Improved Thermal Stability and Operational Lifetime of Blue Fluorescent Organic Light-Emitting Diodes by Using a Mixed-Electron Transporting Layer

Since the chemical and thermal stability of organic thin film layers in organic light-emitting diodes (OLEDs) highly influences the operational device lifetime, a rational advanced design of materials and device structures is quite necessary. Here, we report a significant improvement in the device thermal stability and operational lifetime of blue-fluorescent OLEDs by adopting a mixed electron transporting layer (mETL) of 4,7-diphenyl-1,10-phenanthroline (BPhen) and hydroxyquinolinolato-lithium (Liq). Compared to pristine BPhen film, Liq mixing improved thermal and morphological stabilities of the mETL by increasing the glass-transition temperature and inhibiting crystallization of the ETL, which directly leads to better device performances. Compared to the reference device using the pristine BPhen film as an ETL, the device with mETL containing 50% Liq maintains the device characteristics, with respect to the thermal stress up to 110 °C, which is a 60 °C increase in the thermal stability of the blue device by applying mETL. Accordingly, the operational lifetime of the device with mETL containing 50% Liq is substantially extended by 67 times, ensuring that this remarkable device lifetime enhancement is dominantly driven by the improved thermal and morphological stabilities of mETL.