Determination of electron diffusion lengths in nanostructured oxide electrodes from photopotential maps obtained with the scanning microscope for semiconductor characterization

The scanning microscope for semiconductor characterization, a scanning laser-spot technique, has been used to obtain laterally resolved open circuit photopotential measurements of anatase nanostructured electrodes in contact with acidic solutions of catechol (or salicylic acid). The oxide nanoporous films were supported on a conducting glass plate with a groove cut in it as to remove a strip of the conducting layer, thus defining two electrically isolated areas. Upon 514.5-nm light illumination, the electrons photoinjected from adsorbed catechol spread over the nanoporous anatase thin film. The photopotential of one of the isolated conducting parts was monitored as a function of the laser spot location. A quantitative model is developed to simulate the photoresponse based on the validity of the diffusion equation for electron transport. Accordingly, a value of 0.4 mm was found for the electron diffusion length in the presence of catechol, which probably results from a high average lifetime of the photogenerated electrons.