Electroluminescence of organic materials. Basic principles

The formation of an image on a screen is the ultimate stage in any information visualization process. The ever-increasing importance of communication in today's world is creating new needs and requirements in terms of visualization, which are only partially met by existing technologies. In particular, there is a real interest in space-saving displays that produce large, bright, high-definition color images visible from a wide viewing angle. Currently, the market for thin-film displays is dominated by nematic liquid crystal displays (LCDs). However, despite the remarkable progress made in recent years in the field of LCD displays, these present a number of disadvantages linked to the very physics of the electro-optical effects involved. The restricted viewing angle requires the use of expensive compensating films. The softbox, essential for backlighting, does not allow access to ultra-thin, low-weight systems. Furthermore, the temperature dependence of the physical properties of liquid crystals limits the operating temperature range of LCD displays.

In this context, organic electroluminescent materials are particularly promising, since they combine :

• low addressing voltages (below 5 V);

• very short response times (in the nanosecond range);

• emission possible in the entire visible range by "simple" modification of the chemical structure of the emitting material;

• a wide angle of view due to a Lambertian emission;

• the ability to obtain luminances as high as several thousand candelas per square metre (by way of comparison, the average luminance of a television screen is 200-300 cd/m ² );

• the potential to produce pixels just a few micrometres in size, opening the way to small, high-definition displays;

• finally, the new capacity to produce flexible screens.

For all these reasons, numerous academic and industrial research teams are active in this field today, and the technological advances made since 1995 have removed many technological barriers. Since 2003, mass production of display systems based on organic electroluminescent compounds has become a reality.

In what follows, after a brief history, we recall the physical principles involved in electroluminescence (EL), and introduce the different efficiencies and standard structures of EL diodes. In a second dossier , we present the main organic EL materials studied to date. We then discuss the problems associated with aging structures, addressing and coloring. Finally, the characteristics...