Polysilicon Production Process and Quasi-Mono Technology

In the area of polysilicon production improvement comes in a form of the cheaper processing and new material offering

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This is a second installment of solar industry technology review.

In difficult times, technological innovation is going to separate winners from losers. Based on article below I am reviewing each technology and new changes arrving to solar manufacturing.

https://solarpvinvestor.com/news/spvi-news/83-investors-should-consider-technology-progress-in-solar-industry-not-the-political-propaganda-and-lacklustre-market-treatment-

In the first article, I have explained manufacturing process of solar cells, listing new structural designs leading to higher conversion.

https://solarpvinvestor.com/news/spvi-news/88-cell-manufacturing-process-and-future-cell-technologies

Poly chunks

In the area of polysilicon production improvement comes in a form of the cheaper processing and new material offering. Currently, there are three processes, which are used in production of polysilicon. Siemens process, FBR process and direct purification of MGSi.  There are modifications to those processes, and as with cells, the innovation is being applied to improve the cost as well as the purity of the material. The polysilicon purity is measured in the number of nines after 99%. The purity is essential for manufacture of monocrystalline silicon, which is used to produce higher conversion wafers and subsequently cells. N10 and in some cases N13 purity is being produced for solar needs today.

All three processes start with production of metallurgical grade silicon (MGS) from quartzite gravel, which is a pure form of sand. Mixed with carbon in coke reduction arc furnace, silica is melted and reduced in 1,800^oC resulting in liquid MGSi.  The next step is the production Trichlorosilane (TCS or SiHCl3) out of MGSi with application of other chemical agents.

Two processes are used for this:

1. Hydrochlorination (direct chlorination)

The reaction for TCS synthesis occurs between metallurgical grade silicon (MGSi) and hydrogen chloride (HCl).

MG silicon, in the form of particles, is fluidized in a fluidized bed reactor (FBR) and is reacted with anhydrous HCl.  High efficiency production of TCS for solar grade polysilicon applications has been constrained by the high heat of reaction between MGSi and anhydrous HCl (1200^oC).  Chinese and solar industry called this process hydrogenation just to add confusion to already complex process.

2. Trichlorosilane Production Using Cold Conversion

The process for TCS production involves the chemical reaction of metallurgical grade silicon (MGSi) with STC and Hydrogen.  This process in chemical expression has been referred to as hydrogenation.  In order to keep error above from becoming even a greater confusion this is called hydrochlorination or some call it cold hydrogenation, because STC is converted to TCS at a lower temperature than in the thermal conversion process above. Advantage here is the use of STC as the chlorine source for the production of TCS. STC is produced in large quantities as a co-product in TCS production, and as a co-product in polysilicon deposition (in CVDR). STC produced in this reaction is collected and recycled back to the Cold Conversion Reactor to make more TCS. Unreacted hydrogen from the process is collected and recycled back to the Cold Conversion Reactor as well. 

Both hydrogenation and hydrochlorination are performed in a fluidized bed reactor (FBR). Note  this is not the same reactor as REC’ deposition reactor producing granulates of polysilicon in reaction of silane with hydrogen.  

This technology (#2) is one which Chinese companies Renesola and Daqo are transitioning into currently. GCL uses this method right now. Direct chlorination process is used by LDK and Renesola at their existing plants.

TCS decomposition

Siemens Process

In chemical vapor deposition reactor (CVDR) purified TCS vapor decomposes to produce polysilicon by deposition on heated rods.  The unreacted TCS, along with STC are vaporized and recycled back. Polysilicon rods grow in diameter in the multi-day cycle. When grown to expected size polysilicon rods are cooled, harvested and put through hammer press to be crushed to chunks. New batch starts with slim-rods containing pure silicon and TCS vapor is introduced again.

FBR Process

Purified silane is mixed with hydrogen and fed to a fluidized bed reactor (FBR) where the silane decomposes on the surface of polysilicon granules to eventually produce a 1-5 mm diameter spherical product.  Since the FBR is continuous, there is no need of batching.  In addition to continuing process, there is no need to waste energy to cool the walls as in CVDR. There is more of product deposited than on rods. There are logistical benefits in transport as well improvement in maximizing crucible load weight (in contrast to chunks).  Granules are also considered being better for Czochralski pullers to produce mono ingots.  REC is one company using this process in large scale.

Direct purification of metallurgical silicon

Direct purification takes the MGSi to refining furnace when it is melted to remove impurities. Refined or upgraded (UMG) silicon is moved to DSS furnaces to create ingots of UMG silicon. The understating of the processes taking place in the refining furnace are highly guarded by each company.  Recent pricing pressure on polysilicon due to new capacity and large inventory of modules, has resulted in rapid deterioration of spot pricing for polysilicon, including high purity product. UMG once seen as the viable option to polysilicon, in this pricing environment and at best solar grade purity of N7, is not seen as strong alternative.

Mono and multicrystalline ingots

The production of ingots starts by placing solar grade silicon into crucibles, which are put in special crystallization furnaces where the silicon is molten.
Multicrystalline crystallization starts by gradually cooling the crucibles from the bottom (directional solidification system) or using a Cz-puller, introducing a seed crystal with defined crystal orientation into the bath of molten silicon and extracting it by pulling and rotation to create monocrystalline silicon.

Steps as per SilFab

Monocrystalline ingots:

1. Raw material (typically 130-150 kg/ingot) is placed in a crucible where is added a doping substance (usually boron or phosphorus for n type silicon)

2. Evacuation of the crystal growing chamber

3. Melting of the silicon in inert atmosphere (argon)

4. Dipping of a thin crystal seed into the molten silicon material

5. While the seed and the crucible rotate in opposite directions, the seed is slowly extracted.

6. In this way, a cylindrical monocrystalline ingot is formed with the desired  crystallographic orientation and diameter.

Polycrystalline ingots:

1. Raw material (typically 400-600 kg/ingot) is placed in a crucible to which is added a doping substance (usually boron)

2. Evacuation of the crystal growing chamber

3. Melting of the silicon in inert atmosphere (argon)

4. Directional solidification of the molten silicon is obtained by controlled cooling of the crucible, which leads to formation of the multicrystalline silicon ingot.

Quasi Mono

Quasi Mono technology allows casting of the monocrystalline ingot in the DDS furnace by placing a mono seed at the bottom of the crucible.  Technology was originally developed by Ja Solar in cooperation with JingGong Science and technology.  There is a number of corporations claiming to have DSS quasi mono furnaces through cooperation with JingGong (GCL, SOL) and some as a result of their own development.  Quasi mono offers conversion close to low range of monocrystalline wafers, with cost of the multicrystalline production. In comparison to to quasi-square shape of mono wafer, quasi mono wafer is a full square. Some companies have also showcased full-square mono wafer, without cut corners (ie. Trina QuadMax). Those wafers are obtained from larger ingots still made in the Cz-puller. In those cases conversion is greatly improved, however it comes at high waste of material as a outcome of squaring process, prior to wafering.

Major companies announcing or showing quasi mono technology in their products

Furnace makers: ACME , Jing Gong, GT Technologies, Centrotherm

Users: GCL, Renesola, GET, Solartech, Trina, Jinko, Phoenix, Canadian, Tainergy, Ja Solar.

Yingli Solar has purchased furnances from JingGong this year and it is expected to have quasi mono technology in its product section.

 

Next article will assess process of wafering, load size, diamond saw, reduction of thickness and kerf, n-type wafers.