Enhanced Efficiency in Green PhOLEDs Using a Simplified Three-Layer Architecture with Bipolar Carbazole–Quinazolinone Hosts

With an aim to provide a balanced charge flux for enhanced PhOLED device efficiency, two novel bipolar host materials, 3-CBZ-QZ and 9-CBZ-QZ, incorporating carbazole and quinazolinone as the electron donor and acceptor, respectively, have been synthesized, characterized, and used as efficient host materials for phosphorescent devices. Furthermore, their photophysical, thermal, and electrochemical properties have been investigated to shed the importance on the structure–performance relationship. 3-CBZ-QZ and 9-CBZ-QZ have high triplet energies of 3.09 and 2.54 eV, respectively, which make them suitable host materials for green emitters in the emissive layer (EML). DFT studies reveal high charge mobilities for both synthesized molecules based on electron and hole reorganization energy calculations. This balanced charge-carrying capacity prompted us to fabricate simple three-layer devices with a single hole and electron transport layer across the EML. The 3-CBZ-QZ-based device resulted in a superior electroluminescent (EL) performance with a turn-on voltage of 2.6 V, a maximum luminance of 77973 cd/m2, a maximum power efficiency of 71.0 Im/W, and an EQE of 18.1%. Although devices with the 9-CBZ-QZ host demonstrated a lower EQE of 16.0%, it showed a higher luminance value of 91111 cd/m2 than 3-CBZ-QZ with a significantly lower efficiency roll-off at a higher practical luminance of 100 cd/m2. The EL characteristics significantly varied with the positional changes in the donor–acceptor linkage. Moreover, these recorded performances demonstrate the promising advantage of the carbazole–quinoxaline-based compounds in developing host materials for realizing efficient PhOLEDs.