Photoelectrochemical Response of DSSCs Under Prolonged Reverse Bias and Conduction Band Lowering in Ru‐Complex‐Sensitized TiO2

TiO2-based dye-sensitized solar cells (DSSCs) sensitized with the Ru-dyes N719 and Z907, are subjected to prolonged reverse bias (RB) by imposing a constant current density of 25 mA cm−2 for up to 1000 h. The temporal changes of the DSSCs during the aging process are followed by the periodical recording of current–voltage curves, electrochemical impedance spectra, charge extraction measurements, and photovoltage decays at the various stages of cell aging. Beside the expected decrease in the concentration of during RB aging, the measurements show a surprising shift of the conduction band edge toward lower energies and a concomitant slowing down of the recombination process with the electrolyte for both DSSCs. This leads to a noticeable improvement in the energy conversion efficiency under low illumination conditions. These unexpected results are discussed on the basis of recent literature reports and attributed tentatively to the adsorption of H+ ions, which are produced during the RB aging, onto the TiO2 surface.