| The photoresponse of thin semiconductor films has been discussed, in particular, the dependence of the photocurrent on (1) the physical film thickness, (2) the optical effects at the inner interface, (3) the electronic properties of the inner interface, (4) the optical properties of the bulk, (5) the electronic properties of the bulk, (6) the optical effects at the outer interface, and (7) the electronic properties of the outer interface where the inner and outer interfaces refer to the film/substrate and film/ambient interfaces, respectively. The analysis was illustrated by examining the photoresponse of anodically grown TiO{dollar}sb2{dollar} films on Ti.; The photoresponse dependence on film thickness was shown to be caused by multiple internal reflection effects. The photoresponse dependence on the remaining six factors was examined using local PhotoElectrochemical Microscopy (PEM) measurements on TiO{dollar}sb2{dollar} films grown on polycrystalline Ti substrates, which provided a range of film properties with a minimum variation in film thickness and surface treatment. It was found that the heterogeneous photoresponse was controlled by the electronic properties of the outer interface, although other factors, such as surface roughness at the inner interface and the bulk optical properties, needed to be considered as well. The heterogeneous photoresponse was attributed to a lateral distribution surface states energy levels, located 1.2-1.3 eV below the conduction band. These surface states were assumed to mediate both interfacial charge transfer and surface recombination processes. Further experimental support was provided by PEM measurements in the presence of iodide ions. The presence of iodide ions increased the charge transfer rate due to the more efficient iodine evolution process in comparison with the oxygen evolution process. Hence, the heterogeneity in the photoresponse was eliminated. |
