Nanoscale analysis of molecular photovoltaic thin film structures and interfaces
File(s)
James Gilchrist thesis
Supplementary material
Author(s)
Gilchrist, James
Type
Thesis
Abstract
Thin films of organic semiconducting materials, such as copper phthalocyanine (CuPc) and
C60, can be used in photovoltaic devices. The interface between these materials is the site of
exciton dissociation, and thus a key region of interest in their study. The processes that
occur within these films and at interfaces are governed by the local morphology and
structure. Studying these films and interfaces at high spatial resolution has previously been
challenging given their soft nature and scale.
Using electron transparent cross-sections prepared with a focussed ion beam (FIB), high
resolution transmission electron microscopy (HRTEM) has been used to probe the local
crystallography of three archetypical organic photovoltaic device structures grown on silicon
and indium tin oxide (ITO). In HRTEM images lattice fringes of unprecedented clarity are
observed, validating the optimised FIB method.
HRTEM examination of device structure cross-sections on silicon reveals lattice fringes
throughout pure films of CuPc and C60. The structure of the CuPc thin film can be correlated
with bulk characterisation methods however, the observation of stacking faults
demonstrates film non-uniformity. Lattice fringes in C60 films show an orientation
preference with respect to the interface, which allows conclusions to be made about C60
when grown on molecular films. Mixed films show no lattice fringes.
Structures grown on ITO are more complex than those on silicon, which is attributed the
relatively rougher growth surface. Due to this rougher surface, the morphological changes
occurring result in reduced crystallinity, a conclusion supported by bulk characterisation methods. The cross-sectional methodology has been extended to thicker films, revealing the
presence of structural deviations that lie parallel to the surface.
Scanning transmission electron microscopy, in combination with energy dispersive X-ray
spectroscopy, high resolution quantitative compositional mapping reveals the morphology
of the interface for the structures studied. This been correlated with the morphology of
single CuPc film surfaces, with the conclusion that morphology of the CuPc surface remains
unchanged after C60 film growth.
C60, can be used in photovoltaic devices. The interface between these materials is the site of
exciton dissociation, and thus a key region of interest in their study. The processes that
occur within these films and at interfaces are governed by the local morphology and
structure. Studying these films and interfaces at high spatial resolution has previously been
challenging given their soft nature and scale.
Using electron transparent cross-sections prepared with a focussed ion beam (FIB), high
resolution transmission electron microscopy (HRTEM) has been used to probe the local
crystallography of three archetypical organic photovoltaic device structures grown on silicon
and indium tin oxide (ITO). In HRTEM images lattice fringes of unprecedented clarity are
observed, validating the optimised FIB method.
HRTEM examination of device structure cross-sections on silicon reveals lattice fringes
throughout pure films of CuPc and C60. The structure of the CuPc thin film can be correlated
with bulk characterisation methods however, the observation of stacking faults
demonstrates film non-uniformity. Lattice fringes in C60 films show an orientation
preference with respect to the interface, which allows conclusions to be made about C60
when grown on molecular films. Mixed films show no lattice fringes.
Structures grown on ITO are more complex than those on silicon, which is attributed the
relatively rougher growth surface. Due to this rougher surface, the morphological changes
occurring result in reduced crystallinity, a conclusion supported by bulk characterisation methods. The cross-sectional methodology has been extended to thicker films, revealing the
presence of structural deviations that lie parallel to the surface.
Scanning transmission electron microscopy, in combination with energy dispersive X-ray
spectroscopy, high resolution quantitative compositional mapping reveals the morphology
of the interface for the structures studied. This been correlated with the morphology of
single CuPc film surfaces, with the conclusion that morphology of the CuPc surface remains
unchanged after C60 film growth.
Version
Open Access
Date Issued
2014-10
Date Awarded
2015-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Heutz, Sandrine
McComb, David
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)