Also found in: Dictionary, Thesaurus, Wikipedia.
The conversion of electromagnetic radiation into electric power through absorption by a semiconducting material. Devices based on this effect serve as power sources in remote terrestrial locations and for satellites and other space applications. Photovoltaic--powered calculators and other consumer electronic products are widely available, and solar photovoltaic automobiles and aircraft have been demonstrated.
The basic requirements for the photovoltaic effect are (1) the absorption of photons through the creation of electron-hole pairs in a semiconductor; (2) the separation of the electron and hole so that their recombination is inhibited and the electric field within the semiconductor is altered; and (3) the collection of the electrons and holes, separately, by each of two current-collecting electrodes so that current can be induced to flow in a circuit external to the semiconductor itself.
There are many approaches to achieving these three requirements simultaneously. A very common approach for separating the electrons from the holes is to use a single-crystal semiconductor, for example, silicon, into which a pn junction has been diffused. Silicon is often chosen because its optical band gap permits the absorption of a substantial portion of solar photons via the generation of electron-hole pairs. The fabrication of such a device structure causes a local transfer of negative charges from the n layer into the p layer, bending the conduction and valence bands in the vicinity of the p-n boundary, and thereby creating a rectifying junction. Electrons generated in the p region can lower their energy by migrating into the n region, which they will do by a random walk process in the electric-field-free region far from the junction, or by drift induced by the electric field in the junction region. Holes created in the n region, conversely, lose energy by migrating into the p region. Thus the presence of such a junction leads to the spontaneous spatial separation of the photogenerated carriers, thereby inducing a voltage difference between current-carrying electrodes connected to the p and n regions. This process will continue until the difference in potential between the two electrodes is large enough to flatten the bands in the vicinity of the junction, canceling out the internal electric field existing there and so eliminating the source of carrier separation. The resulting voltage is termed the open-circuit voltage, and approximates the built-in voltage associated with the pn junction in the dark, a value which cannot exceed the band gap of the semiconductor. See Hole states in solids, Semiconductor, Semiconductor diode
In the limit when the device is short-circuited by the external circuit, no such buildup of potential can occur. In this case, one electron flows in the external circuit for each electron or hole which crosses the junction, that is, for each optically generated electron-hole pair which is successfully separated by the junction. The resulting current is termed the short-circuit current and, in most practical photovoltaic devices, approaches numerically the rate at which photons are being absorbed within the device. Losses can arise from the recombination of minority carriers (for example, electrons in the p-type region, holes in the n-type region) with majority carriers. See Electron-hole recombination
For a photovoltaic device to generate power, it is necessary to provide a load in the external circuit which is sufficiently resistive to avoid short-circuiting the device. In this case, the voltage will be reduced compared to the open-circuit voltage because a continuing requirement exists for carrier separation at the junction; thus some band bending and its associated internal field must be retained.
Various multiple-layered device configurations based on doped and undoped alloys of amorphous silicon have been developed for photovoltaic devices used in applications ranging from solar watches and calculators to remote power generators. The photovoltaic effect in these devices is particularly intriguing since it is possible to build up so-called tandem devices by stacking one device electrically and optically in series above another. In addition to the increased voltage and concomitant reduction in the required current-carrying capability of electrode grid structures, such devices permit, in principle, an increased efficiency of solar photovoltaic energy conversion.