The National Renewable Energy Laboratory (NREL) has engineered silicon solar cells using a new nanotechnology technique for an efficiency of 18.2% by cutting light reflection of the silicon solar cell surface.
The “best” and most efficient solar cells are still built from silicon. But the costs have just crushed the buying interest even in the face of a price war stringing out bankruptcies and government bailouts. The government incentives are winding up and the future looks to keep only the fittest businesses in position to answer demand.
The NREL breakthrough should be a major step toward helping lower the cost of solar energy.
The breakthrough is a nanostructured surface ensuring that the light-generated electricity can still be collected efficiently from the solar cell. Typically, solar cell manufacturers must add an extra anti-reflection layer, or two, to their cells, which boosts costs significantly.
The NREL team made nano-islands of silver on a silicon wafer and immersed it briefly in liquids to make billions of nano-sized holes in each square-inch of the silicon wafer surface. The holes and silicon walls are smaller than the light wavelengths hitting them, so the light doesn’t recognize any sudden change in density at the surface and, thus, doesn’t reflect back into the atmosphere as wasted energy. The researchers controlled the nanoshapes and the chemical composition of the surface to reach record solar cell efficiencies for this new ‘black silicon’ material.
The work builds on a previously demonstrated nanostructures that reflected less light than the best anti-reflection layers of a solar cell. But until now, they hadn’t been able to achieve overall efficiency with their black silicon cells that could approach the best scores for other silicon cells.
Hao-Chih Yuan, and Howard Branz first had to determine why the increased surface area of the nanostructures dramatically reduced the collection of electricity and hurt the voltage and current of the cells.
Their experiments demonstrated that the high-surface area, and especially a process called Auger recombination, limit the collection of photons on most nanostructured solar cells. They concluded that this Auger recombination is caused when too many of the dopant impurities put in to make the cell work come through the nanostructured surface.
Having worked that out the new understanding enabled the team to suppress Auger recombination with lighter and shallower doping. Combining this lighter doping with slightly smoother nanoshapes, they can build an 18.2%-efficient solar cell that is black but responds nearly ideally to almost the entire solar spectrum.
Branz, the principal investigator, discusses the implications, “This work can have a big impact on both conventional and emerging solar cell based on nanowires and nanospheres. For the first time it shows that really great solar cells can be made from nanostructured semiconductors. The next challenges are to translate these results to common industrial practice and then get the efficiency over 20%. After that, I hope to see these kinds of nanostructuring techniques used on far thinner cells to use less semiconductor material.”
Yuan said, “Now we have a clear study that shows how optimizing the surface area and the doping together can give better efficiency. The surface area and the doping concentration near the surface affect nano-structured solar-cell performance.”
First author, Jihun Oh, an NREL Postdoctoral Fellow said the NREL study “clearly shows that the right combination of a carefully nano-structured surface and good processing can reduce the cost while cutting unwanted reflection of sunlight.”
Photovoltaic solar needs every break and breakthrough. The new technology is adding up, better solar cells and much less costly inverters will help. But ultimately solar will have to carve out its market on the merits vs. the cost. So far the merits are compelling, but the costs take the balance away, by far.