Extremely stable deep-blue organic light-emitting diodes employing diindolophenazine-based fluorophore with narrow-band emission and a shallow LUMO level

This work demonstrated that outstanding stability could be achieved in deep-blue OLEDs containing a diindolophenazine/diphenylamine derivative, which has a narrow emission bandwidth and a shallow LUMO energy level of −1.97 eV. [Display omitted] •Novel deep-blue emitters with a narrow FWHM of 17 nm are synthesized.•A long LT95 lifetime of over 300 h is achieved at a current density of 25 mA/cm2.•Shallow LUMO energy level of emitter is found to be critical for stable blue OLEDs. The development of narrow emission bandwidth materials is a highly active research area in the field of organic light-emitting diodes (OLEDs). In addition to B- and N-heterocyclic compounds, known as multiple-resonance (MR) emitters, chromophores based on multi-N-heterocycles can also achieve ultra-narrow-band blue emission. However, the reliability of devices containing the latter has been rarely investigated. In this study, we designed a rigid and electron-rich chromophore diindolo[3,2,1-de:3′,2′,1′-kl]phenazine (DIPz) by fusing two 9H-carbazole. This resulted in suppressed geometric distortion and vibration coupling during excitation. Furthermore, DIPz was modified with mesityl and 4-(tert-butyl)-N-phenylaniline groups, which finally evolved into a narrow-band blue emitter tDAmDIPz. The tDAmDIPz-based device showed an emission maximum at 458 nm with a narrow full-width at half-maximum of 17 nm, and a high maximum external quantum efficiency of 9.1%. Because the lowest unoccupied molecular orbital (LUMO) level of − 1.98 eV for tDAmDIPz was much shallower than − 2.47 eV for the anthracene-based host, electron trapping at emitters could be prevented, resulting in a very long LT95 lifetime of 338 h at a current density of 25 mA/cm2, which was higher than devices based on traditional BN-type MR emitters. This study suggested a general relationship between the operational lifetime and the LUMO energy difference between the emitter and the host, which opens a door to extremely stable blue OLEDs.