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 (Alq(3)) 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 Alq(3):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 Alq(3) 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/Alq(3)-based organic light emitting diodes (OLEDs); therefore, it is necessary to control this delicate interface interaction to maximize OLED performance.