To
obtain high-efficiency solution-processed organic light-emitting
diodes (OLEDs), a hole transport material (HTM) capable of solution
processing with excellent charge transport properties is required.
In this study, a new vinyl polymer (PmCP) containing
hole-transporting 1,3-di(9H-carbazol-9-yl)benzene
(mCP) in the side chain was successfully synthesized via radical polymerization. PmCP showed good film-forming ability and thermal stability.
Moreover, PmCP has a higher triplet energy value and
hole mobility than poly(N-vinylcarbazole) (PVK) used
as a reference HTM, which can be applied as a hole transport layer
(HTL) in thermally activated delayed fluorescence (TADF) OLEDs, providing
green and blue emissions. PmCP-based solution-processable
TADF-OLEDs containing green- and blue-emitting layers were easily
fabricated without damaging the lower HTL while using ethyl acetate
as an orthogonal solvent. The corresponding OLEDs possess high external
quantum efficiencies of 29.60% and 11.00% for the green- and blue-emitting
devices, respectively. They show superior performances compared to
PVK-based devices used as a reference. It was confirmed that PmCP as a solution-processable HTM can replace PVK and is
universally applicable to both green- and blue-emitting devices.
Metal–halide perovskite nanocrystals (NCs) have emerged as suitable light‐emitting materials for light‐emitting diodes (LEDs) and other practical applications. However, LEDs with perovskite NCs undergo environment‐induced and ion‐migration‐induced structural degradation during operation; therefore, novel NC design concepts, such as hermetic sealing of the perovskite NCs, are required. Thus far, viable synthetic conditions to form a robust and hermetic semiconducting shell on perovskite NCs have been rarely reported for LED applications because of the difficulties in the delicate engineering of encapsulation techniques. Herein, a highly bright and durable deep‐blue perovskite LED (PeLED) formed by hermetically sealing perovskite NCs with epitaxial ZnS shells is reported. This shell protects the perovskite NCs from the environment, facilitates charge injection/transport, and effectively suppresses interparticle ion migration during the LED operation, resulting in exceptional brightness (2916 cd m−2) at 451 nm and a high external quantum efficiency of 1.32%. Furthermore, even in the unencapsulated state, the LED shows a long operational lifetime (T50) of 1192 s (≈20 min) in the air. These results demonstrate that the epitaxial and hermetic encapsulation of perovskite NCs is a powerful strategy for fabricating high‐performance deep‐blue‐emitting PeLEDs.
Recently, various hosts and emitters for solution-processable thermally activated delayed fluorescence organic lightemitting diodes (TADF-OLEDs) have been developed. However, a few studies have been conducted on hole transport materials (HTMs) with differentiated solubility characteristics for manufacturing multilayer OLEDs using a solution process. Here, three new hole transport (HT) styrene polymers, PICz, PPBCz, and PTPCz, were synthesized by radical polymerization. Each of the polymers exhibited increases in their highest occupied molecular orbital (HOMO) levels and better hole-transporting abilities than poly(9vinylcarbazole) (PVK) as a reference HT polymer. Furthermore, the three HT polymers exhibited different solubilities in toluene. Therefore, it was not possible to use a toluene solution to prepare the emitting layer (EML). To overcome this problem, ethyl acetate (EA), in which the three HT polymers are insoluble, was used as an orthogonal solvent to prepare an EML solution. In EAsolution-processed green-emitting TADF-OLEDs, the three HT-polymer-based devices displayed somewhat low turn-on voltages of 2.8 V and high external quantum efficiencies (EQEs) of >23%. These values are superior to those of a device with a PVK-HT layer. In addition, the devices manufactured with the EA solution showed high-performance reproducibility owing to the stable formation of each layer. In this study, we removed the HTM solubility constraint by dramatically changing the solvent for preparing the EML solution and provided an efficient strategy for the fabrication of OLED devices via solution processes in the future.
Light-emitting diodes (LEDs) are the rapidly developing core components of current display and lighting technology. Metal halide perovskite nanocrystals (MHP NCs) have recently been used as the deep-blue-light-emitting component in LEDs and are considered to have the greatest potential for growth in practical applications. However, the vulnerability of MHP NCs to the environment and the ion migration during the operation of LEDs pose formidable obstacles to the practical application of MHP NCs. Herein, we show that mixed-halide CsPb(Br1-xClx)3 NCs enclosed by epitaxially grown ZnS shells (CPBC/ZnS) are integral to ensuring a stable perovskite-based deep-blue-light-emitting diode (PeLED). We found that epitaxial ZnS shells protect the MHP NCs from the environment, and that the interparticle ion migration between MHP NCs could be effectively suppressed during LED operation, affording an exceptional external quantum efficiency (EQE) of 3.63% at an emission peak of 451 nm and a maximum luminance of 1687 cd m-2. Our results demonstrate that the epitaxial encapsulation of MHP NCs is a powerful strategy for the fabrication of high-efficiency, high-stability PeLEDs with a deep-blue emission.
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