Browsing by Author "Okoshi, Kenji"

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Inhomogeneous Photoluminescence Characteristic in Carbon Nanodots and Electrophotoluminescence Measurements
(2018-03-22)
Hamada, Morihiko
;
Okoshi, Kenji
;
Rana, Shailesh
;
Awasthi, Kamlesh
;
Iimori, Toshifumi
Photoluminescence (PL) spectra, time-resolved PL spectra, and PL decay profiles have been observed for carbon nanodots (CDs) with different excitation wavelengths in an embedded solid film and in solution. PL excitation spectra have been also observed with different monitoring wavelengths. Then, it is found in both solid film and solution that not only the location of the PL spectra but also the peak of the excitation spectra show a significant red shift, as the excitation and monitoring wavelengths become longer, respectively, indicating that the emitting states of the excitation-dependent PL are the real state to which direct absorption occurs from the ground state, not the transient trapped states produced by photoexcitation. It is shown that not only the excitation-dependent PL but also the excitation-independent PL with a peak at similar to 375 nm exist. The lifetimes of both PL emissions are very sensitive to the surroundings. Multiple emitting states that give excitation-dependent PL are ascribed to the inhomogeneous properties in prepared carbon nanodots, which is supported by the fluorescence lifetime image measurements. Electrophotoluminescence spectrum, that is, the electric-field-induced change in PL spectrum, has also been observed for the excitation-independent PL of CDs embedded in a poly(methyl methacrylate) film, and the magnitude of the change in electric dipole moment and molecular polarizability following emission has been determined.
Stark Spectroscopy of Absorption and Emission of Indoline Sensitizers: A Correlation with the Performance of Photovoltaic Cells
(2016-11-24)
Ohta, Nobuhiro
;
Awasthi, Kamlesh
;
Okoshi, Kenji
;
Manseki, Kazuhiro
;
Miura, Hidetoshi
Electric field effects on photoexcitation dynamics and electronic properties of highly efficient indoline sensitizers, DN488, D205, and DN182, embedded in PMMA films have been examined by using electroabsorption (E-A) and electrophotoluminescence (E-PL) spectroscopic techniques and time-resolved photoluminescence (PL) decay measurements in the presence of electric fields. Photovoltaic performances have been also measured for devices constructed using these sensitizers. Then, field-induced quenching of PL and field-induced change in PL decay profile were observed, and it was found that these field effects, which depend on the sensitizers investigated herein, are well correlated with the trend of power conversion efficiencies of the corresponding photovoltaic cells. Electric dipole moment and molecular polarizability of these sensitizers both in the ground state (S-0) and in the excited state have been calculated at the level of B3LYP/6-31G(d), and the differencesof these physical parameters between So and excited state thus obtained have been compared with the ones determined from the E-A and E-PL spectra. The present study of Stark spectroscopy of indoline dyes provides new insights for the exciton dissociation property and carrier mobility of organic dyes, which are important factors to understand the operation mechanism in dye-sensitized solar cells.