The role of charge transport in nanoporous films under electromagnetic irradiation

Recently, attention has been paid to surface plasmon waves occurring at an interface as responsible for selective absorption of light and enhancement of the photoelectron current. Interest for plasmon resonance phenomena in photovoltaics is groving, for applications in very thin film-based devices. We discuss in this paper d-c open circuit voltages of nominally undoped mesoporous films of TiO2 of the type used in dye-sensitized cells, under exposure to visible and ambient electromagnetic radiation. In non-sensitized films we interpret these as arising from absorption of electromagnetic energy at plasmon resonances determined by the morphology of the films and subsequent electronic diffusion through the nanocrystals of TiO2. Typical order of magnitude of voltages V(oc) approximately equal to 1 mV indicates an equivalent temperature T approximately equal to 3 K of the radiation involved, which suggests ambient microwave background as responsabile of the effect. The order of magnitude of short-circuit current I approximately equal to 10(-9) A is consistent with experimental data of the diffusion coefficient of the oxide. We find from these data the efficiency of the process is of typical order 1% for nanocrystals of 10 nm typical size. Confirmation of these results are obtained by a Montecarlo simulation of transport taking into account the overall mechanisms of scattering, including phonons, imperfections and doping centres, and traps. We find that TiO2 is a n-type self doped material, with typical doping concentrations N (I) approximately equal to 10(18) cm(-3). The same route is applied to dye-sensitized cells under exposure to solar light for which we find a maximum efficiency of 10% in agreement with certified results on Graetzel's cells, independent of the nanocrystal size. We conclude that the enhancement of the efficiency of these electrochemical cells may be obtained by optimizing the charge current within the mesoporous films, which can be obtained by maximizing the diffusion coefficient.