Page 408 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for dye-sensitized solar cells 365

Front illumination
10

Efficiency (%) 8 6

Back illumination
4

2
5 10 15 20
Thickness (μm)
Fig. 13.3 Effects of front and backside illumination and TiO 2 thickness on efficiency of
glass-DSC.
Reproduced with permission from Ito S, Ha N-LC, Rothenberger G, Liska P, Comte P,
Zakeeruddin SM, et al. High-efficiency (7.2%) flexible dye-sensitized solar cells with Ti-metal
substrate for nanocrystalline-TiO2 photoanode. Chem Commun 2006;4004–6.

and (3) both PE and CE on plastics. Although stability of metal foils at temperatures
as high as 500°C brings advantages like better electron collection and transport prop-
erties for MOS layer, they require backside illumination that results in lower cell
performance due to light absorption by the CE and electrolyte before reaching the
dye molecules (Fig. 13.3) [19]. Additionally, the risk for corrosion of the metal foils
in contact with the electrolyte and the necessity of cutting the metal substrates into
individual cells to guarantee electric isolation in the production of large-area solar
panels hinder the use of metal substrates in low-cost DSCs. On the other side, the
use of transparent plastic substrates provides advantages like optical transparency,
chemical inertness, and facile electric isolation for the production of low-cost, higher
performance solar cells [14,20]. Therefore, this section will focus only on polymer-
based nanocomposites in flexible sensitized solar cells, and the solar cells on metal
substrates will not be mentioned here.
Plastic films like polyimide (PI), PET, and PEN are widely used in solar cell market
[21–23]. ITO layer is generally coated on the substrates by direct current/radio
frequency (DC/RF) magnetron sputtering methods [24]. In this method, ITO is depo-
sited from sintered ceramic In 2 O 3 targets containing 3–10 wt% SnO 2 . Additionally,
there are industrial R2R sputter coating system for ITO-PET/PEN fabrication.
Fig. 13.4 shows schematic of industrial sputtering R2R system.
ITO-PET or ITO-PEN have higher transparency than ITO-PI and also have nearly
the same electric conductivity, stability against iodine-based electrolytes, and cost as
FTO-glass [26]. PEN films show better physical properties, such as higher T g and T m ,
better oxygen and moisture barrier, and higher tensile strength than PET films [27].
General properties of PET and PEN films are shown in Table 13.1. The differences
in the physical properties of PET and PEN have generally been attributed to