Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

Introduction
Dye-sensitized solar cells (DSC) are an innovative technology for the production of electricity from solar
energy. The DSC was invented by B. O'Regan and M. Grätzel in 1991 at the Ecole polytechnique
fédérale de Lausanne (EPFL) [1,2]. In contrast to conventional silicon semiconductor solar cells, light
is absorbed in DSCs by dye molecules bounded to the surface of a highly porous structure of
nanoparticles of transparent TiO2. Dye excitation is followed by electron injection into the TiO2 and by
dye reduction from a redox electrolyte filling the pores of the TiO2 film. Electrons are transported in the
TiO2 nanoparticles to the front contact, which consists of a transparent conductive oxide layer (TCO).
The contact to the redox electrolyte is made by a (catalyst-coated) back contact.
The production process of DSCs is based on relatively simple and inexpensive techniques, like e.g.
screen printing. In addition no cost-intensive, exhaustible raw materials are needed. Therefore, the
contribution of DSCs could be essential for an economic production of solar energy in the future.
Objectives
The objective of this project is to develop validated models for the dye-sensitized solar cell (DSC), in
order to promote this innovative technology invented in Switzerland. These models aim at providing a
mathematical description of the coupled optical, electrical and electrochemical processes taking place
within the DSC [3,4]. This allows to analyze quantitatively the different loss channels of the energy
conversion process within the DSC. Research and development of this type of solar cells could be
accelerated to a large extent, if these new models are implemented into accurate and efficient numerical
algorithms. It is a project goal to make the numerical algorithms accessible through a user-friendly
software (e.g. with a graphical user interface) for research at academic laboratories and for DSC
manufacturing companies. Such a software will be extremely helpful, in particular for the selection of
appropriate materials for DSC production. In addition, the software should be applicable for the interpretation
of measurement data and for the optimization of the different DSC parameters.
Short description of the project
Accurate modeling of DSCs should lead to a better understanding of the physical and electrochemical
processes responsible for the energy conversion in DSCs [3,4] and to new approaches for their optimization.
With a user-friendly simulation software available, these new approaches could be investigated
in a cost-effective way and on a shorter timescale. Academic laboratories as well as possible
DSC manufacturers could largely benefit from such a software.
The project is carried out by the Institute of Computational Physics (ICP) at the Zurich University of
Applied Science (ZHAW) and the Laboratoire de Photonique et Interfaces at the Ecole polytechnique
fédérale de Lausanne (EPFL). The project is funded by the GEBERT RÜF STIFTUNG. The detailed
project plan contains the following work packages:
� Optical Model: In the optical simulation reflection and absorption losses are calculated and the
spatially resolved generation of excited states is simulated. The optical model is based on thinfilm
optics, ray-tracing, and effective medium theory [5,6].
� 1D through-plane Model: The 1D through-plane model describes the essential optical, electric
and electrochemical processes taking place within the DSC. The most important processes are:
charge carrier injection from the photoexcited dye molecules into the semiconductor, regeneration
of the oxidized dye molecules by the electrolyte, electron transport through the nanoporous semiconductor,
and recombination of electrons at the semiconductor/electrolyte interface, diffusion
and migration of the redox species in the electrolyte. With the 1D through-plane model the DSC
operation at steady-state can be simulated and various quantities characterizing the cell, such as
current-voltage curves or quantum efficiencies can be calculated.
� Time-dependent 1D Model: A simplified time-dependent version of the 1D through-plane model
is formulated for characterization purposes.
� 2+1D Model: With a 2+1D numerical simulation, it is possible to account for lateral losses occurring
in large solar cells. These losses cause a reduction in the efficiency of solar modules in comparison
to small laboratory test cells. In addition, the 2+1D model allows us to simulate new cell
geometries.
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Modeling, simulation and loss analysis of dye-sensitized solar cells, J.O. Schumacher, ZHAW - ICP
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