Effect of Electric Field on Coulomb‐Stabilized Excitons in Host/Guest Systems for Deep‐Blue Electrophosphorescence

Abstract

Here, a study of the electric field induced quenching on the phosphorescence intensity of a deep‐blue triplet emitter dispersed in different host materials is presented. The hosts are characterized by a higher triplet excitonic level with respect to the emitter, ensuring efficient energy transfer and exciton confinement, whereas they differ in the highest occupied molecular orbital (HOMO) alignment, forming type I and type II host/guest heterostructures. While the type I structure shows negligible electric field induced quenching, a quenching up to 25% for the type II at a field of 2 MV/cm is reported. A similar quenching behaviour is also reported for thin films of the pure emitter, revealing an important luminescence loss mechanism for aggregated emitter molecules. These results are interpreted by considering Coulomb stabilized excitons in the type II heterostructure and in the pure emitter, that become very sensitive to dissociation upon application of the field. These results clarify the role of external electric field quenching on the phosphorescence of triplet emitters and provide useful insights for the design of deep‐blue electrophosphorescent devices with a reduced efficiency roll‐off.

High-efficiency and low efficiency roll-off near-infrared fluorescent OLEDs through triplet fusion

“…Such host/guest heterostructures can efficiently suppress the electric field induced quenching and thus minimize the efficiency roll-off in devices. 38 , 49 On the other hand, both Ga 2 (saph) 2 q 2 and NSeD have a bipolar character and thus provides a wide charge recombination region. 50 Overall, the well matched host/guest energy levels and their bipolar characters substantially contribute to the negligible efficiency roll-off of such NIR OLEDs.…”

High-efficiency near-infrared organic light-emitting devices based on an iridium complex with negligible efficiency roll-off

“…As revealed by the single crystalline structure, the bulky aromatic cyclometalated ligands of complex 1 further distort the coordination geometry, thus resulting in a relatively large k r . 47 Moreover, the triplet energy (E T ) of complex 1 is only 1.6 eV, which is much lower than that of any other organic material, including the host material (calculated as 2.2 eV), and NPB (2.3 eV) and Bphen (2.5 eV). 16 In addition to TTA and TPQ, device parameters such as a broad recombination zone and well-matched energy levels have also been reported to help reduce the efficiency roll-off in PHOLEDs.…”

“…Similar to Judd-Ofelt theory, the electricdipole radiative probability is proportional to the asymmetry of the coordination sphere around the central metal ion. 46,47 The energy level diagram of the devices is shown in Fig. As revealed by the single crystalline structure, the bulky aromatic cyclometalated ligands of complex 1 further distort the coordination geometry, thus resulting in a relatively large k r .…”

The influence of type-I and type-II triplet multiple quantum well structure on white organic light-emitting diodes

“…Meanwhile, the larger barrier of 0.7 eV at the interface of [HOMO] EML /[HOMO] BCP results in holes confined in the HOMO energy level of EML. Since electrons and holes are confined in different organic layers, which increase the probability of excitons disassociation and decrease the recombination efficiency of carriers [ 23 ], device C presents inferior EL performances.…”