Explaining the different efficiency behaviors of PHOLEDs with/without a hole injection barrier at the hole transport layer/emitter layer interface

• Efficiency behaviors of devices with/without hole injection barrier are investigated.
• Recombination of electrons and holes on the host leads to higher device efficiency.
• Host e–h recombination is associated with significant efficiency roll-off.
• This efficiency roll-off is mainly due to host–host TTA.

In this work we investigate the different efficiency behaviors of the devices with and without hole injection barrier, utilizing in our investigation the archetypical 4,4′-bis(carbazol-9-yl)biphenyl:Tris(2-phenylpyridine)iridium(III) host–guest PHOLEDs system. The results show that the recombination of electrons and holes on the host material generally leads to higher device efficiency in comparison to the case where recombination happens on the guest material. The results also show that in devices where a hole injection barrier between the HTL and the host material in the EML exists, the emission mechanism gradually changes from one based on host e–h recombination to one based on guest e–h recombination as the guest concentration is increased. When host e–h recombination is dominant, although it tends to produce higher device efficiency, host e–h recombination is generally also associated with significant efficiency roll-off; the latter arises from quenching of the host triplet excitons primarily due to host–host TTA. As the concentration of the guest molecules increases and the creation of host triplet excitons subsides (since most e–h recombination occurs on the guest) host–host TTA decreases, hence also the efficiency roll-off. In such case, quenching is mostly caused by polarons residing on guest sites. At optimum guest concentrations (∼8% Vol.), a balance between host e–h recombination and guest e–h recombination is reached, and thus also minimal TTA and Triplet-Polaron Quenching. On the other hand, in devices where hole injection barrier between the HTL and the host in the EML is insignificant, emission mechanism is always based on host e–h recombination irrespective of the guest concentration, and therefore have higher efficiency and the efficiency does not depend on guest concentration. The absence of the injection barrier in these devices results in a wider recombination zone, and hence a lower exciton concentration in general, which in turn reduces host–host TTA and thus lowers efficiency roll-off. In contrast, guest–guest TTA is not found to play a significant role in device efficiency behavior.

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