ABSTRACT
Figure 6.2 (a) Typical shape of RGB LEDs. Published by PiccoloNamek. (b) Principal structure of an LED. Published by J Nava’s. Both pictures published under Creative Commons Attribution-Share Alike 3.0 Unsorted. OLEDs from the ground up can be easily understood following the steps that led to the development of these innovative devices. Therefore, we will give a schematic overview of the research history that led to the original publication of OLEDs by Tang and Van Slyke
from Kodak Labs in 1987, before we move on to a detailed description of state-of-the-art OLEDs and the variety of possible modifications that are present in current devices. 6.1.1 History
their development has originally been triggered by findings based on inorganic materials. It was in 1907, when the young researcher H. J. Round discovered electrically generated luminescence, electroluminescence, upon application of a voltage of 10 V onto a silicon carbide crystal, also known as carborundum. He applied a voltage of approximately 10 V on silicon carbide, a common grinding material that has been known since the 19th century. Round observed yellowish-to orange-colored light, depending on the way the carbide had been synthesized. These findings did not directly lead to a specific application but caused other researchers to study materials under such special conditions. In 1963, Pope and coworkers reported electroluminescence from an anthracene crystal, that is, an organic material. Unfortunately, high voltages above 100 V were required to reach reasonable intensity. This was due to inefficient charge injection into the crystal in combination with poor charge transport within the material due to grain boundaries and defects inherent in the crystal. In general, the electrical properties of inorganic and organic materials are not alike. Especially in terms of conductivity, things change drastically when moving from inorganic to organic materials. To operate devices with organic materials at low voltages, layer thicknesses of less than 100 nm are common. 6.1.1.2 Inorganic light-emitting diodes
A first triumph in the history of semiconductor physics was the red-emitting GaAs LED developed by Nick Holonyak in 1962. He had been working on diodes with inorganic III-V semiconductors. He used GaAs with different dopings (p-and n-type) to construct a sandwich-type device architecture with an undoped GaAs layer
between a p-type doped and a n-type doped layer, which both had been contacted by two electrodes (see Fig. 6.3). Once operating, this diode structure would emit light from the undoped layer, resulting in the first LED. The layers had been vapor-deposited, which is still the process to commercially produce both LEDs and OLEDs today.
