What do the impurity data of GCL’s granular polysilicon mean?

GCL-Poly Energy is drumming up its granular polysilicon business. On March 9 the company issued the third update on the 10,000-ton fluidized bed reactor (FBR) facility of its polysilicon subsidiary Jiangsu Zhongneng within three months.

The series of process efficiency and product quality data published has to be taken with a grain of salt as GCL’s announcements are known for exaggerations. For instance, the company claimed a monocrystalline yield of 99% for its cast-mono silicon ingots in 2019, which was met with widespread skepticism in the industry.

GCL says that 90% of the granular polysilicon from Jiangsu Zhongneng can be used for monocrystalline materials. We assume that the published impurity data only refer to this portion of the output.

As the chart above shows, the metal and carbon concentrations of GCL’s product are comparable to those on the latest data sheets of TianREC, the Chinese FBR joint venture between U.S.-based polysilicon manufacturer REC Silicon and Shaanxi Non-Ferrous Tianhong New Energy in Yulin, Shaanxi province.

The low concentration of metals is due both to the high purity of monosilane injected into the FBR, compared to trichlorosilane used in the conventional Siemens process, and to a liner that prevents the reactor wall from contaminating the polysilicon granules produced inside the reactor.

Relatively high carbon concentration, but very low hydrogen content

Compared to the best Category I of the PV17 standard on virgin polysilicon for PV applications from Semiconductor Equipment and Materials International (SEMI), however, the granular materials from both GCL and TianREC exhibit higher carbon concentrations. Those originate from the liner coating usually made of silicon carbide.

Although GCL acquired the FBR technology from bankrupt Sunedison in 2017, the granular polysilicon from Jiangsu Zhongneng does not reach the low carbon values that Sunedison’s FBR subsidiary MEMC Pasadena achieved (see chart above) until it shut down operations in late 2015.

In contrast, the hydrogen content of GCL’s material appears very low. If the company’s update does not confuse parts per million by weight (ppm_w) with parts per million atoms (ppm_a), then the low hydrogen concentration of 20 ppm_a seems to correlate to GCL’s boldest claim: a fine powder ratio of “below 0.1%”.

In a typical FBR, 10% to 15% of the injected monosilane decomposes to fine silicon powder (dust). Only the vibrational research and development reactor of Centrotherm Sitec (now insolvent) is said to have reached dust rates of 0.1%. If GCL should actually have achieved the same value, this would be a technological sensation. We have to wait for customers that can – at least indirectly – confirm GCL’s bold claim.