Carbazole-based thermally activated delayed fluorescent emitters for efficient pure blue organic light-emitting diodes

Despite the significant progress in organic light-emitting diodes (OLEDs), it is still challenging to produce blue OLEDs with sufficient color purity, stability, and efficiency for applications [[1], [2], [3]]. Hence, 9,9'-(4'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-2,6-diyl) bis(3,6-di-tert-butyl-9H-carbazole) (DtCz-TRZ) and 9,9'-(4'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-3,5-diyl) bis(3,6-di-tert-butyl-9H-carbazole) (mDtCz-TRZ) were designed and synthesized by introducing an additional benzene moiety to enlarge the distance of the donor and acceptor units. Thanks to the appropriate overlap of frontier molecular orbits (FMOs) and the extended molecular backbone that weakens the intramolecular charge transfer (ICT) process, structural relaxation and spectral redshift can be hindered, and efficient deep blue emission can be realized. Deep blue TADF-OLEDs achieved a maximum external quantum efficiency of 9.37% with an emission peak at 455 nm under a Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.18). To enhance the color purity, target compounds were employed as sensitizer and famous multiple resonances (MR)-TADF molecule v-DABNA as emitter, thermally activated sensitized fluorescence (TSF)-OLED based on mDtCz-TRZ shows EQEmax of 15.31% with electroluminescence (EL) spectrum peak at 473 nm, maximum luminance of close to 5000 cd m−2, FWHM of 20 nm, respectively. The two blue TADF materials have demonstrated superior performance in TADF- and TSF-OLEDs, validating their versatile applications.