Insights into charge balance and its limitations in simplified phosphorescent organic light-emitting devices

Abstract

Simplified phosphorescent organic light-emitting device (PHOLED), which utilizes only two organic layers, showed record-high efficiency when first introduced. It is quite surprising that this device can have such high efficiency without the use of complex carrier and exciton confinement layers that are common in the state-of-the-art PHOLEDs nowadays. Therefore, it is important to understand how good charge balance is in simplified PHOLED and why. In this work, we study the effects of altering charge balance in simplified PHOLED through means of changing layer thickness in the hole transport layer (HTL) and electron transport layer (ETL) as well as intentionally doping hole and electron traps in the HTL and ETL, respectively, on device efficiency. The results show that when using high carrier mobility charge transport materials, changing layer thickness does not impact charge balance appreciably. On the other hand, introducing charge traps in a thin layer within the HTL or ETL can, in comparison, influence charge balance more significantly, and proves to be a more effective approach for studying the factors limiting charge balance in these devices. The results reveal that simplified PHOLEDs are generally hole-rich, and that the leakage of electrons to the counter electrode is also a major mechanism behind the poor charge balance and efficiency loss in these devices. In order to optimize charge balance in simplified PHOLED, it is important to reduce hole transport in the device so that e-h ratio can be brought closer to unity, as well as eliminate electron leakage. Finally, we show that by simply using an electron blocking HTL, the efficiency of the device can be enhanced by as much as 25%, representing the highest reported for simplified PHOLEDs.