2C13
Electric field-induced modification of photoluminescent properties inpolyfluorene thin films(Hokkaido Univ.1 and National Chiao Tung Univ.2) MEHATA, Mohan Singh1; HSU, ChainShu2; LEE, Yuan-Perrn2; OHTA, Nobuhiro1
Poly(9,9-dioctylfluorene), i.e., polyfluorene (PFO) and its derivatives have emerged as
especially attractive conjugated polymers due to their strong blue light emission (blueOLEDs) and excellent electronic properties, which bears large extinction coefficient, highfluorescence quantum yield and high hole mobility, that show great prospect for device
application. Such applications rely on injection and transport of charge carriers.
Electroabsorption(E-A)

andelectrophotoluminescence (E-F) spectra of thinfilms of blue-light-emitting PFO have beenmeasured at temperatures ranging from 25 to 295
K to examine both the optical property andexcitation dynamics of these films in the presenceof external electric fields of 0 - 1.2 MV cm-1. The
applied electric field induces a spectral shift ofabsorption and photoluminescence (PL) spectra ofPFO to yield a line shape represented by the first
derivative of absorption and the PL spectra,
respectively,indicatingthat

themolecularpolarizability is different between the excited andground states. Electric field effects on excitation
dynamics depend on not only applied field strength
but also excitation wavelength and temperature(Figs 1-3). With excitation into lower excited states(i.e. at 344 nm or 402 nm excitation), the PL
intensity is unaffected by F when the applied fieldstrength is as low as 0.4 MVcm-1(Fig. 1), indicatingthat the relaxation rate of the emitting state are

Fig. 1: E-F spectra as well as PL spectra ofPFO film obtained with 344-nm excitation at
different field strengths in the region of 0.4 1.2 MV cm-1. The maximum PL intensity isnormalized to unity in every case.

unaffected by F. At low electric fields, only the Stark shift was observed. As the applied field
becomes stronger, however, the PL of PFO films is quenched by F even with excitation into

low excited states. This quenching is ascribed to
the field-assisted generation of hole-electronpairs followed by efficient charge transport.
With excitation into higher excited states (Fig.
2), i.e. at 298 nm excitation, the PL of PFO isquenched even when the applied field is weak,
and the magnitude of the quenching increases
with increasing field strength, which could be
ascribed to the electron-transfer from highly
excited state through the orbital-overlap.
The field-induced quenching of PL at thelow excited state is less efficient with thedecrease of temperature (Fig. 3), whereas theFig. 2: E-F spectra as well as PL spectra of PFO
film obtained with 298-nm excitation at various fieldstrengths in the region of 0.4 - 1.0 MV cm-1. The
maximum PL intensity is normalized to unity inevery case.

PL quenching at higher excited states is nearly
independentoftemperature.

The

directmeasurements of the field-induced change inemission decay profile confirm that fluorescencequenching arises from a decreased initial
population of the emitting species in bothmechanisms. The present study is strongly
related to a quest for high performance ofmolecular devices, e.g., for the design anddevelopment of efficient and stable light-emitting
devices.
References: M.S. Mehata, C.S Hsu, Y.P. Lee, N. Ohta,J. Phys. Chem. C, 113 (2009) 11907. M. S. Mehata, T.
Iimori, N. Ohta, Chem. Phys. Lett. 457 (2008) 62.

Fig. 3: E-F spectra as well as PL spectra ofPFO film obtained with 344-nm excitation at
various temperatures. The applied fieldstrength is 1.2 MV cm-1.