ABSTRACT
Materials ...................................................................... 153 4.4 Electron-Transporting Amorphous Molecular
Materials ...................................................................... 160 4.5 Hole-Blocking Amorphous Molecular Materials ....... 165 4.6 Charge Transport in Amorphous Molecular
Materials ...................................................................... 168 References............................................................................. 172
4.1 INTRODUCTION
Organic light-emitting diodes (OLEDs) have attracted a great deal of attention, both academic interest and interest in their practical applications for full-color, flat-panel displays and
lighting.1-5 The operation of OLEDs involves charge injection from the electrodes, transport of charge carriers, recombination of holes and electrons to generate electronically excited states, followed by the emission of either fluorescence or phosphorescence. The main factors that determine luminous and quantum efficiencies are the following: efficiency of charge carrier injection from electrodes, charge balance, spin multiplicity of the luminescent state, emission quantum yield, and light output coupling factor. To attain high quantum efficiency for electroluminescence (EL), it is necessary to attain high charge injection efficiency and good charge balance, and to confine charge carriers within the emitting layer to lead to an enhanced recombination probability of the charge carriers. Generally, layered devices consisting of charge-transporting and charge-emitting layers can achieve higher charge injection efficiency and better charge balance than can single-layer devices using emitting materials alone. This is because a suitable combination of charge-transporting and emitting materials in layered devices reduces the energy barrier for the injection of charge carriers from the electrodes into the organic layer and blocking charge carriers from escaping from the emitting layer, leading to better balance in the number of injected holes and electrons, as shown in Figure 4.1.
