Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 63

d n 3 r 4 n g | 9

Figure 4.2 Various hierarchical nanostructures for high eciency DSSCs. (a) Nano-
particle decorated nanowire. 13 (b) Nanoflower. 14 (c) Nanotips. 15

.

(d)

Branched nanowires. 10
Reproduced with permission from ref. 10, 13-15.

''petals'' to the nanowire backbone. Nanoflower films can be grown by a
hydrothermal method at low temperatures, typically by employing a 5 mM
zinc chloride aqueous solution with a small amount of ammonia. 10 These as-
synthesized nanoflowers have dimensions of about 200 nm in diameter. The
solar cell performance of ZnO nanoflower films was characterized by an
overall conversion eciency of 1.9%, a current density of 5.5 mA cm 2 , and a
fill factor of 0.53. These values are higher than the 1.0%, 4.5 mA cm 2 , and
0.36 for films of nanorod arrays with comparable diameters and array
densities that were also fabricated by the hydrothermal method. Nanotip
arrays with different lengths could be synthesized to achieve high eciency
solar cells. The results confirmed that the energy-conversion e ciency of the
cells increased with the length of the ZnO nanotips due to the increase in
surface area of the photoelectrode film. An overall conversion eciency of
0.55% was obtained for 3.2mm long ZnO nanotips. It has been reported that
ZnO nanotips present a maximum overall conversion eciency at higher
light intensities than in the case of TiO 2 nanoparticles. This implies a
nontrap-limited electron transport in the respect that the nanotips provide a