Hong-Kyu Seo

Synergetic electrode architecture for efficient graphene-based flexible organic light-emitting diodes

Graphene-based organic light-emitting diodes (OLEDs) have recently emerged as a key element essential in next-generation displays and lighting, mainly due to their promise for highly flexible light sources. However, their efficiency has been, at best, similar to that of conventional, indium tin oxide-based counterparts. We here propose an ideal electrode structure based on a synergetic interplay of high-index TiO2 layers and low-index hole-injection layers sandwiching graphene electrodes, which results in an ideal situation where enhancement by cavity resonance is maximized yet loss to surface plasmon polariton is mitigated. The proposed approach leads to OLEDs exhibiting ultrahigh external quantum efficiency of 40.8 and 62.1% (64.7 and 103% with a half-ball lens) for single- and multi-junction devices, respectively. The OLEDs made on plastics with those electrodes are repeatedly bendable at a radius of 2.3 mm, partly due to the TiO2 layers withstanding flexural strain up to 4% via crack-deflection toughening.

N-Doped Graphene Field-Effect Transistors with Enhanced Electron Mobility and Air-stability

Although graphene can be easily p-doped by various adsorbates, developing stable n-doped graphene that is very useful for practical device applications is a difficult challenge. We investigated the doping effect of solution-processed (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) on chemical-vapor-deposited (CVD) graphene. Strong n-type doping is confirmed by Raman spectroscopy and the electrical transport characteristics of graphene field-effect transistors. The strong n-type doping effect shifts the Dirac point to around -140 V. Appropriate annealing at a low temperature of 80 ºC enables an enhanced electron mobility of 1150 cm(2) V(-1) s(-1). The work function and its uniformity on a large scale (1.2 mm × 1.2 mm) of the doped surface are evaluated using ultraviolet photoelectron spectroscopy and Kelvin probe mapping. Stable electrical properties are observed in a device aged in air for more than one month.

Efficient Flexible Organic/Inorganic Hybrid Perovskite Light‐Emitting Diodes Based on Graphene Anode

Highly efficient organic/inorganic hybrid perovskite light-emitting diodes (PeLEDs) based on graphene anode are developed for the first time. Chemically inert graphene avoids quenching of excitons by diffused metal atom species from indium tin oxide. The flexible PeLEDs with graphene anode on plastic substrate show good bending stability; they provide an alternative and reliable flexible electrode for highly efficient flexible PeLEDs.

Rapid Fabrication of Designable Large-Scale Aligned Graphene Nanoribbons by Electro-hydrodynamic Nanowire Lithography

A new technique, electro-hydrodynamic nanowire (e-NW) lithography , is demonstrated for the rapid, inexpensive, and efficient fabrication of graphene nanorib bons (GNRs) on a large scale while simultaneously controlling the location and alignment of the GNRs. A series of interesting GNR architectures, including parallel lines, grids, ladders, and stars are produced. A sub-10-nm-wide GNR is obtained to fabricate field-effect transistors that show a room-temperature on/off current ratio of ca. 70.

Individually Position‐Addressable Metal‐Nanofiber Electrodes for Large‐Area Electronics

A individually position-addressable large-scale-aligned Cu nanofiber (NF) array is fabricated using electro-hydrodynamic nanowire printing. The printed single-stranded Cu NF has a diameter of about 710 nm and resistivity of 14.1 μΩ cm and is effectively used as source/drain nanoelectrode in pentacene transistors, which show a 25-fold increased hole mobility than that of a device with Cu thin-film electrodes.

Versatile p-Type Chemical Doping to Achieve Ideal Flexible Graphene Electrodes.

We report effective solution-processed chemical p-type doping of graphene using trifluoromethanesulfonic acid (CF3 SO3 H, TFMS), that can provide essential requirements to approach an ideal flexible graphene anode for practical applications: i) high optical transmittance, ii) low sheet resistance (70 % decrease), iii) high work function (0.83 eV increase), iv) smooth surface, and iv) air-stability at the same time. The TFMS-doped graphene formed nearly ohmic contact with a conventional organic hole transporting layer, and a green phosphorescent organic light-emitting diode with the TFMS-doped graphene anode showed lower operating voltage, and higher device efficiencies (104.1 cd A(-1) , 80.7 lm W(-1) ) than those with conventional ITO (84.8 cd A(-1) , 73.8 lm W(-1) ).

Controllable n‐Type Doping on CVD‐Grown Single‐ and Double‐Layer Graphene Mixture

n-Type doping of mixed single- and double-layer graphene grown by chemical vapor deposition (CVD) using decamethyl-cobaltocene reveals a local-quasilinear relationship between the work function and the logarithm of the dopant solution concentration. The relationship that arises from bandgap opening is deduced by comparing the relationship between the two factors for single- or double-layer graphene. This work has extensive applicability and practical significance in doping CVD-grown graphene.

Laminated Graphene Films for Flexible Transparent Thin Film Encapsulation

We introduce a simple, inexpensive, and large-area flexible transparent lamination encapsulation method that uses graphene films with polydimethylsiloxane (PDMS) buffer on polyethylene terephthalate (PET) substrate. The number of stacked graphene layers (nG) was increased from 2 to 6, and 6-layered graphene-encapsulation showed high impermeability to moisture and air. The graphene-encapsulated polymer light emitting diodes (PLEDs) had stable operating characteristics, and the operational lifetime of encapsulated PLEDs increased as nG increased. Calcium oxidation test data confirmed the improved impermeability of graphene-encapsulation with increased nG. As a practical application, we demonstrated large-area flexible organic light emitting diodes (FOLEDs) and transparent FOLEDs that were encapsulated by our polymer/graphene encapsulant.

Flexible transparent electrodes for organic light-emitting diodes

The use of flexible organic light-emitting diodes (OLEDs) for the next-generation displays and solid-state lightings has been considered, but the widely used transparent conducting electrode (TCE), indium–tin-oxide (ITO), should be replaced by flexible electrodes due to its brittleness and increasing cost. Therefore, many kinds of alternative TCEs have been increasingly studied. In this paper, the properties and applications of the candidate transparent flexible electrodes classified into four categories (conducting polymer, silver nanowire, carbon nanotube and graphene) are described. This paper finally suggests how to develop alternative TCEs for replacing the conventional ITO electrode.

Flexible organic light-emitting diodes for solid-state lighting

Abstract. Flexible organic light-emitting diodes (OLEDs) are candidates for next-generation solid-state lighting because they have merits such as low driving voltage, various color tuning, designable form, and large-area light emission. Although OLEDs’ efficiency, luminance, and lifetime have been improved enough to be commercialized, they are still inflexible despite being based on organic materials. To achieve efficient and reliable flexible OLEDs for solid-state lighting, flexible substrates for OLEDs should be developed. For this purpose, progress must be made in developing good flexible substrates, electrode materials, and encapsulation techniques compatible with these flexible substrates. Here, we review and discuss progress made in these three technologies for solid-state lighting using flexible OLEDs. Addressing the technical challenges associated with the development of high performing flexible substrates, electrode materials compatible with these substrates and good encapsulation techniques would lead to efficient and reliable flexible OLEDs and make flexible solid-state lighting commercially feasible.