String ribbon

String Ribbon solar cells is proprietary technology developed by Evergreen Solar.

Technology description

String Ribbon describes a method of producing high grade silicon wafers suitable for the photovoltaics industry. The name describes the manufacturing process, where strings are pulled through molten silicon and pressed to form a ribbon of silicon crystals. The ribbon is then cut into lengths which are treated to traditional processes to form solar cells.

String Ribbon is advantageous to other wafer production methods as wafers are manufactured to the correct specification avoiding the need for wire sawing of silicon blocks. Silicon accounts for more than 50% of manufacturing costs in producing first generation solar cells, where much of the silicon is discarded as waste at the sawing stage of manufacture. Employing the string ribbon process allows the manufacture of PV grade silicon wafers to the appropriate dimensions while avoiding the waste encountered when sawing wafers from ingots. This manufacturing process uses about half the amount of input silicon required by traditional processes.

String Ribbon technology offers a less expensive manufacturing technique but is not capable of achieving the same electrical performance as wafer technology. Typically a cut wafer will convert 15-16% of the incoming light into electricity where String Ribbon Solar Cells are capable of converting 13-14%. In research laboratories the technology has reached as high as 18.3%, however it cannot be produced commercially to this specification [Fabrication and analysis of high-efficiency String Ribbon Si solar cells. K. Nakayashiki, B. Rounsaville, V. Yelundur, D.S. Kim, A. Rohatgi, R. Clark-Phelps, and J.I. Hanoka, Solid-State Electron. 50, 1406 (2006)] . Wafer technologies have reached as high as 25% in laboratory conditions.

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