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Title: Spectroscopic studies of excited states in polyfluorene:small molecule blend films
Author: Dyer-Smith, Clare
ISNI:       0000 0004 2708 7759
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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In organic solar cells, understanding of interfacial processes and energy losses governing device performance is crucial to the development of new organic photovoltaic materials. This thesis reports on spectroscopic studies addressing the conditions for interfacial excited-state losses in donor-acceptor organic blends based on polyfluorene copolymers. The first chapter of results addresses the process of energy transfer in polyfluorene:fullerene blends. Novel fullerene materials based upon multiply substituted C60 and C70 are used in order to modify the energy levels, packing and photophysics in the blend. In these blends, a limitation becomes apparent in that an increase in the offset between the donor ionisation potential and the acceptor electron affinity (expected to increase the open circuit voltage) is accompanied by a loss in photocurrent and the formation of fullerene singlet and triplet states, above a threshold offset of 1.6 electronvolts. Spectroscopic measurements support a mechanism of resonant energy transfer from polymer to fullerene as a process leading to significant energy loss. In these polymers, the efficiency of hole transfer relative to fullerene intersystem crossing influences charge generation yield. The second chapter of results addresses charge transfer ('exciplex') emission as an important source of information about the photophysics of organic-organic interfaces. Blends of emissive polyfluorenes with n-type silole derivatives exhibit charge transfer emission from an interfacial state. The decrease in oscillator strength of the exciplex emission with exciplex energy is assigned to an increase in charge transfer character, explained either by the presence of electron withdrawing moieties, or an increase in solid-state ordering. The third results chapter presents a photophysical study of a silicon-bridged polyindenofluorene polymer blended with the fullerene PCBM. In this system the lifetime of the polymer triplet displays a dependence upon PCBM content in the film, indicating that the polymer triplet dynamics are influenced by interfacial processes.
Supervisor: Nelson, Jenny ; Haque, Saif Sponsor: Grantham Institute for Climate Change
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral