Abstract
Polaron-induced exciton quenching in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) can lead to external quantum efficiency (EQE) roll-off and device degradation. In this study, singlet-polaron annihilation (SPA) and triplet-polaron annihilation (TPA) were investigated under steady-state conditions and their relative contributions to EQE roll-off were quantified, using experimentally obtained parameters. It is observed that both TPA and SPA can lead to efficiency roll-off in 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) doped OLEDs. Charge imbalance and singlet-triplet annihilation (STA) were found to be the main contributing factors, whereas the device degradation process is mainly dominated by TPA. It is also shown that the impact of electric field-induced exciton dissociation is negligible under the DC operation regime (electric field < 0.5 MV cm−1). Through theoretical simulation, it is demonstrated that improvement to the charge recombination rate may reduce the effect of polaron-induced quenching, and thus significantly decrease the EQE roll-off.
Highlights
Polaron-induced exciton quenching in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) can lead to external quantum efficiency (EQE) roll-off and device degradation
The area corresponding to singlet-triplet annihilation (STA), singlet-polaron annihilation (SPA), and triplet-polaron annihilation (TPA) gradually increases until these loss mechanisms start to dominate over reverse intersystem crossing (RISC), intersystem crossing (ISC), fluorescence, and phosphorescence pathways
It is clear that both STA and SPA can significantly contribute to the density-driven singlet loss mechanism under high bias conditions
Summary
Polaron-induced exciton quenching in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) can lead to external quantum efficiency (EQE) roll-off and device degradation. Despite the high IQEs, both phosphorescent[4,5] and TADF6–8 type emitters exhibit extensive EQE roll-off at high current densities, which is often required for high brightness This decrease in EQE is predominately attributed to exciton quenching through exciton–exciton and exciton–polaron interactions. It is suggested that the efficiency roll-off in phosphorescent OLEDs is predominately due to the long lifetime of triplets—usually in the order of microseconds to milliseconds This can lead to excessive accumulation of triplet excitons under steady-state OLED operations and eventually, density-driven loss mechanisms such as triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA)[9,10,11,12,13,14,15,16]. Strategies to reduce the effect of polaroninduced quenching and EQE roll-off have been discussed briefly
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