A STUDY OF THE RECOMBINATION PROCESSES OF PHOTOGENERATED CHARGE-CARRIERS IN SNO2/ION EXCHANGE POLYMER-ZNTPP/AU CELLS

Theoretical approaches to the description of charge carrier recombination in insulators and semiconductors containing traps have been used to interpret photoconductive decay curves observed after illumination of SnO2/ZnTPP-polyXIO/Au cells. Application of the double exponential solution derived from a simplified photoconductivity model accurately reproduces the decays and points to the involvement of both shallow and deep traps in the overall recombination process, which is consistent with experimental results presented in a previous report. From this approach, relevant parameters such as the relative rates of trapping, detrapping, and recombination for photogenerated charge carriers are estimated. However, the relatively large detrapping rate that this model yields for the deep traps involved outlines its limitation in describing recombination processes in systems of this type. A further analysis of these results, based on a model for bimolecular trapping/annihilation processes where deep traps act solely as recombination centers, helps demonstrate how a single decay can be composed of two distinct recombination mechanisms, in agreement with previously reported data. During about the first 15 s of the decay, bimolecular recombination occurs mainly between charge carriers residing in shallow traps, also referred to as ''free'' carriers. At longer times, the bimolecular recombination becomes mediated by the deeper traps present in the system. The expression derived from the bimolecular/trapping annihilation model for the decay at longer times, which is arrived at by effectively turning off the free carrier recombination, is equivalent to the expression one gets for a process that is of a unimolecular type.