Electrical Transport in Semiconductor Nanocrystal Assemblies

The size-tunable electronic properties and the solution processability of colloidal semiconductor nanocrystals (NCs) have made these materials promising candidates for thin-film optoelectronics. In particular, the tunable absorption and emission of NCs can potentially allow solar cells, light emitting diodes, and photodetectors to be optimized systematically. Towards these applications, significant progress has been made in understanding optical properties of NCs as well as in development of synthetic methods to prepare monodisperse NCs. Despite all the excitement surrounding the potential use of NCs, however, successful incorporation of these materials into practical devices is still challenging since assemblies of NCs are not highly conductive. Also, compared to the extensive studies on optical properties, the charge transport in NC assemblies has been much less explored. Considering that most of these devices typically involve charge conduction between NCs, their charge transport properties should also be well-understood. Furthermore, methods to obtain highly conductive NC assemblies should be developed. These will lead to improved device performance that takes full advantage of the characteristic properties of NCs in optoelectronic applications.;To address these issues, the electrical transport properties of NC assemblies are investigated comprehensively in this thesis. Throughout the entire study, a transistor device structure was employed to control charge density in NC films. To begin with, the charge transport mechanism in films of different sized PbSe NCs was examined at varying temperatures. We observed that the electron transport between NCs underwent a transition in mechanism from Efros-Shklovskii-variable-range-hopping (ES-VRH) to nearest-neighbor-hopping (NNH). Interestingly, this transition in the mechanism was NC size-dependent, such that it occurred at higher temperatures for films with smaller NCs. We also observed that the electron localization length, estimated from the ES-VRH model, was comparable to the NC size and scaled systematically with NC diameter. Furthermore, the activation energy from the NNH regime was also size-dependent which is attributed both to the size-dependent Coulomb effects and the size-distribution of NCs.;Following, the influence of NC size on the charge transport in CdSe NC assemblies was investigated by employing electrolyte gating. We found that the transport parameters in CdSe NC assemblies also varied strongly and systematically with NC diameter. First, a strong correlation was observed between the device turn-on voltage and the size-dependent position of the lowest unoccupied electronic states of NCs. Second, the electron mobility increased with increasing particle diameter. Third, the charge transport following the NNH model exhibited a size-dependent activation energy and a pre-exponential factor for mobility, consistent with the result from PbSe NC assemblies.;Towards improving the electrical conduction in NC assemblies, electrical transport in doped NC assemblies was examined and compared with the results of undoped NC assemblies. The charge transport activation energy obtained within the NNH temperature regime for PbSe NC films was reduced by incorporating Ag atoms into PbSe NCs, which is probably due to the reduced Coulomb penalty for hopping processes. Also, the device turn-on voltage revealed that the Fermi level is shifted upon doping CdSe NCs with either Ag or Al atoms. Ag atoms raised the Fermi level closer to the electron transport level ( n-type doping) with initial doping but then lowered the Fermi level (p-type doping) with further doping. Meanwhile, Al atoms only raised the Fermi level closer to the electron transport level ( n-type doping). Also, a corresponding change in the electron mobility was observed with doping such that the electron mobility of CdSe NC films decreased with p-type doping and increased with n-type doping.;Finally, one potential application of transistors based on NCs is discussed, namely the light-emitting transistors (LETs), which offer a potential approach to efficient electrically-pumped NC lasers with tunable emission. Although, the light emission in NC transistors is not realized in this thesis, efficient transport of both electrons and holes with high carrier densities is demonstrated, which is a critical requirement for obtaining NC-based LETs. This was particularly achieved by employing a high capacitance gate dielectric based on ionic liquids that allowed accumulating high carrier densities at low operation voltages.;Overall, the size- and temperature-dependent charge transport properties of undoped/doped NC films described here provides a more thorough understanding of electrical conduction in NC films. We believe that these results will promote more technological applications of NCs that utilize their characteristic electronic/optical properties.