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Title: Directly probing thin film morphology-optoelectronic property relationships in organic and hybrid solar cells
Author: Wood, Sebastian
ISNI:       0000 0004 6061 4856
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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Solution processable organic semiconductors offer a promising route towards low-cost solar photovoltaics. The performance of these devices is critically dependent on the morphology of the thin film active layer and is very sensitive to both the chemical structure and deposition conditions of the materials. In this thesis a range of complementary techniques are used to characterise the morphology, particularly resonant Raman spectroscopy and atomic force microscopy, in addition to analysis of the device performance. By comparing these results we are able to fulfil the aim of this project, which was to elucidate the fundamental relationships between the thin film morphology and photovoltaic performance for a range of organic and hybrid solar cells. For polymer/polymer blends we consider the impacts of nanowire formation, molecular weight, and thermal annealing on the thin film molecular order. By controlling the interactions between the two polymers we are able to increase the charge carrier mobilities by several orders of magnitude, resulting in reduced bimolecular recombination and enhanced device efficiency. For the hybrid polymer/inorganic devices that we consider, we identify an interfacial region of disordered polymer, which can be partly controlled but not fully overcome. We suggest that this represents an intrinsic limitation, which should be addressed by considering alternative routes to interface formation. Donor-acceptor copolymers are an important class of materials showing promising optoelectronic properties for polymer/fullerene solar cells. We consider how various chemical modifications including fluorination, side chain branching, and heavy atom substitution affect the molecular properties and thin film morphology. In particular, we consider the nature of the electronic absorption transitions of diketopyrrolopyrrole-based copolymers and find that the low energy transition is localised on the diketopyrrolopyrrole unit and is very stable to photodegradation, whereas the high energy transition couples more strongly to the donor unit, which is more vulnerable to photooxidation.
Supervisor: Kim, Ji-Seon ; Nelson, Jenny Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral