How Efficient Is Electron Collection in Dye-Sensitized Solar Cells? Comparison of Different Dynamic Methods for the Determination of the Electron Diffusion Length

The diffusion length of electrons in high efficiency liquid electrolyte dye-sensitized nanocrystalline solar cells has been investigated using two different approaches. The first method is based on measuring the rise and decay times of the small amplitude photovoltage increment generated by a short laser pulse superimposed on a range of steady-state illumination levels. The advantage of this technique is that it allows the simultaneous measurement of the diffusion coefficient and electron lifetime under identical conditions. In addition to transport-controlled substrate charging, direct injection of electrons into the substrate from dye adsorbed at the contact interface was observed at the high laser pulse energies required for measurements at high dc photovoltages. The second method involves using intensity-modulated photocurrent and photovoltage spectroscopies (IMPS and IMVS, respectively) to measure the electron diffusion coefficient and electron lifetime at short circuit and open circuit, respectively, as a function of light intensity. The difference between the electron trap occupancies under open-circuit and short-circuit conditions must be accounted for in this case. The diffusion lengths derived from the study are in the range of 40-70 mu m, which are at least an order of magnitude greater than the film thickness. This indicates that the electron collection efficiency in the cells is close to 100%.