Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 64

faster conduction pathway for electron transport. This feature allows for the
use of ZnO nanotips in the fabrication of more stable and ecient DSSCs
under high illumination. It has also been demonstrated that the overall
conversion eciency could be increased to 0.77% by combining the ZnO
nanotips with a Ga-doped ZnO film as a transparent conducting layer.
Dendritic ZnO nanowires, which possess a fractal structure more compli-
cated than that of nanoflowers, are formed by a nanowire backbone with
outstretched branches, on which the growth of smaller-sized nanowire
backbones and branches is reduplicated. 1 Baxter et al. 8 described a MOCVD
fabrication for dendritic ZnO nanowires by using a route of so-called mul-
tiple-generation growth. They first grew 100 nm diameter ZnO nanowires
with 20 nm secondary nanowire branches that nucleated and grew from the
primary nanowire backbone. It was indicated that each of the nanowires was
crystalline with grain boundaries separating secondary nanowires from the
primary nanowire. The substrate with both primary nanowires and sec-
ondary nanowire branches was then used to continue the nanowire growth,
called ''secondary generation'' growth. During the growth of a second gen-
eration of nanowires, the outstretched nanowire branches act as new nu-
cleation sites for next generation nanowire growth. The growth can also be
continued for third and fourth generations for the attainment of a dendrite-
like branched ZnO nanostructure. DSSC characterization showed that the
short-circuit density increased with increasing growth generation due to the
larger surface area, which in turn led to increased adsorption of dye mol-
ecules. A total improvement of over 250 times in current density and over
400 times in eciency has been observed when the film morphology was
changed from smooth nanowires to branched second-generation nanowires.
The eciencies obtained using fourth-generation dendritic nanowire films
with a branched nanostructure and a thickness of 10 mm displayed an
overall conversion eciency of 0.5%, a more than 600-fold improvement
over smooth nanowires. By integrating ZnO nanoparticles within the film of
dendritic nanowires, the specific surface area was increased, leading to an
improved conversion eciency of 1.1% for these cells. 1 These hierarchical
NWs were grown from seeds formed from Zn(OAc) 2 and still showed a
relatively low eciency of 1.5% due to insucient surface area and lack of
uniformity of the secondary branches that were produced by a randomly
distributed seed layer.
To address these problems encountered during research into hierarchical
nanostructured solar cells, Ko et al. 11 reported that ''nanoforests'' of high
density, long branched ''treelike'' multi-generation hierarchical ZnO nano-
wire photoanodes grown via simple selective hierarchical growth by com-
bining length-wise growth (LG) and branched growth (BG) can significantly
increase the power conversion eciency by 350-500% for the same back-
bone nanowire length as shown in Figure 4.3 and Table 4.1. The eciency
increase was due to a highly enhanced surface area for higher dye loading
and light harvesting, and also due to reduced charge recombination
by direct conduction pathway along the crystalline ZnO ''nanotree''

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