Dependence of the Efficiency Improvement of Black‐Dye‐Based Dye‐Sensitized Solar Cells on Alkyl Chain Length of Quaternary Ammonium Cations in Electrolyte Solutions

Dependence of the suppression of the backward electron transfer reaction from the TiO2 photoelectrode to I3(-) in the electrolyte on the alkyl chain length of the quaternary ammonium cation has been investigated for further efficiency improvement of high-performance cosensitized dye-sensitized solar cells (DSCs). The tetraheptylammonium cation was found to be more effective than the tetraethylammonium and tetrabutylammonium cations for the suppression of the backward electron transfer reaction without changing the conduction band energy of TiO2. 12.0% conversion efficiency, which is the second highest efficiency for DSCs based on ruthenium sensitizers, was achieved in the cosensitized DSC with Black dye and D131 by using an electrolyte solution containing a moderate concentration of tetraheptylammonium iodide.