A novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitised nanocrystalline solar cells

A novel charge extraction method has been developed to study the transport, trapping and back reaction of photogenerated electrons in dye sensitised nanocrystalline cells (Gratzel cells). The cell is illuminated at open circuit until a steady state is reached in which the rates of photogeneration and of back reaction of electrons with tri-iodide are equal. The illumination is then interrupted, and the electron density is allowed to decay for a given time in the dark before short circuiting the cell using a solid state switch. For high efficiency cells, the integrated current measured at short circuit corresponds closely to the remaining electronic charge in the film. Small corrections are required to account for back reaction and substrate charging. The delay time between interruption of the illumination and short circuit charge extraction is varied systematically to follow the decay of electron concentration. Analysis of the time dependence of the electron charge indicates that the back reaction of electrons with I-3(-) is second order in electron density, which is consistent with the formation of I-2(-) as an intermediate. Simultaneous measurement of the charge and photovoltage decay curves shows that the density of trap states decreases exponentially with trap depth. (C) 2000 Published by Elsevier Science S.A.