Ag Reflector Research Articles

Ultrahigh Purcell factor in low-threshold nanolaser based on asymmetric hybrid plasmonic cavity.

A low-threshold nanolaser with all three dimensions at the subwavelength scale is proposed and investigated. The nanolaser is constructed based on an asymmetric hybrid plasmonic F-P cavity with Ag-coated end facets. Lasing characteristics are calculated using finite element method at the wavelength of 1550 nm. The results show that owing to the low modal loss, large modal confinement factor of the asymmetric plasmonic cavity structure, in conjunction with the high reflectivity of the Ag reflectors, a minimum threshold gain of 240 cm−1 is predicted. Furthermore, the Purcell factor as large as 2518 is obtained with optimized structure parameters to enhance rates of spontaneous and stimulated emission.

Electroless Silver Plating Reflectors to Boost the Performance of Vertical Light-Emitting Diodes

In this paper, we experimentally demonstrated a novel approach known as electroless silver plating (ESP) for the fabrication of Ag-only reflectors on vertical light-emitting diodes (VLEDs). The Ag reflector that was obtained through the ESP approach shows a higher reflectance of 83% and a lower contact resistivity of $6\ast 10^{-3} \Omega\cdot\text{cm}^{2}$ with p-GaN, compared with that fabricated through the conventional electron beam (EB) approach, which shows a reflectance of 75% and a contact resistivity of $6.2\ast 10^{-2} \Omega\cdot\text{cm}^{2}$ , respectively. Improvements in the light output intensity-current (L-I) and current-voltage (I-V) results in VLEDs, which originated from the high reflectance and low-contact resistivity of the incorporated ESP–Ag reflector. Our result opens up an alternative and promising way to substitute the traditional EB approach for Ag-based reflector fabrication and, thus, is practically meaningful in real device fabrication, such as with VLEDs.

Dual Effect of ITO‐Interlayer on Inverted Top‐Illuminated Polymer Solar Cells: Wetting of Polyelectrolyte and Tuning of Cavity

Flexible inverted top‐illuminated polymer solar cells (IT‐PSCs) are fabricated by wetting of polyelectrolyte and designing a microcavity structure by laying an indium‐tin‐oxide (ITO) interlayer on top of an Ag reflector. The ITO‐coated Ag makes the surface hydrophilic, thereby improving wettability of polyethyleneimine (PEIE). This increased wettability of PEIE yields a reflective cathode with low work function of 3.73 eV. The ITO layer also tunes the light absorption spectrum in the active layer. Finite‐domain time‐difference simulation provides evidence that the ITO layer played a role in both the shift in resonant wavelength in the microcavity and confinement of the electric field to the active layer. Time‐dependent simulation suggests that the time to reach steady‐state light absorption is longer (6.6 fs) when a microcavity is present than when it is not present (3.8 fs); i.e., the microcavity increases light absorption in the active layer. The designed IT‐PSCs show a maximum photo‐conversion efficiency of 6.4% on plastic film and 6.1% on opaque copper foil; these are the highest values obtained by top‐illuminated PSCs on a metallic substrate. The IT‐PSCs have excellent mechanical flexibility and more stable in air than conventional normal structured devices.

On the interplay of cell thickness and optimum period of silicon thin‐film solar cells: light trapping and plasmonic losses

AbstractLight trapping and photon management in honeycomb‐textured microcrystalline silicon solar cells are investigated experimentally and by modeling of the manufacturing process and the optical wave propagation. The solar cells on honeycomb‐textured substrates exhibit short circuit current densities exceeding 30 mA/cm2 and energy conversion efficiencies of up to 11.0%. By controlling the fabrication process, the period and height of the honeycomb‐textured substrates are varied. The influence of the honeycomb substrate morphology on the interfaces of the individual solar cell layers and the quantum efficiency is determined. The optical wave propagation is calculated using 3D finite difference time domain simulations. A very good agreement between the optical simulation and experimental results is obtained. Strategies are discussed on how to increase the short circuit current density beyond 30 mA/cm2. In particular, the influence of plasmonic losses of the textured silver (Ag) reflector on the short circuit current and quantum efficiency of the solar cell is discussed. Finally, solar cell structures with reduced plasmonic losses are proposed. Copyright © 2015 John Wiley & Sons, Ltd.

Efficient nanorod-based amorphous silicon solar cells with advanced light trapping

We present a simple, low-cost, and scalable approach for the fabrication of efficient nanorod-based solar cells. Templates with arrays of self-assembled ZnO nanorods with tunable morphology are synthesized by chemical bath deposition using a low process temperature at 80 °C. The nanorod templates are conformally coated with hydrogenated amorphous silicon light absorber layers of 100 nm and 200 nm thickness. An initial efficiency of up to 9.0% is achieved for the optimized design. External quantum efficiency measurements on the nanorod cells show a substantial photocurrent enhancement both in the red and the blue parts of the solar spectrum. Key insights in the light trapping mechanisms in these arrays are obtained via a combination of three-dimensional finite-difference time-domain simulations, optical absorption, and external quantum efficiency measurements. Front surface patterns enhance the light incoupling in the blue, while rear side patterns lead to enhanced light trapping in the red. The red response in the nanorod cells is limited by absorption in the patterned Ag back contact. With these findings, we develop and experimentally realize a further advanced design with patterned front and back sides while keeping the Ag reflector flat, showing significantly enhanced scattering from the back reflector with reduced parasitic absorption in the Ag and thus higher photocurrent generation. Many of the findings in this work can serve to provide insights for further optimization of nanostructures for thin-film solar cells in a broad range of materials.

Sulfurization Study on the Ag and Ag-Pd Reflectors for GaN-Based LEDs

In this study, we have established a sulfurization resistance test, which is an immersion process in 0.01 M Na2S solution for 10 min. By using this process, we found that the sulfurized Ag reflector drops significantly over 52%. However, the Ag-Pd alloy reflector does not drop as much as the Ag reflector. The minimum reduction rate (around 20%) occurs at the Ag-Pd reflector with a Pd content of 3.72 at.%. It means that there is an optimal surface Pd content (around 3.72 at.%) for the reduction in reflectivity. Above, two effects of Pd alloying atoms on the sulfurization of the surface Ag atoms have been drawn, which are (1) chemical-state change effect and (2) surface microstructure effect. We found that the chemical-state change effect retards the sulfurization of the surface Ag atoms, but the surface microstructure effect (increase in Ag (111) plane) promotes the sulfurization of the surface Ag atoms. Here, we believe that these two opposite effects explain that there is an optimal Pd content on the Ag-Pd surface for the resistance of the sulfurization on the Ag surface.

Embeded photonic crystal at the interface of p-GaN and Ag reflector to improve light extraction of GaN-based flip-chip light-emitting diode

In this experiment, a flip-chip light-emitting diode with photonic crystal was fabricated at the interface of p-GaN and Ag reflector via nanospheres lithography technique. In this structure, photonic crystal could couple with the guide-light efficiently by reason of the little distance between photonic crystal and active region. The light output power of light emitting diode with embedded photonic crystal was 1.42 times larger than that of planar flip-chip light-emitting diode. Moreover, the embedded photonic crystal structure makes the far-field divergence angle decreased by 18° without spectra shift. The three-dimensional finite difference time domain simulation results show that photonic crystal could improve the light extraction, and enhance the light absorption caused by Ag reflector simultaneously, because of the roughed surface. The depth of photonic crystal is the key parameter affecting the light extraction and absorption. Light extraction efficiency increases with the depth photonic crystal structure rapidly, and reaches the maximum at the depth 80 nm, beyond which light extraction decrease drastically.

On the geometry of plasmonic reflection grating back contacts for light trapping in prototype amorphous silicon thin-film solar cells

We experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. These back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. We vary the period, unit cell, and width of the nanostructures to identify design strategies for optimized light trapping. First, a general correlation between the reduction of the period of the nanostructures down to 550 nm and an increase of the absorptance, as well as external quantum efficiency is found for various unit cells formed by nanostructures. Second, increasing the width of the nanostructures from 200 to 350 nm, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, we identify a design for improved light trapping for the given solar cell parameters within the considered variations. It consists of thin-film solar cells applying a combination of a period of 600 nm and a structure width of 350 nm. The implementation of back contacts with this configuration yields enhanced power conversion efficiency as compared to reference solar cells processed on conventionally used randomly textured substrates. In detail, the enhancement of the short-circuit current density from initially 14.7 to initially 15.6 mA/cm2 improves the power conversion efficiency from 9.1 to 9.3%.

Silver versus white sheet as a back reflector for microcrystalline silicon solar cells deposited on LPCVD‐ZnO electrodes of various textures

AbstractWe compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5‐µm‐thick low‐pressure chemical vapor deposition (LPCVD)‐ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD‐ZnO layer (with thickness below 200 nm). For this latter design, the optical loss in the nano‐rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar‐plasma treatment. Because of its superior lateral conductivity, the thin‐ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD‐ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin‐ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active‐layer thickness of only 1.1 µm. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright © 2014 John Wiley & Sons, Ltd.

Morphological stability of Ag reflector for high-power GaN-based vertical light-emitting diode by addition of Ni layer

We investigated the reasons why Ag reflectors in vertical light-emitting diodes showed much better morphological stability with the addition of an intermediate Ni layer by means of X-ray pole figures, scanning electron microscopy (SEM), and SEM electron backscatter diffraction (EBSD) techniques. The SEM results showed that, unlike the pitted Ag-only contacts, the Ni-combined Ag contacts annealed at 300°C contained only hillocks, even after annealing for 60min. The EBSD results demonstrated that the Ag-only samples were more strongly 〈111〉-textured than the Ni-combined Ag samples after annealing for 60min. The pole-figure results also indicated that, for both the samples, the {111} texture was enhanced by annealing, although the Ag-only samples were more highly 〈111〉-textured than the Ni-combined Ag samples. On the basis of the SEM, EBSD, and pole-figure results, we interpret and discuss the possible mechanisms underlying the improved morphological stability of the Ni-combined Ag reflectors.

Polarity dependence of the electrical characteristics of Ag reflectors for high-power GaN-based light emitting diodes

We report on the polarity dependence of the electrical properties of Ag reflectors for high-power GaN-based light-emitting diodes. The (0001) c-plane samples become ohmic after annealing in air. However, the (11–22) semi-polar samples are non-ohmic after annealing, although the 300 °C-annealed sample shows the lowest contact resistivity. The X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the c-plane samples experiences larger shift toward the valence band than that for the semi-polar samples. The XPS depth profile results show that unlike the c-plane samples, the semi-polar samples contain some amounts of oxygen at the Ag/GaN interface regions. The outdiffusion of Ga atoms is far more significant in the c-plane samples than in the semi-polar samples, whereas the outdiffusion of N atoms is relatively less significant in the c-plane samples. On the basis of the electrical and XPS results, the polarity dependence of the electrical properties is described and discussed

Nanotechnology enhanced solar cells prepared on laser-crystallized polycrystalline thin films (

It is shown in this paper that indeed silicon nanotechnology can significantly improve the performance of solar cells made on low-cost materials like polycrystalline silicon (pc-Si) thin films formed on glass substrates. By proper engineering of the nanowires (cleaning of the metal contamination from the metal-assisted wet chemical etching and passivation by a-Si:H emitter as well as by an ultrathin Al2O3 tunnel layer on the emitter surface) and by proper design of the cell structure (superstrate configuration), the detrimental effects related to the nanowires can be avoided while the light trapping properties remain. In superstrate configuration with an Ag reflector, a prototype solar cell with efficiency of 10% has been realized on a pc-Si thin film with a thickness of only 8µm formed on glass, resulting from an enhanced photocurrent due to light trapping by silicon nanowires. Further optimization steps are discussed and targeted to improve the efficiency and the performance of the solar cells.

(Invited) Vertical-Geometry GaN-Based Light-Emitting Diodes: Improving Current Injection and Light Extraction Efficiencies

The effect of an indium middle layer on the structural, thermal and electrical properties of Ag contacts for high-power GaN-based vertical LEDs was investigated. As the temperature increases, agglomeration is initiated by the formation of hillocks, followed by the generation of holes, leading to agglomeration. The In layer effectively prevents agglomeration. Consequently, the In-inserted Ag reflectors exhibit better reflectivity at 500°C than Ag only contacts. The In-inserted Ag sample was less <111>-textured than the Ag only sample. On the basis of the scanning electron microscopy, and X-ray phi scan and pole figure results, the improved stability is described by means of the <111>-texture and the modification of the surface energy of Ag films. The In-inserted Ag reflectors exhibit better specific contact resistivity at 500°C than Ag only contacts. The output power (at 20 mA) of LEDs with the In-inserted Ag contacts is higher than that of LEDs with Ag only contacts.

Enhanced absorption in elliptical silicon nanowire arrays for solar energy harvesting

A crystalline Si thin film solar cell with elliptical silicon nanowire (SiNW)-decorated surface is presented. The elliptical SiNW solar cell performs broadband light absorption in the low energy region while maintaining strong absorption in the high energy region. For the elliptical SiNW solar cells with 1-μm underlying Si substrate, J sc can be improved to 24.2 mA/cm 2 , which is 16% higher than that of the circular SiNW-decorated solar cells. After integration with an Ag reflector, the elliptical SiNW solar cells can achieve higher J sc of 26.3 mA/cm 2 , demonstrating a photocurrent enhancement of 12.6% as compared with circular SiNW control device.

Delayed texture for improving the thermal stability of Ag reflectors for high-performance GaN-based light-emitting diodes

The thermal stability of Ag-based reflectors for GaN-based light-emitting diodes (LEDs) was significantly improved by inserting an Mg layer. LEDs fabricated with the Mg-inserted reflector exhibited 22% higher output power (at 20 mA) than LEDs with the Ag-only reflector. The Mg-inserted Ag sample was less textured than the Ag-only sample. Based on scanning electron microscopy and X-ray results, the improved stability is attributed to the delay in the texturing and the modification of the surface energy of Ag films.

“Seeing” the Resonant SPP Modes Confined in Metal Nanocavity via Cathodoluminescne Spectroscopy

ABSTRACT:Surface plasmon polaritons (SPPs), which are coupled excitations of electrons bound to a metal-dielectric interface, show great potential for application in future nanoscale photonic systems due to the strong field confinement at the nanoscale, intensive local field enhancement, and interplay between strongly localized and propagating SPPs. The fabrication of sufficiently smooth metal surface with nanoscale feature size is crucial for SPPs to have practical applications. A template stripping (ST) method combined with PMMA as a template was successfully developed to create extraordinarily smooth metal nanostructures with a desirable feature size and morphology for plasmonics and metamaterials. The advantages of this method, including the high resolution, precipitous top-to bottom profile with a high aspect ratio, and three-dimensional characteristics, make it very suitable for the fabrication of plasmonic structures. By using this ST method, boxing ring-shaped nanocavities have been fabricated and the confined modes of surface plasmon polaritons in these nanocavities have been investigated and imaged by using cathodoluminescence (CL) spectroscopy, which has been turned out to be a powerful means to characterize the resonant SPPs modes confined in metal nanocavities [1∼5] . The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons. Quality factors can be directly acquired from the spectra induced by the ultrasmooth surface of the cavity and the high reflectivity of the silver (Ag) reflectors. Because of its three-dimensional confined characteristics and the omnidirectional reflectors, the nanocavity exhibits a small modal volume, small total volume, rich resonant modes, and flexibility in mode control. Numerous applications, such as plasmonic filter, nanolaser, and efficient light-emitting devices, can be expected to arise from these developments.

Improving the output power of near-ultraviolet InGaN/GaN-based light emitting diodes by enhancing the thermal and electrical properties of Ag-based reflector

We report on the improved performance of near-ultraviolet (385nm) InGaN/GaN multi-quantum well light emitting diodes (LEDs) by enhancing the thermal and electrical properties of Ag-based reflector. It is shown that after annealing at 500°C, indium (In)-overlaid Ag contact exhibits considerably higher reflectivity at 385nm than Ag only contact. After annealing at 500°C, both the Ag only and In-overlaid Ag contacts are ohmic with a specific contact resistance of 6.7×10−4 and 8.7×10−5Ωcm2, respectively. Near-UV (385nm) LEDs fabricated with annealed Ag only and In-overlaid Ag reflectors show a forward-bias voltage of 3.4 and 3.37V at an injection current of 20mA, respectively. The LEDs with the annealed In-overlaid Ag reflector exhibit 24.7% higher light output power (at 20mA) than the LEDs with the 500°C-annealed Ag only reflector. X-ray photoemission spectroscopy was performed to understand the improved electrical properties of the In-overlaid Ag reflectors. The enhanced thermal stability of In-overlaid Ag reflector is also briefly described.

Optimization of the planar microcavity structure in bottom-emitting gallium nitride light-emitting diode

An optimized microcavity structure is proposed to enhance the emission efficiency of the bottom-emitting gallium nitride (GaN) light emitting diode. This structure contains a SiO 2 layer below the n -GaN layer to enhance transmission through the bottom layer and a SiO 2 /Ag reflector coated on the p -GaN layer to raise the reflection on the upper surface of the GaN layer. The thicknesses of the two SiO 2 layers are optimized to improve the light extraction efficiency and internal quantum efficiency. The 200-nm-thick p -GaN layer signifies high current injection efficiency. The calculations reveal that the TM-polarized intensity in the normal direction from the optimized sample is about eight times stronger than that from the reference sample. Our designation offers the advantages of simplifying the fabricating process and reducing the manufacturing cost.

Optical characterisation of a spectrally tunable plasmonic reflector for application in thin-film silicon solar cells

We have investigated the interaction between a random two-dimensional array of Ag islands near a Ag reflector, with the aim of producing a plasmonic back reflector structure with high diffuse reflectivity in the near-infrared, 600–1100nm wavelength, region. We have demonstrated the ability to tune the power scattered and absorbed by varying the distance between the plasmonic layer and the reflector. Finite-difference-time-domain (FDTD) simulations demonstrate the tunability of the scattered and absorbed power with separation distance for a single Ag nanosphere near a planar Ag reflector. The tunability of the optical properties can be attributed to the modulation in the electric field driving the plasmonic resonance with separation distance. The simulation results indicate an intermediate distance where the scattered power peaks with minimal absorption losses. Random arrays of metal-islands were fabricated on varying thicknesses of a ZnO separation layer on a Ag reflector. Compared to a conventional textured Ag reflector, which has ∼2% diffuse reflectance in the near-infrared spectral region, the fabricated plasmonic reflector with ∼200nm sized Ag metal islands at 100nm separation distance from the Ag reflector shows a relatively higher, ∼24%, integrated diffuse reflectance in the near bandgap, 600–1100nm wavelength, region for thin film silicon solar cells.

Effect of an In layer on the thermal stability of Ag reflector for vertical GaN-based light-emitting diodes

We investigated the effect of an indium (In) middle layer on the thermal and electrical properties of Ag contacts for high-power GaN-based vertical light-emitting diodes (LEDs). It is shown that with increasing temperature, agglomeration is initiated by the formation of hillocks, followed by the generation of holes, eventually leading to agglomeration. The presence of the In layer effectively delays agglomeration. As a result, the In-combined Ag reflectors exhibit better specific contact resistance (6.0×10−5Ωcm2) and reflectance (∼75% at 440nm) when annealed at 500°C, as compared to Ag only contacts. Blue (440nm) LEDs fabricated with the 500°C-annealed In-combined Ag contacts exhibit a forward voltage of 2.99V (at an injection current of 20mA) lower than that of LEDs with the 500°C-annealed Ag only contacts. The output power (at 20mA) of the LEDs with the 500°C-annealed In-combined Ag contacts is 29% higher than that of LEDs with the 500°C-annealed Ag only contacts.