Ultrahigh efficiency fluorescent single and bi-layer organic light emitting diodes: The key role of triplet fusion

Abstract

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T1≤Tn and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent-based OLEDs with the added advantage of much improved stability. A new family of anthracene core emitters where the lowest triplet state has less than half the energy of the next highest triplet level is developed. The triplet fusion becomes highly efficient, resulting in fluorescent organic light emitting diode (OLED) devices of very simple structure, that far exceed the 25% external quantum efficiency limit due to the efficient singlet generation from triplet fusion in the device. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.