Porous Silicon Antireflection Coatings Research Articles

Oxidation Effects on Graded Porous Silicon Anti-Reflection Coatings

Efficient anti-reflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband anti-reflection properties can be fabricated. In this work, ageing of graded PS has been studied using Spectroscopic Ellipsometry, Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS elements such as pure Si (Si0), Si2O (Si+), SiO (Si2+), Si2O3 (Si3+), and SiO2 (Si4+) are present. In addition both hydrogen and carbon is introduced to the PS in the form of Si3SiH and CO. The oxide grows almost linearly with time when exposed to oxygen, from an average thickness of 0–3.8 nm for the surface PS. The oxidation is then correlated to the optical stability of multi-layered PS ARCs. It is found that even after extensive oxidation, the changes in the optical properties of the PS structures are small.

Ellipsometry, Reflectometry, and XPS Comparative Studies of Oxidation Effects on Graded Porous Silicon Antireflection Coatings

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband antireflective properties can be fabricated. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate ARCs optimized for use in solar cells. In this work, the effect of ageing on the reflection properties of multilayered PS ARCs is studied. The reflection is correlated to the oxidation of the structures through the use of x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). It is found that even after extensive oxidation, very small changes in reflectivity are measured.

Ellipsometric study of the influence of chemical etching on thin porous silicon structures

The effect of chemical etching on Porous Silicon (PS) samples is studied and quantified by using variable angle spectroscopic ellipsometry (VASE). The main aim of this work is to assess the impact of such etching on the physical properties of electrochemically etched, thin PS antireflection coatings (ARC) for solar cell applications. In this study, detailed models of PS layers etched at constant current densities are created using a graded uniaxial Bruggeman Effective Medium Approximation (BEMA). Changes in porosity, thickness, and optical anisotropy of the PS samples due to chemical etching are determined as a function of etching time after PS formation. Three series of PS films, etched at three different current densities, are investigated. It is shown that significant changes in physical properties occur for chemical etching times longer than ~60s. The anodic etching process for fabricating PS ARC structures can be performed in less than 10s. Therefore, chemical etching does not lead to significant deviations from the intended PS structure and is not seen as a hindrance to accurate control of processes for fabricating thin PS ARCs.

Ellipsometry and XPS Comparative Studies of Oxidation Effects on Graded Porous Silicon Antireflection Coatings

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Applications of porous silicon thin films in solar cells and biosensors

An overview of the applications of porous silicon (PS) thin films, as antireflection coatings (ARC) in silicon solar cells and transducers in biosensors, is presented. The reflectance spectra of PS films have been compared with other conventional ARCs (such as SiNx TiO2/MgF2 and ZnS), and optimal PS ARC with minimum reflectance has been obtained. The implementation of PS into an industrially compatible screen-printed (SP) solar cell by both the electrochemical etching (ECE) and chemical etching (CE) methods are reviewed. Porous silicon films, formed via ECE for short anodization times, on textured n+ emitter ofc-Si solar cell having SP front and back contacts, lead to improvements in the performance of solar cells and demonstrate their viability in industrial applications.

Optimization of multilayer porous silicon antireflection coatings for silicon solar cells

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. In this work, multilayered refractive index stacks optimized for antireflection, in bare air and within modules, are modeled. The relation between porous silicon (PS) etching parameters and PS structure is carefully investigated using spectroscopic ellipsometry, gravimetry, x-ray photoelectron spectroscopy, and scanning electron microscopy. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate the optimized ARCs. Limits imposed by efficient application in photovoltaics, such as thickness restrictions and the angular distribution of incident light, are examined and accounted for. Low reflectance multilayer ARCs are fabricated with integrated reflectances of ∼3% in air and 1.4% under glass in the wavelength range 400–1100 nm.

Porous silicon antireflection layer for solar cells using metal‐assisted chemical etching

AbstractPorous silicon antireflection coatings were generated on fully processed screen‐printed monocrystalline silicon solar cells using metal‐assisted chemical etching in HF:H2O2:ethanol. In this method, interaction of the HF acid with the cell front grid contact results in the dissolution of silver nanoparticles which provide the catalytic sites required to initiate porous silicon formation. Control of the etching time and HF concentration was found critical to attain good antireflection proper‐ ties while avoiding front contact degradation. The optimised porous silicon layer reduced the solar cell effective reflectance from 44% to 12% in the spectral range 400–1100 nm and increased the short‐circuit current density by 46%. The solar cell efficiency was raised by 3% absolute. The simplicity of the method makes it promising for cost‐effective reduction of reflection losses in silicon solar cells. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Screen-printed multicrystalline silicon solar cells with porous silicon antireflective layer formed by electrochemical etching

The latest results on the use of porous silicon (PS) as an antireflection coating (ARC) in simplified processing for multicrystalline silicon (mc-Si) solar cells are presented. A PS layer with optimal antireflection characteristics was obtained for charge density (Q) of 5.2 C∕cm2. The weighted reflectance was reduced to 4.7% in the range of wavelengths between 400 and 1000 nm. Also, the optimization of a PS selective emitter formation results in a 13.2% efficiency mc-Si cell (2×2 cm2) with the electroplating method. Specific attention is given to the implementation of a PS ARC into a commercially compatible screen-printed solar cell process.

Anodic and optical characterisation of stain-etched porous silicon antireflection coatings

Porous silicon (PS) antireflection coatings have been obtained by stain etching of crystalline Si in HF:HNO 3:H 2O solutions at different etching times and HNO 3 concentrations. The morphology of the PS samples was characterised by anodic oxidation and revealed the increase of the porous surface area with the HNO 3 concentration. Upon a certain critical concentration of HNO 3, the surface of crystalline Si immersed in the solution becomes polished, and a decrease of its surface area is observed. The samples were also characterised by a spectrophotometer in the range 300–900 nm, showing an effective reflectance below 4% in most of the spectral range for samples obtained in an etching solution, where the HNO 3 is below its critical concentration. Anodic oxidation after etching can be used as an useful tool for the qualitative and quantitative analysis of porous surface areas.

Firing through Porous Silicon Antireflection Coating for Silicon Solar Cells

The new idea to realize contacts through a porous silicon (PS) antireflection coating (ARC) is tested for the first time from the viewpoint of PS eligibility towards a rapid high-temperature process. The morphological analysis (TEM and SE) reveals that the thickness of PS after the thermal treatment is slightly increased accompanied by an increase of its porosity. As a result no significant changes in reflectance characteristics are observed. Therefore it can be successfully used as an alternative way for preparation of PS ARC for silicon solar cells.

Wide Bandwidth Porous Silicon Coatings for Silicon

Porous silicon anti-reflection coatings for silicon substrate, with a reflectance of less than 2% in the spectral range of 400–1000 nm, were fabricated. The wide bandwidth operation was realized by introducing a smooth refractive index gradation at the porous silicon–substrate interface to suppress multiple reflection fringes. Low current density anodization was adopted to form the high refractive index layer needed for a smooth refractive index gradation.

Design of porous silicon antireflection coatings for silicon solar cells

A porous silicon (PS) layer formed electrochemically in the outer part of the n + emitter of p-n + Si junctions can be used as an efficient antireflection coating (ARC). A two-step procedure is presented which can determine the electrochemical parameters leading to the formation of an optimized single-layer PS ARC. Single-layer PS ARCs achieving ≈7% effective reflectance between 400 and 1000 nm are obtained on shallow p–n + junction solar cells. To reduce the reflectance further, the design of double-layer ARCs based on PS is investigated. PS layers with different porosities can be realized in a single experiment by modulating the current density during the electrochemical process. It is shown theoretically and experimentally that such PS structures can lead to an effective reflectance below 3%.

Use of porous silicon antireflection coating in multicrystalline silicon solar cell processing

The latest results on the use of porous silicon (PS) as an antireflection coating (ARC) in simplified processing for multicrystalline silicon solar cells are presented. The optimization of a PS selective emitter formation results in a 14.1% efficiency multicrystalline (5/spl times/5 cm/sup 2/) Si cell with evaporated contacts processed without texturization, surface passivation, or additional ARC deposition. Specific attention is given to the implementation of a PS ARC into an industrially compatible screen-printed solar cell process. Both the chemical and electrochemical PS ARC formation method are used in different solar cell processes, as well as on different multicrystalline silicon materials. Efficiencies between 12.1 and 13.2% are achieved on large-area (up to 164 cm/sup 2/) commercial Si solar cells.