Twisted Acceptor Core Molecular Design with Phenoxazine and Phenothiazine Donors Enabled Yellow Thermally Activated Delayed Fluorescent Emitters/Sensitizers for Long-Lifetime Solution-Processed Organic Light-Emitting Diodes Exceeding 31% External Quantum Efficiency

Abstract

Organic light-emitting diodes (OLEDs) with high external quantum efficiency and long operational lifetimes that have been solution-processed are still in their infancy. In this context, two new thermally activated delayed fluorescent (TADF) emitters, KCPOZ and KCPTZ, are designed using a new design strategy consisting of an interlocked unsymmetrical dual acceptor core for solution-processed yellow OLEDs. Unsymmetrical and twisted molecular structure aided twisted intramolecular charge transfer in their films. Narrow ΔE_ST in both the emitters enabled efficient triplet exciton population and fast reverse intersystem crossing to manufacture high-efficiency OLED devices. A doped (5% in CBP) OLED device based on KCPOZ showed the best performance between both. The 5.0 wt % KCPOZ-doped device exhibited a PE_max of 85.6 lm/W, CE_max of 95.2 cd/A, EQE_max of 31.5%, and L_max of 18,240 cd/m². Both emitters were also employed as sensitizers for TBRb, an orange TADF emitter, to improve the orange device performance. EQE_max increased from 5% to 20 and 18.0% when KCPOZ and KCPTZ concentrations climbed from 0 to 10%. At 100 cd/m², the KCPOZ-based device had an estimated half-lifetime of 19,844 h, while the KCPTZ-based device had a lifetime of 10,550 h. This work demonstrates using unconventional ways to design molecular core structures integrated with appropriate donors to enable high efficiency in the OLED device with a longer lifetime.