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Article

Dye-Sensitized SnO₂ Electrodes with Iodide and Pseudohalide Redox Mediators

Bryan V. Bergeron,^† Andras Marton,^† Gerko Oskam,^‡ and Gerald J. Meyer*^†

Department of Chemistry and Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, and Departamento de Fsica Aplicada, CINVESTAV-IPN, Mrida, Yuc. 97310, Mexico

J. Phys. Chem. B, 2005, 109 (2), pp 937–943

DOI: 10.1021/jp0461347

Publication Date (Web): December 21, 2004

†

The Johns Hopkins University.

‡

CINVESTAV-IPN.

To whom correspondence should be addressed.

Abstract

Dye-sensitized mesoporous nanocrystalline SnO₂ electrodes and the pseudohalogen redox mediator (SeCN)₂/SeCN^- or (SCN)₂/SCN^- or the halogen redox mediator I₃^-/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, Γ₀ = 1.7 × 10^-8 mol/cm², and surface-adduct formation constant, K_ad = 2 × 10⁵ 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 > 10⁸ 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_oc) were comparable for (SeCN)₂/SeCN^- and I₃^-/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 50−60 mV/decade. Capacitance measurements of the SnO₂ electrode in the presence of I₃^-/I^-, (SeCN)₂/SeCN^-, or (SCN)₂/SCN^- are reported.