The Stanford Photonics Research Center

March 14, 2009

 
We are developing film crystal silicon technology on inexpensive foreign substrates  for photovoltaics. Silicon is abundant, non-toxic and highly manufacturable, but the wafer presently accounts for about half the photovoltaic module cost.  Our goal is an film silicon alternative with the efficiency of crystal silicon and a cost structure more like amorphous silicon. However, there are many materials science and engineering challenges to be overcome.
 
Epitaxial silicon films grown on high-quality seed layers can have good crystalline quality; thicknesses between 2 and 10 microns with excellent light trapping will be required. This technology requires high-rate, high-quality, silicon epitaxy in the 600 - 750°C temperature range to which low-cost substrates such as borosilicate glass can be heated for hours without significant deformation. We have demonstrated scalable hot-wire chemical vapor deposition (HWCVD) epitaxy growth from silane onto wafers, layer-transferred silicon-on-insulator,  borosilicate glass coated with a thin Al-induced crystalline silicon seed layer, and other seeds. With HWCVD epitaxy at about 700°C, we have grown layers up to 40 microns thick at 200 nm/min on the [100] silicon surface, without observing the epitaxy breakdown that occurs below 500°C. Epitaxial layers were also grown at 300 nm/min and growth modeling suggests that even higher rates can be reached by increasing the hot tungsten wire surface area. Cathodoluminescence reveals dislocation densities below 10^6 cm-2 in epitaxial layers grown at 700°C. Transmission electron microscopy images suggest that most remaining defects nucleate at the wafer surface and might therefore be reduced by improved surface cleaning and pretreatment. We have demonstrated controlled gas-phase doping with both n-type and p-type impurites and measured Hall mobilities near the impurity and phonon-scattering limit. First 2-micron-thick epitaxial solar cells have open-circuit voltage above 500 mV and efficiency above 4% without light trapping.