Use of delayed electroluminescence as a tool to investigate the emission mechanism of phosphorescent organic light emitting devices

Abstract

Delayed electroluminescence (Delayed EL) measurements are used to investigate the emission mechanism in typical Phosphorescent Organic Light Emitting Devices (PHOLEDs). In this work, emission mechanism of PHOLED based on CBP (4,4'-bis(carbazol-9-yl)biphenyl : Ir(ppy)₃ (Tris(2-phenylpyridine)iridium(III)) host:guest system is studied. The technique is used to understand whether the EL emission arises from charge recombination directly on guest sites or from recombination on host material followed by host to guest energy transfer. The results show that the emission mechanism changes with the emission layer (EML) thickness. In the case of devices with a thick EML, strong host triplet-triplet annihilation (TTA) is observed, suggesting the presence of significant energy transfer from host to the guest. For the case of devices with thin EML, on the other hand, host TTA is insignificant, suggesting that direct charge trapping on the guest is the main emission mechanism. The results also show that the thickness of the hole transport layer (HTL) has negligible effect on the emission mechanism of PHOLED, showing that the emission mechanism mainly depends on the EML.