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Title: Improving exciton dissociation and charge transport in organic photovoltaic cells
Author: Tolk, M.
Awarding Body: University College London (University of London)
Current Institution: University College London (University of London)
Date of Award: 2013
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I have divided this dissertation into three chapters: introduction to organic solar cells, thermo-chemical lithography of a conjugated polymer, and triplet emitters in organic solar cells (OSCs). The first chapter introduces OSCs giving the background necessary to understand the problem of simultaneous optimisation of exciton dissociation and charge transport. The second chapter deals with scanning thermo-chemical lithography (SThL) of PPV on indium-tin oxide (ITO) by means of a thermal AFM, i.e. an AFM that has a hot probe scanning across the surface, to ultimately pattern the active layer of an OSC. I investigate the influence of the thermal conductivity of the substrate on the lithography by combining finite element simulations of the heat transfer and experimental results. The model explains the rather substrate-independent feature size observed during experiments and it is found that for the highest resolution features, there exists a small gap of unconverted polymer near the substrate, which is why SThL is possible on high thermal conductivity substrates such as gold. In the third chapter I report experimental findings regarding the inclusion of triplet emitters in organic photovoltaic cells. The idea is to increase the exciton diffusion length (L) of the primary photoexcitations by converting them into triplet excitons, which are known to have longer lifetimes and hence offer the potential of increased exciton diffusion lengths. Several host systems were chosen, among them P3HT:PC61BM, MDMO-PPV:PC61BM and PBTTT:bis-PC61BM. As phosphorescent molecules I used Cu-complexes and different Ir-complexes. Results on MDMO-PPV:PC61BM blends and bilayer devices showed a promising increase in the short-circuit current density (Jsc) partly supported by an increase in the incident photon to current efficiency peak in the polymer absorption wavelength range. The overall achievability of the idea is critically discussed and a 1D random walk model used to estimate possible improvements of Jsc upon increases in L.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available