In this work we present spectroscopic and photoluminescent properties of nanostructured polymeric composite films of nanocrystalline TiO2 and polyfuorenes (PF+nc-TiO2). The films were deposited onto glass and KBr substrates by spin-coating PF-in-toluene solution with embedding of 5-nm thick nc-TiO2 particles. The nc-TiO2 wt concentration embedded in PF was chosen as 1%, 2% and 4%. The FTIR, FTR, UV-VIS-NIR and photoluminescence characterization were carried-out to study the influence of oxide nanoparticles on the absorption spectra, molecular structures and emissive luminescence of the composites. It was shown that the presence of TiO2 nanoparticles results in improvement of charge separation of carriers generated by light excitation. Under a short wavelength excitation, in some samples a PL enhancement was observed. Under normal or long wavelength excitation, PF fluorescence quenching occurred for all the composites. Aging process of the composites was also investigated

Chi, Le Ha

Chung Thuy, Tran Thi

Dinh, Nguyen Nang

Nguyen, Thien Phap

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 21, No. 1 (2011), pp. 51-56
SPECTROSCOPIC AND PHOTOLUMINESCENT PROPERTIES OF
NANOSTRUCTURED POLYFLUORENES/TiO2 COMPOSITE
FILMS USED FOR OLEDS
NGUYEN NANG DINH, LE HA CHI, AND TRAN THI CHUNG THUY
College of Technology, VNU Hanoi, Vietnam
THIEN PHAP NGUYEN
Institut des Mate´riaux Jean Rouxel, Nantes, France
Abstract. In this work we present spectroscopic and photoluminescent properties of nanostruc-
tured polymeric composite films of nanocrystalline TiO2 and polyfuorenes (PF+nc-TiO2). The
films were deposited onto glass and KBr substrates by spin-coating PF-in-toluene solution with
embedding of 5-nm thick nc-TiO2 particles. The nc-TiO2 wt concentration embedded in PF was
chosen as 1%, 2% and 4%. The FTIR, FTR, UV-VIS-NIR and photoluminescence characteri-
zation were carried-out to study the influence of oxide nanoparticles on the absorption spectra,
molecular structures and emissive luminescence of the composites. It was shown that the pres-
ence of TiO2 nanoparticles results in improvement of charge separation of carriers generated by
light excitation. Under a short wavelength excitation, in some samples a PL enhancement was
observed. Under normal or long wavelength excitation, PF fluorescence quenching occurred for all
the composites. Aging process of the composites was also investigated.
I. INTRODUCTION
Conducting polymers and polymer-based devices have been increasingly investigated
due to their many potential applications such as optoelectronics, OLED displays, solar flex-
ible cells, etc [1-2]. For OLED, it is very important to improve the luminescence efficiency
of the emission layer. Efficient device operation requires optimization of three factors: (i)
equalization of injection rates of positive (hole) and negative (electron) charge carriers (ii)
recombination of the charge carriers to form singlet exciton and (iii) radiative decay of the
excitons. Of the two carriers, electrons have the lower mobility and hence limit the current
conduction process. By adding a hole transport layer (HTL) to the three-layer device one
can improve the equalization of injection rates of hole and electron, consequently to obtain
a higher electroluminescent efficiency of the OLED. Recently, among the nanostructured
materials there are polymeric nanocomposites have been studied strongly day-by-day, be-
cause these materials can change the properties of the devices built-up from them. By
embedding inorganic nanocrystalline oxides into polymer matrices one can enhance the
efficiency and service duration of the devices. Being in nanoparticle form, the oxides can
substantially influence both the electrical and optical properties of the polymer in which
they are embedded. For instance, TiO2 nanocrystals were embedded in conjugate polymer
MEH-PPV to form a composite thin film that was studied as photoactive material in [3].
52 NGUYEN NANG DINH, LE HA CHI, TRAN THI CHUNG THUY, AND THIEN PHAP NGUYEN
It has been shown that MEH-PPV luminescence quenching was much strongly with rod-
like TiO2 in comparison with dot-like one. It is seen that polyfluorines (PF) with different
additives can emit blue or red light, they are also a good candidate for OLED because of
their high chemical stability and the versatile chemical structures, which make them ideal
for chemical modifications [4]. By embedding oxide nanoparticles into PF, one can expect
enhancement in the emission property of this polymer. The aim of this paper is thus to
study spectroscopic properties such as FTIR spectra, absorption and photoluminescence
of PF+nc-TiO2 nanocomposites films, which are used for preparation of both the OLEDs
and the polymeric solar cells.
II. EXPERIMENTS
The glass substrates used for spin coating nanocomposite films were ultrasonically
cleaned in distilled water, followed in ethanol and acetone. KBr crystalline substrates were
used for characterization of photoluminescence (PL) properties. To deposit the composites
layers onto glass, PF solution was prepared by dissolving PF powder in toluene with a
ratio as 10 mg of PF in 1 ml of toluene. Then, TiO2 nanoparticles were embedded in these
solutions according to the weight ratio of TiO2/PF (wt) as 0, 0.01 and 0.04 (i. e. PF
alone, PF+1%TiO2, PF+2%TiO2 and PF+4%TiO2, further called shortly PF00, PF01
and PF04, respectively). To obtain homogenously dispersion of TiO2 in polymer, the
solutions were mixed for 12 hours by using magnetic stirring. These liquid composites
further used for spin-coating and casting. The conditions for spin coating are following:
delay time is 120 s, rest spin time is 30 s, spin speed is 1500 rpm and accelerate is 500
rpm. Finally, the waiting time for drying is 2 min. The films used for PL characterization
have been deposited by casting onto KBr tablets with diameter of 10 mm, using 50 ?l of
the PF solution. To dry the films, the samples were put in a flow of dried gaseous nitrogen
for 12 hours. All the samples were put in a fore vacuum box until the measurements.
FTIR data were obtained by the measurements on a “Vertex-70 FTIR system in IMN
(Institut de Matriaux de Nantes), Nantes, France. UV-VIS absorption and PL spectra
were carried-out, respectively on a “Cary-5G and a FL3-2 spectrophotometer using a
Xe-lamp as excitation source.
III. RESULTS AND DISCUSSIONS
The IR absorption spectra of PF composites films are shown in Fig. 1. As reported
in [5, 6] the IR bands of the PF can be grouped into three main regions; the region around
3000 cm?1 which is associated with CH stretching vibrations, the region of CH scissoring
vibrations below 1600 cm−1, and the region between 1600 and 1800 cm−1 where two
features evolve by heating in air. In our samples of PF and composites there are also three
groups of the bands, two end regions (below 1600 cm−1 and around 3000 cm−1) have strong
bands, in the range of 1600 to 2800 cm-1 there are not any band for PF0, PF01 and PF02,
but for PF04 (4% TiO2) there is one rather strong band at 2360 cm
−1 (Fig. 1). The band
at 1630 cm−1 of PF is relative with oxygen atom in PF, this feature disappeared in the
spectra of composite films, and instead, the other features were grown up. This difference
in IR of the two types of the samples can be attributed to the presence of nc-TiO2; the
NANOSTRUCTURED POLYFLUORENES/TIO2 COMPOSITE FILMS USED FOR OLEDS 53
last can bond with oxygen atoms of PF. As depicted in Fig. 2 (absorption spectra), all
In
te
n
s
it
y
(
a
.u
.)
Wave number cm-1
Fig. 1. FTIR absorption spectra of PF com-
posites vs. TiO2 content.
Fig. 2. Absorption spectra of PF composite
films vs. content of nc-TiO2.
the PF00, PF01 and PF04 samples exhibit an absorption peak at wavelength of 380 nm,
corresponding to 3.26 eV, which is known that this absorption was attributed to the first
vibronic pi − pi∗ transition. A very broad peak at 540 nm (2.30 eV) corresponding to the
second vibronic pipi∗ transition was also observed. This is related to emission phenomenon
of the PF composites, as observed during the heating cycles while the emission near 380
nm is reduced , a second broad emission band around 530 550 nm evolves [7].
The last transition is strongly For as-prepared samples, absorption coefficient of
PF-alone (0.76) is smaller than that of PF01 (0.82) and of PF04 (0.92) composites. The
PF04 is strongest absorbed. It is seen that TiO2 nanoparticles have enabled PF be much
better absorbed.
Figure 3 presents aging processes of the composites which shows the decrease of
absorption maxima for three composites samples: PF00, PF01 and PF04. From this
figure one can see that for the first day the PF04 composite aged faster compared with
the pure polymer PF and PF01. But from the fourth day on, the process seemed to be
opposite: the sample with 1%TiO2 ages slowly, the sample with 4%TiO2 exhibits stabilized
state at a value as high as 0.67. Comparing with the pure polymeric PF, nanocomposites
exhibit slower aging process. Indeed, this is a typical advantage of composite materials,
for nanocomposites in particular. Using nanocomposites of PF for OLEDs one can prolong
their displaying time.
From the absorption spectra, it is seen that for both the pure PF and composites
there have been revealed two peaks, one strong at 380 nm and another weak at 540 nm.
Thus, in the study of photoluminescence properties, we used two excitation regions nearly
above mentioned wavelengths. Fig. 4 shows the PL spectra of a PF04 composite obtained
with excitation light sources at 340 nm and 540 nm wavelengths (Xe lamp). This figure
shows that the larger energy of excitation photon (shorter wavelength), the larger PL
intensity of the composites was obtained. From the emission curve of the Xe lamp, it is
seen that the ratio between intensities of light at 540 nm and 340 nm is about 1.5. So the
difference in PL intensities can not be attributed due to excitation powers, but it can be
54 NGUYEN NANG DINH, LE HA CHI, TRAN THI CHUNG THUY, AND THIEN PHAP NGUYEN
Fig. 3. Aging of PF vs. TiO2 content in com-
posites.
Fig. 4. PL spectra of PF04 sample with differ-
ent excitation wavelengths.
explained by stronger absorption of the PF and PF-composites at 340 nm. PL emission
spectra having maxima at ca. 662 nm, corresponding to 1.87 eV and a shoulder near 648
nm (1.91 eV) showed that all the samples emitted intense red light upon photo-excitation
at the corresponding absorption maximum. The photoluminescence spectral features of
the polymer are not significantly changed when embedded with TiO2 nanocrystals. In
Fig. 5, the relative PL quenching of polyfluorene films emission is shown as a function
of TiO2 nanoparticle concentration in the precursor solution. In this spectral region, the
TiO2 does not show any detectable absorption. All deposited films exhibit PF absorption
feature (with a maximum peak at approx. 648 nm) which is similar to that reported for
the polymer in solution. As reported in [8], for MEH-PPV composites with the rod-like
TiO2 nanocrystals, the quenching of fluorescence is significantly higher than in the case of
spherical TiO2 nanoparticles. In our samples, TiO2 particles with 5 nm size are so small,
that can be seen as inorganic oxide dots. Presence of these dots in PF has results in the
fluorescence quenching clearly as observed for the rod-like nano TiO2 in MEH-PPV. This
phenomenon is currently attributed to the transfer of the photogenerated electrons to the
inorganic semiconductor [9]. As in dot-like nanocrystals, the surface-to-volume ratio is
higher than in spheres, the occurrence of enhanced emission quenching can be explained
by an increased probability of charge transfer at the PF/TiO2 interface. In addition, the
higher delocalization of carriers in dots may help in preventing the back recombination of
the electrons transferred on TiO2 with holes in the polymer.
With an excitation of a short wavelength light the photoluminescence for the com-
posites exhibits an opposite feature. Instead of the PL quenching of the luminescence
of PF, an enhancement in PL intensities was observed (Fig. 6). The largest increase of
the PL was obtained for the PF04 composite. Although, the PL enhancement has been
rarely mentioned, one can suggest that the increase PL intensity for such a PF04 film
can be attributed to the large absorption coefficient for TiO2 dots. This phenomenon
was explained due to the non-radiative Frster resonant energy transfer (FRET) [10] from
TiO2 nanorods to polymer with excitation of wavelength less 350 nm. From Fig. 5 and
NANOSTRUCTURED POLYFLUORENES/TIO2 COMPOSITE FILMS USED FOR OLEDS 55
Fig. 5. Fluorescence quenching of PF com-
posites with excitation at 540 nm wavelength.
Fig. 6. Enhancement in the PL intensity of
PF composite films when excited at 340 nm
wavelength.
Fig. 6 one can see that in these samples the larger enhancement in PL intensity (under
short wavelength excitation), the stronger fluorescence quenching (under normal excita-
tion) occurred. It is known [11] that the fluorescence quenching of polymeric composites
such as MEH-PPV+nc-TiO2 results in a charge-separation at interfaces of nanocrystalline
oxide/polymer, consequently reducing the barrier height at the last. Figure 7 exhibits
schematically the mechanism of charge transfer at the PF/TiO2-nanoparticle interfaces.
Under excitation by photons of energy larger than the gap between HUMO and LUMO
levels, in the polymer electrons from the HOMO jumped to the LUMO level. As the
LUMO level is higher than the conducting band of TiO2, these electrons transfer to the
last, creating such a called charge-separation zone. This phenomenon indicates that, be-
sides being used for OLEDs, PF composite films can be served as a good material for a
photovoltaic solar cell.
Fig. 7. Schematic representation of charge transfer at the PF/TiO2-dot inter-
faces. The vacuum level is defined at 0 eV.
56 NGUYEN NANG DINH, LE HA CHI, TRAN THI CHUNG THUY, AND THIEN PHAP NGUYEN
IV. CONCLUSIONS
Nanostructured polyfluorene + nc-TiO2 composite films were prepared by spin-
coating. Spectroscopic properties such as absorption and photoluminescence spectra vs.
the TiO2 concentration were investigated. Exciting by a short wavelength of 340 nm, the
enhancement in PL intensity was obtained the best for the composite film with 4%TiO2
embedded in PF and with an excitation of a 540 nm wavelength, PF fluorescence quenching
occurred strongest also in this sample. From the characterization, it is seen that PF
composites films can be suggested as a candidate for both OLEDs and photovoltaic solar
cell applications.
ACKNOWLEDGMENT
This work was partially supported by the Vietnam Korea Protocol on Scientific
Research in the period of 2009 2010 and by the Vietnam National Foundation for Science
and Technology Development (NAFOSTED) in the period 2010 2011 (Project Code:
103.02.88.09).
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Spectroscopic and Photoluminescent Properties of Nanostructured Polyfluorenes/TiO2  Composite Films used for OLEDs

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