Competition between charge transport and energy barrier in injection-limited metal/quantum dot nanocrystal contacts

Injection-limited contacts in many of electronic devices such as light-emitting diodes (LEDs) and field effect transistors (FETs) are not easily avoided. We demonstrate that charge injection in the injection-limited contact is determined by charge transport properties as well as the charge injection energy barrier due to vacuum energy level alignment. Interestingly, injection-limited contact properties were observed at 5 nm diameter lead sulfide (PbS) quantum dot (QD)/Au contacts for which carrier injection is predicted to be energetically favorable. To probe the effect of charge transport properties on carrier injection, the electrical channel resistance of PbS nanocrystal (NC) FETs was varied through thermal annealing, photoillumination, ligand exchange, surface treatment of the gate dielectric, and use of different sized PbS NCs. Injection current through the PbS/Au contact varied with the FET mobility of PbS NC films consistent with a theoretical prediction where the net injection current is dominated by carrier mobility. This result suggests that the charge transport properties, that is, mobility, of QD NC films should be considered as a means to enhance carrier injection along with the vacuum level energy alignment at the interface between QD NCs and metal electrodes. Photocurrent microscopic images of the PbS/Au contact demonstrate the presence of a built-in potential in a two-dimensionally continuous PbS film near the metal electrodes.