Differences in Electron and Hole Injection and Auger Recombination between Red, Green, and Blue CdSe-Based Quantum Dot Light Emitting Devices.

Despite the significant progress that has been made in recent years in improving the performance of quantum dot light-emitting devices (QLEDs), the effect of charge imbalance and excess carriers on excitons in red (R) vs green (G) vs blue (B) QLEDs has not been compared or systematically studied. In this work we study the effect of changing the electron (e)/hole (h) supply ratio in the QDs emissive layer (EML) in CdSe-based R-, G-, and B-QLEDs with inverted structure in order to identify the type of excess carriers and investigate their effect on the electroluminescence performance of QLEDs of each color. Results show that in R-QLEDs, the e/h ratio in the EML is >1, whereas in G- and B-QLEDs, the e/h ratio is <1 with charge balance conditions being significantly worse in the case of B-QLEDs.

Changes in Hole and Electron Injection under Electrical Stress and the Rapid Electroluminescence Loss in Blue Quantum-Dot Light-Emitting Devices.

Blue quantum dot light-emitting devices (QLEDs) suffer from fast electroluminescence (EL) loss when under electrical bias. Here, it is identified that the fast EL loss in blue QLEDs is not due to a deterioration in the photoluminescence quantum yield of the quantum dots (QDs), contrary to what is commonly believed, but rather arises primarily from changes in charge injection overtime under the bias that leads to a deterioration in charge balance. Measurements on hole-only and electron-only devices show that hole injection into blue QDs increases over time whereas electron injection decreases.

Molecular-Additive-Assisted Tellurium Homogenization in ZnSeTe Quantum Dots.

Addition of aqueous hydrohalic acids during the synthesis of colloidal quantum dots (QDs) is widely employed to achieve high-quality QDs. However, this reliance on the use of aqueous solutions is incompatible with oxygen- and water-sensitive precursors such as those used in the synthesis of Te-alloyed ZnSe QDs. Herein, it is shown that this incompatibility leads to phase segregation into Te-rich and Te-poor regions, causing spectral broadening and luminescence peak shifting under high laser irradiation and applied electrical bias.

Influence of Encapsulation on the Efficiency and Positive Aging Behavior in Blue Quantum Dot Light-Emitting Devices.

Encapsulating blue quantum dot light-emitting devices (QLEDs) using an ultraviolet curable resin is known to lead to a significant increase in their efficiency. Some of this efficiency increase occurs immediately, whereas some of it proceeds over a period of time, typically over several tens of hours following the encapsulation, a behavior commonly referred to as positive aging. The root causes of this positive aging, especially in blue QLEDs, remain not well understood.

Inverted Solution-Processed Quantum Dot Light-Emitting Devices with Wide Band Gap Quantum Dot Interlayers.

Despite its benefits for facilitating device fabrication, utilization of a polymeric hole transport layer (HTL) in inverted quantum dots (QDs) light-emitting devices (IQLEDs) often leads to poor device performance. In this work, we find that the poor performance arises primarily from electron leakage, inefficient charge injection, and significant exciton quenching at the HTL interface in the inverted architecture and not due to solvent damage effects as is widely believed. We also find that using a layer of wider band gap QDs as an interlayer (IL) in between the HTL and the main QDs' emission material layer (EML) can facilitate hole injection, suppress electron leakage, and reduce exciton quenching, effectively mitigating the poor interface effects and resulting in high electroluminescence performance.

Significant Lifetime Enhancement in QLEDs by Reducing Interfacial Charge Accumulation via Fluorine Incorporation in the ZnO Electron Transport Layer.

ZnO nanoparticles are widely used for the electron transport layers (ETLs) of quantum dots light emitting devices (QLEDs). In this work we show that incorporating fluorine (F) into the ZnO ETL results in significant enhancement in device electroluminescence stability, leading to LT50 at 100 cd m of 2,370,000 h in red QLED, 47X longer than the control devices. X-ray photo-electron spectroscopy, time-of-flight secondary ion mass spectroscopy, photoluminescence and electrical measurements show that the F passivates oxygen vacancies and reduces electron traps in ZnO.

Significant enhancement in quantum-dot light emitting device stability a ZnO:polyethylenimine mixture in the electron transport layer.

The effect of adding polyethylenimine (PEI) into the ZnO electron transport layer (ETL) of inverted quantum dot (QD) light emitting devices (QDLEDs) to form a blended ZnO:PEI ETL instead of using it in a separate layer in a bilayer ZnO/PEI ETL is investigated. Results show that while both ZnO/PEI bilayer ETL and ZnO:PEI blended ETL can improve device efficiency by more than 50% compared to QDLEDs with only ZnO, the ZnO:PEI ETL significantly improves device stability, leading to more than 10 times longer device lifetime. Investigations using devices with marking luminescent layers, electron-only devices and delayed electroluminescence measurements show that the ZnO:PEI ETL leads to a deeper penetration of electrons into the hole transport layer (HTL) of the QDLEDs.

Modulating the luminance of organic light-emitting diodes via optical stimulation of a photochromic molecular monolayer at transparent oxide electrode.

Self-assembled monolayers (SAMs) deposited on bottom electrodes are commonly used to tune charge carrier injection or blocking in optoelectronic devices. Beside the enhancement of device performance, the fabrication of multifunctional devices in which the output can be modulated by multiple external stimuli remains a challenging target. In this work, we report the functionalization of an indium tin oxide (ITO) electrode with a SAM of a diarylethene derivative designed for optically control the electronic properties.

Optically switchable organic light-emitting transistors.

Organic light-emitting transistors are pivotal components for emerging opto- and nanoelectronics applications, such as logic circuitries and smart displays. Within this technology sector, the integration of multiple functionalities in a single electronic device remains the key challenge. Here we show optically switchable organic light-emitting transistors fabricated through a judicious combination of light-emitting semiconductors and photochromic molecules.

Modifying the Size of Ultrasound-Induced Liquid-Phase Exfoliated Graphene: From Nanosheets to Nanodots.

Ultrasound-induced liquid-phase exfoliation (UILPE) is an established method to produce single- (SLG) and few-layer (FLG) graphene nanosheets starting from graphite as a precursor. In this paper we investigate the effect of the ultrasonication power in the UILPE process carried out in either N-methyl-2-pyrrolidone (NMP) or ortho-dichlorobenzene (o-DCB). Our experimental results reveal that while the SLGs/FLGs concentration of the NMP dispersions is independent of the power of the ultrasonic bath during the UILPE process, in o-DCB it decreases as the ultrasonication power increases.