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Posted: Apr 14, 2011
Natcore Successfully Passivates "Black Silicon" Solar Cells
(Nanowerk News) Scientists at Natcore Technology Inc. have been able to demonstrate the effectiveness of its liquid phase deposition (LPD) process in passivating the surface of "black silicon" solar cells.
"Black silicon" refers to the apparent color of the surface of a silicon wafer after it has been etched with nano-scale pores. The etching takes place in a matter of a few minutes in a liquid solution at room temperature; the black color is not a color at all but results from the absence of reflected light from the porous wafer surface. Black silicon solar cells have been under intensive study for years because of their potential for significantly improved performance compared to standard production cells now available.
The reflectivity of a polished silicon wafer surface approaches 40%, giving the wafer its shiny appearance. Adding the typical solar cell industry antireflective coating reduces the average reflectivity to approximately 6% and gives the cells their distinctive dark blue color. The black silicon process, however, has been shown by Natcore scientists, the Barron Group (working with Natcore funding) at Rice University, and researchers at the National Renewable Energy Laboratory (NREL) to reduce the average reflectivity to less than 1.5%.
A key impediment to turning a solar cell's increased light absorption into increased power output, however, is a significantly increased area of exposed silicon on the sidewalls of the pores and on the small mesas that remain at the top surface of the wafer itself. This increased area must be passivated, or treated to keep it from trapping the light-generated electric charges as they migrate toward the contacts of the solar cell, a process that robs the cell of output power.
Natcore-sponsored research, conducted by Professor Andrew Barron's group in combination with research by Natcore scientists working at the Ohio State University, resulted in a 20-fold increase in a parameter called minority carrier lifetime for the LPD-coated black silicon, compared to the uncoated black silicon surface. According to research by the National Renewable Energy Laboratory (NREL), such a result would enable the power gains promised by using black silicon for solar cell manufacturing to actually be realized.
Passivation is the process of filling the dangling atomic bonds at the surface of the solar cell, as well as reducing the numbers of defects that always exist in the upper region of the cell body. It is critical to enabling production of long-term, high-performance silicon solar cells. In September 2010, Natcore scientists announced their success in passivating standard commercial silicon solar cells on which a silica film had been grown using Natcore's liquid phase deposition (LPD) process.
"It is likely that a thermal oxidation process could also accomplish the passivation of black silicon," says Natcore President and CEO Chuck Provini. "But that would be costly, and it's not practical for implementation in a high-volume solar cell fabrication line. The ability of our LPD process to passivate black silicon eliminates the need for thermal oxidation. It also enables use of an all-liquid phase process for creating ultralow reflectivity, high-performance silicon solar cells at high volume production rates. That translates to lower cost than is possible with current solar cell manufacturing technology."
Because of the great potential offered by black silicon solar cells, Natcore is currently in discussions with the NREL as well as industry groups to refine and accelerate Natcore's advances in this area.