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Time resolved luminescence spectroscopy of conjugated polymers
JOURNAL OF
LUMINESCENCE ELSEVIER
Journal of Luminescence 60&6l (1994) 479—481
Time resolved luminescence spectroscopy of conjugated polymers R.F. Mahrtb, U. Lemmera
*
A. Greiner”, Y. Wadaa, H. Bässlerb, E.O. Göbela, R. Kerstinge, K. Leoc, H. Kurzc
aEachbereich Physik, Philipps-Univer.citdt Marburg, D-35032 Marburg, Germany hFaChbereich Phvsikalische Chemie und Zen trum .Thr Materialwissen.schaften, Philipps-Universitëzt Marburg, D-35032 Marhurg, Germany ~Institut fur Haibleitertechnik II, RWTH Aachen, Sommerfeldsirajie, D-52074 Aachen, Germany
Abstract The dynamics of photoexcitations in conjugated polymers and polymer blends is studied by means of time resolved luminescence spectroscopy. After excitation into the vibronic progression, the luminescence rises quasi-instantaneously.
The luminescence decay depends strongly on the detection energy and the polymer concentration. The results are explained within a hopping model.
1. Introduction Conjugated polymers have attracted much interest from both fundamental and technological points of view. Among these materials the phenylenevinylene family offers a great potential for luminescent devices [1]. In spite of the large progress in device technology, a detailed understanding of the relaxation and recombination processes is still lacking. The different models used to describe the electronic properties in conjugated
after optical excitation of an electron is considered to be distortion of the lattice leading to polaron formation. However, it has been pointed out that the disorder present in amorphous polymers significantly alters the static and especially the dynamic electronic properties [3,4]. It was shown that hopping processes of the optical excitations may play a central role for the relaxation behavior. Here we address the energy relaxation of photoexcitations and the nature of the recombination processes. We report on time resolved luminescence spectroscopy
polymers predict different processes being domi-
on poly(p-phenylenevinylene) (PPV) and its sol-
nant for the relaxation of optical excitations. The commonly used one electron semiconductor band model is based on the work of Su et al. [2]. This model views the polymer backbone as an infinite one-dimensional system with strong electron— phonon coupling. The main relaxation process
uble derivative poly(phenyl-p-phenylenevinylene) (PPPV). After optical excitation into the higher lying vibronic progressions, an ultrafast (<200 fs) vibrational relaxation is observed. Subsequently, the dynamics of the luminescence is governed by spectral relaxation within the inhomogenously broadened density of states distribution (DOS) and an energy dependent trapping of mobile excitations.
*
Corresponding author,
0022-2313/94/S07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-2313(93)E0384-A
45))
RI. tla/iri
(‘1 (if.
Journal if / ,,ijnciccfricc 60&6
2. Experimental
,...,
/
/994
1 .2
.~
We have used the fenitosecond luminescence upconversion as well as the picosecond streak-camera technique for the time resolved detection of the luminescence. In the up-conversion experiment the sample was excited with laser pulses derived from the frequency doubled output of a Kerr-lens modelocked Ti: sapphire laser. The laser produced pulses with a duration of lSOfs at a photon energy of 1.56eV. The temporal profile of the luminescence was monitored via sum-frequency generation with a delayed reference pulse within a nonlinear crystal of beta-barium borate (BBO). The sum-frequency signal was dispersed in a monochromator and detected with a single photon counting system. For the experiments with the streak-camera technique. the samples were excited with 7 Ps laser pulses produced by a Coumarin 102 dye laser which was synchronously pumped by a frequency tripled modelocked Nd:YLF-laser. All the experiments were performed on solution cast films of PPV prepared via the sulphonium precursor route and films of PPPV synthesized via the Heck-reaction. By blending PPPV with polycarbonate (PC), homogeneous optical quality films with different concentrations were prepared in order to investigate the effect of dilution on the relaxation dynamics.
3. Results and discussion After excitation with 3.12 eV laser pulses with a pulse duration of l50fs, the luminescence rises quasi-instantaneously over the whole emission band as can be seen in Fig. 1(a) for four different detection energies. Already during the excitation pulse the spectrum resembles the time integrated luminescence spectrum. This indicates an ultrafast vibrational relaxation out of the initially excited higher vibronic levels of the first excited electronic state. The observed fast dissipation of the vibrational energy is explained by the high vibrational mode density in these large organic molecules, After this initial step, the subsequent luminescence decay depends strongly on the detection energy. For the decay to I/c of the initial intensity decay times as short as 300fs are observed for states energetically
(I)
0 8
~
I
I
‘ 0.4 .~ —
4 ‘O 477 I
.::~-~
~ ~_
~,
“
2 4 e4
“
~
2 6
j ~
2 7
0.0
Tme (ps) ~‘
b)
100
.~‘
10
10K 30K .
0.0
0.5
1.0
1.5
2.0
T1m6 (n5) l~ ig. I.
a) lime resolved luminescence traces measured in the
up-conversion experiment for different detection energies measured T = 300 K). For the excitation ISO fs laser pulses at 3.12eV were used. (‘C denotes the cross correlation between theexciting and the gating pulse. hI Temperature dependent luminescence decay of PPV measured with the sireak camera at a detection
LnLrg5
high in the DOS. For lower energies the decay time increases gradually by almost two orders of magnitude. These experimental findings can be explained within a hopping model. Since the depopulation at high energies is controlled by the number of states available for downward hopping, a strongly energy dependent decay is observed. Fig. 1(b) shows luminescence traces on a larger time scale detected at the emission maximum for different temperatures. Nonexponential decays are observed for all temperatures with characteristic times of 140 and 7Ops for low temperature and room temperature. respectively. The faster decay at higher temperatures is accompanied by a decrease of the quanturn efficiency. This indicates an additional quenching process which becomes more prominent at higher temperatures. In order to investigate whether the luminescence quenching is mainly due to an intra or interchain process. we have performed experiments on diluted systems. Fig. 2 shows low temperature luminescence
RE. Mahrt et al. / Journal of Luminescence 60&61 (1994) 479 48/
oligomers of PPV. This similarity gives further evidence that the conjugated polymers can be modelled as arrays of chromophores identified as
1 .2 a: 1% pppv/pc b: 50% PPPV/PC C
08
~C 0.4 0.0
C: EL
PPPV =2
subunits of the polymer chain.
a
4. Conclusion
500
1000
1500
__________________________
Time
481
2000
(~)
Fig. 2. Low temperature time resolved luminescence traces for PPPV/ PC polymer blends with various concentrations. The samples were excited with 7 Ps laser pulses at an energy of 2.63 eV. The time resolution of the system was about 40 ps.
traces detected at the same energy obtained for the different concentrations. The decay gradually slows down for lower concentrations and becomes closer to an exponential. In contrast to previous work [5], we have to conclude that interchain processes play a significant role in the relaxation and recombination dynamics. Measurements of the relative quanturn efficiencies have provided higher values for the diluted systems. Thus there is strong evidence that the fast component of the luminescence decay is due to a nonradiative process which involves interchain transport of mobile excitations. In PPV where the lowest quantum efficiencies are observed, this process is even more pronounced (see Fig. 1(b)). The temperature dependence is in agreement with this model. For higher temperatures the excitations can undergo thermally activated hopping transport to so far unidentified traps. For the 1% PPPV/PC sample this transport is strongly hampered and the observed luminescence decay reflects the intrinsic radiative lifetime. The decay time of 940ps is close to the value of 1.3 ns reported for the
luminescence In conclusion, measurements we have onfamily. conjugated out time resolved polymers vibronic of the relaxation phenylenevinylene on a carried time scale ofVery less rapid than 200fs is followed by a hopping mediated spectral relaxation of the optical excitations within the inhomogenously broadened density of states. Additionally energy dependent trapping of mobile excitations quenches the luminescence. This effect, also evident in the quantum efficiencies, leads to the shorter luminescence decay times in PPV as cornpared to PPPV and especially the PPPV/PC polymer blend.
Acknowledgements We thank H. Martelock and M. Gailberger for preparing the samples. We gratefully acknowledge financial support by the Stiftung Volkswagenwerk.
References [I] [2] [3] [4] [5]
R.H. Friend. D.D.C. Bradley and A. Holmes, Physics World, Vol. 5 (1992) p 42. W.P. Su, J.R. Schrieffer and A.J. Heeger. Phys. Rev. Lett. 42 (1979) 1698. U. Rauscher, H. Bassler, D.D.C. Bradley and M. Hennecke, Phys. Rev. B 42 (1990) 9830. R. Kersting, U. Lemmer, R.F. Mahrt, K. Leo, H. Kurz, H Bãssler and ED Göbel, Phys. Rev. Lett. 70(1993) 3820 M. Furukawa. K. Mizuno, A. Matsui. S.D.D. Rughooputh and W.C. Walker, J. Phys. Soc. Jap. 58(1989) 2976.
LUMINESCENCE ELSEVIER
Journal of Luminescence 60&6l (1994) 479—481
Time resolved luminescence spectroscopy of conjugated polymers R.F. Mahrtb, U. Lemmera
*
A. Greiner”, Y. Wadaa, H. Bässlerb, E.O. Göbela, R. Kerstinge, K. Leoc, H. Kurzc
aEachbereich Physik, Philipps-Univer.citdt Marburg, D-35032 Marburg, Germany hFaChbereich Phvsikalische Chemie und Zen trum .Thr Materialwissen.schaften, Philipps-Universitëzt Marburg, D-35032 Marhurg, Germany ~Institut fur Haibleitertechnik II, RWTH Aachen, Sommerfeldsirajie, D-52074 Aachen, Germany
Abstract The dynamics of photoexcitations in conjugated polymers and polymer blends is studied by means of time resolved luminescence spectroscopy. After excitation into the vibronic progression, the luminescence rises quasi-instantaneously.
The luminescence decay depends strongly on the detection energy and the polymer concentration. The results are explained within a hopping model.
1. Introduction Conjugated polymers have attracted much interest from both fundamental and technological points of view. Among these materials the phenylenevinylene family offers a great potential for luminescent devices [1]. In spite of the large progress in device technology, a detailed understanding of the relaxation and recombination processes is still lacking. The different models used to describe the electronic properties in conjugated
after optical excitation of an electron is considered to be distortion of the lattice leading to polaron formation. However, it has been pointed out that the disorder present in amorphous polymers significantly alters the static and especially the dynamic electronic properties [3,4]. It was shown that hopping processes of the optical excitations may play a central role for the relaxation behavior. Here we address the energy relaxation of photoexcitations and the nature of the recombination processes. We report on time resolved luminescence spectroscopy
polymers predict different processes being domi-
on poly(p-phenylenevinylene) (PPV) and its sol-
nant for the relaxation of optical excitations. The commonly used one electron semiconductor band model is based on the work of Su et al. [2]. This model views the polymer backbone as an infinite one-dimensional system with strong electron— phonon coupling. The main relaxation process
uble derivative poly(phenyl-p-phenylenevinylene) (PPPV). After optical excitation into the higher lying vibronic progressions, an ultrafast (<200 fs) vibrational relaxation is observed. Subsequently, the dynamics of the luminescence is governed by spectral relaxation within the inhomogenously broadened density of states distribution (DOS) and an energy dependent trapping of mobile excitations.
*
Corresponding author,
0022-2313/94/S07.00 © 1994 — Elsevier Science B.V. All rights reserved SSDI 0022-2313(93)E0384-A
45))
RI. tla/iri
(‘1 (if.
Journal if / ,,ijnciccfricc 60&6
2. Experimental
,...,
/
/994
1 .2
.~
We have used the fenitosecond luminescence upconversion as well as the picosecond streak-camera technique for the time resolved detection of the luminescence. In the up-conversion experiment the sample was excited with laser pulses derived from the frequency doubled output of a Kerr-lens modelocked Ti: sapphire laser. The laser produced pulses with a duration of lSOfs at a photon energy of 1.56eV. The temporal profile of the luminescence was monitored via sum-frequency generation with a delayed reference pulse within a nonlinear crystal of beta-barium borate (BBO). The sum-frequency signal was dispersed in a monochromator and detected with a single photon counting system. For the experiments with the streak-camera technique. the samples were excited with 7 Ps laser pulses produced by a Coumarin 102 dye laser which was synchronously pumped by a frequency tripled modelocked Nd:YLF-laser. All the experiments were performed on solution cast films of PPV prepared via the sulphonium precursor route and films of PPPV synthesized via the Heck-reaction. By blending PPPV with polycarbonate (PC), homogeneous optical quality films with different concentrations were prepared in order to investigate the effect of dilution on the relaxation dynamics.
3. Results and discussion After excitation with 3.12 eV laser pulses with a pulse duration of l50fs, the luminescence rises quasi-instantaneously over the whole emission band as can be seen in Fig. 1(a) for four different detection energies. Already during the excitation pulse the spectrum resembles the time integrated luminescence spectrum. This indicates an ultrafast vibrational relaxation out of the initially excited higher vibronic levels of the first excited electronic state. The observed fast dissipation of the vibrational energy is explained by the high vibrational mode density in these large organic molecules, After this initial step, the subsequent luminescence decay depends strongly on the detection energy. For the decay to I/c of the initial intensity decay times as short as 300fs are observed for states energetically
(I)
0 8
~
I
I
‘ 0.4 .~ —
4 ‘O 477 I
.::~-~
~ ~_
~,
“
2 4 e4
“
~
2 6
j ~
2 7
0.0
Tme (ps) ~‘
b)
100
.~‘
10
10K 30K .
0.0
0.5
1.0
1.5
2.0
T1m6 (n5) l~ ig. I.
a) lime resolved luminescence traces measured in the
up-conversion experiment for different detection energies measured T = 300 K). For the excitation ISO fs laser pulses at 3.12eV were used. (‘C denotes the cross correlation between theexciting and the gating pulse. hI Temperature dependent luminescence decay of PPV measured with the sireak camera at a detection
LnLrg5
high in the DOS. For lower energies the decay time increases gradually by almost two orders of magnitude. These experimental findings can be explained within a hopping model. Since the depopulation at high energies is controlled by the number of states available for downward hopping, a strongly energy dependent decay is observed. Fig. 1(b) shows luminescence traces on a larger time scale detected at the emission maximum for different temperatures. Nonexponential decays are observed for all temperatures with characteristic times of 140 and 7Ops for low temperature and room temperature. respectively. The faster decay at higher temperatures is accompanied by a decrease of the quanturn efficiency. This indicates an additional quenching process which becomes more prominent at higher temperatures. In order to investigate whether the luminescence quenching is mainly due to an intra or interchain process. we have performed experiments on diluted systems. Fig. 2 shows low temperature luminescence
RE. Mahrt et al. / Journal of Luminescence 60&61 (1994) 479 48/
oligomers of PPV. This similarity gives further evidence that the conjugated polymers can be modelled as arrays of chromophores identified as
1 .2 a: 1% pppv/pc b: 50% PPPV/PC C
08
~C 0.4 0.0
C: EL
PPPV =2
subunits of the polymer chain.
a
4. Conclusion
500
1000
1500
__________________________
Time
481
2000
(~)
Fig. 2. Low temperature time resolved luminescence traces for PPPV/ PC polymer blends with various concentrations. The samples were excited with 7 Ps laser pulses at an energy of 2.63 eV. The time resolution of the system was about 40 ps.
traces detected at the same energy obtained for the different concentrations. The decay gradually slows down for lower concentrations and becomes closer to an exponential. In contrast to previous work [5], we have to conclude that interchain processes play a significant role in the relaxation and recombination dynamics. Measurements of the relative quanturn efficiencies have provided higher values for the diluted systems. Thus there is strong evidence that the fast component of the luminescence decay is due to a nonradiative process which involves interchain transport of mobile excitations. In PPV where the lowest quantum efficiencies are observed, this process is even more pronounced (see Fig. 1(b)). The temperature dependence is in agreement with this model. For higher temperatures the excitations can undergo thermally activated hopping transport to so far unidentified traps. For the 1% PPPV/PC sample this transport is strongly hampered and the observed luminescence decay reflects the intrinsic radiative lifetime. The decay time of 940ps is close to the value of 1.3 ns reported for the
luminescence In conclusion, measurements we have onfamily. conjugated out time resolved polymers vibronic of the relaxation phenylenevinylene on a carried time scale ofVery less rapid than 200fs is followed by a hopping mediated spectral relaxation of the optical excitations within the inhomogenously broadened density of states. Additionally energy dependent trapping of mobile excitations quenches the luminescence. This effect, also evident in the quantum efficiencies, leads to the shorter luminescence decay times in PPV as cornpared to PPPV and especially the PPPV/PC polymer blend.
Acknowledgements We thank H. Martelock and M. Gailberger for preparing the samples. We gratefully acknowledge financial support by the Stiftung Volkswagenwerk.
References [I] [2] [3] [4] [5]
R.H. Friend. D.D.C. Bradley and A. Holmes, Physics World, Vol. 5 (1992) p 42. W.P. Su, J.R. Schrieffer and A.J. Heeger. Phys. Rev. Lett. 42 (1979) 1698. U. Rauscher, H. Bassler, D.D.C. Bradley and M. Hennecke, Phys. Rev. B 42 (1990) 9830. R. Kersting, U. Lemmer, R.F. Mahrt, K. Leo, H. Kurz, H Bãssler and ED Göbel, Phys. Rev. Lett. 70(1993) 3820 M. Furukawa. K. Mizuno, A. Matsui. S.D.D. Rughooputh and W.C. Walker, J. Phys. Soc. Jap. 58(1989) 2976.