Graphite and platinum's catalytic selectivity for disulfide/thiolate (T2/T−) and triiodide/iodide (I3−/I−)

Dye-sensitized solar cells (DSSCs) using pure graphite sheet as the substrate and catalytic material were first fabricated based on a disulfide/thiolate (T2/T−) redox couple. It can present up to 4.79% of the power-conversion efficiency (PCE), compared with the 2.33% of triiodide/iodide (I3−/I−) redox couple at the same experimental condition under AM 1.5 illumination (100 mW cm−2). In contrast, when FTO/Pt is used as the counter electrode, PCE values are 3.97% and 7.13% based on the T2/T− and I3−/I−redox couples, respectively. With respect to the above dramatic difference, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) are applied to study the catalytic behavior at electrolyte (T2/T− and I3−/I−)/counter electrode (graphite sheet and FTO/Pt) interface. In particular, EIS indicated that the charge transfer resistance (RCT) (1.1 Ω cm2) of the graphite sheet for T2/T− is less than one-ninth of its RCT (10.2 Ω cm2) for I3−/I−. However, the RCT of FTO/Pt (7.7 Ω cm2) for T2/T− is more than twice the RCT (3.0 Ω cm2) of I3−/I−. This demonstrates the high catalytic selectivity of graphite and platinum (Pt) for the two redox couples. These results show the great potential of carbon materials to replace Pt by using this novel electron-transfer mediator in DSSCs.