Photocurrent distribution in graphene-CdS nanowire devices.

In order to realize solar cells with technologically useful performance, intensive efforts are being directed towards the development of novel device architectures and components. So far, high power conversion effi ciencies exceeding 5% have been reached with dye-sensitized [ 1 ] or polymer blend-based solar cells, [ 2 ] wherein very fast interfacial charge transfer occurs. However, charge transport is often limiting the performance of these devices. In particular for blendbased solar cells, the photocurrent depends strongly on the properties of the percolation network. [ 3 ] A promising strategy to overcome this limitation involves nano-structured solar cells which provide well-defi ned, separate pathways for carrier transport, thus minimizing recombination losses. Examples include vertically aligned arrays of ZnO, [ 4 ] TiO 2 , [ 5 ] Si nanowires, [ 6 ] or TiO 2 functionalized CNTs. [ 7 ] Especially promising are CNTs decorated with semiconductor nanoparticles, since the former provide a close-to-ideal transport pathway for carriers. However, it is diffi cult to obtain a high quality electrical connection between nanotube and semiconductor without disrupting the carbon framework of the nanotubes. Accordingly, studying the interface between sp 2 -bonded carbon materials and semiconductors is important for further improving the performance of CNT based solar cells. More recently, also the closely related graphene has attracted increasing interest toward photovoltaic applications. [ 8 ] It has been chemically modifi ed by the attachment of TiO 2 nanoparticles [ 9 ] or CdS quantum dots, [ 10 ] albeit only little is known about the interface between graphene and inorganic or organic semiconductors, in contrast to the metal–graphene interaction. [ 11 – 13 ] In fact, while ultrafast electron transfer from CdS dots to graphene has been detected by time-resolved photoluminescence spectroscopy, [ 14 ] the suitability of these nanocomposites for light harvesting applications remains to be evaluated. Here, we investigate the photoelectric properties of the interface between graphene as a carbon nanostructure and