The properties of TiO2 and polyphenyl oxide (PPO) blocking layers were compared using a highly positive redox shuttle in dye-sensitized solar cells. The dark current versus applied potential curve was found to be composed of two separate current features. Cyclic voltammetric and impedance measurements were performed to identify the source of the two features. The first feature results from electron transfer from the TiO2 blocking layer and the first layer of the TiO2 nanoparticle film contacting the substrate. The second onset of dark current results from the transport resistance in the nanoparticle film decreasing, thus allowing electron transfer to occur throughout the film. It is further demonstrated that PPO prevents back electron transfer from the conductive substrate to the redox shuttle across the entire potential range studied; however, the TiO2 blocking layer is active and participates in electron transfer at rates similar to those of the TiO2 nanoparticle film.