Hierarchical Nanostructures for Solar Cells - Hierarchical Nanostructures for Energy Devices - page 65

d n 3 r 4 n g | 9

Figure 4.3 (a) Schematic structure and (b) IV curve of a dye-sensitized solar cell with

a nanoforest ZnO nanowire.
Reproduced with permission from ref. 11. Copyright 2013, American
Chemical Society.

multi-generation branches. This approach mimics branched plant struc-
tures with the objective to capture more sunlight.
The hierarchically branched nanowires grow in the normal direction on
the vertically oriented 1st generation backbone nanowire surface. The
diameter of the branched nanowire was usually 30-50 nm, which is much
smaller than that of 1st generation backbone nanowires. The length of the
branched nanowire was 2-10 mm after a single growth step. The diameter of
the branched nanowires was always smaller than that of backbone nano-
wires. This may be attributed to the fact that the branched nanowires are
based on one of the faces of the hexagonal backbone nanowires. Both
backbone nanowires and branched nanowires have a hexagonal cross sec-
tion and grow along the c axis of the wurtzite crystal. While the branched
growth nanowire and the backbone nanowires have a monolithically single
crystalline relationship for CVD grown comb-like ZnO nanostructures, 57,58
the hydrothermally grown secondary branched ZnO NW did not originate
from the backbone NW but from the ZnO seed nanoparticles in the c axis
direction with a single crystal wurtzite structure. 11 The eciency increase
can be explained by a combination of several effects. First, the enhanced
photon absorption associated with the augmented surface area results in
increased dye loading and corresponds to a large J sc increase. Length-wise
growth can grow only upstanding nanowires by adding extra length to the 1st
generation backbone nanowire. However, branched growth can grow multi-
branched hierarchical nanowires from just a single 1st generation backbone
nanowire, thus surface area can be increased dramatically. The measured
nanowire number density for upstanding conventional nanowires (10 9 /cm 2 )
could be increased by 1-2 orders of magnitude (10 11,12 /cm 2 ) by branched

.