Time-Resolved Electroluminescence of Dye Sensitized Solar Cells

Abstract

The electroluminescence of dye sensitized solar cells is measured under forward bias as a function of time after the cells are connected to and disconnected from an external voltage source. At low applied voltages, which correspond to forward currents smaller than the short circuit current under AM1.5 illumination, the electroluminescence intensity reaches its steady state within a few hundred milliseconds. The observed signals are very similar to time dependent photocurrents measured after the illumination is switched on. In this voltage range the electroluminescence intensity increases exponentially with the applied voltage in accordance with the predictions of the generalized Planck equation. Surprisingly, at larger applied voltages the electroluminescence intensity exhibits a maximum as a function of time and its steady-state value decreases with increasing applied voltage. A simple numerical model and measurements with cells containing different electrolytes suggest that this unexpected feature of the dye sensitized solar cell can be assigned to the depletion of the oxidizing species of the redox couple in the electrolyte. Energy transfer reactions from the excited state of the dye to TiO2 conduction band electrons, which have been proposed for this observation, are rather improbable.