Investigation of the kinetics of the back reaction of electrons with tri-iodide in dye-sensitized nanocrystalline photovoltaic cells

A novel technique has been developed to study the kinetics of the back reaction of electrons with I-3(-) in dye-sensitized nanocrystalline cells. A solid-state switched operational amplifier feedback circuit controlling the cell is used to alternate open-circuit and short-circuit conditions. In the experiment, electrons are injected by the photoexcited dye during illumination at open circuit, and then the subsequent decay of the open-circuit photovoltage in the dark is recorded up to a given time, at which the cell is short-circuited abruptly. The electron charge extracted at short circuit is measured by a current amplifier and integrator device. The kinetics of electron decay at open circuit has been studied by varying the delay between interrupting the illumination and short-circuiting the cell. Analysis of the time dependence of the electron density has established that the decay process is second-order in the total electron concentration. This is consistent with a mechanism involving the formation of I-2(-.) as an intermediate. The pseudo-second-order rate constant for the reaction of electrons with tri-iodide was found to be 1.9 x 10(-17) cm(3) s(-1) for [I-3(-)] = 0.05 M at 24 degrees C.