Chul-Hyun Kim

Highly efficient uniform ZnO nanostructures for an electron transport layer of inverted organic solar cells

A highly uniform and predesigned ZnO nanostructure fabricated by single step direct nanoimprinting was used as the efficient electron transport layer (ETL) in inverted bulk heterojunction organic solar cells. Improved photovoltaic cell efficiency with long-term stability can be observed due to the large interface between the active layer and nanostructured ZnO ETL.

Mobility improvement of P3HT thin film by high-voltage electrostatic field-assisted crystallization

High-temperature annealing, assisted by the in-plane application of a high-voltage electric field, was used to form thin films of poly(3-hexylthiophene) (P3HT), boasting increased crystallite size and enhanced charge mobility. In-plane crystallite growth and surface morphology were characterized by atomic force microscopy. X-ray diffraction analyses showed that the application of an electric field decreased film thickness and increased the dimensions of the crystalline domains by 25.6% during subsequent high-temperature annealing. Crystalline domains were 13.4% larger, and exhibited 70% higher mobility, than those obtained with thermal annealing in the absence of an electric field.

Direct Nanoimprint of Metal Bilayer for Tunnable Metal Photonic Properties

Tunable metal optical properties were realized by directly nanoimprinting metal onto silver-coated poly(methyl methacrylate) (PMMA) bilayer glass substrates. Imprinting at elevated temperatures enabled the molding of a stamp pattern consisting of a hexagonal pillar array with a 265 nm diameter and 530 nm pitch size, which produced a corrugated-metal nanohole array. The transmittance of the sample imprinted at 200 °C and 50 bar was uniquely improved from below 20% to more than 30% in the visible–infrared region, and its reflectance was reduced by more than 65% compared to a reference sample. The optical properties of the investigated metal depended significantly on the imprint temperature and relatively less on the imprint pressure and deposited silver film thickness. The improved transmittance was obtained only for the direct-imprinted silver–PMMA and not when the silver was simply deposited onto the predefined PMMA pattern. The presented fabrication strategy enables the provision of accessible optical tunability as well as improved cost-labor effectiveness for metal nanostructuring.

Imprinted Pattern Profile-Dependent Optical Properties of Metal Nanostructures

As-imprinted right-edged pillar structures were reconfigured into a tapered sidewall profile by CHF3-based reactive ion etch. The transmittance spectra improved as the sidewall became more tapered for longer etch times. The effect was most distinctive (28.4%) in the transmittance trench zone at wavelengths from 460 to 470 nm owing to the reduced diffraction scattering loss. The transmittance enhancement for silver-coated corrugated nanostructures was even greater (57.0%) for a tapered sidewall structure, in close agreement with the predictions of simulations. The infrared transmittance was notably reduced compared with that in the UV–visible zone, suggesting the possible applications of the structures in heat-insulated windows.

Bilayer hybrid nanoimprinting method for fabricating embedded silver nanostructure arrays with enhanced photoluminescence

Abstract. A bilayer hybrid nanoimprinting (NI) method was developed for fabricating embedded metal nanopatterns with greater processability and improved reliability for enhanced photoluminescence (PL) in optoelectronic devices. Bilayer hybrid NI consists of the following: (a) spin-coating ultraviolet (UV) and thermally curable NI resists in sequence, (b) high-pressure thermal NI and UV exposure while maintaining the stamp in a pressed position, and (c) silver (Ag) deposition and lift-off using a thermal NI resist on the upper surface to create embedded Ag nanoarrays. Reference samples with no Ag nanopatterns and with protruding Ag dot-shaped nanopatterns were also fabricated for comparison. The transmittance and PL of all samples were measured. All samples containing Ag nanopatterns exhibited improved PL compared with reference samples with no Ag. For all pattern sizes, the samples with the embedded Ag nanoarrays exhibited the highest PL; the relative PL enhancements compared with samples with Ag dot-shaped nanoarrays were 32.2%, 36.1%, and 62.7% for pattern sizes of 150, 200, and 265 nm, respectively.