Light Trapping Effect Research Articles

Impact of Silver Surface Morphology on the Wall Plug Efficiency of Blue Vertical Light-Emitting Diodes

Due to merits of good thermal conductivity and vertical current distribution, vertical light-emitting diodes (VLEDs) have been widely studied for many years. However, the wall plug efficiency (WPE) remains limited by the light trapping effect in GaN cavity and difficulties to achieve low-resistance ohmic contacts to p-GaN. As one of the solutions, Ni-/Ag-based mirror layers as ohmic contacts have attracted much attention due to the high work function of Ni and high reflectivity of Ag for blue light. In this paper, a Ni/Ag/Ni/Ag/Ti metal stack was deposited separately on the surface of p-GaN. A high WPE of 45.7% for GaN-based blue VLEDs on Si substrates has been achieved at the mean diameter (69 nm) of Ag grains and root-mean-square (rms) surface roughness (2.06 nm) of Ag mirror layers. There are three impacts on VF: formation of NiO, growth of Ag grain, and increases of Ag oxide. The impacts of surface topography on optical properties are investigated by the Mie scattering theory for Ag grains and surface scattering theory for roughness surface of Ag mirror layer. The conclusion shows that: 1) with growth of Ag grain, the absorption of Ag grains is heightened obviously and the intensity distribution of scattering light with scattering angle is changed from uniformity to irregularity and 2) since the magnitudes of specular reflectance are dramatically larger than the scattering about 1011 times, the specular reflectance of Ag roughness surface totally covers the attribution of scattering and is only decided by rms values and wavelength of incident light.

Quasiepitaxy Strategy for Efficient Full‐Inorganic Sb2S3 Solar Cells

AbstractAntimony sulfide (Sb2S3) as a wide‐bandgap, nontoxic, and stable photovoltaic material reveals great potential for the uppermost cells in Si‐based tandem cell stacks. Sb2S3 solar cells with a compatible process, acceptable cost, and high efficiency therefore become the mandatory prerequisites to match silicon bottom cells. The performance of vacuum processed Sb2S3 device is pinned by bulk and interfacial recombination. Herein, a thermally treated TiO2 buffer layer induces quasiepitaxial growth of vertical orientation Sb2S3 absorber overcoming interface defects and absorber transport loss. Such novel growth could pronouncedly improve the open‐circuit voltage (Voc) due to the superior interface quality and intraribbon transport. The epitaxial rough Sb2S3 surface shows a texturized‐like morphology. It is optimized by tuning the grain sizes to form strong light trapping effect, which further enhances the short‐circuit current density (Jsc) with a 16% improvement. The final optimal device with high stability obtains a power conversion efficiency of 5.4%, which is the best efficiency for full‐inorganic Sb2S3 solar cells. The present developed quasiepitaxy strategy supports a superior interface, vertical orientation, and surface light trapping effect, which provides a new perspective for efficient noncubic material thin film solar cells.

Plasmonically Engineered Textile Polymer Solar Cells for High-Performance, Wearable Photovoltaics.

A practically applicable type of wearable polymer solar cells (PSCs) is presented with the enhanced performance by exploiting simply embodied, plasmonic nanostructures on a commercially available textile platform of optically opaque, geometrically uneven, and physically permeable woven fabrics that are commonly not compatible with organic photovoltaics. On a conformable fabric substrate preferentially processed with organic/inorganic multilayers for both planarization and encapsulation, the fabrication of top-illuminated, inverted type of PSCs with a transparent top electrode consisting of optimized dielectric/metal/dielectric multilayers is conducted, where a nanostructure of disorderly distributed elliptical hemispheres is implanted at an opaque bottom silver electrode by spin-coated silica nanoparticles in advance of depositing this electrode. The nanostructured bottom electrode promotes the light trapping effect at wavelengths of the surface plasmon resonance, as well as reduces the electrical Ohmic loss, thereby achieving a device with the power conversion efficiency of ∼8.71% at the given plasmonic device, where a net improvement of the efficiency is ∼1.46% compared to the planar device comprising otherwise same constituent layers. Systematic studies on optical properties and associated photovoltaic performance in experiments, together with analytic numerical modeling, allow quantitative understanding of the underlying physics, providing optimal rules for tailoring random nanostructures to the textile PSCs in the context of high-performance wearable photovoltaics.

Recent Progress on Metal Sulfide Composite Nanomaterials for Photocatalytic Hydrogen Production

Metal sulfide-based photocatalysts have gained much attention due to their outstanding photocatalytic properties. This review paper discusses recent developments on metal sulfide-based nanomaterials for H2 production, acting as either photocatalysts or cocatalysts, especially in the last decade. Recent progress on key experimental parameters, in-situ characterization methods, and the performance of the metal sulfide photocatalysts are systematically discussed, including the forms of heterogeneous composite photocatalysts, immobilized photocatalysts, and magnetically separable photocatalysts. Some methods have been studied to solve the problem of rapid recombination of photoinduced carriers. The electronic density of photocatalysts can be investigated by in-situ C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra to study the mechanism of the photocatalytic process. The effects of crystal properties, nanostructure, cocatalyst, sacrificial agent, electrically conductive materials, doping, calcination, crystal size, and pH on the performance of composite photocatalysts are presented. Moreover, the facet effect and light trapping (or light harvesting) effect, which can improve the photocatalytic activity, are also discussed.

Improved responsivity performance of ZnO film ultraviolet photodetectors by vertical arrays ZnO nanowires with light trapping effect

The light trapping effect of zinc oxide (ZnO) ultraviolet photodetectors (UV PDs) has been established as a promising way to optimize the performance of optoelectronic devices. In this paper, we report a light trapping fabricated metal–semiconductor–metal structure, consisting of a ZnO nanowire array as a top layer light absorber supported by a ZnO film. The ZnO film is bridged between two interdigitated metal electrodes for collecting photo-generated carriers. In this connection, high-dense ZnO nanowires can be used as a light trapping unit and to transmit the photogenerated carriers towards the ZnO film. The photogenerated carriers diffuse along the longitudinal direction of the ZnO nanowire and then to the ZnO film and are collected by the applied bias electrode. Compared to present ZnO thin film UV PDs, our device has an effective light trapping effect and the enhancement of photo-generated carriers at the top interface by a ZnO nanowire array structure are highly beneficial to UV light detection as they can provide a long optical path and more surface area. In addition, when the device was connected with nanowires, a 10 times augment of responsivity appeared accompanied by a giant photo-to-dark current ratio (1.6 × 103). This novel work not only enhanced fundamental improvement of nanowires to ZnO film UV PDs, but also provided a distinct contrast between light trapping UV PDs and ZnO film UV PDs.

Anti-reflective and anticorrosive properties of laser-etched titanium sheet in different media

The effects of femtosecond laser etching in different media (air, deionized water, and anhydrous ethanol) on the morphology, anti-reflection, and anticorrosion of titanium sheet surface were studied. Ripples, ripples overlapped by grooves and micro-bumps, were, respectively, obtained by laser etching in the air, ethanol, and water. Compared to the untreated titanium, on the surface of titanium sheet, Ti6O and Ti3O are obtained by laser etching in water, but Ti6O was obtained by laser etching in ethanol. The reflectivity of laser-etched Ti surface in ethanol or water is reduced to less than 17% within the 200 nm–1200 nm spectral range. The etched surface in ethanol shows better reflectivity in the ultraviolet–visible range with a minimum value of 3%, and the etched surface in water exhibits better anti-reflection performance in the near-infrared spectral range 800 nm–1200 nm with the lowest value of 7.6%. The differences of anti-reflection are ascribed to the competition between the light trapping effects and the effective medium effect. Furthermore, the anticorrosion property of titanium is also improved by laser etching in water or ethanol. Ecorr of the laser-etched samples is about 0.05 v higher than that of untreated Ti, while Icorr is reduced by about 0.5 order of magnitude. The better anticorrosion is mainly due to the improved hydrophobicity in 3.5% NaCl solution. The improvement of these properties allows titanium to have more important application value in solar collector panels, optoelectronic devices, thermal photovoltaic power generation, and so on.

Large-scale, adhesive-free and omnidirectional 3D nanocone anti-reflection films for high performance photovoltaics

An effective and low-cost front-side anti-reflection (AR) technique has long been sought to enhance the performance of highly efficient photovoltaic devices due to its capability of maximizing the light absorption in photovoltaic devices. In order to achieve high throughput fabrication of nanostructured flexible and anti-reflection films, large-scale, nano-engineered wafer molds were fabricated in this work. Additionally, to gain in-depth understanding of the optical and electrical performance enhancement with AR films on polycrystalline Si solar cells, both theoretical and experimental studies were performed. Intriguingly, the nanocone structures demonstrated an efficient light trapping effect which reduced the surface reflection of a solar cell by 17.7% and therefore enhanced the overall electric output power of photovoltaic devices by 6% at normal light incidence. Notably, the output power improvement is even more significant at a larger light incident angle which is practically meaningful for daily operation of solar panels. The application of the developed AR films is not only limited to crystalline Si solar cells explored here, but also compatible with any types of photovoltaic technology for performance enhancement.

PdSe2 Multilayer on Germanium Nanocones Array with Light Trapping Effect for Sensitive Infrared Photodetector and Image Sensing Application

AbstractIn this study, a sensitive infrared photodetector (IRPD) composed of a germanium nanocones (GeNCs) array and PdSe2 multilayer is presented, which is obtained by a straightforward selenization approach. The as‐assembled PdSe2/GeNCs hybrid heterojunction exhibits obvious photovoltaic behavior to 1550 nm illumination, which renders the IRPD a self‐driven device without external power supply. Further device analysis reveals that the PdSe2/GeNCs hybrid based IRPD exhibits high sensitivity to 1350, 1550, and 1650 nm illumination with excellent stability and reproducibility. The responsivity and external quantum efficiency is as high as 530.2 mA W−1 and 42.4%, respectively. Such a relatively good device performance is related to the strong light trapping effect of GeNCs array, according to the theoretical simulation based on finite‐difference time‐domain. It is also found that the IRPD shows an abnormal sensitivity to IR illumination with a wavelength of 2200 nm. Finally, the present individual IRPD can also record the simple “F” image produced by 1550 nm, suggesting the promising application of the PdSe2/GeNCs hybrid device in future infrared optoelectronic systems.

Optimum Luminescent Down-Shifting Properties for High Efficiency and Stable Perovskite Solar Cells

In recent years, the discovery of the excellent optical and electrical properties of perovskite solar cells (PSCs) made them a main focus of research in photovoltaics, with efficiency records increasing astonishingly fast since their inception. However, problems associated with the stability of these devices are hindering their market application. UV degradation is one of the most severe issues, chiefly caused by TiO2’s photogenerated electrons that decompose the perovskite absorber material, coupled with the additional intrinsic degradation of this material under UV exposure. The solution presented here can minimize this effect while boosting the cells’ generated photocurrent, by making use of combined light-trapping and luminescent down-shifting effects capable of changing the harmful UV radiation to higher wavelengths that do not affect the stability and can be effectively “trapped” in the cell. This work focuses in the optimization of the photocurrent gains that can be attained by emulating the change...

High-temperature stable refractory nanoneedles with over 99% solar absorptance

Solar absorber coatings have widely been investigated for solar-thermal technologies including concentrated solar power and solar thermochemistry. While various nanostructures such as nanowires and nanotubes have been commonly used for high solar absorptance owing to their potent light trapping effect, the high temperature stability of these nanostructures has yet to be established due to either coarsening of nanostructures or oxidation of certain materials in air (e.g., Si and C). In this work, we developed a nanostructured solar absorber from a family of high-temperature refractory spinel oxides, Co3O4 and CuCo2O4, with ultra-high solar absorptance over 99%. Once coated with a thin layer of HfO2 or SiO2 through atomic layer deposition, the Co3O4 and CuCo2O4 nanoneedles preserve their high aspect ratio and sharp tips, allowing the solar absorbers to maintain the superior absorptivity and excellent thermal stability at an elevated temperature for an extended period (650 °C and 800 °C for 100 h for passivated Co3O4 and Cu1Co2O4, respectively). These results suggest that solar absorbers made from refractory spinel oxide nanoneedles can be used for high-temperature solar thermal applications with ultrahigh absorptance.

High-Efficient Solar Cells Textured by Cu/Ag-Cocatalyzed Chemical Etching on Diamond Wire Sawing Multicrystalline Silicon.

We reported a novel texture method through one-step Cu/Ag-cocatalyzed chemical etching which can be widely used in the photovoltaic industry because of its simple and low-cost process. The etching mechanism of an inverted rectangular pyramid is the cooperation of Ag-catalyzed vertical etching and Cu-catalyzed lateral etching. In our texture method, neither saw damage removal nor post-treatment is needed. During the etching process, the digging holes by Ag-catalyzed etching and enlarging holes by Cu-catalyzed etching completed at the same step. Benefiting from the excellent light-trapping and passivation effect of the inverted rectangular pyramid, diamond wire sawing multicrystalline silicon (mc-Si) Al-BSF solar cells with a super high efficiency of 19.49% had been obtained.

Comprehensive Study of Au Nano-Mesh as a Catalyst in the Fabrication of Silicon Nanowires Arrays by Metal-Assisted Chemical Etching

Silicon nanowires (SiNWs) arrays have become one of low-dimensional structural nanomaterials for the preparation of high-performance optoelectronic devices with the advantages of highly efficient light trapping effect, carrier multiplication, and adjustable optical bandgap. The controlled growth of SiNWs determines their electrical and optical properties. The morphology of silicon nanowires fabricated by conventional metal-assisted chemical etching (MACE) involving the Ag-based etching process cannot be precisely controlled. Ultra-thin anodic aluminum oxide (AAO) is one of the new-pattern nanostructure assembly systems for the synthesis of nanomaterials. The synthesized nanostructure arrays can be tuned to exhibit different optical and electrical properties in a certain wavelength range by adjusting the AAO membrane parameters. In this paper, we demonstrate an ultra-thin Au nano-meshes array from a single hexagonal AAO membrane as a replication master instead of conventional Ag particles as etching catalyst. The extended ordered silicon nanowires arrays are fabricated by the selective chemical dissolution of nanoscale noble metal meshes that exhibit excellent anti-reflection performance in broadband wavelengths and a wide incidence angle.

Influence of nanobranched growth on photoelectrochemical performance of TiO2 nanotree arrays films

TiO2 nanotree arrays films were obtained on fluorine doped tin oxide (FTO) substrates through a facile two-step hydrothermal method. The synthesized nanostructures were investigated by means of X-ray diffractometer, field-emission scanning electron microscopy and high-resolution transmission electron microscopy. The effect of TiO2 nanotree morphology on optical and photoelectrochemical performance was systematically studied using UV–Vis absorption spectra, electrochemical impedance spectroscopy (EIS), Mott-Schottky, linear voltammetry and transient photocurrent measurements. The results revealed that a TiO2 nanotree arrays film sample synthesized at 120 ℃ for nanobranched growth time 0.5 h showed a higher light trapping effect than TiO2 nanorod arrays film, and achieved relatively lower charge transport resistance and higher photocurrent density compared with the other TiO2 nanotree arrays samples. These behaviors can be mainly attributed to the formation of appropriate nanobranches manipulated the TiO2 nanotree arrays by varying the hydrothermal reaction time to improve photoexcited electrons rate under the premise of high electronic transmission performance.

Plasmon enhanced light trapping in thin film GaAs solar cells by Al nanoparticle array

Light trapping is a crucial factor to enhance the performance of thin film solar cells. For effective light trapping, we introduced Al nanoparticle array on the top and rear surface of thin film GaAs solar cells. The effect of both array on the optical absorption and current density of solar cells is investigated by using finite difference time domain (FDTD) method. The optimization process of top and rear array in solar cells is done systematically. The results indicate that by plasmonic action of arrays, the optical absorption is significantly enhanced and optimized structure yields a current density of 25.77 mA/cm2. These enhancements are mainly attributed to surface plasmon effects induced by Al nanoparticles and the light grating properties of the arrays.

18.78% hierarchical black silicon solar cells achieved with the balance of light-trapping and interfacial contact

18.78% efficiency of crystalline silicon (Si) solar cells was achieved through introducing the combined nanopore/pyramid textures. These hierarchical structures possessed the ultra-low reflectivity with values <6% under the various illumination angles from 0 to 60°, evidencing their remarkable omnidirectional light-trapping capability. Although the greatly improved light-trapping effect of the hierarchical structures was demonstrated, we found that the increased formation durations of Si nanopores led the substantial increase of photoluminescent characteristics that could limit the efficient separation of photogenerated carriers through charge recombination. Moreover, the positive correlation in surface roughness of nanopore arrays with respect to the elongated etching durations was evidenced, and these issues could increase the series resistance of solar cells owing to poor interfacial contact between nanopores and front electrodes. Thus, there existed the optimal combination between two-scale textures critically for both photonic and electrical management of cell design. By optimizing the etching durations for the controlled nanopore formation, the improved light-trapping characteristics and fairly unchanged contact resistance with front electrode was achieved, possessing the improved conversion efficiency with >0.58% of increased value beyond the typical microtexture-based solar cells.

Light trapping effect of textured FTO in perovskite solar cells

Hybrid organic–inorganic halide perovskites solar cells (PVSCs) have made great progress in just a few years. With maximum efficiencies evolving from 3.8% to 22.1%. The ongoing efficiency improvements are obtained mainly through the optimization of material process and the enhancement of interface. Whereas the conflict between generating photo carriers and collecting carriers is still a challenge for PVSCs. Light trapping design provides a promising way to solve the problem. However, there are few reports about light trapping effect in PVSCs. In this contribution, FTO substrates with different texture etched by laser were used to fabricate PVSCs. The influence of roughness on absorption of the perovskites layer and extraction properties of the carriers across the interface were discussed. The results showed that the device on the FTO substrate etched by laser with 4A current intensity showed a highest power conversion efficiency, with an enhancement of 13.2% compared with that on the unetched substrate due to the improved scattering of light in the photoactive layer. Our result is expected to open up a new way to enhance the PCE of PVSCs.

Improved power conversion efficiency of silicon nanowire solar cells based on transition metal oxides

An n-type Si nanowire (SiNW) solar cell based on transition metal oxides V2O5 and TiO2 has been fabricated. The cell is demonstrated in the ITO/V2O5/n-SiNWs/TiO2/Al heterojunction structure, in which the V2O5 and TiO2 work as the hole- and the electron-selective layers, respectively. Parameters of cells with different lengths of nanowires have been investigated. Owing to the excellent light trapping effect of nanowire arrays and superior carrier transporting properties of thin V2O5 and TiO2 layers, an improved power conversion efficiency of 12.7% has been achieved in a cell with 1.45-μm-long nanowires. Due to the large work function difference between V2O5 and n-Si, a build-in potential of 0.75 V was obtained from the capacitance-voltage measurement, which implies the possibility of achieving a large open circuit voltage for these nanowire solar cells. The efficiency loss is mainly caused by the recombination via the defects on the unpassivated surface of SiNWs. By solving this problem, the cell efficiency may be improved further and realize the application in PV industries.

Broadband light-trapping enhancement of graphene absorptivity

Low optical absorptivity of graphene limits the quantum efficiency of graphene-based optoelectronic devices. We demonstrate a broadband enhancement of absorptivity in an architecture of graphene intercalation inside a simple-cubic woodpile photonic crystal. Significant light-trapping effect is achieved due to spectrally broad and dense resonances in the architecture, especially modes propagating nearly parallel to the interface between the photonic crystal and the background material. We demonstrate an overall absorptivity $\ensuremath{\sim}10$ to 17 times larger than the free-standing bare graphene monolayer with bandwidth to central frequency ratio 0.3, and $\ensuremath{\sim}5$ to 11 times larger with bandwidth to central frequency ratio 1.33. A distributed intercalation of three graphene monolayers proves a $\ensuremath{\sim}70%$ overall absorptivity with bandwidth to central frequency ratio 0.3.

Synergetic light trapping effects in organic solar cells with a patterned semi-transparent electrode.

The dielectric/ultra-thin metal/dielectric structure has been widely used for semi-transparent electrodes in organic solar cells (OSCs) due to its potential replacement of the transparent conductive oxide indium tin oxide. Here, we introduce the dual light trapping structures, i.e. the nanopatterned MoO3/Ag/MoO3 (MAM) as the anode and a short-pitched metallic grating as the cathode, to cooperatively improve the OSC performance. The optical and electrical properties of the OSCs have been investigated via solving the coupled Maxwell and semiconductor equations by the finite-difference method. The results indicate that the optical light absorption of the active layer and the electrical carrier collection have been significantly enhanced after the adoption of the proposed dual light trapping structures. We have shown that the optical and electrical improvements are attributed to the synergetic effects of surface plasmon resonance of the grating patterned cathode and scattering effect of the nanopatterned MAM anode. Our results have further revealed that the short-pitched metal grating can induce considerable field confinement due to the mode coupling and hybridization of the surface plasmons in-between the adjacent short-distance metal strips. With the optimized structural parameters of the dual light trapping structure, the power conversion efficiency (PCE) of the OSCs has been substantially enhanced by 39% in comparison with a flat cell. Besides the efficiency improvement, the OSCs with the proposed dual light harvesting structures reveal an alleviated angular dependence of electrical properties as the light is beyond the normal incident angle. Our results contribute to the further development of ITO-free OSCs and are promising for semi-transparent optoelectronics.

New high-efficiency protective coating containing glycidyl-POSS nanocage for improvement of solar cell electrical parameters

Various antireflection thin films are often used to cover glass to increase solar cell electrical parameters. In the recent years many efforts have been done to develop and improve of solar cell films with high electrical output. One of the most important challenges of obtaining of high-efficiency thin films of solar cells is creation an effective light trapping system. The new polymeric protecting materials are called polyhedral oligomeric silsesquioxanes (POSS) that can improve the solar cells properties. In the present work, firstly Tetraglycidyloxypropyl tetramethyl POSS was synthesized via sol-gel method and used to prepare TGTM POSS-silica nanocomposite, then it was applied on solar cell slides. The structural, morphological, thermal, optical and electrical properties were investigated by TEM, FTIR, NMR, FE-SEM, TGA, UV-VIS spectroscopy and solar simulator. Evaluation of the obtained results showed that the efficiency of solar cells was improved twice or more (50%) compared with that without POSS coating. The coatings transmittances were equal 100% that are ideal for solar cell films. The solar cells stability to solar light with POSS coating was also evaluated. The obtained results showed that the degradation of solar cell transmission and efficiency after exposure to UV was negligible (lower than 1%).