Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 60

solar cells (OPV) are produced in a cost-effective way by using a simple
solution process and they possess a moderately high eciency due to their
large surface areas. The compatibility and easy applicability of the solution-
based process to nanostructured materials have made the nanostructures
the most promising materials in realizing high eciency third generation
solar cells. Therefore, to show the applicability of hierarchical nano-
structures for solar cell applications, we will focus mainly on third gene-
ration DSSC and OPV solar cells.
Since the successful high eciency DSSC demonstration by Gr¨tzel in
1991 3 with a highly porous TiO 2 nanocrystalline film, the development of
new materials (such as dyes, electrolytes, catalysts etc.) has been the main
research topic in high eciency solar cell development. Research into the
conversion eciency enhancement that could be obtained from the
development of new materials was slow, so researchers turned their
attention to smart nanomaterial structuring, which could allow a dramatic
photo-conversion eciency even with the same materials. One of the major
research trends in smart nanomaterial structuring was the introduction of
nanowires, which were expected to have more favorable electron transport
with reduced recombination loss than nanoparticle-based solar cells.
The nanowire-based solar cell research started from a simple structure
consisting of a vertically-grown nanowire forest, and progressed to more
complex structures including 2D and 3D hierarchical nanostructures to
achieve high photo-conversion eciencies. The major objectives of hier-
archical 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 for DSSCs, and (3) a light scattering layer to capture the
sunlight more eciently by multiple scattering.
In this chapter, various research trends will be introduced including how
smart material structuring will lead to a photo-conversion e ciency increase
in solar cells, especially by introducing hierarchical nanostructures.

d n 3 r 4 n g | 9

.

4.2 Hierarchical Nanostructured Solar Cell
Compared with conventional silicon-based solar cells, excitonic solar cells, 4 in
which small molecules, polymers, or quantum dots are used as light absorbing
materials (for example DSSC and OPV), can benefit greatly from functional
nanostructured materials that orthogonalize the geometry (make the junction
normal to the substrate). 4 The critical dimension for these photovoltaic devices
is the exciton diffusion length, which is typically much shorter—roughly 10 nm
or less in polymers and up to a micrometre in high-quality small molecule
films—than the minority carrier diffusion length in silicon. 4 This orthogona-
lized device structure maximizes the volume of the absorber material and/or
interfacial area, which contribute to charge generation while providing high-
mobility channels through which these charges can be extracted. 4 Excitonic
cells are promising due to their being comprised of inexpensive organic