Electrical transport in organic semiconductors: Role of electrical self-heating, doping, and chemical structure

Organic semiconductors are an interesting class of materials for application in a wide range of emerging opto-electronics and electrical devices, such as light emitting diodes, thin-film transistors, solar cells, sensors, and thermoelectrics. However, despite the tremendous efforts on the development of the high-performance materials, device architectures, and manufacturing techniques; crucial challenges still persist and will need to be addressed in order to fully exploit the potential of this class of materials. In this respect, the underlying physics of charge transport in organic semiconductors is still not fully understood. Particularly, solution processed organic semiconductors have complex microstructures. In contrast to inorganic semiconductors, carrier transport in these partially ordered films cannot be described using models for either amorphous or crystalline semiconductors due to their uncontrolled morphology and alignment in the charge transport domain. Further, organic semiconductors are intrinsic and must be doped either with chemical or molecular dopants, carrier injection via external contacts, or photoexcitation in order to transport charge. In this regard, current flow may cause unwanted device heating in organic electrical devices due to high resistance of organic layers. Again, the low thermal conductivity of organic semiconductors prevents rapid heat dissipation from the device, which can have negative consequences on device lifetime and performance. Doping in organic semiconductors can significantly impact the molecular backbone, thin film morphology, and corresponding charge transport nature. The challenge is to experimentally untangle these intertwined physical and chemical phenomena. This thesis extensively investigates charge transport in conjugated organic semiconductors, with a focus on macromolecules such as conjugated polymers that are applied in opto-electronic and energy applications. The goal was to elucidate the impact of electrical and molecular doping in different conjugated systems. I also studied the role of different chemical structures on charge transport efficiency.
Chapter 1 provides the general background on electronic structure, charge transport, current-voltage characteristics, and vibrational spectroscopy of organic semiconductors. Chapter 2 includes the details of the materials, device structures, and the experimental procedures used for the experiments. Chapter 3 describes how electrical doping impacts the charge transport of organic diodes. Here, I used a novel approach based on in-situ Raman spectroscopy coupled with electrical measurements to unravel the effects of electrical-self heating in the organic semiconductor layer of a working diode. Chapter 4 describes the influence of p-type and n-type doping in molecular structure, thus charge transport of a donor and acceptor-type organic semiconductor, respectively. Especially, this chapter shows how oxidation (p-doping) and reduction (n-doping) of a pristine conjugated chain via doping influences the molecular structure and property by using vibrational spectroscopy, and electrical study. Chapter 5 demonstrates how the structural functionalization of conjugated polymer substrates modifies the charge transport in the working diodes and solar cells. Further, this chapter provides the characterization of novel acceptor polymers for all-polymer solar cell and single-component polymer solar cell applications. Finally, Chapter 6 summarizes the research described in this thesis and outlines the future directions toward an improved understanding of the device physics of emerging organic opto-electronics.