Page 416 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer nanocomposites for dye-sensitized solar cells 373

mediator and to get high solar cell performance [48]. Additionally, for effective CEs,
electric conductivity and stability against the electrolyte system are other important
issues. In this section, we will review three types of materials used for the formation
of CE, namely, Pt- and carbon-based materials and conductive polymers.

Pt-based flexible CEs
In rigid DSCs, platinized TCO-glass is commonly used due to its high catalytic activ-
ity, electric conductivity, and chemical stability [49]. For the production of platinized
TCO-glass (Pt-TCO-glass), electron beam evaporation, thermal decomposition, or
sputtering methods are generally used. In thermal decomposition, commonly,
hexachloroplatinic acid (H 2 PtCl 6 ) is coated on TCO-glass substrate, and subse-
quently, high-temperature annealing (300–400°C) is applied. According to Hauch
and Georg [50], the performance of Pt-CEs is directly related with the thickness of
Pt layer and the deposition method. They investigated the charge-transfer resistance
(R ct ) on platinized TCO CEs prepared by either electron beam evaporation, sputter, or
thermal decomposition methods. Their results reveled that thermally produced Pt-CEs
give better performance. In the thermal decomposition method, an R ct of 1.3 Ω was
obtained from only 10 nm-thick Pt layer that was obtained from 40 nm-thick Pt layer
in the sputtering method. However, in case of plastic substrates, sputtered Pt has often
been the preferred choice [14].
The instability of TCO-PET or PEN against high temperatures, the high cost of Pt,
and the required vacuum systems for coating Pt onto flexible substrates are main
obstacles for flexible and cheap solar cells. In order to reduce the cost and to be able
to produce flexible CE, methods like electrochemical deposition [51] dipping [52],
spraying [34,52], and screen printing [18,53] and different materials like carbon
and its derivatives [18,54–56], conductive polymers [18,57–59], or inorganic mate-
rials [60–64] have been introduced.
Wei et al. [51] compared the electron-beam-evaporated Pt film with FTO-glass
(Pt-FTO-glass) and room-temperature pulsed electrochemical deposited (ECD)
Pt-NPs with ITO-PET in terms of catalytic activity and performance of DSC. As stated
in the results, Pt-FTO-glass and the corresponding DSC showed the highest catalytic
activity and PCE (5.5%). In the case of pulsed ECD, the catalytic activity and perfor-
mance of the corresponding DSC were increased with increasing the number of depo-
sition cycles. Fig. 13.8A and B shows the dependency of magnitude of the catalytic
peak current, J sc and FF to the number of deposition cycles. This effect was attributed
2
to the increased surface roughness. The highest J sc (10.98 mA cm ), V oc (0.65 V),
FF (60.2), and PCE (4.30%) were obtained from the DSC that used 200 cycles pulsed
ECD Pt-NPs on ITO/PET-based CE.
Wang et al. [52] prepared low-temperature Pt-based CE on ITO-PEN substrate
using simple dip-coating method. They produced p-octyl polyethylene glycol phenyl
ether (Triton X-100)-capped Pt-NP-based ink by adding certain amount of sodium
borohydride (NaBH 4 ) into H 2 PtCl 6 -Triton X-100-deionized water solution. An
ITO-PEN film was then immersed into this ink for 5 min at room temperature and
dried at 130°C for 30 min. For comparison, they produced Pt-ITO-PEN film by CR