MAXIMUM OPEN CIRCUIT VOLTAGE

The band gap of semiconducting carbon nanotubes is related to the nano­tube’s diameter d through

Подпись: (3)Eg = ^(a/d)

where y0 = 2.45eV is the nearest-neighbour overlap integral and aCC is the carbon-carbon distance [30], [34]-[36]. Note that the band gap of semi­conducting nanotubes does not depend on the chiral angle. The band gap for a 1.5 nm nanotube, the average diameter of our nanotube material, therefore amounts to ~500meV. The band diagram is drawn in figure 1d. The ‘work function’ of the electrolyte is Oe = 4.85eV [37], while the work function for carbon nanotubes is Ont =4.5eV [36]. We therefore expect the maximum attainable open circuit voltage VOCmax ~200 – 290 mV, where the range indicates variations due to the diameter. We observe a maximum voltage of ~200mV. This is expected as nanotubes with a slightly smaller band gap will ‘short out’ the effect of nanotubes with a slightly larger bandgap.