Interfacial electron transfer from dye molecule excited states into acceptor states of mesoporous metal oxide thin films is the instigating event for current generation in dye-sensitized solar cells. As such, this interfacial electron transfer is important to understand. In this study, electron transfer into three mesoporous metal oxide thin films sensitized with Ru-polypyridyl dyes was quantified as the oxide Fermi energies were tuned with an external applied bias. The conducting material tin-doped indium oxide (ITO) showed strikingly different behavior than did the well-studied wide band gap semiconductor TiO2. It was shown that ITO reductively quenches excited states, transferring an electron from the ITO to the dye. In sharp contrast, TiO2 exclusively quenches by oxidative electron transfer from the dye to the film. The direction for electron transfer was reversed in the conducting material from that in the semiconducting material. The rate of this reductive quenching by ITO was controlled by external bias. Further, reductive quenching was also observed with the semiconductor SnO2, but only under large forward bias conditions. Ultimately, it was shown that the direction of electron flow between metal oxide electrodes and bound dyes was determined by the electronic structure of the oxide, specifically the density of low energy acceptor states. These states overlap with the dye excited state reduction potential, allowing the observed reductive quenching. This novel reactivity of conducting metal oxide electrodes allows cathodic behavior and will be discussed in terms of implications for use in dye-sensitized solar cell applications.

Metal oxide electronic structure determines the direction of electron transfer at dye-sensitized interfaces

2019 Fall National Meeting

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