Abstract A composite material of poly(phenylenevinylene) (PPV) doped by dye aggregates was prepared. A very efficient and temperature activated energy transfer (ET) from the PPV to the dye aggregates was attributed to F . orster ET accompanied by exciton diffusion. A clear complementary relation between the photoluminescence (PL) and electroluminscence (EL) images was observed for thin (15 nm) PPV-based OLEDs. So-called &apos;&apos;black spots&apos;&apos; in EL become bright ones when the photoluminescence of the same area was excited. This effect was attributed to the presence of an insulating layer between the polymer and aluminium. 

A G Vitukhnovsky

I G Scheblykin

L S Lepnev

M Van Der Auweraer

CiteSeerX

Journal of Luminescence 94–95 (2001) 461–464
Electroluminescence and optical properties of
poly(phenylenevinylene)/J-aggregate composites
I.G. Scheblykina,*, L.S. Lepnevb, A.G. Vitukhnovskyb, M. Van der Auweraera
aLaboratory for Molecular Dynamics and Spectroscopy, K.U. Leuven, Celestijnenlaan 200 F, 3001 Heverlee, Belgium
bP.N. Lebedev Physics Institute RAS and P.N. Lebedev Research Centre in Physics, Leninsky pr. 53, 119991GSP1 Moscow, Russia
Abstract
A composite material of poly(phenylenevinylene) (PPV) doped by dye aggregates was prepared. A very efficient and
temperature activated energy transfer (ET) from the PPV to the dye aggregates was attributed to F .orster ET
accompanied by exciton diffusion. A clear complementary relation between the photoluminescence (PL) and
electroluminscence (EL) images was observed for thin (15 nm) PPV-based OLEDs. So-called ‘‘black spots’’ in EL
become bright ones when the photoluminescence of the same area was excited. This effect was attributed to the presence
of an insulating layer between the polymer and aluminium. r 2001 Elsevier Science B.V. All rights reserved.
Keywords: Exciton dissociation; Exciton diffusion; J-aggregates; Black spots
1. Introduction
Doping of conjugated polymers by fluorescent
dyes is mainly used to tune the colour and increase
the emission efficiency of organic light emitting
diodes (OLEDs) [1]. In the current contribution we
present a study of photoluminescence (PL) and
electroluminescence (EL) of thin light emitting
diodes made of poly(phenylenevinylene) (PPV)
and of PPV doped by 3,30-bis-[3-sulfopropyl]-
5,50-dichloro-9-ethylthiacarbocyanine (THIATS)
and its J-aggregates [2]. Dye aggregates being
dispersed in an electroactive polymer act as
extended traps for excitons and as very effective
fluorescent recombination centres [3,4]. The choice
of THIATS allowed us to prepare a stable solution
of J-aggregates and the PPV precursor in water
while this was not possible for another dye [4] due
to incompatibility of the pH-sensitivity of the
mixed compounds. The formation of the so-called
‘‘black spots’’ on the active area of an operating
OLED is a well-known phenomenon [5] respon-
sible for EL degradation. ‘‘Black spots’’ (BS) are
usually circular areas with very low EL efficiency
as compared to the surrounding. In distinction to
the majority of studies on OLEDs, we used a very
thin active layer of about 15 nm (instead of 100 nm
in common OLEDs) which allowed us to demon-
strate a complementary relation between electro-
luminescent (EL) and photoluminescent (PL)
properties of OLED areas subjected to BS
degradation.
2. Experimental
The positively charged water-soluble tetrahy-
drothiophenium PPV precursor (pre-PPV) was
*Corresponding author. Fax: +32-16-327990.
E-mail address: scheb@sci.lebedev.ru (I.G. Scheblykin).
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 3 7 - 4
synthesized as described elsewhere [6,7]. Mixing an
aqueous solution (ca. 104 M) of the negatively
charged THIATS dye with an aqueous solution of
the positively charged pre-PPV (ca. 102 M) was
found to cause extensive aggregation. The absorp-
tion spectrum maximum shifted from 550 nm,
corresponding to dye monomers in water, to
625 nm (Fig. 1(a)), which is a typical absorption
band of J-aggregates [2], while the width of the
band was reduced to 15 nm (400 cm1). In this
solution, J-aggregates revealed a high fluorescence
quantum yield of ca. 40%, and a narrow
fluorescence band around 630 nm. Films were
prepared by spincoating from a pure pre-PPV
solution with a concentration of monomer
of 2 102 M and PPV/THIATS composite
solutions where the PPV/THIATS monomer con-
centration ratio was varied from 40/1 to 6000/1.
Thermal conversion of pre-PPV into the conju-
gated polymer was carried out by heating in
103 Torr at 1501C for 11 h. The thickness of the
films was estimated from topographic profiles of
‘‘scratched’’ samples measured on an atomic force
microscope (Topometrics Explorer) and was
found as small as 15 nm. OLEDs of the following
arrangement were fabricated: ITO/PVK(20 nm)/
PPV(15 nm)/Al; ITO/PPV(15 nm)/Al and ITO/
PVK(20 nm)/[PPV+THIATS composite (10–
15 nm)]/PPV(15 nm)/Al. The layer of poly(N-
vinylcarbazole) (PVK) layer was used in order to
reduce the hole injection from ITO resulting in a
more balanced charge injection into the device.
Photo-images of OLEDs excited by 430 nm light
(PL-image) and by electrical current (EL-image)
were obtained by the use of an optical microscope
with episcopic fluorescence attachment coupled to
a photo-camera. Temperature dependent fluores-
cence spectra exited by a CW He–Cd laser
(lex ¼ 442 nm) were measured in a liquid helium
cryostat. In these experiments polymer films
deposited on glass substrates were always covered
by a 100–200 nm aluminium layer to prevent fast
photodegradation of the material when exposed to
air [7].
3. Results and discussion
The absorption spectrum of the PPV/THIATS
composite film changed drastically after the heat
treatment (Fig. 1(a)). A wide band at 400–450 nm,
corresponding to the PPV absorption [6], with
optical density of about 0.2 appeared. The band of
the J-aggregates became much broader and less
red shifted (585 nm) compared to the data
obtained before heating. We attribute it to short
aggregates where only a few dye molecules are
coherently coupled. Moreover, a monomer band
was observed at about 550 nm. That disintegration
of aggregates we attribute to substantial changes
happening in the film morphology and dielectric
properties as well by the disappearance of the
charge during the conversion of the precursor to
the conjugated polymer.
Fig. 1. (a) Absorption spectrum (bottom) of the composite film
before (thin line) and after thermal conversion (thick dashed
line). PL spectra of the PPV/THIATS and PPV films at
different temperatures (top); (b) temperature dependence of the
total fluorescence yield (open squares and solid circles) and of
the parameter d for the same film (large open circles). The lines
are eye guides.
I.G. Scheblykin et al. / Journal of Luminescence 94–95 (2001) 461–464462
The fluorescence spectra of the PPV/THIATS
(1300/1) composite film, excited at 442 nm via the
PPV band, are shown in Fig. 1(a) for different
temperatures. The spectrum exhibits both the
PPV-related vibrational progression with bands
at 510 and 550 nm and a wide red band in the
range of 600–650 nm. We attribute this long-
wavelength fluorescence to aggregated dye species.
The excitation spectrum of the 610 nm band
almost coincides with the absorption band of
PPV. The total fluorescence quantum yield of
the PPV/THIATS films on glass substrates was
estimated to be 6–10%. One can, therefore,
conclude that very efficient energy transfer (ET)
occurs from the initially excited polymer to the dye
species. An example of the temperature depen-
dence of the fluorescence spectra of a PPV/
THIATS composite film (1300/1) is shown in
Fig. 1(a). It is clearly seen that the contribution of
the dye aggregates to the total fluorescence
increases with temperature. The total fluorescence
yield of the films was found to decrease with
increasing temperature (Fig. 1(b)). The efficiency
of the ET can be characterized by ratio d of the
fluorescence spectrum integrated over the band of
aggregates to the integral of the total fluorescence
spectrum. According to the definition d ¼ 1
corresponds to 100% energy transfer to the
aggregates while d ¼ 0 shows that no ET occurs.
As it is shown in Fig. 1(b) at room temperature
60% of light originated from the aggregates, while
at T ¼ 5K the contribution of the aggregates is
o15%. One could expect the F .orster ET to be
quite important due to the substantial overlap of
the PPV fluorescence spectrum and the absorption
band of dye aggregates. However, it is quite hard
to obtain the observed temperature activated
acceptor fluorescence in the framework of the
F .orster model. We suggest that exciton diffusion
in PPV [8] toward the dye species is the main
mechanism of ET responsible for observed tem-
perature dependence. This process is temperature
activated as far as it is an intrinsic feature of the
exciton diffusion in disordered solids [9].
PL and EL of PPV/THIATS composite OLEDs
were found to be very similar. This suggests that
the ET process does not depend on the way (by
light or by electric field) the film is excited.
Due to the very small thickness of the polymer
layer the degradation of OLEDs, fabricated as
described in the experimental section, occurred
within tens of minutes. A typical current density
ranged from 5 to 100 mA/cm2 at a biased voltage
from 2 to 8 V for fresh and aged samples,
respectively. PL and EL images of a PPV-only
OLED are presented in Fig. 2. The EL-image
reveals the classical black spot picture [5]. Com-
paring the EL and PL images of the same samples
clearly shows that these images are complemen-
tary. In other words, the EL image is a negative of
the PL image. The occurrence of this phenomenon
did not depend on the size of dark areas in the EL
image and it was observed for PPV/THIATS
composite OLEDs as well as for OLEDs without
THIATS. In contrast to thicker samples usually
studied, for thin layers the bulk fluorescence does
no longer hide the phenomena affecting the
fluorescence originating at the interfacial region.
Moreover the low optical density (less than 0.25)
makes it possible to carry out photoexcitation of
the region near the interface between the polymer
and the metal. There are practically no indications
of such an effect in the literature. To our knowl-
edge, only McElvain et al. [5] noticed a slight
increase of the BS brightness in the PL image of
Alq3-based OLEDs. However, no further attention
was paid to and no explanation was suggested for
that interesting phenomenon. The anticorrelation
between PL and EL images can be rationalized in
terms of the contact-assisted quenching of singlet
excitations. More specifically, we attribute this
effect to the formation of an insulating intermedi-
ate layer between the polymer and the aluminium
electrode. For more details we refer the reader to
Ref. [7]. Such a modification of the interface, on
the one hand, leads to a reduction of the electron
Fig. 2. PL and EL images of the thin ITO/PPV/AL OLED.
I.G. Scheblykin et al. / Journal of Luminescence 94–95 (2001) 461–464 463
injection and to a concomitant decrease of the EL
intensity and, on the other hand, it suppresses the
PL quenching via exciton dissociation at the metal
surface [10] and enhances PL.
4. Conclusions
An efficient and temperature activated energy
transfer from the conjugated polymer to the dye
aggregates was attributed to exciton diffusion. A
clear complementary relation between PL and EL
images was observed in thin PPV-based OLEDs.
Black spots on the OLED’s active area, observed
during the operation, become bright ones when the
photoluminescence of the same area was excited.
This phenomenon was explained by formation of a
thin insulating layer between the polymer and the
aluminium contact reducing the electron injection,
and PL quenching due to exciton dissociation at
the metal–polymer interface.
Acknowledgements
This work was supported by NATO Grant
SfP97-1940 and RFBR Grants 00-02-16607, 99-02-
17326, 00-15-96707. I.G. Sch. thanks the
K.U. Leuven for a ‘‘Junior Research Fellowship’’
and the ‘‘Fonds voor Wetenschappelijk Onderzoek
Vlaanderen’’ (F.W.O.) for a postdoctoral Fellow-
ship. The authors gratefully acknowledge the
continuing support from DWTC (Belgium)
through Grant IUAP-IV-11, the F.W.O.-Vlaande-
ren, the Nationale Loterij, the research Council of
the K.U. Leuven through GOA 1996 and COST
D14. The authors thank Dr. H. Zhang for help
with AFM measurements and Dr. Vladimir
Arkhipov for discussions.
References
[1] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem.
Int. Ed. 37 (1998) 402.
[2] I.G. Scheblykin, et al., J. Phys. Chem. B 105 (2001)
4636.
[3] E.I. Mat’tsev, et al., Appl. Phys. Lett. 75 (1999) 1896.
[4] S. Kirstein, et al., Isr. J. Chem. 40 (2000) 129.
[5] J. McElvain, H. Antoniadis, M.R. Hueschen, J.N. Miller,
D.M. Roitman, J.R. Sheats, R.L. Moon, J. Appl. Phys. 80
(1996) 6002.
[6] D.R. Gagnon, J.D. Capistran, F.E. Karasz, R.W. Lenz,
S. Antoun, Polymer 28 (1987) 567.
[7] I.G. Scheblykin, V.I. Arkhipov, M. van der Auweraer,
F. de Schrÿver, Helv. Chim. Acta (2001), accepted for
publication.
[8] M. Yan, et al., Phys. Rev. Lett. 73 (1994) 744.
[9] V.M. Arganovich, M.D. Galanin, Electronic Excitation
Energy Transfer in Condensed Matter, North Holland,
Amsterdam, 1982.
[10] M. Pope, C.E. Swenberg, Electronic Processes in Organic
Crystals and Polymers, Oxford University Press, Oxford,
1999, pp. 326–330 (Chapter 2E).
I.G. Scheblykin et al. / Journal of Luminescence 94–95 (2001) 461–464464


Electroluminescence and optical properties of poly(phenylenevinylene)/J-aggregate composites

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=CA845482CA10C607D8734E24FF9A7A21?doi=10.1.1.1073.1915&rep=rep1&type=pdf

Electroluminescence and optical properties of poly(phenylenevinylene)/J-aggregate composites

Abstract

Similar works

Full text

Available Versions

CiteSeerX