Graphene
Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick, having a number of unique electronic and mechanical properties. This is because of the fact that electrons whiz through graphene at extremely high speeds, behaving like "Dirac" particles with little resistance. Graphene is also transparent to light due to its Dirac electrons and can absorb light of any color.
Solar cells
Researchers so far have made solar cells from graphene but the power conversion efficiency is quite low at around 1.9%. But researchers at the University of Florida in Gainesville have succeeded in fabricating the most efficient graphene-based solar cells ever by adding an organic dopant to the graphene layer in the devices. The power conversion efficiency of the new solar cells reaches nearly 9%, compared to just fewer than 2% for cells that use undoped graphene.
Structure
These cells are made of graphene sheet doped with the organic compound (trifluoromethanesulphonyl) amide, or TFSA, placed atop a silicon wafer to make a graphene/silicon Schottky junction. Transferring graphene onto silicon causes minimal disturbance at the graphene surface and hence the interface remains pristine. A clean interface is important because any disorder in this area acts as a trap for separated charges, so reducing their lifetime, which means that they cannot be collected as efficiently.
Working
Such photovoltaic devices work by producing electron-holes pairs when exposed to sunlight. The electrons and holes are then separated by the Schotky interface and collected by electrodes contacted onto the oppositely charged graphene and silicon. The current produced by the flowing electrons and holes allows the device to generate power.
Doping graphene with TFSA changes the Fermi level of the graphene which has the effect of readjusting the charges at the graphene/silicon junction. This increases the strength of the electric field across the interface and allows electrons and holes to be collected more efficiently, ultimately leading to an increase in the amount power generated.