Charge transport and trap states in lead sulfide quantum dot field-effect transistors

Lead sulfide quantum dots (PbS QDs) have shown great potentials for a wide range of electronic devices such as solar cells, sensors, and light-emitting devices. The quantum confinement in these materials leads to the quantization of energy and the size-tunability of their band-gap. These materials are also compatible with solution-processed deposition methods, which allows the fabrication of low cost and low energy consuming electronic devices. PbS QDs have been also intensively investigated in field-effect transistors (FETs), one of main components of logic circuits. However, FETs based on these materials still suffer from low carrier mobility (~10-2 cm2V-1s-1) due especially to the high number of carrier traps. Main obstacle for further mobility improvement is the lack of understanding charge transport and carrier traps in the devices. This thesis aims to investigate charge transport and the role of carrier traps in FETs based on PbS QDs.
Several methods were used including the introduction of molecular dipoles on the dielectric surface, the use of dielectric with better surface properties, and the utilization of high-k dielectrics. These methods were found to influence the electrical characteristics and the electronic structures of carrier traps in the devices.
Chemical doping using organic molecules was also performed which allows tuning the device polarity and obtaining mobility as high as 0.64 cm2V-1s-1. The fabrication of FETs on plastic substrates allowed introducing mechanical strain, with which we modulated the inter-QD distance and further improved carrier mobility to 3.0 cm2V-1s-1, the highest mobility reported in flexible FETs based on PbS QDs.