Harvesting Triplet Excitons from Fluorescent Blue Emitters in White Organic Light‐Emitting Diodes

Abstract

A novel concept for white organic light emitting diodes (OLEDs) enabling the utilization of all electrically generated excitons for light generation and achieving a high luminance at very low voltage is introduced. The key feature is a fluorescent blue emitter with high triplet energy, rendering it possible to harvest its triplet excitons by letting them diffuse to an orange phosphorescent iridium complex. This results in a total internal quantum efficiency of almost unity. The high potential of this concept is demonstrated in that a total external power efficiency of 57.6 lm W -1 at a brightness of 100 Cd m -2 (20.3 % external quantum efficiency) and 37.5 lm W -1 (16.1 %) at 1000 Cd m -2 were achieved. This is the most efficient white OLED reported in scientific literature so far. Lighting uses a significant part of the world's energy resources, with a large share still consumed by inefficient incandescent lamps. White organic light emitting diodes (OLEDs) show promise as major role-players in future ambient lighting [1][2][3] due to their favorable properties such as homogenous large-area emission, good color rendering, and potential realization on flexible substrates. Although white OLED efficiencies superior to those of incandescent lamps have been reported, it remains a challenge to reach or surpass the efficiency of fluorescent tubes. A key requirement for such high efficiencies is that all electrically generated excitons, i.e., both singlet and triplet excitons are employed for emission. While suitable molecules have been found for the green and red, stable and efficient blue phosphorescent emitters are still a challenge. [4] Recently, OLED concepts for white light generation, which combine fluorescent blue emitters with phosphorescent green and red ones, have shown an improved operational stability. However, up until now, devices comprising fluorescent blue do not utilize all the generated excitons for light emission, thus giving rise to rather low efficiencies at an illuminationrelevant brightness. [5][6][7] A new concept consisting in the transfer of singlet excitons generated on a host to a fluorescent blue emitter as well as triplets to phosphorescent guests [5] has improved exciton harvesting. Nevertheless, this concept suffers from the disadvantage of the triplet excitons, which are either directly generated on or transferred by Dexter transfer to the blue emitter, being lost. This is a result of the excitons' inability to leave the blue emitter because of its low-lying triplet state. Here, we report on a novel concept for highly efficient white OLEDs. We demonstrate the possibility of harvesting triplet excitons on the fluorescent blue emitter since an emitter molecule with very high triplet energy is used. This permits the transfer of the triplet excitons to the lower-energy colors of the white spectrum. In particular, we demonstrate the achievement of an internal quantum efficiency of almost unity, which may render it possible to completely dispense blue phosphorescent emitters in future high-eff...

Highly efficient hybrid warm white organic light-emitting diodes using a blue thermally activated delayed fluorescence emitter: exploiting the external heavy-atom effect

“…The increase of k ISC can be calculated using Equation (2) and is shown in Supplementary Table S1. More triplet excitons will be formed in these fluorophors, and the radiative decay of the triplet excitons will be improved; therefore, such a system has been widely used to measure the triplet energies of fluorophors.…”

“…[11][12][13] This process could be another possible mechanism of energy loss in hybrid WOLEDs, especially for the WOLED with a single EML, even though this process has been ignored in previous studies. 1,2,14 The quenching process between the fluorophor and the EHA perturber is rather complicated and not fully understood. 12 Multiple factors would affect the fluorescence quenching, such as the temperature, the distance between the fluorophor and the EHA perturber, and the nature of the fluorophor and perturber, respectively.…”

“…6 Therefore, a 'triplet harvesting' strategy by doping a blue fluorophor and red/green phosphors in separated regions of high-triplet-energy host materials has been proposed 1, 10 and is further improved by incorporating blue fluorophors with high triplet energy (T 1 ) to prevent the undesirable 'back energy transfer' from the phosphors to the blue fluorophors. 2,7,8 However, the heavy metal in the phosphors introduce an external heavy-atom effect (EHA) in the emissive layer (EML) and may lead to quenching of the fluorescence due to the increased singlet (S 1 )-triplet (T 1 ) intersystem crossing (ISC) rate (k ISC ) constant and the radiative or non-radiative decay rate of triplet excitons (k p ) of the fluorophors. [11][12][13] This process could be another possible mechanism of energy loss in hybrid WOLEDs, especially for the WOLED with a single EML, even though this process has been ignored in previous studies.…”

“…[1][2][3][4] For practical use, high efficiency and long lifetime are the most critical parameters. Blue phosphors can obtain high luminous efficiency but suffer from short lifetime, while blue fluorophors are exactly the opposite.…”

“…6 Therefore, a 'triplet harvesting' strategy by doping a blue fluorophor and red/green phosphors in separated regions of high-triplet-energy host materials has been proposed 1,10 and is further improved by incorporating blue fluorophors with high triplet energy (T 1 ) to prevent the undesirable 'back energy transfer' from the phosphors to the blue fluorophors. 2,7,8 However, the heavy metal in the phosphors introduce an external heavy-atom effect (EHA) in the emissive layer (EML) and may lead to …”