by Paula Doe, Contributing Editor
Japan’s burgeoning solar market is attracting new players with new technologies, and that’s helping to bring down costs to spur further growth. The two big new players about to muscle in to the solar cell business are both banking on innovations in equipment and materials for their success.
“We’re aggressively adopting new equipment and materials because our style is to do what no one else does,” Yasuhiro Suzuki, who’s in charge of Honda Soltec’s solar cell development, told SST‘s partner Nikkei Microdevices. Motonobu Kawai reports that Honda will use coating tools adopted from the automotive industry — not a vacuum process — for the CIGS thin film cells it will start producing later this year at a new 27.5MW plant. Fuji Electric officials also say they’re aggressively adopting new materials to distinguish the amorphous silicon cells on flexible film they’re now sampling.
Suppliers from a range of other fields are stepping up with offerings to the more mainstream crystalline silicon solar cell makers: new types of lower cost silicon, more efficient wire saws, and better encapsulants to drive down the price of solar energy. JFE Steel provides polysilicon made by a metallurgical process that drives off the impurities in the silicon (first with an electron beam in a vacuum, then with a plasma torch in an argon atmosphere) and refines and cools the melt in a continuous process similar to steelmaking. These metallurgical polysilicon furnaces can be brought online in small increments relatively quickly and cheaply, though they do turn out product of only “six nines” (99.9999%) purity — less than the eleven nines of the conventional Siemens process, but apparently sufficient for at least current-generation solar cells.
Chisso Corp. is reviving an improved version of its old zinc reduction process for making polysilicon of “seven nines” (99.99999%) purity, which costs slightly more than the metallurgical process, but still costs 30%-40% less than the Siemens method. The chemical maker has formed a venture with Nippon Mining Holdings and Toho Titanium to commercialize the process, now in pilot production and yet to be demonstrated in high volume. Chisso’s process reacts the molten Si with Cl2 to make SiCl4, and then reduces the SiCl4 with Zn vapor to get polysilicon, with ZnCl2 as the byproduct. This ZnCl2 is then recycled back into Cl and Zn to feed back into the reaction, using Toho Titanium’s molten salt electrolysis technology.
Other suppliers are finding ways to reduce the amount of silicon wasted by sawing the silicon ingots or blocks into wafers with the typical 0.14-0.16mm wire. Yasunaga Wire Saw Systems, which supplies saws using ultrathin 0.07mm wire and tight uniformity control to the compound semiconductor sector, saw that the solar customers buying its slurry reclamation systems (accounting for some 80% of the company’s systems for recycling polishing slurry) needed better saws as well. So it introduced a commercial solar silicon wire saw in April that uses 0.12mm wire to cut two 125mm x 125mm x 280mm ingots at the same time, reportedly yielding 6.8% more useable silicon than 0.14mm wire, increasing output from 1436 to 1534 wafers.
Mitsubishi Electric, a major supplier of wire electrodischarge machining systems for precision cutting, is now applying that technology to the solar industry as well. The technology uses the heat from a pulse discharge between the wire and the substrate to melt and cut the substrate precisely, without contact. Though wire electrodischarge does precision cutting well, and eliminates the need for abrasive slurry, it has typically only worked for cutting metals where it’s easy to get the pulse discharge. Mitsubishi gets the process to work with silicon as well, by using a high frequency, single-pole source. The company reports that tests show exposing the wafers to the additional heat from this cutting process has no impact on conversion efficiency of the finished solar cells.
Meanwhile, Dainippon Printing is applying its technology for food packaging plastics to solar cell encapsulants and backsheets. The company says its new thermoplastic olefin film is a significantly better water-vapor barrier than other heat-processed films, without the need for the usual curing at 150 degrees C for 30 min., and at no additional cost. DNP reports that major solar-cell makers in Japan and abroad have qualified the material, and it expects wide adoption after it ramps production during the middle of this year. — P.D.