display devices. Therefore, much efforts have been devoted in improving their performances and, as the result, better and more reliable materials, particularly the emitters, have been developed in both academic and industrial sectors. [1] Among the emissive materials, Ir(III) metal complexes have been recognized for their remarkable stability and efficient green and red luminescence and, nowadays, they have been employed as an integral component of commercially viable OLED devices. However, much less progresses have been made on the respective blue emitters. [2] It is due to the high emission gap than that of green and red counterparts, which unavoidably caused greater instability and inferior luminous efficiency during operation. This shortfall prompted more research on both the blue emissive transition-metal-based phosphors [3] and thermally activated delayed fluorescence (TADF) emitters [4] for future advancement of OLED technology.In general, the Ir(III) metal atom is capable to promote the facile intersystem crossing facilitated by spin-orbit coupling, allowing full utilization of both the electro-generated singlet and triplet excitons. [5] There are two possible class of Ir(III) complexes suitable for making the demanded blue phosphors. One involves functional cyclometalating chelates linked to N-donor fragment such as Homoleptic fac-substituted Ir(III) carbene complexes exhibit higher emission energy (in purple region) in comparison to their mer-counterparts, prohibiting them to be employed in fabrication of blue emissive organic light-emitting diode (OLED) devices. Now, the design of two distinctive CF 3 -functionalized purin-8-ylidene Ir(III) complexes, namely, m-and f-CF 3 and m-and f-PhCF 3 , from new carbene motifs, 9-(3-(tert-butyl)phenyl)-7-isopropyl-2-(trifluoromethyl)-7,9-dihydro-8H-purin-8-ylidene (A4) and 9-(3-(tert-butyl) phenyl)-7-methyl-6-phenyl-2-(trifluoromethyl)-7,9-dihydro-8H-purin-8-ylidene (B7), having notably stabilized lowest unoccupied molecular orbital energy levels is reported. Hence, the corresponding f-isomers f-CF 3 and f-PhCF 3 exhibit electroluminescence with peak max. at 478 and 495 nm, max. external quantum efficiencies (EQEs) of 10.4% and 12.8%, respectively. By using f-CF 3 as assistant dopant to convey its energy to terminal emitter t-DABNA and from f-PhCF 3 donor to 2TCzBN acceptor, two hyper-OLED devices are successfully fabricated, giving high max. EQE of 23.8%, full-width at half-maximum (FWHM) of 30 nm, and CIE x,y coordinate of (0.13, 0.14) for the acceptor t-DABNA, and max. EQE of 24.0%, FWHM of 28 nm, and CIE x,y of (0.11, 0.36) for the acceptor 2TCzBN, confirming the advantages of these purin-8-ylidene Ir(III) complexes.

Four isomeric, homoleptic iridium(III) metal complexes bearing 5-(trifluoromethyl)imidazo[4,5-b]pyridin-2-ylidene and 6-(trifluoromethyl)imidazo[4,5-b]pyridin-2-ylidene-based cyclometalating chelates are successfully synthesized. The meridional isomers can be converted to facial isomers through acid induced isomerization. The m-isomers display a relatively broadened and red-shifted emission, while f-isomers exhibit narrowed blue emission band, together with higher photoluminescent quantum yields and reduced radiative lifetime relative to the mer-counterparts. Maximum external quantum efficiencies of 13.5% and 22.8% are achieved for the electrophosphorescent devices based on f-tpb1 and m-tpb1 as dopant emitter together with CIE coordinates of (0.15, 0.23) and (0.22, 0.45), respectively. By using f-tpb1 as the sensitizing phosphor and t-DABNA as thermally activated delayed fluorescence (TADF) terminal emitter, hyperluminescent OLEDs are successfully fabricated, giving high efficiency of 29.6%, full width at half maximum (FWHM) of 30 nm, and CIE coordinates of (0.13, 0.11), confirming the efficient Förster resonance energy transfer (FRET) process.

We demonstrate a kind of perovskite/organic hybrid white electroluminescent device, where an ultrathin doping-free organic phosphorescent interlayer is embedded between a p-type hole transport layer and a ntype electron transport layer to give an organic p−i−n heterojunction unit, which is superimposed layer by layer onto a quasi-two-dimensional perovskite layer. The unique carrier transport character of the p-type hole transport layer leads to a broad carrier recombination region approaching the p−i−n heterojunction unit. As a result, pure-red emission from the perovskite layer and sky-blue emission from the organic p−i−n heterojunction were simultaneously achieved to generate white emission with a peak external quantum efficiency of 7.35%, Commission Internationale de L'Eclairage coordinates of (0.424, 0.363), and a low correlated color temperature of 2868 K. More importantly, excellent spectral stability and a greatly enhanced operating lifetime (10-fold longer than those of perovskite-only LEDs) are simultaneously achieved, providing a new path for the development of high-performance white LEDs.

High‐quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll‐off blue solution‐processed organic light‐emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T1 state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high‐performance blue OLED devices. The smaller T1 energy gap between host and guest allows the quenching of long‐lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet‐triplet and triplet‐triplet annihilations. Triplet‐energy‐mediated hosts with high enough T1 and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQEmax) of 20.8%, long operational lifetime (T50 of 398.3 h @ 500 cd m−2), and negligible efficiency roll‐off (EQE of 20.1% @ 1000 cd m−2) are achieved for bluish‐green TADF OLEDs. Additionally introducing a narrowband emission multiple‐resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQEmax of 23.1%, suppressed roll‐off and extended lifetime of 456.3 h are achieved for the sky‐blue sensitized OLEDs at the same brightness.

Sulfone-embedded heterocyclics are of great interest in organic light-emitting diodes (OLEDs), however, exploring highly efficient narrowband emitters based on sulfone-embedded heterocyclics remains challenging. Herein, five emitters with different sulfur valence state and molecular rigidity, namely tP, tCPD, 2tCPD, tPD and tPT, are thoroughly analysed. With restricted twisting of flexible peripheral phenyl by strengthening molecular rigidity, molecular emission spectra can be enormously narrowed. Further, introducing the sulfone group with bending vibration in lowfrequency region that suppresses high-frequency vibration, sharp narrow full-widths at half-maximum of 28 and 25 nm are achieved for 2tCPD and tPD, respectively. Maximum external quantum efficiencies of 22.0 % and 27.1 % are successfully realized for 2tCPDand tPD-based OLED devices. These results offer a novel design strategy for constructing narrowband emitters by introducing sulfone group into a rigid molecular framework.