The Photovoltaic Effect

        The photovoltaic effect was discovered more than 150 years. The direct conversion of sunlight into electricity for the first time succeeded Alexandre Edmond Becquerel, French physicist in 1839. For many years this effect has been exploited only in the photograph at the exposure. The most important application today is the photovoltaic power generation with solar cells. 

       The solar cells consist of two superposed layers consisting mostly of crystalline silicon. On the outside there are contacts that are made of metal. Since the two silicon layers treated with different materials such as boron and phosphorus were, they have different electrical properties. In the so-called boundary layer, which is only about one thousandth of a millimeter is strong there is an electric field with positive and negative poles. Now, if light falls on the boundary layer, the electrons gain energy from the photons of light, loose binding of the crystal and move in positive direction. The flowing current can now be removed by the consumer. Light (Greek: Photo) thereby causing an electrical voltage. Since one is measuring the voltage in volts it up the term photovoltaic (PV too).
The photovoltaic effect is observed only in semiconductors, which can absorb photons at all on the one hand, on the other hand, can simultaneously produce charged particles which have enough energy to move through the material to leave the semiconductor and thus to produce electricity.

      By absorption of a photon of appropriate energy, an electron goes from the ground state to an excited state, whereby a free electron-hole pair is created. Is there a sufficiently strong electric field as it exists for example in a pn junction of a semiconductor, the two charge carrier drift in the opposite direction. The electrons migrate to the n-layer and the holes in the p-layer. By the charge transfer results in a photovoltage which causes a corresponding photocurrent, if the two layers are connected via an external circuit, where electrical energy from light is produced (semiconductor photoelectric effect).