Polycrystalline Silicon Sheets Research Articles

Numerical Simulation and Fabrication of Silicon Sheet via Spin Casting

A spin-casting process for fabricating polycrystalline silicon sheets for use as solar cell wafers is proposed, and the parameters that control the sheet thickness are investigated. A numerical study of the fluidity of molten silicon indicates that the formation of thin silicon sheets without a mold and via spin casting is feasible. The faster the rotation speed of graphite mold, the thinner the thickness of sheet. After the spread of the molten silicon to cover the graphite mold with rotation speed of above 500 rpm, the solidification has to start. Silicon sheets can be produced by using the centrifugal force under appropriate experimental conditions. The spin-cast sheet had a vertical columnar microstructure due to the normal heat extraction to the substrate, and the sheet lifetime varied from 0.1 microS to 0.3 microS measured by using the microwave photoconductance decay (MW-PCD) to confirm that the spin-cast silicon sheet is applicable to photovoltaics.

Formation of Silicon Sheet on a Rotating Substrate

A spin casting process to fabricate polycrystalline silicon sheets for use as solar cell wafers is presented and the parameters that control the sheet thickness are investigated. The computational model for the spin casting is proposed in order to understand the melt flow and solidification behaviors in the mold. The effect of the rotating speed of the mold and substrate morphology on the silicon sheets is studied via computer simulations, and the simulation results are compared with the experimental results. The numerical study of the fluidity and solidification behavior of the silicon predicted that the formation of rectangular sheets via spin casting is feasible, and the subsequent experiment confirmed this prediction. Using a square mold, rectangular silicon sheets can be produced under appropriate experimental conditions. Microstructural analyses verified the presence of long columnar structures on the sheets.

Residual stresses in polycrystalline silicon sheet and their relation to electron-hole lifetime

This letter summarizes research on the characterization of residual stresses and electron-hole lifetimes of polycrystalline silicon sheet for photovoltaic applications. Full-field polariscopy, scanning room temperature photoluminescence, and surface photovoltage are used to characterize the polycrystalline silicon sheet. An orientation dependent stress-optic coefficient has been developed and used to extract the in-plane residual stresses from analysis of transmitted near-infrared light. The characteristics of the spatial distribution and the quantitative correlation between the residual stresses and the electron-hole lifetime are presented.

Progress on 15‐MW single‐thread silicon‐film™ solar cell manufacturing systems

AbstractAdvances in solar cell manufacturing systems for large‐area Silicon‐Film™ polycrystalline silicon sheet material are reviewed. Recent progress in continuous rapid thermal junction diffusion and cassetteless wet chemical processing is discussed. A single Silicon‐Film™ sheet generation system operating at 3.1 m/min now has a capacity of 15 MW of silicon sheet per year. This sheet generation system is used as the basis for an advanced single‐thread production line design for manufacturing the next generation of Silicon‐Film™ solar cells whose area is nominally 400 cm2. These solar cells are being used to design large‐area solar modules that can produce up to 220 W of power. Copyright © 2002 John Wiley & Sons, Ltd.

Fabrication of poly-crystalline silicon films using plasma spray method

We have developed a new fabrication technique of poly-crystalline silicon (poly-Si) sheet for solar cells, namely the DC-RF hybrid plasma spray method. It has some advantages such as high deposition rate of more than 10 μm/s and large grain size of the obtained poly-Si films. Poly-Si films with a grain size of more than 20 μm and a defect density of 106–107 cm−2 have been obtained at the initial stage trial. The solar cell conversion efficiency of 4.3% has been obtained using the plasma sprayed poly-Si. It is considered that the reasons for the low conversion efficiency are metallic impurity contamination, regions of micro grain due to rapid nucleation, and many defects in the films due to thermal stress.

Spectroscopic study of SiC-like structures formed on polycrystalline silicon sheets during growth

Edge-defined film-fed grown polycrystalline silicon sheets, grown with one face exposed to oxidizing CO gas added to the inert Ar atmosphere, were studied. Interaction of CO with molten silicon surface during growth produced SiC-like structures in a thin layer on the surface exposed to CO. Infrared spectroscopy results suggest that this layer is constituted of good quality SiC; however, Raman and x-ray photoelectron spectroscopy showed that it consists of Si1−xCx in the form of small crystallites mixed with C- and O-rich silicon.

In situ Cr gettering in polycrystalline silicon sheets obtained by edge-defined film-fed growth

Gettering of Cr during the growth of silicon sheets from a Cr-doped melt is observed when the solid/liquid interface region is exposed to CO or CO2 gases. The gettering occurs within a region about 1-μm wide at the surface of the crystal, where a large accumulation of carbon and oxygen is detected. Mechanisms for carbon and oxygen participation in forming gettering sites for Cr are examined.

Homogeneity of carbon microdistribution in edge-defined film-fed grown polycrystalline silicon

Edge-defined film-fed grown polycrystalline silicon sheets are known to contain carbon impurities in a state of extremely high supersaturation. During very rapid growth, carbon is incorporated with an unusually high effective segregation coefficient close to unity; therefore it is of interest to study its microdistribution over the sheet. We performed a scanning IR absorption microanalysis over macroscopic distances in order to evaluate the homogeneity of carbon microdistribution in as-received samples grown in an inert or oxidizing atmosphere, as well as on samples annealed for 72 h in air at different temperatures, up to 1150 °C. As-received samples revealed a very good homogeneity of carbon distribution, which is not even disturbed by the presence of oxygen. However, the homogeneity of carbon microdistribution was found to be significantly disturbed upon annealing. This must be taken into consideration when studying the electrical characteristics of solar cells produced from such material upon various thermal treatments.

Growth of polycrystalline silicon sheet by Hoxan cast ribbon process

Polycrystalline silicon sheet for low-cost solar cells has been obtained by the Hoxan cast ribbon process which has a high growth rate. This process involves a new ribbon growth technique. The silicon sheet is formed in a mold, rather than using the capillary action of silicon in a die, by controlling temperature gradients along all three mold axes. The silicon sheet obtained had a thickness of 0.3 mm and widths of 25 mm or 100 mm. The growth rate was 200–400 mm/min; the average grain size was 2–3 mm and it had a columnar structure. Using these sheets, solar cells with efficiencies as high as 10.9% (2.5 cm×2.5 cm, AM1.5, 100 mW/cm 2) were obtained.

Silicon sheets for solar cells grown from silicon powder by the SSP technique

The purpose of the SSP (silicon sheets from powder) project is to grow large area polycrystalline silicon sheets (100 × 100 mm 2) for solar cells with high conversion efficiency ( > 13%). In the SSP process, a thin layer of silicon powder is converted into a coarse-grained sheet material by two consecutive melting steps using focussed incoherent light as the heat source. The feasibility of the technique was demonstrated by fabricating sheets up to 80 × 150 mm 2 with a thickness of only 350 μm. Recently, a new machine has been built which allows continuous processing of ribbons of 100 mm width. The same sequence of process steps and the same heating technique are used. The sheets are characterized by metallographic and electrical methods. Grain sizes extend to some cm in length and some mm in width, showing a predominance of 〈 211〉 orientation in growth direction. Strong variations in the etch-pit density (10 3−10 6 cm -2) are correlated to variations in EBIC contrast. Test solar cells (20 × 20 mm 2 area) from these sheets demonstrate the quality of the SSP material, with conversion efficiencies of up to 13.1% achieved.

A new spin cast process for mass-production of polycrystalline silicon sheets

A spin cast process for producing polycrystalline silicon sheets for low-cost and high-efficiency solar cells and equipment for its application to mass production are discussed. This process makes it possible to produce high-quality silicon sheets with a production rate of 15 s/sheet. A maximum solar cell efficiency of over 13% is obtained. To reduce production time, a crystal growth zone occurs in the equipment between the spinning zone and the cooling zone. The crystal grows after the spinning process with a physical growth rate of 20 mm/min when mold modules in the crystal growth zone are transferred to the cooling zone.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of oxygen on polycrystalline silicon crystal for solar cells

The influence of oxygen on the characteristics of polycrystalline silicon sheets for solar cells, produced by the spin cast method, has been investigated. It was confirmed that if the oxygen concentration was more than 7.5 X 10 17 atoms/cm 3 after the cell fabrication process, the resistivity of the crystal became higher than before cell fabrication process. To obtain solar cells which have an energy conversion efficiency of more than 10%, the oxygen content should be less than 7.5 X 10 17 atoms/cm 3

Transmission electron microscopy studies in relation to production of solar cells from polycrystalline silicon sheets (R.A.D. process).

Ribbon against drop (R.A.D.) ribbons exhibit a polycrystalline texture similar to that of most melt-grown ribbons; we review here the defects of as-grown materials as well as after the solar cells' production.

Mould shaping silicon crystal growth with a mould coating material by the spinning method

The authors have proposed a mould shaping technique for forming silicon sheets - directly from the silicon melt - the so-called spinning method. Using a combination of Si 3N 4 and SiO 2. for the mould coating material, shaped silicon crystal growth was carried out. Polycrystalline silicon sheets, 10 cm × 10 cm and 0.5 mm thick, have been obtained without adhesion to the mould. The sheets had an oxygen content of 1.7 ppm, carbon content of 0.33 ppm and lower levels of other contamination. The mould was reusable up to 10 times.

Improved polycrystalline silicon sheet resistance by rapid thermal annealing prior and subsequent to ion implantation

Polycrystalline Si films on oxidized Si wafers have been subjected to a rapid thermal processing anneal prior to As ion implantation. After ion implantation the films are given another rapid thermal processing anneal to activate the As. The preimplant anneal causes the as-deposited grain size to increase by ∼ a factor of 10. These films have a 20–30% lower sheet resistance than films that were post-implant annealed only. The increase in grain size by the preimplant anneal reduces the grain boundary area and therefore, minimizes the amount of dopant in the grain boundary relative to the grain.

Releasing Material for the Growth of Shaped Silicon Crystals

A mold coating material for a melt‐shaped crystal growth technique has been developed. After studying the contact angle and reaction between the molten silicon and other various high melting point materials, it was found that for shaped crystal growth with a lower impurity contamination level, a combination coating of and is suitable for a mold coating material. A double layer coating was applied to the graphite mold base. The bottom layer in the mold, which had a 100–150 μm thickness, consisted of that had been formed by nitriding a spray‐coated Si powder. The 200 μm thick top layer consisted of a material. It had been produced by oxidizing a sprayed‐on coating consisting of powder mixed in a silanol liquid. Using the mold prepared with these coatings, a melt‐shaped crystal growth technique was carried out by the spinning‐casting process. Polycrystalline silicon sheets, with a rectangular dimension and 0.5 mm thickness the size of a mold cavity, were obtained without adhesion to the mold. The sheet had an oxygen content of 1.7 ppm and a carbon content of 0.33 ppm. The mold was usable repeatedly up to 10 times.

Residual stresses of thin, short rectangular plates

The analysis of the residual stresses in thin, short rectangular plates is presented. The analysis is used in conjunction with a shadow Moire interferometry technique by which residual stresses are obtained over a large spatial area from a strain measurement. The technique and analysis are applied to a residual stress measurement of polycrystalline silicon sheet grown by the edge-defined film growth technique.

Polycrystalline silicon sheets for solar cells by the improved spinning method

Large-grain polycrystalline silicon sheets, 10 × 10-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 0.1-0.5-mm thick, have been obtained directly by spinning the silicon melt and diffusing it into mold cavities. The sheet has a crystal structure with average grain size of 3.0 mm and contains a few dendrites. Production time from melt is about 4 sheets per 15 s. For solar cell efficiency, this sheet has a greater potential to meet the goal of a low-cost solar cell than other recent unconventional silicon sheet technologies.

Electron channeling and EBIC studies of edge-supported pulling silicon sheets

The dominant grain structure in edge-supported pulling silicon sheets has been studied by electron channeling in a scanning electron microscope. For unseeded polycrystalline silicon sheets in which equilibrium grain structures have been attained, we found that the dominant grain structure is long, narrow grains with surface normals less than twenty degrees off the [011] direction. The plane that is parallel to the growth direction and perpendicular to the surfaces for most of these grains is very close to

Large-grain silicon sheets by the improved spinning method

Large-grain polycrystalline silicon sheets, 50 mm square and 0.1 to 0.5 mm thick, have been obtained directly by spinning the silicon melt and diffusing it into mold cavities. The sheet has a crystal structure with average grain size of 3.0 mm and contains a few dendrites. Production time from the melt is about 4 sheets per minute. For solar cell efficiency, this sheet has a greater potential to meet the goal of a low-cost solar cell than even recent unconventional silicon sheet technologies.