Nanoscale Characterisation and Neutron Damage Testing of Organic semiconductors

Abstract

In this thesis work, I employ a wide range of morphological, structural and dynamical tools to investigate how the intermolecular interactions in a variety organic semiconductors (OSCs) can influence heavily the nanoscale arrangement and govern the tolerance against external stimuli (i.e. radiation) of these promising materials and related devices. Among the experimental techniques used, this work emphasises on the use of neutron scattering, due to the strong propensity of highly hydrogenated materials to scatter these particles, and thus reveal precious information about their inner structural and dynamical features. I exploit the power of intermolecular interactions, to grow large and solvent-free single crystals of the electron-acceptor of choice of organic solar cells, the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The structure and quality of these crystals have been characterised by X-rays diffraction and metrological atomic force microscopy experiments. High-quality and solvent-free crystals of organic semiconductors can be a reliable platform for studying the intrinsic optical and electronic properties of these materials. The nanoscale structure and dynamics of poly (thiophene):fullerene blends has been investigate by means of neutron reflectivity and quasi-elastic neutron scattering. In particular, I show that the intercalation of fullerene particles in between polymer side-chain depends on degree of order of the polymer. Furthermore, I find that polymer:fullerene interaction has a great effect on polymer dynamics, leading to a retardation of side-chain motion. The interaction between polymer and fullerene particles and their nanoscale mutual arrangement is of great interest for improving the charge phogeneration process in organic solar cells. Finally, I studied for the first time the neutron radiation tolerance of two benchmark poly(thiophenes), observing that polymer crystallinity has a prominent role in determining the neutron radiation tolerance of conjugated polymers. Radiation hardening studies on organic electronics can pave the way to the integration of this class of devices in space and avionic applications.