Efficiency limitations in dye-sensitized solar cells caused by inefficient sensitizer regeneration

Abstract

It is widely believed that the prototypical ruthenium dyes N719 and Z907 are regenerated by iodide with near unity quantum yield following photo-oxidation in dye-sensitized solar cells (DSCs). However, the incident photon-to-current efficiency (IPCE) of DSCs using these dyes decreases with increasing forward bias, limiting power conversion efficiency (η) compared to the hypothetical constant-IPCE case. This phenomenon could arise due to incomplete regeneration, but despite the important implications for cell efficiency, it has received little attention. DSCs employing electrolytes with different iodide concentrations and the Z907 sensitizer have been characterized using complementary photoelectrochemical techniques to test whether the decrease in IPCE is caused by inefficient regeneration. The results strongly suggest that this is the case, even for abnormally high iodide concentrations, where η is reduced by as much as 30% by the effect. Similar results are obtained with the N719 sensitizer. Interestingly, the predicted reduction in photovoltage is partially offset by a change in the electrostatic potential drop across the Helmholtz layer at the TiO2-electroyte interface, which has an estimated microscopic areal capacitance in the range 2.3-9.3 μF cm -2. These findings suggest that it will be important to carefully consider sensitizer regeneration kinetics and interfacial electric fields to further improve the efficiency of DSCs. © 2011 American Chemical Society.