AbstractTo achieve high‐efficiency deep‐blue electroluminescence satisfying Rec.2020 standard blue gamut, two thermally activated delayed fluorescent (TADF) emitters are developed: 5‐(2,12‐di‐tert‐butyl‐5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐10,10‐diphenyl‐5,10‐dihydrodibenzo[b,e][1,4]azasiline (TDBA‐PAS) and 10‐(2,12‐di‐tert‐butyl‐5,9‐dioxa‐13b‐boranaphtho[3,2,1‐de]anthracen‐7‐yl)‐9,9‐diphenyl‐9,10‐dihydroacridine (TDBA‐DPAC). Inheriting from their parented organoboron multi‐resonance core, both emitters show very promising deep‐blue emissions with relatively narrow full width at half‐maximum (FWHM, ≈50 nm in solution), high photoluminescence quantum yield (up to 92.3%), and short emission lifetime (≤2.49 µs) with fast reverse intersystem crossing (>10⁶ s⁻¹) in doped films. More importantly, replacing the spiro‐centered sp³ C atom (TDBA‐DPAC) with the larger‐radius sp³ Si atom (TDBA‐PAS), enhanced conformational heterogeneities in bulky‐group‐shielded TADF molecules are observed in solution, doped film, and device. Consequently, OLEDs based on TDBA‐PAS retain high maximum external quantum efficiencies ≈20% with suppressed efficiency roll‐off and color index close to Rec.2020 blue gamut over a wide doping range of 10–50 wt%. This study highlights a new strategy to restrain spectral broadening and redshifting and efficiency roll‐off in the design of deep‐blue TADF emitters.