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 highresolution 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 electrondonating 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 nearunity 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 LT 99 over 110 h at the initial brightness of 3000 cd m −2 .

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