On the Mass Transport in Apparently Iodine-Free Ionic Liquid Polyaniline-Coated Carbon Black Composite Electrolytes in Dye-Sensitized Solar Cells

Volatile electrolytes are a stability concern in dye solar cells (DSCs) due to their tendency for leakage. A composite electrolyte consisting of iodide-based ionic liquid and polyaniline-coated carbon black has been previously reported to provide good current transport while being leakage proof due to a quasi-solid structure and the absence of volatile constituents. In this paper we investigate the operating principle of this type of electrolyte and especially its exceptional feature of operating efficiently without added iodine. The absence of additive iodine is significant due to the fact that it is usually required to form the current carrying the I-/I-3(-) redox couple. We modified an electrolyte mass transport model from the literature to estimate the upper limit for the charge transport capability of the composite electrolyte. Comparison of experimental results with the estimated upper limit for the diffusion limiting current density shows clearly that the high current densities observed experimentally with the composite electrolyte can not be explained with normal diffusion even in the case that every feasible source and transport mechanism of free I-3(-) known until now is considered, including photogeneration of I-3(-), shortened diffusion layer thickness, impurity I-3(-), and accumulation of I-3(-) to the photoelectrode from the counterelectrode pores and electrolyte edge regions. This intriguing result suggests a currently unknown I-3(-) source or transport mechanism in this type of DSC.