Electron OLEDS

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

The materials used as the electron and hole injecting electrodes play a crucial role in the overall performance of the device and therefore cannot be neglected even in a brief review of the materials used in OLEDs. The primary requirements for the function of the electrodes is that the work function of the cathode be sufficiently low and that of the anode sufficiently high, to enable good injection of electrons and holes, respectively. In addition, at least one electrode must be sufficiently transparent to permit the exit of light from the organic layer. 

The processes of charge injection, transport, and recombination dictate the recombination efficiency h(/), which is the fraction of injected electrons that recombine to give an exciton. The recombination efficiency, which is a function of the device current, plays a primaty role in determining the amount of emitted light, therefore determining the OLED figurcs-of-meril. For example, the quantum efficiency /y(/) (fraction of injected electrons that results in the emission of a photon from the device) is, to a first approximation, given by ... 

Figure 13-11. (a) A diagram showing ihc spatial distribution of lire relative hole and electron currents in an OLED. The recombination efficiency h is equal to the fraction of the electron (hole) current that docs not make it to the anode (cathode) (b) cll icicncy-currcni balance diagram for OLEDs. Sec text for details. 

To date, most small molecule-based OLEDs are prepared by vapor deposition of the metal-organic light-emitting molecules. Such molecules must, therefore, be thermally stable, highly fluorescent (in the solid state), form thin films on vacuum deposition, and be capable of transporting electrons. These properties limit the number of metal coordination compounds that can be used in OLED fabrication. 

The discovery of the use of A1Q3 as an electron-transport-emitting layer is undoubtedly the most significant achievement in the research that led to the development of stable OLEDs.180,181 It is very stable and can be sublimed without decomposition at 350 °C,188 and its thin-film PL quantum efficiency at room temperature is about 32%, independent of film thickness between 10 nm and 1,350 nm.189... 

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

Shibusawa, M. Kobayashi, M. Hanari, J. Sunohara, K. Ibaraki, N. 2003. A 17-inch WXGA full-color OLED display using the polymer ink jet technology. IEICE Trans. Electron., E86-C(ll) 2269-2274. 

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