Page 412 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for dye-sensitized solar cells 369

final size, and the morphology of the TiO 2 particles are determined by water/TTIP
ratio [45]:
ð
Ti ORÞ +4H 2 O ! 2Ti OHð Þ + 4ROH hydrolysisÞ (13.1)
ð
4
4
ð
ð
Ti OHÞ ! TiO 2 xH 2 O+ 2 xð ÞH 2 O condensationÞ (13.2)
4
In a similar fashion, Kim and Hwang [38] prepared TiO 2 paste by mixing certain
amount of P25 TiO 2 particles, ethanol, deionized water, and TTIP in order to reduce
the required sintering temperature. In their study, they prepared pastes with different
TTIP/TiO 2 molar ratios and found 0.1:1 as the optimal ratio in terms of dispersion
stability, transparency, microstructure quality, and flexibility strength. Flexible
photoanodes were produced by simple dip-coating method followed by annealing
at 120°C for 1 h. The PCE and V oc values for this flexible DSC were 1.96% and
0.65 V, respectively. They also produced DSC on ITO-glass at low temperature by
using the same paste and obtained 2.93% PCE. They attributed this lower PCE
1
of the flexible device to the higher sheet resistance of ITO-PEN (20–25 Ω sq )in
1
comparison with ITO-glass (5 Ω sq ).
New film formation/transfer methods
Several methods have been used for low-temperature synthesis of TiO 2 films that can
provide solar cells with enhanced PCE. The following section reviews these methods,
which can be categorized as mechanical pressing, etch transfer, friction transfer, and
UV or UV-O 3 irradiation.
Mechanical pressing is a low-temperature route for the production of porous TiO 2
films with highly interconnected particles on plastic substrates. In this method, firstly,
TiO 2 film is deposited by doctor-blade method using ethanol-based TiO 2 slurry. After
the evaporation of ethanol from the deposited suspension, the coated substrate is com-
2
pressed between two planar steel press plates under pressure of like 1000 kg cm .
Lindstr€ om et al. [39] reported a sandwich-type, flexible DSC with 4.9% PCE fabri-
cated by mechanical pressing of TiO 2 and Pt-SnO 2 powders on ITO-PET substrates.
They explained that the press technique provides the production of films that have
similar mechanical properties and pore structures with the high-temperature sintered
ones and can be achieved in a much shorter time. Yamaguchi et al. [40] also used
pressing technique (100 MPa) to produce flexible DSC and obtained 6.5% and
7.1% PCE using ethanol- and water-based TiO 2 slurries, respectively. According to
them, even after pressing and heat treatment (150°C, 10 min), some organic residues
stayed within the photoanode structure causing reduction in the efficiency.
In order to use the advantages of high-temperature annealing, Yoo et al. [32]
introduced a new method to construct flexible, TCO-free DSC that includes high-
temperature annealed working and CEs on plastic substrates. In their method, after
coating and annealing of mesoporous TiO 2 layer onto a glass, they transferred the
layer on plastic substrate with the help of thermal adhesive (Bynel) film and HF solu-
tion. To achieve this, the deposited TiO 2 layer was firstly attached to a PEN substrate
via Bynel film, and then, the affixed substrates were dipped in 5 wt% aqueous solution