Determination of the density and energetic distribution of electron traps in dye-sensitized nanocrystalline solar cells

Electron transport and recombination in dye-sensitized nanocrystalline solar cells (DSCs) are strongly influenced by the presence of trapping states in the titanium dioxide particles, and collection of photoinjected electrons at the contact can require times ranging from milliseconds to seconds, depending on the illumination intensity. A direct method of determining the density and energetic distribution of the trapping states responsible for slowing electron transport has been developed. It involves extraction of trapped electrons by switching the cell from an open circuit to a short circuit after a period of illumination. An advantage of this charge extraction method is that it is less sensitive than other methods to shunting of the DSC by electron transfer at the conducting glass substrate. Results derived from charge extraction measurements on DSCs (with and without compact TiO2 blocking layers) are compared with those obtained by analysis of the open circuit photovoltage decay.