Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 61

materialssuchasdyesandpolymers,andthosethatemployinorganiccom-
ponents are generally made by scalable solution syntheses. 4 Obtaining the size,
spacing and aspect ratio required for ecient excitonic devices is currently
accessible only through bottom-up synthetic approaches 4 and this is why
hierarchical nanostructuring is becoming so important. Therefore, we are
mainly going to discuss the applicability of hierarchical nanostructures to
DSSCs and OPVs rather than other types of solar cells.

d n 3 r 4 n g | 9

4.2.1 Hierarchically-branched Nanowires for Solar Cells
Various 1D nanostructures have been developed to reduce the recombination
rate in excitonic solar cells. 1D nanostructures are usually crystalline materials
that can serve to increase the electron diffusion length by providing a direct
pathway for electron transport in the interior of the continuous crystal without
grain boundary scattering from the point of electron injection to the substrate
of the collection electrode. 1 This differs from the nanoporous materials or
nanoparticles because the electrons take a random path among the nano-
particles and undergo many collisions at the grain boundary between nano-
particles or the semiconductor/electrolyte interface. In those cases, vertically
aligned nanowire arrays (nanowires, nanotubes, nanotips) were developed to
achieve functional 1D nanostructures and these were demonstrated by Law
et al. in 2005 as shown in Figure 4.1. 6 However, the performance of the 1D
nanostructured solar cell did not exceed expected values. A further enhance-
ment of the solar cell eciency could be realized by the development of 3D
hierarchical nanostructures such as nanoflowers and nanoforests.
The vertically aligned 1D nanowire arrays were expected to show high
photo-conversion eciency due to a direct electron pathway with much re-
duced recombination loss by orthogonalizing the geometry. However, the
insucient surface area limited the conversion eciency of that class of
solar cells. The surface area could be increased dramatically by adding extra
dimensions to the vertically aligned 1D nanowire arrays such as nano-
flowers, nanotrees, nanoforests, nanodendrimers etc. In this regard, 3D
branched nanostructures with larger surface areas in comparison to 1D
nanowires have attracted increasing attention. Suh et al. 7 and Baxter et al. 8
have presented a dendritic ZnO NW DSSC. They grew ZnO NWs by expensive
chemical vapor deposition (CVD), and they showed relatively low eciency
(0.5%) due to insucient surface areas as shown in Figure 4.2. Jiang et al. 9
reported a ZnO nanoflower photoanode and Cheng et al. 10 reported hier-
archical ZnO NWs produced via a hydrothermal method as shown in
Figure 4.2. The nanoflower photoanodes consist of upstanding nanowires
and outstretched branches. This is based on the consideration that the
nanowires alone may not capture the photons completely due to the exist-
ence of intervals inherent in the morphology. 1 Nanoflower hierarchical
nanostructures, however, have nanoscale branches that stretch to fill these
intervals and, therefore, provide both a larger surface area and a direct
pathway for electron transport along the channels from the branched

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