Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 105

the fibrous membranes are found to peel off from the TCO substrate. Recently, we
demonstrated the hierarchical hollow nanofiberous electrode with nanotube
branches which balancing the trade-off between the fibrous thickness and
mechanical stability [ 108 ]. Assembling NT arrays on a highly interconnected 3D
fibrous backbone would reinforce the stability of the electrode. Figure 11 shows
the fabrication stages of hierarchical 3-D hollow TiO 2 nanofibers (H-TiO 2 -NFs)
and the detailed experimental procedure explained in elsewhere [ 108 ].
Figure 12 a shows the QD-sensitized 3-D TiO 2 nanotubes branched on TiO 2
hollow nanofibers (H-TiO 2 NFs). The high resolution TEM images reveal that the
spatially decorated TiO 2 NT arrays on TiO 2 NFs have good contact with the TiO 2
backbone. Furthermore, TEM images (Fig. 12 a, b) suggest that the TiO 2 tubular
branches have sufficiently large pore channels for electrolyte filling as well as good
structural stability. This 3-D photoelectrode was tested and compared with con-
ventional vertically grown TiO 2 nanotube on TCO (TiO 2 -NT). The optical
reflectance spectra (Fig. 12 c) shows high reflectance compared to TiO 2 -NTs in the
wavelength range of 380-800 nm. This might be attributed to the multiple scat-
tering of incident light at the hierarchical TiO 2 NT branches, thus drastically
enhancing the reflectance of the electrode. The photovoltaic performance of
TiO 2 -NT and 3-D H-TiO 2 NFs electrodes were compared in Fig. 12 d. The TiO 2 -
NTs directly grown on a FTO electrode resulted in a photoconversion efficiency of
g = 0.9 % with photovoltage, V oc = 0.62 V, photocurrent, J sc = 2.5 mA cm -2 ,
and fill factor, FF = 58.3 %. As anticipated, the hierarchical TiO 2 nanotube
branches grown on a hollow NF backbone show unprecedentedly promoted to
g = 2.8 % with V oc = 0.61 V, J sc = 8.8 mA cm -2 and F.F. = 50.3 %. It is
clearly evident that the TiO 2 NTs spatially assembled on the hierarchical
3D-nanofibrous backbone promote the QDSSC performance by a factor of three
compared to the TiO 2 NTs directly grown on a TCO substrate. We can relate the
enhancement of photocurrent generation with the H-TiO 2 NF photoanodes to
several contributions: (a) higher effective surface area and consequently higher QD
loading and light harvesting; (b) highly efficient charge collection throughout the
photoanode with fewer boundary layers and (c) the multiple scattering effects of
the comb-like hierarchical NT arrays, in particular, red photon harvesting.
4.2 3-D Tree-Like Branched Hierarchical Nanowire
Designing vertically grown 1-D nanostructures such as nanowire [ 109 - 112 ] and
nanotube [ 113 - 117 ] which directly attached to the charge collectors (TCO) pro-
vides high charge collection efficiency in DSSCs. However, the larger voids
presence in between the nanowire/tube channels lowers the internal surface area of
the electrode [ 118 ]. Promoting internal surface area through introducing hierarchal
branches on 1-D nanostructured stems are beneficial approach for achieving 3-D
photoanodes and offers simultaneously improved surface area and favorable
electron transportation. The resultant 3-D complex nanoarchitectures show many