Current Density versus Potential Characteristics of Dye-Sensitized Nanostructured Semiconductor Photoelectrodes. 2. Simulations

The impact of changes in various parameters on the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes has been evaluated through a series of simulations. The model parameters can be divided into three classes, designated as type I, type II, and type III, respectively. Type I parameters primarily affect the open-circuit potential without changing the overall shape of the J−E curves. Type II parameters primarily affect the limiting quantum yield for photocurrent production. Rate constants for injection and quenching of the excited state of the dye by the contacting phase are type II parameters, whereas the rate constant for the direct electrolyte reduction reaction, the reduced equilibrium constant for iodine formation, and the rate constant for recombination are type I parameters. The rate constant for regeneration affects both the shape of the J−E curves and the limiting quantum yield for photocurrent flow, and it is therefore designated as a type III parameter. Variation of the diffusion coefficient parameter for electrons in the semiconducting membrane produces a mixed response having both type I and II characteristics. Comparisons between simulations and experimental data have delineated the factors that should be manipulated to increase the energy conversion efficiency of photoelectrochemical cells that utilize nanostructured TiO_2 electrodes