Operation mechanisms of thin film organic electroluminescent diodes

Optical and electrical characteristics of single- and double-layer (DL) organic electroluminescent diodes based on an aromatic diamine (TPD) and 8-hydroxyquinoline aluminium complex (Alq3) were investigated to compare their operation mechanisms. From the optical spectra of thin films and of single-layer (SL) and DL devices, the nature of emitting states is revealed and their energy level scheme is derived. The injection-controlled electroluminescence mode of the operation of SL and DL devices provides a powerful experimental approach for investigating the process of charge injection. It is demonstrated that the physical mechanism responsible for charge injection is the primary carrier penetration over the image force barrier at high electric fields, and field-assisted thermionic injection in the low-field regime of operation of these devices. The role of material and device structure for charge injection parameters is pointed out in this context.