Sunho Kim

Polyethylenimine Ethoxylated-Mediated All-Solution-Processed High-Performance Flexible Inverted Quantum Dot-Light-Emitting Device

We report on an all-solution-processed fabrication of highly efficient green quantum dot-light-emitting diodes (QLEDs) with an inverted architecture, where an interfacial polymeric surface modifier of polyethylenimine ethoxylated (PEIE) is inserted between a quantum dot (QD) emitting layer (EML) and a hole transport layer (HTL), and a MoOx hole injection layer is solution deposited on top of the HTL. Among the inverted QLEDs with varied PEIE thicknesses, the device with an optimal PEIE thickness of 15.5 nm shows record maximum efficiency values of 65.3 cd/A in current efficiency and 15.6% in external quantum efficiency (EQE). All-solution-processed fabrication of inverted QLED is further implemented on a flexible platform by developing a high-performing transparent conducting composite film of ZnO nanoparticles-overcoated on Ag nanowires. The resulting flexible inverted device possesses 35.1 cd/A in current efficiency and 8.4% in EQE, which are also the highest efficiency values ever reported in flexible QLEDs.

Facile patterning using dry film photo-resists for flexible electronics: Ag nanowire networks and carbon nanotube networks

In this study, we explored the use of a dry film photo-resist (DFR) in the patterning of Ag nanowire and carbon nanotube (CNT) networks for the first time. With a simple lamination process, the DFR was uniformly coated on the Ag nanowire and CNT networks on poly(ethylene terephthalate) (PET) substrates without a post-thermal baking process. Furthermore, a Na2CO3-based developer enabled the networks to be patterned without loss of conductivity. Scanning electron microscopy images revealed that the Ag nanowire and CNT networks were successfully patterned with a pattern width up to ∼30 μm. The patterned Ag nanowire networks were confirmed to follow the percolation theory, showing a logarithmically linear increase in resistance as the pattern width decreased. The results indicated that there was no harmful effect on the Ag nanowire network during the patterning process. In addition, the mechanical reliability under bending fatigue was explored, which revealed a pattern-size dependent bending fatigue behavior, where the Ag nanowire networks with smaller pattern widths showed a higher increase in resistance during bending fatigue. This simple patterning method using DFRs combined with the roll-to-roll process is expected to lead the future patterning technology of flexible electronics.

Enhancement of electrical conductivity of silver nanowires-networked films via the addition of Cs-added TiO2

This study proposes a novel method of improving the electrical conductivity of silver nanowires (NWs)-networked films for the application of transparent conductive electrodes. We applied Cs-added TiO2 (TiO2:Cs) nanoparticles onto Ag NWs, which caused the NWs to be neatly welded together through local melting at the junctions, according to our transmission and scanning electron microscopy analyses. Systematic comparison of the sheet resistance of the samples reveals that these welded NWs yielded a significant improvement in conductivity. OLED devices, fabricated by using the NW film planarized via embedding the wires into PMMA, demonstrated device performance was comparable with the reference sample with indium tin oxide electrode.

Employment of gold-coated silver nanowires as transparent conductive electrode for organic light emitting diodes

This study proposes a simple method of Au coating on silver nanowires (Ag NWs) transparent conductive films as the anode of organic light emitting diodes (OLED) to increase the work function of the film and thus enhance hole transport. We carefully engineer the process conditions (pretreatment, solution concentrations, and coating number) of the coating using a diluted HAuCl4 solution on the Ag NWs film to minimize etching damage on Ag NWs accompanying the galvanic replacement reaction. Ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy show work function increase of Ag NWs upon Au coating. OLED devices based on Au-coated Ag NWs show a lower turn-on voltage and higher luminance, compared with pristine Ag NWs device. Although the Ag NWs device displays poor efficiencies in the low luminance range due to a high leakage, some of the Au-coated Ag NWs devices showed efficiencies higher than those of the ITO device in a high luminance.

Effects of alkali treatments on Ag nanowire transparent conductive films

In this study, we employ various alkali materials (alkali metals with different base strengths, and ammonia gas and solution) to improve the conductivity of silver nanowire (Ag NW)-networked films. The alkali treatment appears to remove the surface oxide and improve the conductivity. When applied with TiO2 nanoparticles, the treatment appears more effective as the alkalis gather around wire junctions and help them weld to each other via heat emitted from the reduction reaction. The ammonia solution treatment is found to be quick and aggressive, damaging the wires severely in the case of excessive treatment. On the other hand, the ammonia gas treatment seems much less aggressive and does not damage the wires even after a long exposure. The results of this study highlight the effectiveness of the alkali treatment in improving of the conductivity of Ag NW-networked transparent conductive films.