Researchers from Peking University discovered that graphene-silicon solar cells can be made more efficient by coating it with an anti-reflective layer (they used Titanium Dioxide).
The researchers say that normal silicon solar cells have an efficiency of about 15%. Adding graphene (as an electrode and also as the charge-carrying layer) can make the cells cheaper, but these are less efficient (at 8.6%). Coating the graphene-silicon structure with a 65-nm-thick layer of titanium dioxide decreases the reflected light from over 30% to less than 10%, which means that more light is converted to electricity - and the efficiency of these new cells is 14.6%.
Currently the cells that the researchers are using are only a few square millimeters in area. Larger graphene films are required before this technology can be used to make real PV cells. In addition, the cells are not stable - after 20 days the efficiency starts to decrease.
Interestingly, titanium dioxide, used in dye-sensitized solar cells, also gets an efficiency boost from adding graphene. In April 2012 Michigan Technological University researchers discovered that the addition of graphene augmented the conductivity of titanium dioxide, thus increasing the electricity production in a dye-sensitized solar cells by 52.4%.