Measuring charge transport from transient photovoltage rise times. A new tool to investigate electron transport in nanoparticle films

Charge transport rate at open-circuit potential (V-oc) is proposed as a new characterization method for dye-sensitized (DS) and other nanostructured solar cells. At V-oc, charge density is flat and measurable, which simplifies quantitative comparison of transport and charge density. Transport measured at V-oc also allows meaningful comparison of charge transport rates between different treatments, temperatures, and types of cells. However, in typical DS cells, charge transport rates at V-oc often cannot be measured by photocurrent transients or modulation techniques due to RC limitations and/or recombination losses. To circumvent this limitation, we show that charge transport at V-oc can be determined directly from the transient photovoltage rise time using a simple, zero-free-parameter model. This method is not sensitive to RC limitation or recombination losses. In trap limited devices, such as DS cells, the comparison of transport rates between different devices or conditions is only valid when the Fermi level in the limiting conductor is at the same distance from the band edge. We show how to perform such comparisons, correcting for conduction band shifts using the density of states (DOS) distribution determined from the same photovoltage transients. Last we show that the relationship between measured transport rate and measured charge density is consistent with the trap limited transport model.