Page 410 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for dye-sensitized solar cells 367

Table 13.1 Properties of PET- and PEN-based films [27]
Unit PEN PET

Chemical structure O
O O
O
O O O
O
n n
Tensile strength MPa 280 230
Glass transition °C 155 110
temperature (T g )
Melting point (T m ) °C 269 258
Density g cm 3 1.36 1.40

photoactive dye molecules. TiO 2 layer is generally applied via doctor blade, screen
printing, or spin-coating methods onto TCO-glass and subsequently annealed at tem-
perature of 450°C. The obtained layer has mesoporous structure composed of crystal-
line, well-interconnected particles. The mesoporous character of the structure
provides high dye loading capacity and high surface area for efficient charge gener-
ation and transfer, while its highly crystalline, well-interconnected particles enhance
the charge-transport rate [34].
In the case of flexible DSCs on plastic substrates, annealing at 450°C is not possible
due to the thermal degradation of the substrates at this high temperature. Reports
related to low-temperature-annealed TiO 2 without any additional treatment showed
low performance of the solar cells. Hu et al. [35] produced TiO 2 layer on ITO-PET
substrate by screen printing and subsequent annealing at 100°C for 10 min. The
maximum efficiency they obtained was 0.25%. Due to the poor interconnection
between the particles and the substrate, the whole photocurrent cannot be collected,
2
and thus, J sc has a value of only 1.11 mA cm . Additionally, the low annealing
temperature inhibits elimination of all organic residues from the TiO 2 film [36].
The limiting factors of TiO 2 -based photoanode for flexible solar cells can be summa-
rized as follows:
– Thermal degradation of plastic substrates at high sintering temperature.
– Undesired organic residues in the mesoporous layer.
– Poor interconnection of particles, poor adhesion of TiO 2 to the substrate, and unsuitable
pore structure when sintering at low temperatures.
Therefore, studies have focused on low-temperature synthesis of TiO 2 film with
good mechanical and electric properties. Usage of binder-free precursor solutions
[37,38], new film formation/transfer methods (such as mechanical pressing [39,40],
low-temperature hydrothermal method [41], photoanode transfer [34], and elec-
trophoretic deposition (EPD) [42]), and different posttreatment applications
(UV radiation [43]) are some of the reported techniques in the literature. In this