Joong Mok Park

A New Architecture for Transparent Electrodes: Relieving the Trade‐Off Between Electrical Conductivity and Optical Transmittance

Transparent conducting electrodes with the combination of high optical transmission and good electrical conductivity are essential and desirable in solar energy harvesting and electric lighting devices including organic solar cells and light-emitting diodes (LEDs) as well as in their inorganic counterparts. Currently, indium tin oxide (ITO) coated glass is most often used because ITO has relatively high transparency to visible light and low sheet resistance for electrical current conduction. However, ITO is costly due to limited resources, is brittle, [ 1 ] and has poor chemical compatibility with the active organic materials. [ 2 ] These disadvantages have motivated the search for other conducting electrodes with similar or better optical and electrical properties. In recent research efforts, carbon nanotube networks, unpatterned thin metal fi lms, random silver metal nanowire meshes, graphene fi lms, and patterned metal nanowire grids have been evaluated as potential replacements for ITO electrodes. [ 3‐11 ] Although these alternative transparent electrode approaches do have the potential to replace ITO, they still suffer from the classic trade-off between the optical transmittance and electrical conductivity. Thicker layers offer higher conductivity, but this comes at the expense of optical transmittance, and vice versa. Here, we report a new architecture for transparent electrodes, which leads to quasi-elimination of this tradeoff. This architecture consists of high-aspect-ratio metallic ribbons with nanoscale thickness and microscale width, spaced at desired periodicities and held in place by a polymer matrix to provide a fl at top surface for fabrication of active layers in solar cells or LEDs. By design, the light path is only obstructed by the nanoscale thickness of the ribbons, thus decoupling the conductivity and transmittance properties from each other. Catrysse and Fan performed theoretical investigations on similar nanopatterned metallic structures, and their simulations indicate that such structures have excellent optical and electrical properties for potential use as transparent conductive electrodes. [ 12 ] Our experimental results show that the novel structure is very promising for such applications.

Soft holographic interference lithography microlens for enhanced organic light emitting diode light extraction

Very uniform 2 μm-pitch square microlens arrays (μLAs), embossed on the blank glass side of an indium-tin-oxide (ITO)-coated 1.1 mm-thick glass, are used to enhance light extraction from organic light-emitting diodes (OLEDs) by ~100%, significantly higher than enhancements reported previously. The array design and size relative to the OLED pixel size appear to be responsible for this enhancement. The arrays are fabricated by very economical soft lithography imprinting of a polydimethylsiloxane (PDMS) mold (itself obtained from a Ni master stamp that is generated from holographic interference lithography of a photoresist) on a UV-curable polyurethane drop placed on the glass. Green and blue OLEDs are then fabricated on the ITO to complete the device. When the μLA is ~15 × 15 mm(2), i.e., much larger than the ~3 × 3 mm(2) OLED pixel, the electroluminescence (EL) in the forward direction is enhanced by ~100%. Similarly, a 19 × 25 mm(2) μLA enhances the EL extracted from a 3 × 3 array of 2 × 2 mm(2) OLED pixels by 96%. Simulations that include the effects of absorption in the organic and ITO layers are in accordance with the experimental results and indicate that a thinner 0.7 mm thick glass would yield a ~140% enhancement.

Effects of Nanometer-Scale Photonic Crystal Structures on the Light Extraction From GaN Light-Emitting Diodes

This paper reports on the effect of nanometer-scale photonic crystal structures on the enhancement of the light extraction in GaN light-emitting diodes. Photonic crystals with hole or pillar-patterned structures with lattice constants of 460, 600, 750, and 920 nm are fabricated on indium-doped tin oxide (ITO) electrodes and/or p-GaN layers using laser holography and reactive ion etching. It is found that the light extraction efficiency depends strongly on the distance between the photonic crystal and the active layer, as well as the lattice constant for both structures. Photonic crystal light-emitting diodes (LEDs) with a lattice constant of 750 nm and hole depths of 260 nm in the ITO layer show an increase in light extraction of up to 32%, compared to conventional LEDs, without degradation in the electrical properties while a maximum enhancement of 26% is obtained from the device with a lattice constant of 460 nm and pillar heights of 60 nm on the p-GaN layer. The dependence of the extraction efficiency on the lattice constant is also calculated using a 3-D finite-difference time-domain method and compared with experimental results.

Metal-nanowall grating transparent electrodes: Achieving high optical transmittance at high incident angles with minimal diffraction

A novel architecture has been employed to fabricate transparent electrodes with high conductivity and high optical transmittance at high incident angles. Soft lithography is used to fabricate polymer grating patterns onto which thin metallic films are deposited. Etching removes excess metal leaving tall walls of metal. Polymer encapsulation of the structure both protects the metal and minimizes diffraction. Transmission is dependent upon the height of the walls and encapsulation and varies from 60% to 80% for structures with heights of 1400 nm to 300 nm. In encapsulated structures, very little distortion is visible (either parallel to or perpendicular to standing walls) even at viewing angles 60° from the normal. Diffraction is at characterized through measurement of intensity for zeroth through third order diffraction spots. Encapsulation is shown to significantly reduce diffraction. Measurements are supported by optical simulations.

Change of Fermi-surface topology in Bi2Sr2CaCu2O8+o with doping

We report the observation of a change in Fermi-surface topology of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} with doping. By collecting high-statistics angle-resolved photoemission spectroscopy data from moderately and highly overdoped samples and dividing the data by the Fermi function, we answer a long-standing question about the Fermi-surface shape of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} close to the ({pi},0) point. For moderately overdoped samples (T{sub c}=80 K) we find that both the bonding and antibonding sheets of the Fermi surface are hole like, but for a doping level corresponding to T{sub c}=55 K we find that the antibonding sheet becomes electron like. Similar observations of a topology change were observed in La{sub 2-x}Sr{sub x}CuO{sub 4} and in Bi{sub 2}Sr{sub 2}CuO{sub 6+{delta}}. On the other hand, whereas this critical doping value in single-layer materials corresponds to a T{sub c} near zero, it occurs at a smaller doping value in the double-layer case where T{sub c} is still quite high (the difference in doping levels is due to the bilayer splitting). This argues against a van Hove singularity scenario for cuprate superconductivity.

Optical and magneto-optical properties of AuMnSn

We have measured room-temperature magneto-optical properties of AuMnSn on a single-crystalline sample. The maximum polar Kerr rotation was predicted to be very large, about −0.7° at 1.2eV [L. Offernes, P. Ravindran, and A. Kjekshus, Appl. Phys. Lett. 82, 2862 (2003)]. We found the experimental maximum Kerr rotation and ellipticity were about three times smaller than predicted and appeared at energies about 0.6eV higher than predicted, which is possibly due to inaccurate handling of the theory based on the local spin-density approximation to density-function theory for the localized 4d and 5d orbitals in AuMnSn.

Reflectance anisotropy of Gd5Si2Ge2 and Tb5Si2.2Ge1.8

Reflectance difference (RD) spectra for the a–b plane of the single crystals of Gd5Si2Ge2 and b–c planes of Gd5Si2Ge2 and Tb5Si2.2Ge1.8 were obtained in the photon energy range of 1.5–5.5 eV. Several peaks were observed for these crystals in the measured spectrum range. Similar features were observed in the RD spectra for the b–c planes of Gd5Si2Ge2 and Tb5Si2.2Ge1.8, while different features were observed for the a–b plane and b–c plane of Gd5Si2Ge2. The RD spectra for the crystals arise not only from the surface anisotropy but also from the bulk anisotropy due to the monoclinic structure of the bulk crystal.

High aspect ratio nanoscale metallic structures as transparent electrodes

A novel technique based on the two polymer micro-transfer molding (2-P μTM) for fabricating one dimensional (1D) high aspect ratio nanoscale metallic structures is presented and experimental characterization is described. Glancing angle metal deposition and physical argon ion milling (etching) techniques were also employed in processing. The resulting metallic structures have high transmission (~80%) in the visible spectrum and have superior electrical conductivity (resistance from 2.4 -7.3 Ω) compared to standard indium-tin oxide (ITO) glass. Thus, the high aspect ratio metallic structures are a promising alternative with potentially superior performances to ITO glass as transparent electrodes for organic solar cells.