PhD defence of Pegah Ghamari - Development of Next Generation Photoelectrochemical and Polymer Transistor Devices

Abstract

This research thesis focuses on two main studies involving micro/nano structural engineering of organic/inorganic semiconductor devices for development of next generation high performance and stable electronics. In the first project, we have explored the potential of GaN-based nanocrystals for the development of artificial photosynthesis devices for the conversion of CO₂ to syngas, a mixture of CO and H₂, and one of the promising future solar fuels. By integrating the Pt/TiO₂ cocatalyst with the strong light harvesting of p-n Si junction and efficient electron extraction effect of GaN nanowires, we demonstrated an efficient and stable photoelectrochemical (PEC) reduction of CO₂ into syngas product with controlled composition. It was found that the metal/oxide interface provides multifunctional catalytic sites that are inaccessible with the individual components, which structurally and electronically facilitate CO₂ conversion into CO. As a result, a record solar-to-syngas (STS) efficiency of 0.87 % and a benchmark turnover number (TON) of 24800 are achieved. In addition, we developed a decoupling strategy involving Au-Pt dual cocatalysts to achieve high energy conversion efficiency with controlled syngas composition. By integrating spatially separated a CO-generating catalyst (Au) and an H₂-generating catalyst (Pt) with GaN nanowires on planar Si photocathode, we achieved a record photon-to-current efficiency of 1.88 % and controllable syngas product with tunable CO/H₂ ratio (0–10) under one-sun illumination. Our designed PEC system exhibited highly stable syngas production in the 10 h duration test.

In the second project we investigated the improvement of organic field-effect transistors (OFETs) performance and stability using doping strategy. OFETs are emerging as promising building blocks for large-area printable and flexible electronics. However, they have yet to be implemented in practical applications due to operational challenges such as low mobility and device instability, both of which are linked to charge carrier trapping phenomena. Intentional molecular doping has been found to be an effective approach for mitigating trap states and enhancing the charge transport. However, unresolved issues such as unwanted off current and limited library of applicable molecular dopants have limited the effectiveness of the doping technique in addressing OFETs operational challenge. Here, we have introduced nitrofluorene (NF) acceptors as novel p-dopants for polymer OFETs due to superior solubility, air stability, and ease of energy level tunability. The addition of NFs to a standard commercial DPP-DTT polymer showed outstanding device performance, including an ∼5-fold enhancement in the saturation field-effect mobility (up to ∼8 cm²V⁻¹ s ⁻¹), lowering threshold voltage, and one order of magnitude decrease in contact resistance. The NF-doping mechanism was investigated via spectroscopic, microscopic, and electrical characterization, which revealed the synergetic effect of filling deep traps and modified microstructure on significantly improved performance OFETs. In continue, we evaluated the environmental and operational stability of pristine and doped transistors. By exploring the impact of air exposure on pristine OFET performance, we found that suppression of electron-induced traps by oxygen doping, as well as diffusion of water molecules to semiconductor networks, lead to device environmental instability. We demonstrate that TeNF doping suppresses both effects, resulting in environmentally independent performance and good long-term stability of unencapsulated devices in ambient air (10% deterioration after 4 months storage). The doped OFETs also show significantly reduced bias stress effect and hysteresis. Such improvement of the environmental and operational stabilities is achieved by suppressing the majority-carrier traps (including electron-induced deep traps), and better microstructural order in TeNF doped polymer films.