High‐Performance,Solution‐Processed,and Insulating‐Layer‐Free Light‐Emitting Diodes Based on Colloidal Quantum Dots

Quantum‐dot light‐emitting diodes (QLEDs) may combine superior properties of colloidal quantum dots (QDs) and advantages of solution‐based fabrication techniques to realize high‐performance, large‐area, and low‐cost electroluminescence devices. In the state‐of‐the‐art red QLED, an ultrathin insulating layer inserted between the QD layer and the oxide electron‐transporting layer (ETL) is crucial for both optimizing charge balance and preserving the QDs' emissive properties. However, this key insulating layer demands very accurate and precise control over thicknesses at sub‐10 nm level, causing substantial difficulties for industrial production. Here, it is reported that interfacial exciton quenching and charge balance can be independently controlled and optimized, leading to devices with efficiency and lifetime comparable to those of state‐of‐the‐art devices. Suppressing exciton quenching at the ETL–QD interface, which is identified as being obligatory for high‐performance devices, is achieved by adopting Zn_0.9Mg_0.1O nanocrystals, instead of ZnO nanocrystals, as ETLs. Optimizing charge balance is readily addressed by other device engineering approaches, such as controlling the oxide ETL/cathode interface and adjusting the thickness of the oxide ETL. These findings are extended to fabrication of high‐efficiency green QLEDs without ultrathin insulating layers. The work may rationalize the design and fabrication of high‐performance QLEDs without ultrathin insulating layers, representing a step forward to large‐scale production and commercialization.     

Recent Advances in Alternating Current‐Driven Organic Light‐Emitting Devices

Organic light‐emitting devices (OLEDs), typically operated with constant‐voltage or direct‐current (DC) power sources, are candidates for next‐generation solid‐state lighting and displays, as they are light, thin, inexpensive, and flexible. However, researchers have focused mainly on the device itself (e.g., development of novel materials, design of the device structure, and optical outcoupling engineering), and little attention has been paid to the driving mode. Recently, an alternative concept to DC‐driven OLEDs by directly driving devices using time‐dependent voltages or alternating current (AC) has been explored. Here, the effects of different device structures of AC‐driven OLEDs, for example, double‐insulation, single‐insulation, double‐injection, and tandem structure, on the device performance are systematically investigated. The formation of excitons and the dielectric layer, which are important to achieve high‐performance AC‐driven OLEDs, are carefully considered. The importance of gaining further understanding of the fundamental properties of AC‐driven OLEDs is then discussed, especially as they relate to device physics.     

Electrically Pumped White‐Light‐Emitting Diodes Based on Histidine‐Doped MoS2 Quantum Dots

MoS₂ quantum dots (QDs)‐based white‐light‐emitting diodes (QD‐WLEDs) are designed, fabricated, and demonstrated. The highly luminescent, histidine‐doped MoS₂ QDs synthesized by microwave induced fragmentation of 2D MoS₂ nanoflakes possess a wide distribution of available electronic states as inferred from the pronounced excitation‐wavelength‐dependent emission properties. Notably, the histidine‐doped MoS₂ QDs show a very strong emission intensity, which exceeds seven times of magnitude larger than that of pristine MoS₂ QDs. The strongly enhanced emission is mainly attributed to nitrogen acceptor bound excitons and passivation of defects by histidine‐doping, which can enhance the radiative recombination drastically. The enabled electroluminescence (EL) spectra of the QD‐WLEDs with the main peak around 500 nm are found to be consistent with the photoluminescence spectra of the histidine‐doped MoS₂ QDs. The enhanced intensity of EL spectra with the current increase shows the stability of histidine‐doped MoS₂ based QD‐WLEDs. The typical EL spectrum of the novel QD‐WLEDs has a Commission Internationale de l'Eclairage chromaticity coordinate of (0.30, 0.36) exhibiting an intrinsic broadband white‐light emission. The unprecedented and low‐toxicity QD‐WLEDs based on a single light‐emitting material can serve as an excellent alternative for using transition metal dichalcogenides QDs as next generation optoelectronic devices.     

Air‐Stable and High‐Performance Solution‐Processed Organic Light‐Emitting Devices Based on Hydrophobic Polymeric Ionic Liquid Carrier‐Injection Layers

A lot of research, mostly using electron‐injection layers (EILs) composed of alkali‐metal compounds has been reported with a view to increase the efficiency of solution‐processed organic light‐emitting devices (OLEDs). However, these materials have intractable properties, such as a strong affinity for moisture, which cause the degradation of OLEDs. Consequently, optimal EIL materials should exhibit high electron‐injection efficiency as well as be stable in air. In this study, polymer light‐emitting devices (PLEDs) based on the commonly used yellow‐fluorescence‐emitting polymer F8BT, which utilize poly(diallyldimethylammonium)‐based polymeric ionic liquids, are experimentally and analytically investigated. As a result, the optimized PLED employing an EIL comprising poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (poly(DDA)TFSI), which is expected to display good moisture resistance because of water repellency of fluorocarbon groups, exhibits excellent storage stability in air and electroluminescence performance with a low turn‐on voltage of 2.01 V, maximum external quantum efficiency of 9.00%, current efficiency of 30.1 cd A^?1, and power efficiency of 32.4 lm W^?1. The devices with poly(DDA)TFSI show one of the highest efficiencies as compared to the reported standard PLEDs. Moreover, poly(DDA)TFSI is applied as a hole‐injection layer (HIL). The optimized PLED using poly(DDA)TFSI as the HIL exhibits performances comparable to those of a device that uses a conventional poly(3,4‐ethylenedioxy‐thiophene):poly(4‐styrenesulfonate) HIL.