Exciplex formations at the HTL/Alq₃ interface in an organic light-emitting diode: Influence of the electron-hole recombination zone and electric field

In this work, we investigate the influence of weak intermolecular interactions, which have not previously been carefully considered, in hole transport material (HTM)/tris(8-hydroxyquinoline)aluminum (Alq ₃)-based organic light-emitting diodes (OLEDs). Although such weak interactions quench Alq₃ fluorescence, no significant spectral shift is identified. Electroluminescence of OLEDs containing HTM:Alq₃ codeposited (mixed) emitter is quenched by the formation of such exciplexes. In general, the electroluminescence quantum efficiency of OLEDs correlates closely with the photoluminescence quantum yields of HTM:Alq₃ codeposited films. In contrast, in an OLED containing a layered structure of HTM/Alq ₃, exciplexes are less effective at quenching the electroluminescence of Alq₃. Because exciplexes form only at the interface between the HTM and Alq₃ layers in HTM/Alq₃-based OLEDs, exciplex formation is affected not only by the electron donating nature of the HTM but also by the position of the electron-hole recombination zone and the application of an external electric field during OLED operation.