Photoelectrochemical response of DSSCs under prolonged reverse bias and conduction band lowering in Ru-complex sensitized TiO2

TiO2-based DSSCs sensitized with the Ru-dyes N719 and Z907, have been subjected to prolonged reverse bias 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 have been followed by means of the periodical recording of I-V curves, electrochemical impedance spectra, charge extraction measurements and photovoltage decays at the various stages of cell aging. Beside the expected decrease of the concentration of I3- during RB aging, the measurements evidenced for both DSSCs a surprising shift of the conduction band edge toward lower energies and a concomitant slowing down of the recombination process with the electrolyte. This led to a noticeable improvement of 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, produced during the RB aging, on the TiO2 surface.