Local charge compensation of electrons in dye-sensitized mesopo-rous TiO2 studied by Stark effects

Wenxing Yang^a, Meysam Pazoki^a, Anna Eriksson^a, Yan Hao^a, Gerrit Boschloo^a

^aUppsala University, Sweden, Uppsala, Sweden

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Wenxing Yang, 400
Publication date: 5th February 2015

Electrons in mesoporous TiO₂ electrodes are charge compensated by cations in the surrounding electrolyte. In dye-sensitized electrodes, the electric field between electrons and cations causes a spectral shift in the absorption spectrum of the adsorbed dye molecules through a Stark effect. This implies that the charge compensating cations are at least partially located outside the dye monolayer. Using the organic dye LEG4, transient absorption measurements upon light or electrical modulation reveal a relatively slow (t > 0.1 s) disappearance of the Stark bleach during the modulation period, which can be attributed to penetration of Li⁺ ions into the adsorbed LEG4 monolayer, leading to a more localized charge compensation of the electrons in TiO₂. Probably these ions are adsorbed onto the electrode surface (electrosorption). When the modulation is removed, a reverse Stark effect is firstly observed, which is attributed to penetrated (absorbed) lithium ions now being charge compensated by anions in the bulk electrolyte, giving a reversed electric field across the dye layer. These findings give new insights into the mesoporous TiO2/dye/ electrolyte interface and charge compensation. It is proposed that electrosorbed cations can act as electrostatic trap states for the electrons in the mesoporous TiO2 electrode.
Schematic picture of charge compensation process proposed at the TiO2/dye/electrolyte interface

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