Surface trap passivation and characterization of lead sulfide quantum dots for optical and electrical applications

Abstract

Quantum dots (QDs) are semiconductor nanocrystals having a size comparable to or smaller than its exciton Bohr radius. The small size of QDs leads to the quantum confinement effects in their electronic structures. Their unique optical properties, including a tunable emission from UV to IR, make QDs attractive in optoelectronic applications. However, further improvements in device performance are required to make them competitive. One well-known factor that presently limits the performance of QD thin film devices is sub-band-gap states, also referred to as trap states. For instance, trap states impair optical properties and device performance by providing alternative pathways for exciton quenching and carrier recombination. Chemical modification of QDs has been commonly used for passivating trap states and thereby improving QD devices. However, the influence of chemical modifications of ligands, QD surfaces, or synthetic routes on electrical properties of QD thin films is not sufficiently characterized. Suppressing the trap states in QD thin films is a key to improve the performance of QDbased optoelectronics. This requires fundamental understanding of trap state source, which is lacking in these materials. In this thesis, I pursue to find a systematic method to control density of trap states by exploring different characterization techniques to investigate trap states in QD thin films. These attempts provide insight to develop a rationale for fabricating better performing QD devices. This thesis focuses on the trap states in IR emitting lead sulfide (PbS) QD thin films, which have great potential for application in photovoltaics, light emitting diodes (LEDs), photodetectors, and bio-imaging. Previously, QD thin films are treated with different ligands to passivate trap states and thereby improve the device performance. Through my work, I pursued to unveil the electrical characteristics and chemical origin of trap states, and develop a strategy to suppress the trap states. First, I hypothesize that surface dangling bonds are a major source of trap states. An inorganic shell layer comprised of cadmium sulfide (CdS) is introduced to PbS QDs to passivate the surface states. Addition of CdS shell layers on PbS QDs yields an enhanced stability and quantum yield (QY), which indicates decreased trap-assisted exciton quenching. These PbS/CdS core/shell QDs have a potential for deep-tissue bio-imaging in shortwavelength IR windows of 1550-1900 nm. However, the shell layer acts as a transport barrier for carriers and results in a significant decrease in conductivity. This hinders the incorporation of the core/shell QDs in electrical applications. An improved reaction condition enables the synthesis of PbS/CdS QDs having a monolayer-thick CdS shell layer. These QDs exhibit QY and stability comparable to thick-shell PbS/CdS QDs. Incorporation of these thin-shell QDs improves external quantum efficiency of IR QD-LEDs by 80 times compared to PbS core-only QDs. In the second phase of my work, I explore capacitance-based measurement techniques for better understanding of the electrical properties of PbS QD thin films. For in-depth analysis, capacitance-based techniques are introduced, which give complementary information to current-based measurements that are widely used for the characterization of QD devices. Nyquist plots are used to determine the dielectric constant of QD films and impedance analyzing models to be used for further analysis. Mott-Schottky measurements are implemented to measure carrier concentration and mobility to compare PbS core-only and PbS/CdS core/shell QD thin films. Drive-level capacitance profiling is employed to characterize the density and energy level of trap states when QD films are oxidized. Lastly, I investigate the chemical origin of trap states and use this knowledge to suppress the trap states of PbS QD thin films. Photoluminescence spectroscopy and X-ray photoelectron spectroscopy show that standard ligand exchange procedures for device fabrication lead to the formation of sub-bandgap emission features and under-charged Pb atoms. Our experimental results are corroborated by density functional theory simulation, which shows that the presence of Pb atoms with a lower charge in QDs contributes to sub-bandgap states. The trap states generated after ligand exchange were significantly reduced by oxidation of under-charged Pb atoms using 1,4-benzoquinone. The density of trap states measured electrically with drive-level capacitance profiling shows that this reduces the electrical trap density by a factor of 40. In this thesis, I characterized trap states and showed that by suppressing the trap states we can modify the electrical properties of QD thin films, which influence the performance of QD devices directly. This work is a starting point to fully analyze the trap states in QD thin devices and thereby provides insight to design a rationale for fabricating better performing QD devices.