New, Edward G. (2013) Design of multi-heterojunction organic photovoltaic devices. PhD thesis, University of Warwick.

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Abstract

Organic photovoltaic (OPV) devices have shown great potential as an inexpensive
source of renewable energy. There are many factors that affect the efficiency of an
OPV device, and understanding them can help the design of new architectures to
boost the efficiency towards a commercially viable level. Multijunction OPV devices
have been realised as a key route to higher efficiencies. The work presented in this
thesis focuses on the design of multi heterojunction OPV devices based upon small
molecule organic semiconductors.
A small molecule tandem OPV device consisting of two donor materials with
complementary absorption spectra, boron subphthalocyanine chloride (SubPc) and
aluminium phthalocyanine chloride (ClAlPc) combined with C60 as the acceptor is
used as a test device architecture.
The recombination layer between the sub-cells was investigated first, with the layer
thickness and deposition conditions optimised. Modification of the photoactive
layers by co-deposition of the donor and acceptor, and the use of a templating layer
is proven to increase the device efficiency.
α-NPD co-deposited with MoOx was used as a hole transport and optical spacer layer
in single junction and tandem OPV devices. The electrical, optical and
morphological properties of the α-NPD and the co-deposited layers are investigated.
The hole transporting properties of α-NPD:MoOx are shown in a SubPc / C60 device
with the optical spacing properties shown for a ClAlPc:C60 bulk heterojunction
(BHJ) device. Implementation of the α-NPD:MoOx optical spacer layer is shown as a
tool for current matching in a SubPc:C60 BHJ tandem OPV device. An optical
simulation predicts the use of an optical spacer will improve the device performance
as it positions the sub-cells in an optimum optical field position for current matching.
Experimental results show good agreement with the optical simulation and an
improvement in short circuit current (JSC) is shown with the α-NPD:MoOx optical
spacer layer. The tandem OPV device is also shown with a larger active area which
displays good operational stability characteristics.
A multi-heterojunction cascade device was designed using α-sexithiophene (α-6T) as
an interfacial layer with phthalocyanine / C60 OPV devices. The α-6T is shown to
dissociate excitons in the phthalocyanine layer and contribute to the photocurrent of
the device without affecting the phthalocyanine / C60 interface. The dual exciton
dissociation in the cascade device leads to an increased JSC compared to the bilayer
devices.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QD Chemistry T Technology > TK Electrical engineering. Electronics Nuclear engineering
Library of Congress Subject Headings (LCSH):	Photovoltaic cells, Organic semiconductors, Photovoltaic power generation, Heterojunctions
Official Date:	May 2013
Dates:	Date Event May 2013 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Jones, T. S. (Tim S.)
Sponsors:	University of Warwick Postgraduate Research Scholarship
Extent:	168 leaves : illustrations.
Language:	eng

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