Recent advances in organic light-emitting diodes toward smart lighting and displays are reviewed in terms of materials, devices, fabrication and applications.

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Solution-processed non-doped blue and white OLEDs with record-high EQEs of 25.8% and 27.3% are realized with a hetero-donor tert-butyl substitution strategy.

Scheme 1. Synthetic routes of the TADF emitters of 1PXZ-BP, 2PXZ-BP, and 3PXZ-BP, respectively.

TADF) materials without using heavy metals have gained great attention as the third-generation organic light-emitting diode (OLED) emitters. [8][9][10][11][12][13][14][15][16][17][18][19][20] The use of pure organic TADF emitters allows almost 100% internal electroluminescence (EL) quantum efficiency by harvesting the triplet excitons (T 1 ) to the emissive singlet excited states (S 1 ) via reverse intersystem crossing (RISC). To date, high external quantum efficiency (EQE) exceeding 20% has been achieved for blue, green, and red TADF emitters in OLEDs. [8][9][10][11][12][13][14][15][16][17][18][19][20] However, achieving high-performance deep blue TADF OLEDs is still challenging, since most of them suffer from poor color purity, low brightness, severe EL efficiency roll-off, as well as short operational stability. [21][22][23][24][25] It remains an urgent demand to develop new blue TADF emitters that can simultaneously achieve high efficiency, high brightness, low driving voltage, long operational lifetime, and low Commission Internationale de l'Eclairage (CIE) y coordinate (CIEy) for various applications.Several strategies have been proposed to realize highly efficient blue TADF emitters. Generally, the TADF molecules are designed with a donor-acceptor-type rigid and symmetrical molecular configuration, exhibiting high optical bandgap (E g ), small S 1 -T 1 energy splitting (ΔE ST ), and high photoluminescence quantum yield (PLQY). [26][27][28] For narrowband emission, Thermally activated delayed fluorescence (TADF) provides great potential for the realization of efficient and stable organic light-emitting diodes (OLEDs). However, it is still challenging for blue TADF emitters to simultaneously achieve high efficiency, high brightness, and low Commission Internationale de l'Eclairage (CIE) y coordinate (CIEy) value. Here, the design and synthesis of two new benzonitrile-based TADF emitters (namely 2,6-di(9H-carbazol-9-yl)-3,5-bis(3,6-diphenyl-9H-carbazol-9-yl)benzonitrile (2PhCz2CzBn) and 2,6-di(9H-carbazol-9-yl)-3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzonitrile (2tCz2CzBn)) with a symmetrical and rigid heterodonor configuration are reported. The TADF OLEDs doped with both the emitters can achieve a high external quantum efficiency (EQE) over 20% and narrowband blue emission of 464 nm with a CIEy < 0.2. Moreover, the incorporation of a terminal tert-butyl group can weaken the intermolecular π-π stacking in the nondoped TADF emitter, and thus significantly suppress self-aggregation-caused emission quenching for enhanced delayed fluorescence. A peak EQE of 21.6% is realized in the 2tCz2CzBn-based nondoped device with an extremely low turn-on voltage of 2.7 V, high color stability, a high brightness over 20 000 cd m −2 , a narrow fullwidth at half-maximum of 70 nm, and CIE color coordinates of (0.167, 0.248).