. Practically all photovoltaic devices incorporate a pn junction in a semiconductor across which the photovoltage is developed. These devices are also known as solar cells. Light absorption occurs in a semiconductor material. The semiconductor material has to be able to absorb a large part of the solar spectrum. Dependent on the absorption properties of the material, the light is absorbed in a region more or less close to the surface. When light quanta are absorbed, electron hole pairs are generated, and if their recombination is prevented they can reach the junction where they are separated by an electric field. Even for a weakly absorbing semiconductor like silicon, most carriers are generated near the surface. This leads to the typical solar cell structure of Fig. 1.1.

The pn junction that separates the emitter and base layer is very close to the surface in order to have a high collection probability for the photogenerated charge carriers. The thin emitter layer above the junction has a
relatively high resistance which requires a well designed contact grid, also shown in the figure.
For practical use solar cells are packaged into modules containing either a number of crystalline Si cells connected in series or a layer of thin-film material which is also internally series connected. The module serves two purposes: It protects the solar cells from the ambient and it delivers a higher voltage than a single cell, which develops only a voltage of less than 1Volt. The conversion efficiencies of today’s production cells are in the range of 13 to 16%, but module efficiencies are somewhat lower. The best laboratory efficiency of crystalline silicon achieved so far is 24.7%, which approaches the theoretical limit of this type of solar cell.
As we shall see in Chap. 5, pn junctions and semiconductors are not the only way to achieve photovoltaic conversion. The future may hold many new materials and concepts. (A. Goetzberger, V.U. Hoffmann 2005)