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Hesse, Holger (2011): Supramolecular assembly and nanoscale morphologies for organic photovoltaic devices. Dissertation, LMU München: Faculty of Physics
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Abstract

Organic photovoltaics is a field of rapidly growing activity in both research and industry. Flexibility, light-weight, and low-cost render this technique an appealing alternative to silicon based devices for solar energy power conversion. Recently, several donor-acceptor systems have been investigated as active materials for organic photovoltaic devices and it has been shown that the morphology of these heterojunction systems has a severe impact on the device performance. This work focuses on the correlation of nanoscale morphologies of organic donor-acceptor systems with resulting spectroscopic and electronic properties of organic photovoltaic devices. Discotic molecules are used in the active layer of the devices exhibiting a number of properties highly desired for the application in organic photovoltaic devices: The planar core shape of this class of materials allows an assembly to 1-D molecular wires showing anisotropic and exceptionally high charge carrier mobility.It is the aim to establish supramolecular assemblies of the discotic molecules and to realize nanoscale interface morphologies between donor and acceptor compounds in order to optimize the photovoltaic performance of the resulting devices. The impact of residue modifications attached to the disc shaped molecules on morphology, current generation and recombination is analyzed and design rules for these solution processable small molecule blend mixtures are derived. Vacuum sublimation is discussed as an alternative processing route facilitating the fabrication of devices with mixed but also bi-layered active material stacking. Using a dye sensitization method the exciton harvesting and photovoltaic performance can be significantly increased in these thin film devices. A highly ordered nanoscale morphology at the donor-acceptor interface, demonstrated using a template assisted imprinting approach, offers high potential towards photovoltaic devices with interdigitated interfaces and superior power conversion efficiency.