light emitting polymer (LEP)
The technology of light-emitting polymers (LEP) is comparable to that of OLEDs and is used in displays. Corresponding LEP displays are extremely flat and flexible and are suitable for color displays.
LEP technology is based on the fact that certain plastics emit light when a voltage is applied to them. This effect was first demonstrated by Cambridge University in 1989 in a carbon polymer, leading to the name Cambridge Display Technology( CDT). In this technique, when a sufficiently high voltage is applied, electrons are torn out of the composite and travel through the material. The missing electrons leave behind positive holes. When the positive holes are occupied by freely moving electrons, energy is released in the form of light.
Technologically, LEP displays consist of a thin glass plastic substrate coated with a transparent indium tin oxide electrode. Above the transparent electrode is a two- layer polymer coating. One part made of Poly Phenylene Vinylene( PPV) is used for hole transport and the other for light emission. This part is called Cyano-Substituted Poly Phenylene Vinylene( CN-PPV). When an electric field is generated across the electrodes, the electrons and holes travel in opposite directions. When electrons and holes recombine, the released energy is converted into light in the CN-PPV layer. The color emission depends on the energy difference between the electrons and holes. The higher the energy difference, the more red components are emitted.
The color emission of the primary colorsred, green and blue can also be generated by doping the polymers with molecules of the corresponding fluorescence. In displays, the individual pixels are arranged next to each other and controlled individually. LEPs are thus self-luminous, light-emitting polymers characterized by low power consumption and a wide viewing angle. They are flexible, can be bent and manufactured in printed electronics.