A deep blue thermally activated delayed fluorescence emitter: balance between charge transfer and color purity

An efficient blue organic light-emitting diode (OLED) device with high color purity is a challenging issue with technological relevance. Herein, we demonstrated the role of the donor substitution position in a thermally activated delayed fluorescence (TADF) emitter to achieve deep blue emission with improved color purity without reducing the device performance. A novel (4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(6-(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridin-3-yl)methanone (3BPy-pDTC) was synthesized, where two tert-butyl carbazolyl (DTC) donors linearly connected at the para position of the benzoyl pyridine (3BPy) acceptor core. The 3BPy-pDTC emitter shows a low Delta E-ST value of 0.19 eV with a subsequent overlap between holes and electrons which leads to a high photoluminescence quantum yield (PLQY) of 93%. Besides, this emitter exhibits narrow emission spectra with a full-width at half maximum (FWHM) of 58 nm. The OLED device using the 3BPy-pDTC dopant provided a high external quantum efficiency (EQE) of 25% with deep blue emission at lambda(max) of 458 nm. 3BPy-pDTC shows color purity in deep blue emission as compared to its meta substituted counterpart (3BPy-mDTC) due to the locked geometry via intramolecular H-bonding and weak excited state charge transfer (CT) interactions. The current study paves the way for developing deep-blue TADF emitters with high efficiency and color purity.