Operating lifetime recovery in organic light-emitting diodes having an azaaromatic hole-blocking/electron-transporting layer

Abstract

Azaaromatic compounds (AACs) are widely used in organic light-emitting diodes (OLEDs), especially as efficient electron transporters. Yet, the operating lifetime of OLEDs is always compromised when AACs are involved in anything more than electron transport (e.g., hole blocking). We show (i) the operating lifetime of OLEDs incorporating AACs as a hole-blocking/electron-transporting layer (HBETL) depends strongly on the charge-conducting ability and excited state energy of the light-emitting layer (LEL) materials and (ii) shifting the charge recombination zone away from the LEL∣HBETL interface deeper into the LEL can recover the lost lifetime. Thus, a pure red fluorescent OLED is demonstrated having 5.3 V drive voltage, 6.5% external quantum efficiency, 6.6 cd/A electroluminescent yield, and ∼125 000 h half-life, all at 20 mA/cm2. This device utilizes an AAC as HBETL followed by an aluminum triquinolate (Alq) ETL doped with Li metal. Alternatively, the lifetime recovery might be assigned to the presence of the n-dopant (Li) at the LEL∣HBETL interface (e.g., effected by diffusion of Li) because such presence stabilizes a wide range of OLEDs, e.g., those based on Alq mixed with polycyclic aromatic hydrocarbons and those based on 9,10-diarylanthracenes. Yet, this rationale is discarded in view of a strong deleterious interaction between the triazine-based AAC used in this work and n-dopants.