Efficient photovoltage multiplication in carbon nanotubes

Carbon nanotubes are direct-bandgap materials that are not only useful for nanoelectronic applications(1,2), but also have the potential to make a significant impact on the next generation of photovoltaic technology(3-5). A semiconducting single-walled carbon nanotube (SWCNT) has an unusual band structure, as a result of which high-efficiency carrier multiplication effects have been predicted and observed(6,7) and films of SWCNTs with absorption close to 100% have been reported(8). Other features that are also important for photovoltaic applications include high mobility(9,10) and the availability of ohmic contacts for both electrons(11,12) and holes(13). However, the photo-voltage generated from a typical semiconducting SWCNT is less than 0.2 V, which is too small for most practical photovoltaic applications. Here, we show that this value may be readily multiplied by using virtual contacts at the carbon nanotube. In one example, more than 1.0 V is generated from a 10-mm-long carbon nanotube with a single-cell photovoltage of similar to 0.2 V.