Silicon Ribbon Research Articles

Silicon processing for photovoltaics based on incoherent radiation power

AbstractIt is well known that impurity contamination is a very important factor in silicon processing. Contamination arises from crucibles or graphite susceptors during crystal growth, from the use of fibers or dies as in the case of String ribbon or EFG technologies respectively. Furthermore, every hot part in a crystal growth system will contribute to increase impurity contamination. Optical processing using halogen lamps provides clean method, with available fast rise and fall ramps, cold wall furnaces, with great flexibility in many situations related to materials processing and in particular in photovoltaics. Optical power provided by halogen lamps is very cheap, which is an important aspect for an industry that needs to reduce process costs. Techniques based in lamp power radiation have been in use in our laboratory at the University of Lisbon for many years. Several examples of the use of such optical techniques are presented, illustrating its application to silicon ribbon growth, kerfless techniques and on its successful application for photovoltaics in a gas‐to‐wafer path, bypassing both the Siemens and the traditional wafering steps. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

N-type silicon RST ribbon solar cells

In this work, we report for the first time on the electronic properties and the solar cell fabrication on N-type silicon ribbons grown by the RST method. We have investigated the majority charge carriers' parameters (resistivity and mobility) and the minority carrier lifetime (MCL) of N-type silicon RST ribbons. The MCL values are deduced using the photoconductivity decay method on RST wafers, the surface of which was passivated by an iodine/ethanol solution. The effect of hydrogen plasma treatment on the electronic properties of the as-grown RST ribbons was also studied. We show that the bulk passivation following hydrogen plasma treatment, carried out at temperatures above 350°C for more than 1h, strongly improves the MCL. Conventional solar cells were prepared on RST ribbons grown in optimized conditions and passivated by plasma hydrogenation. Two types of boron emitters were tested, namely those obtained by boron-spin-coating and by ion implantation, while the N+ region serving as a back surface field was made by phosphorus spin coating. The highest conversion efficiency values obtained for boron-implantation (B-II) and boron-spin-coating (B-SOD) emitters on a ∼120μm thick RST were 13.8% and 12.7% respectively, with no surface texturing, or optimized surface passivation. Suns-Voc method applied to these RST cells gave, for B-II and B-SOD emitters conversion efficiencies of about 15.1% and 13.6%, respectively. The former value represents to our knowledge the highest efficiency value obtained for ultrathin N-type multi-crystalline silicon ribbon based cells.

Highly Stretchable Alkaline Batteries Based on an Embedded Conductive Fabric

Recent progress in the fabrication of ultrathin silicon ribbons and novel architectures have enabled devices that can stretch, bend, and twist without mechanical fatigue or changes in operational performance. [ 1–5 ] These advances have lead to compliant, conformable electronics for health monitoring and sensing purposes. [ 6 , 7 ] For true autonomous operations, these devices require an equally accommodating power source. Existing commercially available power sources are too bulky and negate the advantages of these compliant/fl exible devices. We demonstrate a stretchable battery with electrochemically active materials embedded in a compliant conductive fabric, which acts as a support for the material. The assembled manganese dioxide (MnO 2 ) zinc (Zn) stretchable cell with a polyacrylic acid (PAA) based polymer gel electrolyte (PGE) had an open circuit potential (OCV) of 1.5 V and a capacity of 3.875 mAh/cm 2 . The capacity remained constant when tested under strain as high as 100%. Two cells connected in series continuously powered an LED when stretched to 150% and twisted by 90 degrees. In the past decade, stretchable electronics with a wide variety of functionality such as biological sensors, [ 8 ] solar cells, [ 9 ] polymer light-emitting devices, [ 10 ] transistors, [ 11 , 12 ] active matrix displays, [ 13 , 14 ] and photo-detectors [ 15 ] have been demonstrated. While there has been progress on power sources with similar mechanical properties, there is still a signifi cant gap. Previously, stretchable supercapacitors based on SWNT deposited on stretched PDMS, [ 16 ] CNTs embedded in fabric [ 17 ] and conducting polymer on compliant substrates [ 18 ] have been demonstrated, but these devices are suited to short term energy storage and cannot be used to power standalone devices. A stretchable MnO 2 -Zn primary battery with a stretchable carbon oil current collector was demonstrated, however the discharge capacity decreased by ∼ 55% when the battery was stretched by 50%. The discharge profi le showed a high ohmic potential drop (I × R) at the start of discharge for the strained battery. Drop in potential can be accounted by decrease in electrical conductivity of the carbon oil current collector and loss in electrical contact in the electrode when stretched. [ 19 ]

Electron Backscattered Diffraction and Transmission Electron Microscopy Analysis of Ribbon Silicon

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Ribbon Silicon Material for Solar Cells

Solar cell is one of most important renewable energy. But now it is not be widely used because of its high cost compared with traditional resource. Ribbon silicon is one new low cost solar cell material avoiding ingot casting and slicing. It is a promising silicon wafer fabrication technology alternative to traditional ingot casting and slicing. Using ribbon silicon can make solar cell production cost greatly reduced. In this paper EFG, String Ribbon and a novel silicon wafer are discussed.

+0.4% Efficiency gain by novel texture for String Ribbon solar cells

This work introduces metal assisted etching for surface texturing of saw-damage free crystalline silicon independent from the crystallographic orientation. A noble metal serves as a catalyst for the chemical etching in an aqueous solution containing hydrofluoric acid and nitric acid. Metal assisted textured String Ribbon solar cells show a reduction ΔReff=−16%rel for the effective reflection Reff compared to untextured String Ribbon solar cells. This improvement results in an increase ΔJsc=+1.4mA/cm2 of the short circuit current density Jsc. Therefore, the median solar cell efficiency η increases by Δη=+0.4% from ηuntex=14.1% to ηtex=14.5%.

Effect of Processing Parameters on Thickness of Columnar Structured Silicon Wafers Directly Grown from Silicon Melts

In order to obtain optimum growth conditions for desired thickness and more effective silicon feedstock usage, effects of processing parameters such as preheated substrate temperatures, time intervals, moving velocity of substrates, and Ar gas blowing rates on silicon ribbon thickness were investigated in the horizontal growth process. Most of the parameters strongly affected in the control of ribbon thickness with columnar grain structure depended on the solidification rate. The thickness of the silicon ribbon decreased with an increasing substrate temperature, decreasing time interval, and increasing moving velocity of the substrate. However, the blowing of Ar gas onto a liquid layer existing on the surface of solidified ribbon contributed to achieving smooth surface roughness but did not closely affect the change of ribbon thickness in the case of a blowing rate of <svg style="vertical-align:-1.29163pt;width:42.525002px;" id="M1" height="12.4875" version="1.1" viewBox="0 0 42.525002 12.4875" width="42.525002" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(1.25,0,0,-1.25,0,12.4875)"> <g transform="translate(72,-62.01)"> <text transform="matrix(1,0,0,-1,-71.95,63.35)"> <tspan style="font-size: 12.50px; " x="0" y="0">≥</tspan> <tspan style="font-size: 12.50px; " x="12.027886" y="0">0</tspan> <tspan style="font-size: 12.50px; " x="18.279387" y="0">.</tspan> <tspan style="font-size: 12.50px; " x="21.405136" y="0">6</tspan> <tspan style="font-size: 12.50px; " x="27.656635" y="0">5</tspan> </text> </g> </g> </svg>&#x2009;Nm<sup >3</sup>/h because the thickness of the solidified layer was already determined by the exit height of the reservoir.

Piezoresistive Strain Sensors and Multiplexed Arrays Using Assemblies of Single-Crystalline Silicon Nanoribbons on Plastic Substrates

This paper describes the fabrication and properties of flexible strain sensors that use thin ribbons of single-crystalline silicon on plastic substrates. The devices exhibit gauge factors of 43, measured by applying uniaxial tensile strain, with good repeatability and agreement with expectation based on finite-element modeling and literature values for the piezoresistivity of silicon. Using Wheatstone bridge configurations integrated with multiplexing diodes, these devices can be integrated into large-area arrays for strain mapping. High sensitivity and good stability suggest promise for the various sensing applications.

SEM Investigation of the electrical properties of silicon ribbons

The structure and electrical properties of silicon ribbons grown on a substrate by the Ribbon Growth on Substrate (RGS) method method for solar cell applications have been investigated in secondary electron and electron beam induced current modes of scanning electron microscopy. The growth method and growth conditions have provided the formation of the coarse-grained structure of silicon, in which the majority of grains are separated by twin boundaries and the dislocation density does not exceed 106 cm−2. According to the electron beam induced current investigations, the recombination contrast from twin boundaries is extremely low at 300 K, only a small amount of twin boundaries show an increase in the contrast upon cooling, and the contrast from dislocations is almost absent in the temperature range from 100 to 300 K.

Towards Cheaper Solar Cells: A New Path to Grow Silicon Ribbons

Towards Cheaper Solar Cells: A New Path to Grow Silicon Ribbons

Recombination Activity of Twin Boundaries in Silicon Ribbons

Investigations of silicon layers grown on carbon foil were carried out using the Electron Beam Induced Current (EBIC) methods. The most of grain boundaries in these ribbons are (111) twin boundaries elongated along the direction. The EBIC measurements showed that the recombination contrast of dislocations and of the most part of twin boundaries at room temperature is practically absent and only random grain boundaries and very small part of twin boundaries produce a noticeable contrast. At lower temperatures a number of electrically active twin boundaries increases but the most part of them remains inactive. A contamination with iron increases the recombination activity of random boundaries but not the activity of twin boundaries.

실리콘 나노리본을 이용한 유연한 패시브 매트릭스 소자 제작

대표적인 반도체 소재인 실리콘을 유연소자로 이용하기 위하여 매우 얇은 나노리본 형태로 제작하였다. p-타입과 n-타입 도핑 그리고 고유한 영역으로 구성된 실리콘 소자(p-i-n 접합소자)를 가로/세로 100라인씩 연결하여 총 10,000개의 어레이 소자를 구현하였고 그 크기는 대각선 1인치에 달했다. 이 패시브 매트릭스 소자는 p-n 접합 소자에 비해 교차 혼선에 의한 역전류가 적어 정류비가 <TEX>$10_{4}$</TEX> 이상의 값을 나타내었다. 완성된 소자는 불산 처리를 통해 기판으로부터 쉽게 떼어낼 수 있으며, 각각 PDMS 와 유연한 PET 필름에 전이할 수 있었다. Thin silicon ribbon was used for fabricating flexible silicon p-i-n junction devices, consisting of 100<TEX>${\times}$</TEX>100 arrays of pixels in 1 inch on the diagonal. Those passive matrix devices exhibited the rectification ratio <TEX>$>10^{4}$</TEX> owing to smaller cross-talking current than that of p-n junction devices. P-i-n devices fabricated on silica/silicon substrates are easily detached by treatment with hydrofluoric acid and are subsequently transferred onto both PDMS and flexible PET film.

Understanding the sprayed boric acid method for bulk doping of silicon ribbons

The sprayed boric acid (SBA) method for bulk doping of silicon ribbons is investigated. Experimental procedures and main results are reviewed. Computational fluid dynamics and experimental tests using partial spraying suggest the role of gas transported evaporated boron oxide to explain the boron incorporation profiles along the sample. The industrial applicability of the SBA method is discussed.

Thermoresponsiveness of Integrated Ultra‐Thin Silicon with Poly(N‐isopropylacrylamide) Hydrogels

Stimuli-sensitive polymer materials have limited device functionality, design and manufacturing flexibility although they are pushed to enable smart device applications. Here we demonstrate the capability of integrating thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels with silicon nanoribbons, and enable the stiff silicon ribbons to become adaptive and drivable by the soft environmentally sensitive substrate, such as becoming mechanically stretched and compressed on temperature change. These and related soft/hard smart devices and systems may open new opportunities in biomedical applications.

Correlating internal stresses, electrical activity and defect structure on the micrometer scale in EFG silicon ribbons

In the present paper, we study the influence of defects through their stress fields on the electrical activity and residual stress states of as-grown edge-defined film-feed (EFG) multicrystalline silicon (mc-Si) ribbons. We apply a combination of micro-Raman spectroscopy, electron beam induced current, defect etching and electron backscatter diffraction techniques that enables us to correlate internal stresses, recombination activity and microstructure on the micrometer scale. The stress fields of defect structures are considered to be too small (several tens of MPa) to influence directly the electrical activity, but they can enhance it via stress-induced accumulation of metallic impurities. It is commonly found that not all recombination-active dislocations on grain boundaries (GBs) and within grains are accompanied by internal stresses. The reason for this is that dislocations interact with each other and tend to locally rearrange in configurations of minimum strain energy in which their stress fields can cancel partially, totally or not at all. The outcome is a nonuniform distribution of electrical activity and internal stresses along the same GB, along different GBs of similar character as well as inside the same grain and inside different grains of similar crystallographic orientations. Our work has implications for developing crystal growth procedures that may lead to reduced internal stresses and consequently to improved electrical quality and mechanical stability of mc-Si materials by means of controlled interaction between structural defects.

Recrystallization of silicon polygonal tubes using an electric closed molten zone

This article describes a process for generating and controlling a closed molten zone with an induced electrical current, and using it for silicon ribbon tube recrystallization. The silicon tube is the secondary loop of a transformer in which the current is generated by electrical induction. The Joule heat caused by the induced current generates a closed molten line along a cross section of the silicon tube; scanning this molten zone along the tube axis results in material recrystallization, with no contact with foreign materials. From the recrystallized silicon tube faces test solar cells were produced, revealing minority carrier diffusion lengths around 100 μm.

Characterization of silicon ribbon by the SEM methods

AbstractInvestigation of silicon layers grown on carbon foil was carried out using the Electron Backscatter Diffraction (EBSD) and Electron Beam Induced Current (EBIC) methods. The EBSD investigations showed that the surface of silicon ribbon is close to (110) and that most of grain boundaries (GBs) are twin boundaries with the misorientation angle of 60° between the neighbouring grains. The diffusion length in Si ribbon studied was obtained to exceed 35 μm. The EBIC measurements showed that the recombination contrast of dislocations and twin GBs at room temperature is practically absent and only random GBs produced a noticeable contrast. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

저가 다결정 EFG 리본 웨이퍼의 표면 반사도 특성 최적화

Ribbon silicon solar cells have been investigated because they can be produced with a lower material cost. However, it is very difficult to get good texturing with a conventional acid solution. To achieve high efficiency should be minimized for the reflectance properties. In this paper, acid vapor texturing and anti-reflection coating of SiNx was applied for EFG Ribbon Si Wafer. P-type ribbon silicon wafer had a thickness of 200 μm and a resistivity of 3 Ω-㎝. Ribbon silicon wafers were exposed in an acid vapor. Acid vapor texturing was made by reaction between the silicon and the mixed solution of HF : HNO3.. After acid vapor texturing process, nanostructure of less than size of 1 μm was formed and surface reflectance of 6.44% was achieved. Reflectance was decreased to 2.37% with anti-reflection coating of SiNx.

Metal Assisted Surface Texture for String Ribbon Solar Cells and Modules

This work presents metal assisted etching for surface texturing of industrial size String Ribbon wafers. Palladium serves as a catalyst for chemical etching in an aqueous solution containing hydrofluoric acid and nitric acid. String Ribbon solar cells textured by metal assisted etching show a decreased effective reflection ΔReff = 16%rel compared to untextured solar cells. The decreased reflection results in a solar cell efficiency increase Δηcell = + 0.4%abs. The improvement in solar cell efficiency is mainly due to an increase ΔJsc = + 0.9 mA/cm2 in the average solar cell short circuit current density Jsc and an increase ΔFF = 0.7%abs in fill factor FF. Due to enhanced optical coupling, the efficiency increases additional Δηmodule ≈ 0.1%abs after encapsulation of the cells behind glass.

Influence of Thickness Deviation on crystalline Silicon Solar Cell Performance

Abstract This paper presents a novel method to measure the local thickness of silicon wafers spatially resolved. A camera detects the transmission of infrared radiation through a silicon wafer, which transforms to a local thickness map. Experiments with String Ribbon solar cells show that the local contact resistance depends on the local wafer thickness. The high resolution thickness map is therefore transformed into a local contact resistance map. A 3D-multi-diode array models the efficiency loss due to an inhomogeneous contact resistance. The fill factor loss ΔFF due to the thickness deviation of solar cells amounts up to ΔFF = -1% abs , or an efficiency loss Δη = -0.2% abs .