Researchers from the University of Oslo have used a bunch of “wonderful tricks” to produce silicon solar cells that are twenty times thinner than commercial solar cells. This breakthrough means that solar cells can be produced using 95% less silicon, reducing production costs considerably — both increasing profits (which are almost nonexistent at the moment), and reducing the cost of solar power installations.
Standard, commercial photovoltaic solar cells are fashioned out of 200-micrometer-thick (0.2mm) wafers of silicon, which are sliced from a large block of silicon. This equates to around five grams of silicon per watt of solar power, and also a lot of wastage — roughly half of the silicon block is turned into sawdust by the slicing process. With solar cells approaching 50 cents per watt (down from a few dollars per watt a few years ago), something needs to change.
Reducing the thickness of solar cells obviously makes a lot of sense from a commercial point of view, but it introduces another issue: As the wafer gets thinner, more light passes straight through the silicon, dramatically reducing the amount of electricity produced by the photovoltaic effect. This is due to wavelengths: Blue light, which has a short wavelength (450nm), can be captured by a very thin wafer of silicon — but red light, with a longer wavelength (750nm), can only be captured by thicker slabs of silicon. This is part of the reason that current solar cells use silicon wafers that are around 200 micrometers — and also why they’re mirrored, which doubles the effective thickness, allowing them to capture more of the visible spectrum.
In essence, the University of Oslo researchers have devised methods for trapping those longer wavelengths, even when the silicon wafers are just 10 micrometers thick. The first trick is using microbeads — very small plastic spheres, uniform in size, that create an almost perfect periodic pattern on the silicon. These beads force the sunlight to “move sideways,” increasing the apparent thickness of the silicon by 25 times.
The University of Oslo is also experimenting with asymmetric microindentations on the back of the silicon wafer. “Cylinders, cones and hemispheres are symmetrical shapes. We have proposed a number of structures that break the symmetry. Our calculations show that asymmetrical microindentations can trap even more of the sunlight”, says Erik Marstein, one of the researchers involved with the work.
The researchers are in talks with industrial partners to investigate whether these methods can be scaled up to industrial production. The researchers seem quite confident that their technology could come to market within five to seven years.
Research paper: dx.doi.org/10.4229/27thEUPVSEC2012-2CV.7.22 – “Light management in thin crystalline silicon solar cells”