Influence of External Electric Fields on Photoluminescence and Charge Carrier Dynamics of pi-Conjugated Polymer P3HT in Multilayer Films with Heterojunctions to TiO2 and Sb2S3

Abstract

pi-conjugated polymers are actively studied as a promising component for developing inexpensive dye sensitized solar cells (DSSCs). DSSCs utilizing pi-conjugated polymers as p-type organic semiconductor and metal chalcogenides as sensitizers have attracted interest due to high absorbance and stability in comparison to DSSCs using conventional molecular dyes. Here, we report the study of the excited state and charge carrier dynamics in poly(hexylthiophene) (P3HT) with junction structures of FTO/TiO2/P3HT/PMMA/Ag and FTO/TiO2/Sb2S3/P3HT/PMMA/Ag, where fluorine-doped tin oxide (FTO) and silver (Ag) are used as electrodes and poly(methyl methacrylate) (PMMA) is used as an insulating film. We measured electroabsorption spectra and the effects of external electric fields on photoluminescence (PL) of P3HT in these two different systems. Two PL bands emitted from different emitting states are observed in both of the systems. As the quadratic electric field effect, fluorescence quantum yield and lifetime become larger and longer in both of the PL bands, respectively, in the presence of external electric fields, indicating that nonradiative decay processes are depressed by applied electric fields in both of the systems. We also measured the linear electric field effects on PL of P3HT, and PL enhancement and quenching of PL intensity as well as lengthening and shortening of PL lifetime were observed, depending on the direction of the applied electric fields. It is shown that the mechanism of the linear field effect is different between the two emitting states in both of the systems; i.e., the population of the emitting state is affected by application of electric field for one PL band but not for the other PL band. The anisotropy of the field-induced change in PL with respect to the field direction of the applied electric fields can be explained in terms of the synergy effect between internal electric field which exists in the P3HT film and externally applied electric field on the charge separation of hot excitons generated following photoexcitation to free electron and hole (polaron) and on nonradiative decay rate of the PL emitting states.