Belt Furnace Gettering and Passivation of n-Web Silicon for High-Efficiency Screen-Printed Front-Surface-Field Solar Cells

摘要

Six different resistivities (0.32. 0.57, 1.2, 2.2, 9.1 and 20 #OMEGA# cm) were investigated to understand the dopant-defect interaction in n-type, antimony-doped, dendritic web silicon ribbon, and to study its response to gettering and passivation during belt furnace processing (BFP). The as-grown lifetime was found to be a strong function of resistivity with higher resistivity displaying higher lifetime. Phosphorus gettering at 925 deg C/6 min raised the as-grown lifetime of approx 1 #mu#s in 20 #OMEGA# cm n-web to 5.4 #mu#s. A combination of phosphorus gettering followed by simultaneous Al gettering and SiN hydrogenation raised the 20 #OMEGA# cm n-web lifetime to 78 #mu#s. unlike the as-grown web, the processed lifetime was greater than 75 #mu#s for all resistivities, with no clear doping dependence. This is attributed to the very effective gettering and passivation during the belt furnace processing. Front surface field (FSF) n~+-n-p~+ cells were fabricated by spin-on phosphorus diffusion on the front and screen-printed Al on the back. A lifetime value of over 100 #mu#s was obtained in a 14.2 percent screen-printed FSF n-web solar cell fabricated on 100-#mu#m-thick 20 #OMEGA# cm substrate. The screen-printed FSF cell fabricated on (111) FZ gave an efficiency of 14.9 percent with a fill factor of 77.6 percent. These results are supported by model calculations, which revealed a maximum efficiency of approx 15 percent for 100-#mu#m-thick planar screen-printed FSF cells and their insensitivity to bulk lifetime above 60 #mu#s.