Nature of photovoltaic action in dye-sensitized solar cells

We explain the cause for the photocurrent and photovoltage in nanocrystalli ne, mesoporous dye-sensitized solar cells, in terms of the separation, reco mbination, and transport of electronic charge as well as in terms of electr on energetics. On the basis of available experimental data, we confirm that the basic cause for the photovoltage is the change in the electron concent ration in the nanocrystalline electron conductor that results from photoind uced charge injection from the dye. The maximum photovoltage is given by th e difference in electron energies between the redox level and the bottom of the electron conductor's conduction band, rather than by any difference in electrical potential in the cell, in the dark. Charge separation occurs be cause of the energetic and entropic driving forces that exist at the dye/el ectron conductor interface, with charge transport aided by such driving for ces at the electron conductor/contact interface. The mesoporosity and nanoc rystallinity of the semiconductor are important not only because of the lar ge amount of dye that can be adsorbed on the system's very. large surface, but also for two additional reasons: (1) it allows the semiconductor small particles to become almost totally depleted upon immersion in the electroly te (allowing for large photovoltages), and (2) the proximity of the electro lyte to all particles makes screening of injected electrons, and thus their transport, possible.