Photovoltaic Effect at Dye–Photoconductor Interfaces

Photovoltages at the junction of a solid-dye film and a cadmium sulfide film were measured where the dye was charged positively with respect to the cadmium sulfide. For blue pinacyanole the photovoltage is approximately equal to the contact potential difference between the dye and cadmium sulfide and is only weakly affected by illumination. On the other hand, for violet pinacyanole and salts of crystal violet the photovoltage is strongly illumination dependent and is only a measure of the relative density of charge carriers in the cadmium sulfide and the dye. In the latter case, equilibrium exists across the junction in the dark, while for blue pinacyanole the behavior suggests that in the dark no charge carriers are exchanged between the dye and cadmium sulfide. The photovoltaic results are consistent with a localized conduction process in organic dyes and a charge-transfer mechanism of dye sensitization. However, only in the case of blue pinacyanole do the photovoltages have any predictive value for dye sensitization.