Browsing by Author "Mehata, Mohan Singh"

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Electric Field Effects on Photoluminescence of Polyfluorene Thin Films: Dependence on Excitation Wavelength, Field Strength, and Temperature
(2009-07-09)
Mehata, Mohan Singh
;
Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
Electroabsorption (E-A) and electrophotoluminescence (E-F) spectra of thin films of blue-light-emitting poly(9,9-dioctylfluorene) (PFO) have been measured at temperatures ranging from 25 to 295 K to examine both the optical property and excitation dynamics of these films in the presence of external electric fields of 0-1.2 MV cm(-1). Electric field effects on excitation dynamics depend on not only applied field strength but also excitation wavelength and temperature. For photoexcitation at 344 or 402 nm, E-F spectra observed with low applied fields show only the Stark shift, whereas fluorescence quenching is induced only by strong electric fields. For photoexcitation at a shorter wavelength of 298 nm, on the other hand, field-induced quenching of fluorescence is observed even with a weak electric field at any temperature. The presence of a nonradiative process from highly excited states that is effectively affected by electric fields is suggested.
Electric-Field-Induced Enhancement/Quenching of Photoluminescence of pi-Conjugated Polymer S3-PPV: Excitation Energy Dependence
(2010-05-20)
Mehata, Mohan Singh
;
Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
The effects of electric field on absorption and photoluminescence (PL) of films of sulfide-substituted PPV derivative S3-PPV, poly[2-(phenyl)-3-(4'-(3,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene-co-2-(11'-decyl sulfanylundecanyloxy)-5-methoxy-1,4-phenylene vinylene], were investigated. Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of S3-PPV show the Stark shifts, indicating a significant alternation in the molecular polarizability (Delta(alpha) over bar) associated with the optical transitions. Field-induced enhancement or quenching is also observed for PL of S3-PPV, depending on the photoexcitation energy, whereas the shape of the PL spectra is independent of the excitation wavelength. The field effects on the decay profiles of PL indicate that the quenching results from a diminished population of the emitting states on excitation at 300 nm, whereas the PL is enhanced on excitation at 471 nm because the emitting state has an increased lifetime. The efficiency of field-assisted generation of electron-hole pairs produced through excitons monotonically increases with increasing excitation energy, and the nonradiative decay rate in the emitting state is diminished by electric fields in S3-PPV. The photoirradiation of S3-PPV in ambient air resulted in rapid degradation of the polymer film.
Electroabsorption and Electrophotoluminescence of Poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene)
(2012-07-19)
Mehata, Mohan Singh
;
Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
Electric-field-induced changes of the absorption, photoluminescence, and photoluminescence excitation spectra and the photoluminescence decay profile have been measured for poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene). The magnitudes of the change of electric dipole moment and molecular polarizability of this compound have been determined for excitation into the first excited state having a strong absorption, and a large charge-separated character, which is relevant for photovoltaic devices, is confirmed. The photoluminescence of this compound is quenched by application of an electric field, and the quenching efficiency increases with decreasing excitation wavelength. Field-assisted dissociation is considered to occur from the photoexcited Franck-Condon state, resulting in the field-induced decrease in the population of the emitting state, which induce the quenching of photoluminescence.
"Electroabsorption and Electrophotoluminescence of Poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene)"
(1970-01-01)
Mehata, Mohan Singh
;
Hsu, Chain-Shu
;
Lee, Yuan-Pern
;
Ohta, Nobuhiro
;
應用化學系
"Electric-field-induced changes of the absorption, photoluminescence, and photoluminescence excitation spectra and the photoluminescence decay profile have been measured for poly(2,3-diphenyl-5-hexyl-p-phenylene vinylene). The magnitudes of the change of electric dipole moment and molecular polarizability of this compound have been determined for excitation into the first excited state having a strong absorption, and a large charge-separated character, which is relevant for photovoltaic devices, is confirmed. The photoluminescence of this compound is quenched by application of an electric field, and the quenching efficiency increases with decreasing excitation wavelength. Field-assisted dissociation is considered to occur from the photoexcited Franck-Condon state, resulting in the field-induced decrease in the population of the emitting state, which induce the quenching of photoluminescence."