Trapping Nanostructures Research Articles

Enhancing light trapping for improved efficiency of perovskite solar cells: design and analysis

Abstract A hybrid organic-inorganic perovskite solar cells (PSCs) is explored for its potential to achieve high-efficiency and cost-effective solar energy conversion, especially through improved light management. In this paper, COMSOL Multiphysics is utilized to explore PSCs with light trapping nanostructures which will enhance the optical and electrical properties. A nanometer-scale concave structure is proposed in this paper to direct and capture light. This configuration is compared to a planar structure to see how it affect PSCs optoelectronic performance. Electrical simulations showed that concave nanostructured PSCs increase carrier generation and collect more carriers. The short-circuit current rose from 3.91 mA for the planar structure to 4.95 mA for the concave structure. Power conversion efficiency for concave PSCs increased 11% compared to the planar construction after choosing optimal height of concave structure. Thus, the performance investigation by combined optical and electrical analysis of nanostructures shows that it has the ability to build high-efficiency PSCs with light-trapping approach.

Plasmon-enhanced parabolic nanostructures for broadband absorption in ultra-thin crystalline Si solar cells.

Sub-wavelength plasmonic light trapping nanostructures are promising candidates for achieving enhanced broadband absorption in ultra-thin silicon (Si) solar cells. In this work, we use finite-difference time-domain (FDTD) simulations to demonstrate the light harvesting properties of periodic and parabola shaped Si nanostructures, decorated with metallic gold (Au) nanoparticles (NPs). The active medium of absorption is a 2 μm thick crystalline-silicon (c-Si), on top of which the parabolic nanotextures couple incident sunlight into guided modes. The parabola shape provides a graded refractive index profile and high diffraction efficiencies at higher order modes leading to excellent antireflection effects. The Au NPs scatter light into the Si layer and offer strong localized surface plasmon resonance (LSPR) resulting in broadband absorption with high conversion efficiency. For wavelengths (λ) ranging between 300 nm and 1600 nm, the structure is optimized for maximum absorption by adjusting the geometry and periodicity of the nanostructures and the size of the Au NPs. For parabola coated with 40 nm Au NPs, the average absorption enhancements are 7% (between λ = 300 nm and 1600 nm) and 28% (between λ = 800 nm and 1600 nm) when compared with bare parabola. Furthermore, device simulations show that the proposed solar cell can achieve a power conversion efficiency (PCE) as high as 21.39%, paving the way for the next generation of highly efficient, ultra-thin and low-cost Si solar cells.

Nanostructured interfacial dipole layers for high-performance and highly stable nonvolatile organic field-effect transistor memory

An interfacial dipole layer formed between the solution-processed charge trapping nanostructures and the active layer enables significant improvement of the charge storage capability and stability.

Avalanche photodetectors with photon trapping structures for biomedical imaging applications.

Enhancing photon detection efficiency and time resolution in photodetectors in the entire visible range is critical to improve the image quality of time-of-flight (TOF)-based imaging systems and fluorescence lifetime imaging (FLIM). In this work, we evaluate the gain, detection efficiency, and timing performance of avalanche photodiodes (APD) with photon trapping nanostructures for photons with 450 nm and 850 nm wavelengths. At 850 nm wavelength, our photon trapping avalanche photodiodes showed 30 times higher gain, an increase from 16% to >60% enhanced absorption efficiency, and a 50% reduction in the full width at half maximum (FWHM) pulse response time close to the breakdown voltage. At 450 nm wavelength, the external quantum efficiency increased from 54% to 82%, while the gain was enhanced more than 20-fold. Therefore, silicon APDs with photon trapping structures exhibited a dramatic increase in absorption compared to control devices. Results suggest very thin devices with fast timing properties and high absorption between the near-ultraviolet and the near infrared region can be manufactured for high-speed applications in biomedical imaging. This study paves the way towards obtaining single photon detectors with photon trapping structures with gains above 106 for the entire visible range.

Design and Analysis of High Efficiency Perovskite Solar Cells with Light Trapping Nano-textured Substrates

Recently, the utilization of hybrid organic-inorganic perovskite solar cells under advanced light management designs have attracted intensive attention. In this study, a three-dimensional (3D) finite element method (FEM) technique was used in the COMSOL Multiphysics simulation package to investigate coupled optical and electrical characteristics of perovskite solar cells (PSCs) with light trapping nanostructures. Upon the use of nano-textured fluorine-doped tin oxide (FTO) substrates, we propose two architectures which can guide and trap the light at nanometer dimensions. Two proposed PSCs i.e. concave and trapezoidal structures are compared to the planar structure in order to investigate the effects of using nanostructured substrates on the optoelectronic performance of PSCs. Optical analysis reveals that using optimized concave and trapezoidal structures can enhance the light absorption up to 32 and 26%, respectively at the wavelength of 550 nm. Electrical simulations have shown that in addition to enhanced total carrier generation, the generated carriers can be effectively collected in the proposed nanostructured PSCs. Accordingly, the short-circuit current has risen from 20 mA for planar structure to 25.7 mA for concave and 23.2 mA for trapezoidal PSCs. After analyzing various heights and adopting optimum values, the power conversion efficiency for concave and trapezoidal PSCs experienced substantial increase of 5.5 and 3.5%, compared to the planar structure. These drastic improvements analyzed by coupled optical and electrical modelling of nanostructures can pave the way for further studies to fabricate high efficiency PSCs with nano-textured substrates as a light-trapping technique.

Random nanohole arrays and its application to crystalline Si thin foils produced by proton induced exfoliation for solar cells

We report high efficiency cell processing technologies for the ultra-thin Si solar cells based on crystalline Si thin foils (below a 50 µm thickness) produced by the proton implant exfoliation (PIE) technique. Shallow textures of submicrometer scale is essential for effective light trapping in crystalline Si thin foil based solar cells. In this study, we report the fabrication process of random Si nanohole arrays of ellipsoids by a facile way using low melting point metal nanoparticles of indium which were vacuum-deposited and dewetted spontaneously at room temperature. Combination of dry and wet etch processes with indium nanoparticles as etch masks enables the fabrication of random Si nanohole arrays of an ellipsoidal shape. The optimized etching processes led to effective light trapping nanostructures comparable to conventional micro-pyramids. We also developed the laser fired contact (LFC) process especially suitable for crystalline Si thin foil based PERC solar cells. The laser processing parameters were optimized to obtain a shallow LFC contact in conjunction with a low contact resistance. Lastly, we applied the random Si nanohole arrays and the LFC process to the crystalline Si thin foils (a 48 µm thickness) produced by the PIE technique and achieved the best efficiency of 17.1% while the planar PERC solar cell without the Si nanohole arrays exhibit 15.6%. Also, we demonstrate the ultra-thin wafer is bendable to have a 16 mm critical bending radius.

Light trapping nano structures with over 30% enhancement in perovskite solar cells

Abstract Organic–inorganic metal halide perovskites have drawn a great deal of attention due to their supreme optical and electrical properties and their potential in future application in optoelectronic devices. Here, we carry out finite-difference time-domain (FDTD) simulation on different experimentally realistic structures of perovskite solar cells (PSC) and optimize their parameters with assistance of neural network (NN). We find an optimized structure with 30.48% enhancement comparing to planar structure and the fact that with properly design, 300-nm-thick nano-textured structure can outperform 900-nm-thick planar structure. We believe that light trapping structure is essential in thin film PSCs and also has a great potential in lead-free PSCs.

A high-performance cupric oxide photocatalyst with palladium light trapping nanostructures and a hole transporting layer for photoelectrochemical hydrogen evolution

The high recombination rate of photogenerated electron–hole pairs, poor photocorrosion stability, and the discrepancy between the optical absorption length and charge collection efficiency of cupric oxide (CuO) are the main limiting factors of CuO photocatalysts.

Dopant-free random inverted nanopyramid ultrathin c-Si solar cell via low work function metal modified ITO and TiO2 electron transporting layer

Ultrathin c-Si solar cells with light trapping nanostructures attract tremendous research interest for their flexibility and high specific power density. However, the performance of the ultrathin c-Si solar cell is limited by a big light absorption loss due to the reduced thickness. Here, we report a novel ultrathin c-Si solar cell through the direct deposition of TiO2 electron transporting layer and indium tin oxide (ITO) electrode modified with ultrathin low workfunction (WF) metal subsequently onto random inverted nanopyramids (INPs) texture. The random INPs are fabricated through the well-known two-step Ag assisted chemical etching method followed by a post nanostructure rebuilding process. The TiO2 thickness, deposition temperature and metal layer thickness are changed to optimize cell performance. With the optimized parameters, a high short-current density (Jsc) (30.66 mA/cm2) and energy-conversion efficiency (11.36%) are achieved on random INPs based 45 μm ultrathin c-Si solar cell by choosing 2 nm Mg layer as the modifying ultrathin metal layer, which are 37.65% and 36.4% respectively higher than that in planar one. All the findings not only offer additional insight into the mechanism of TiO2 electron transporting layer based ultrathin c-Si solar cell but also introduce a promising new approach for next-generation cost effective flexible photovoltaics.

Broadband enhancement of dielectric light trapping nanostructure used in ultra-thin solar cells

A dielectric fishnet nanostructure is designed to increase the light trapping capability of ultra-thin solar cells. The complex performance of ultra-thin cells such as the optical response and electrical response are fully quantified in simulation through a complete optoelectronic investigation. The results show that the optimized light trapping nanostructure can enhances the electromagnetic resonance in active layer then lead to extraordinary enhancement of both absorption and light-conversion capabilities in the solar cell. The short-circuit current density increases by 49.46% from 9.40 mA/cm2 to 14.05 mA/cm2 and light-conversion efficiency increases by 51.84% from 9.51% to 14.44% compared to the benchmark, a solar cell with an ITO-GaAs-Ag structure.

Quick design of high efficiency light trapping nanostructures for thin film silicon solar cells

Photonic nanostructures are now widely investigated as light trapping textures to achieve significant absorption in thin film solar cells. In this paper, we quickly designed a high efficiency photonic structure for thin film silicon solar cells. Based on the coupled mode theory, we compared classic lattice photonic structures and demonstrated triangular lattice structure exhibits better light trapping performance. Through analysis of short circuit current density, unit cells with heart-pentagon polygon arrangement were verified to offer superior absorption in entire silicon absorption spectrum. Finally, the comparisons with other reported textures validated our design.

Towards nanostructured perovskite solar cells with enhanced efficiency: Coupled optical and electrical modeling

Third generation photovoltaic technologies based on perovskites have demonstrated an exceptional progress in solar energy conversion since their first use in 2009. Herein, we investigated the effect of using light trapping nanostructures on the absorption, carrier collection, and overall efficiency of perovskite (CH3NH3PbI3) solar cells using three dimensional (3D) finite element method (FEM) technique. A combined optical-electrical model was constructed to full characterize the proposed devices. Upon the use of nanotubular architecture, the optimized active area absorption enhanced by 6% and the total generation rate increased by 7% compared to the planar architecture. Under one sunlight illumination (AM1.5G), with normal incident angle, the solar cells containing nanostructured light trapping architecture showed a drastic enhancement in the short circuit current (Jsc), the quantum efficiency (EQE), and the overall efficiency compared to the planar film-based solar cell. The obtained enhancements would open a new route for integrating light trapping nanostructures in CH3NH3PbI3 perovskite-based solar cells for better efficiency.

The Effect of Reaction Time on Optical Trapping Nanostructure Formation on the Multi-Crystalline Silicon by Metal-Assisted Chemical Etching

The surface texturing has been considered as an important process for the high-efficiency solar cell fabrication. A two-step metal-assisted chemical etching method was used to produce light trapping nanostructure on multi-crystalline silicon wafers. And the effect of the reaction time on the optical trapping properties of the nanostructures was investigated. Both the density and the size of Ag particles are highly dependent on the deposition time. The etching time instead of the Ag particle deposition time dominates the reflectance of the nanostructures when the etching time is above 30 s. The lowest spectrum-weighted average reflectance obtained in this study is about 9.2%.

Trapping Nanostructures on Surfaces through Weak Interactions.

The assembly of imidazole-functionalized phenanthroline-strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl3 , THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three-dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface-molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.

Fabrication of the replica templated from butterfly wing scales with complex light trapping structures

The polydimethylsiloxane (PDMS) positive replica templated twice from the excellent light trapping surface of butterfly Trogonoptera brookiana wing scales was fabricated by a simple and promising route. The exact SiO2 negative replica was fabricated by using a synthesis method combining a sol–gel process and subsequent selective etching. Afterwards, a vacuum-aided process was introduced to make PDMS gel fill into the SiO2 negative replica, and the PDMS gel was solidified in an oven. Then, the SiO2 negative replica was used as secondary template and the structures in its surface was transcribed onto the surface of PDMS. At last, the PDMS positive replica was obtained. After comparing the PDMS positive replica and the original bio-template in terms of morphology, dimensions and reflectance spectra and so on, it is evident that the excellent light trapping structures of butterfly wing scales were inherited by the PDMS positive replica faithfully. This bio-inspired route could facilitate the preparation of complex light trapping nanostructure surfaces without any assistance from other power-wasting and expensive nanofabrication technologies.

Efficient Degradation of Methylene Blue over Two‐Dimensional Au/TiO2 Nanosheet Films with Overlapped Light Harvesting Nanostructures

Two‐dimensional TiO2 nanosheet films with visible light trapping nanostructures were successfully fabricated by alkali hydrothermal reaction using Ti sheet as precursor. Metallic Au nanoparticles (NPs) were then deposited on the surface of TiO2 film through a microwave‐assisted reduction process. The investigations reveal that the localized surface plasmon resonance (LSPR) of Au NPs is greatly enhanced by the overlapped light harvesting nanostructures between TiO2 film and Au NPs, resulting in an enhanced LSPR‐absorption with two peaks at 389 nm and 540 nm. The photocatalytic performance of the samples was evaluated by degradation of methylene blue (MB) as a model pollutant. The experimental results indicate that the photocatalytic performance of TiO2 is greatly promoted by a synergetic effect between the overlapped light harvesting nanostructures and the improved charge carrier separation processes. The MB degradation over the optimal sample is much faster than that of pure TiO2 film by a factor of 3.0 and 5.7 under UV light and UV + visible light irradiation, respectively. This study provides a simple strategy to develop film‐shaped plasmonic photocatalysts with high efficiency.

How to assess light trapping structures versus a Lambertian Scatterer for solar cells?

We propose a new figure of merit to assess the performance of light trapping nanostructures for solar cells, which we call the light trapping efficiency (LTE). The LTE has a target value of unity to represent the performance of an ideal Lambertian scatterer, although this is not an absolute limit but rather a benchmark value. Since the LTE aims to assess the nanostructure itself, it is, in principle, independent of the material, fabrication method or technology used. We use the LTE to compare numerous proposals in the literature and to identify the most promising light trapping strategies. We find that different types of photonic structures allow approaching the Lambertian limit, which shows that the light trapping problem can be approached from multiple directions. The LTE of theoretical structures significantly exceeds that of experimental structures, which highlights the need for theoretical descriptions to be more comprehensive and to take all relevant electro-optic effects into account.

Laser-induced jets of nanoparticles: exploiting air drag forces to select the particle size of nanoparticle arrays

In this study, we developed a new method-based on laser-induced jets of nanoparticles (NPs) and air drag forces-to select the particle size of NP arrays. First, the incident wavelength of an excimer laser was varied to ensure good photo-to-thermal energy conversion efficiency. We then exploited air drag forces to select NPs with sizes ranging from 5 to 50 nm at different captured distances. Controlling the jet distances allowed us to finely tune the localized surface plasmon resonance (LSPR) wavelength. The shifting range of the LSPR wavelengths of the corresponding NP arrays prepared using the laser-induced jet was wider than that of a single NP or an NP dimer. We further calculated the relationship between the air drag force and the diameter of the NPs to provide good control over the mean NP size (capture size ≧ 300 μm) by varying the capture distance. Laser-induced jets of NPs could also be used to fabricate NP arrays on a variety of substrates, including Si, glass, plastic, and paper. This method has the attractive features of rapid, large-area preparation in an ambient environment, no need for further thermal annealing treatment, ready control over mean particle size, and high selectivity in the positioning of NP arrays. Finally, we used this method to prepare large NP arrays for acting hot spots on surface-enhanced Raman scattering-active substrates, and 10(-12) M R6G can be detected. Besides, we also prepare small NP arrays to act as metal catalysts for constructing low-reflection, broadband light trapping nanostructures on Si substrates.

Fabrication and optical characterization of light trapping silicon nanopore and nanoscrew devices

We have fabricated nanotextured Si substrates that exhibit controllable optical reflection intensities and colors. Si nanopore has a photon trapping nanostructure but has abrupt changes in the index of refraction displaying a darkened specular reflection. Nanoscrew Si shows graded refractive-index photon trapping structures that enable diffuse reflection to be as low as 2.2% over the visible wavelengths. By tuning the 3D nanoscale silicon structure, the optical reflection peak wavelength and intensity are changed in the wavelength range of 300–800 nm, making the surface have different reflectivity and apparent colors. The relation between the surface optical properties with the spatial features of the photon trapping nanostructures is examined. Integration of photon trapping structures with planar Si structure on the same substrate is also demonstrated. The tunable photon trapping silicon structures have potential applications in enhancing the performance of semiconductor photoelectric devices.

Extreme Superomniphobicity of Multiwalled 8 nm TiO2 Nanotubes

We report unprecedented superomniphobic characteristics of nanotube-structured TiO(2) surface fabricated by electrochemical etching and hydrothermal synthesis process, with the wettability contact angles for water and oil both being ∼174° or higher. A tangled forest of ∼8-nm-diameter, multiwalled nanotubes of TiO(2) was produced on the microtextured Ti surface, with the overall nanotube length controlled to 150 nm by adjusting the processing time. Wettability measurements indicate that the nanotube surface is extremely nonwettable to both water and oil. The contact angle of the 8 nm TiO(2) nanotube surface after perfluorosilane coating is extremely high (178°) for water droplets indicating superhydrophobic properties. The contact angle for oil, measured using a glycerol droplet, is also very high, about 174°, indicating superoleophobic characteristics. These dual nonwetting properties, superomniphobic characteristics, are in sharp contrast to the as-made TiO(2) nanotubes which exhibit superhydrophilic properties with a contact angle of essentially ∼0°. Such an extreme superomniphobic material made by a simple and versatile method can be useful for a variety of technical applications. It is interesting to note that all three properties can be obtained with identical nanotube structures. A nanometer-scaled structure introduced by hydrothermally grown TiO(2) nanotubes is an effective air trapping nanostructure in enhancing the amphiphobic (superomniphobic) wettability.