Solution processable carbazole-benzophenone derivatives as bipolar hosts enabling high-efficiency stable green TADF organic LEDs

Abstract

Solution-processable bipolar materials acting as hosts, soluble in common organic solvents, are key materials for thermally activated delayed fluorescence (TADF) emitters. Bipolar hosts are importantly anticipated to encourage solution-processable high-efficiency TADF organic light-emitting diodes (LEDs). Herein, we have successfully designed and synthesized solution-processable bipolar carbazole-benzophenone derivatives as host materials, i.e. 4,4′-di(3-phenylcarbazol-9-yl)benzophenone (BPBCzO) and 4,4′-di[3-[4-(carbazol-9-yl)phenyl] carbazol-9-y] benzophenone (BCzBCzO). These newly synthesized hosts exhibited high-triplet energies, balanced charge-transporting properties, suitable molecular orbital energy levels, good thermal stability, and good solubility, which are required to realize green TADF based organic LEDs. Initially, we fabricated two different types of TADF organic LEDs using these hosts and commercially available guest 4CzIPN as a green dopant and compared their device characteristics. A green TADF based organic LED employing BPBCzO host displayed excellent performance with a maximum external quantum efficiency (EQE_max), current efficacy (CE_max), and power efficacy (PE_max) as high as 23.2%, 70.7 cd A⁻¹, and 55.6 l m W⁻¹, respectively. In particular, over 90% of EQE was reserved (EQE of 21.3%) at the practical luminance of 1000 cd m⁻², which is advantageous for display technology. At last, green OLED was also fabricated with a cross-linkable hole transport material, i.e. 3,6-bis(4-vinylphenyl)-9-ethylcarbazole (VPEC) and realized PE_max of 63.6 l m W⁻¹ with EQE of 25.3%, which could be very effective for lighting and display devices. These excellent outcomes demonstrate the big potential of the carbazole-benzophenone derivatives as host materials for next-generation solution-processable display and lighting technologies.