Enhanced Efficiency\nin Green PhOLEDs Using a Simplified\nThree-Layer Architecture with Bipolar Carbazole–Quinazolinone\nHosts

With an aim to provide a balanced charge flux for enhanced\nPhOLED\ndevice efficiency, two novel bipolar host materials, <b>3-CBZ-QZ</b> and <b>9-CBZ-QZ</b>, incorporating carbazole and quinazolinone\nas the electron donor and acceptor, respectively, have been synthesized,\ncharacterized, and used as efficient host materials for phosphorescent\ndevices. Furthermore, their photophysical, thermal, and electrochemical\nproperties have been investigated to shed the importance on the structure–performance\nrelationship. <b>3-CBZ-QZ</b> and <b>9-CBZ-QZ</b> have\nhigh triplet energies of 3.09 and 2.54 eV, respectively, which make\nthem suitable host materials for green emitters in the emissive layer\n(EML). DFT studies reveal high charge mobilities for both synthesized\nmolecules based on electron and hole reorganization energy calculations.\nThis balanced charge-carrying capacity prompted us to fabricate simple\nthree-layer devices with a single hole and electron transport layer\nacross the EML. The <b>3-CBZ-QZ</b>-based device resulted in\na superior electroluminescent (EL) performance with a turn-on voltage\nof 2.6 V, a maximum luminance of 77973 cd/m<sup>2</sup>, a maximum\npower efficiency of 71.0 Im/W, and an EQE of 18.1%. Although devices\nwith the <b>9-CBZ-QZ</b> host demonstrated a lower EQE of 16.0%,\nit showed a higher luminance value of 91111 cd/m<sup>2</sup> than <b>3-CBZ-QZ</b> with a significantly lower efficiency roll-off at\na higher practical luminance of 100 cd/m<sup>2</sup>. The EL characteristics\nsignificantly varied with the positional changes in the donor–acceptor\nlinkage. Moreover, these recorded performances demonstrate the promising\nadvantage of the carbazole–quinoxaline-based compounds in developing\nhost materials for realizing efficient PhOLEDs.