Achieving High Electroluminescence Efficiency and High Color Rendering Index for All‐Fluorescent White OLEDs Based on an Out‐of‐Phase Sensitizing System

Abstract Sensitizing conventional fluorescence (CF) dopants with thermally activated delayed fluorescence (TADF) materials has achieved considerable progress, by which the advantages of TADF materials and CF dopants can be fully harnessed. However, the usually used co‐phase configuration of CF dopant‐engaged sensitizing systems often encounters exciton loss due to Dexter energy transfer (DET). Herein, an effective out‐of‐phase configuration is proposed to sensitize CF dopants in the fabrication of white organic light‐emitting diodes (WOLEDs). Based on a new efficient sky‐blue TADF luminogen DCP‐BP‐DPAC which has an electroluminescence (EL) peak at 486 nm and an EL efficiency of 26.6%, a green TADF material BDMAC‐XT, and a red CF dopant DBP sensitized by BDMAC‐XT through an out‐of‐phase configuration without interlayer, efficient WOLEDs are successfully fabricated. By further adopting orange TBRB or 4CzTPNBu as intermediate sensitizers, more efficient energy transfer to DBP is achieved via Förster energy transfer. Through step‐by‐step energy transfer and elimination of excess DET process, high‐performance all‐fluorescent WOLEDs are achieved, providing excellent EL efficiencies over 23.0%, and highly stable white light with a high color rendering index of 87. The outstanding EL performance and high‐quality emission color demonstrate the great potential of the proposed out‐of‐phase design for sensitizing systems of WOLEDs.