Sungjun Kim

Enhanced Light Out‐Coupling of Organic Light‐Emitting Diodes: Spontaneously Formed Nanofacet‐Structured MgO as a Refractive Index Modulation Layer

Organic light-emitting diodes (OLEDs) have attracted attention owing to their potential applications in full color fl at panel displays, as a back-lighting source for liquid-crystal displays. in fl exible display devices, and in solid-state lighting. [ 1–3 ] Typical bottom-emitting OLEDs are composed of a glass substrate, a transparent indium-tin-oxide (ITO) anode, thin organic multilayers, and a refl ective metal cathode. [ 4 ] The internal quantum effi ciency and lifetime have seen dramatic improvements since reports of organic fl uorescent OLEDs, and devices using phosphorescent emitting materials are operating at nearly 100% internal quantum effi ciencies. [ 5 ] However, the majority of the light generated in the organic material is confi ned in the ITO anode and glass substrate due to the large difference in the refractive indices n between their layers ( n ITO = 1.9, n glass = 1.5). [ 2 ] As explained by classical ray optics theory (i.e., Snell’s law), this results in out-coupling effi ciencies ( η out ) – expressed as the ratio of surface emission to all emitted light – of only around 20%. [ 6,7 ] The remaining 80% of the photons are trapped in the organic and substrate layers. This low out-coupling effi ciency has become one of the main limitations to highly effi cient OLEDs. Hence, the greatest potential for a substantial increase in external quantum effi ciency and power effi ciency is to enhance the light out-coupling effi ciency of OLEDs. Many techniques, such as microlenses on the glass substrate, photonic crystals, high refractive index substrates, low-index grids, and low-index-silica aerogels, have been studied to enhance η out . [ 6 , 8–12 ] Although these methods can increase the η out , they have several limitations, such as a shifted output spectra, changes in the electrical properties, complex processing techniques, and high-cost fabrication procedures. [ 13 , 14 ]

Design of dielectric/metal/dielectric transparent electrodes for flexible electronics

Abstract. A flexible transparent electrode (FTE) is one of the most essential parts for the next generation of flexible optoelectronic devices, including solar cells, displays, and solid-state lighting devices. Although a lot of candidate materials for the FTE such as metallic nanowires, carbon nanotube, and graphene have been investigated, each material has fundamental limits as FTE applications, such as low transmittance (70% to 80%), high sheet resistance (>100  ohm/sq) and rough surface morphology. Dielectric/metal/dielectric (DMD) electrode structure is a promising candidate for next-generation flexible transparent electrodes. Compared with other transparent electrodes, DMD electrodes show best performance in terms of optical transparency, sheet resistance, and mechanical flexibility. In addition, it has been also reported that the device performances can be significantly enhanced by the microcavity effects with the DMD electrodes. We review the relevant principles and discusses recent progress in DMD electrodes.

Expression of Streptomyces peucetius genes for doxorubicin resistance and aklavinone 11-hydroxylase in Streptomyces galilaeus ATCC 31133 and production of a hybrid aclacinomycin.

The aklavinone 11-hydroxylase gene and two doxorubicin resistance genes cloned from Streptomyces peucetius subsp. caesius ATCC 27952 were introduced into doxorubicin-sensitive Streptomyces galilaeus ATCC 31133, an aclacinomycin producer. The doxorubicin resistance genes drrA and drrB endowed S. galilaeus with high-level resistance to doxorubicin, indicating that the resistance mechanism for doxorubicin might be different from that for aclacinomycin A. Transformation of S. galilaeus ATCC 31133 with plasmid pMC213 containing the aklavinone 11-hydroxylase gene (dnrF) resulted in the production of many red pigments. A new metabolite was purified, and the position of the newly introduced hydroxyl group was determined. This result indicated that the aklavinone 11-hydroxylase gene was stably expressed in S. galilaeus ATCC 31133 and that it gave rise to a hybrid aclacinomycin A which showed highly specific in vitro cytotoxicity against leukemia and melanoma cell lines.

Design of broadband transparent electrodes for flexible organic solar cells

Broadband transparent electrodes, Ta2O5/Ag/WO3−x, are successfully designed to enhance light absorption and carrier transport properties in organic solar cells (OSCs) as an alternative to ITO/PEDOT:PSS. Employing the optical constant matching layer, zero reflection conditions could be achieved over a broad range of wavelengths. Moreover, the non-stoichiometric WO3−x could induce a large density of gap states near the Fermi level via quick thermal deposition, acting as the transport path of carriers. Significantly improved current densities were achieved, increasing the power conversion efficiency from 2.1% to 2.9%, values which are comparable to conventionally fabricated devices on ITO/PEDOT:PSS.

Effect of Oxygen Plasma Treatment on Crystal Growth Mode at Pentacene/Ni Interface in Organic Thin-Film Transistors

We report how treatment of nickel (Ni) with O(2) plasma affects the polarity of Ni surface, crystallinity of pentacene film on the Ni, and electrical properties of pentacene organic thin-film transistors (OTFTs) that use Ni as source-drain electrodes. The polar component of surface energy in Ni surface increased from 8.1 to 43.3 mJ/m(2) after O(2)-plasma treatment for 10 s. From X-ray photoelectron spectra and secondary electron emission spectra, we found that NiO(x) was formed on the O(2)-plasma-treated Ni surface and the work function of O(2)-plasma-treated Ni was 0.85 eV higher than that of untreated Ni. X-ray diffraction and atomic force microscopy measurements showed that pentacene molecules are well aligned as a thin-film and grains grow much larger on O(2)-plasma-treated Ni than on untreated Ni. This change in the growth mode is attributed to the reduction of interaction energy between pentacene and Ni due to formation of oxide at the Ni/pentacene interface. Thus, O(2)-plasma treatment promoted the growth of well-ordered pentacene film and lowered both the hole injection barrier and the contact resistance between Ni and pentacene by forming NiO(x), enhancing the electrical property of bottom-contact OTFTs.

Phase-controllable copper oxides for an efficient anode interfacial layer in organic light-emitting diodes

We investigated phase-controllable copper oxide (CuOx) as a hole injection layer for improving the electrical and optical properties of organic light emitting diodes (OLEDs). The phase of CuOx is changed from CuO to Cu2O by elevating the deposition rate during thermal evaporation, and a non-stoichiometric film mixing with two phases occurs in the intermediate condition. In non-stoichiometric films, unbonded oxygen atoms with a large density of gap states are induced near the Fermi level, acting as a hole transport path. Thus, holes can be more easily injected into the highest occupied molecular orbital states of α-NPD, lowering the hole injection barrier significantly. Films of non-stoichiometric CuOx are inserted at the interface of α-NPD with an Ag electrode, serving as the hole injection layer. Significantly improved current densities and luminance were achieved, reducing the operation voltage at 1 mA cm−2 from 6.1 V to 3.2 V.

Design of red, green, blue transparent electrodes for flexible optical devices.

Controlling the wavelength of electrodes within a desirable region is important in most optoelectronic devices for enhancing their efficiencies. Here, we investigated a full-color flexible transparent electrode using a wavelength matching layer (WML). The WMLs were able to adjust the optical-phase thickness of the entire electrode by controlling refractive indices and were capable of producing desirable colors in the visible band from 470 to 610 nm. Electrodes with tungsten oxide (WO(3)) having a refractive index of 1.9 showed high transmittance (T = 90.5%) at 460 nm and low sheet resistance (R(s) = 11.08 Ω/sq), comparable with those of indium tin oxide (ITO, T = 86.4%, R(s) = 12 Ω/sq). The optimum structure of electrodes determined by optical simulation based on the characteristic matrix method agrees well with that based on the experimental method. Replacing the ITO electrode with the WO(3) electrode, the luminance of blue organic light-emitting diodes (λ = 460 nm) at 222 mA/cm(2) increased from 7020 to 7200 cd/m(2).

MgO nano-facet embedded silver-based dielectric/metal/dielectric transparent electrode

We replace Indium Tin Oxide (ITO) with an MgO nano-facet Embedded WO(3)/Ag/WO(3)(WAW) multilayer for electrodes of high efficiency OLEDs. WAW shows higher values for transmittance (93%) and conductivity (1.3×10(5) S/cm) than those of ITO. Moreover, WAW shows higher transmittance (92.5%) than that of ITO (86.4%) in the blue region (<500 nm). However, due to the large difference in refractive indices (n) of glass (n=1.55) and WO(3) (n=1.95), the incident light has a small critical angle (52°). Thus, the generated light is confined by the glass/WAW interface, resulting in low light outcoupling efficiency (~20%). This can be enhanced by using a nano-facet structured MgO (n=1.73) layer and a ZrO(2) (n=1.84) layer as a graded index layer. Using these optimized electrodes, ITO-free, OLEDs with various emission wavelengths have been produced. The luminance of OLEDs using MgO/ZrO(2)/WAW layers is enhanced by 24% compared to that of devices with ITO.

Eco-friendly graphene synthesis on Cu foil electroplated by reusing Cu etchants

Graphene film grown by chemical vapor deposition using Cu substrate is promising for industrial applications. After etching the Cu substrate, which is essential step in graphene transfer process, the etchant solution must be chemically treated to prevent water pollution. Here we investigated that a method of reusing Cu etchant used to synthesize graphene, the synthesis of graphene on the resulting reused Cu films (R-G), and the application of R-G to organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). The turn-on voltage of OLEDs based on the R-G electrode was 4.2 V, and the efficiencies of OPVs based on the R-G electrode were 5.9–5.95%, that are similar to or better than those of the indium-tin-oxide-based devices. These results suggest that the reusing of Cu foil by the electroplating method could reduce the cost of graphene synthesis, thus opening a wide range of applications in graphene electronics.

Indium-tin-oxide free transparent electrodes using a plasmon frequency conversion layer

Transparent electrodes to enhance external quantum efficiency (EQE) in optoelectronic devices are proposed based on the suppression of surface plasmons (SPs) at the metal–dielectric (or metal–organic) interface using a frequency conversion layer. Plasmonic absorption at metal-based electrodes causes severe optical losses in the planar stacks of optoelectronic devices. Even though Ag is suitable for transparent electrodes owing to its lowest absorption coefficient compared to other metals, the surface plasmon resonant frequency (SPRF) of Ag is located in the visible region (i.e., ωSP ∼ 3.9 eV, λSP = 500–550 nm). Thus, incident light is absorbed by surface plasmon resonance (SPR) at the interface between Ag and dielectric materials. These plasmonic resonances could be dramatically suppressed by adding a 2 nm-thick Al interlayer with resonance frequency out of the visible region (i.e., ωSP ∼ 15 eV, λSP = 250–300 nm), which results in an extreme enhancement of the optical transmittance of Ag-based electrodes from 68% to 91% at 470 nm. These approaches for highly transparent and conductive multilayer stacks are applicable to universal optoelectronics because they are straightforward, cost-effective and reliable even in large area fabrication.