TADF Host Engineering for Improved Pure Blue Matrix‐Free Hyperfluorescent OLEDs with Ultranarrow Emission

Abstract Hyperfluorescence has been proposed as a promising approach for simultaneously efficient and stable blue electroluminescence. Two‐component matrix‐free hyperfluorescence (MFHF) is particularly appealing to simplify device fabrication but is limited by the availability of suitable blue thermally activated delayed fluorescent (TADF) hosts. In this work, blue TADF materials are developed with high non‐doped OLED performance and apply them as hosts in MFHF devices. High maximum external quantum efficiencies (EQEs) of up to 23.2% are achieved from blue non‐doped TADF organic light emitting diodes (OLEDs), facilitating efficient MFHF devices with ultranarrow pure blue emission (EQE > 20%, CIE_y ≈ 0.15 and full‐width at half‐maximum ≤15 nm) upon incorporation of a narrowband terminal emitter with multi‐resonance spectral features. Steady‐state and transient photophysical studies support the high performance of non‐doped TADF and MFHF OLEDs. Our results indicate that optimizing TADF hosts for intrinsic non‐doped device efficiency is a viable direction to realize high‐performing pure blue MFHF OLEDs based on a simple host‐guest emissive system.