Nematic liquid crystals for nano-structured organic photovoltaic

Alkhalifah, Manea S.

Physics; Physical sciences
February 2010

Thesis or dissertation

© 2010 Manea S Alkhalifah. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

A range of novel liquid crystals and amorphous organic conjugated semiconductors were synthesised by the chemistry branch of the Organophotonics group at Hull University. These compounds are studied electrochemically and optically to investigate the suitability of these materials as good donor/acceptor with suitable pairs energy levels for use in organic photovoltaic devices. Liquid crystal compounds with a fluorene-thiophene structure were identified as potential electron donors in combination with perylene based compounds as electron acceptors.Time-of-flight was used to study the charge transport of organic semiconductors in this thesis. The nature of the functional groups of the molecules was found to have a significant influence on the charge carrier mobility. The incorporation of a reactive end group with spacer affected the charge carrier mobility of electron donors negatively, suggesting that the mobility depends on the intermolecular separation. Based on the need to correlate the charge transport of the donor/acceptor blends with photovoltaic devices, the electron and hole mobility were studied for blends. For all blends the hole mobility is lower than that of the pure electron donor. The electron mobility of the blends is much higher than that of the pure electron acceptor. The thermal activation of charge transport in the liquid crystals is investigated by applying the Gill model. The result shows that charge transport in the liquid crystals is thermally activated and the activation energy is field dependent. The Gaussian disorder model and correlated disorder model were used to analyze the mobility data of four liquid crystals compounds with the same conjugated core and different end groups.We show that the thin film nanoscale morphology and the phase separation of the donors/acceptors blends depend on the chemical structures of donors and acceptors, the casting solvents and the annealing temperature of the film. The functional groups of the perylene bisimide are found to influence the roughness. The surface roughness of the blended thin film is minimum and its phase separation finest when the electron donors component has short terminal aliphatic groups rather than long polymerisable chains. Chlorobenzene shows the best performance as coasting solvent. The annealing temperature is significant in controlling the nanoscale morphology and the phase separation of an intermixed network of the blends.We successfully demonstrate photovoltaic performance using blends of our novel donors and acceptors. The annealing temperature is very important to optimise the solar cell performance by optimisation of the phase separation. The perylene based liquid crystals have disappointing performance as electron acceptors. The donor with the shortest terminal end group gives the best result. The device performance fully correlates with the blend nanoscale morphology of the blends; the blend with the smallest domains gives best power conversion efficiency; the best device has a value of 1.1%.

Department of Physical Sciences, The University of Hull
O'Neill, Mary
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