Nanometrology for controlling and probing organic semiconductors and devices

Abstract

Organic semiconductors continue to attract interest due to their potential applications in printable, low-cost, high throughput, flexible electronic devices, which could profoundly impact the large area electronics and semiconductor industries. The performance and stability of such devices is strongly influenced by the chemical composition and molecular packing of the organic semiconducting material. This thesis considers a range of solution processing techniques, which can be used to control the molecular packing of polymeric and small molecular organic semiconductors, and employs a range of complementary spectroscopic and electronic characterisation techniques to elucidate the complex relationships between thin film morphology and optoelectronic device performance. By understanding these effects, we are able to control the deposition processes in order to yield optimised morphologies for high-performance organic field effect transistors, magnetic tunnel junctions, and photovoltaic devices. We first develop a highly controlled solution deposition system that offers precise control of the thermal conditions and rate of solution deposition. The zone-casting technique is optimised to print uniaxially aligned needle-like domains of small molecule organic semiconductors. We establish a robust structural probe, Raman spectroscopy, to interpret the well-defined packing motifs. The characterisation of molecular packing is extended to vacuum deposited organic thin films. For both pentacene and phthalocyanine materials, we identify strong interactions between the organic layer and the substrate, which can be modified using self-assembled monolayers, inorganic interlayers and temperature control. We are able to produce films with uniform molecular packing and an optimised molecular orientation such that the charge carrier mobility or magnetoresistance of transistor or tunnel junction devices are increased by several orders of magnitude. Finally we consider organic photovoltaic devices, where modifications to the chemical structures of conjugated polymers provide an effective means of controlling both molecular packing and device stability. We investigate the impacts of a number of structural modifications on the molecular packing and stability of the benchmark material poly(3-hexyl)thiophene. We consider the operational stability of these materials and find that increased planarity of the conjugated backbone and a deeper ionisation potential do not necessarily result in long device lifetimes. For polymer/fullerene blend films, we identify how the size and loading of the acceptor molecules influences the polymer molecular packing. Both the specific conclusions drawn in this thesis and the techniques for morphological control and characterisation are highly relevant to a broad range of organic semiconducting materials and device architectures. Our findings have particular relevance to the issues of optimised solution deposition and device stability, which are of critical importance to the emerging industry of printed electronics.