Browsing by Author "Chiou, Chin-Shiun"

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Effect of an External Electric Field on the Fluorescence of a pi-Conjugated Polymer Film of P3HT Sandwiched between FTO and PMMA Published as part of The Journal of Physical Chemistry virtual special issue "Hai-Lung Dai Festschrift"
(2019-05-23)
Awasthi, Kamlesh
;
Chiou, Chin-Shiun
;
Iimori, Toshifumi
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
We report the photoexcitation dynamics of poly(hexylthiophene-2,5-diyl) (P3HT) as a solid film sandwiched between conducting films of a fluorine-doped tin-oxide-coated glass substrate (FTO) and an insulator film of poly(methyl methacrylate) (PMMA), from measurements of the effects of an external electric field on the absorption and photoluminescence (PL) spectra of P3HT. An Ag film was coated on the PMMA film; the FTO and Ag films served as electrodes to apply an external electric field to P3HT. According to the effects of a quadratic electric field, the lifetime of the emitting state of P3HT increased in the presence of that external field, resulting in an enhanced fluorescence quantum yield. An enhancement and quenching of fluorescence were observed also on applying an external electric field having the opposite polarity to the P3HT film between FTO and Ag films, which results from a field-induced change of both the population of the emitting state following photoexcitation and the rate of nonradiative decay at the emitting state. The dependence of the field-induced change of photoluminescence on the polarity of the applied external electric field between the FTO and Ag films was interpreted as an influence of an internal field located in the P3HT film sandwiched between FTO and PMMA films. These results of the effects of an electric field on the PL of a P3HT film sandwiched between FTO and PMMA films are compared with the effects on PL of P3HT sandwiched between a semiconductor film of TiO2 or a Sb2S3 film and a PMMA film.
Fluorescence enhancement induced by quadratic electric-field effects on singlet exciton dynamics in poly(3-hexylthiophene) dispersed in poly(methyl methacrylate)
(2019-03-14)
Iimori, Toshifumi
;
Awasthi, Kamlesh
;
Chiou, Chin-Shiun
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
The dynamics of the exciton generated by photoexcitation of a regioregular poly(3-hexylthiophene) (P3HT) polymer dispersed in a poly(methyl methacrylate) (PMMA) matrix was examined using electro-photoluminescence (E-PL) spectroscopy, where electric field effects on the photoluminescence (PL) spectra were measured. The quadratic electric-field effect was investigated using the modulation technique, with field-induced changes in the PL intensity monitored at the second harmonic of the modulation frequency of the applied electric field. Absorption and PL spectra indicated the formation of both ordered crystalline aggregates and amorphous regions of P3HT polymer chains. Although previous studies of electric field effects on -conjugated polymers have generally shown that the PL intensity is decreased by electric fields, we report that the PL intensity of P3HT and PL lifetime increased with the quadratic electric-field effect. The magnitude of the change in PL intensity was quantitatively explained in terms of the field-induced decrease in the nonradiative decay rate constants of the exciton. We proposed that a delayed PL, originating from charge carrier recombination, was enhanced in the presence of electric fields. The rate constant of the downhill relaxation process of the exciton, which originated from the relaxation in distributed energy levels due to an inherent energetic disorder in P3HT aggregates, was implied to decrease in the presence of electric fields. The radiative decay rate constant and PL quantum yield of P3HT dissolved in solution, which were evaluated from the molar extinction coefficient and the PL lifetime, were compared with those of P3HT dispersed in a PMMA matrix.
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
(2018-11-01)
Iimori, Toshifumi
;
Awasthi, Kamlesh
;
Chiou, Chin-Shiun
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
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.