Howells, Thomas J. (2011) Asymmetric tandem organic solar cells. PhD thesis, University of Warwick.

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Abstract

Organic photovoltaics (OPVs) is an area that has attracted much attention recently as
a potential low cost, sustainable source of energy with a good potential for full-scale
commercialisation. Understanding the factors that determine the efficiency of such
cells is therefore a high priority, as well as developing ways to boost efficiency to
commercially-useful levels. In addition to an intensive search for new materials,
significant effort has been spent on ways to squeeze more performance out of
existing materials, such as multijunction cells. This thesis investigates double
junction tandem cells in the context of small molecule organic materials.
Two different organic electron donor materials, boron subphthalocyanine chloride
(SubPc) and aluminium phthalocyanine chloride (ClAlPc) were used as donors in
heterojunctions with C60 to create tandem cells for this thesis. These materials have
been previously used for solar cells and the absorption spectra of the donor materials
complement each other, making them good candidates for tandem cell architectures.
The design of the recombination layer between the cells is considered first, with
silver nanoparticles demonstrated to work well as recombination centres for charges
from the front and back sub-cells, necessary to avoid a charge build-up at the
interface. The growth conditions for the nanoparticles are optimised, with the tandem
cells outperforming the single heterojunction architecture.
Optical modelling is considered as a method to improve the understanding of thin
film solar cells, where interference effects from the reflective aluminium electrode
are important in determining the magnitude of absorption a cell can achieve. The use
of such modelling is first demonstrated in hybrid solar cells based on a SubPc donor
with a titanium oxide (TiOx) acceptor; this system is ideal for observing the effects
of interference as only the SubPc layer has significant absorption. The modelling is
then applied to tandem cells where it is used to predict the short-circuit current (Jsc)
generation of the sub-cells, which is not accessible experimentally. Current-matching
is then used to predict the Jsc of the complete tandem device.
As a support to the optical modelling, ellipsometry measurements of thin films of
ClAlPc are presented. These films of known thickness are analysed to extract the
complex refractive index for use in optical modelling calculations. A dependence of
the complex refractive index on film thickness and substrate is also noted.
Finally, the external quantum efficiency (EQE) technique is considered as applied to
solar cells, and an additional method is proposed to characterise current balancing in
asymmetric tandem cells under illumination. This technique is verified
experimentally by two separate sets of data.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	T Technology > TK Electrical engineering. Electronics Nuclear engineering
Library of Congress Subject Headings (LCSH):	Solar cells -- Design
Official Date:	2011
Dates:	Date Event 2011 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Jones, Tim
Sponsors:	Kurt J. Lesker Company
Extent:	xv, 157 leaves : ill., charts
Language:	eng

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