Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 106

Fig. 11 Schematic illustration of H-TiO 2 NF photoanode fabrication stages of (a) TiO 2 hollow
nanofibers (TiO 2 -NFs), (b) ZnO NR templates grown on TiO 2 -HNFs, (c) TiO 2 nanotube branches
grown on TiO 2 -NFs through ZnO NR templates, and (d) QD-sensitized H-TiO 2 NF photoanode.
(Han et al. [ 108 ] reprint permission from RSC publishers)

advantages in sensitizer-type solar cells including: (i) the enhancement in the light-
harvesting probability through large surface area for sensitizer loading and photon
localization arising from the random multiple scattering of the light within the pine
tree network (ii) the direct charge extraction pathways throughout the device
thickness (fast electron transport from sensitizer to collecting terminal), and (iii)
the huge 3-D porous network that allows better electrolyte filling, with possible
beneficial implications for preparation of solid devices.
Among the many 3-D hierarchical anode architectures, ''tree-like'' morphology
is a versatile candidate, since the tree-like configuration can be fabricated in large
scale and each electronic interfaces being assembled at separate stages without any
complication. The schematic structure of proximally coated sensitizers on tree-like
framework is illustrated in Fig. 13 . Under irradiation the excited electrons at
sensitizer can inject to wide bandgap branches (WB-PA), and subsequently the
electrons were being rapidly reached to collector substrate through conducting
backbone nanowire (stem) [ 119 ].
Macroscopically, assembling tree-like crystalline framework results 3-D
''nanoforest'' structure, which offer ample room for more quantity of sensitizer
loading compare to branch-free nanowire electrodes. This approach is simply
mimicking

branched plant structures, ultimately, to capture more sunlight.