Photoconductive Atomic Force Microscopy Maps Photocurrent in Solar Cells
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The researchers have also demonstrated that graphene can serve as a supercurrent transistor. Heersche and colleagues have created a device in which the supercurrent can be regulated by a voltage, similar to the control exhibited in a conventional semiconductor transistor in which the current is controlled by a gate electrode. Graphene is not a semiconductor, but rather a semi-metal. This means that not only the size of the supercurrent can be regulated, but also the type of charge carrier, according to the researchers. This can be electron Cooper pairs (with a negative charge) or hole Cooper pairs (with a positive charge). The researchers therefore report the fabrication of a bipolar supercurrent transistor. The researchers said that, in sweeping the position of the Fermi level from the valence band to the conduction band, they sweep through the Dirac point. They said that, even then, the supercurrent remains finite, which they argue demonstrates that the electronic transport in graphene is phase coherent, even when the Fermi level is located at the Dirac point. The researchers also discuss the effects of time reversal symmetry in graphene in the superconducting state based upon
occupation of the two different K-points in the band structure by the two Cooper electrons. They contrast this to graphene in the normal state in which transport is determined by a single-valley band structure.
Photoconductive Atomic Force Microscopy Maps Photocurrent in Solar Cells Solar cells made from organic semiconductor blends are currently low in power conversion efficiency. Typically, bound excitons created from absorbed light must dissociate into free charges at donor– acceptor interfaces. However, efficient light absorption requires a ~100-nm film thickness, but excitons can only travel ~10 nm before they decay. This so-called exciton bottleneck can be overcome with nanostructured organic solar cells with large internal surface areas. For example, donor and acceptor materials can be processed in a common solvent to a phase-separated film. Solar cell performance has been shown to depend very much on film morphology, but conventional characterization tools either lack the resolution needed to approach the exciton diffusion length, or do not provide direct information on the correlation between structure and local per-
formance. Recently, however, assistant professors D.S. Ginger and G.P. Bartholomew, along with their students D. Coffey, O. Reid, and D. Rodovsky at the Department of Chemistry, University of Washington, Seattle, have demonstrated that photoconductive atomic force microscopy (pcAFM) can map local photocurrents in polymer-fullerene blend solar cells with ~20-nm resolution. As reported in the March issue of Nano Letters (p. 738; DOI: 10.1021/nl062989e), Ginger and co-researchers applied pcAFM to poly[2-methoxy-5-(3’,7’dimethyloctyl-oxy)-1,4-phenylene vinylene]:(6,6)-phenyl-C61-butyric acid methyl ester (MDMO-PPV:PCBM)—a well known photovoltaic blend. Standard, mm-sized photovoltaic cells were fabricated by spincoating
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