High Efficiency Graphene Solar Cells by Chemical Doping
  We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide, exceeds the native(undoped) device performance by a factor of 4.5 and the best previously reported PCE in similar devices by a factor of nearly 6. Current-voltage, capacitance-voltage and external quantum efficiency measurements show the enhancement to be due to the doping induced shift in the graphene chemical potential which increases the graphene carrier density (decreasing the cell series resistance) and increases the built-in potential. 
Xiaochang Miao, Sefaattin Tongay, Maureen K. Petterson, Kara Berke,   Andrew G. Rinzler, Bill R. Appleton and Arthur F. Hebard
cond-mat.mtrl-sci