Dae-Geun Choi

Ultraviolet nanoimprinted polymer nanostructure for organic light emitting diode application

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

Solution-processable polymer solar cells from a poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acidmethyl ester concentration graded bilayers

Polymer photovoltaic (PV) device prepared with a vertical phase separation has intensified the research on the effectiveness of the concentration graded active layer. In this paper, a polymer PV device with a poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acidmethyl ester (P3HT/PCBM) bilayers active film with a concentration gradient has been fabricated via solution process. The concentration variation has been confirmed by the Auger spectroscopy. The devices showed an enhanced photocurrent density and power conversion efficiency compared to those of the bulk heterojunction PV prepared under the same fabrication condition.

Superamphiphobic surface by nanotransfer molding and isotropic etching.

We present a novel method of fabricating superhydrophobic and superoleophobic surfaces with nanoscale reentrant curvature by nanotransfer molding and controlled wet etching of the facile undercut. This method produces completely ordered re-entrant nanostructures and prevents capillary-induced bundling effects. The mushroom-like, re-entrant, overhanging structure demonstrates superhydrophobic and superoleophobic characteristics, as tested by water droplet bouncing and contact angle measurements, and has high transparency on a flexible substrate. Widespread use as self-cleaning surfaces is expected in the near future.

High‐Durable AgNi Nanomesh Film for a Transparent Conducting Electrode

Uniform metal nanomesh structures are promising candidates that may replace of indium-tin oxide (ITO) in transparent conducting electrodes (TCEs). However, the durability of the uniform metal mesh has not yet been studied. For this reason, a comparative analysis of the durability of TCEs based on pure Ag and AgNi nanomesh, which are fabricated by using simple transfer printing, is performed. The AgNi nanomesh shows high long-term stability to oxidation, heat, and chemicals compared with that of pure Ag nanomesh. This is because of nickel in the AgNi nanomesh. Furthermore, the AgNi nanomesh shows strong adhesion to a transparent substrate and good stability after repeated bending.

13.2% efficiency Si nanowire/PEDOT:PSS hybrid solar cell using a transfer-imprinted Au mesh electrode

In recent years, inorganic/organic hybrid solar cell concept has received growing attention for alternative energy solution because of the potential for facile and low-cost fabrication and high efficiency. Here, we report highly efficient hybrid solar cells based on silicon nanowires (SiNWs) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using transfer-imprinted metal mesh front electrodes. Such a structure increases the optical absorption and shortens the carrier transport distance, thus, it greatly increases the charge carrier collection efficiency. Compared with hybrid cells formed using indium tin oxide (ITO) electrodes, we find an increase in power conversion efficiency from 5.95% to 13.2%, which is attributed to improvements in both the electrical and optical properties of the Au mesh electrode. Our fabrication strategy for metal mesh electrode is suitable for the large-scale fabrication of flexible transparent electrodes, paving the way towards low-cost, high-efficiency, flexible solar cells.

Effect of two-step sol–gel reaction on the mesoporous silica structure

In the present study, we investigated the effects of two-step sol-gel reaction by abrupt pH change on the SBA-15 and mesocellular silica foams (MCF). Mesoporous silica was fabricated by using triblock copolymer templates (poly(ethylene oxide) and poly(propylene oxide)). The prepared silica structure was characterized by X-ray diffraction, transmission electron microscopy, and N(2) sorption experiment. Specifically, we prepared SBA-15 with long-range two-dimensional hexagonal arrangement of 3 to 6-nm feature spacing and MCF with larger pores of a few tens of nanometers. The pore size and ordering were influenced by pH change in a two-step sol-gel reaction and the concentration of organic solvent. Although well-ordered hexagonal arrangement of mesopores was prevalent in acidic conditions, the materials synthesized by a single-step reaction in neutral or basic conditions possessed gel-like structure without mesopores. However, the present two-step reaction (low pH sol-gel reaction followed by high pH reaction) not only produced mesoporous materials but also provided controllability of the pore size. In particular, mesoporous structures with pore sizes as large as those of MCF were successfully fabricated by the two-step reaction without using organic swelling agents. As expected, when xylene was added as a swelling agent, the pore size increased with the xylene/copolymer weight ratio.

Micropatterning of thin P3HT films via plasma enhanced polymer transfer printing

We have developed a simple but robust process, plasma enhanced polymer Transfer printing (PEPTP), for fabricating micropatterns of semi-conducting poly(3-hexyl thiophene) (P3HT) thin films. The method is based on transferring thin P3HT film spin cast directly on a pre-patterned elastomeric poly(dimethyl siloxane) (PDMS) mold. Printing is accomplished by the application of oxygen plasma on both P3HT film and substrate for surface energy modulation at ambient conditions without additional pressure. The control of the relative interfacial surface energy by the plasma enables us to micro/nanopattern a wide range of polymers from P3HT to conventional insulating ones on various substrates including Si, glass and polymers over large areas. A bottom contact organic thin film transistor with the printed P3HT patterns using PEPTP exhibits a carrier mobility of approximately 0.02 cm2V−1 s−1 with a relatively high on/off current ratio of 6 × 103.

Graphoepitaxy of Block‐Copolymer Self‐Assembly Integrated with Single‐Step ZnO Nanoimprinting

A highly efficient, ultralarge-area nanolithography that integrates block-copolymer lithography with single-step ZnO nanoimprinting is introduced. The UV-assisted imprinting of a photosensitive sol-gel precursor creates large-area ZnO topographic patterns with various pattern shapes in a single-step process. This straightforward approach provides a smooth line edge and high thermal stability of the imprinted ZnO pattern; these properties are greatly advantageous for further graphoepitaxial block-copolymer assembly. According to the ZnO pattern shape and depth, the orientation and lateral ordering of self-assembled cylindrical nanodomains in block-copolymer thin films could be directed in a variety of ways. Significantly, the subtle tunability of ZnO trench depth enabled by nanoimprinting, generated complex hierarchical nanopatterns, where surface-parallel and surface-perpendicular nanocylinder arrays are alternately arranged. The stability of this complex morphology is confirmed by self-consistent field theory (SCFT) calculations. The highly ordered graphoepitaxial nanoscale assembly achieved on transparent semiconducting ZnO substrates offers enormous potential for photonics and optoelectronics.

Photovoltaic devices with an active layer from a stamping transfer technique: single layer versus double layer.

In this study, organic photovoltaic devices with single or double-layered active film were prepared from a stamping transfer technique. A P3HT/PCBM single-layered active layer and a ratio-controlled P3HT/PCBM double-layered active can be successfully fabricated with the help of ultraviolet curable polycarbonate films via a stamping transfer technique. The maximum conversion efficiency values 2.85 for a single active layer transferred device and 3.24% for an optimized double active layer transferred device. Even though transferred double layers should have a sharp interface boundary, an intermixed zone with a concentration gradient was generated by the interpenetration of a donor-rich layer and an acceptor-rich layer in a thermal annealing process. The generation of the intermixed zone is confirmed by Auger electron spectroscopy. The enhanced conversion efficiency levels are attributed to the increased efficiency of the carrier transporting process, which is due to the fact that the concentration gradient is combined with the efficient charge generation from the bulk heterojunction layers.

Photo-induced hybrid nanopatterning of titanium dioxide via direct imprint lithography

A novel ultraviolet (UV)-assisted imprinting procedure that employs photosensitive titanium(IV) di-n-butoxide bis(2-ethylhexanoate) is presented for the fabrication of well-ordered titanium dioxide (TiO2) nanostructures at room temperature. The main novelty of this technique is the use of the photosensitive titanium organic compound, rather than a commonly used UV-curable resin, for direct UV-assisted nanoimprint lithography. Fourier transform infrared and X-ray photoelectron spectroscopy studies suggest that exposure to UV light resulted in the gradual removal of organic groups from films prepared from titanium(IV) di-n-butoxide bis(2-ethylhexanoate) photochemically and successively converted the films to TiO2 at room temperature. This approach allows direct fabrication of TiO2 nanopatterns with lines down to 35 nm in width, hole arrays of 265 nm in diameter, and three-dimensional TiO2 hybrid micro/nano-patterns without observable defects for use in applications where ordered surface nanostructures are required, such as photovoltaics, photonics, and optical waveguides.