Myoung Hoon Song

Workfunction-Tunable, N-Doped Reduced Graphene Transparent Electrodes for High-Performance Polymer Light-Emitting Diodes

Graphene is a promising candidate to complement brittle and expensive transparent conducting oxides. Nevertheless, previous research efforts have paid little attention to reduced graphene, which can be of great benefit due to low-cost solution processing without substrate transfer. Here we demonstrate workfunction-tunable, highly conductive, N-doped reduced graphene film, which is obtainable from the spin-casting of graphene oxide dispersion and can be successfully employed as a transparent cathode for high-performance polymer light-emitting diodes (PLEDs) as an alternative to fluorine-doped tin oxide (FTO). The sheet resistance of N-doped reduced graphene attained 300 Ω/□ at 80% transmittance, one of the lowest values ever reported from the reduction of graphene oxide films. The optimal doping of quaternary nitrogen and the effective removal of oxygen functionalities via sequential hydrazine treatment and thermal reduction accomplished the low resistance. The PLEDs employing N-doped reduced graphene cathodes exhibited a maximum electroluminescence efficiency higher than those of FTO-based devices (4.0 cd/A for FTO and 7.0 cd/A for N-doped graphene at 17,000 cd/m(2)). The reduced barrier for electron injection from a workfunction-tunable, N-doped reduced graphene cathode offered this remarkable device performance.

Combination of Titanium Oxide and a Conjugated Polyelectrolyte for High‐Performance Inverted‐Type Organic Optoelectronic Devices

Organic semiconductor-based optoelectronic devices, such as polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs), have attracted considerable attention because of their cost-effective, low-temperature, and solution-based fabrication over a large area; light weight; chemically tunable optoelectronic properties; and mechanical fl exibility. [ 1 , 2 ] Balanced charge injection and transport are a basic requirement for highly effi cient optoelectronic devices. Poor electron injection continues to be a serious problem for realizing highly effi cient PLEDs. Although there have been remarkable advances in conventional PSCs and PLEDs using low-work-function cathodes, such as Ca or Ba, [ 3 , 4 ]

Efficient Single‐Layer Polymer Light‐Emitting Diodes

Organic/polymeric light emitting diodes (LEDs) have been actively investigated in recent years for display and solid-state lighting due to their rapidly improving effi ciency and performance. [ 1 ] Amongst other critical aspects, the interfaces between the electrodes and emissive semiconductors play important roles in determining their operating characteristics and stability. [ 2 , 3 ] We and others have shown that ZnO can provide adequate electron injection into F8BT [ 4–10 ] Coating of ZnO with a thin layer of Cs 2 CO 3 has been shown to further improve the current effi ciency in hybrid PLEDs. [ 8 ] Ohmic hole injection into the deep HOMO level of F8BT ( ∼ 5.8 eV, chemical structure shown in Figure 1a ) is diffi cult using high work function metals or a layer of the conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). Recent work on hybrid PLEDs has demonstrated that MoO 3 is a potential candidate to achieve good hole injection into F8BT. [ 4 , 6 , 7 ] It has recently been shown in photoelectron spectroscopy study that the work-function of MoO 3 is as large as 6.9 eV. [ 11 ] This enables ohmic hole injection into materials with ionisation potentials signifi cantly deeper than that for F8BT. [ 12 ] MoO 3 has also been utilized in OLEDs [ 13 ] and in transistor [ 14 ] structures for improved hole injection. The hybrid inverted PLED structures, as shown in Figure 1b , have metal-oxides as charge transporting and injection layers. Most of the studies in this area are focused on the bottom n-type metal-oxides layer for example compact TiO 2 , [ 4 , 6 ] mesoporous TiO 2 , [ 4 , 9 , 15 ] ZnO [ 4 , 5 , 7 , 8 , 16 ]

High‐Performance Planar Perovskite Optoelectronic Devices: A Morphological and Interfacial Control by Polar Solvent Treatment

Highly efficient planar perovskite optoelectronic devices are realized by amine-based solvent treatment on compact TiO2 and by optimizing the morphology of the perovskite layers. Amine-based solvent treatment between the TiO2 and the perovskite layers enhances electron injection and extraction and reduces the recombination of photogenerated charges at the interface.

Amine-Based Polar Solvent Treatment for Highly Efficient Inverted Polymer Solar Cells

The interfacial dipolar polarization in inverted structure polymer solar cells, which arises spontaneously from the absorption of ethanolamine end groups, such as amine and hydroxyl groups on ripple-structure zinc oxide (ZnO-R), lowers the contact barrier for electron transport and extraction and leads to enhanced electron mobility, suppression of bimolecular recombination, reduction of the contact resistance and series resistance, and remarkable enhancement of the power conversion efficiency.

Highly efficient polymer light-emitting diodes using graphene oxide as a hole transport layer.

We present an investigation of polymer light-emitting diodes (PLEDs) with a solution-processable graphene oxide (GO) interlayer. The GO layer with a wide band gap blocks electron transport from an emissive polymer to an ITO anode while reducing the exciton quenching between the GO and the active layer in place of poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS). This GO interlayer maximizes hole-electron recombinations within the emissive layer, finally enhancing device performance and efficiency levels in PLEDs. It was found that the thickness of the GO layer is an important factor in device performance. PLEDs with a 4.3 nm thick GO interlayer are superior to both those with PEDOT:PSS layers as well as those with rGO, showing maximum luminance of 39 000 Cd/m(2), maximum luminous efficiencies of 19.1 Cd/A (at 6.8 V), and maximum power efficiency as high as 11.0 lm/W (at 4.4 V). This indicates that PLEDs with a GO layer show a 220% increase in their luminous efficiency and 280% increase in their power conversion efficiency compared to PLEDs with PEDOT:PSS.

Efficient hybrid organic-inorganic light emitting diodes with self-assembled dipole molecule deposited metal oxides

We investigate the effect of self-assembled dipole molecules (SADMs) on ZnO surface in hybrid organic-inorganic polymeric light-emitting diodes (HyPLEDs). Despite the SADM being extremely thin, the magnitude and orientation of SADM dipole moment effectively influenced the work function of the ZnO. As a consequence, the charge injection barrier between the conduction band of the ZnO and the lowest unoccupied molecular orbital of poly(9,9′-dioctylfluorene)-co-benzothiadiazole could be efficiently controlled resulting that electron injection efficiency is remarkably enhanced. The HyPLEDs modified with a negative dipolar SADM exhibited enhanced device performances, which correspond to approximately a fourfold compared to those of unmodified HyPLEDs.

Highly efficient inverted polymer light-emitting diodes using surface modifications of ZnO layer

Organic light-emitting diodes have been recently focused for flexible display and solid-state lighting applications and so much effort has been devoted to achieve highly efficient organic light-emitting diodes. Here, we improve the efficiency of inverted polymer light-emitting diodes by introducing a spontaneously formed ripple-shaped nanostructure of ZnO and applying an amine-based polar solvent treatment to the nanostructure of ZnO. The nanostructure of the ZnO layer improves the extraction of the waveguide modes inside the device structure, and a 2-ME+EA interlayer enhances the electron injection and hole blocking in addition to reducing exciton quenching between the polar-solvent-treated ZnO and the emissive layer. Therefore, our optimized inverted polymer light-emitting diodes have a luminous efficiency of 61.6 cd A(-1) and an external quantum efficiency of 17.8%, which are the highest efficiency values among polymer-based fluorescent light-emitting diodes that contain a single emissive layer.

Surface modification of metal oxide using ionic liquid molecules in hybrid organic–inorganic optoelectronic devices

We demonstrate enhanced device performance by surface modification of n-type ZnO using ionic liquid molecules (ILMs) in hybrid organic–inorganic polymeric light-emitting diodes (HyPLEDs) and solar cells (HySCs). Spontaneously aligned dipole polarization within the thin ILMs layer reduces the electron injection barrier, and significantly enhances the electron injection efficiency in HyPLEDs and the open-circuit voltage (VOC) in HySCs.

Improving the Stability and Performance of Perovskite Light-Emitting Diodes by Thermal Annealing Treatment

A perovskite LED with a perovskite film treated under optimum thermal annealing conditions exhibits a significantly enhanced long-term stability with full coverage of the green electroluminescence emission due to the highly uniform morphology of the perovskite film.