Simulation of the transient photocurrent response of polycrystalline photocells based on equivalent circuit analysis

Abstract

We propose an equivalent circuit representation of the photogenerated charge separation and propagation in dye sensitized polycrystalline semiconductor in contact with a redox electrolyte. The suggested equivalent circuit for this type of photocell is based on an electrical transmission-line, and uses distributed photodiode model for the semiconductor-redox electrolyte interface. It was found that for small signal conditions, diodes can be replaced by equivalent elements, each consisting of a resistor and a capacitor connected in series. The equivalent circuit also provides for Beer-Lambert characteristics of light absorption. The simulation of the transmission line equivalent circuit, subjected to short pulse illumination, allows us to reproduce the experimentally found difference between the photocurrent responses of the photocell to illumination from the electrolyte side, and its response to illumination through the semitransparent back electrode, to which the polycrystalline semiconductor layer is attached. We suggest an electrochemical model for photogenerated charge separation, whereby the electrical field across the Helmholtz electrostatic double layer at the polycrystalline phase - electrolyte interface separates the photogenerated carriers.