Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 69

branches on the backbone ZnO nanowires, up to 400% for the same back-
bone nanowire length. The overall light conversion eciency increased as
the length of the branches increased because the dense network of crystal-
line ZnO NWs can increase the electron diffusion length and electron col-
lection. However, doubling or tripling the branch length did not double or
triple the cell eciency. When the branch length was over 10 mm, the cell
eciency increase with the branch length slowed down. This could be at-
tributed to the recombination loss during the electron transport from the
point of injection to the collection electrode and the partial collapse of the
branches as the branches got longer. Branches could bundle during the
drying process after the nanowire hydrothermal growth. The bundled
branches could provide less surface area for dye attachment than the cal-
culated expected values for a weeping willow nanotree structured solar cell
with well separated branches.
TiO 2 and ZnO are the two most popular materials for hierarchical
nanostructured solar cell fabrication. ZnO is a wide-band-gap semi-
conductor that possesses an energy-band structure and physical properties
similar to those of TiO 2 , but has higher electronic mobility that would be
favorable for electron transport, with reduced recombination loss when used
in DSSCs. 1 Many studies have already been reported on the use of ZnO
materials for application in DSSCs. Although the conversion eciencies of
0.4-5.8% obtained for ZnO are much lower than 11% for TiO 2 , ZnO is still
thought of as a distinguished alternative to TiO 2 due to its ease of crystal-
lization and anisotropic growth. These properties allow ZnO to be produced
in a wide variety of nanostructures. Very similar approaches to ZnO bran-
ched nanostructures were demonstrated for TiO 2 as shown in Figure 4.5. 16,17

d n 3 r 4 n g | 9

.

4.2.2 Hierarchically Structured Porous Materials for Solar
Cells
The first generation of high eciency DSSCs was demonstrated with uni-
form sized TiO 2 nanoparticle-based nanoporous structures. Since then,
hierarchical nanoparticles at different scales have been studied to enhance
the surface area and to capture the sunlight more eciently by a light-
scattering mechanism. The performance of solar cells with hierarchically
structured ZnO films can be significantly affected by either the average size
or the size distribution of the nanopores.
Natural materials have developed highly ecient hierarchical
structures over a very long time to eciently capture, convert and store
sunlight energy. For example, green leaves and certain photosynthetic plants
have hierarchical structures optimized for ecient light harvesting and
sunlight conversion to chemical energy by photosynthesis. By learning
from nature, materials with hierarchical porosity and structures have
been heavily involved in newly developed energy storage and conversion
systems. Owing to meticulous design and ingenious hierarchical