Broadband Light Trapping Research Articles

High-Performance Planar Broadband Hot-Electron Photodetection through Platinum-Dielectric Triple Junctions.

Recently, planar and broadband hot-electron photodetectors (HE PDs) were established but exhibited degraded performances due to the adoptions of the single-junction configurations and the utilizations of absorbable films with thicknesses larger than the electronic mean free path. In this work, we present a five-layer design for planar HE PDs assisted by triple junctions in which an ultrathin Pt layer dominates the broadband and displays strong optical absorption (>0.9 from 900 nm to 1700 nm). Optical studies reveal that the optical admittance matching between optical admittances of designed device and air at all interested wavelengths is responsible for broadband light-trapping that induces prominent energy depositions in Pt layers. Electrical investigations show that, benefitting from suppressed hot-electron transport losses and increased hot-electron harvesting junctions, the predicted responsivity of the designed HE PD is up to 8.51 mA/W at 900 nm. Moreover, the high average absorption (responsivity) of 0.96 (3.66 mA/W) is substantially sustained over a broad incidence angle regardless of the polarizations of incident light. The comparison studies between five-layer and three-layer devices emphasize the superiority of five-layer design in strong optical absorption in Pt layers and efficient hot-electron extraction.

Broadband light trapping in perovskite solar cells: Optimization and enhancement through exploiting multi-resonant Mie resonators

We present multi-resonant Mie resonators comprising dielectric nanopyramid arrays (DNPAs) for efficient broadband light trapping (LT) in perovskite solar cells (PSCs). Absorption in a 100 nm-thick active layer is significantly enhanced over a broad range of wavelengths through the simultaneous resonant excitation of electric and magnetic moments. Through optimization, a PSC integrated with a front-located DNPA LT structure having a base edge of 320 nm and a spacing of 180 nm at an aspect ratio of 1.5 yields a short-circuit current density (Jsc) of 18.13 mA/cm2, which is about 29 % higher than that of a planar PSC. Multipole resonance contributions and radiation patterns of DNPs are also investigated. The spectral overlap of the multipole resonances of the DNPAs can be easily tuned, thereby providing performance enhancements in diverse applications, such as nanoantennas, metasurfaces, photodiodes, and other solar cells.

Amber rainbow ribbon effect in broadband optical metamaterials

Using the trapped rainbow effect to slow down or even stop light has been widely studied. However, high loss and energy leakage severely limited the development of rainbow devices. Here, we observed the negative Goos-Hänchen effect in film samples across the entire visible spectrum. We also discovered an amber rainbow ribbon and an optical black hole due to perfect back reflection in optical waveguides, where little light leaks out. Not only does the amber rainbow ribbon effect show an automatic frequency selection response, as predicted by single frequency theoretical models and confirmed by experiments, it also shows spatial periodic regulation, resulting from broadband omnidirectional visible metamaterials prepared by disordered assembly systems. This broadband light trapping system could play a crucial role in the fields of optical storage and information processing when being used to construct ultra-compact modulators and other tunable devices.

Integrated solar evaporator based on photothermal Cu-CuOx/NC with heat-insulated polyurethane foam enabling highly efficient salt-tolerant desalination and water purification

Renewable solar energy meeting photothermal materials provides a promising approach for freshwater production via interfacial steam generation. The design and manufacture of high-performance evaporators are still highly anticipated for practical applications. Herein, the robust and durable solar evaporator is proposed and fabricated by integration of photothermal Cu-CuOx/NC (CNC) and three-dimensional porous structure heat-insulated polyurethane (PU) foam with polyvinyl alcohol (PVA)/sodium alginate (SA) modifications. The designed PVA-SA-CNC/PU simultaneously possesses fast water transmission, broadband light trapping, satisfactory mechanical robustness and effective heat confinement. When applied as a solar evaporator, the PVA-SA-CNC/PU exhibits a high evaporation rate of 1.99 kg m−2 h−1 and operates stably within ten consecutive cycles under one-sun illumination, which can also handle salt tolerant desalination of seawater and saline water to sustainably produce freshwater. In addition, the functional versatility of the fabricated evaporator is explored to purify various wastewater. This study establishes a highly efficient integrated evaporator with multitasking water purification towards the solution of water shortage.

Plasmonic Bullseye Nanocavities for Broadband Light Localization and Multi‐Wavelength SERS

AbstractPlasmonic nanostructures capable of broadband light trapping and field enhancement have promising applications in a wide range of fields. This study presents a platform for broadband, polarization‐independent field enhancement in the visible regime through the use of width‐graded nanocavities in a bullseye configuration. The fabrication procedure utilizes electron beam lithography (EBL) to achieve fine control over the nanocavity geometry and template stripping to enable rapid and low‐cost production. The utility of these devices as substrates for multi‐wavelength surface enhanced Raman spectroscopy (SERS) is demonstrated through molecular detection in a 10 μM solution at two excitation wavelengths. The impact of bullseye geometry on both the broadband spectral response and multi‐wavelength SERS performance is examined. The measured SERS enhancement factor (EF) is shown to depend primarily on the plasmonically active surface area of the device, regardless of the local electromagnetic field strength within the nanocavities. These results highlight not only the utility of the width‐graded bullseye as a broadband platform for SERS and other applications but also provide design guidelines to optimize the enhancement factor and broadband performance of similar devices.

Efficient Broadband Light-Trapping Structures on Thin-Film Silicon Fabricated by Laser, Chemical and Hybrid Chemical/Laser Treatments

Light-trapping structures formed on surfaces of various materials have attracted much attention in recent years due to their important role in many applications of science and technology. This article discusses various methods for manufacturing light-trapping "black" silicon, namely laser, chemical and hybrid chemical/laser ones. In addition to the widely explored laser texturing and chemical etching methods, we develop a hybrid chemical/laser texturing method, consisting in laser post-texturing of pyramidal structures obtained after chemical etching. After laser treatments the surface morphology was represented by a chaotic relief of microcones, while after chemical treatment it acquired a chaotic pyramidal relief. Moreover, laser texturing of preliminarily chemically microtextured silicon wafers is shown to take five-fold less time compared to bare flat silicon. In this case, the chemically/laser-treated samples exhibit average total reflectance in the spectral range of 250-1100 nm lower by 7-10% than after the purely chemical treatment.

Facile Synthesis of Vertically Arranged CNTs for Efficient Solar-Driven Interfacial Water Evaporation.

Solar-driven evaporation of water is a sustainable and promising technology for addressing the crisis of clean water. Herein, novel vertically arranged carbon nanotube (V-CNT) aerogels with a tree branch structure is facilely synthesized through an ice templating method. The V-CNT-based photothermal evaporator exhibits efficient broadband light trapping and super-hydrophilicity. Owing to the unique structure and ultrafast water transportation, a high evaporation rate of 3.26 kg m-2 h-1 was achieved by the three-dimensional V-CNT-based evaporator under a solar illumination of 1 kW m-2. More significantly, the V-CNT shows excellent recycling stability and salt-resistant performance in seawater and may provide a novel strategy to the practical sustainable technique of water purification applications.

Monolithic perovskite/black-silicon tandems based on tunnel oxide passivated contacts

<h2>Summary</h2> Perovskite/silicon tandems offer a promising pathway to increase the efficiency beyond the Auger limit of silicon at minimal additional cost. However, it is challenging to accommodate the solution-processed perovskite without flattening or reducing pyramidal texture of silicon subcells. Herein, we report the first monolithic perovskite/silicon tandem featuring an industrially applicable front-side-nanostructured black silicon with a tunnel oxide passivated contact (TOPCon). The TOPCon together with the surface reconstruction of black silicon contributes to the high-level surface passivation without sacrificing the broadband light trapping. Additionally, the reconstructed nanotexture significantly facilitates the wetting of perovskite and acts as a nanoconfining scaffold to guide the vertical growth of perovskite. The resulting tandem yields a certified efficiency of 28.2%, representing one of the highest efficiencies reported on either perovskite/TOPCon or double-side-textured perovskite/silicon tandems.

Light absorption enhancement in ultrathin perovskite solar cells using light scattering of high-index dielectric nanospheres.

Arrays of high-index dielectric nanoparticles supporting both electrical and magnetic resonances have gained increasing attention for their excellent light-trapping (LT) effects, thus greatly improving the performance of ultrathin solar cells. This work explores front-located, high-index dielectric subwavelength nanosphere arrays as an efficient and broadband LT structure patterned on top of an ultrathin perovskite solar cell (PSC) for a greatly enhanced absorption. Combined strong light scattering and anti-reflection properties achieved by optimized geometrical parameters of the LT structure lead to a broadband absorption enhancement in the ultrathin thickness of a photoactive layer (100 nm) yielding the short-circuit current density (Jsc) of 18.7 mA/cm2, which is 31.7% higher than that of a planar counterpart. Moreover, effects of the LT structure on far-field radiation patterns, scattering cross-sections, multipoles' contributions, and asymmetry parameters along with the incidence angle and polarization dependence are investigated. The present strategy could be applied to diverse applications, such as other ultrathin or semitransparent solar cells, absorbers and photodetectors.

Advanced damage-free neutral beam etching technology to texture Si wafer with honeycomb pattern for broadband light trapping in photovoltaics

Advanced damage-free neutral beam etching technology to texture Si wafer with honeycomb pattern for broadband light trapping in photovoltaics

Photonic non-Bloch quadrupole topological insulators in coupled ring resonators

We investigate the second-order topological phases in a two-dimensional ring resonator array with each plaquette occupied by \ensuremath{\pi} gauge flux and imaginary gauge field. The real and imaginary gauge fields are induced by shifting the displacement and integrating gain or loss into the two half perimeters of the auxiliary rings. The system supports topological corner modes with their emergence being determined by the non-Bloch topological invariant due to skin effects. The bulk modes, exhibiting second-order skin effects in both trivial and nontrivial phases, are accumulated at opposite corners depending on whether clockwise or counterclockwise modes are excited. By introducing an interface with different imaginary gauge fields, we show the bulk modes exist at the interface while the topological corner modes are localized at the physical corners. Furthermore, the skin effects are also presented in the passive ring resonators. The study may find applications in lasers and broadband light trapping.

Optimized design of silver nanoparticles for broadband and high efficiency light trapping in thin film solar cells

This paper reports a high-efficiency approach to improve the photoelectric-conversion efficiency of thin-film solar cells by plasmonic scattering and local near-field amplification of silver nanoparticles. We employ a three-dimensional (3D) electromagnetic model and use the finite-difference time-domain (FDTD) and rigorously coupled-wave analysis methods to investigate the interaction of light with such a metallic particle. The numerical results show that the absorption and scattering spectra depend upon the properties of the embedded particles and the refractive index of the surrounding material. Strong redshifts and high-order modes are observed in the response spectrum with the increase of the particle size and the refractive index of the surrounding material. With an optimized design having [Formula: see text], [Formula: see text], and [Formula: see text] nm, the performance of cell device is improved over a broad spectral range. Moreover, some of the absorption, in the resonance region, is beyond the Yablonovitch limit. The corresponding light-generated photocurrent is increased from 14.2 mA/cm2 to 18.3 mA/cm2, with a 28.9% enhancement compared with conventional cells with antireflective coatings (ARCs).

Flat photonics for broadband light-trapping

The ability to trap light in an ultrathin photoactive layer has been of great significance for applications ranging from optoelectronics, energy to spectroscopy. However, the current broadband light trapping suffers from undesirable direct Joule heat output with plasmonics or bulkiness with photonics. Here, we report a light-trapping photonic structure using an ultrathin all-dielectric super-absorbing metasurface. This presented photonic structure features asymmetrically coupled magnetic resonances, which eliminate reflection and transmission simultaneously by introducing destructive interference between the backscattered field of the resonance and the direct reflected field of the highly reflective Fabry–Pérot background. In particular, this photonic structure enables broadband light trapping by placing nanostructures of different sizes in a supercell. As a proof of concept, we experimentally demonstrate broadband (550–1280 nm) super absorption (&amp;gt;50%) within an ultrathin (∼200 nm) all-dielectric germanium metasurface. This work provides a design paradigm for harvesting light through flat photonic structures at the nanoscale and paves the way for cost-effective light management.

Photonic integration of uniform GaAs nanowires in hexagonal and honeycomb lattice for broadband optical absorption

We present an experimental approach toward the realization of GaAs nanowires in the form of square, hexagonal, and honeycomb lattices for photonic integration toward enhanced optical properties. We have carried out a design and fabrication process on GaAs wafers using electron beam lithography patterning, reactive ion etching for hard mask removal, and inductively coupled plasma etching of the material. The resulting photonic crystals are analyzed by field emission scanning electron microscopy. Nanowire array designs in a square, hexagonal, and honeycomb lattice with a variable height of nanowires have been studied. Using finite-difference time-domain simulation, we can derive the comparative optical absorption properties of these nanowire arrays. A very high broadband absorbance of &amp;gt;94% over the 400 nm–1000 nm wavelength range is studied for hexagonal and honeycomb arrays, while a square lattice array shows only a maximum of 85% absorption. We report a minimum of 2% reflectance, or 98% optical absorbance, over 450 nm–700 nm and over a wide angle of 45° through hexagonal and honeycomb lattice integration in GaAs. These results will have potential applications toward broadband optical absorption or light trapping in solar energy harvesting.

Dynamic bouncing mode cavity for integratable broadband light trapping and release

We propose a dynamic process based on a new structure of bouncing mode cavity to realize integratable long-term light pulse trapping and release on a waveguide. To break the delay-bandwidth limit, the idea of a bouncing mode cavity is to bind a guided light pulse bouncing back and forth inside. Being compatible with complementary metal-oxide-semiconductor (COMS) processing, an optimized boundary mirror design with an ultra-low loss and a broad bandwidth is presented. Using the dynamic bouncing mode cavity composed of a switchable boundary mirror, the simulation of controlled optical pulse storage is demonstrated on a two-dimensional waveguide. The results show that the long lifetime of 32 ns and the wide bandwidth of 16.11 THz can be obtained simultaneously. All the results are verified with finite-difference time-domain numerical calculation. With progress in manipulating the optical properties of materials, this approach may contribute to integrated photonic devices for optical information processing.

Effects of the ambient medium and structure parameter on the optical properties of tapered silicon nanowire

Tapered silicon nanowire (T-SiNW) is expected to have a wider application than a cylindrical one in optical sensors, photovoltaics, etc. due to its broadband light-trapping. However, there is little research on the effect of ambient medium and structure parameters on the optical properties of T-SiNW. First, based on numerical simulations it is found that, compared with that in air, when a T-SiNW is embedded in ambient medium with refraction index of 1.284: the absorbed light by it under given irradiation becomes much less; the scattered light by it shows a wider angular distribution; the nearfield light around it becomes stronger in a wider area. These mean that T-SiNW is more desirable for light-trapping in the ambient medium. Then, as the bottom diameter of T-SiNW increases, its light-collecting waveband red-shifts; at the same time, its scattered light shows a broadened angular distribution range. Hence, T-SiNW with larger bottom diameter is more desirable for light-trapping, especially for longwave light which is sometimes difficult to be utilized in photovoltaic devices. Besides, we found that the light-collecting and -absorption multiples of T-SiNW are both proportional to its length approximatively, which provides a very convenient way to regulate its light-trapping performance. The insights in this work are helpful to the design of devices using T-SiNW as the main block.

A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, ~150 µm for silicon and 2–4 µm for Cu(In,Ga)(S,Se)2, CdTe or III–V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry–Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target.

Synergetic Transparent Electrode Architecture for Efficient Non-Fullerene Flexible Organic Solar Cells with >12% Efficiency.

Flexible organic solar cells (OSCs) are considered one key component in wearable, intelligent electronics due to the unique capacity for highly flexible renewable energy sources. However, it is urgently required to enhance their efficiency, as it is far inferior to that of their conventional, glass-based counterparts. To boost the performance of flexible OSCs on plastic substrates, we here present a synergetic transparent electrode structure, which combines electrically conductive silver nanowires, a sol-gel-derived ZnO planarization layer, and imprinted light-trapping nanostructures. This synergetic composite electrode exhibits good properties in terms of optical transparency, electrical conductivity, mechanical flexibility, and low-temperature processability. As a result, the single-junction non-fullerene-based flexible OSCs achieve a power conversion efficiency exceeding 12% due to the synergetic interplay between broadband light trapping and suppressed charge recombination loss. Moreover, these flexible OSCs are repeatedly bendable in both inward and outward bending directions, retaining over 60% of the initial efficiency after 1000 cycles of the bending test at a 3.0 mm radius. These results convey a clear depiction of the practicality of flexible OSCs in a variety of high-performance flexible applications.

Quantitative analysis of gradient effective refractive index in silicon nanowires for broadband light trapping and anti-reflective properties

Controlling the light reflection using vertically-aligned nanowires has great importance in fundamental research with interesting applications in photonic devices. Here, we discuss the spatial- and polarization-dependent reflectivity measurements from the cross-sectional as well as from the top surface of vertically-aligned disordered silicon nanowires. The gradient variation in an effective refractive index along the nanowire length is estimated using the cross-sectional reflectivity measurements. We have studied the gradient variation of an effective refractive index profile and its tunability with the nanowire length. The reflectivity is measured to be as low as 5% irrespective of spatial directions and the polarization of incident light in a broad wavelength range. This constitutes the signature of a broadband omnidirectional anti-reflector that is scalable with the nanowire length. The reflectivity measurements are in good agreement with theoretical calculations. Such omnidirectional anti-reflection in a broad wavelength range is useful for applications such as photon management in photovoltaic devices and disorder-induced light scattering.

Experimental demonstration of broadband light trapping by exciting surface modes of an all-dielectric taper

We design an all-dielectric taper and then excite its surface modes by illuminating a plane wave upon the taper to achieve broadband light trapping spanning from 20 to 100 GHz. Via Lewin’s theory, such excitation of surface modes could be analogous to “trapped rainbow”, i.e., activation of negative Goos-Hänchen effect within a negative refractive waveguide. To further reinforce this statement, the corresponding power flow distributions within the all-dielectric taper are recorded in finite-integration time domain simulation and suggest that a chromatic incident pulse is indeed trapped at different critical thicknesses of the taper, a character of the negative refractive waveguide. Finally, the transmittance is measured and compared to the simulated ones. The two followed the similar trend.