An Effective Approach toward Yellow‐to‐Orange Multi‐Resonance TADF Emitters by Integrating Strong Electron Donor into B/N‐Based Polycyclic Architecture: High Performance OLEDs with Nearly 40% EQE.

B/N‐based multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters and the corresponding narrow band emissive organic light‐emitting diodes (OLEDs) exhibit great potential for next‐generation high‐resolution displays. Nonetheless, designing MR‐TADF emitters with emission wavelength over 550 nm remains challenging. Herein, an effective approach toward yellow‐to‐orange MR‐TADF emitters by integrating a strong electron‐donating indolophenazine building block into the B/N‐doped polycyclic aromatic hydrocarbons is proposed. The investigation of photophysical properties reveals that the electron‐donating difference between the donor segments of MR framework has a dramatic influence on the luminescent features, including the emission wavelength and full‐width at half‐maximum (FWHM). These TADF emitters display excellent photophysical characteristics such as near‐unity photoluminescence quantum yields and almost 100% horizontal dipole ratio. As a result, yellow and orange OLEDs employing these emitters achieve state‐of‐the‐art device performances with an ultrahigh external quantum efficiency of up to nearly 40%, power efficiency of 163 lm W−1, and luminance close to 120 000 cd m−2, which set a record among MR‐TADF based OLEDs with emission peaks over 550 nm. More impressively, the fabricated device presents outstanding operational stability of LT99 over 110 h at the initial brightness of 3000 cd m−2. [ABSTRACT FROM AUTHOR]