High‐Performance Nondoped Blue Delayed Fluorescence Organic Light‐Emitting Diodes Featuring Low Driving Voltage and High Brightness

Abstract

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).

Efficient and Stable Deep‐Blue Fluorescent Organic Light‐Emitting Diodes Employing a Sensitizer with Fast Triplet Upconversion

“…To reduce ΔE 3LE-de,3CT , one strategy is to lower the 3 LE energy with a secondary donor. [11,14] However, this leads to the redshifted emission. Another strategy is to increase the CT energy to approach the high-lying 3 LE by reducing the number of Cz moieties, though this negates the advantages of the multiple Ds motif and thus correspondingly leads to reduced k RISC s though a blueshifted emission.…”

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

“…As a result, the 13-based fully solution-processed blue OLED realizes a maximum EQE of 22.6%, which is almost 20 times higher than that of the 14-based OLED. With the same emission core, Tang et al 59 also reported two 4CzBN-based blue TADF emitters 15 and 16 by introducing steric groups. Both emitters exhibit blue emission at 464 nm with a CIE y < 0.2 and an EQE over 20% in doped devices.…”

Emerging Biomedical Applications of Organic Light‐Emitting Diodes

“…Consequently, those materials possess larger bandgaps and thus devices based on them typically require higher drive voltages than those using fluorescent emitters. (Note that there is increasing research on using nondoped phosphorescent [ 131 ] and TADF emitters, [ 132,133 ] which may reduce drive voltages compared to host–guest systems. [ 133 ] ) Low drive voltages are important for biomedical applications, not only for reducing power consumption and for integration with electronic drivers, but also for reducing resistive heating of the environment.…”

“…(Note that there is increasing research on using nondoped phosphorescent [ 131 ] and TADF emitters, [ 132,133 ] which may reduce drive voltages compared to host–guest systems. [ 133 ] ) Low drive voltages are important for biomedical applications, not only for reducing power consumption and for integration with electronic drivers, but also for reducing resistive heating of the environment. Especially applications involving direct contact of the light source with live cells and in particular brain tissue need to retain the environment at physiological temperatures, which means that only a temperature increase up to around 1 °C is tolerated.…”