Interlayer structure and morphology study of semiconducting polymer thin film devices

Semiconductors are now the foundation of modern electronic devices, like mobile phones, computers, etc, which are becoming an indispensable part of people s daily life. Polymeric materials are fast developing to be a promising candidate for the manufacturing of semiconducting devices. They have numbers of advantages like flexibility, light weight, low cost, etc, over the conventional silicon option. Similar to metal alloys or composite, use of blends of polymers in organic devices is feasible to modify the product properties, sometimes even possessing new features which not present in either component. The electronic properties of semiconducting polymeric materials have been extensively studied, and well understood. However the physical structure in such devices is more difficult to investigate and thus less well understood. Since blends of polymers are becoming a common option in manufacturing the devices, it is important to gain more understanding of the devices physics. In this work, the interface structure and morphology changes in bilayer systems consisting semiconducting polymers and ordinary polymers have been studied. The literature survey chapter introduces the origin of conjugated polymers and the photovoltaic related properties of semiconducting polymers are also introduced. The development of semiconducting polymer applications, light emitting diodes, solar cells and field effect transistors, is reviewed. Fundamental knowledge of polymer physics, and its relation to the thin film devices are introduced. The results part consists of three chapters. The first chapter is a report of a neutron reflectivity study on bilayer devices containing poly(2,7-(9,9)-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene))(TFB). Neutron data analysis revealed unexpected mixing at the interface between two immiscible polymer layers, forming an insoluble layer after thermal annealing and solvent rinsing. This layer has been found to improve the device performance according to Cambridge Display Technology Ltd. The fitting also indicated phase segregation in the poly(styrene sulfonate)/poly(ethyl dioxy thiophene) (PSS/PEDOT) blend polymer layer might be occurring. The second results chapter is an investigation on bilayer polymer thin film systems including poly(styrene sulfonate)(PSS) and polystyrene(PS) or poly(methyl methacrylate)(PMMA). Similar thermal treatment as on the TFB based system was applied on these bilayer systems. Vibration spectroscopy, surface morphology and device structure characterisations were applied to the system following the treatment process. Evidence of a new insoluble layer formation was reported for PMMA/PSS system. The final results chapter is about the study on polythiophene (PT)/polyethylene (PE) thin film devices. Samples included pure polymer and blends with different weight ratio. Neutron reflectivity measurements were taken at ISIS, Oxford. Results didn t give good specular data, indicating a rough sample surface. Reflective optical microscopy study showed non-homogeneous mixing in the PT/PE blend samples, with clear phase separation observed. Thermal treatment was applied to all samples, and the microscopy images taken afterwards showed limited differences with the pre-annealed ones.