High-efficiency solar cells using HEM silicon

Developments in Heat Exchanger Method (HEM) technology for production of multicrystalline silicon ingot production have led to growth of larger ingots (55 cm square cross section) with lower costs and reliability in production. A single reusable crucible has been used to produce 18 multicrystalline 33 cm square cross section 40 kg ingots, and capability to produce 44 cm ingots has been demonstrated. Large area solar cells of 16.3% (42 cm{sup 2}) and 15.3% (100 cm{sup 2}) efficiency have been produced without optimization of the material production and the solar cell processing

Slicing of silicon ingots with reduced kerf. Final report (Phase II)

The Fixed Abrasive Slicing Technique (FAST) has been developed as an effective wafering technique by combining the advantages of diamond as an abrasive like internal diamond (ID), a reciprocating bladehead as in Multi-Blade Slurry (MBS) and wire slicing as in multiwire slicing (MWS) techniques. During the development stage, it was necessary to develop a slicer, bladepacks and the technique. It was recognized that high pressures were required between the diamond abrasive and the crystal to achieve effective slicing. This was achieved by rocking the workpiece to minimize the contact length. During the present work, the cutting effectiveness was enhanced by increasing the rotary speed of the crystal in the range of 2,500 to 5,000 rpm and the reciprocating rate of the bladehead to about 100 cycles/minute. The combination of high speed rotation of the crystal and FAST slicing have produced more effective slicing by increasing the cutting rate by a factor of 50 to produce multiple wafers wit h low kerf, low surface damage and high accuracy. In addition, this technique is now applicable for soft as well as very hard crystals. Based on these developments, Crystal Systems is currently commercializing FAST for slicing of a wide variety of hard and soft crystals

Heat exchanger-ingot casting/slicing process. Silicon Sheet Growth Development for the Large Area Silicon Sheet Task of the Low Cost Silicon Solar Array Project. Eighth quarterly progress report, July 1, 1977--September 30, 1977

Graded crucibles have been developed which are dense enough to avoid penetration of the molten silicon and weak enough to fracture during the cool-down cycle. These crucibles have been used to cast crack-free silicon ingots up to 3.3 kg. Significant progress has been made in the crystallinity of the samples cast. Solar cells made from one of the ingots have yielded over 9% conversion efficiency. The source of silicon carbide in the cast silicon has been identified, both theoretically and experimentally, to be associated with the use of graphite retainers in contact with the crucible. Both 45 ..mu..m and 30 ..mu..m diamonds can be used for efficient slicing of silicon. Wafers sliced with 45 ..mu..m diamond plated wire show a surface roughness of +-0.5 ..mu..m and extent of damage of 3 ..mu..m. In an effort to avoid diamond pullout from impregnated wire it was found that a layer of 0.3 mil thick plating is sufficient to encapsulate the diamonds. A projected cost analysis has shown that the add-on cost of casting and slicing of silicon is $11.57 per square meter

Heat exchanger method: ingot casting; fixed abrasive method: multi-wire slicing (Phase II). Silicon sheet growth development for the Large Area Silicon Sheet Task of the Low Cost Silicon Solar Array Project. Quarterly progress report No. 1, November 21, 1977--December 31, 1977

A high degree of crystallinity has been achieved in ingots cast. Since most of the growth took place near the solidification temperature, the top portion of the ingot was solidified by freezing from the surface. The thickness of this layer was reduced with the control of thermal flow characteristics in the furnace. The crucibles used in this study have a nonuniform bottom which is not conducive to proper heat transfer during solidification. In an effort to achieve high packing density of solar cells in the module with maximum material utilization, an attempt was made to cast a square cross-section ingot. Even though some minor cracking occurred in the first ingot, it appears feasible to cast square cross-section ingots by the Heat Exchanger Method. Higher feed forces results in higher cutting rates. However, this is accompanied by wire wander and increased surface damage depth. It has been established that the life of an impregnated blade can be prolonged by plating it after impregnation

Silicon ingot casting: heat exchanger method. Multi-wire slicing: fixed abrasive slicing technique. Phase III. Quarterly progress report No. 4, July 1-September 30, 1979

This contract is for casting silicon ingots by the Heat Exchanger Method (HEM) and slicing by multi-wire Fixed Abrasive Slicing Technique (FAST). Significant advancements have been made in the area of crystal casting. It has been demonstrated that nearly single crystal ingots can be cast with a single HEM solidification of upgraded metallurgical grade silicon. The impurities were rejected to the last material to freeze-near the wall of the crucible. The resistivity of the silicon after directional solidification by HEM was 0.1 to 0.2 ..cap omega..-cm. Macroscopic impurities, presumably SiC, did not break down the solid-liquid interface and, in some cases, caused only localised twin formation. This material may be used for making solar cells directly. The HEM process has been scaled up to solidify 22 cm x 22 cm square cross-section ingots weighing up to 10.5 kg. The 22 cm square cross-section ingot is nearly all single crystal material. For silicon slicing using FAST significant progress has been made in demonstrating high throughput of the slicer and extended life of the wires. Cutting rates exceeded 1986 goals by more than 40%. This has been achieved with the combination of high speeds of the slicer and improvement in the blade. Emphasis in the area of blade development has been with impregnation using CSI technology of impregnating diamonds only in the cutting edge

Production of solar grade (SoG) silicon by refining liquid metallurgical grade (MG) silicon: Annual Report: June 10 1998--October 19, 1999

Pyro-metallurgical refining techniques are being developed for use with molten metallurgical-grade (MG) silicon so that directionally solidified refined MG silicon can be used as solar-grade (SoG) silicon feedstock for photovoltaic applications. The most problematic impurity elements are B and P because of their high segregation coefficients. Refining processes such as evacuation, formation of impurity complexes, oxidation of impurities, and slagging have been effective in removal of impurities from MG silicon. Charge sizes have been scaled up to 60 kg. Impurity analysis of 60-kg charges after refining and directional solidification has shown reduction of most impurities to <1 ppma and B and P to the 10-ppma level. It has been demonstrated that B and P, as well as other impurities, can be reduced from MG silicon. Further reduction of impurities will be necessary for use as SoG silicon. The procedures developed are simple and scaleable to larger charge sizes and carried out in a foundry or MG silicon production plant. Therefore, SoG silicon production using these procedures should be at low cost

Silicon ingot casting: Heat Exchanger Method (HEM)/multi-wire slicing: Fixed Abrasive Slicing Technique (FAST), Phase IV. Quarterly progress report No. 2, April 1, 1980-June 30, 1980

Silicon ingot size cast by HEM has been extended to 34 cm x 34 cm x 10 cm. A 20 kg ingot has been solidified at 3 kg/hr with no crucible attachment or ingot cracking problems. Another ingot of 26 kg weight has also been solidified. The heat treatment used to develop a graded structure caused cracking on the inside surface of the first large crucibles. The thermal conditions were altered to minimize high gradients and the cracking was eliminated. A high degree of single crystallinity has been maintained as the size of the ingots has been increased. A graphite retainer made out of flat plates was used to produce an ingot with flat sides and rounded curves. It is now possible to electroplate diamonds only on the cutting edge of the wire. The advantages associated with diamonds on the cutting edge only are lower kerf, improved accuracy by improved seating in the support rollers, and less degradation of the rollers. This has resulted in less wander of wires and will reduce costs by using less diamonds and less degradation of rollers. The main failure mechanism of wires - diamond pullout - has been minimized by using filler diamonds to prevent erosion of the nickel matrix. It has been shown that an electroplated wirepack can be used to slice three 10 cm diameter silicon ingots without significant diamond pullout. IPEG analysis of value added costs of sheet formation using conservative and optimistic extension of HEM and FAST technologies yields $27.05/m/sup 2/ ($0.191/w) and $13.49/m/sup 2/ (0.095/w), respectively. Assuming cost goals of other tasks are met, the projected costs are$0.654/w, conservatively, and $0.539/w, optimistically, for photovoltaic modules