The performance of dye-sensitized solar cells operating at the maximum power point depends on competition between electron transport and back transfer to the redox electrolyte or hole conducting medium. Most methods to study electron transport involve measuring transient photocurrents under short circuit conditions. However, the distribution of electrons in the cell at the maximum power point differs substantially from the distribution at short circuit, so the question arises whether the transport properties measured at short circuit are relevant to the operating cell. We have used a method introduced by O’Regan eta al (1) in which transient charging of the capacitance of the substrate electrode in response to pulsed illumination under open circuit conditions is used to derive the apparent diffusion coefficient of electrons (the illumination pulse is superimposed on steady state illumination in order to obtain a small increment of photovoltage that allows linearization of the system response). The subsequent decay of the photovoltage allows determination of the apparent electron lifetime. As a consequence, it is possible to measure the electron diffusion length under conditions relevant to the cell operation. Measurements were carried out over a wide range of cells with and without a blocking layer of TiO₂. The charging process was also modeled numerically within the framework of the multiple trapping model. The results give new insights into the behavior of dye-sensitized solar cells under working conditions.

1. O'Regan, B. C.; Bakker, K.; Kroeze, J.; Smit, H.; Sommeling, P.; Durrant, J. R.
J. Phys. Chem. B.; 2006; 110(34); 17155-17160.