Electroluminescent (EL) devices based on organic thin layers have recently attracted much attention because of their potential uses in large-area flat-panel displays and light-emitting diodes (LED). Organic LEDs (OLEDs) have been made with both low molecular-weight organic materials and with polymers. The performance of these devices is significantly influenced by the charge balance between electrons and holes from opposite electrodes. The charge can be balanced by using a bilayer structure including a hole transport layer and an electron transport layer. One or both of these layers can be luminescent.
An important quality of organic EL materials is their durability, i.e., thermal and morphological stability. Thus, it is desirable that organic EL materials are not only light-emitting and hole-transporting, but also robust. In addition, it is desirable that an amorphous (glassy) thin film remain homogenous without crystallization or coagulation under the high temperatures reached during operation of the OLED.