Up to now, noble metal-loaded substrates have already been widely used as the standard for the CE of DSCs, due to their unique properties, including (1) high electrochemical activity that can reduce the voltage loss due to charge-transfer overpotential of CE; (2) a low charge transfer resistance which can lead to minimum energy loss. A thin layer of noble metals, e. g. Pt, Au, is well established as the catalyst on CE substrate, such as TCO glass and metal foil. One of the important roles of noble metal in CE is to catalyze the reduction of triiodide (I3-) ions. This means that the available catalytic surface in the electrode plays a crucial role in determining the overall device current. So the rough/porous electrodes, which are characterized by a higher surface, are expected to assure a higher number density of catalytic sites. It is obvious that the use of metal nanoparticle film results in CE with high surface area. Therefore, the preparation methods of noble metal films will influence the final film structure/properties.
Generally all the synthetic methods of the noble metal film are roughly divided into two categories: the physical and chemical approaches. The difference between the two approaches arises from the starting point in the synthetic route to prepare the films. For the physical approaches, the film is prepared by the macroscopic precursors through subsequent subdivision in ever smaller particles by strong milling of solids or through lithographic processes including sputtering, laser ablation, vapor phase deposition, lithography, etc. While the chemical approaches start from their atomic and molecular precursors, through chemical reactions and modulating their self-assembling in order. The physical approaches (Terauchi et al., 1995) are generally quite expensive and resourceconsuming, while chemical methods are generally cheaper and better fit for large scale applications. Moreover, since chemical methods allow in principle the control at a molecular level, they allow a fine size and polydispersity control and can be implemented to prepare 2D and/or 3D nanoparticle arrays to enhance the available catalytic site.
The catalytic activity is expressed in terms of the exchange current density (Jo), which is calculated from the charge-transfer resistance (Rct) using the equation Rct = RT/nFJ0, in which R, T, n, and F are the gas constant, temperature, number of electrons transferred in the elementary electrode reaction (n = 2) and Faraday constant, respectively. Yoon et al (Yoon et al., 2008) used Nyquist plots to investigate Rct on platinized TCO CEs prepared by either the electrochemical deposition (ED) method, the sputter-deposited (SD) method, or the thermal deposited (TD) method. The Nyquist plots suggest qualitatively that Rct of a cell increases in the order of Rct of ED-Pt film < Rct of SD-Pt film < Rct of TD-Pt film. The DSC fabricated with the ED-Pt CE rendered the highest power conversion efficiency of 7.6%, compared with approximately 6.4% of the cells fabricated with the SD-Pt or most commonly-employed TD – Pt CEs. The improved performance of DSC with the ED-Pt CE is attributed to the improved catalytic activity of the reduction reaction (I3- +2er^3I-) and the decreased charge transfer resistance at CE/ electrolyte interface. Hauch et al (Hauch & Georg, 2001) also used impedance spectra to investigate Rct on platinized TCO CEs prepared by either the electron – beam evaporation (EB), SD or TD method. The 450 nm thick platinum electrode prepared by SD method gave the lowest Rct of of 0.05 ohm/cm2. The TD of a Pt film (<10 nm thick), using H2PtCl6 as a precursor on a TCO substrate, produced a low Rct of 1.3 ohm/cm2 comparable to the Rct of the 40 nm thick sputtered Pt, confirming the superiority of the TD method.