Evolution of the conductivity of potassium-doped pentacene films
  We have investigated the evolution of the temperature-dependent conductivity of electron-doped pentacene (PEN) thin films, in which the electron density is controlled by means of potassium (K) intercalation. We find that the conductivity is high and exhibit metallic temperature dependence in a broad range of concentrations up to approximately 1 K/PEN. At this value, charge transfer from potassium to PEN saturates, leaving the lowest unoccupied molecular orbital at half filling. This causes a sharp drop of the conductivity, concomitantly with a re-entrance into an insulating state. Our observations are consistent with the occurrence of a Mott-Hubbard insulating state driven by strong electron-electron interaction in agreement with theoretical predictions. 
M. F. Craciun, S. Rogge, and A. F. Morpurgo