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Electrons can be injected into TiO2 nanocrystals by a surface-adsorbed Ru-based dye in a well-known photosensitization process. Here we present time-resolved microwave conductivity measurements of the decay of electrons out of an electrically interconnected network of such TiO2 nanocrystals, following charge injection, and in the presence of an electron donor to the dye cation that prevents direct recombination. The time scale for this decay process is hundreds of milliseconds to seconds and explains the high current collection efficiency in devices based on these materials, since the long time scale allows for slow charge transport through the network.

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