Sang-Mook Kim

Fabrication of an Efficient Light‐Scattering Functionalized Photoanode Using Periodically Aligned ZnO Hemisphere Crystals for Dye‐Sensitized Solar Cells

Since the pioneering work of O’Regan and Grätzel in 1991[1] numerous studies have investigated dye-sensitized solar cells (DSSCs) as an alternative next generation solar cell. This evolution has continued to progress, and solar light-to-electricity conversion efficiencies (power conversion efficiency, PCE) have now exceeded 11%, which was attained with a 12 μm thick TiO2 nanoparticulate photoanode.[2] DSSCs have recently attracted increasing attention as an ideal photovoltaic concept; the advantages of DSSCs are their low-cost, transparency, color rendition, eco-friendly process, biocompatibility and simplicity.[3,4] Generally, improvements in overall PCE have focused on increasing the photovoltage through the modification of the oxide layer, improving the photocurrent with new dye molecules, and increasing the stability by controlling the cell configurations.[4,5] A transparent mesoporous TiO2 nanoparticulate layer is a well-known photoanode material used in conventional DSSCs. However, the small size of TiO2 nanoparticles (diameter ∼ 20 nm) makes this layer transparent to visible light, and thus weakly light scattering due to the small particle size. As a result, a substantial amount of the incident light passes through the TiO2 nanoparticulate layer without being captured and utilized to produce photocurrent. Many studies have focused on capturing more light from the photoanode layer by using sub-micron poly-dispersed oxide particle aggregates, which act as effective scattering centers,[6,7] and/or by using gradient scattering layers consisting of TiO2 nano-particles with different radii along the light path.[8] Although the utilization of the large size aggregates within the photoanode film with a thickness of ∼9 μm and a cell area of

Increase of light extraction from GaN based light emitting diodes incorporating patterned structure by colloidal lithography

The light extraction efficiency of light emitting diodes (LEDs) was enhanced by incorporating nanoscale patterns inside the LED structure. A hole patterned p-GaN layer and a pillar patterned indium-tin-oxide (ITO) contact layer were fabricated by using colloidal lithography with size-tunable polystyrene spheres. It was found that the light output power (at 20mA) of the LEDs with the hole patterned p-GaN layer and the pillar patterned ITO contact layer were enhanced by 21% and 10%, respectively, compared with the conventional LED due to the increase of the extraction probability of the internally reflected photons through the patterns.

All‐Solution‐Processed Transparent Thin Film Transistor and Its Application to Liquid Crystals Driving

All-solution-processed transparent thin film transistors (TTFTs) are demonstrated with silver grid source/drain electrodes, which are fabricated by printing and subsequent silver nanoparticles solution coating, which allows continuous processing without using high vacuum systems. The silver grid electrode shows a reasonable transmittance in visible range, moderate electrical conductance and mechanical strength. The TTFTs are employed to drive liquid crystal cells and demonstrate a successful switching operation.

Light Extraction Enhancement of GaN-Based Light-Emitting Diodes Using Volcano-Shaped Patterned Sapphire Substrates

A new volcano-shaped patterned sapphire substrate (VPSS), which enhances the light extraction efficiency (LEE) of GaN-based light emitting diodes (LEDs), was presented. The Monte Carlo ray-tracing method shows that the VPSS with a crater slope angle of about 50° has the highest LEE. To compare the optical characteristics, 380 nm ultraviolet LEDs were grown on an optimized VPSS, hemispherical PSS (HPSS), and planar sapphire substrate by metal-organic chemical vapor deposition (MOCVD). As a result, the extraction efficiency of the LED grown on the optimized VPSS was estimated to be almost 2.8 times larger than that of the planar sapphire substrate and was enhanced 1.6 times compared with that of the LED grown on the HPSS.

Electroluminescence emission from light-emitting diode of p-ZnO/(InGaN/GaN) multiquantum well/n-GaN

We report on the fabrication and characteristics of light-emitting diodes (LEDs) which consist of antimony (Sb) doped p-ZnO, (InGaN/GaN) multiquantum well (MQW), and n-GaN. An electroluminescence (EL) emission at a wavelength of 468 nm is observed from the hybrid LEDs after thermal annealing at 750 °C, showing that Sb-doped p-ZnO can be used as a hole supplying layer in hybrid LEDs. Furthermore, the EL peaks are redshifted as the injection current is increased, indicating that the compressive strain in MQW layers is relaxed due to Sb-doped p-ZnO layer.

Effects of Patterned Sapphire Substrates on Piezoelectric Field in Blue-Emitting InGaN Multiple Quantum Wells

The strain and piezoelectric fields in InGaN blue light-emitting diodes on a GaN layer, which is grown on a planar sapphire substrate or patterned sapphire substrates (PSSs), such as a microsized PSS and a nanosized PSS (NPSS), are investigated by micro-Raman spectroscopy and electroreflectance (ER) spectroscopy. The obtained piezoelectric field in InGaN multiple quantum wells (QWs) grown on the planar substrate is 0.83 MV/cm, and it is 0.70 MV/cm for the case of the NPSS. These results are attributed to the fact that the GaN layers on the PSSs have a smaller residual strain compared to that on the planar sapphire, and thus, strain reduction in the GaN layer can reduce the piezoelectric field in the InGaN QWs grown on top of it.

Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes.

We describe the fabrication of corrugated inorganic oxide surface via direct single step conformal nanoimprinting to achieve enhanced light extraction in light emitting diodes (LEDs). Nanoscale zinc oxide (ZnO) and indium tin oxide (ITO) corrugated layer were created on a nonplanar GaN LED surface including metal electrode using ultraviolet (UV) assisted conformal nanoimprinting and subsequent inductively coupled plasma reactive ion etching (ICP-RIE) treatment. The total output powers of the surface corrugated LEDs increased by 45.6% for the patterned sapphire substrate LED and 41.9% for the flat c-plane substrate LED without any degradation of the electrical characteristics. The role of the nanoscale corrugations on the light extraction efficiency enhancement was examined using 3-dimensional finite-difference time-domain (FDTD) analysis. It was found that light scattering by subwavelength scale surface corrugation plays important role to redirect the trapped light into radiative modes. This straightforward inorganic oxide imprint method with inherent flexibility provides an efficient way to generate nanoscale surface textures for the production of high power LEDs and optoelectronic devices.

Epitaxial Growth of Crack-Free GaN on Patterned Si(111) Substrate

High-quality GaN epilayers were grown on patterned Si(111) substrates by metalorganic chemical vapor deposition. Crack-free GaN epilayers were obtained by growing the GaN on the plateau of the patterned Si(111) substrate. A high-temperature AlN buffer layer improved the crystalline quality of the GaN epilayer. When the growth temperature of the AlN buffer layer was 1000 °C, the GaN epitaxial layer showed a smooth surface and high crystalline quality. The dislocation density of the subsequently grown GaN layer was 2×109 cm-2, which is comparable to those of conventional GaN epilayers grown on sapphire substrate.

Nanopatterned aluminum nitride template for high efficiency light-emitting diodes

Nanopatterned aluminum nitride (NP-AlN) templates were used to enhance the light extraction efficiency of the light-emitting diodes (LEDs). Here, the NP-AlN interlayer between the sapphire substrate and GaN-based LED was used as an effective light outcoupling layer at the direction of bottom side and as a buffer layer for growth of GaN LEDs. The cross-sectional transmission electron microscopy (TEM) analysis showed that the formation of stacking faults and voids could help reduce the threading dislocations. Micro Raman spectra also revealed that the GaN-based epilayer grown on the NP-AlN template had smaller residual stress than that grown on a planar sapphire substrate. The normalized electroluminescence (EL) spectra at the top and bottom sides of device revealed that the enhancement of the bottom side emission of the LED with the NP-AlN interlayer was more notable than a planar sapphire substrate due to the graded-refractive-index (GRIN) effect of the NP-AlN.

Spontaneous nanoscale polymer solution patterning using solvent evaporation driven double-dewetting edge lithography

We develop an innovative solution processable edge lithography, which we call double-dewetting edge lithography (DDEL). The polymer solution spontaneously dewets the hydrophobic regions and covers only hydrophilic regions on a surface energy-engineered substrate, which is achieved by a combination of conventional photolithography and a subsequent hydrophobic treatment of the exposed areas. Then, the secondary dewetting occurs through a coffee stain effect during the solvent evaporation, leaving polymer edge patterns behind. The whole double-dewetting phenomenon is complete within 1 s. This technique is a fast, cost-effective and easy direct solution patterning method, which enables nanoscale polymer edge patterns to be produced from various micron-scale platforms including lines, angular and irregular shapes.