Polymer/fullerene solution processing: impact on thin film morphology

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Title: Polymer/fullerene solution processing: impact on thin film morphology
Authors: Dattani, Rajeev
Item Type: Thesis or dissertation
Abstract: Polymer:fullerene blends are commonly used in organic solar cells (OSCs) and solution based processing is currently the most widely used fabrication method (Vandewal et al., Macromolecules, 2013, Thompson et al. Angewandte Chemie, 2008) [1, 2]. However, the solution behaviour, polymer chain conformation and thermodynamic interactions within these systems, which critically impact thin film morphology, is not well understood. In this work, a model ternary system is systematically investigated polystyrene (PS), C60 and toluene. PS was chosen as it is a flexible polymer and has been extensively studied in solution, thin films and melt, and C60 is an exceptionally well defined molecule. A range of PS molecular weights is investigated, from 18 to 1000 kg/mol, dilute to semidilute polymer solution regimes and the fullerene solutions (below and above its miscibility limit in toluene). No change in the polymer chain dimensions (Rg, Rh or ξ) is found. C60 is shown to aggregate in solution below the miscibility limit in toluene and is shown to be dependent on PS Mw, PS concentration and time. The results are consistent with polymer/colloid theory in the protein-limit. (Dattani et al. Macromolecules, 2014) [3] Thin films with thicknesses ranging from 60 to 130 nm are fabricated from solutions in a homogenous and precipitated regime and the consequences for film formation and morphology under thermal annealing is investigated and quantified. C60 clusters are grown in solution via light exposure and are the result of light-induced oxidation. C60 epoxides are present in the solution and an aggregation mechanism is proposed. C60 epoxidation and aggregation is found to be dependent on C60 concentration, laser power, solvent, temperature and time. The knowledge gained from the model PS/C60/toluene system is applied to systems containing the much-studied poly-3-hexylthiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) pair, with qualitatively similar results observed. The change in film morphology, accessible by a simple change in processing conditions, offers a unique method for tuning device efficiencies with respect to the size of the fullerene domains.
Content Version: Open Access
Issue Date: Sep-2014
Date Awarded: Jan-2015
URI: http://hdl.handle.net/10044/1/23958
Supervisor: Cabral, Joao
Stingelin, Natalie
Nelson, Jenny
Sponsor/Funder: Engineering and Physical Sciences Research Council
Funder's Grant Number: EP/G037515/1
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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