Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 74

mesoporous underlayer with an optically active 3D photonic crystal over-
layer. Self-assembly material synthesis on two length-scales enabled fab-
rication of a double layer DSSC with electric and pore connectivity at the
mesoporous and the microporous level simultaneously. This construct
allows effective dye sensitization, electrolyte infiltration, and charge col-
lection from both the mesoporous and the photonic crystal layers. Due to
the intimate physical contact between the layers, photonic crystal-induced
resonances can significantly contribute to absorption enhancement in a
specific part of the spectrum. 34

d n 3 r 4 n g | 9

4.2.3 Other Hierarchically Structured Solar Cells
Besides hierarchical nanoforest and nanoporous structures, nanostructures
with other morphologies, such as nanosheets, nanobelts, and nanotetrapods
as shown in Figure 4.9, have also been studied for high eciency solar cell
applications on account of the fact that they also have a large specific
surface area. 1 However, for these nanostructures, the specific surface area
is not the only factor that determines the photovoltaic eciency of the
DSSC. 1 Solar cell performance is also believed to be significantly affected
by the geometrical structure of the photoelectrode films, which provides

.

SEM images of nanostructured ZnO films: (a) Dispersed nanosheets. 45
(b) Nanosheet-assembled spheres. 43 (c) Nanobelt array. 44 (d) A ZnO
tetrapod formed in a three-dimensional structure with four arms ex-
tending from a common core. 41 (e) Networked film with interconnected
ZnO tetrapods. 42
Reproduced with permission from ref. 1. Copyright 2013, Wiley-VCH.

Figure 4.9