Charge transport in PbSe nanocrystal arrays

We report electrical transport measurements of arrays of PbSe nanocrystals forming the channels of field-effect transistors. We measure the current in these devices as a function of source-drain voltage, gate voltage, and temperature. Annealing is necessary to observe measurable current, after which a simple model of hopping between intrinsic localized states describes the transport properties of the nanocrystal solid. We find that the majority carriers are holes, which are thermally released from acceptor states. At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder. At high source-drain voltages, the activation energy is given by the former only. The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the $1S_h$ state, and this is confirmed by field-effect measurements, which give a density of states of approximately 8 per nanocrystal as expected from the degeneracy of the $1S_h$ state.