Omnidirectional Light Absorption Research Articles

Omnidirectional, Broadband Light Absorption in a Hierarchical Nanoturf Membrane for an Advanced Solar‐Vapor Generator

AbstractSolar‐thermal materials have been intensively studied in the context of production and localization of thermal energy, targeting an industry level application. Although photonic and optical strategies for enhancing light absorption have increased the efficiency of photo excitation/conversion into thermal energy, most of them have several limitations such as large area fabrication, thermal stability and broadband/omnidirectional light absorption. In this study, a gold‐coated hierarchical nanoturf membrane (Au/h‐Nanoturf membrane) incorporated with randomly distributed high aspect ratio (AR) nanostructures and micro‐through holes is proposed. The Au/h‐Nanoturf has peculiar black structures that provide advantages in forming a membrane with a large area and in absorbing broadband solar light spectrum. Furthermore, the membrane is combined with micro‐cone array which makes it exhibit exceptionally omnidirectional light absorption properties. Using computational analysis, it is confirmed that the micro‐cone array substantially contributes to the omnidirectional solar absorption irrespective of the wavelength. The optimized structural parameters for the maximum efficiency of the solar‐thermal materials are also found. The prepared solar‐vapor generator with the optimized structural features exhibits 91% average conversion efficiency under one sun condition. The efficiency is sustainable for up to four weeks. The highly efficient and omnidirectional broadband‐absorbing solar‐thermal membrane can be readily employed, targeting an industry level application.

Integrated Evaporator for Efficient Solar-Driven Interfacial Steam Generation.

Solar-driven interfacial steam generation is a promising technique for clean water production because it can minimize thermal loss by localizing solar-to-heat conversion at the air/liquid interface. Here we report an integrated solar evaporator by partially growing 2D polypyrrole microsheets within a melamine foam through chemical vapor polymerization. These microsheets can induce multiple light reflections within the foam, enable omnidirectional light absorption, provide abundant surfaces to promote heat transfer, and achieve spatially defined hydrophobicity to facilitate vapor escape. Meanwhile, the inherent hydrophilicity of the bottom part of the foam promotes spontaneous upward water transport and suppresses heat loss. The composite foam exhibits an excellent apparent evaporation rate of ∼2 kg/(m2·h) and solar-to-vapor efficiency of ∼91%. The combined advantages of large surface area, high efficiency, low cost, all-weather application, excellent durability, and scalable manufacturing make our integrated design promising for fabricating large-scale solar steam generation systems that are suitable for practical clean water production.

Highly Efficient Semiconductor-Based Metasurface for Photoelectrochemical Water Splitting: Broadband Light Perfect Absorption with Dimensions Smaller than the Diffusion Length

In this paper, we demonstrate a highly efficient light trapping design that is made of a metal-oxide-semiconductor-semiconductor (nanograting/nanopatch) (MOSSg/p) four-layer design to absorb light in a broad wavelength regime in dimensions smaller than the hole diffusion length of the active layer. For this aim, we first adopt a modeling approach based on the transfer matrix method (TMM) to find out the absorption bandwidth (BW) limits of a simple hematite (α-Fe2O3)-based metal-oxide-semiconductor (MOS) cavity design. Our modeling findings show that this design architecture can provide near-perfect absorption in shorter wavelengths. To extend the absorption toward longer wavelengths, a nanostructured semiconductor is placed on top of this MOS design. This nanostructure supports the Mie resonance and adds a new resonance in longer wavelengths without disrupting the lower wavelength absorption capability of MOS cavity. By this way, a polarization-insensitive absorption above 0.8 can be acquired up to λ=565 nm. Moreover, to have a better qualitative comparison, the water-splitting photocurrent of this design has been estimated. Our calculations show that a photocurrent as high as 10.6 mA cm−2 can be achieved with this design that is quite close to the theoretical limit of 12.5 mA cm−2 for hematite-based water-splitting photoanode. This paper proposes a design approach in which the superposition of cavity modes and Mie resonances can lead to a broadband, polarization-insensitive, and omnidirectional near-perfect light absorption in dimensions smaller than the carrier’s diffusion length. This can be considered as a winning strategy to design highly efficient and ultrathin optoelectronic designs in a variety of applications including photoelectrochemical water splitting and photovoltaics.

All-solution-processed Si films with broadband and omnidirectional light absorption

Silicon photonic structures have attracted a great deal of attention due to their potential benefits of efficient light management in optoelectronic applications. In this paper, we demonstrate broadband optical absorption enhancement in solution-processed amorphous silicon (a-Si) by leveraging the advantages of silicon photonic structures. Graded refractive index silicon multi-layer structures are employed by modulating optical constants with simple process optimization, resulting in significantly improved reflectance over a broad range of visible wavelengths. In addition, nanopatterning flexibility of solution-processed silicon provides benefits for tailoring silicon optical properties. With the incorporation of the two-dimensional submicron pattern into silicon films, the absorptivity of silicon films improves considerably below the wavelength of the bandgap (λ ∼ 800 nm), and the limited bandwidth of absorptivity in silicon films can be extended to near-infrared wavelengths by coating with a thin gold layer. The methodology is generally applicable to a platform for improving the broadband optical absorption of photonic and optoelectronic devices.

Omnidirectional and Broadband Light Absorption Enhancement in 2-D Photonic-Structured Organic Solar Cells

The effect of 2-D photonic structures on omnidirectional and broadband light absorption enhancement in organic solar cells (OSCs) is analyzed using a combination of theoretical simulation and experimental optimization. The photonic structures in the active layer, with a blend system of poly[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-bA]dithiophene-2,6-diyl][3-fluoro-2-[(2- ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]:[6,6]-phenyl-C70- butyric-acid-methyl-ester (PTB7:PC70BM), were prepared by the nanoimprint method. It shows that the 2-D photonic structures enable not only broadband but also omnidirectional absorption enhancements in the PTB7:PC70BM-based OSCs over a broader angle range of the incident light, leading to >11% increase in the power conversion efficiency, as compared to an optimal planar control cell. A weak angular dependency on light absorption is a unique feature of the photonic-structured OSCs, which is useful for different applications.

Fabrication of Orientation-Tunable Si Nanowires on Silicon Pyramids with Omnidirectional Light Absorption.

In this work, the different orientation of SiNWs on Si pyramids by a two step MACE method have been fabricated. By tuning the structure of Ag catalyst film and controlling the concentration of H2O2 or the etching temperature, the tunability of the orientation of SiNWs from <111> to <100> on Si pyramids was realized. Si structures composed of Si pyramids and SiNWs exhibit better omnidirectional light-trapping ability by multiple reflections. Si structures with structural tunability and enhanced light harvesting performance will find a wide variety of significant applications in solar cells, photodetectors, and optoelectronic devices.

Wide-angle polarization-free plasmon-enhanced light absorption in perovskite films using silver nanowires.

Since the successful implementation of organic-inorganic hybrid perovskites as light-absorbing materials, stunning progresses have been made towards the efficiency boost of perovskite solar cells. To build upon these successes, further impetus may derive from revisits to the intrinsic properties of perovskites, such as their optical properties. Herein, we introduce periodic Ag nanowire (AgNW) structures into perovskite films to optimize their solar absorption efficiency through plasmonic interactions. Numerical simulations show a remarkable integrated solar absorption enhancement of 25.9% attained by incorporating properly tailored AgNW arrays into perovskite films. The AgNW crosses are further introduced to achieve polarization-independent light harvesting capability. The omnidirectional light absorption enhancement ability of the AgNW embedded perovskite films is also demonstrated.

Broadband omnidirectional light detection in flexible and hierarchical ZnO/Si heterojunction photodiodes

The development of flexible photodetectors has received great attention for future optoelectronic applications including flexible image sensors, biomedical imaging, and smart, wearable systems. Previously, omnidirectional photodetectors were only achievable by integration of a hemispherical microlens assembly on multiple photodetectors. Herein, a hierarchical photodiode design of ZnO nanowires (NWs) on honeycomb-structured Si (H-Si) membranes is demonstrated to exhibit excellent omnidirectional light-absorption ability and thus maintain high photocurrents over broad spectral ranges (365 to 1,100 nm) for wide incident angles (0° to 70°), which enabled broadband omnidirectional light detection in flexible photodetectors. Furthermore, the stress-relieving honeycomb pattern within the photodiode micromembranes provided photodetectors with excellent mechanical flexibility (10% decrease in photocurrent at a bending radius of 3 mm) and durability (minimal change in photocurrent over 10,000 bending cycles). When employed in semiconductor thin films, the hierarchical NW/honeycomb heterostructure design acts as an efficient platform for various optoelectronic devices requiring mechanical flexibility and broadband omnidirectional light detection.

Improved broadband and omnidirectional light absorption in silicon nanopillars achieved through gradient mesoporosity induced leaky waveguide modulation

Omnidirectional and polarization insensitive light coupling through mesoporosity induced waveguiding in Si nanopillars, fabricated by metal assisted chemical etching and nanosphere lithography.

Extremely Thin Photoelectrode Architectures for Photocatalysis

Introduction Concepts from metamaterials, plasmonics and nanophotonics are highly promising to improve the design of future solar energy conversion devices. Here, I will describe how we use these concepts to design advanced photoelectrode architectures for driving two challenging photocatalytic reactions: water splitting and CO2 reduction. We focus on the light management aspect in extremely thin absorber structures and in plasmonic metal nanostructures, to achieve broadband omnidirectional solar light absorption while carefully choosing materials systems that allow for efficient charge separation and catalytic activity. Such thin-film absorber photoelectrodes hold promise for achieving enhanced charge carrier extraction, increased photovoltages, and the possibility to exploit hot carriers for purposes of driving chemical reactions. In addition, architectures made from two-dimensional materials building blocks allow us to tailor new electronic, catalytic and optical properties by engineering materials one layer at a time. However, light absorption in single-layer materials is generally small, and efficient photoconversion structures require nanophotonic concepts to enhance their optical properties. Experimental We are exploiting nanophotonics concepts to realize extreme light absorption in two-dimensional (2D) transition metal dichalcogenides. I will discuss the analytical models and three-dimensional electromagnetic simulations that we employ to engineer light absorption in these 2D materials and in plasmonic metal nanostructures, and describe our progress towards the experimental realization and characterization of such structures. Complementing these materials and device design efforts, we are developing an experimental characterization toolbox, including photoelectrochemical and spectroscopic techniques. Materials We have designed both plasmonic- and dielectric-based ultrathin photoelectrode structures 1. To create centimeter-scale nanostructured photoelectrodes, we have developed the synthesis of Anodic Aluminum Oxide (AAO) templates as generic deposition masks for plasmonic and non-plasmonic materials. The AAO masks can be transferred to a wide range of substrates (including rough transparent conducting oxides and metal oxides) and e-beam evaporation of metals of various sizes, shapes and periodicities can be accomplished. The AAO templates can also be used for lateral nanostructuring of two-dimensional materials. Sample characterization were performed by SEM, XRD, AFM. Photoelectrochemical measurements of water splitting were performed on plasmonic nanostructures, combined with suitable metal oxides for efficient charge separation. Results and Discussion Full-field electromagnetic simulations of our absorber structures suggest that they are suitable to substantially enhance the optical absorption over a broad wavelength range, covering regions of the solar spectrum that contain a large portion of the solar energy flux. The ultrathin nature of our structures also renders them relatively immune to efficiency reductions under non-normal incidence illumination. We will discuss first results on photoelectrode fabrication, and their materials-, optical and photoelectrochemical characterization. Conclusions This work is aimed at developing and characterizing advanced photoelectrode architectures for two challenging photocatalytic reactions, water splitting and CO2 reduction. Such structures will ultimately allow the deployment of solar-based approaches for driving large-scale applications in photoelectrochemistry and photocatalysis, and may hold promise for new generations of opto-electronic devices.

Omnidirectional absorption enhancement of symmetry-broken crescent-deformed single-nanowire photovoltaic cells

Single semiconductor nanowires enable a diverse range of applications in sensing, photodetection, and photovoltaics. Here we report that a delicate modification to the circular cross-section by eliminating a nanocrescent part can substantially strengthen the light-harvesting capability of a single nanowire. Despite the reduced photoactive material, the photocurrent density of the symmetry-broken crescent-deformed free-standing silicon single-nanowire solar cell can be counter-intuitively improved by over 45%. The excellent light-harvesting ability of the crescent-deformed nanowire can be maintained over a broad spectral band at a wide range of incident angle as corroborated by its dispersion relation. Under assistance of a metallic back reflector, the photocurrent enhancement ratio can be up to 66.2% without taking into account the material reduction. Our electrical evaluation shows that the new design leads to an enhancement ratio of light-conversion efficiency by 40.8%. This novel design by breaking structure symmetry represents a new pathway to realize highly efficient and compact photovoltaic devices with omnidirectional light absorption enhancement.

Omnidirectional and broadband optical absorption enhancement in small molecule organic solar cells by a patterned MoO3/Ag/MoO3 transparent anode

We designed and calculated a novel organic solar cell (OSC) with MoO3/Ag/MoO3 (MAM) grating as transparent anode and the patterned copper phthalocyanine (CuPc)/fullerence (C60) as active layer. The numerical results indicate that a broadband, omnidirectional light absorption enhancement is realized by utilizing such a one-dimensional (1D) grating with core–shell structure. The total absorption efficiency of the active layer over the wavelength range from 400 to 900nm is enhanced by 178.88%, 19.44% and 99.16% relative to the equivalent planar cell considering the weight of air-mass 1.5 global (AM 1.5G) solar spectrum at normally incident transverse magnetic (TM), transverse electric (TE) and TM/TE hybrid polarized light, respectively. The improved light trapping is attributed to the multiple modes hybridization of propagating surface plasmon polaritons (SPPs), localized surface plasmons (LSPs) and the strong coupling of SPP waves at TM polarization along with the Floquet modes at TE polarization. Furthermore, the proposed optimized architecture also exhibits an expected short-circuit current density (Jsc) with the value of 11.11mA/cm2 in theory, which is increased by 116.6% compared with that of the planar control device.

Quasi-total omnidirectional light absorption in nanostructured gold films

A series of experiments and simulations have been performed to evidence the omnidirectional light absorption of planar gold structures containing a twodimensional lattice of spherical beads or pores. We show that more than 90 % of incident light is absorbed at angles of incidence up to 65° for optimum values of the gold film thickness. We also report the tunability of the absorption wavelength by varying the size of the beads/pores.

Toward omnidirectional light absorption by plasmonic effect for high-efficiency flexible nonvacuum Cu(In,Ga)Se2 thin film solar cells.

We have successfully demonstrated a great advantage of plasmonic Au nanoparticles for efficient enhancement of Cu(In,Ga)Se2(CIGS) flexible photovoltaic devices. The incorporation of Au NPs can eliminate obstacles in the way of developing ink-printing CIGS flexible thin film photovoltaics (TFPV), such as poor absorption at wavelengths in the high intensity region of solar spectrum, and that occurs significantly at large incident angle of solar irradiation. The enhancement of external quantum efficiency and photocurrent have been systematically analyzed via the calculated electromagnetic field distribution. Finally, the major benefits of the localized surface plasmon resonances (LSPR) in visible wavelength have been investigated by ultrabroadband pump-probe spectroscopy, providing a solid evidence on the strong absorption and reduction of surface recombination that increases electron-hole generation and improves the carrier transportation in the vicinity of pn-juction.

Omnidirectional light absorption of disordered nano-hole structure inspired from Papilio ulysses

Butterflies routinely produce nanostructured surfaces with useful properties. Here, we report a disordered nano-hole structure with ridges inspired by Papilio ulysses that produce omnidirectional light absorption compared with the common ordered structure. The result shows that the omnidirectional light absorption is affected by polarization, the incident angle, and the wavelength. Using the finite-difference time-domain (FDTD) method, the stable omnidirectional light absorption is achieved in the structure inspired from the Papilio ulysses over a wide incident angle range and with various wavelengths. This explains some of the mysteries of the structure of the Papilio ulysses butterfly. These conclusions can guide the design of omnidirectional absorption materials.

Omnidirectional and polarization-insensitive light absorption enhancement in an organic photovoltaic device using a one-dimensional nanograting

A novel poly-3-hexylthiophene (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) solar cell was designed with both the silver cathode and the organic layers structured into one-dimensional periodic patterns. Its optical behavior was investigated systematically by the finite element method. The numerical results indicate that a broadband, omnidirectional, and polarization-insensitive light absorption enhancement is realized by utilizing this structure. This is attributed to the simultaneous excitation of both plasmonic and photonic modes by particular structural designs. Moreover, the increased absorption allows for the use of very thin active layers, resulting in decreased losses of charge carriers transporting to the electrodes. Therefore, this proposal is expected to find potential applications in organic solar cells for improving the overall device performance.

Infrared radiative properties of two-dimensional square optical black holes with materials of insulators and semiconductors

Optical black hole (OBH) is a special optical structure in which the dielectric function or refractive index of media becomes gradually larger from the outside to the core. Square optical black hole (SOBH) has been proven to be a special and remarkably useful optical structure with broadband omnidirectional light absorption, and a device that is easier to fabricate than circular optical black hole (COBH). The main goal of this paper is to find potential applicable materials of SOBH due to fabrication and infrared radiative properties consideration. The infrared radiative properties of an optical slab with periodic two-dimensional SOBHs with several insulators and semiconductors materials are numerically studied with the finite-difference time-domain (FDTD) method. The results show that the spectral radiative properties of the optical slab change with different number of materials and dimensions of SOBHs. When the incidence wavelength increases to 10μm, the absorptance of the optical slab decreases as SOBH dimension decreases. This is due to the diffraction effect. The material selection and spectral radiative property requirement should be taken into account for device designs and fabrication.

Enhancement of light-matter interactions in slow-wave metasurfaces

We show that an ultrathin metamaterial can trap photons for a long time, with the measured longest trapping time being \ensuremath{\sim}5 ns for a microwave pulse with a center frequency of 5.5 GHz. Such photon-trapping effect is governed by the anomalous dispersion and surface plasmon excitations of the system, which is more significant in thinner samples owing to stronger mode hybridizations. Light-matter interactions are remarkably enhanced inside these structures, leading to perfect omnidirectional light absorption and dramatically enhanced nonlinear generations, which are demonstrated by microwave experiments and full-wave simulations.

Omnidirectional light absorption in thin film silicon solar cell with dual anti-reflection coatings

Abstract A theoretical model for an all-in-one thin film silicon solar cell (TFSSC) design with the anti-reflection (AR) coatings, the transparent electrodes, the silicon and the back reflective coatings all deposited on one piece of glass is proposed and the optical performance for the design is numerically simulated. The calculated average reflectance over the wavelength range of 0.4–1.0 μm and incident angles from 0° to 75° for the optimized dual-AR coatings as a whole is 3.67% – ranking among the lowest values for current AR coatings on silicon solar cells. Furthermore, the spectrum-averaged absorptance (SAA) in the 5-μm-thick cell in the 0.3–1.2 μm wavelength range decreases only by 2.58% when the incident angle varies from 0° to 75°, clearly showing the omnidirectional characteristics of the dual-AR coatings. With reasonable assumption of the internal quantum efficiency (near unity), the 5-μm cell produces a spectrum averaged external quantum efficiency (EQE) of 81.4% at 75° angle of incidence – 6.7% larger than the experiment result of the best planar bulk cell. Under equal sunshine conditions, TFSSCs generally give higher output voltages than their bulk counterpart if equal light absorption is assumed, so the 5-μm cell has the potential to reach the highest experimental conversion efficiency of the best planar bulk cell. By using a special mathematical technique, we are able to prove the angle-dependent absorption curves in the longer wavelength region tend to converge to a small neighborhood of the upper limit irrespective of significant differences between the transmission profiles. This indicates the AR requirement in the longer wavelength region can be significantly relaxed.

Infrared radiative properties of two-dimensional square optical black holes

Optical black hole (OBH) is a special optical structure, in which the dielectric function or refractive index of media becomes gradually larger from the outside to the core. The circular optical black holes (COBH) have been proved remarkably useful for broadband omnidirectional light absorption. The main goal of this paper is to propose an alternative square structure of OBH due to fabrication consideration. The infrared radiative properties of two-dimensional SOBHs are numerically studied with geometric optics approximation (GOA) and the finite-difference time-domain (FDTD) method. A critical wavelength is found in this paper: when the wavelength is smaller than half of the inner core side length, the Poynting vectors obtained by the FDTD method agree well with the ray trajectories calculated by the GOA, and the absorptance is nearly 100% with a proper refractive index gradient. While the wavelength is increasing, the net energy flows do not agree with the ray trajectories and the absorptance will be decreased. The reason is attributed to the diffraction effect, which is discussed in detail in this paper.