In the September issue of Nature Materials, I have a News and Views piece about a really neat article by Sakanoue and Sirringhaus of the Cambridge University organic electronics group. My apologies to those without subscriptions - here's a brief summary:
Transport in organic semiconductors is generally poor when compared with that in inorganic semiconductors. Disorder and purity are major concerns, and electronic conduction (parametrized by the mobility of the charge carriers) very often is thermally activated, so that decreasing temperature leads to an exponential worsening of charge transport. This is in contrast to the situation in clean, nice materials like Si or GaAs, when lowering T leads to improving mobility, as scattering of carriers by thermal phonons is reduced. The Cambridge investigators have successfully made transistors from high quality spin-cast films of TIPS-pentacene, a small molecule organic semiconductor. These films actually do show improving conduction as T is reduced down to 140 K. At high source-drain electric fields and high carrier densities, transport becomes pretty temperature independent down to cryogenic temperatures.
Most importantly, however, the Cambridge group has also done "charge modulation spectroscopy" - optical spectroscopy measurements on the films as well as on the molecules in solution. By combining the optical measurements with the transport experiments, they are able to make rather strong statements about how localized the charge carriers are. They can thus rule out exotic physics or voltage-driven metal-insulator transitions as the origin of the good conduction regime.
This work shows the power of combining complementary techniques. Relying only on transport, we had made similar arguments here. However, the addition of the optical data greatly enhances the scientific arguments - what we had argued as "consistent" is totally nailed down here, thanks to the additional information from the spectra.