Fabrication of micro/nanoscale structure via direct imprinting of functional material and its application to optoelectronic device

Abstract

however, the inherent deficiencies of ITO hinder applications in flexible optoelectronic devices. Although there have been increasing efforts in developing alternative materials of TCs, including conductive polymers, graphene and carbon nanotubes, their performance are insufficient for the next-generation flexible optoelectronic applications in terms of conductivity and stability. Metal grids have been found to provide superior electrical and optical properties, comparable to ITO. A direct imprinting of colloidal metal NPs can fabricate metal-based micro/nanoscale structures at low cost and high throughput. However, there are two challenges to fabricate high-performance metal grid TCs using the direct imprinting of colloidal metal NPs. Firstly, colloidal metal NPs during the direct imprinting process suffers from NP agglomerates and low metal concentration. Secondly, a sufficient filling of the concentrated ink inside the mold cavity should be achieved with the minimum residual layers. We developed a direct imprinting of thermally reduced Ag NPs using a reservoir-assisted mold in order to fabricate the high-performance metal grid TCs. A grid patterned mold was created to have a macroscale cavity by designing a “reservoir” that captured an outgoing ink and injected the captured ink into the grid patterned mold cavity by the roof deformation. The ink supply from the reservoir contributed to not only improving the ink filling, but also decreasing the linewidth of the grid patterned mold cavity due to the sidewall deformation on the liquid film. These behaviors led to not only lowering the sheet resistance ($R_s$), but also enhancing the transmittance (T). The metal grid TCs were embedded into a large-scale, flexible, and transparent films, which showed a reasonable electromechanical stability under repeated bending. The metal grid embedded TCs were applied to touch touch screen panels. Also, Ag NW networks were incorporated into the metal grids, which were fabricated via a direct imprinting process using deformation-driven ink injection. The solution-processed hybrid TCs consist of integration of microscale metal grids and Ag NW networks, which shows superior performance than the only Ag NW networks. We estimated the $R_s$-T performance of the Ag grids using geometric calculation, and demonstrated their effect on the en-hancement in the performance of the Ag NW networks. The hybrid TCs were successfully transferred into a flexible polymer matrix, which reduced the surface roughness of metal structures and showed the reasonable electromechanical stability. The hybrid Ag gird and NW embedded TCs were applied to a flexible organic solar cell (OSC) as an anode electrode. A one-step micro/nanoscale patterning method of metal NPs via the mold deformation and slip behavior was developed. The dramatic size reduction (~ 10 times) of metal NP structures was generated inside the de-formed cavity where its sidewall deformation and slip behavior led to decreasing the width of the mold cavity. To predict the mold deformation by considering the slip effect, the mold deformation was simulated using finite element analysis with assuming Neo-Hookean hyperelastic material and coefficient of friction. This phenomenon allowed fabrication of various, complex and isolated micro/nanoscale metallic structures, which were applied to organic field effect transistors for device demonstration. Our approach provides a promising tool to fabricate and manipulate micro/nanoscale structures of functional materials at large area, low cost and high throughput. Also, a micro/nanoscale patterning method of QDs via direct imprinting over a large area at low temperatures and low pressures was demonstrated as an alternative to conventional vacuum deposition and photolithography methods. More complex QD patterning could be demonstrated by expanding the QD direct imprinting process for multiple colored QDs and patterning on the multiple layers. A self-alignment scheme was developed to pattern multiple layers without the need for laborious alignment steps. Our approach may be useful for fabrication of QD-based optoelectronic device and patterning on large flexible substrates due to the low-temperature requirements of this process.

Micro/nanoscale structures of functional materials such as metal nanoparticle (NP), quantum dot (QD) and polymer are important factors to improve the overall performance in biological devices, photonic devices, and optoelectronic devices. The cost-effective micro/nanoscale structures are generally fabricated by nanoimprinting lithography as an alternative of traditional expensive vacuum deposition and photolithography processes. However, oxygen reactive ion etching, which is required for eliminating unwanted residual layers after the nanoimprinting process, increases the cost of manufacturing and offsets the advantages of the nanoimprinting process. Polydimethylsiloxane (PDMS)-based patterning methods can directly produce micro/nanoscale structures without any post process at low cost and high throughput. Among the PDMS-based patterning methods, a direct imprinting of functional inorganic materials provides tremendous potential to fabricate micro/nanoscale structures due to several advantages such as the minimum residual layers by conformal contact and the ease of demolding by low surface energy. Also, the PDMS mold deformation, such as roof deformation and sidewall deformation, can be used to not only improve the device performance, but also fabricate scale-down structures for use in a variety of next-generation flexible optoelectronic devices. Transparent conductors (TCs) are key components of various optoelectronic devices, such as photovoltaic cells, organic light­emitting diodes, or touch screen panels. Conventionally, indium tin oxide (ITO) is the most widely used as a transparent conductive material due to its low sheet resistance and excellent optical transparency