Hancox, Ian (2013) Utilising high work function metal oxides as hole extracting layers for organic photovoltaic cells. PhD thesis, University of Warwick.

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Abstract

A substantial amount of research has already been undertaken towards
creating commercially viable organic photovoltaics (OPVs). This is due to the
potential use of OPV cells as an inexpensive source of renewable energy. There are
many factors to consider in OPV cell design, including photo-active materials, cell
architecture and electrode selection. However, additional interlayers for use between
the photo-active materials and the electrodes were identified to be as important and
need to be developed to optimise cell performance. The work presented here focuses
on the influence of various metal oxide hole extracting layers in different OPV
systems.
Metal oxides such as molybdenum oxide (MoOx) have shown great promise
in polymer cells as a hole extracting layer, and here we investigate their use in small
molecule cells. An optimised MoOx layer thickness of 5 nm provides a ~ 60 %
increase in overall power conversion efficiency (ηp) for chloroaluminium
phthalocyanine (ClAlPc) / fullerene (C60) cells in comparison to those fabricated on
bare ITO. A similar improvement of ηp is reported when using the MoOx layer in a
boron subphthalocyanine chloride (SubPc) / C60 system. For both high ionisation
potential donor materials, the cells containing MoOx achieve a significantly higher
open circuit voltage (Voc). Conversely, cells utilising the lower ionisation potential
donor materials such as copper phthalocyanine (CuPc) and pentacene produce
similar Voc values when deposited on both ITO and MoOx. Hence, the ηp is
marginally reduced with the MoOx layer. To attain a deeper understanding, the
factors behind these performance differences were explored by UV-vis absorption
spectroscopy, ultra-violet photoemission spectroscopy (UPS), X-ray diffraction
(XRD) and atomic force microscopy (AFM).
Thermally evaporated vanadium oxide (V2Ox) was used as an alternative hole
extracting layer to MoOx, achieving analogous performance to MoOx when used in
SubPc / C60 and CuPc / C60 cells. The electronic properties of the V2Ox layer are
investigated using UPS, and it is demonstrated to have substoichiometric n-type
character in contrast to the p-type behaviour previously reported. Additionally, the
in-situ fabrication and characterisation of organic layers using UPS indicate Fermi
level pinning of the organic to the metal oxide.
A solution processed vanadium oxide (V2Ox(sol)) layer was developed and
characterised as an alternative method of layer fabrication. The atmospheric
processing conditions are found to have a dramatic effect on cell performance, and
are studied using x-ray photoelectron spectroscopy (XPS). Layers spin-coated under
a nitrogen atmosphere exhibit a larger composition of V4+ states. Kelvin probe and
UPS experiments indicate the V2Ox(sol) is also a high work function, n-type layer,
with the V2Ox(sol) hole extracting layer producing similar cell performance to the
thermally evaporated metal oxide layers. Cells deposited on the V2Ox(sol) layer
demonstrate good operational stability characteristics, outperforming a commonly
used solution processable hole extracting layer.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry T Technology > TP Chemical technology
Library of Congress Subject Headings (LCSH):	Photovoltaic cells, Metallic oxides, Extraction (Chemistry)
Official Date:	March 2013
Dates:	Date Event March 2013 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Jones, Tim S.
Sponsors:	Engineering and Physical Sciences Research Council (EPSRC); Asylum Research UK
Extent:	xvi, 174 leaves : illustrations, charts.
Language:	eng

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