Heeyoung Jung

Multifunctional Dendrimer Ligands for High-Efficiency, Solution-Processed Quantum Dot Light-Emitting Diodes

We present multifunctional dendrimer ligands that serve as the charge injection controlling layer as well as the adhesive layer at the interfaces between quantum dots (QDs) and the electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs). Specifically, we use primary amine-functionalized dendrimer ligands (e.g., a series of poly(amidoamine) dendrimers (PADs, also referred to PAMAM)) that bind to the surface of QDs by replacing the native ligands (oleic acids) and also to the surface of ZnO ETL. PAD ligands control the electron injection rate from ZnO ETL into QDs by altering the electronic energy levels of the surface of ZnO ETL and thereby improve the charge balance within QDs in devices, leading to the enhancement of the device efficiency. As an ultimate achievement, the device efficiency (peak external quantum efficiency) improves by a factor of 3 by replacing the native ligands (3.86%) with PAD ligands (11.36%). In addition, multibranched dendrimer ligands keep the QD emissive layer intact during subsequent solution processing, enabling us to accomplish solution-processed QLEDs. The approach and results in the present study emphasize the importance of controlling the ligands of QDs to enhance QLED performance and also offer simple yet effective chemical mean toward all-solution-processed QLEDs.

Semiconductor nanocrystals in fluorous liquids for the construction of light-emitting diodes

This communication reports a materials handling strategy based on fluorous materials chemistry using CdSe/CdS/CdZnS core–shell type semiconductor nanocrystals. When the crystals were treated with the semi-perfluoroalkanethiol ligands in the presence of iPr2EtN, the surface-modified nanocrystals (RF1-NC and RF2-NC) became soluble in the fluorous liquids. Solutions of RF1-NC and RF2-NC in HFE-7500 enabled the solution-casting of nanocrystalline films on top of a small-molecular hole-transporting layer and provided layered structures suitable for light-emitting diode fabrication.

Highly soluble fluorous alkyl ether-tagged imaging materials for the photo-patterning of organic light-emitting devices

A photolithographic patterning scheme for organic light-emitting diode (OLED) pixels was proposed, requiring highly soluble imaging materials in fluorous solvents. The attachment of perfluoroether chains onto a previously reported acid-cleavable resorcinarene and the addition of a semi-perfluoroalkoxy moiety to a naphthalimide backbone proved to be effective toward achieving imaging materials with enhanced solubility in fluorous solvents. These materials exhibited good film-forming and photo-imaging properties when layered onto a hole-transporting 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA) layer without seriously damaging it. With the proposed scheme and benign imaging materials, it was possible to construct OLEDs with patterned pixels as small as 50 μm.

Analysis of Interfacial Layer-Induced Open-Circuit Voltage Burn-In Loss in Polymer Solar Cells on the Basis of Electroluminescence and Impedance Spectroscopy

Stable and robust open-circuit voltage (VOC) is essential to achieve a long lifetime for polymer solar cells (PSCs). Here, we investigate the VOC burn-in loss mechanism on the basis of the analysis of electroluminescence quantum efficiency (EQEEL) and impedance measurements in amorphous PSCs, with an inverted structure having different electron transport layers (ETLs) of ZnO nanoparticles (NPs) and the sol-gel processed ZnO layer. We found that both charge recombination and energetic disorder account for a substantial proportion of the VOC burn-in loss. Moreover, varying the ETL significantly affected the degree of VOC burn-in loss, although relative contribution of these two factors remained constant. To accurately extract charge recombination-induced VOC loss, we applied a novel yet effective method that relates the EQEEL of PSCs to charge recombination-induced VOC loss. Additional analyses, including those focused on light intensity (Plight)-dependent VOC and density of states, will provide an inclusive perspective on the degradation mechanism of VOC and development of stable PSCs.

Unraveling the Origin of Operational Instability of Quantum Dot Based Light-Emitting Diodes

We investigate the operational instability of quantum dot (QD)-based light-emitting diodes (QLEDs). Spectroscopic analysis on the QD emissive layer within devices in chorus with the optoelectronic and electrical characteristics of devices discloses that the device efficiency of QLEDs under operation is indeed deteriorated by two main mechanisms. The first is the luminance efficiency drop of the QD emissive layer in the running devices owing to the accumulation of excess electrons in the QDs, which escalates the possibility of nonradiative Auger recombination processes in the QDs. The other is the electron leakage toward hole transport layers (HTLs) that accompanies irreversible physical damage to the HTL by creating nonradiative recombination centers. These processes are distinguishable in terms of the time scale and the reversibility, but both stem from a single origin, the discrepancy between electron versus hole injection rates into QDs. Based on experimental and calculation results, we propose mechanistic models for the operation of QLEDs in individual quantum dot levels and their degradation during operation and offer rational guidelines that promise the realization of high-performance QLEDs with proven operational stability.

Ligand-Asymmetric Janus Quantum Dots for Efficient Blue-Quantum Dot Light-Emitting Diodes

We present ligand-asymmetric Janus quantum dots (QDs) to improve the device performance of quantum dot light-emitting diodes (QLEDs). Specifically, we devise blue QLEDs incorporating blue QDs with asymmetrically modified ligands, in which the bottom ligand of QDs in contact with ZnO electron-transport layer serves as a robust adhesive layer and an effective electron-blocking layer and the top ligand ensures uniform deposition of organic hole transport layers with enhanced hole injection properties. Suppressed electron overflow by the bottom ligand and stimulated hole injection enabled by the top ligand contribute synergistically to boost the balance of charge injection in blue QDs and therefore the device performance of blue QLEDs. As an ultimate achievement, the blue QLED adopting ligand-asymmetric QDs displays 2-fold enhancement in peak external quantum efficiency (EQE = 3.23%) compared to the case of QDs with native ligands (oleic acid) (peak EQE = 1.49%). The present study demonstrates an integrated strategy to control over the charge injection properties into QDs via ligand engineering that enables enhancement of the device performance of blue QLEDs and thus promises successful realization of white light-emitting devices using QDs.

Effect of polystyrenesulphonate and electrolyte concentration for electrical properties of polypyrrole film on aluminium alloy using conductive AFM

Abstract The electrochemical synthesis of polystyrenesulphonate (PSS) doped polypyrrole (PPy) film onto aluminium alloy (AA 2024-T3) under galvanostatic conditions at current densities of 1 mA cm−2 was studied. In this study, conductive atomic force microscopy (C-AFM) was performed to investigate the electrical properties of PPy film on AA 2024-T3 depending on the concentration of PSS as dopant and nitric acid as electrolyte during electrochemical synthesis. The addition of HNO3 to aqueous electrolyte solution is found to allow the electrochemical synthesis of well adhering homogeneous PPy film in the presence of PSS on AA 2023-T3. The PPy film in the presence of nitric acid alone can be synthesised, although this film showed the poor quality of the electrical properties of PPy film. According to C-AFM, the study confirmed that the conductivity of PPy film is significantly increased with increasing the PSS, nitric acid concentration and electrochemical deposition time.