ELECTROLUMINESCENCE

production of light by the flow of electrons, as within certain crystals. Electroluminescence is one of the few instances in which a direct conversion of electric energy into visible light takes place without the generation of heat, such as occurs in the incandescent lamp. There are two distinct mechanisms that can produce electroluminescence in crystals: pure or intrinsic (also called the Destriau effect, after Georges Destriau, a 20th-century French physicist) and charge injection. The principal differences between the two mechanisms are that in the first, no net current passes through the phosphor (electroluminescent material) and in the second, luminescence prevails during the passage of an electric current. Destriau Electroluminescence In Destriau electroluminescence, thermal activation and the electric field liberate atomic electrons (from donor levels) into the conduction band. Many of these conduction electrons are accelerated by the field until they collide with luminescent centres, ionizing them (i.e., ejecting electrons from their atoms). Light is emitted in the normal way as soon as an electron recombines with an ionized atom of the centre. Because the effect dies away when constant voltage is applied, an alternating voltage may be used to create a sustained light emission. The Destriau effect may be extended over large areas so that electroluminescent cells may be made with which to panel walls and ceilings of a room for general illumination. The Figure shows a typical cell in which a phosphor, such as zinc sulfide (ZnS), is mixed with various small particles, or dopants, and embedded in an insulating film. The insulating film is then sandwiched between two electrodes. The front electrode consists of a thin transparent film of tin (IV) oxide (SnO2), which is coated onto a glass plate. The rear electrode is coated black to render it opaque, thereby making the emitted light rays easier to see. When alternating current is applied to the cell, an intense electric field is generated, which activates the dopants within the phosphor. The colour of the emitted light depends upon the composition of these dopants as well as the voltage frequency. For example, a mixture of copper (Cu) and chlorine (Cl) produces blue light; copper and aluminum (Al) produce green light; and copper, chlorine, and manganese (Mn) together produce yellow light. Electroluminescence can also result from charge injection, as when an electrode contacts a crystal to provide a flow of electrons or holes (electron extraction) or a voltage is applied to a pn junction causing a current to flow; i.e., electrons flow from the n-type material into the p-type material. In both cases, the electrons lose energy when recombining with centres or positive holes accompanied by the emission of light.