Thin silicon string ribbon

Evergreen Solar is a new solar cell company that is commercializing a silicon sheet growth method known as String Ribbon, a method for forming continuous polycrystalline silicon ribbon directly from the melt. Recently, modifications to the growth environment by means of a tunable afterheater have enabled Evergreen to grow flat, 5.6 cm wide and 100 mu m thick ribbon with low enough stress to permit solar cell fabrication. The development work was aided by using finite element analysis with a thermoelastic model. Initial work on forming cells on this material was quite promising. The best solar cell made on the 100 mu m thick, p-type silicon ribbon (rho = 0.1 Ohm cm) was 14.7% efficient, It had a V-oc of 639 mV and a fill factor of 0.798 with a J(sc) of 28.8 mA/cm(2). Significant increases in lifetime during processing were not unusual. With better starting lifetimes from higher bulk resistivity values, PCID modeling indicated that 16% efficient cells should be readily achievable on this material.