Solar cells based on the Schottky junction between graphene and silicon could provide a useful alternative to silicon diode-based cells, as they are less costly to make and the graphene can act as both a transparent electrode and an active layer. This cell design has recently been given new promise by X. Miao and co-workers at the University of Florida, whose article in the June 13 issue of Nano Letters (DOI: 10.1021/nl204414u; p. 2745) describes how a new record in power-conversion efficiency has been set by chemically doping the graphene layer.
The team first prepared a silicon substrate by depositing a frame of gold-chromium and etching the rest of the surface to remove the insulating oxide layer. Graphene grown on copper foil by chemical vapor deposition was then transferred to the substrate using a poly(methyl methacrylate) (PMMA) support, and was placed so that the edges made electrical contact with the metallic frame, while the rest of the sheet formed a Schottky junction with the exposed silicon (see Figure). After removing the PMMA in an acetone vapor bath, chemical p-doping of the graphene was achieved by spin coating the electron acceptor bis(trifluoromethanesulfonyl)-amide (TFSA) over the device.
Under illumination, the TFSA-doped devices yielded a power-conversion efficiency of 8.6% as compared with 1.9% for the undoped devices. This represents the highest reported figure for a graphene-based solar cell, where the increase in efficiency is attributed to the fact that doping leads to a reduction in graphene sheet resistance, improving charge transport and increasing the graphene work function. Photons that are absorbed by the silicon generate electron–hole pairs that can then be separated by the electric field associated with the Schottky junction at the material interface. Increasing the work function of the graphene increases the voltage drop at the interface, which leads to more efficient separation of electron–hole pairs into useful current. The hydrophobic dopant also acts to protect the device, endowing it with superior environmental stability as compared with pristine graphene solar cells, which degrade over time.
The factor of 4.5 increase in efficiency achieved though TFSA doping represents a significant improvement in performance, which could lead to these cells acting as viable alternatives to expensive silicon diode cells and less stable organic cells. Alternatively, the doped graphene layer could itself be applied to a range of other substrates including flexible polymer semiconductors.
(a) The structure of a pristine graphene solar cell and (b) one with bis(trifluoromethanesulfonyl)-amide (TFSA) doping. Reproduced with permission from Nano Lett. 12 (2012), DOI: 10.1021/nl204414u; p. 2745. © 2012 American Chemical Society.
