Kristen P. Constant

A New Architecture for Transparent Electrodes: Relieving the Trade‐Off Between Electrical Conductivity and Optical Transmittance

Transparent conducting electrodes with the combination of high optical transmission and good electrical conductivity are essential and desirable in solar energy harvesting and electric lighting devices including organic solar cells and light-emitting diodes (LEDs) as well as in their inorganic counterparts. Currently, indium tin oxide (ITO) coated glass is most often used because ITO has relatively high transparency to visible light and low sheet resistance for electrical current conduction. However, ITO is costly due to limited resources, is brittle, [ 1 ] and has poor chemical compatibility with the active organic materials. [ 2 ] These disadvantages have motivated the search for other conducting electrodes with similar or better optical and electrical properties. In recent research efforts, carbon nanotube networks, unpatterned thin metal fi lms, random silver metal nanowire meshes, graphene fi lms, and patterned metal nanowire grids have been evaluated as potential replacements for ITO electrodes. [ 3‐11 ] Although these alternative transparent electrode approaches do have the potential to replace ITO, they still suffer from the classic trade-off between the optical transmittance and electrical conductivity. Thicker layers offer higher conductivity, but this comes at the expense of optical transmittance, and vice versa. Here, we report a new architecture for transparent electrodes, which leads to quasi-elimination of this tradeoff. This architecture consists of high-aspect-ratio metallic ribbons with nanoscale thickness and microscale width, spaced at desired periodicities and held in place by a polymer matrix to provide a fl at top surface for fabrication of active layers in solar cells or LEDs. By design, the light path is only obstructed by the nanoscale thickness of the ribbons, thus decoupling the conductivity and transmittance properties from each other. Catrysse and Fan performed theoretical investigations on similar nanopatterned metallic structures, and their simulations indicate that such structures have excellent optical and electrical properties for potential use as transparent conductive electrodes. [ 12 ] Our experimental results show that the novel structure is very promising for such applications.

Fabrication of photonic band gap materials using microtransfer molded templates

A method of manufacturing photonic band gap structures operable in the optical spectrum has been presented. The method comprises the steps of creating a patterned template for an elastomeric mold, fabricating an elastomeric mold from poly-dimethylsiloxane (PDMS) or other suitable polymer, filling the elastomeric mold with a second polymer such as epoxy or other suitable polymer, stamping the second polymer by making contact with a substrate or multilayer structure, removing the elastomeric mold, infiltrating the multilayer structure with ceramic or metal, and heating the multilayer structure to remove the second polymer to form a photonic band gap structure.

Soft holographic interference lithography microlens for enhanced organic light emitting diode light extraction

Very uniform 2 μm-pitch square microlens arrays (μLAs), embossed on the blank glass side of an indium-tin-oxide (ITO)-coated 1.1 mm-thick glass, are used to enhance light extraction from organic light-emitting diodes (OLEDs) by ~100%, significantly higher than enhancements reported previously. The array design and size relative to the OLED pixel size appear to be responsible for this enhancement. The arrays are fabricated by very economical soft lithography imprinting of a polydimethylsiloxane (PDMS) mold (itself obtained from a Ni master stamp that is generated from holographic interference lithography of a photoresist) on a UV-curable polyurethane drop placed on the glass. Green and blue OLEDs are then fabricated on the ITO to complete the device. When the μLA is ~15 × 15 mm(2), i.e., much larger than the ~3 × 3 mm(2) OLED pixel, the electroluminescence (EL) in the forward direction is enhanced by ~100%. Similarly, a 19 × 25 mm(2) μLA enhances the EL extracted from a 3 × 3 array of 2 × 2 mm(2) OLED pixels by 96%. Simulations that include the effects of absorption in the organic and ITO layers are in accordance with the experimental results and indicate that a thinner 0.7 mm thick glass would yield a ~140% enhancement.

Effects of Nanometer-Scale Photonic Crystal Structures on the Light Extraction From GaN Light-Emitting Diodes

This paper reports on the effect of nanometer-scale photonic crystal structures on the enhancement of the light extraction in GaN light-emitting diodes. Photonic crystals with hole or pillar-patterned structures with lattice constants of 460, 600, 750, and 920 nm are fabricated on indium-doped tin oxide (ITO) electrodes and/or p-GaN layers using laser holography and reactive ion etching. It is found that the light extraction efficiency depends strongly on the distance between the photonic crystal and the active layer, as well as the lattice constant for both structures. Photonic crystal light-emitting diodes (LEDs) with a lattice constant of 750 nm and hole depths of 260 nm in the ITO layer show an increase in light extraction of up to 32%, compared to conventional LEDs, without degradation in the electrical properties while a maximum enhancement of 26% is obtained from the device with a lattice constant of 460 nm and pillar heights of 60 nm on the p-GaN layer. The dependence of the extraction efficiency on the lattice constant is also calculated using a 3-D finite-difference time-domain method and compared with experimental results.

Three-dimensional metallic photonic crystals fabricated by soft lithography for midinfrared applications

We present an efficient method of fabricating freestanding three-dimensional metallic photonic crystals using soft lithography. Low cost and ease of fabrication are achieved through gold sputter deposition on a freestanding woodpile polymer template. We compare experimental results to theoretical calculations for tetragonal and face-centered-tetragonal structures as a function of the number of layers. The photonic crystals behave like full metallic structures with a photonic band edge at a wavelength of 3.5μm. The rejection rates of the structures are about 10dB/layer.

Fabrication of metallic nanowires and nanoribbons using laser interference lithography and shadow lithography

Ordered and free-standing metallic nanowires were fabricated by e-beam deposition on patterned polymer templates made by interference lithography. The dimensions of the nanowires can be controlled through adjustment of deposition conditions and polymer templates. Grain size, polarized optical transmission and electrical resistivity were measured with ordered and free-standing nanowires.

Diffracted moiré fringes as analysis and alignment tools for multilayer fabrication in soft lithography

We studied the first-order diffracted moire fringes of transparent multilayered structures comprised of irregularly deformed periodic patterns. By a comparison study of the diffracted moire fringe pattern and detailed microscopy of the structure, we show that the diffracted moire fringe can be used as a nondestructive tool to analyze the alignment of multilayered structures. We demonstrate the alignment method for the case of layer-by-layer microstructures using soft lithography. The alignment method yields high contrast of fringes even when the materials being aligned have very weak contrasts. The imaging method of diffracted moire fringes is a versatile visual tool for the microfabrication of transparent deformable microstructures in layer-by-layer fashion.

Electrical characterization of nanocrystalline titania—I: (Impedance spectroscopy studies between 300 K and 473 K)

Abstract The electrical properties of nanocrystalline titania with gold electrodes were characterized using impedance spectroscopy in the frequency range 10 −2 to 10 6 Hz and temperature range 300 K to 500 K. An attempt has been made to correlate the microstructural properties of these specimens to the electrical response of the material over the different ranges of temperatures. The results indicate that the conductivity of nanocrystalline titania is dependent on the porosity, grain size, and grain boundaries. The impedance and polarization behavior of the samples, especially at the electrodes, is also affected by the humidity. The electrical behavior of the nanocrystalline material makes it a candidate for use as a low temperature gas sensor.

Layer-by-layer photonic crystal fabricated by low-temperature atomic layer deposition

Layer-by-layer three-dimensional photonic crystals are fabricated by low-temperature atomic layer deposition of titanium dioxide on a polymer template created by soft lithography. With a highly conformal layer of titanium dioxide, a significantly enhanced photonic band gap effect appears at 3.1μm in transmittance and reflectance. From optical investigations of systematically shifted structures, the robust nature of the photonic band gap with respect to structural fluctuations is confirmed experimentally. With angle-resolved Fourier-transform spectroscopy, the authors also demonstrate that the fabricated photonic crystal can be a diffraction-free device as the photonic band gap exists over the diffracting regime.

Fabrication of submicron metallic grids with interference and phase-mask holography

Complex, submicron Cu metallic mesh nanostructures are made by electrochemical deposition using polymer templates made from photoresist. The polymer templates are fabricated with photoresist using two-beam interference holography and phase mask holography with three diffracted beams. Freestanding metallic mesh structures are made in two separate electrodepositions with perpendicular photoresist grating templates. Cu mesh square nanostructures having large (52.6%) open areas are also made by single electrodeposition with a photoresist template made with a phase mask. These structures have potential as electrodes in photonic devices.