Dye-sensitized SnO2 electrodes with iodide and pseudohalide redox mediators

Dye-sensitized mesoporous nanocrystalline SnO2 electrodes and the pseudohalogen redox mediator (SeCN)(2)/ SeCN- or (SCN)(2)/SCN- or the halogen redox mediator I-3(-)/I- were implemented for regenerative solar cell studies. Adsorption isotherms of the sensitizers Ru(deeb)(bpy)(2)(PF6)(2), Ru(deeb)(2)(dpp)(PF6)(2), and Ru(deeb)(2)-(bpz)(PF6)(2), where deeb is 4,4'-diethylester-2,2'-bipyridine. dpp is 2,3-dipyridyl pyrazine, and bpz is bipyrazine, binding to the SnO2 surface were well described by the Langmuir model from which the saturation coverage. Gamma(0) = 1.7 x 10(-8) mol/cm(2), and surface-adduct formation constant, K-2d = 2 x 10(5) M-1, were obtained. Following excited-state interfacial electron transfer. the oxidized sensitizers were reduced by donors present in the acetonitrile electrolyte as shown by transient absorption spectroscopy. With iodide as the donor. a rate constant k > 10(8) s(-1) was measured for sensitizer regeneration. In regenerative solar cells, it was found that the incident photon-to-current conversion efficiencies and open circuit voltages (V-alpha) were comparable for (SeCN)(2)/SeCN- and I-3(-)/I- for all three sensitizers. The V-oc varied linearly with the logarithm of the short circuit photocurrent densities (J(sc)), with typical correlations of similar to50-60 mV/decade. Capacitance measurements of the SnO2 electrode in the presence of I-3(-)/I-, (SeCN)(2)/SeCN-, or (SCN)(2)/SCN- are reported.