Hao Ou

Light‐emitting electrochemical cells (LECs) have attracted considerable attention owing to their unique luminescence mechanisms and device structures, making them suitable for solution processing. However, these cells encounter problems related to color purity, which plays an important role in efficient and high‐resolution displays. Here, two types of solution‐processed LECs are reported that exhibit ultrapure blue emissions at 467 and 452 nm with narrow emission full‐widths at half‐maximum (FWHM) values of 21 and 30 nm, corresponding to the Commission Internationale de l'éclairage (CIE) coordinates of (0.12, 0.15) and (0.16, 0.10), respectively. These values are obtained using the multi‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials, namely, ν‐DABNA and DABNA‐4TBC. The suppression of structural relaxation and vibronic coupling in these MR‐TADF materials results in high‐color‐purity emissions with a small FWHM. Host‐guest‐type LECs employing these molecules as guest emitters in a spin‐coated single active layer exhibit both ultrapure blue electroluminescence (EL) and solution processability, strongly supporting their use in building a sustainable society in the future.