The Dark Current at the TiO2 Electrode of a Dye-Sensitized TiO2 Photovoltaic Cell

One of the major factors limiting the performance of the dye-sensitized TiO2 photovoltaic cell is the rapid increase in dark current at potentials above 0.5 V. This current is due to the reduction of the oxidant within the electrolyte at the dye-sensitized TiO2 electrode. Owing to the porous nature of the TiO2 film, the dark reaction can occur at either the TiO2 surface or the surface of the conducting glass. Dark current measurements were made on Libbey -Owens-Ford TEC 10 conducting glass and on the same glass coated with electrophoretically deposited Degussa P-25 TiO2 covered with a monolayer of the sensitizing dye cis-di( thiocyanato ) bis (2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) in an electrolyte consisting of the I3-/I- couple in 1:1 propylene carbonate/ethylene carbonate. The contribution to the dark current resulting from the reaction at the surface of the conducting glass was relatively small at ambient temperatures. The dark reaction at the surface of the dye-sensitized TiO2 was found to make the major contribution to the dark current under these conditions. Tentative results for the Tafel parameters which indicate that the dark reaction occurs via the reduction of both I3- and I2, which are in equilibrium, are also presented.