Solution processed materials, such as organic molecules and semiconductor quantum dots (QDs), have been widely explored for large area electronics and optoelectronics applications, such as field effect transistors (FETs), solar cells, photodetectors, LEDs, etc. Compared to the traditional crystalline/polycrystalline semiconductors (Si, GaAs, etc.), these novel devices have the advantage of being low-cost and flexible. However, a major limitation in device performance is the significant amount of unintentional defects that are believed to hinder charge transport and induce electron-hole recombination, detrimental to optoelectronic devices. The chemical origin of these defects is largely unknown, and is difficult to characterize. One major goal of my PhD research is to investigate the nature of the defects in solution processed semiconductor materials, and to control the defects and doping for improving the large area electronic/optoelectronic device performance.

Semiconductor QDs can be viewed as “artificial atoms” that can form “artificial solids” when assembled together. By controlling the QD size, shape, surface chemistry, and the way QDs are connected or fused together, we are able to design the artificial QD solids to have the desired electronic functionalities. These materials are typically viewed in a coarse-grained way as electronically homogeneous, for the ease of understanding the electrical properties using traditional semiconductor theory. However, in the presence of various possible defects, the electronic structure and charge distribution can be inherently heterogeneous, which may have a dominant effect on the transport properties. Therefore, the other important goal in this thesis is to microscopically image the charge transport pathways, and to control them for device applications.

Our technical approach is using scanning probe-based microscopy and spectroscopy, to characterize the materials and devices. With the obtained novel mechanisms, we go back to engineer the properties to achieve high efficiency devices. Specifically, we developed a technic to use Kelvin probe force microscopy to probe the surface potential of an FET channel, from which the density of in-gap states (IGS, electronic states inside the band gap) are extracted. Using this technique and other complimentary characterization tools, we probed the IGS in a thiophene-based organic FET and PbS quantum dots, and attribute their origin to hydroxyl and molecular oxygen species, respectively. We also imaged the charge percolation pathways of PbS QD solids and CdTe polycrystalline films made by sintering CdTe nanocrystals, and found novel impurity conduction and grain boundary assisted conduction mechanisms. Mesoscale engineering of percolation pathways further lead to ultra-high gain CdTe photoconductors, where the high gain comes from the heterogeneous dopant distribution and high mobility in the grain boundary.