Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching

Abstract

Delayed electroluminescence measurements are used to probe and differentiate between triplet-triplet-annihilation (TTA) and triplet-polaron-quenching (TPQ) processes and their correlation with efficiency roll-off in fac-tris(2-phenylpyridine) iridium-based phosphorescent organic light emitting devices. Investigations on devices employing 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl) triphenylamine, two widely used host materials, show that the efficiency roll-off is primarily due to TPQ processes. Guest-guest TTA, on the other hand, is found to play no major role, contrary to speculations, especially at low guest concentrations. Evidence of host-host TTA in certain cases, and its possible contribution to exciton quenching in the case of devices with CBP host, is also reported.

Tailoring Optoelectronic Properties of Phenanthroline‐Based Thermally Activated Delayed Fluorescence Emitters through Isomer Engineering

“…The DF lifetimes of x-PXZP doped into CBP films (3 wt%) at 300 K are in the sequence of 2.82 μs (o-PXZP) > 1.70 μs (m-PXZP) > 1.42 μs (p-PXZP), which is in good agreement with the tendency of ΔE ST values for x-PXZP both in experiment and theoretical prediction. This result suggests that the reduced ΔE ST could favor RISC progress from T 1 to S 1 and shorten the DF lifetimes, and thereby possibly suppress quenching processes of electro-generated triplet excitons in OLEDs, such as triplet-triplet annihilation, [53][54][55] singlet-triplet annihilation, [24,56] and triplet-polaron quenching. [55,57] …”

Monte Carlo study of efficiency roll-off of phosphorescent organic light-emitting diodes: Evidence for dominant role of triplet-polaron quenching

“…TPQ occurs upon the creation of an excited polaron state after close encounter with an exciton, after which the excess energy is lost by thermalization, whereas TTA is caused by the fusion of two triplet excitons to a high-energy excitonic state, followed again by a loss of the excess energy by thermalization, so that effectively one triplet exciton is lost. TTA 2 as well as TPQ 5,7 have been argued to be the predominant cause of the roll-off. The experimental evidence to support either of these points of view has been based on time-dependent photoluminescence, 4 and steady state 2 and time-dependent 7 electroluminescence measurements.…”

“…2,3 Elucidating the origin of this roll-off and disentangling the roles of both types of loss processes has been a subject of intensive study. [2][3][4][5][6][7] However, so far, no widely accepted picture has emerged. Obtaining such a picture is hampered by the need to carefully consider the strong spatial non-uniformity of the charge and exciton densities in actual devices.…”

Effect of Förster-mediated triplet-polaron quenching and triplet-triplet annihilation on the efficiency roll-off of organic light-emitting diodes

“…Considerable effort has been made to study the rate of the individual quenching processes thought to contribute to the roll-off in phosphorescent OLEDs: triplet-polaron quenching (TPQ) and triplet-triplet-annihilation (TTA). [1][2][3][4][5][6] However, due to the complex interplay of the various processes and the spatial non-uniformity inherent to organic semiconductor materials and devices, it has been difficult to link the results of individual experiments to actual device performance.…”

“…We note that in the case of fluorescent emitters, for which F€ orster-mediated TTA is not possible, Dexter-type TTA is non-negligible at high triplet densities, giving rise to delayed fluorescence. 12 Whereas various studies have provided evidence for a significant contribution of TPQ to the roll-off of OLEDs 3,5,13,14 and whereas effective TPQ rate coefficients have been determined for various host-guest systems, 1 the underlying mechanism (Dexter- Published by AIP Publishing. 119, 163102-1 the effective TPQ rate is not only determined by the TPQ F€ orster-radius but also by the polaron diffusion coefficient.…”