Stark effects in D35-sensitized mesoporous TiO2: influence of dye coverage and electrolyte composition

Strong Stark effects are visible in the spectra of the organic donor-pi-acceptor dye D35 when it is adsorbed onto mesoporous TiO2, both under steady state conditions and under modulated light conditions. The addition of lithium cations to the electrolyte results in a significant red shift of the D35 absorption spectrum, which is attributed to adsorption of Li+ at the TiO2 surface, resulting in a change of the electric field across the adsorbed dye molecules. The dye molecules must therefore be located inside the Helmholtz double layer at the TiO2/electrolyte interface. In photoinduced absorption (PIA) spectroscopy, modulated light is used to excite dye molecules. A significant Stark bleach is found in PIA spectra, which corresponds to a blue shift of the dye absorption spectrum upon addition of electrons to TiO2, due to an electric field across the dye monolayer. The observed bleach is reduced when the concentration of supporting electrolyte is increased, indicating local charge compensation of the electrons in TiO2 by adsorbed cations. Transient absorption studies reveal that screening of the interfacial electric field is faster at lower dye coverage compared to full monolayer coverage. Analysis of the Stark effect in dye-sensitized solar cell gives valuable information on the mechanism of charge compensation of electrons in mesoporous electrodes.