Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 76

4.2.3.4 Mesh-based 3D Electrodes
Rustomji et al. 46 demonstrated a mesh-based 3D photoelectrode in an at-
tempt to maximize the surface area of a photoelectrode comprised of TiO 2
nanotubes. By anodizing a commercial metal mesh of titanium (99.5 wt%
Ti), TiO 2 nanotubes on the surface of titanium fibers were produced. The
TiO 2 nanotubes grew in different orientations due to the use of a mesh as a
substrate, and the mesh produced presented a surface area much larger than
that of a flat titanium foil. A solar cell constructed with such a 3D mesh
with TiO 2 nanotubes 37 microns in length and 110 nm in diameter showed
5.0% eciency.

d n 3 r 4 n g | 9

4.3 Fabrication/Synthesis Methods
Many studies on synthetic approaches towards hierarchical nanostructures
have been reported for applications in high eciency solar cells. The pro-
duction of these structures can be achieved through sol-gel synthesis, 47
hydrothermal/solvothermal growth, 48 physical or chemical vapor de-
position, 49,50 low-temperature aqueous growth, 51,52 chemical bath de-
position, 53 and electrochemical deposition. 54-56 Interestingly, the majority
of the hierarchical nanostructured solar cell studies apply wet chemistry
instead of vacuum synthesis (for example, chemical vapor deposition) to
realize a low cost solar cell fabrication process. This is very important be-
cause cost as well as eciency is the most important factor in solar cell
production. Solution processing becomes more important especially for
DSSCs because DSSCs are low-cost at the cost of having a moderately lower
eciency compared with silicone-based crystalline solar cells, which have a
high eciency but are very expensive to produce.
One of the most notable hierarchical nanoforest synthesis methods was
demonstrated by Ko et al. 11 and Herman et al. 12 by the hydrothermal growth
of branched ZnO nanowires. A nanoforest of hierarchical ZnO nanowires is
grown by a modified hydrothermal growth approach as illustrated in
Figure 4.10. Depending on the growth conditions, there are two types of
growth modes: length-wise growth (LG) and branched growth (BG). LG can
yield nanowires of increased length by extending the growth at the tip of the
backbone nanowire while BG produces highly branched nanowires by
multiple generation hierarchical growth on nanowire side surfaces. The first
generation (backbone) nanowires are grown from ZnO quantum dot seeds
deposited on a substrate in an aqueous precursor solution. ZnO quantum
dots (3-4 nm) in ethanol are drop casted on a substrate to form uniform
seeds for nanowire growth. Nanowires were grown by immersing the seeded
substrate in aqueous solutions containing 25 mM zinc nitrate hydrate
[Zn(NO 3 ) 2 6H 2 O], 25 mM hexamethylenetetramine (C 6 H 12 N 4 , HMTA) and
5-7 mM polyethylenimine (PEI) at 65-95 1C for 3-7 hours. After the reaction
was complete, nanowires grown on the substrate were thoroughly rinsed
with MilliQ water and dried in air to remove any residual polymer. Longer

.