Intrinsic ambipolar carrier transport in beryllium complex semiconductors for high performance simple organic light-emitting diodes

Beryllium complexes, such as bis(10-hydroxybenzo[h]quinoline) beryllium complex (BeBq2), have been widely employed in organic light-emitting diodes (OLEDs) as electron transport materials due to their good electron mobilities. However, there is significant misunderstanding in their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), leading to the neglection of their bipolar transporting capabilities and incorrect interpretation of device mechanisms. In this study, the HOMO and LUMO of BeBq2 were determined to be −6.04 and −3.29 eV, which is about 0.5 eV lower than commonly cited values. It was further identified that its hole and electron mobilities are of the same order, indicating BeBq2 is a good ambipolar transporting material, which opens emergent opportunities for better device design. Simple OLEDs with BeBq2 serving as the hole transporter, electron transporter, and phosphorescent host were fabricated, which delivered a promising external quantum efficiency of 15% for deep red emission at 629 nm. Moreover, the device exhibited a LT50 lifetime of 164 h under an initial luminance of 5000 cd/m2, surpassing that of multilayered devices.