String Ribbon - Technology Description

Technology Description

String ribbon is a method of producing multi-crystalline silicon strips suitable for the photovoltaic industry.

The name describes the manufacturing process, where high temperature resistant wires are pulled through molten silicon to form a multi-crystalline ribbon of silicon crystals. The ribbon is then cut into lengths which are treated with traditional processes to form solar cells. The process was developed in the 1970s by Mobil-Tyco, Solar Energy Corp., Energy Materials, Corp., Motorola and IBM. Ribbons 4-5 inches wide and less than 1/100th of an inch thick were made. It is similar to the dendrite process, developed by Westinghouse in the 1970s, which had the further advantage of having no ribbon (graphite or ceramic die), using only two special seed crystals, or "dendrites," which were dipped side by side into molten silicon and pulled up slowly. These demonstrated efficiencies upwards of 16% by 1980.

String Ribbon has the capability of using less silicon compared to other wafer production methods as wafers are manufactured to the approximately correct specification avoiding the need for 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 approximate 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. Wafer technologies have reached as high as 25% in laboratory conditions.

While String Ribbon technology has certain advantages as to the shape of the crystals, the overall thickness varies enough so that not every 'silicon strip' can be processed directly into a solar cell. In addition to this drawback, the growth process is thermally very inefficient. The radiating area/gram of crystal is extremely high, leading to very high energy expenses which offset the reduced silicon use/expense.