Front Surface Reflection Research Articles

Technological process for a new silicon solar cell structure with honeycomb textured front surface

Abstract This paper presents a new silicon solar-cell structure improved by texturisation of the front surface using silicon micromachining technologies. A ‘honeycomb’-textured front surface has been obtained through a photolithographical process to generate patterns (disc holes) on the front surface followed by isotropic etching (in HNO 3 : HF: CH 3 COOH) until the wells joined together. For front-surface loss characterisation, the spectral dependence of the front-surface reflectivity has been investigated by spectrophotometrical measurements. The surface reflectivity was lowered under 10% and this value was a good one compared to the reflectivity of silicon monocrystalline wafer untextured surface. The p–n junction made by phosphorous diffusion at ∼0.8 μm follows the honeycomb profile. In order to obtain low series resistance, a p+ boron diffusion on the back of the structure was made. The fabrication process was completed with an ohmic contact (Al on top and on the back surface).

Minimizing lifetime degradation associated with thermal oxidation of upright randomly textured silicon surfaces

Thermally grown silicon dioxide is commonly used in high-efficiency mono-crystalline silicon solar cell designs as a diffusion mask, electroless plating mask, passivation layer or as a rudimentary anti-reflection coating. Some of these device designs also use upright random texturing, to minimize front surface reflection, in combination with thermally grown silicon dioxide layers. The volume expansion associated with oxidation, generates large stresses on the silicon substrate when oxidizing sharp surface features. If this stress exceeds the fracture stress of silicon, defects are generated within the bulk region. This paper reviews oxidation-induced stress, and highlights two techniques for reducing lifetime degradation during the growth of silicon dioxide on textured wafers: (i) reducing the generation of stress, and (ii) decrease the relaxation time of the silicon dioxide layer. This paper experimentally investigates, and evaluates, several practical methods for implementing these techniques on upright random textured p-type float zoned silicon using photoconductance lifetime.

Self-Organized Corrugated Interface for Barrier Heterostructures to Solar Cells Application

This report is devoted to the investigation of self-organized microrelief morphology control and optimization of the microrelief processing leading to obtain hotosensitivity of solar cells (SC) enhancement. The use of the heterostructures (HS) with the corrugated interface as active element allows one to increase SC photoresponse due to the front surface reflection reduction (optical losses). Surface barrier HS of Au/GaAs were used as model structures to reveal general relationship between the scattered light intensity and morphology peculiarities of HS interfaces.

Modeling Ultrasonic Pulse-Echo Signals from a Flat-Bottom Hole in Immersion Testing Using a Multi-Gaussian Beam

This paper proposes an ultrasonic measurement model that can predict the pulse-echo signals from a flat-bottom hole in an isotropic, homogeneous solid specimen immersed in water in a computationally efficient manner. To develop such a model, a measurement model approach is adopted based on two important assumptions: the paraxial approximation for the transducer beam and the small flaw assumption for the flat-bottom hole. The modular model that results from these two assumptions contains three terms: a diffraction correction term, a far-field scattering amplitude term and a system efficiency factor term. The diffraction correction is defined based on a multi-Gaussian beam model which allows the rapid evaluation of the wave field incident on the hole. The far-field scattering amplitude of the flat-bottom hole is obtained using the Kirchhoff approximation together with the small flaw assumption. The system efficiency factor is determined by deconvolution of an experimental front surface reflection signal by a reference reflector model. Here, the contribution of each of these three terms to the overall measurement model are described in detail and the accuracy of the proposed model is verified by the comparison of the model-based predictions to experiments.

Thin‐film (25.5 μm) solar cells from layer transfer using porous silicon with 32.7 mA/cm2 short‐circuit current density

AbstractWe fabricate a 25.5‐μm‐thick monocrystalline Si solar cell with a confirmed power conversion efficiency of 15.4% and an area of 3.88 cm2 using a layer transfer process with porous Si. The process is free of photolithography and contains no high‐temperature oxidation steps. We investigate three design features that improve the short‐circuit current density to a value of 32.7 mA/cm2 under AM1.5 illumination. The detached back reflector contributes 2 mA/cm2, a reduced front‐surface reflectance accounts for an additional 2 mA/cm2 and a reduced base doping increases the current density by 1 mA/cm2. Copyright © 2002 John Wiley & Sons, Ltd.

Optimal Absorbance for Transmission or Reflection Spectra Measured under Conditions of Constant Detector Noise in the Presence of Stray Radiation

The signal-to-noise ratio ( S/N) for absorption spectrometers can be expressed as absorbance/absorbance noise ( Am/ SAm), where Am is the measured absorbance and Smm is the standard deviation in Am, or as concentration/concentration noise ( c/sc) calculated from Beer's law. For measurements made under ideal conditions, Am/ SAm. However, when stray radiation, or an analogous phenomenon, such as detector nonlinearity in FT-IR measurements or surface reflection in diffuse reflection spectrometry, contributes to the spectrum, C/ Sc ≠ Am/ SAm. It is known that the optimum value of Am/ SAm in the absence of stray light is found at an absorbance of 0.4816 when noise from the reference spectrum is taken into account and 0.4343 when noise from the reference spectrum is excluded from the calculations. In the present paper we discuss the effect of stray light on the absorbance and concentration S/N. The maximum value of c/sc is found at lower absorbance than is the case if stray light is absent. While this effect is negligibly small for the typical levels of stray light found with most contemporary monochromators, it can be significant when, for example, the response of a detector used in Fourier transform spectrometry is nonlinear or when the front-surface reflection in diffuse reflection measurements is large. For effective stray light levels of 10%, the maximum value of c/sc is found at an absorbance that is 22% lower than when no source of stray light is present.

Industrially attractive front contact formation methods for mechanically V-textured multicrystalline silicon solar cells

Abstract The formation of the front contact is one major topic for solar cell industry. In addition, for multicrystalline silicon (mc Si) there has been no breakthrough in establishing an efficient and cost effective surface texturisation technology for considerable front surface reflection reduction yet. This paper gives an overview on the integration of the V-texturisation into industrial solar cell processes applying the following economical attractive metallisation methods: screen-printing, roller-printing and buried contact formation. For these metallisation methods cell efficiencies of 16.0–16.3% on BAYSIX mc Si (area 100–156 cm 2 ) have been reached with 4–7% rel. efficiency increase as compared to untextured reference cells. In addition solar cell simulations based on realistic assumptions for the contact geometry and electrical performance are given. Finally, a comparison between solar cell concepts with homogeneous and selective emitter including a mechanical V-texture is presented.

Design considerations for thin-film silicon solar cells from the porous silicon (PSI) process

Our monocrystalline thin-film solar cells grown on textured porous silicon are almost planar on one side and are textured with random, inverted pyramids on the other side. We compare solar cells illuminated through the textured side with solar cells that are illuminated through the almost planar side. Two-dimensional numerical modeling of the measured internal quantum efficiency shows that the textured front side increases the collection efficiency of the base. This effect, together with the reduction of the front surface reflectance, makes a textured front surface advantageous over a planar front surface.

Two-Dimensional Measurements of Lubricant Spreading on Diamond-Like-Carbon Surface Using Image Processing on Fringe Patterns Formed by Michelson Interferometry

A Michelson interferometer based on the Linnik microscope configuration has been constituted and applied to the measurement of lubricant spreading on diamond-like-carbon (DLC) surface. Performing image processing on the fringe patterns formed by the interferometer, the lubricant thickness, as well as the spreading behavior can be investigated. This method provides complete line profiles in two dimensions and permits direct observation of the spreading phenomena, in contrast to the conventional point-by-point scanning method. To improve the measurement accuracy, a fringe following technique and a noise suppression technique were introduced. The fringe following technique successfully prevented a fringe shift over the visual field and thus suppressed the pseudo phase shift caused by a fringe shift. The spatially fixed noise due to non-uniform laser beams and flaws in optical components was also efficiently suppressed by the noise compensation method. From the calculation of multiple beam interference in a stratified medium, it is found that reduction in sensitivity due to additional reflections from underlayers is effectively prevented if the lubricant is observed through a glass disk using reflections on the back surfaces of the glass and lubricant, instead of being observed on a disk using the front surface reflections. Utilizing this advantage, a novel scheme was employed to give maximum sensitivity in which lubricant was coated on the back side of a glass disk whose front surface was coated with non-reflecting film and whose back surface was sputtered with the proper thickness of DLC. The enhanced resolution attained by the new scheme was experimentally confirmed by observing lubricant on the back surface of the glass disk and the front surface of a magnetic disk.

Test of the Representative Layer Theory of Diffuse Reflectance Using Plane Parallel Samples

Equations of Benford used in the Representative Layer Theory are able to describe spectroscopic remission from layered plane parallel samples (plastic sheets) quite effectively. Losses due to reflection directly back in the direction of the incident beam are a major cause of discrepancies. Non-compositional variation and experimental errors tended to produce linear changes in the absorption coefficient, with the remission coefficient being more drastically affected. The remission coefficients obtained experimentally, in general, vary inversely to the absorption coefficient, although, as predicted by theory, front surface reflectance causes a direct variation. In transflectance, the log(1/ R) spectrum of the thinnest samples is the one that is most like the absorption coefficient curve, but the shape of the Kubelka–Munk (absorption/remission) spectra are less affected by sample thickness, especially in the absence of surface reflection.

Evaluation of the Quality of Solid Sugar Samples by Fluorescence Spectroscopy and Chemometrics

It has been shown that fluorescence spectroscopy of sugar in aqueous solution carries important quality and process information related to beet sugar factories, which is accessible by multivariate analyses. A method for measuring crystalline sugar directly on-line in the process should be advantageous. In this paper we compare the solution measurement technique with two methods of fluorescence measurement on solid sugar. Surprisingly, it was possible to measure fluorescence through the sugar crystals by using the same transmission techniques with 90° detection as with the sugar solutions. This method was compared with a 45° front-surface reflection method. Sugar samples from six different sugar factories were examined. The spectral responses were reasonable, but they were influenced by the heterogeneous sample composition and the sample geometry. It was possible with the two methods to separate sugar samples according to factory with the use of principal component analysis (PCA). Seasonal time trends were found in weekly samples from the same factory. Partial least-squares regression (PLS) was used to predict quality parameters, where color (range: 6–41), ash (range: 0.003–0.018), and α-amino-N (range: 0.28–5.07) could be modeled with errors of 2.3–2.6, 0.0015–0.0016, and 0.40–0.42, respectively. Model errors for similar solution data have been determined to 2.4, 0.0012, and 0.266, respectively.

Optical Characterization Of Industrially Sputtered Nickel–Nickel Oxide Solar Selective Surface

Tandem absorbers are often used in the design of solar absorbers for photo thermal conversion. They consist of a thin coating, selectively absorbing in the wavelength range of the solar spectrum, on a metal substrate. The optical performance of a tandem absorber depends on the optical constants and thickness of the absorbing coating and also on the reflectivity of the underlying metal. A very high solar absorptance is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the front surface. This type of composition suppresses coating interference and gives a low front surface reflection if the refractive index at the front surface is low. We report on optical analysis of a solar absorber with a graded index coating of sputtered nickel–nickel oxide deposited on aluminium. The optical constants have been determined from reflectance, transmittance and ellipsometry data by fitting the data to a two-layer model of the coating. The optical constants of the two layers can be regarded as effective optical constants for the lower and upper part of the graded index coating respectively. It is found that the effective refractive index of the top layer is somewhat lower than for the base layer. The extinction coefficient is higher in the lower part of the coating. Both effective refractive index and extinction coefficient of the base layer increase monotonically with increasing wavelength as for metallic materials.

The effect of gloss on color

A model was developed for the effect of gloss on reflected color, taking the measurement geometry of the color measuring device into account. It considers the total front surface reflectance of a given sample to be independent of gloss and determined only by its refractive index and the angle of the incident beam. Although the total front surface reflectance is fixed, its specular component increases (and the diffuse component decreases) with increasing sample gloss. Therefore, the diffuse reflectance factor measured by a spectrophotometer decreases with sample gloss. In this article, a defined curve was fitted to the measured reflectance factor of black xerographic print samples having a range of gloss values for the 0/45 and diffuse spectrophotometer geometries, and this curve was used to predict the CIELAB color values for samples of the other colors at various gloss levels. The predicted color was compared to the measured color for cyan, magenta, yellow, red, green, and blue images that spanned a wide range of gloss. The RMS color error between the predicted and measured color of all of these images was found to be about 3 CIELAB ΔE* units. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 369–376, 1999

A new approach to the characterization of molecular orientation in uniaxially and biaxially oriented samples of poly(ethylene terephthalate)

The usual approach that is used to characterize the molecular orientation in biaxially oriented samples by infrared spectroscopy is to measure spectra with polarization in all three directions: machine, transverse and normal (or thickness). However, the latter measurement is rather difficult to make experimentally. In the present work we propose a new approach to characterizing the molecular orientation in both uniaxially and biaxially oriented samples of PET, based on the use of front-surface reflection spectra. It makes use of the ratio of the absorption bands at 1330–1240 and 1729 cm −1, the first of which shows parallel dichroism and the second perpendicular dichroism. An equation is developed that relates this ratio to the molecular orientation with respect to the direction of measurement. Thus, it is possible to determine individually the orientation functions with respect to the machine and transverse directions. The validity of functions determined in this way is confirmed by comparison with birefringence results.

Ray-tracing of arbitrary surface textures for light-trapping in thin silicon solar cells

A new ray-tracing program is introduced for modelling the anti-reflection and light-trapping properties of arbitrary surface textures. The validity of the ray-tracing model is discussed. The program is used to ray-trace thin crystalline silicon solar cells that might be deposited on to chemically etched glass. The scattering effectiveness of the textured surface is much greater by reflection than by transmission. Thus for best light-trapping properties (which might approach the lambertian ideal), the rear surface of the cell should be textured. Front surface reflection losses for a module under typical yearly averaged illumination conditions are, however, quite significant. A range of metal and detached reflectors are considered, and also bifacial modules. For a 10 μm thick film, a maximum J sc of 34–35 mA cm 2 seems likely under real conditions.

Observation and analysis of acoustic signals following the second or third front surface reflections in an acoustic microscope

In the normal mode of operation of an acoustic microscope, the properties of the sample are inferred from the time delay of the leaky Rayleigh wave signal with respect to the first specular reflection from the sample surface. In this paper, observations of similar leaky Rayleigh wave signals following the second and third front surface signals from the sample are reported. Included is a ray analysis of their origin, which identifies the second front surface as involving a Rayleigh wave in the lens and the third as involving a Rayleigh wave in the sample. It is suggested that the latter could be of significant practical interest since, for a given lens, leaky Rayleigh waves can be excited on samples with lower velocities than those following the first specular reflection.

Specular reflectance: A convenient tool for polymer characterization by FTIR-microscopy?

Front-surface specular reflectance Fourier transform infrared (FTIR) measurements at near normal incidence are being increasingly used to study both the chemical composition and morphology of ‘optically-thick’ polymer samples. They provide a convenient, non-contact, non-destructive method of characterising, both qualitatively and quantitatively, the chemical and physical properties of thick polymer samples, or at least their surface layers. These measurements are readily made with FTIR-microscope combinations, which facilitate the easy analysis of awkwardly shaped materials or small samples, and point-by-point mapping across a sample surface. Applications of specular reflectance FTIR-microscopy to obtaining structural and morphological data on polymeric materials are illustrated and discussed in this paper.

ＳＡＩＬモデルにおける葉の表と裏の反射率の違いの影響

Canopy reflectance model is a useful tool to correct the effect of measuring conditions like as the changes in observation angle and solar irradiance angle. So that, if we use the corrected reflectance data by the model for estimating vegetation index such as NDVI, we can derive more accurate information of vegetative distribution from satellite data.Nowadays, the famous model for canopy reflectance is the SAIL model. (Verhoef, 1984) This study notices that the leaf reflectance parameter of the SAIL model is considered only front surface of leaves. And so, we survey the influence on canopy reflectance. As a result of it, we find that it is not adequate to predict canopy reflectance by using only front surface reflectance of leaves.

Characterization of Surface Orientation in Poly(Ethylene Terephthalate) by Front-Surface Reflection Infrared Spectroscopy

The use of front-surface specular reflection FT-IR spectroscopy to characterize surface orientation in thick samples of poly(ethylene terephthalate) (PET) has been investigated. It has been shown that, even for samples whose surface uniformity is less than perfect, absorption index spectra of excellent quality are obtained from the Kramers-Kronig transformation. These spectra provide detailed information, both qualitative and quantitative, on the molecular conformation and orientation of the polymer. Of particular interest are the strongly absorbing bands such as the carbonyl and ester peaks, which are generally saturated in transmission spectra. The reflection technique opens up the possibility of obtaining new information on conformational changes and orientation in PET through a detailed study of these peaks.

A new texturing geometry for producing high efficiency solar cells with no antireflection coatings

Abstract Various schemes to trap weakly absorbed light into solar cells have been proposed. These schemes include texturing the cell, texturing the cover glass and geometric arrangements of the individual cells. The perpendicular slats geometry is considered to be the best cell texturing design for light trapping. In this paper a new cell surface texturing design is proposed which, without the use of anti-reflection coatings, can outperform the perpendicular slats geometry with a double layer anti-reflection coating by virtue of efficient internal light trapping and a decrease in the front surface reflectance. The single sided texture uses three perpeendicular planes on the front surface and a planar back surface. The three perpendicular planes provide a triple bounce for the incoming light and efficient confinement for light which has entered the cell. TEXTURE, a raytracing program for textured cells, was used to predict the performance of this new design. A quantitative comparison with other texturing schemes is also provided. It is shown that for a cell without an anti-reflection coating on the front and a 98% effective back surface reflector, the new design produces a maximum short circuit current density of 40.99 mA/cm2 as compared to 41.46 mA/cm2 and 35.16 mA/cm2 for the perpendicular slats geometry and flat surfaces, respectively, with a conventional single layer AR coating on the front. Effects of different front surface reflection coefficients are examined to show that as the front reflectance is decreased by improved antireflection coatings, the importance of the triple bounce is reduced and most promising surface texturing schemes approach the same value of maximum current.