Page 411 - Polymer-based Nanocomposites for Energy and Environmental Applications

368 Polymer-based Nanocomposites for Energy and Environmental Applications

section, different photoanode formation and transfer techniques for deposition of TiO 2
on flexible polymeric substrates will be discussed in detail, and examples will
be given.

Binder-free TiO 2 paste
The first major challenge for DSC fabrication on plastic substrates is the preparation
of suitable TiO 2 pastes with specific rheological properties without addition of any
organic binder or dispersant. In general, binders allow the production of viscous pastes
and stable and efficient films on plastic substrates, while dispersants prevent crack
formation in the annealed film by inhibiting agglomeration of TiO 2 nanoparticles
(NPs). However, these organic materials remain in the film structure after low-
temperature heat treatment and adversely affect the PCE. Weerasinghe et al. [37] pre-
pared binder-free TiO 2 pastes by mixing commercial 21 nm-sized TiO 2 (P25 TiO 2 )
powders and ethanol either by ball milling or in a mortar. The prepared pastes were
deposited onto ITO-PEN substrates by doctor-blade and spin-coating methods
followed by heat treatment at 150°C for 30 min. Ball milling of P25 TiO 2 powder
was found to increase the film quality, its mechanical stability, and the surface area
of the TiO 2 films. Additionally, they observed higher photovoltaic performance and
dye loading as the milling time increased. As a final product, they obtained a plastic-
based DSC with maximum PCE of 4.2%.
In other film production methods, such as screen printing or gravure printing, pas-
tes with high viscosity are required. The commonly used water- or alcohol-based
binder-free TiO 2 slurries usually exhibit very low viscosity. However, their rheolo-
gical behavior can be modified by addition of ammonia, hydrochloric acid (HCl),
and water. In another study, Weerasinghe et al. [44] investigated the effect of acidity
and basicity of TiO 2 -ethanol slurry and its water content on its rheological properties.
They observed that water addition (up to 20%) has larger effect on the viscosity than
the addition of HCl and ammonia. Hence, better solar cell performance was achieved
for the ethanol-based pastes containing acid or water. PCE values of 4.9% and 5.0%
were reported for the plastic-based DSCs fabricated using water and acid-added
slurries, respectively.
Another way to improve the bonding of the titanium particles and the viscosity of
the paste is addition of small amount of Ti precursors into the paste slurry. Titanium
isopropoxide (TIPP, Ti{OCH(CH 3 ) 2 } 4 ), titanium tetrachloride (TiCl 4 ), and titanium
oxysulfate (TiOSO 4 ) are some of the Ti precursors used. Among them, the titanium
tetraisopropoxide (TTIP) that has metal’oxygen’carbon linkages is the most widely
used metal-organic compounds. Alike other metal alkoxides, the popularity of TTIP
originated from its ability to hydrolysis and condensation reactions that provide a
binder-like effect for the presynthesized TiO 2 particles. The chemical reactions of
TTIP with water are shown in Eq. (13.1), Eq. (13.2). Due to the high reactivity of
alkoxide ligands, hydrolysis reaction can take place within seconds. In order to slow
down the reaction rate, alkoxides are generally diluted with alcohol and some addi-
tives (such as citric acid and acetic acid). The stability of the solution, the