Broadband Light Absorption Research Articles

Nanofiber based origami evaporator for multifunctional and omnidirectional solar steam generation

Interfacial solar steam generation is considered as an effective and environment-friendly technology for clean water production. However, the preparation of an evaporator with omnidirectional light harvesting, high evaporation rate, catalytic and antibacterial properties simultaneously remains a great challenge. Herein, we report the Ag nanoparticles decorated MXene nanosheets/polyacrylonitrile (Ag@MXene/PAN) nanofiber based evaporators for high-efficiency, multifunctional and omnidirectional solar steam generation by solution electrospinning technology. The conjunction of Ag nanoparticles and MXene nanosheets not only enhances the broadband light absorption and heat production, but also contributes to the good catalytic and antibacterial performance. Furthermore, the flexibility and foldability of Ag@MXene/PAN nanofiber membranes enable the design of 3D fine-structure evaporator with increased evaporation surface area and optimized light absorption. As a proof of concept, Ag@MXene/PAN nanofiber based origami evaporator is constructed, which demonstrates relatively stable evaporation rates during a wide range of incident angle from 30° to 150°. The maximum evaporation rate reaches 2.08 kg m−2 h−1 under 1 sun illumination, far superior to the state-of-the-art solution-electrospun nanofiber based evaporators. Integration of structure flexibility, high performance and multi-functionality makes the designed nanofiber based evaporator promising for practical clean water production.

Study on the optical characteristics of gradient Al component AlGaN nanowires for ultraviolet photocathode

We numerically studied the effect of the geometric structure and Al component on the optical capture performance of gradient Al component AlxGa1 − xN photocathodes. The effects of geometric parameters, such as base radius (R), wire-to-wire spacing, cone rate, and angle of incident light, on the optical response were systematically studied based on the finite element method. In the radial direction, we study the optical response of rectangular periodic structure and hexagonal periodic structure. Simulation results show that pencil nanostructure can achieve omnidirectional and broadband light absorption of AlxGa1 − xN nanowires with hexagonal periodic structure. In addition, we used the Spicer three-step emission model to establish the photoemission efficiency of the AlxGa1 − xN nanostructure. As a result, the photocathode achieves optimal quantum efficiency when the Al component is in the range of 0 to 0.75 and sublayer thickness of 240, 180, 120, and 60 nm.

Improving the light absorption efficiency in thin-film plasmonic tandem solar cell

Recently, thin-film solar cells have received much attention due to low production cost. However, such cells suffer from low efficiency due to which they cannot be used for practical applications. To cater this problem, we have numerically investigated thin-film tandem solar cells comprising of double active layers made of amorphous silicon (a-Si) and crystalline silicon (c-Si) of different energy bandgaps. The tandem cell is found to absorb large amount of solar photons compared to a single a-Si and c-Si solar cells. To further improve the absorption efficiency, we efficiently utilized the surface plasmon resonance (SPR) effect of plasmonic nanoparticles, which are deposited in the form of a square lattice in the top a-Si active layer. The absorption characteristics of the cell are enhanced and tuned by optimizing the material and size of the nanoparticles. Moreover, periodic disorders are incorporated, which allow mixing of plasmonic modes having different angular momenta, resulting in a super broadband light absorption from 300 to 1100 nm and yielding a short circuit current density of 33.917 mA/cm2 in the active layers. The proposed plasmonic tandem cell can be advantageous for most of the photovoltaic applications.

Graphene and its Derivatives-Based Optical Sensors.

Being the first successfully prepared two-dimensional material, graphene has attracted extensive attention from researchers due to its excellent properties and extremely wide range of applications. In particular, graphene and its derivatives have displayed several ideal properties, including broadband light absorption, ability to quench fluorescence, excellent biocompatibility, and strong polarization-dependent effects, thus emerging as one of the most popular platforms for optical sensors. Graphene and its derivatives-based optical sensors have numerous advantages, such as high sensitivity, low-cost, fast response time, and small dimensions. In this review, recent developments in graphene and its derivatives-based optical sensors are summarized, covering aspects related to fluorescence, graphene-based substrates for surface-enhanced Raman scattering (SERS), optical fiber biological sensors, and other kinds of graphene-based optical sensors. Various sensing applications, such as single-cell detection, cancer diagnosis, protein, and DNA sensing, are introduced and discussed systematically. Finally, a summary and roadmap of current and future trends are presented in order to provide a prospect for the development of graphene and its derivatives-based optical sensors.

Analytic formulas and numerical simulations for non-uniform AlxGa1-xN nanostructure on electro-optical properties for UV Photodetectors

In this paper, we numerically studied the influence of the geometry of gradient Al component AlxGa1-xN nanowires on their optical response. Based on the finite element simulation software COMSOL Multiphysics, we systematically studied the influence of geometric parameters of base radius (R), nanowires spacing and angle of incident light (AOI) on the optical response. To this end, we designed various gradient Al component AlxGa1-xN nano-trapped structure in the wavelength range of lambda (300–400 nm). The simulation results show that the optimal optical absorption spectra of hexagonal pyramid nanostructure arrays range from 300 to 400 nm and the average optical absorption efficiency is 97.87%, achieving omnidirectionality and unified broadband light absorption for AlxGa1-xN ultraviolet photocathode. In addition, we adjust the distribution and thickness of the Al component in the nanowires to achieve the optimization of the quantum efficiency and collection efficiency of the gradient Al component AlxGa1-xN. Using Spicer’s three-step emission theoretical model, the photoemission efficiency of variable Al component AlxGa1-xN nanowires array was calculated. As a result, when the Al component range is 0–0.19, the external electric field is 2.5 (V/μm) and the sublayer thickness is 60 nm, 120 nm, 180 nm, 240 nm, respectively, the optimal quantum efficiency of the AlGaN photocathode is 38.16% and the optimal collection efficiency is 31.12%. All these findings not only indicate that the gradient Al component AlxGa1-xN material has great potential advantages for UV band detection, but also provide theoretical support for the experiment and preparation of AlGaN photocathodes.

Optical absorption enhancement in GaN nanowire arrays with hexagonal periodic arrangement for UV photocathode

We investigated the influence of geometric parameters on the light absorption of gallium nitride (GaN) nanoarrays with hexagonal periodic arrangement by the finite element analysis method. In this article, we simulated the various structural parameters of GaN nanostructure to obtain the optimum optical efficiency over the substrate filling rates (FRs) ranging from 0.2 to 0.9. It was found that non-uniform hexagonal pyramid nanostructure achieved more significant and broadband light absorption than that of uniform structures in ultraviolet wavelength range from 200 to 400 nm. Simulation results indicate that the non-uniform hexagonal pyramid nanostructure can reduce the reflection on the substrate surface and increase the optical absorption. The distribution non-uniformity can be used as an attempt to improve the light absorption efficiency for optoelectronic device performance. We also analyzed the effects of cross- sectional shape, angles of incident light, and wire-to-wire spacing on the optical absorption of various GaN structures. In conclusion, non-uniform hexagonal pyramid nanostructure (FR = 0.8) with hexagonal periodic arrangement shows significant enhancement of optical absorption. All the results provide an effective solution to enhance optical properties for GaN ultraviolet photocathode.

Surface morphological and optical properties of flexible black silicon fabricated by metal-assisted chemical etching

Flexible black silicon (b-Si) is a promising candidate to reduce technology cost while maintaining superior light absorption and high photocurrent in crystalline silicon (c-Si) solar cells. In this study, surface morphological and optical properties of flexible b-Si fabricated by metal-assisted chemical etching (MACE) have been investigated. Monocrystalline silicon (mono c-Si) wafers are thinned down to 65 μm by concentrated potassium hydroxide (KOH) solution to produce flexible wafers. Then, b-Si nanowires (b-Si NWs) are fabricated using one-step MACE process with different etching durations (5–25 min), utilizing silver (Ag) as catalyst. The flexible b-Si etched for 20 min exhibits nanowires with length of about 1.61 μm, which leads to the lowest average weighted reflectance (AWR) and the highest broadband absorption within 300–1100 nm wavelength region. The enhanced broadband light absorption is attributed to refractive index grading effect at the air/b-Si interface which increases light-coupling into the flexible b-Si absorber. With the 20 min etching, the maximum potential short-circuit current density (Jsc(max)) in the b-Si absorber increases to 40 mA/cm2, or 70.2% relative enhancement when compared to the c-Si reference.

Broadband light absorption by metal nanoparticle or quantum dot-coated silicon nanostructures for solar cell applications

Light absorption by metal nanoparticle (MNP) and quantum dot (QD) coated nanostructures is theoretically investigated for crystalline silicon (Si) solar cells. Despite light trapping by bare Si nanostructures, the near infrared region (NIR) of the solar spectrum remains unharvested owing to the indirect bandgap (1.1 eV) and low absorption coefficient of Si beyond visible wavelengths. In this work, periodic Si nanostructures, in the form of vertical nanowires (NWs), nanopyramids, and flat-topped nanocones, have been modeled, with their sidewalls decorated with spherical gold (Au) NPs. MNPs scatter light into neighboring Si and introduce localized plasmonic effects, thereby, offering strong broadband absorption and high conversion efficiency. Finite-difference time-domain (FDTD) analysis shows enhanced absorption beyond visible wavelengths, in the presence of Au NPs, for varying NP size and nanostructure periodicity. For 700 nm < λ < 1600 nm, the average absorption by Au NP coated Si nanopyramids and flat-topped nanocones is 5× higher than their bare counterparts. Moreover, Si QD/Si NW hybrid structures display better absorption characteristics than the Au NP/Si NW combination. The findings can be used to design and optimize highly efficient Si solar cells that combine light trapping nanostructures with broader, size tunable absorption profiles of plasmonic NPs and QDs.

An ordered copper nanowell-array film with near-perfect broadband absorption from ultraviolet to near-infrared light

A copper film with the well arrays structure is prepared by magnetron sputtering in an anodic aluminum oxide (AAO) template. The ultraviolet–visible absorption spectra indicate that the light absorbance of the film is as high as 99.6% in the wavelength range of 200–2500 nm. The copper nanowell-array film exhibits a strong absorptance at the spectral range of 200–2500 nm owing to light trapped in the nanowell-array structures, the interband transition of copper, and the surface plasmon resonances of copper nanowell-arrays. This near-perfect broadband light absorption from UV–vis–NIR might improve greatly the utilization of light.

Optimization of etching time for enhanced broadband absorption in flexible black silicon on stainless steel foil

This paper presents optimization of etching time in metal-assisted chemical etching (MACE) process for enhanced broadband absorption in flexible black silicon (b-Si) on stainless steel (SS) foil. Flexible monocrystalline silicon (mono c-Si) wafers with 65 μm thickness are used in this work. For the optimization of etching time, one-step MACE process is utilized with 10−25 min etching to fabricate the b-Si absorber. The SS foil beneath the b-Si also acts as a back reflector (BR) which ensures zero transmission loss in the long wavelength region (above 800 nm). After 20 min etching, b-Si with average nanowires (NWs) length of 982 nm is formed. This condition exhibits the lowest average weighted reflection (AWR) and the highest broadband light absorption in the b-Si within 300−1100 nm wavelength region, owing to refractive index grading effect at the air/b-Si interface. This corresponds to the maximum potential short-circuit density (Jsc(max)) of 39.3 mA/cm2, or 62.4 % enhancement when compared to the planar c-Si reference. The findings demonstrate the potential of flexible b-Si on SS foil to deliver high photocurrent in the future solar cells.

Designs of photoabsorption-enhanced variable Al component GaN nanostructure for UV photodetectors

Abstract In this paper, we study the effect of geometric structure of gradient Al component AlxGa1-xN nanowires on their optical response. Based on the finite element simulation software Comsol Multiphysics, we systematically studied the optical properties of different geometric parameters of base radius (R), axis inclination (θ) and Angle of incident light (AOI). In the radial mode, we investigate the optical response of nanopillar and nanohole. Simulation results show that the AlxGa1-xN cathode can be omnidirectional and unified broadband light absorption by pillar nanostructured arrays with an axial inclination of 24°. In addition, Three-step Spicer emission model was used to establish the photoemission efficiency of graded Al component AlxGa1-xN nanostructure. All these findings not only indicate that the gradient Al component AlxGa1-xN material has a significant potential advantage over the UV photocathode, but it also provides an effective broadband and omnidirectional light absorption UV photocathode.

Construction of hierarchical 2D/2D Ti3C2/MoS2 nanocomposites for high-efficiency solar steam generation

Solar steam generation has been considered one of the most promising approaches for dealing with the energy and freshwater resource crises in recent years. However, achieving high efficiency in photo-thermal conversion remains a considerable challenge. Here, a series of hierarchical Ti3C2/MoS2 nanocomposites were designed for steam generation by a hydrothermal method. When the mass fraction of MoS2 reached 65 wt% (TM-3), the Ti3C2/MoS2 nanocomposite presented a strong broad-band light absorption of 92.4% from the UV to NIR region because of the accordion-like layered structure. The evaporation rate and solar-thermal conversion efficiency of the TM-3 with as-fabricated evaporator could reach 1.36 kg·m−2·h−1 and 87.2% under 1 kW/m2, due to the excellent light absorption ability of TM-3 and the low thermal energy loss (8.8%) of the evaporator. Meanwhile, TM-3 permits the evaporator to have remarkable cycle stability because of its hydrophobic properties. Moreover, TM-3 showed excellent seawater desalination and wastewater treatment abilities. Thus, the excellent light absorption ability, photo-thermal conversion efficiency, and stability of the overall system suggested that these nanocomposites show great potential applications in synergetic solar desalination and sewage treatment.

A MXene‐Based Hierarchical Design Enabling Highly Efficient and Stable Solar‐Water Desalination with Good Salt Resistance

AbstractA solar‐thermal water evaporation structure that can continuously generate clean water with high efficiency and good salt rejection ability under sunlight is highly desirable for water desalination, but its realization remains challenging. Here, a hierarchical solar‐absorbing architecture is designed and fabricated, which comprises a 3D MXene microporous skeleton with vertically aligned MXene nanosheets, decorated with vertical arrays of metal–organic framework‐derived 2D carbon nanoplates embedded with cobalt nanoparticles. The rational integration of three categories of photothermal materials enables broadband light absorption, efficient light to heat conversion, low heat loss, rapid water transportation behavior, and much‐improved corrosion and oxidation resistance. Moreover, when assembling with a hydrophobic insulating layer with hydrophilic channel, the MXene‐based solar absorber can exhibit effective inhibition of salt crystallization due to the ability to advect and diffuse concentrated salt back into the water. As a result, when irradiating under one sun, the solar‐vapor conversion efficiency of the MXene‐based hierarchical design can achieve up to ≈93.4%, and can remain over 91% over 100 h to generate clean vapor for stable and continuous water desalination. This strategy opens an avenue for the development of MXene‐based solar absorbers for sustainable solar‐driven desalination.

Properties of indium tin oxide on black silicon after post-deposition annealing for heterojunction solar cells

Indium tin oxide (ITO) serves as an excellent anti-reflective coating (ARC) and transparent contact in optoelectronic devices including in crystalline silicon (c-Si) solar cells. To enhance broadband light absorption in the c-Si solar cells, black silicon (b-Si) surfaces can be used to reduce light reflection within 300–1100 nm wavelength region. In this work, properties of 300 nm-thick ITO on b-Si after post-deposition annealing with different temperatures (200–500 °C) are investigated for application in heterojunction solar cells. Annealing the ITO at 400 °C produces the highest crystalline quality, moderate surface reflection and optimum electrical properties. For electrical characterization, ITO/b-Si/NiO and ITO/c-Si/NiO reference solar cells are illuminated with white light LED-based solar simulator at illumination of 47 mW/cm2 and temperature of 25 °C. The ITO/b-Si/NiO solar cells demonstrate higher short-circuit current density (Jsc) and open-circuit voltage (Voc) when compared to the reference solar cells.

Quantum-assisted photoelectric gain effects in perovskite solar cells

Further boosting the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) without excessively increasing production expenses is critical for practical applications. Here, we introduce silicon quantum dots (SiQDs) to enable perovskites to harvest additional sunlight without changing PSC processes. These SiQDs can convert shorter wavelength excitation light (300–530 nm) into visible region light and reflect longer wavelength perovskite-unabsorbed visible light (550–800 nm), leading to broadband light absorption enhancement in PSCs. As a result, the SiQD-based photocurrent gain can improve the external quantum efficiencies of PSCs over a wide wavelength range of 360–760 nm, yielding relatively enhanced short-circuit current density (+1.66 mA/cm2) and PCE (+1.4%). Surprisingly, even the PSC with a low-purity perovskite layer shows an ultrahigh PCE improvement of 5.6%. Our findings demonstrate QD-assisted effects based on earth-abundant and environmentally friendly silicon, leading to effective optical management that remarkably promotes the performance of PSCs and enables the balance of costs to be substantially addressed.

Oxygen-enriched tubular carbon for efficient solar steam generation

Carbon materials with excellent light-harvesting capacity are promising light absorbers for solar-thermal conversion. However, developing advanced carbon materials with tailored morphology and properties that are suitable for solar steam generation remains challenging. Herein, we have successfully synthesized oxygen-enriched tubular carbon with uniform hollow architecture and some defective structure by pyrolysis of a coordination complex (PEG-CaCl2 precursor). Briefly, PEG molecules can promote the anisotropic growth of CaCl2 nanorods during pyrolysis through coordination interaction (Ca–O), and simultaneously serve as the carbon and oxygen sources for in-situ growth of self-doped tubular carbon using the as-formed nanorods as ideal templates. The resulting tubular carbon with one-dimensional hollow structure, possesses a large specific surface area (613 m2 g−1) for light absorption, low thermal conductivity (0.0474 W m−1 K−1) for heat localization, and abundant hydrophilic oxygen-containing groups (8.62 at.% oxygen) for efficient water delivery. Thus, the self-floating photothermal membrane based on the tubular carbon, fabricated by the addition of agar binder, exhibits broadband light absorption (96%) and achieves a high evaporation efficiency of 91.3% under one sun illumination. This work provides a new insight to design and synthesize functionalized carbon with controlled morphology and the desired structure for potential application in solar steam generation.

Ray tracing of inverted pyramids for light-trapping in thin crystalline silicon for solar cells

In this work, ray tracing of inverted pyramids is conducted to investigate different light-trapping schemes in thin crystalline silicon (c-Si) for solar cells. Monocrystalline silicon (mono c-Si) wafer with thickness of 30 μm is used as the substrate. The light-trapping schemes include front inverted pyramids (Scheme I), rear inverted pyramids (Scheme II) and inverted pyramids on both sides of the c-Si (Scheme III). Each light-trapping scheme is equipped with a front silicon nitride (SiNx) anti-reflective coating (ARC). Planar c-Si is used as a reference. The light-trapping performance is investigated within 300-1200 nm wavelength region, utilizing AM1.5 G solar spectrum at normal incidence as the illumination. From the absorption profile, the maximum potential short-circuit current density (Jmax) is calculated throughout the entire region, assuming unity carrier collection. Planar c-Si exhibits Jmax of 22.5 mA/cm2. Light-trapping with Scheme I demonstrates the highest broadband light absorption, owing to superior light scattering by the front pyramids and effective light-coupling by the SiNx ARC. This results in Jmax of 39.1 mA/cm2, which corresponds to 74% enhancement when compared to the planar c-Si reference. The findings from this work demonstrate the potential of the thin c-Si as a promising PV technology for the future.

High-efficiency solar-driven water desalination using a thermally isolated plasmonic membrane

This study presents an experimental demonstration of a highly efficient solar-driven interfacial evaporation system for potable water production. The engineered evaporation system consist of a photothermal structure (plasmonic titanium nitride nanoparticles (TiN NPs) coated on a hydrophilic porous membrane), and a thermally insulating nano silica aerogel. During the solar-driven vapor generation test, a hydrophilic membrane functioned as a porous support and drew underlying water to the surface through its microporous channels; the TiN NPs coated on a membrane surface functioned as a photothermal layer and generated localized heat at the water–vapor interface upon light irradiation; and an aerogel mat positioned between the photothermal membrane and the underlying bulk water served as a thermally insulating barrier, to suppress parasitic heat dissipation. The results reveal that the optimized TiN photothermal membrane when tested in a thermally-insulated system, efficiently produced clean water at a rate of 1.34 kgm−2h−1 that corresponds to a solar-thermal conversion efficiency of 84.5% under 1 sun. A superior efficiency of the system was primarily attributed to the broadband light absorption and superior light to heat conversion properties of plasmonic TiN NPs, as well as to the suppressed heat loss from the heated surface to the underlying water. It is believed that the application of TiN-membranes fabricated via a simple and scalable method presents a concrete step for solar-assisted off-grid desalination, particularly at remote locations with limited or no access to electricity.

Broadband, wide-angle, and polarization-insensitive enhancement of light absorption in monolayer graphene over whole visible spectrum

By precise control of the resonance positions of four magnetic dipole modes in metamaterials, we numerically exhibit a broadband light absorption enhancement of monolayer graphene in the whole visible region. In the wavelength range from 450 to 800 nm, the minimum and maximum absorption efficiencies of monolayer graphene exceed 40% and 70%, respectively, and the averaged absorption efficiency can reach as high as about 65%, which suggests great potentials in various optoelectronic devices. Thanks to the localization nature of magnetic dipole modes, the light absorption in monolayer graphene under oblique incidence is, on the whole, less dependent on the incidence angle and the polarization direction of light. Besides ultra-broad bandwidth, other good performances of the light absorption enhancement of monolayer graphene, such as wide incidence-angle and polarization insensitivity, are also desired for graphene-based optoelectronic devices in some cases.

Black paints covered with multidielectrics: light absorbers.

Black paints are commonly used to provide broadband light absorbers in high-precision optics. We show how multidielectric coatings improve the performances of these absorbers. The coated rough paints still exhibit a quasi-lambertian diffuse reflection, but this scattering pattern can be reduced by several orders of magnitude, which strongly enhances absorption. Predictions are based on an exact electromagnetic theory of light scattering from arbitrary rough multilayers. Results are also compared to useful approximate theories.