We employ a model of charged species transport to assess the performance characteristics of a dye-sensitized solar cell. The DSSC is distinguished from conventional semiconductor-based photovoltaics in two key ways. One difference is that an electrolyte phase with a redox reaction pair is substituted for one of the semiconductors that form the p-n junction in conventional photovoltaics. Thus the role of the p-n junction is played by the electrochemical interface between electrolyte and semiconductor. The other difference is that photons are captured by a monolayer of photo-sensitive dye applied to the surface of the semiconductor, rather than collected in the semiconductor bulk. The purpose of the dye is to allow more efficient photon collection by decoupling this step from the material characteristics of the semiconductor. Since ions serve as charge carriers over part of the circuit, the DSSC is a type of galvanic cell.

We consider a system here in which high surface area at the junction is achieved by immersing an array of ZnO semiconducting nanowires into a bath of lithium-iodide-triiodide electrolyte confined between transparent conducting oxide contacts. The nanowires, which function as the anode during cell operation, are in ohmic contact with one electrical contact. Electrolyte separates the nanowires from the other contact, which serves as the cathode. To model this system we solve drift-diffusion equations for ions in the electrolyte and electrons in the nanowire, together with Poisson's equation for electric potential in both phases. Boundary conditions are formulated to represent open and closed circuit cases for both dark and illuminated conditions. Oxidation of iodide at the anode and reduction of triiodide at the cathode are treated using Butler-Volmer kinetics. Voltage-current relationships are computed to determine the internal resistance of the cell, a critical factor in cell efficiency, as a function of operating conditions and cell geometry.