Design of highly transparent ohmic contact to N face n-GaN for enhancing light extraction in GaN-based micro LED display.

In GaN-based vertical micro LEDs, conventional metal n-contacts on the N face n-GaN suffer from a low aperture ratio due to the high reflection of metals, resulting in low-light extraction efficiencies. Great efforts have been devoted to enhancing transparency by employing transparent conducting oxides for n-contacts, but they exhibited poor Ohmic behavior due to their large work functions. Herein, we introduce an InN/ITO n-contact to achieve both superior contact property and high transparency. At the initial stage, the ITO with thin In interlayer was utilized, and the change in contact properties was observed with different annealing temperatures in the N2 atmosphere. After annealing at 200 °C, the In/ITO n-contact exhibited Ohmic behavior with high a transparency of 74% in the blue wavelength region. The metallic In transformed into InN during the annealing process, as confirmed by transmission electron microscopy. The formation of InN caused polarization-induced band bending at the InN/GaN interface, providing evidence of enhanced Ohmic properties. In the application of vertical GaN µLED, the EQE increased from 6.59% to 11.5% while operating at 50 A/cm2 after the annealing process.

Subwavelength-scale nanorods implemented hexagonal pyramids structure as efficient light-extraction in Light-emitting diodes.

Subwavelength-scale nanorods were implemented on the hexagonal pyramid of photochemically etched light-emitting diodes (LEDs) to improve light extraction efficiency (LEE). Sequential processes of Ag deposition and inductively coupled plasma etching successfully produce nanorods on both locally unetched flat surface and sidewall of hexagonal pyramids. The subwavelength-scale structures on flat surface offer gradually changed refractive index, and the structures on side wall of hexagonal pyramid reduce backward reflection, thereby enhancing further enhancement of the light extraction efficiency. Consequently, the nanorods implemented LED shows a remarkable enhancement in the light output power by 14% compared with that of the photochemically etched LEDs which is known to exhibit the highest light output power. Theoretical calculations using a rigorous coupled wave analysis method reveal that the subwavelength-scale nanorods are very effective in the elimination of TIR as well as backward reflections, thereby further enhancing LEE of the LEDs.

Nano-imprinting of refractive-index-matched indium tin oxide sol-gel in light-emitting diodes for eliminating total internal reflection.

Refractive-index (RI)-matched nanostructures are implemented in GaN-based light-emitting diodes (LEDs) for enhancing light output efficiency. The RI-matched indium tin oxide (ITO) nanostructures are successfully implemented in GaN-based lateral LEDs by using ITO sol-gel and nanoimprint lithography. The ITO sol-gel nanostructures annealed at 300 °C have RI of 1.95, showing high transparency of 90% and high diffused transmittance of 34%. Consequently, the light output power in LEDs with the RI-matched nanostructures increases by 8% in comparison with that in LEDs containing flat ITO. Ray tracing and finite-difference time-domain (FDTD) simulations show that the RI-matched nanostructures on the transparent current spreading layer dramatically reduce Fresnel reflection loss at the interface of the current spreading layer with the nanostructure and extract confined waveguide lights in LEDs.

Monolithic Nanoporous In-Sn Alloy for Electrochemical Reduction of Carbon Dioxide.

Nanostructured metal catalysts to convert CO2 to formate, which have been extensively studied over decades, have many problems such as durability, lifetime, high process temperature, and difficulty in controlling the morphology of nanostructures. Here, we report a facile method to fabricate monolithic nanoporous In-Sn alloy, a network of nanopores, induced by electroreduction of indium tin oxide nanobranches (ITO BRs). The electroreduction process concentrated a local electric field at the tip of the nanostructure, leading to current-assisted joule-heating to form a nanoporous In-Sn alloy. Scanning electron microscopy images showed that the nanopore size of In-Sn alloy could be controlled from 1176 to 65 nm by tuning the electroreduction condition: the applied potential and the time. As a result, formate Faradaic efficiency could be improved from 42.4% to 78.6%. Also, current density was increased from -6.6 to -9.6 mA/cm2 at -1.2 VRHE, thereby resulting in the highest HCOO- production rate of 75.9 µmol/(h cm2). Detachment of catalysts from the substrate was not observed even after a long-term (12 h) electrochemical measurement at high potential (-1.2 VRHE). This work provides a design rule to fabricate highly efficient and stable oxide-derived electrocatalysts.

Flexible Near-Field Wireless Optoelectronics as Subdermal Implants for Broad Applications in Optogenetics.

In vivo optogenetics provides unique, powerful capabilities in the dissection of neural circuits implicated in neuropsychiatric disorders. Conventional hardware for such studies, however, physically tethers the experimental animal to an external light source, limiting the range of possible experiments. Emerging wireless options offer important capabilities that avoid some of these limitations, but the current size, bulk, weight, and wireless area of coverage is often disadvantageous. Here, we present a simple but powerful setup based on wireless, near-field power transfer and miniaturized, thin, flexible optoelectronic implants, for complete optical control in a variety of behavioral paradigms. The devices combine subdermal magnetic coil antennas connected to microscale, injectable light-emitting diodes (LEDs), with the ability to operate at wavelengths ranging from UV to blue, green-yellow, and red. An external loop antenna allows robust, straightforward application in a multitude of behavioral apparatuses. The result is a readily mass-producible, user-friendly technology with broad potential for optogenetics applications.

Parallel Aligned Mesopore Arrays in Pyramidal-Shaped Gallium Nitride and Their Photocatalytic Applications.

Parallel aligned mesopore arrays in pyramidal-shaped GaN are fabricated by using an electrochemical anodic etching technique, followed by inductively coupled plasma etching assisted by SiO2 nanosphere lithography, and used as a promising photoelectrode for solar water oxidation. The parallel alignment of the pores of several tens of micrometers scale in length is achieved by the low applied voltage and prepattern guided anodization. The dry etching of single-layer SiO2 nanosphere-coated GaN produces a pyramidal shape of the GaN, making the pores open at both sides and shortening the escape path of evolved gas bubbles produced inside pores during the water oxidation. The absorption spectra show that the light absorption in the UV range is ∼93% and that there is a red shift in the absorption edge by 30 nm, compared with the flat GaN. It also shows a remarkable enhancement in the photocurrent density by 5.3 times, compared with flat GaN. Further enhancement (∼40%) by the deposition of Ni was observed due to the generation of an electric field, which increases the charge separation ratio.

Visible Color Tunable Emission in Three-Dimensional Light Emitting Diodes by MgO Passivation of Pyramid Tip.

We demonstrated visible color tunable three-dimensional (3D) pyramidal light emitting diodes by depositing the MgO on and near the tip of the pyramid as an insulating layer. Here, we show that the degradation of the materials (i.e., p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region is responsible for the large leakage current observed in LEDs. Confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, a blue-shift by 10.3 nm in the wavelength. This technique is simple and scalable, providing a promising solution for developing 3D pyramidal LEDs with low leakage current and controllable light emission.

A Challenge Beyond Bottom Cells: Top-Illuminated Flexible Organic Solar Cells with Nanostructured Dielectric/Metal/Polymer (DMP) Films.

Top-illuminated flexible organic solar cells with a high power conversion efficiency (≈6.75%) are fabricated using a dielectric/metal/polymer (DMP) electrode. Employing a polymer layer (n = 1.49) makes it possible to show the high transmittance, which is insensitive to film thickness, and the excellent haze induced by well-ordered nanopatterns on the DMP electrode, leading to a 28% of enhancement in efficiency compared to bottom cells.

Flexible a-Si:H Solar Cells with Spontaneously Formed Parabolic Nanostructures on a Hexagonal-Pyramid Reflector.

Flexible amorphous silicon (a-Si:H) solar cells with high photoconversion efficiency (PCE) are demonstrated by embedding hexagonal pyramid nanostructures below a Ag/indium tin oxide (ITO) reflector. The nanostructures constructed by nanoimprint lithography using soft materials allow the top ITO electrode to spontaneously form parabolic nanostructures. Nanoimprint lithography using soft materials is simple, and is conducted at low temperature. The resulting structure has excellent durability under repeated bending, and thus, flexible nanostructures are successfully constructed on flexible a-Si:H solar cells on plastic film. The nanoimprinted pyramid back reflector provides a high angular light scattering with haze reflectance >98% throughout the visible spectrum. The spontaneously formed parabolic nanostructure on the top surface of the a-Si:H solar cells both reduces reflection and scatters incident light into the absorber layer, thereby elongating the optical path length. As a result, the nanopatterned a-Si:H solar cells, fabricated on polyethersulfone (PES) film, exhibit excellent mechanical flexibility and PCE increased by 48% compared with devices on a flat substrate.