Myung Gyu Kang

A Facile Route to Polymer Solar Cells with Optimum Morphology Readily Applicable to a Roll‐to‐Roll Process without Sacrificing High Device Performances

A new fabrication method for polymer solar cells that can produce optimized vertical distribution of components is reported. The favorable donor–acceptor morphology showing a well-organized photo-induced charge transporting pathway with fine nanodomains and high crystallinity is achieved. This process is also readily scalable to a large-area and high-speed roll-to-roll process without sacrificing high device performances, even without a PEDOT:PSS layer.

Choice of electrode geometry for accurate measurement of organic photovoltaic cell performance

The relationship between the performance and the electrode geometry of organic photovoltaic devices was investigated to establish the proper electrode geometry for reproducible and accurate performance measurement. Photovoltaic cells (ITO∕PEDOT:PSS∕P3HT+PCBM∕LiF∕Al) having crossbar-type and island-type electrode geometries were fabricated. The crossbar-type cells varied greatly in performance depending on the illuminated light beam size relative to the overlap area of the crossbar-type electrodes due to excess photocurrent generated from the cell region outside the overlapped electrode area, where PEDOT:PSS serves as anode. We systematically investigated the relationship between the conductivity of the PEDOT:PSS, the illumination area, and the amount of excess photocurrent generated.

Large Area High Density Sub-20 nm SiO2 Nanostructures Fabricated by Block Copolymer Template for Nanoimprint Lithography

We developed simple fabrication methods to effectively transfer the block copolymer nanopatterns to a substrate material. High aspect ratio, sub-20 nm nanopillar and nanohole structures are successfully fabricated in a SiO(2) layer in large area format, and the versatile utilities of these nanostructures as nanoimprint molds are studied. Nanoimprint lithography using these molds makes it possible to easily replicate densely packed block copolymer nanotemplate patterns on arbitrary substrates in a short processing time by using a large variety of polymer materials, including functional materials such as conjugated polymers. In addition, the PDMS soft stamps with both nanohole and nanopillar pattern polarities, which are useful tools for soft lithography and transparent template applications, are also successfully fabricated using the pillar- and hole-type SiO(2) molds. These soft stamps provide an effective way to fabricate controllable as well as reproducible plasmonic metal nanostructures with tunable surface plasmon resonances.

Semitransparent Cu electrode on a flexible substrate and its application in organic light emitting diodes

A semitransparent nanomesh Cu electrode on a polyethylene terephthalate (PET) substrate using metal transfer from a polydimethylsiloxane (PDMS) stamp and nanoimprint lithography is reported. A nanoscale dense mesh pattern is replicated by using a high modulus PDMS stamp. It is found that a uniform pressure of 30 psi and a temperature of 100 °C are needed for the transfer of the Cu mesh structure from the PDMS stamp onto the PET substrate. A fabricated semitransparent Cu electrode exhibits high transmittance in the visible range and good electrical conductivity. The authors show that the transmittance is increased by reducing the linewidth of the mesh pattern and an average transmittance of 75% is achieved. An organic light emitting diode on a flexible substrate is fabricated to demonstrate the potential use of a semitransparent Cu electrode as a transparent conducting electrode.

Metal transfer assisted nanolithography on rigid and flexible substrates

A nanolithography technique based on metal transfer printing is demonstrated. The transferred metal conveniently acts as an etch mask for pattern transfer to a substrate. This lithography technique can be used on flexible plastic substrate as well as on rigid substrate due to the low pressure and temperature used in the metal transfer process. Dense nanosize metal particle arrays with different shapes such as square, diamond, and nanobar were created with high yield over large area, and localized surface plasmon spectra of those particle arrays were measured. Pattern linewidth was reduced to 50 nm in metal grating by depositing metals on the poly(dimethysiloxane) grating sidewall using a shadow evaporation process.

Fabrication of high aspect ratio Si nanogratings with smooth sidewalls for a deep UV-blocking particle filter

To measure space plasmas and neutral particles one must filter out high-energy ultraviolet photons that would increase background count or damage sensors. To enable sensitive neutral particle measurements, a photon-to-particle rejection rate of 1014 is desired, far exceeding the requirements of prior filters. The authors propose a high-aspect ratio Si grating with densely packed, sub-100 nm slits. In this article, the authors report the development of a new technique for fabricating sturdy, self-supported transmission gratings in silicon using nanoimprint lithography and deep reactive ion etching, resulting in grating slits with scalloping under 7 nm and high (8.5:1) aspect ratios.

Subwavelength grating structures with magnetic resonances at visible frequencies fabricated by nanoimprint lithography for large area applications

Transmission of TM-polarized light through a subwavelength metal-dielectric grating structure exhibits strong resonance in the visible range. Simulations by finite-difference time-domain and finite-element methods show that the resonance can be attributed to the magnetic response. Further simulation shows that the grating structure can be optimized by adding an index matching dielectric layer to produce negative refractive index response in the visible band. Easily fabricated using nanoimprint lithography and conveniently able to be excited by incident light normal to the fabrication plane, such metal-dielectric grating structure could find potential use in large area negative refractive index applications.

Enhanced efficiency of organic solar cells with silver nanowire electrodes

Surface plamon resonance (SPR) in metallic nanostructures offers a promising way to enhance the power conversion efficiency (PCE) of organic solar cell as it exhibits strongly enhanced electromagnetic fields in the vicinity of the metal, which can lead to increased optical absorption in the organic film. Here we demonstrate the SPR enhanced photo-current and PCE of organic solar cells using periodic Ag nanowires as transparent electrodes as compared to the device with conventional ITO electrode. Photo-currents and external quantum efficiencies (EQE) are enhanced as much as 40 % and 2.5 fold at a wavelength of 570 nm, respectively, resulting in 35% overall increase in power conversion efficiency than the ITO control device. The use of plasmonic transparent Ag nanowire electrode may help to realize low cost and high performance organic solar cells.