Chemistry Reference
In-Depth Information
particular properties in terms of
the electron transport and/or light
propagation.
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4.2.3.1 Tetrapods
Tetrapod nanostructures for solar cell applications were first studied by
Alivisatos's group at UC Berkeley.
39
Tetrapods possess a three-dimensional
structure consisting of four arms extending from a common core. With the
unique mechanical structures, tetrapods are shaped like schoolyard
''jacks'' since they come to rest on their three downward pointing legs. The
tetrapod length of the arms can be adjusted within the range of 1-20 mm,
while the diameter can be tuned from 100 nm to 2 mm by changing the
substrate temperature and oxygen partial pressure during vapor de-
position.
40
Multiple-layer deposition can result in tetrapods connected to
each other so as to form a porous network with a large specific surface area.
Tetrapod structures could be better for plastic solar cell applications,
because the rods need to point in the same direction, and tetrapods
will always do that. By simply scattering the tetrapods on a surface, they
will all point upwards. The ZnO tetrapod-based DSSCs have achieved
overall conversion eciencies of 1.20-3.27%.
41,42
It was reported that the
internal surface area of tetrapod films could be increased further by in-
corporating nanoparticles with these films,
leading to significant
improvement in the solar cell performance.
41
.
4.2.3.2 Nanosheets
Nanosheets are thin film quasi-2D structures. The connected patches of ZnO
nanosheet used in a DSSC have been shown to possess a relatively low
conversion e
ciency of 1.55%, possibly due to an insu
cient internal sur-
face area. It seems that ZnO nanosheet spheres prepared by hydrothermal
treatment using oxalic acid as the capping agent may provide a significant
enhancement in internal surface area, resulting in a conversion eciency of
up to 2.61%.
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As for nanosheet spheres, the performance of the solar cell is
also believed to benefit from a high degree of crystallinity and, therefore, low
resistance with regards to electron transport.
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4.2.3.3 Nanobelts
Nanobelt array films prepared through an electrodeposition method were
also studied for DSSC applications.
44
In fabricating these nanobelts, poly-
oxyethylene cetylether was added in the electrolyte as a surfactant. The ZnO
nanobelt array obtained shows a highly porous striped structure with
a nanobelt thickness of 5 nm, a typical surface area of 70 m
2
/g, and
a photovoltaic eciency as high as 2.6%.
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