Dependence of the electrocatalytic performance of platinized counter electrodes on the redox mediator employed in dye-sensitized solar cells

The electrocatalytic properties of platinized counter electrodes (Pt CEs) prepared by various coating methods were investigated with respect to the redox mediator, including the widely used iodide/tri-iodide (I^?/I ₃ ^? ) and the more recently introduced cobalt(II/III)tris(2,2′-bipyridine) (Co(bpy) ₃ ^2+/3+ ), for application in dye-sensitized solar cells (DSCs). The coating methods controlled Pt loading and the surface morphology of the Pt CEs. For a high-performance DSC with a fill factor >0.7, the charge-transfer resistance at the Pt CE/electrolyte interface should be <4.5 Ω cm² for both redox mediators. The I^?/I ₃ ^? -mediated DSCs were insensitive to Pt loading as low as 0.001 mg cm^?2, while the Co(bpy) ₃ ^2+/3+ -mediated DSCs required relatively large Pt loadings of > 0.005 mg cm^?2. Our results indicated that care should be taken in the preparation of Pt CEs with high transparency and low loading to obtain high-performance DSCs employing cobalt–ligand redox electrolyte.