Research on interfacial change and regulation of organic light-emitting diodes under thermal stress

The injection and balance of charge carriers in organic light-emitting diodes (OLEDs), as well as the contact at the organic/metal interface, are essential to OLED under thermal stress. We employed electron transporting layers (ETLs) with different glass transition temperature (T_g) to investigate the effect of thermal stress on the organic/metal interface and proposed a strategy of realizing highly thermally stable OLEDs of using 4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine:8-quinolin-olato lithium (B3PYMPM:Liq) bulk heterojunction as ETL. We found that the thermal stability of the organic/metal interface was strengthened with the increase of the T_g of the organic material. OLEDs with low-T_g ETL were destroyed after being heated at 100 °C, while the performance of the OLED with high-T_g ETL could be maintained. By combining the advantages of the good electron transport ability of Liq and high T_g of B3PYMPM, the OLED with B3PYMPM:20% Liq bulk heterojunction ETL obtained the best thermal stability, with an optimal current efficiency (CE) of 59.22 cd/A under 85 ℃ heating. The CE of the optimized OLED after heating at 100 ℃ was still 1.49% higher than its original performance. The strategy of employing the heterojunction ETL consisting of materials with good electron transporting properties and thermal stability is an effective way to improve the thermal stability of OLED.