Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 80

fast evaporation of the solvent makes the process kinetically controlled,
and the chemistry has to be carefully modulated to get self-organiza-
tion. 32,33 If hierarchical porous films are synthesized by self-assembly to
obtain organized mesoporosity, the other templates should not interfere
with self-organization.

d n 3 r 4 n g | 9

4.4 Summary
Hierarchical nanostructures, such as branched nanoforest and nanoporous
structures, have changed the recent research trend in developing high e-
ciency solar cells. Traditional research had focused on the development of
new materials such as dyes, electrolytes, catalysts and so on. However,
conversion eciency enhancement by new material development has slowed
down and new research trends to enhance solar cell eciency by smart
nanostructuring from the same material have started to garner tremendous
attention. Nanowire-based solar cells have ignited this nanostructuring re-
search and further progress with 2D and 3D hierarchical nanostructures has
drawn noticeable solar cell eciency enhancement. The major objectives of
hierarchical nanostructuring in solar cells are: (1) high carrier mobility
(mostly electron mobility in photoanodes) along the nanowire structures
with less recombination, (2) a large surface area to capture more sunlight
and adsorb more dye molecules, and (3) a light scattering layer to capture the
sunlight more eciently by multiple scattering. A large surface area and
high carrier mobility are the requirements for most energy-related devices.
Therefore, it is evident that hierarchical nanostructures can be applied to
emerging energy conversion and storage fields, such as photocatalysis,
photoelectrochemical water splitting, Li ion batteries, supercapacitors, fuel
cells, thermoelectric devices, piezoelectric devices as well as solar cells.
Furthermore, using 3D branched structures to harvest various types of am-
bient power, such as thermal, wind, vibration and electromagnetic energy,
would also be very promising, which provides a potential endless source
of energy. 5
Even though there are a lot of published papers on 3D hierarchical
nanostructured solar cell devices, further developments in this research field
require improvements in synthetic methods and novel fabrication processes
to provide better control of the structural complexity, composition uni-
formity, surface chemistry and interface electronics and last but not least,
the yield, of hierarchical nanostructures. 5 These factors are directly related
to the sustainability, high eciency and production costs at an affordable
level for the public for practical applications. This is why developing simple,
economic and environmentally friendly hierarchical nanostructure mass
production methods are of great interest. One of the most promising eco-
nomical approaches usually consists of a solution process without using any
expensive and complex vacuum-based vapour phase methods.
There are also drawbacks to using 3D branched nanostructures in energy
applications,

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in that they bring challenges in quantifying the charge