Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 28

Fig. 9 SEM images of a 20-lm-long TiO 2 nanotube array from anodization of the Ti film
sputter-deposited on FTO-coated glass: a cross-sectional view and b top view. The current-
voltage characteristics (c) and the incident photon-to-current conversion efficiency (IPCE) spectra
of DSSCs fabricated using transparent nanotube array films of various lengths. (Reprinted with
permission from Ref. [ 188 ]. Copyright American Chemical Society)
incoming light using back illumination mode is partially reflected by the counter
electrode and partially absorbed by the counter electrode and iodine in the elec-
trolyte before striking TiO 2 nanotubes, leading to a loss of *25 % of the incident
solar energy [ 180 ]. Furthermore, sintering of the TiO 2 for transformation from
amorphous phase to crystalline phase (i.e. anatase or rutile) can introduce the
formation of a barrier layer between the nanotubes and the underlying Ti substrate.
This enables recombination of electrons and holes when the nanotube layers are
used for photoelectrochemical water splitting [ 181 ]. Fortunately, several strategies
have been explored to solve this deadlock. One straightforward alternative solution
is to deposit titanium as a thin film on a transparent substrate (e.g., FTO glass)
before anodizing. The deposition process is usually performed by physical
methods, for example, radio-frequency (RF) or direct-current (DC) magnetron
sputtering [ 182 - 187 ]. Through anodization of sputtered titanium into nanotube
layers on transparent substrate (Fig. 9 a, b), a power conversion efficiency of 6.9 %
for the resulting DSSCs was obtained (Fig. 9 c, d) [ 188 ]. The length of the
nanotubes is limited by the difficulty of growing a high quality, thick Ti film on the
conductive glass via sputter deposition [ 189 ]. Therefore, a large-area free-standing