Front Surface Reflection Research Articles

Upright Pyramid Surface Textures for Light Trapping and MoOx Layer in Ultrathin Crystalline Silicon Solar Cells

In this work, ray tracing is used to investigate the optical characteristics of various surface structures in ultrathin crystalline silicon (c-Si) for solar cells. Ultrathin c-Si with a thickness of 20 μm is used as the substrate. The light trapping includes front upright pyramids with a molybdenum oxides (MoOx) anti-reflection (AR) layer. Planar ultrathin c-Si (without a MoOx AR layer and upright pyramids) is used as a reference. The wafer ray tracer was developed by a photovoltaic (PV) lighthouse to model the MoOx AR layer to reduce the front surface reflectance and impacts of the AR layer on ultrathin Si solar cells. The optical properties are calculated on the AM1.5 global solar energy spectrum across the 200–1200 nm wavelength region. From the absorbance profile, the photogenerated current density (Jph) in the substrate is also calculated with various surface structures. The front upright pyramids with the MoOx layer result in the largest absorbance enhancement due to the enhanced light scattering by the pyramids and MoOx AR layer. The Jph of 37.41 mA/cm2 is improved when compared to the planar ultrathin c-Si reference. This study is significant as it illustrates the potential of ultrathin c-Si as a promising PV module technology in the future.

Frequency-domain searching algorithm in deflectometry for measuring the surface of a transparent planar element.

This Letter presents the frequency-domain searching algorithm in deflectometry (FSAD). By encoding specialized multi-frequency fringe patterns and employing a correlation searching algorithm, the limitations of existing frequency-domain methods can be overcome to some extent, thereby separating front and back surface reflections to obtain complete measurement data. The principles of FSAD are described in detail. In the experiment, a piece of window glass with thickness of 10 mm and a square area of 96 × 96 mm is measured to verify the proposed method.

Minimizing annual reflection loss in fixed-tilt photovoltaic modules using graded refractive index (GRIN) anti-reflective glass

This study evaluates the performance of graded refractive index (GRIN) anti-reflective (AR) structures on photovoltaic (PV) modules across twenty global locations and compares them with conventional thin film AR coatings and bare glass. Optimized glass nanocones were chosen to represent the GRIN structures. Electrodynamic simulations were conducted to assess reflection properties at multiple angles of incidence. GRIN AR structures dramatically reduce total solar-integrated reflection to 0.2% at normal incidence, compared to 3.8% for bare glass and 0.8% for optimized thin film AR coatings, and maintained low reflection across a range of incident angles, demonstrating only 0.7% reflection at 60 degrees compared to significantly higher values for other surfaces. Detailed hourly simulations for twenty global locations revealed that GRIN AR coatings substantially reduce annual reflection loss to just 0.86±0.19% and increase expected annual energy output by up to 11.15±1.00% compared to bare glass at optimal tilt angles. The benefits of GRIN AR coatings are more substantial at sub-optimal panel orientations such as horizontal or vertical due to its broad angle AR properties. These findings highlight the potential of GRIN AR technology to approach the theoretical limit where front surface reflection loss is completely eliminated and enhance solar power conversion efficiencies. Additionally, a generalized model to approximate annual reflection loss for various materials is presented. This research encourages further exploration into GRIN AR coatings to optimize power conversion efficiencies and reduce reflection in PV modules.

Simulation of Anti-Reflective TiO2/SiO2 Coating for Silicon Photovoltaic Application by Ray Tracing

n solar systems, anti-reflective coatings are used to reduce reflection and increase efficiency. However, the front surface on the solar cells alone is not effective because most of the light from the sun is reflected and very less energy absorption into the solar cells occur. An anti-reflective coating (ARC) of a sufficient thickness can greatly reduce front surface reflectance. Nanoscale surface texturing, on the other hand, can efficiently capture a higher ratio of incident light to boost optical absorption. In this study, the light trapping scheme within the wavelength of 300 to 1200 nm was used to improve the overall efficiency of silicon solar cells. A thin layer of TiO2 and SiO2 anti-reflective coating with different thicknesses was stacked alternatingly due to their different refractive index with TiO2 having a high refractive index and SiO2 with a low refractive index. Solar irradiance spectrum AM1.5G at normal incidence was used in this present work. For the ray-tracing simulation, the front planar with multilayer ARC with different thicknesses were investigated to obtain the optimum value for optical properties and current density. All the four combination arrangements of SiO2 and TiO2 were evaluated and the maximum potential photocurrent density (Jmax) was calculated. The Jmax value of thin crystalline silicon, c-Si (without ARC) was 24.93 mA/cm2 and increased to 30.28 mA/cm2when ARC was used on the front surface. This represents an increasing of 21.46 % enhancement compared to the Jmax of the c-Si reference. Simulation of Anti-Reflective TiO2/SiO2 Coating for Silicon Photovoltaic Application by Ray TracingImran Al-Haqeem Bin Jaffar1, Siti Hajar Mohmad Salleh2*, Mohd Zaki Mohd Yusoff31,2Faculty of Applied Sciences, Universiti Teknologi MARA, Cawangan Perlis, Campus Arau, 02600 Perlis, Malaysia3Faculty of Applied Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia*Corresponding author’s E-mail: sitiha2902@uitm.edu.myReceived: 08 August 2022 Accepted: 17 February 2023Online First: 17 March 2023

Performance Improvement of npn Solar Cell Microstructure by TCAD Simulation: Role of Emitter Contact and ARC

In the current study, the performance of the npn solar cell (SC) microstructure is improved by inspecting some modifications to provide possible paths for fabrication techniques of the structure. The npn microstructure is simulated by applying a process simulator by starting with a heavily doped p-type substrate which could be based on low-cost Si wafers. After etching deep notches through the substrate and forming the emitter by n-type diffusion, an aluminum layer is deposited to form the emitter electrode with about 0.1 µm thickness; thereby, the notches are partially filled. This nearly-open-notches microstructure, using thin metal instead of filling the notch completely with Al, gives an efficiency of 15.3%, which is higher than the conventional structure by 0.8%. Moreover, as antireflection coating (ARC) techniques play a crucial role in decreasing the front surface reflectivity, we apply different ARC schemes to inspect their influence on the optical performance. The influence of utilizing single layer (ZnO), double (Si3N4/ZnO), and triple (SiO2/Si3N/ZnO) ARC systems is investigated, and the simulation results are compared. The improvement in the structure performance because of the inclusion of ARC is evaluated by the relative change in the efficiency (Δη). In the single, double, and triple ARC, Δη is found to be 12.5%, 15.4%, and 17%, respectively. All simulations are performed by using a full TCAD process and device simulators under AM1.5 illumination.

Suppression of front and back surface reflections in ultrasonic analytic-signal responses from composites

Pulse-echo ultrasound testing is the most prevalent method for inspection of composite materials in industry although evolving designs combined with the anisotropic nature of composites demands the constant development of more advanced signal-processing techniques and testing equipment. One problem that is frequently encountered in ultrasonic inspection, in pulse-echo mode, is the masking effect that occurs due to the strong surface reflections. This can prove critical for the detection of near-surface defects and accurate ply-tracking of the first and last two plies. The purpose of this study is to suppress the front- and back-surface reflections by first removing them from the measured ultrasonic response using the analytic signal and its instantaneous parameters. The first in the series of key steps the method includes is determining the shape of the input pulse using the measured front-surface reflection. After obtaining the input pulse, the front- and back-surface reflections are constructed artificially. The front-surface reflected pulse is the product of a complex reflection coefficient and the input pulse, while the back-surface reflected pulse is the product of a complex reflection coefficient, an attenuation term, and the incident pulse at the back surface. The next step involves subtracting the constructed surface pulses from the original response and substituting a reflection from a resin layer embedded in composite at the front and back surfaces. Those reflections are added back to the signal in order to make the ply extraction work consistently in the internal layers. The method has been tested using both simulated and real data. Subtraction of the front-surface was highly successful in a range of material configurations, but subtraction of the back-surface required algorithm refinements to cope automatically with all the scenarios tested. The ability of the method to improve detectability of defects and tracking of near-surface plies is demonstrated using data from real samples with near-surface delaminations, tape gaps and overlaps, and internal wrinkling.

Rapid anisotropic chemical etching for quick formation of novel octagonal pyramids on silicon surface for photovoltaics

A novel octagonal micro pyramid structure was formed by etching a monocrystalline silicon wafer (100) by employing a wet chemical anisotropic etching process. The main objective is to reduce the silicon wafer's front surface reflectance, improve surface morphology, enhance wettability, and increase the coverage area of the octagonal pyramids on the surface of silicon wafers. The reaction time of the etching process was kept precise under strict observation and control. The experimental results demonstrate a significant reduction in silicon wafers' optical surface reflectance, achieving the lowest reflectance of 8.87%, along with improved surface morphology, periodicity, and coverage of more than 88%. While adding hydrofluoric acid together with magnetic stirring (mechanical agitation) at 300 rpm was advantageous in forming the octagonal silicon pyramid with a high etch rate of 0.41 um/min and significantly reducing the reaction time.

Optical analysis of light management for finger designs in CPV systems

AbstractConcentrated photovoltaic (CPV) systems can achieve high energy conversion efficiencies due to their use of concentrating optics and highly efficient solar cells; however, power losses arising from the front surface reflection and metallization shading typically contribute a significant fraction to the total power loss for these devices. In flat silicon photovoltaic (PV) modules, the front‐metal finger shape is often shaped to minimize shading losses. Finger shape is even more relevant for CPV cells where the current density and front metal shading are much higher. This study compares the optical performance of five light management finger designs in CPV systems with two different refractive homogenizers. The optical efficiency and loss analysis were determined using ray‐tracing simulations. It is shown that shaped, light diverting metal fingers can improve the optical efficiency in CPV systems by up to 9.9% absolute compared to rectangular fingers, corresponding to an absolute cell efficiency increase of 3.0%. Although sharp edged triangular finger structures may be challenging to fabricate, significant optical gains can still be achieved with trapezoid or rounded cap finger shapes.

Impact of different parameters on the performance of GaAs solar cell using PC1D simulation

In this work, GaAs solar cell has been analyzed by using PC1D simulation software. Optimization of thickness and doping concentration of emitter and absorber layer of solar cell is done. Effect of front and rear surface recombination velocity is analyzed in this work. When surface recombination velocity increases, the value of efficiency decreases due to increase in recombination process. The optimum value of surface recombination velocity are Sn = Sp = 102 cm/s. Front surface reflection is minimized by the use of TiO2 single layer ARC and double layer LiF/HfO2 ARC. Increase in efficiency around 2.71% is achieved as compared to cell without ARC after applying anti-reflection coating of optimum thickness. Simulation results shows GaAs solar cell with efficiency of 26.68%.

Copper-assisted catalyzed etching for nanotextured black silicon with enhanced photoelectric-conversion properties.

Black silicon contains high-aspect-ratio micro/nanostructures with greatly suppressed front-surface reflection, thus possessing superior property in photoelectric devices. In this report, by a two-step copper-assisted chemical etching method, we have fabricated pyramid n+p-black silicon with optimized morphology and anti-reflectance capability, through systematically tuning the concentration of both copper ions and reducing agents, as well as the etching time. The improved optical absorption and superior charge transfer kinetics validate n+p-black silicon as a highly active photocathode in photoelectrochemical cells. The onset potential of 0.21 V vs. RHE and the saturation photocurrent density of 32.56 mA/cm2 are achieved in the optimal n+p-black silicon. In addition, the nanoporous structure with lower reflectance is also achieved in planar p-silicon via the same etching method. Moreover, the photodetectors based on planar p-black silicon show significantly enhanced photoresponsivity over a broad spectral range. This study offers a low-cost and scalable strategy to improve the photoelectric-conversion efficiency in silicon-based devices.

The Role of Metal-Catalyzed Chemical Etching Black Silicon in the Reduction of Light- and Elevated Temperature-Induced Degradation in P-Type Multicrystalline Wafers

Light- and elevated temperature-induced degradation (LeTID) of silicon wafers reduces the energy yield of photovoltaic modules in the field. A previous study demonstrated that this degradation is greatly reduced in samples with a surface textured with black silicon formed via reactive ion etching. However, the reason for this reduction is currently not understood. This article extends that result to a range of nano-textures formed using commercial metal-catalyzed chemical etching. Lifetime test structures are fabricated on multicrystalline wafers with six different chemical nano-textures and the results are compared to planar reference samples. The article demonstrates a reduction in the extent of light-induced degradation compared to planar reference samples for all six textures. The extent of LeTID is observed to reduce with decreasing front surface reflectance and increasing front surface area. The article finds that wafer thickness, phosphorus gettering, and fast-firing cannot explain the result.

Pre-Texturing Thermal Treatment for Saw-Damage-Removal-Free Wet Texturing of Monocrystalline Silicon Wafers

The etching of Si wafers significantly influences the efficiency of photovoltaic devices. Texturing can effectively decrease front surface reflection and improve device performance. Saw damage removal (SDR) is necessary to yields uniform random pyramidal surfaces without the appearance of saw marks, it entails significant consumption of chemical solutions and complicated cleaning steps. Herein, an alternative process of pre-texturing thermal treatment was carried out at 800 °C for 10 min, followed by anisotropic texturing, and a uniform pyramidal surface over a large area of the textured surface was obtained without saw marks. Compared with that of as-cut mono-Si wafers (30.7%), the weighted average reflectance of the samples textured with or without thermal treatment decreased to 11.2% and 11.9%, respectively, and further to 3% and 3.4%, respectively, when anti-reflection coatings were applied. In addition, saw marks on the wafer surface were used as gettering sites during thermal treatment, and the bulk lifetime was more than doubled from 42.6 µs before the treatment to 93.8 µs after. The simple, SDR-free method presented herein for enhancing the textural uniformity of Si wafers and, hence, solar cell performance, can be employed on an industrial scale without necessitating additional investment in equipment.

Pyramid textured Si{100} surface with low reflectivity in CMOS compatible solution

et anisotropic etching of Si{100} results in micro-pyramids/hillocks bounded by four {111} planes. These kinds of geometrical structures are needed to reduce the front surface reflectance from silicon by taking multiple internal reflections of the incident ray and thus increase the efficiency of the solar cells. In this work, surface texturing of Si{100} is performed at different temperatures in a very low concentration CMOS compatible tetramethylammonium hydroxide (TMAH) without using any additive and agitation. The present research is aimed to significantly reduce the etching time in an extremely low concentration TMAH to texture the silicon surface for obtaining the lowest possible reflectance. Etching temperature is varied from 80 to 95°C with a step of 5°C. For each temperature, the etching time is also varied from 5 to 40 min with an interval of 5 min, whereas etching concentration is made a fix to 0.5 wt%. Surface morphology with dense and uniform pyramidal structures is achieved on the silicon samples etched at 85°C for 25 min. These samples provide the average solar weighted reflectance (Rsw) of around 10% and the lowest reflectance of 6.7% at 790 nm.

Effect of Etching Temperature on the Surface Texturing of Si{100} in an Extremely Low Concentration Tmah for Minimized Reflectivity

Among various renewable energy resources, solar energy is one of the most abundant energy available in nature. The solar energy can be utilized by converting photon energy into electrical energy by using solar cells as energy harvesters. Silicon-based solar cells dominate the current photovoltaic market because of its higher efficiency, excellent stability, non-toxicity, lower processing cost and excellent reliability in the outdoor conditions.1 However, the silicon solar cell efficiency can be further increased by reducing the front surface reflection from silicon, which is greater than 35%. In order to reduce the reflection from silicon surface, surface texturing is done where silicon surface is intentionally made rough. Wet anisotropic etching is most widely used in surface texturing of silicon because of its high throughput and lower cost. Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are two most extensively used alkaline solution for surface texturing of silicon. 2-4 In these two etchants, TMAH is preferred because it is fully compatible with CMOS process and also provide high etch selectivity between silicon and silicon dioxide. Surface texturing using low concentration TMAH (<5%) results in high surface roughness. This is a remarkable observation that can be applied in solar cell industries to minimize the reflection from silicon front surface.In this work, we have studied the effect of very low concentration TMAH (1.0-0.1 wt%) without adding any additive and agitation during etching for surface texturing of Si{100} surface. The main objective of this study is to achieve an optimized etched surface topography at lowest possible etching concentration and etching time to minimize the reflectance for application in solar cells. The average solar weighted reflectance (Rsw) of around 10% is obtained in a very mild etching conditions. Compared to the previous results reported in literatures and also the mild etching conditions, reflectance values obtained in this work are quite promising for large area surface texturing of silicon solar cell panels for enhancement in their efficiency. In addition, the use of very low concentration TMAH minimizes the chemical waste. This also helps in adopting the technique for fabricating anti-reflective silicon surfaces at large scale.

A Radial Basis Function Neural Network Based Approach to Mitigate Soiling from PV Module

Solar power is available in abundance and is free of cost but despite this fact photovoltaic (PV) systems are not reliable as the efficiency of PV module is still not up to the mark. Amount of sun light absorbed by the module is the fuel for the PV modules, there are many factors which play a vital role such as front surface reflectivity, solar beam incidence angle and the most important is the transmissivity of the front surface. Objective of this paper is to examine the function of a neural network based model of a module to mitigate the soiling and improve the accuracy of dust prediction over the module and enhance the efficiency which was degraded by soiling. One of the techniques in artificial intelligence named, radial basis function neural networks (RBFNN) is utilized here to predict the dust data by reading the data of solar irradiance, module temperature, PV voltage, current and power and generates the control signal to the windshield wiper motor that drives the wiper to wipe the dust from the panel. A considerable measure of accessible exploratory information was utilized for the preparation of the RBFNN, and a back propagation algorithm was utilized.

Blade-Coated Perovskites on Textured Silicon for 26%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells

Summary Integrating perovskites onto textured silicon provides a pathway to 30% tandem solar cells. However, deposition of 0.5–1 μm thick perovskite layers from solution onto textured silicon with typical pyramid heights of 3–10 μm remains a challenge. We propose a new tandem architecture that enables scalable, solution-based blading of perovskites onto silicon wafers textured with pyramids less than 1 μm in height. These pyramids are rough enough to scatter light within the silicon nearly as efficiently as large pyramids but smooth enough to solution-process a perovskite film. A nitrogen-assisted blading process deposits both a conformal hole transport layer and a planarizing perovskite layer that fully covers the textured silicon, at a speed of 1.5 m/min. With a textured light-scattering layer added to the top of the tandem to reduce front-surface reflectance, we achieve a perovskite/silicon tandem cell with an efficiency of 26% on textured silicon.

Highly Efficient and Less Time Consuming Additive Free Anisotropic Etching of Silicon Wafers for Photovoltaics

Etching of silicon wafers has a great effect on increasing the efficiency of photovoltaic devices. Texturization can effectively reduce the front surface reflection and improve device performance by enhancing light trapping and harvesting. In this paper, we report a highly efficient aqueous etching solution for silicon wafers. This etching solution has been studied in terms of reflectance, wettability and surface morphology of silicon wafers. Experimental results demonstrate tremendous improvement in surface morphology as well as decrease in the front surface reflection. The resulting pyramids were periodic with uniform distribution. The lowest surface reflectance of 10.76% was achieved with a significant improvement in the wettability of silicon wafers. These results proved the potential of this etching solution as well as paves a way towards the fabrication of highly efficient and low-cost photovoltaic devices.

TiO2 Self-Assembled, Thin-Walled Nanotube Arrays for Photonic Applications.

Two-dimensional arrays of hollow nanotubes made of TiO are a promising platform for sensing, spectroscopy and light harvesting applications. Their straightforward fabrication via electrochemical anodization, growing nanotube pillars of finite length from a Ti foil, allows precise tailoring of geometry and, thus, material properties. We theoretically investigate these photonic crystal structures with respect to reduction of front surface reflection, achievable field enhancement, and photonic bands. Employing the Rigorous Coupled Wave Analysis (RCWA), we study the optical response of photonic crystals made of thin-walled nanotubes relative to their bare Ti foil substrate, including under additional charge carrier doping which might occur during the growth process.

Anti-reflective structures for photovoltaics: Numerical and experimental design

The effects of different anti-reflective structures on the photovoltaic performance of the silicon solar cell were studied using finite-element modelling and numerical simulations for which experiment alone does not provide a full description. The front surface reflectivity may be mitigated significantly by an anti-reflective coating (ARC) of a suitable thickness. Alternatively, nanoscale surface texturing can effectively trap a greater ratio of incident light to increase optical absorption. The optimized layer thicknesses of the ZnO single layer and SiO2/Si3N4 double layer films were calculated for minimum reflectivity, with the former grown by magnetron sputter deposition and characterized using specular X-ray reflectivity measurements. Based on geometric ray-tracing and solutions to the semiconductor equations, the theoretical photovoltaic performance was simulated and compared for a range of incident angles at an optical intensity of 0.1 Wcm−2, revealing a limit to the angular collection efficiency of the ARC at a grazing incidence angle of 30°. Using ZnO or SiO2/Si3N4 ARCs or surface texturing increases the power conversion efficiency by 20%, 24% and 30% respectively at normal incidence. The insights provided by physical-based modelling on the optimized design parameters of the anti-reflective structures confer a promising pathway for enhancing the external quantum efficiency of photovoltaic devices.

Quasi-crystal photonic structures for fullband absorption enhancement in thin film silicon solar cells

To further increase the efficiency of thin film solar cells, it is critical to enhance the absorption over the full spectral wavelength range in which solar cells generate electricity. In this letter, we present a fullband absorption enhancement method for n-i-p thin film silicon solar cells based on a Quasi-Crystal Structure (QCS) by superimposing Ag random nanotextures on periodically patterned Micro-Cone Substrates. Both light in-coupling and light trapping abilities are significantly improved thanks to the reduction of front-surface reflection originated from the gradually changed refractive index of preserved micro-cone profile after film deposition and the richer guided mode resonances caused by the QCS. An initial efficiency of 10.4% is obtained for QCS-based hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells, which outperforms the planar (efficiency of 7.5%) and randomly nanotextured (efficiency of 8.7%) counter part by 38.7% and 19.5%, respectively. The QCS can also be duplicated for other thin film photovoltaic devices and provides a new approach for creating high-efficiency thin-film solar cells.