Improved Light Trapping Research Articles

Enhanced photoelectrochemical water splitting by a 3D hierarchical sea urchin-like structure: ZnO nanorod arrays on TiO2 hollow hemisphere

A hierarchical sea urchin-like hybrid metal oxide nanostructure of ZnO nanorods deposited on TiO2 porous hollow hemispheres with a thin zinc titanate interface layer is specifically designed and synthesized to form a combined type I straddling and type II staggered junctions. The HHSs, synthesized by electrospinning, facilitate light trapping and scattering. The ZnO nanorods offer a large surface area for improved surface oxidation kinetics. The interface layer of zinc titanate (ZnTiO3) between the TiO2 HHSs and ZnO nanorods regulates the charge separation in a closely coupled hierarchy structure of ZnO/ZnTiO3/TiO2. The synergistic effects of the improved light trapping, charge separation, and fast surface reaction kinetics result in a superior photoconversion efficiency of 1.07% for the photoelectrochemical water splitting with an outstanding photocurrent density of 2.8 mA cm−2 at 1.23 V versus RHE.

Magnetically separable MnFeCoNiCu-based high entropy alloy nanoparticles for photocatalytic oxidation of antibiotic cocktails in different aqueous matrices

Destroying persistent organic pollutants via photocatalytic advanced oxidation processes is critical for sustainable water management. In spite of the tremendous progress in developing highly effective and durable photocatalysts in recent decades, inferior optical absorption and rapid charge recombination considerably restrict their application on a commercial scale. In order to address these issues, herein we have constructed MnFeCoNiCu-based high entropy alloy (HEA) nanoparticles (NPs), through a facile and straightforward procedure, and showcase its superiority over conventional photocatalysts in degrading some typical antibiotics (viz., sulfamethoxazole, SMX; ofloxacin, OFX; and ciprofloxacin, CFX). The equimolar HEA-NPs feature n-type semiconducting properties, improved light trapping ability, and reduced charge recombination. Consequently, a high degradation of 95 %, 94 %, and 89 % for SMX, OFX, and CFX, respectively, can be achieved in the presence of MnFeCoNiCu NPs under visible light illumination, without any serious risks of secondary contamination. In order to better evaluate the photocatalytic performance of MnFeCoNiCu NPs, particularly from a practical perspective, the removal of antibiotic pollutants from multicomponent systems is also investigated. The average treatment efficiency for a ternary mixture of SMX, OFX, and CFX in different aqueous matrices decreases in the order of deionized water (∼82 %) < tap water (∼86 %) < river water (∼78 %) < hospital wastewater (∼60 %) < domestic wastewater (∼57 %) < pharmaceutical industry wastewater (∼47 %). Further, the expended HEA-NPs can be conveniently recovered from the reaction system by applying an external magnetic field, allowing recurrent usage, without any substantial alteration in their microstructure and performance. These demonstrations of satisfactory photocatalytic activity under a diverse spread of realistic environmental conditions, excellent durability, and inexpensive retrieval endorse the application potential of MnFeCoNiCu NPs in decentralized water treatment systems.

A facile synthesis of shell-shaped GeOx (x≤2) islands by metal-assisted chemical etching of Ge and their optoelectronic properties

Here we report a facile synthesis of GeOx islands on Ge substrates, having a unique shell-shaped microstructure, and investigate its optoelectronic properties. In this study, Ge substrates are subjected to metal-assisted chemical etching (MaCE) method, leading to the formation of GeOx islands. Interestingly, dimension of the GeOx islands and their coverage on Ge substrates can be tuned by various etching parameters. Formation of hexagonal phase of crystalline GeOx islands is confirmed by x-ray diffraction studies. We also observe an increasing trend in the band-gap values with higher coverage of GeOx islands on Ge substrates. While scanning electron microscopy and atomic force microscopy measurements reveal the formation of shell-shaped microstructure of GeOx islands, x-ray photoelectron spectroscopy, micro-Raman, and energy dispersive x-ray spectroscopic data confirm the occurrence of GeOx islands during the MaCE processing of Ge substrates. The specular reflectance data of GeOx-decorated Ge surface show up to ∼28% reduction in the optical reflectance value compared to the pristine one. Due to their improved light trapping capacity, GeOx-decorated Ge surfaces can therefore be used as an anti-reflecting layer suitable for optoelectronic applications.

Improved light trapping and electrical properties from ZnO:Al seed layer/Ag nanowires/ZnO:Al nanorods composite structure for transparent conducting films

A novel ZnO:Al seed layer/Ag nanowires/ZnO:Al nanorods (AZO SL/Ag NWs/AZO NRs) composite structure for transparent conducting films was successfully obtained to enhance the light trapping and electrical properties concurrently. The morphology of Ag NWs and AZO NRs interlaced composite structure on the AZO SL was observed. Compared with the simplex AZO SL/AZO NRs films, the films with AZO SL/Ag NWs/AZO NRs composite structure had lower average sheet resistance (13.8 Ω/sq) and higher haze value (0.33) at 550 nm, which exhibited the effective improvement of both electrical and light trapping properties. The studies on the AZO SL/Ag NWs/AZO NR composite films in this work provide a novel structure for transparent conducting films applied in solar cells.

Suppressed charge carrier trap states and double photon absorption in substitutional Ta-doped TiO2-NT array

Anatase-TiO2 nanotubes (A-TiO2 NTs) represent a great opportunity for the electron transport materials used in perovskite solar cells because of several intrinsic advantages, e.g. an improved light trapping effect, an inherent ion-blocking layer, a directed electron transmission channel without interfacial random scattering. Nevertheless, its severe double-photon absorption and charge carrier trap states badly jeopardize the stability and electron transport of the perovskite active layers (PALs) under visible light, representing a major obstacle for practical applications. In this paper, we introduce Ta to substitute Ti position in A-TiO2 NTs lattice through a simple fluorination process, and reveal its underneath mechanism on suppresing the abovementioned limiting factors of charge carrier trap states and double-photon absorption. Moreover, we use the effect of double-photon absorption of studied NTs to excite the photogenerated carriers under a modulated sinusoidal visible light with small amplitude, which can perturb the transport dynamics of photo-induced charge carriers and simulate the dynamic process of charge carriers at the interface between electron transport layer (ETL) and PALs in real time. These achievements highlight the unique potential of substitutional Ta doping for interfacing engineering of perovskite solar cells.

Efficient hydrothermal growth of high-performance MoS2/pyramid-Si photocathodes by surface hydrophilicity engineering

Compared with traditional precious metal catalysts, MoS2 is regarded as the promising hydrogen evolution reaction catalyst for Si photocathodes, because of its higher photochemical stability and lower hydrogen adsorption free energy. However, the layers of MoS2 are connected by van der Waals force, and the interaction force between the layers of MoS2 and Si is weak. Therefore, the homogeneous MoS2 nanosheets directly synthesized on Si by the hydrothermal method remain challenging. Herein, we report a facile strategy for the fast and efficient growth of vertically standing MoS2 on pyramid Si via surface hydrophilicity tailoring by inserting a thin TiO2 layer. Thanks to the improved light trapping and catalytic kinetics, the optimal MoS2/TiO2/Si shows a short-circuit photocurrent density of 9.79 mA/cm2 and an onset potential of 0.31 V vs reversible hydrogen electrode under 100 mW/cm2 Xe-lamp illumination. By comparing with different oxide buffer layers, surface hydrophilicity is found to be essential for promoting the homogenous nucleation and growth of vertically standing MoS2 by the hydrothermal method. These results not only provide valuable insight into the growth kinetics of transition metal chalcogenides in aqueous solution but also deliver an efficient route for assembling noble-metal free catalysts on silicon-based photoelectrodes.

Enhanced Light Trapping in Conformal CuO/Si Microholes Array Heterojunction for Self-Powered Broadband Photodetection

In this letter, we demonstrate the fabrication of a conformal CuO/Si microholes array heterojunction through the DC reactive magnetron sputtering from a high-purity Cu target. By using the monolayer graphene as the top electrode, the device served well as a self-powered vis-NIR photodetector, showing a high responsivity of 301.5 mA W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , specific detectivity of 7.96×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> Jones and a fast response speed (rise time 9.9 μ s and fall time 10 μs) upon 530 nm illumination. Compared to its planar counterpart, the responsivity was remarkably enhanced over the broadband region. The underlying reason should be ascribed to the improved light trapping in the microholes via the longitudinal Fabry-Perot (F-P) cavity resonance, according to theoretical simulation by finite-difference time-domain (FDTD) solution. Such an effect gave rise to an enhanced light-matter interaction, leading to the improved photoresponse. This work also opens up an effective way for the on-chip high performance photodetection due to the well compatibility with the current complementary metal-oxide-semiconductor (CMOS) technology.

Nonparametric design of nanoparticles with maximum scattering using evolutionary topology optimization

• A free form nanoparticle is designed for maximum scattering using topology • optimization. • The optimization objectives increased 2-35 times compared to standard shapes. • Spectral cross sections increased between 1-10 times compared to standard shapes. • A checkerboard control scheme is proposed in addition to the filtering techniques. The interaction between light and subwavelength structures provides tailorable optical properties that can be useful in many engineering applications. These properties strongly depend on the material shape, which provides obtaining unique scattering characteristics when rigorously designed. However, the conventional design methods require precise modeling and characterization of the shapes of the scattering objects, thus requiring a lot of intuition and knowledge about light radiation at small scales, as well as many rounds of experimental trial and error. We propose a framework to discover new nanoparticle designs for improved scattering based on topology optimization. The framework allows us to maximize the scattering cross section of the particle domain. Increased scattering cross-section at nanoscale leads to improved light trapping, which is critical in many applications such as thin film solar cells and biological imaging. Topology optimization offers a knowledge independent design procedure, therefore revealing relationships between specific regions in the design domain and the light behavior for maximum scattering cross section.

Direct Growth of Pyramid‐Textured Perovskite Single Crystals: A New Strategy for Enhanced Optoelectronic Performance

AbstractEven though there have been a few reports of substrate surface texturing of thin film perovskite solar cells to enhance their light trapping, there has been no direct texturing of the perovskite material, let alone perovskite single crystals (SCs). Herein, a method to prepare a pyramid‐structured perovskite CH3NH3PbX3 (CH3NH3+ = MA, X = I, Br) SC surface with minimized light reflection and maximized incident light harvesting is reported. Specifically, a hard template is used to directly transfer the pyramidal texture onto the MAPbX3 SC during its growth. A well‐shaped pyramidal texture is formed on the single‐crystal surface leading to improved light trapping. The textured MAPbBr3 SC shows good crystallinity, prolonged carrier lifetime, and improved carrier mobility. Furthermore, the photodetector made from the textured SC shows enhanced responsivity of 321 A W−1 and external quantum efficiency of 7191%, about two times higher than that of a control device. This method may be used to directly fabricate desired textures on general single crystal surfaces.

Achieving cadmium selenide-decorated zinc ferrite@titanium dioxide hollow core/shell nanospheres with improved light trapping and charge generation for photocatalytic hydrogen generation

We report the rational design and fabrication of magnetically separable zinc ferrite@titanium dioxide (ZnFe2O4@TiO2) hollow core/shell nanospheres as photocatalysts for efficient H2 evolution by loading the TiO2 shell layer on the prepared ZnFe2O4 hollow nanospheres using the kinetics-controlled coating method. Meanwhile, the incident light absorption, photogenerated charge transfer and separation and photocatalytic hydrogen evolution activity were remarkably improved by well anchoring cadmium selenide (CdSe) quantum dots on the ZnFe2O4@TiO2 hollow core/shell nanospheres. This unique design integrates the structural and functional merits of the ZnFe2O4, TiO2, and CdSe quantum dots into porous hollow nanospheres with the double-shell heterostructure. This design significantly accelerates the separation and transport of photogenerated charge carriers, enhances the light absorption, and offers more active sites for the photocatalytic H2 evolution reaction. Benefitting from the unique structural and component merits, the optimized magnetically separable ZnFe2O4@TiO2/CdSe hollow core/shell nanospheres exhibit excellent photocatalytic hydrogen evolution performance with a high H2 generation rate (266.0 μmol h−1·g−1) and high stability.

Hematite coated, conductive Y doped ZnO nanorods for high efficiency solar water splitting

For the first time, hematite (α-Fe2O3) crystals were electrochemically deposited over vertically aligned conductive zinc oxide nanorods (NR) to form a specially designed 3D heterostructure with a unique triple layer structure. The structure formed with a thin layer of ZnFe2O4 sandwiched between the hematite and the ZnO, which forms a barrier to reduce the back migration of holes. Hence, the charge separation is significantly improved. The small unequal bandgaps of α-Fe2O3 and ZnFe2O4 help to enhance and broaden visible light absorption. The electron transportation was further improved by yttrium doping in the ZnO (YZnO) NRs, resulting in increased conductivity. This allowed the vertically aligned NRs to perform as electron highways, which also behave as effective optical waveguides for improved light trapping and absorption, since ZnO absorbs little visible light. All these benefits made the unique structures suitable for high performance photoelectrochemical (PEC) water splitting. Optimisation of α-Fe2O3 thickness led to a photocurrent density improvement from 0.66 to 0.95 mA cm−2 at 1.23 VRHE. This was further improved to 1.59 mA cm−2 by annealing at 550 °C for 3 h, representing a record-breaking photocurrent for α-Fe2O3/ZnO systems. Finally IPCE confirmed the successful generation and transfer of photoelectrons under visible light excitation in the specifically designed heterostructure photoanode, with 5% efficiency for blue light, and 15% for violet light.

Improving light harvesting and charge extraction of polymer solar cells upon buffer layer doping

It is an important strategy to improve the performance of polymer solar cells (PSCs) by incorporating metal nanoparticles into functional layers. Herein, gold (Au)@titanium dioxide (TiO2) plasmonic core-shell nanoparticles (PCSNPs) are synthesized and doped into zinc oxide (ZnO) as hybrid electron transport layer in PSCs. Improved light trapping and electrical conductivity are achieved through localized surface plasma resonance of Au@TiO2 PCSNPs. As a result, the short-circuit current density is apparently enhanced, while remaining the unchanged open-circuit voltage. The maximum PCE of 8.801% is obtained when 1.5 wt% Au@TiO2 PCSNPs are introduced into the ZnO layer. This study provides a new inspiration for the development of high efficiency PSCs.

Photoresponse characteristics of MoS2 QDs/Si nanocone heterojunctions utilizing geometry controlled light trapping mechanism in black Si

A unique light trapping mechanism associated with nano-conical textured black Si templates has been utilized to achieve improved photoresponse in MoS2QDs/Si heterojunctions over a wide wavelength range from visible to near infrared. Black Si templates have been fabricated by a simple and cost effective metal assisted chemical etching technique followed by spin-coating of colloidal MoS2 quantum dots (QDs) to form the heterojunction. A peak responsivity of as high as ∼1.39 A W−1 at ∼665 nm for a bias of 5 V has been achieved. The responsivity value is higher as compared to recently published results having similar device structure. The combination of MoS2 QDs and black Si has resulted in a broader spectral response with enhanced optical absorption in the nano-conical heterojunction devices. Finite element based optical simulation results revealed the superiority of MoS2 QDs/Si nano-conical heterojunctions due to improved light trapping. The results appear attractive for next generation Si CMOS compatible broad band photodetectors using two dimensional semiconductors.

High-Responsivity MSM Solar-Blind UV Photodetector Based on Annealed ITO/Ag/ITO Structure Using RF Sputtering

In this paper, a novel ITO/Ag/ITO (IAI) multilayer UV photodetector was successfully prepared via RF magnetron sputtering technique to achieve the dual-benefit of improved photoresponse and outstanding visible blindness properties. The influence of the annealing process on the performance of the elaborated IAI UV photodetector was carried out. Moreover, the structural and photoelectrical properties of the sputtered tri-layered structures with and without annealing effects were also analyzed. It was found that the elaborated UV -photodetector provides near perfect UV absorbance of 96%. This achievement is attributed to the improved light trapping capability and tunable plasmonic resonance effects induced by the efficient optical coupling between free-space UV photons and Ag plasmons. It was also revealed that the heat treatment paves a new path toward preparing efficient visible-blind UV photodetectors, where the annealed sample demonstrated a superior rejection ratio of 225 as compared to that of the unannealed counterpart. Besides, electrical characterization underlined the ability of the annealed IAI UV photodetector at 500°C for offering exciting opportunities, where it exhibits a high responsivity of 3.8mA/W at self-powered condition. Therefore, by well optimizing both IAI multilayer geometry and annealing conditions, we were able to elaborate an ultrathin photodetector suitable for high-performance and flexible UV sensing applications.

Large-Scale Ligand-Free Synthesis of Homogeneous Core-Shell Quantum-Dot-Modified Cs4PbBr6 Microcrystals.

An organic ligand-free solution method is developed for preparing homogeneous core-shell quantum-dot (QD)-modified pure Cs4PbBr6 microcrystals on a large scale (∼12 g) at room temperature. The ligand-free Cs4PbBr6 microcrystals show a high green photoluminescence quantum yield of 76% with 360 nm of excitation light, which is attributed to their unique microarchitecture, with several features including quantum confinement of the outer QDs, stability of the inner Cs4PbBr6 microcrystals, improved light trapping, and interfacial recombination. UV-vis-near-IR and photoluminescence analyses provide valued evidence to support the ligand-free Cs4PbBr6 with synergy between the QDs and microcrystals.

3D highly efficient photonic micro concave-pit arrays for enhanced solar water splitting.

The construction of three-dimensional (3D) photonic micro/nanostructures is regarded as one of the most promising approaches to develop highly efficient photoelectrodes for solar water splitting. Here, we report the design and fabrication of an indium tin oxide glass with 3D micro concave-pit arrays (MCPAs) as an effective photonic substrate for dramatically enhanced photoelectrochemical (PEC) water splitting. Compared with the planar counterpart, more than three-fold photocurrent density can be obtained for the 3D photoelectrodes with In2S3 nanosheet arrays grown on the inner surfaces of the MCPAs, mainly ascribable to their largely improved light trapping ability and increased surface area for charge separation and extraction. The PEC performance is further elevated by constructing an effective In2S3/ZnO heterojunction to accelerate the photocarrier separation. As a result, the 3D MCPA-based photoanodes demonstrate a maximum incident photon to current efficiency of 11.7% at 380 nm, which is about four times higher than that of the planar counterpart. The significant advancement demonstrated here provides a facile and low-cost route for the large-scale fabrication of 3D photonic electrodes aiming to achieve highly efficient PEC water splitting.

Enhanced Electrochemical Stability of TiO2-Protected, Al-doped ZnO Transparent Conducting Oxide Synthesized by Atomic Layer Deposition.

Transparent, conductive coatings on porous, three-dimensional materials are often used as the current collector for photoelectrode designs in photoelectrochemical applications. These structures allow for improved light trapping and absorption in chemically synthesized, photoactive overlayers while minimizing parasitic absorption in the current collecting layer. Atomic layer deposition (ALD) is particularly useful for fabricating transparent conducting oxides (TCOs) like Sn-doped In2O3 (ITO) and Al-doped ZnO (AZO) for structured materials because the deposition is specific to exposed surfaces. Unlike line-of-sight deposition methods (evaporation, spray pyrolysis, sputtering), ALD can access the entire complex interface to make a conformal transparent conductive layer. While ITO and AZO can be grown by ALD, they are intrinsically soluble in the acidic and basic environments common for electrochemical applications like water splitting. To take advantage of the unique characteristics of ALD in these applications, it is important to develop strategies for fabricating TCO layers with enhanced chemical stability. Ultrathin coatings of stable materials can be used to protect otherwise unstable electrochemical interfaces while maintaining the desired function. Here, we describe experiments to characterize the chemical and electrochemical stability of ALD-deposited AZO TCO thin films protected by a 10 nm TiO2 overlayer. The addition of a TiO2 protection layer is demonstrated to improve the chemical stability of AZO by orders of magnitude compared to unprotected, yet otherwise identically prepared, AZO films. The electrochemical stability is enhanced accordingly in both acidic and basic environments. We demonstrate that TiO2-protected AZO can be used as a TCO for both the cathodic hydrogen evolution (HER) and anodic water oxidation (OER) half-reactions of electrochemical water splitting in base and for HER in acid when the appropriate electrocatalysts are added. As a result, we show that ALD can be used to synthesize a chemically stable TCO heterostructure, expanding the range of materials and electrochemical environments available for building complex photoelectrode architectures.

Progress and limitations of thin-film silicon solar cells

The major issues of thin-film silicon solar cells have been the light-induced metastability of hydrogenated amorphous silicon (a-Si:H) and the weak infrared light absorption of hydrogenated microcrystalline silicon (μc-Si:H). In order to overcome these challenges, we have developed a novel deposition process for a-Si:H and an improved light trapping scheme for μc-Si:H light absorbers, leading to four record independently-confirmed conversion efficiencies with single-junction and multijunction device structures. In this article, we review some technological progresses that led to the notable improvement in thin-film silicon solar cells, and address some limitations imposed by the absorber layer quality as well as the cell design.

Polyimide for silicon solar cells with double-sided textured pyramids

Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping over cells with front-side only texture. However, increased surface area, roughness and exposed <111> crystal planes of textured surfaces not only causes increased recombination, but also makes cells susceptible to shunting through pinholes in the dielectric at the sharp peaks and valleys of the textured pyramids. A polyimide film as an insulating interlayer film is investigated to circumvent the tradeoff between improved light trapping, increased recombination and increased shunt paths. When applied at the rear of the interdigitated back contact silicon solar cell structure, the polyimide film provides an excellent electrical insulation (> 1000 MΩ of insulation resistance) and increases photocurrent (~ 1.1 mA/cm2) owing to an increased rear internal reflectance. The polyimide is also compatible with metal annealing of passivating dielectrics such as silicon nitride. Optical simulation and experimental results are combined in a 3D semiconductor simulation (Quokka) to quantify the possible gain of implementing the double-sided texture in high efficiency silicon solar cells.

Trade-off between Photon Management Efficacy and Material Quality in Thin-Film Solar Cells on Nanostructured Substrates of High Aspect Ratio Structures

Although texturing of the transparent electrode of thin-film solar cells has long been used to enhance light absorption via light trapping, such texturing has involved low aspect ratio features. With the recent development of nanotechnology, nanostructured substrates enable improved light trapping and enhanced optical absorption via resonances, a process known as photon management, in thin-film solar cells. Despite the progress made in the development of photon management in thin-film solar cells using nanostructures substrates, the structural integrity of the thin-film solar cells deposited onto such nanostructured substrates is rarely considered. Here, we report the observation of the reduction in the open circuit voltage of amorphous silicon solar cells deposited onto a nanostructured substrate with increasing areal number density of high aspect ratio structures. For a nanostructured substrate with the areal number density of such nanostructures increasing in correlation with the distance from one edge of the substrate, a correlation between the open circuit voltage reduction and the increase of the areal number density of high aspect ratio nanostructures of the front electrode of the small-size amorphous silicon solar cells deposited onto different regions of the substrate with graded nanostructure density indicates the effect of the surface morphology on the material quality, i.e., a trade-off between photon management efficacy and material quality. This observed trade-off highlights the importance of optimizing the morphology of the nanostructured substrate to ensure conformal deposition of the thin-film solar cell.