Exciplex formations between tris(8-hydoxyquinolate)aluminum and hole transport materials and their photoluminescence and electroluminescence characteristics

We investigated exciplex formations between tris(8-hydoxyquinolate)aluminum (Alq₃) and hole transport materials (HTMs) that have various highest occupied molecular orbital (HOMO) levels (-6.0 to -5.1 eV). The investigation focused on the photoluminescence and electroluminescence characteristics of Alq₃:HTM codeposited films. Intermolecular interaction between Alq₃ and the HTMs correlated with the HOMO levels of the HTMs. In the HTMs with HOMO levels ranging between -5.3 and -5.1 eV, the exciplex formation was clearly observed. No appreciable exciplex formation was observed in the HTMs with HOMO levels between -6.0 and -5.4 eV. The HTMs with -5.5 and -5.4 eV of HOMO levels, which are in an equivocal gray zone, showed weak intermolecular interaction, leading to a slight decrease in Alq₃ fluorescence efficiency without a significant spectral shift. The quenching of Alq ₃ fluorescence by such weak intermolecular interaction was signaled by an external electric field. We observed that this type of weak exciplex formation also affected the electroluminescence quantum efficiency of HTM/Alq₃-based organic light emitting diodes (OLEDs); therefore, it is necessary to control this delicate interface interaction to maximize OLED performance.