Translating semiconductor device physics into nanoparticle films for electronic applications

Abstract

This thesis explores and quantifies some of the important device physics, parameters, and mechanisms of semiconductor nanocrystal quantum dot (QD) electronic devices, and photovoltaic devices in particular. This involves a variety of characterization techniques and their adaptations, as well as careful evaluation of their results to assure the validity of assumptions. Chapter 1 provides an introduction of semiconductor band bending from a chemistry perspective and bulk semiconductor solar cells to establish context for the QD analogs. Chapter 2 discusses the tradeoff between absorption and conduction for QD thin films in a stacked architecture and the absorption percentage is calculated as a function of film thickness for the solar spectrum. Chapter 3 presents a quantitative measurement of the number of trapped carriers and a measurement of exciton quenching to assess limiting mechanisms for current losses in PbS-quantum-dot-based photovoltaic devices. The trapped-carrier density ranges from one in 10 to one in 10,000 quantum dots, depending on ligand treatment, and non-radiative exciton quenching, as opposed to recombination with trapped carriers, is likely the limiting mechanism in these devices. Chapter 4 presents a thorough study of the dielectric constant of PbS QD films as a function of the volume fraction of QDs. A capillary small-angle x-ray scattering (SAXS) technique is used to create a reliable QD sizing curve, a pair-distribution function for QD spacing is extracted from SAXS measurements of thin films, and a stacked-capacitor geometry is used to measure the AC capacitance and determine the film dielectric constant. The resulting data yield values of dielectric constants as a function of volume fraction of QDs in the thin films that do not fit within any simple model that applies the bulk dielectric constant of PbS, which suggests that surface or other size effects may play a role in altering the dielectric constant of the individual QDs. Appendix A quantifies the number of ligands per particle present in a QD thin film using thermogravimetric analysis to find that, in general, QD films can be heated under nitrogen to ~200°C without significant mass loss. The number of ligands per QD in a thin film ranges from 300-3000 for 4.94-nm-diameter particles, which suggests that there are many ligands in the thin film that are not directly bound to the QD. Appendix B discusses the behavior of QD thin films in thin-film transistors and the information that can and cannot be extracted from these measurements. These discussions are accompanied by QD-FET transfer curve data for devices with each gold and titanium electrodes that show distinct differences for the two electrodes. In Appendix C, steady-state and time-dependent photoluminescence are evaluated as a metric for functioning QD-PV devices. The steady-state photoluminescence varies as a function of voltage without shifting the peak position; application of forward bias increases the total photoluminescence and application of reverse bias decreases the total photoluminescence without reducing it to zero. Time-dependent photoluminescence studies show only minimal changes in PL decay time as a function of applied bias. Finally, Appendix D and E establish liftoff techniques to create crack-free nanoscale patterns of QDs (Appendix D) and controlled placement of small clusters of QDs (Appendix E) for use in smaller-scale optoelectronic devices and experiments.