Backside Reflector Research Articles

Design of photonic crystals for light‐emitting diodes

AbstractPhotonic crystals (PCs) can greatly enhance the optoelectronic performance of light‐emitting diodes (LEDs) due to their distinctive color, photonic band gap, etc. Therefore, many scholars have conducted extensive research based on the high light extraction efficiency, good monochromaticity, and other excellent optoelectronic properties of PC LEDs. This review discusses the main principles of photonic crystals to improve the optoelectronic performance of LEDs and summarizes 12 structural applications of photonic crystal LEDs, such as PC slabs, Bragg grating, backside reflectors, surface PC, embedded PC, dual PC, PC beads, CPC, PC thin films, LIPC, defective PC, and composite architectures with other materials that boost LED optoelectronic qualities. In summary, it is found that photonic crystals can not only greatly improve the light extraction efficiency of LEDs but also improve other optoelectronic properties such as luminescent color and directional radiation angle, and reduce the manufacturing cost of LEDs. Photonic crystal LEDs are expected to be a strong candidate for future lighting technology. Finally, the prospects and challenges of PC LEDs are summarized.

See‐Through Flexible Photovoltaic Modules of Ultrathin Silicon Solar Microcells Enhanced by Synergistic Luminescent and Reflective Near‐Infrared Backplanes

AbstractUltrathin single‐crystalline silicon solar microcells assembled into see‐through and flexible modules have exhibited broadly applicable cost‐effective photovoltaics with reliable durability and efficiency. However, their module output power has suffered from inherent limitations caused by the inferior photon absorption property and the inactive see‐through windows. Herein a strategy is presented to enhance the performance of see‐through flexible ultrathin silicon microcell modules by employing visibly transparent but near‐infrared‐manipulatable optical backplanes, such as a luminescent waveguide gathering near‐infrared photons and a backside reflector mitigating near‐infrared photon escape. Simultaneous integration of these near‐infrared backplanes provides more photovoltaic gains to silicon solar modules than the sum of each backplane capability, and this synergy is due to the promoted properties of the photoluminescence process and waveguiding. The experimental module with ≈3‐µm thick silicon solar microcells can achieve up to ≈40%‐boosted output power generation when assisted by these backplanes. Detailed studies of optical and photovoltaic characterizations for near‐infrared backplanes and their integration modules elucidate the effectiveness of designed backplanes for see‐through flexible silicon photovoltaics.

Modeling selective narrowband light absorption in coaxial InAs-GaAs0.1Sb0.9 nanowires with partial shell segment coverage

Vertical III-V nanowire (NW) arrays are promising candidates for infrared (IR) photodetection applications. Generally, NWs with large diameters are required for efficient absorption in the IR range. However, increasing the NW diameter results in a loss of spectral selectivity and an enhancement in the photodetector dark current. Here, we propose a nanophotonic engineering approach to achieving spectrally-selective light absorption while minimizing the volume of the absorbing medium. Based on simulations performed using rigorous coupled-wave analysis (RCWA) techniques, we demonstrate dramatic tunability of the short-wavelength infrared (SWIR) light absorption properties of InAs NWs with base segments embedded in a reflective backside Au layer and with partial GaAs0.1Sb0.9 shell segment coverage. Use of a backside reflector results in the generation of a delocalized evanescent field around the NW core segment that can be selectively captured by the partially encapsulating GaAs0.1Sb0.9 shell layer. By adjusting the core and shell dimensions, unity absorption can be selectively achieved in the 2 to 3 μm wavelength range. Due to the transparency of the GaAs0.1Sb0.9 shell segments, wavelength-selective absorption occurs only along the InAs core segments where they are partially encapsulated. The design presented in this work paves the path toward spectrally-selective and polarization-dependent NW array-based photodetectors, in which carrier collection efficiencies can be enhanced by positioning active junctions at the predefined locations of the partial shell segments.

Optical investigation of sol–gel-synthesized and spin-coated SiO2/TiO2 multilayers

In this work, silicon dioxide (SiO2)/titanium dioxide (TiO2) multilayer films were deposited on silicon substrates through the sol–gel spin-coating technique using the precursors titanium butoxide and tetraethyl orthosilicate. After the preparation, the various optical and morphological properties of the films were studied by using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and ultraviolet–visible (UV–Vis) spectroscopy. The XRD patterns showed intense diffraction peaks, which were found to correspond to anatase titanium dioxide and it matched well the standard Joint Committee on Powder Diffraction Standards file. The crystallite sizes of the prepared samples were calculated as 24 nm for three stacks of silicon dioxide/titanium dioxide thin films and 30 nm for five stacks of silicon dioxide/titanium dioxide thin films using Debye–Scherrer’s formula. The FTIR transmittance spectra confirmed Si–O–Ti vibrational modes at 800 and 960 cm−1 with related functional groups. A reflectance of 86% was measured by using UV–Vis spectroscopy. These dielectric multipliers will be useful in solar cells as backside reflectors for an improved light-harvesting mechanism.

Enhancement of slope efficiency of a dually modulated photonic-crystal surface-emitting laser over a wide range of emission angles by introducing a backside reflector

Dually modulated photonic-crystal surface-emitting lasers (DM-PCSELs) are a new type of semiconductor laser that enable on-chip, mechanical-free, high-power, high-beam-quality 2D beam scanning over a wide field of view. These lasers are attracting attention for application in light detection and ranging, and the improvement of their slope efficiency is desired for this application. Thus far, the highest experimentally demonstrated slope efficiency is approximately 0.4 W/A at wavelengths of around 940 nm, which was limited by the fact that roughly half of the laser light was emitted toward and absorbed by the backside electrode. In this work, in order to improve the slope efficiency of DM-PCSELs, we utilize the light emitted toward the backside of the device by introducing a distributed Bragg reflector (DBR) as a backside reflector. In consideration of this laser’s ability to emit beams over a wide field of view, we design laser structures that facilitate a commensurate enhancement of efficiency at various emission angles. Next, we discuss the effect of the DBR on the confinement of transverse modes in thickness direction as well as the suppression of higher-order transverse modes. Then, we analyze the emission characteristics of a DM-PCSEL with a DBR and calculate that the theoretical slope efficiency is enhanced to 0.7–0.8 W/A, which is twice that of the device without a DBR, over a wide range of emission angles from 0° to 30°. We then fabricate the devices and experimentally demonstrate the emission of a single-lobed beam with a high slope efficiency of 0.7–0.8 W/A.

Ultrathin Film Amorphous Silicon Solar Cell Performance using Rigorous Coupled Wave Analysis Method

The issues related to global energy needs and environmental safeties as well as health crisis are some of the major challenges faced by the human, which make us to generate new pollution-free and sustainable energy sources. For that the optical functional nanostructures can be manipulated the confined light at the nanoscale level. These characteristics are emerging and leading candidate for the solar energy conversion. The combination of photonic (dielectric) and plasmonic (metallic) nanostructures are responsible for the development of better optical performance in solar cells. Here, the enhancement of light trapping within the thin active region is the primary goal. In this work, we have studied the influence of front-ITO (rectangular) and back-Ag (triangular) nanogratings were incorporated with ultrathin film amorphous silicon (a-Si) solar cell by using rigorous coupled wave analysis (RCWA) method. The improvement of light absorption, scattering (large angle), diffraction and field distributions (TE/TM) were demonstrated by the addition of single and dual nanogratings structures. Significantly, the plasmonic (noble metal) nanogratings are located at the bottom of the cell structure as a backside reflector which is helpful for the omni-directional reflection and increased the path length (life time) of the photons due to that the collection of the charge carriers were enhanced. Further, the proposed solar cell structure has optimized and compared to a back-Ag, front-ITO and dual nanogratings based ultrathin film amorphous silicon solar cell. Finally, the obtained results were evidenced for the assistance of photonic and plasmonic modes and achieved the highest current density (Jsc) of 23.82 mA/cm2(TE) and 22.75 mA/cm2 (TM) with in 50 nm thin active layers by integration of (dual) cell structures.

Design of hybrid MoS2/photonic devices compatible with technological constraints

The integration of transition metal dichalcogenide layers into photonic devices is a current challenge in the field of 2D materials. Based on numerical simulations, this work explores the design of devices combining an MoS2 monolayer with planar photonic gratings sustaining localized optical resonances. A special attention is paid to the technological constraints. The optical response of six devices is compared taking into account the limitations imposed by the growth conditions of the MoS2 layer and the processing of the resonant optical gratings. The reported photonic devices composed of grating filters and a backside reflector on silicon and silica substrates exhibit a theoretical absorption by the MoS2 layer between 85 and 99% at 532 nm. The numerical simulations further show that the addition of an Al2O3 encapsulation layer, to protect the MoS2 monolayer, results in an increase of the performance of the devices. These hybrid MoS2 based photonic devices are promising technological platforms for the study of the optical properties of integrated MoS2 monolayers.

Optical investigation on dielectric multilayers prepared by sol-gel method

The challenging optical performance of the thin film has depending on the selected materials, the number of alternative layers (or stacks), thin film deposition methodologies, etc. In this paper reporting, the experimental investigations of the alternate stacking of TiO2/SiO2 multilayers on the silicon substrates by using sol-gel and spin-coating method. The prepared seven (7S) and nine stacks (9S) of dielectric reflectors (TiO2/SiO2) were characterized to examine the reflectance and related functional groups by UV–Visible spectrophotometer and Raman spectroscopy. The higher and broader photonic bandgap (PBG) achieved in the ultraviolet and visible spectral region by enhanced number of stacks (9S) which was showed the highest reflectance. Further, the Raman spectra confirmed the existence of silicon (512 cm−1), anatase-TiO2 (Eg, 144 cm−1) and week bands of SiO2 (410, 487 & 919 cm−1). Finally, the relative reflectance enhancement of 54.3% was noticed by the highest number of stacks which will be useful as a back side reflector in solar cells.

Performance enhancement of amorphous silicon solar cell using 1D photonic crystal as back reflector

In the present work, design and optimization of a highly reflective light-trapping nanostructures were remarkably enhanced the efficiency of thin-film amorphous silicon (a-Si) solar cells by using rigorous coupled wave analysis (RCWA) method. One-dimensional photonic crystal (1DPC) is also known as distributed Bragg reflectors (DBRs) used as backside reflector. The property of photonic crystal has depends on the various parameters such as refractive index, thickness, center wavelength (λc) and the selection of the materials. In this work, the reflectivity of DBRs (Si/SiO2) was determined by various center (incidence) wavelength (λc) such as 400, 600, and 800 nm. The maximum reflectance of 100% achieved by 5DBR alternative layers (or stacks). The proposed solar cell consists of anti-reflection coating-ITO, top-grating (ITO), absorber layer (a-Si) and an optimized DBRs. Further, investigated the careful design optimization and presented the enhanced light trapping mechanism within 40 nm thick a-Si absorber region.

Study of ultrathin‐film amorphous silicon solar cell performance using photonic and plasmonic nanostructure

The photonic and plasmonic nanostructures are highly feasible for enhanced light trapping mechanisms. These nanostructures hold great promises for better photovoltaic performance by yielding the highest light-harvesting photons within the few nanometer absorber regions. The shed light on the nano-scaled structures (thin films and nanogratings) is responsible for the highest scattering mechanism with the omnidirectional diffraction angles and enhanced life time of the photons. In this research work, we have focused on improved ultrathin film amorphous silicon (a-Si) solar cell performance, which was integrated by top-SiO2 and bottom-Ag nanogratings as a backside reflector by using rigorous coupled-wave analysis method. The SiO2 antireflection coating, nanogratings, and absorber (a-Si) layer thicknesses were optimized for better photovoltaic performance. With the influence of optimization parameters, the highest current density of 27.03 and 33.53 mA/cm2 were obtained from transverse electric and transverse magnetic polarization conditions due to the surface guided-mode, Fabry–Perot resonance, surface excitation, localized fields, and surface plasmon polariton modes.

Light-harvesting enhancement in ultrathin film silicon solar cells using photonic and plasmonic nanostructures

Past few decades, nanostructures are useful in the active region and light-harvesting layers in thin film solar cells. In this work, the high reflectance alternative Si/SiO2 distributed Bragg reflectors (DBRs), and aluminum (Al) thin film layers were integrated as a backside reflectors in ultrathin film silicon solar cells to improve the light trapping mechanism using finite-difference time-domain (FDTD) method. DBR is known as one-dimensional photonic crystal (1DPC) used as backside reflectors with the addition of SiO2 buffer layer. Further, the transverse electric (TE) and magnetic (TM) field distribution shown the strong light scattering in the silicon active layer region by integration of top (ITO) and bottom (Al) nanogratings. Both polarization conditions yielded the highest current density (Jsc) of 22.03 (TE) and 30.42 mA/cm2 (TM) achieved within 50 nm active region thickness.

Study of a GaN-Based Light-Emitting Diode with a Specific Hybrid Structure

A specific hybrid structure, including 45° sidewalls, a shallower (0.25 μm in depth) microhole array, and a 3-D like backside reflector, is used to fabricate GaN-based light-emitting diodes (LEDs). The 3-D like backside reflector is formed by the deposition of 100 nm-diameter SiO2 nanospheres (NSs) and an Al layer on the sapphire substrate. Based on the use of 45° sidewall and microhole array, the reduced total internal reflection (TIR) and Fresnel reflection and the increased scattering probability and the opportunity of photons to find escape cones can be expected. In addition, the use of 3-D like backside reflector can effectively reflect scatter, and redirect the downward photons emitted from multiple quantum well (MQW) region into arbitrary directions. Thus, the light extraction performance can be substantially improved. In experiment, under an injection current of 400 mA, the studied Device D with this hybrid structure shows enhancements of 47.4%, 47.7%, 33.1%, and 47.8% in light output power (LOP), external quantum efficiency (EQE), luminous flux, and wall-plug efficiency (WPE), respectively, as compared to a conventional LED. Moreover, the related electrical properties are not degraded for the studied Device D. Therefore, this hybrid structure shows a promise for high-performance GaN-based LED applications.

Single-layer metamaterial bolometer for sensitive detection of low-power terahertz waves at room temperature

This study demonstrates a metamaterial bolometer that can detect terahertz (THz) waves by measuring variations in electrical resistance. A metamaterial pattern for enhanced THz waves absorption and a composite material with a high temperature coefficient of resistance (TCR) are incorporated into a single layer of the bolometer chip to realize a compact and highly sensitive device. To detect the temperature change caused by the absorption of the THz waves, a polydimethylsiloxane mixed with carbon black microparticles is used. The thermosensitive composite has TCR ranging from 1.88%/K to 3.11%/K at room temperature (22.2-23.8°C). In addition, a microscale metamaterial without a backside reflector is designed to enable the measurement of the resistance and to enhance the sensitivity of the bolometer. The proposed configuration effectively improves thermal response of the chip as well as the absorption of the THz waves. It was confirmed that the irradiated THz waves can be detected via the increment in the electrical resistance. The resistance change caused by the absorption of the THz waves is detectable in spite of the changes in resistance originating from the background thermal noise. The proposed metamaterial bolometer could be applied to detect chemical or biological molecules that have fingerprints in the THz band by measuring the variation of the resistance without using the complex and bulky THz time-domain spectroscopy system.

Near-field thermophotovoltaic energy conversion by excitation of magnetic polariton inside nanometric vacuum gaps with nanostructured Drude emitter and backside reflector

In this work, we study the effect of magnetic polariton (MP) on the conversion performance of a near-field thermophotovoltaic (TPV) system made of a nanostructured Drude grating emitter and a nanometer-thick photovoltaic (PV) cell with a lossless metal as the backside reflector. Near-field radiative heat flux is calculated through scattering matrix theory coupled with rigorous coupled-wave analysis based on fluctuational electrodynamics. It is theoretically shown that MP can be excited inside the nanometric vacuum gap and the 100-nm PV cell to spectrally enhance the heat flux above the cell bandgap. The lossless metal backing, which could practically serve as electrode for charge collection, is employed to reflect photons back into the cell for recycling. Contour plots of energy transmission coefficient and an inductor-capacitor circuit model along with magnetic field distribution are used to verify and understand the MP excitation in the near-field. Effects of PV cell thickness and vacuum gap distances, are systematically investigated on the near-field radiative heat flux, electrical power output, and energy conversion efficiency. It is found that the conversion efficiency increases from 0.7% to 12.2% when the semi-infinite In0.18Ga0.82Sb cell is replaced by an ultra-thin PV cell supported by a backside reflector with the emitter at 1000 K and the cell at 300 K separated by 200 nm gap. The conversion efficiency can be also further improved to 17.6% using a nanostructured Drude grating emitter over a planar emitter. This work would open up a new way to improve the performance of TPV devices with nanostructures with near-field thermal radiation.

Analysis of Photocurrent Generation within a Schottky-Junction-Based Near-Field Thermophotovoltaic System

Numerous studies have reported performance enhancement of a thermophotovoltaic (TPV) system when an emitter is separated by nanoscale gaps from a TPV cell. Although a $p$-$n$-junction-based TPV cell has been widely used for the near-field TPV system, a Schottky-junction-based near-field TPV system has drawn attention recently with the advantage of the easy fabrication. However, existing studies mostly focused on the generated photocurrent only in the metal side due to the fact that required energy for the metal-side photocurrent (i.e., Schottky barrier height) is smaller than the bandgap energy. Here, we suggest the precise performance analysis model for the Schottky-junction-based near-field TPV system, including photocurrent generation on the semiconductor side by considering the transport of minority carriers within the semiconductor. It is found that most of the total photocurrent in the Schottky-junction-based near-field TPV system is generated in the semiconductor side. We also demonstrate that further enhancement in the photocurrent generation can be achieved by re-absorbing the usable photon energy in the metal with the help of a backside reflector. The present work will provide a design guideline for the Schottky-junction-based near-field TPV system taking into account three types of photocurrents.

Performance enhancement methods of an ultra-thin silicon solar cell using different shapes of back grating and angle of incidence light

The goal of this research is to evaluate the efficiency dependence of a silicon based ultra-thin film solar cell on angle of incident of light. Here, the optical path length of incoming light inside the active layer is increased using a distributed Bragg reflector (DBR) and a rectangular, triangular or cylindrical shaped grating horizontally arranged and place on top of DBR. This arrangement of DBR and horizontal grating is used to control the light inside the absorber. It is shown that by implementing these structures, it is possible to enhance the efficiency of a 3 µm thin silicon solar cell near up to %20. In addition, when incidence angles of 0°–30° are considered, it is learned that cylindrical shaped backside reflector, when is combined with the DBR results in best efficiency.

Optimization of a near-field thermophotovoltaic system operating at low temperature and large vacuum gap

The present work successfully achieves a strong enhancement in performance of a near-field thermophotovoltaic (TPV) system operating at low temperature and large-vacuum-gap width by introducing a hyperbolic-metamaterial (HMM) emitter, multilayered graphene, and an Au-backside reflector. Design variables for the HMM emitter and the multilayered-graphene-covered TPV cell are optimized for maximizing the power output of the near-field TPV system with the genetic algorithm. The near-field TPV system with the optimized configuration results in 24.2 times of enhancement in power output compared with that of the system with a bulk emitter and a bare TPV cell. Through the analysis of the radiative heat transfer together with surface-plasmon-polariton (SPP) dispersion curves, it is found that coupling of SPPs generated from both the HMM emitter and the multilayered-graphene-covered TPV cell plays a key role in a substantial increase in the heat transfer even at a 200-nm vacuum gap. Further, the backside reflector at the bottom of the TPV cell significantly increases not only the conversion efficiency, but also the power output by generating additional polariton modes which can be readily coupled with the existing SPPs of the HMM emitter and the multilayered-graphene-covered TPV cell.

Optimization of wave-guided luminescence for higher efficiency of bifacial thin-film microscale GaAs solar cells

In pursuit of capturing more wave-guided luminescence for surface-printed bifacial GaAs μ-cells, the pyramid structure has been incorporated with specular back side reflector (BSR) to change the direction of photon propagation. Based on ray tracing model, the calculated photon capturing efficiency of GaAs μ-cells from back side via pyramid, dependent on the parameters of pyramid structure, achieve the largest 1.7× increase for dye absorption peak of 480 nm compared to the case without pyramid. More significantly, the short circuit current in experiment has been improved from original 16.5 mA/cm2 to 23.75 mA/cm2 for the AM 1.5G solar spectrum. Further experiment demonstrates that the optimized pyramid structure enables the integrated luminescent intensity to reach ∼3× increase in a smaller distance of optical transport, which means the advantages in photon capturing efficiency for cells with higher aspect ratio. The calculation further confirms that the cells with higher aspect ratio, among all cells with the same area, realize the higher concentration ratio for the same geometric gain. This provides a guideline for design of cell geometries to guarantee a higher power output in terms of cell modules.

Light Extraction Enhancement of GaN-Based Light-Emitting Diodes With Textured Sidewalls and ICP-Transferred Nanohemispherical Backside Reflector

Textured-sidewall GaN-based light-emitting diodes (LEDs) with convex and 45° patterns and an inductively coupled plasma (ICP)-transferred nanohemispherical backside reflector, formed using an ICP etching process, are fabricated and studied. For devices with textured sidewalls, the scattering probability of photons at the GaN/air interface is increased and the light extraction efficiency is enhanced since photons are allowed to find escape cones in the horizontal direction. With the ICP-transferred nanohemispherical backside reflector, reflected photons can be easily scattered and redirected in arbitrary directions for light extraction and thus have more opportunities to escape the devices. The LED with 45° sidewalls and a backside reflector exhibited the significant improvements of 55.8%, 49.3%, 47.2%, and 55.4% in light output power, luminous flux, external quantum efficiency, and wall-plug efficiency, respectively, compared to those of a conventional LED without these specific designs (Device A). In addition, the higher intensities in a light emission mapping image and improved far-field patterns are obtained for the studied device. The enhanced optical performance is mainly attributed to the increased light extraction in all directions due to a significant reduction in the total internal reflection by the textured sidewalls and a backside reflector. Therefore, textured-sidewall structures and an ICP-transferred nanohemispherical backside reflector are promising for high-power GaN-based LED applications.

Multiple-Wavelength Detection in SOI Lateral PIN Diodes With Backside Reflectors

This research details the potential of a microhotplate photo sensor, based on a silicon-on-insulator (SOI) lateral PIN (P+/P–/N+) diode and a microheater, fabricated on a thin suspended membrane from a commercial 1.0-μm SOI complementary metal oxide semiconductor technology. A local annealing (30-min. microheating, at elevated temperature ∼250 °C) is directly carried out onto the suspended diode to optimize device characteristics (e.g., leakage current, output optical response), for device long-term stability and industrial application. The optical performances of such SOI lateral PIN diodes with four different backside reflectors placed below them are fully investigated. Under same incident illumination, four specific output photocurrents and responsivities are therefore obtained due to the varied light absorption in the active Si film. By combining the photodiodes responses with the four backside reflectors (i.e., gold, aluminum, silicon substrate, and black silicon), multiple-wavelength detection can be straightforwardly achieved within the 450–900-nm wavelength range, which makes the SOI photodiode highly promising in red-green-blue sensing, gas analyzing or plasma monitoring applications.