Enhancing Charge Carrier Mobility in Colloidal Quantum Dots For Technological Applications

Colloidal quantum dots (QD) are promising semiconducting materials to engineer photovoltaic and optoelectronic devices due to tunable size-dependent absorption and emission properties. These materials are important as they don’t need complicated equipment and huge setup investment for industrial applications. If formulated into a kind of stable nano-ink, these QDs can be incorporated into devices using the most economical processing technologies, spray or roll to roll printing. More importantly, these are compatible with thin-film stacked devices and circuitry that can be formed on heat-sensitive and flexible substrates to make flexible wearable devices and sensors that are difficult to achieve with crystalline-based material in existence. QD processing technology consists of three main steps (I) Synthesis (II) Purification (III) and ligand exchange and device fabrication. With well-established synthetic procedures, great effort has been done on ligand exchange and device fabrication but there has been negligible attention given to purification strategies. Another major hurdle for their industrial applications is very short-lived post-ligand exchange QD solution stability that compromises the QD ink quality even before device fabrication. This thesis is divided into six chapters. First, I will introduce QD chemistry, applications, and post-synthesis purification en route to device fabrication. Second, I’ll introduce Gel Permeation Chromatography (GPC) as a purification technique in parallel to the established precipitation/re-dispersion (PR) method. This section will demonstrate the effectiveness of GPC in removing byproducts and unbound ligands from PbS QDs, and the subsequent applicability of the GPC-purified QDs in optoelectronic devices. In the third and fourth chapters, I will present highly stable 3-mercaptopropionic acid (MPA) capped and halide capped PbS QDs, dispersed in a single non-coordinating organic solvent, to form printable p-type and n-type nano-inks. These inks are stable and suitable for making standalone, heterojunction, and p-n junction solar cell and photodetection devices. These inks should make QDs a viable option for industrial-scale manufacturing of QD devices through spray, or roll-to-roll printing processes. Chapter 5 will introduce AgBiS2 based QDs ink as environment friendly alternative to eliminate toxicity concerns associated with the current state of the art PbS QD system. This ink has been utilized to fabricate flexible photodetectors to show its broad applicability in sensitive areas such as food processing and biomedical applications. Lastly, Chapter 6 of this thesis demonstrates scanning photocurrent microscopy (SPCM) as a diagnostic technique to characterize III-nitride (GaN/AlGaN) based high electron mobility transistor (HEMT) structures for growth defects and current conduction mechanisms via sub-bandgap excitation