Biomedical Engineering Reference
In-Depth Information
2.5.1 Block Copolymers in the Dye-Sensiized Solar Cell
Dye-sensitized solar cells (DSCs) are a paricularly successful
example of a bulk heterojunction cell architecture. A wide bandgap
inorganic semiconductor (typically a metal oxide) is sensitized to
the solar spectrum by attaching a surface-adsorbed monolayer of an
organic or organic-metal complex dye [96, 97]. If the redox potential
of the photoexcited dye lies above the conduction band edge of the
inorganic semiconductor then an electron may be injected into the
layer. The oxidized dye is regenerated by electron transfer from a
surrounding donor species. The first, and still most successful
version of this concept (though there are now many variations) uses
a donor species dissolved in a liquid electrolyte, usually an organic
or ionic liquid solvent containing the I
−
/I
−
redox couple [98]. The
oxidized donor diffuses away to a counterelectrode where it is
subsequently reduced to complete the cell circuit.
The problem of exciton diffusion is avoided since all excited
states are generated directly at a charge-separating interface.
However, the confinement of the cell's absorbing component to
a single interfacial monolayer makes it very difficult to achieve
significant light absorption in a flat layer. The bulk heterojunction
concept again offers a solution in which a highly structured (or
porous) inorganic layer provides the massively increased surface
required to load with dye. Using a typical transition metal complex
dye, a surface area enhancement of around 1000 over the flat
surface equivalent is needed to achieve significant (
3
90%) light
harvesting. Electron extraction occurs via the diffusion of electrons
through the nanocrystalline inorganic layer [99], again requiring a
fully continuous network structure. This well-studied and modeled
system provides an ideal testing ground for probing both the
device performance of BCP morphologies and the dependence of
important optoelectronic characteristics such as charge transport
and recombination rates on the semiconductor topology.
Fig. 2.26a-f summarizes the templated BCP morphologies
accesible using electrochemical replication of TiO
2
ca.
detailed in
Section 2.4.6.2. In addition to standing nanowires and the gyroid
network, a disordered mesoporous layer of thermally sintered,
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