Page 428 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for dye-sensitized solar cells 385

Polymer electrolytes
Polymer electrolytes are generally obtained by dissolution of inorganic salts (LiI, NaI,
LiClO 4 , LiCF 3 SO 3 , LiSCN, NaSCN, NaClO 4 , LiPF 6 , etc.) into a high-molecular-
weight, polar polymer hosts (e.g., polyethylene oxide (PEO)). Coordination interac-
tions between the metal ions and the polar groups of the polymer are the main
dissolution mechanism [92–94]. This type of electrolytes should not be confused with
PGEs and polyelectrolytes. In PGEs, polymeric gelling agents and plasticizers are
added into liquid electrolytes in order to promote solidification of the electrolyte
and to increase the ionic conductivity, respectively. However, encapsulated liquid
electrolytes in the gel pores can cause volatilization or leakage as a result of the
increased vapor pressure when the temperature increases [95]. In the case of polyelec-
trolytes, cationic or anionic groups are chemically bonded to a polymer chain, and the
counter ions of these groups are solvated into solvents with high dielectric constant,
hence making the polymers conductive [92]. In this section, firstly, solid-state
polymer electrolyte and then PGE-related studies will be reviewed.
After the measurements of ionic conductivity in PEO polymer-inorganic salt
mixtures done by Wright and coworkers in 1975, PEO attracted big attention due
to its polar and chemically stable nature in polymer electrolytes [96]. According to
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the literature reports, ionic conductivity values of at least 10 –10 4 Scm 1 at room
temperature are applicable for DSCs, which is much higher than that of pure PEO
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(10 –10 4 Scm 1 at 40–100°C) [20]. Additionally, polymer electrolytes in amor-
phous structure are preferable than in crystalline one because of their conduction
mechanism. The mechanism consists of their quick segmental motion and the
Lewis-type acid-base interactions between the cation and the donor atom present in
their structure [95]. However, due to the low crystallization temperature of PEO
(below 60°C) [97] and its low conductivity, many reports have been established to
the increment of ionic conductivity of PEO by incorporation of inorganic nanopar-
ticles [97–99], comonomers [100,101], carbon materials [102], etc. Apart from PEO,
different polymers such as poly(vinyl pyrrolidone) (PVP) [103], poly(ethylene glycol)
(PEG) [104], poly(butyl acrylate) (PBA) [105], and poly(dimethylsiloxane) (PDMS)
[106] and their derivatives have been also applied in DSCs.
The first study related with the addition of inorganic nanoparticles into polymer
electrolytes was reported in 1998 by Croce et al. [97]. In their study, they used
nanometer-sized (nm-sized) TiO 2 and Al 2 O 3 powders as solid plasticizer to kineti-
cally inhibit the crystallization of PEO below 60°C and thus to increase the ionic
3
conductivity at room temperature. They obtained 10 times higher ionic conducti-
vity at 30°C by addition of these nanometer-sized powders due to enlargement of
the amorphous phase in the polymer matrix. After that study, the improvement of
ionic conductivity of polymer electrolytes by addition of inorganic nanoparticles
took its toll. Stergiopoulos et al. [98] have used commercial titania (P25) nano-
particles in PEO-LiI-I 2 -based polymer electrolytes. The addition of TiO 2 particles
separated the polymer chains and arranged them in three-dimensional (3D) network
that creates free space and voids into which the iodide/triiodide anions can easily
3
pass through. TiO 2 /Ru535/(PEO-TiO 2 (I -I ))-based DSC showed 4.2% PCE

(at 65.6 mW cm 1 2 ).