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Picosecond hopping relaxation in conjugated polymers
Volume 209, number 3
CHEMICAL. PHYSICS LETTERS
2 July 1993
Picosecond hopping relaxation in conjugated polymers U. Lemmer, R.F. Mahrt, Y. Wada *, A. Greiner, H. Biisslerand E-0. GGbel Fuchbereich Physik und Physikalische Chemie und Zentrum fir Materialwissenschaften der Philipps-Universitiit, Renihof 5, W-3550 Marburg, Germany Received 18 February 1993
The energy relaxation of optical excitations in poly (pphenyl-phenylenevinylene) (PPPV) and PPPV blended in polycarbonate (PC) is investigated by means of picosecond time-resolved luminescence spectroscopy. In PPPV a pronounced dependence on the photon energy of the excitation laser is found. For excitation into the tail states ofthe inhomogeneously broadened density of states distribution, the spectral shape of the luminescence remains almost constant with time. However, a transient red-shift is observed for excitation at higher energies. Experiments on the diluted system PPPV/PC allow us to distinguish between intraand inter-chain pmcesse~ The experimental results can be explained with a hopping model.
1. Introduction The fundamental interest in conjugated polymers combined with their increasing technological importance has created a strong impetus for studying their linear and noniinear optical properties [ 1,2]. Their optical properties are commonly discussed on the basis of a one-ciectron semiconductor band model as first proposed by Su et al. [ 31. The polymer backbone is considered to be an infinite one-dimensional ( 1D) system with strong electron-phonon coupling and negligible interchain coupling. Photon absorption is attributed to the transition of an electron from the r-type valence band of the chain to the 1c*-conduction band followed by rapid structural distortion (polaron formation) in the vicinity of the charge. Photoluminescence is attributed to the radiative decay of the so-called polaron-exciton [ 4,5 1. However, it has been pointed out that Coulomb and electroncorrelation effects also play an important role [ 6 1. An alternative approach is to consider conjugated polymers as arrays of chromophores where the electronic transitions arc considerably inhomogeneously broadened due to disorder. This model is supported by the systematic variation of the absorption spectra ’ Permanent address: National Institute for Research in Inorganic Materials, Tsukuba, Japan. 0009-2614/93/S
of oligomeric and polymeric phenylenevinylenes with chain length [ 71 and by site-selective fluorescence spectroscopy [ 81. Within this model, the elementary excitations can execute a random walk within the inhomogeneously broadened distribution of states (DOS) with the electron and hole being strongly Coulomb correlated. We report time-resolved photoluminescence (PL) experiments which allow the study of the picosecond dynamics of the optical excitations. Where polaron formation is the dominant process, the relaxation of the primary optical excitation is expected to be fast. Thus the spectral shape of the luminescence spectra probed on a time scalar larger than the polaron formation time (which is expected to be less than 1 ps) should be time independent, apart from a drop of the overall intensity due to recombination. Recent time-resolved studies have pointed out that migration of the optical excitations may occur in conjugated polymers [ 9,101. Energy relaxation due to hopping of neutral excitations within a DOS of localizedstates with random energies will result in more sophisticated behavior as compared to the self-trap ping ( polaron-formation ) process. This is basically due to the fact that the mean hopping time of an excitation is strongly dependent on its energy [ 111. We therefore can define a demarcation energy .F,, sep arating states where radiative recombination is more
06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
243
Volume 209, number 3
CHEMICAL PHYSICS LETTERS
2 July 1993
probable than hopping to sites with lower energy. For excitation with photon energy considerably larger than the demarcation energy, a transient red-shift of the luminescence spectra is expected due to energy relaxation. This red-shift should be absent for excitation with photon energy close to or smaller than EIOC-
Experiments were performed on 10 urn thick solution cast films of a fluorescent conjugated polymer, poly-(pphenyl-phenylenevinylene) (PPPV). The material was prepared via the Heck reaction. For comparison, we have also investigated a homogeneous polymer blend consisting of II PPPV in polycarbonate (PC). A tunable coumarin 102 dye laser, synchronously pumped by a frequency-tripled modelocked Nd: YLF laser providing 7 ps pulses was used for the excitation. To avoid nonlinear annihilation effects the output power was reduced to 1 mW (pulse rate was 76 MHz) and focused onto a spotsize of 1.5 mm diameter. The fluorescence was passed through a 0.64 m monochromator and detected by a streak camera with 20 ps time resolution.
. ‘;
PPPV
2. Experimental
1.5
2.0
2.5
3.0
Energy
(eV)
3.5
4.0
Fig. 1. Low-temperature (IO K) luminescence and absorption spectra of PPPV. E,, denotes the demarcation energy (see text)
astaken from site-selectivefluorescenceexperiments.
3. Results and discussion Absorption and cw photoluminescence spectra of PPPV at low temperature (T= 10 K) are depicted in fig. 1. The broad absorption band contains the fundamental S, cS,-, electronic transition as well as different vibronic bands at higher energies, which are not resolved due to the inhomogeneous and homogeneous broadening. The emission at lower photon energies reveals the fundamental electronic transition at the high-energy side as well as several different vibronic sidebands at lower energies. The demarcation energy E,, as determined from siteselective fluorescence experiments [ 121 is also indicated in fig. 1. The time dependence of the photoluminescence after excitation with two different photon energies is summarized in fig. 2, where normalized luminescence spectra of the high-energy emission band are shown for different delay times after pulsed excitation. For excitation with photon 244
T=lOK
2.4
2.5
2.6 Energy
-
2.7
(eV)
Fig. 2. Time-resolved spectra of the high-energy luminescence band of PPPV after pulsed excitation with different photon encrgies E,.
energy considerably larger than E,, (upper part) the data reveal a clear red-shift of the photoluminescence with time while this red-shift is absent for excitation close to E,, (lower part). The spectral shift for excitation with high photon energy (E,,>E,,,) is also clearly reflected in the PL decay monitored at different emission energies as displayed in fig. 3 for an excitation energy of E,,= 2.66 eV. The PL decay is fast at high energy close to the excitation energy
-500
0
2 July 1993
CHEMICAL PHYSICS LETTERS
Volume 209, number 3
500 Time
1000
1500
(ps)
Fig. 3. Decay of the luminescence in PPPY for different luminacence energies after excitation with a photon energy of 2.66 eV.
and becomes gradually slower at lower energies. Decay constants decreasing from 330 ps for a photon energy of 2.4 eV to 55 ps at 2.6 eV can be deduced. These results can be interpreted consistently in terms of a random walk concept of neutral excitations. It predicts for an excitation created close to the center of the DOS (which is in our case at even higher energy) rapid energy relaxation due to downward hopping. At lower energies, well in the tail of the DOS, energy relaxation is slowed down due to the decrease of the density of states and the corresponding decrease of the number of nearest neighbours with lower energies. At these energies radiative decay becomes the most probable process. Observing a temporal shift of the PL in PPPV if E,,,> Eloc yet not if E,,, - E,, represents clear evidence for the occurrence of energy-dependent hopping of the optical excitations. In the presence of extrinsic traps, which may act as recombination centers, nonradiative recombination may occur in addition to the relaxation. This nonradiative recombination, however, will be essential only for states above E,=, where the excitations are mobile and thus can migrate to trap states [ 9 1. Our results also show that the Stokes shift between the purely electronic absorption and emission bands has to be essentially attributed to energy relaxation. We would like to point out that the relaxation behavior observed here is similar to the energy relaxation in amorphous semiconductors [ 131. Yet, different to amorphous semiconductors, in the case of PPPV E,,
does not correspond to a mobility edge as seen from electroabsorption measurements indicating that in PPPV absorbing states are localized both below and above E,, [ 141. The experimental results for the polymer blend ( 1%PPPV in PC) are depicted in fig, 4. For the excitation 2 ps laser pulses at Eat 3.6 eV were used. Already on a time scale comparable to the temporal resolution of the experimental apparatus a broad spectrum evolved. This shows that intra-chain relaxation relaxation processes play the dominant role for the initial relaxation. The subsequent evolution of the luminescence spectrum in the polymer blend, however, is different from that in the homopolymer. Due to the hampered inter-chain transport in the diluted system, even for excitation at high photon energies no significant red-shift of the luminescence spectra can be observed after the ultrafast intra-chain relaxation. The relaxation into energetically deeper states as well as the trapping process is strongly suppressed as compared with pure PPPV. In the polymer blend the luminescence decay is almost independent of the detection energy. Due to the suppression of the nonradiative recombination channel the excitations decay predominantly radiatively. A decay time of 630 ps is deduced from the luminescence trace at 2.6 eV, which is shown in the
r,
c
z
: 5
‘\
P+ -
1.. 2.6
1..
2.7
.
1..
2.8
Energy
.
-
5oops ooops 4ops
1 *. 2.9
* 3.0
(eV)
Fig. 4. Time-resolved spectra of the high-energy side of the luminescence of the polymer blend ( 1% PPPV in PC) after pulsed excitation. The excitation energy is 3.6 eV. The inset shows the transient luminescence at 2.6 eV.
245
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CHEMICALPHYSICSLETTERS
inset. This is more than one order of magnitude larger than in the homopolymer at the same detection energy, giving direct evidence for a reduction of energy relaxation as well as nonradiative recombination, In summary, we have demonstrated that the dynamics of optical excitations in PPPV can be qualitatively described by a random walk of the excitations within an inhomogenously broadened distribution of states assigned to subunits of the polymer backbone. Comparison of the excitation dynamics of pure PPPV and of a polymer blend consisting of 1% PPPV in PC demonstrates that interchain relaxation dominates on the picosecond time scale in the homopolymer. We also confirm our previous conclusion that the essential contribution to the Stokes shift between the purely electronic absorption and emission band comes from energy relaxation.
Acknowledgement We gratefully acknowledge H. Martelock for preparing the polymer samples. We are indebted to the Deutsche Forschungsgemeinschaft and the Stiftung Volkswagenwerk for financial support.
246
2 July 1993
References [ 1] H.G. Kiess, ed., Conjugated conducting polymers (Springer, Berlin, 1992). [2] T. Kobayashi, ed., Nonlinear optics of organics and semiconductors (Springer, Berlin, 1989). [3) W.P. Su, J.R. Schrieffer and A.J. Heeger, Phys. Rev. Letters 42 (1979) 1698. 14) N.F. Colaneri, D.D.C. Bradley, R.H. Friend, P.L. Bum, A.B. Holmes and C.W. Spangler, Phys. Rev. B 42 ( 1990) 11670. [51K. Fesser, A.R. Bishop and D.K. Campbell, Phys. Rev. B 27 (1983) 4804. [6] S. Mukamel and H.X. Wang, Phys. Rev. Letters 69 (1992) [7] Ei. Mahrt, J. Yang, A. Greiner, H. Blssler and D.D.C. Bradley, Macromol. Chem. Rapid Commun. 11 ( 1990) 415. [ 81U. Rauscher, H. Bissler, D.D.C. Bradley and M. Hennecke, Phys. Rev. B 42 (1990) 9830. [9] U. Lemmer, R.F. Mahrt, Y. Wada, A. Greiner, H. BIssler and E.O. G&l, Appl. Phys. Letters 62 (1993), in press. [lo] I.D.W. Samuel, B. Crystall, G. Rumbles, P.L. Burn, A.B. Holmes and R.H. Friend, Synth. Metals 54 ( 1993) 281. [ 111 B. Movaghar, M. Griinewald, 9. Ries, H. B%sler and D. Wiinz, Phys. Rev. B 33 (1986) 5545. [ 121 S. Heun, R.F. Mahrt, A. Greiner, U. Lemmer, H. Bjissler, D.A. Hall&y, D.D.C. Bradley, P.L. Bum and A.B. Holmes, J. Phys. C 5 (1993) 247. [ 131 R. Fischer, E.O. GBbel, G. Noll, P. Thomas and A. Weller, in: Hopping and related phenomena, Vol. 2, ed. H. Fritzsche (World Scientific, Singapore, 1990) p. 403. [ 141 A. Horvath, H. BLsler and G. Weiser, Pbys. Stat. Sol. (a) 173 (1992) 755.
CHEMICAL. PHYSICS LETTERS
2 July 1993
Picosecond hopping relaxation in conjugated polymers U. Lemmer, R.F. Mahrt, Y. Wada *, A. Greiner, H. Biisslerand E-0. GGbel Fuchbereich Physik und Physikalische Chemie und Zentrum fir Materialwissenschaften der Philipps-Universitiit, Renihof 5, W-3550 Marburg, Germany Received 18 February 1993
The energy relaxation of optical excitations in poly (pphenyl-phenylenevinylene) (PPPV) and PPPV blended in polycarbonate (PC) is investigated by means of picosecond time-resolved luminescence spectroscopy. In PPPV a pronounced dependence on the photon energy of the excitation laser is found. For excitation into the tail states ofthe inhomogeneously broadened density of states distribution, the spectral shape of the luminescence remains almost constant with time. However, a transient red-shift is observed for excitation at higher energies. Experiments on the diluted system PPPV/PC allow us to distinguish between intraand inter-chain pmcesse~ The experimental results can be explained with a hopping model.
1. Introduction The fundamental interest in conjugated polymers combined with their increasing technological importance has created a strong impetus for studying their linear and noniinear optical properties [ 1,2]. Their optical properties are commonly discussed on the basis of a one-ciectron semiconductor band model as first proposed by Su et al. [ 31. The polymer backbone is considered to be an infinite one-dimensional ( 1D) system with strong electron-phonon coupling and negligible interchain coupling. Photon absorption is attributed to the transition of an electron from the r-type valence band of the chain to the 1c*-conduction band followed by rapid structural distortion (polaron formation) in the vicinity of the charge. Photoluminescence is attributed to the radiative decay of the so-called polaron-exciton [ 4,5 1. However, it has been pointed out that Coulomb and electroncorrelation effects also play an important role [ 6 1. An alternative approach is to consider conjugated polymers as arrays of chromophores where the electronic transitions arc considerably inhomogeneously broadened due to disorder. This model is supported by the systematic variation of the absorption spectra ’ Permanent address: National Institute for Research in Inorganic Materials, Tsukuba, Japan. 0009-2614/93/S
of oligomeric and polymeric phenylenevinylenes with chain length [ 71 and by site-selective fluorescence spectroscopy [ 81. Within this model, the elementary excitations can execute a random walk within the inhomogeneously broadened distribution of states (DOS) with the electron and hole being strongly Coulomb correlated. We report time-resolved photoluminescence (PL) experiments which allow the study of the picosecond dynamics of the optical excitations. Where polaron formation is the dominant process, the relaxation of the primary optical excitation is expected to be fast. Thus the spectral shape of the luminescence spectra probed on a time scalar larger than the polaron formation time (which is expected to be less than 1 ps) should be time independent, apart from a drop of the overall intensity due to recombination. Recent time-resolved studies have pointed out that migration of the optical excitations may occur in conjugated polymers [ 9,101. Energy relaxation due to hopping of neutral excitations within a DOS of localizedstates with random energies will result in more sophisticated behavior as compared to the self-trap ping ( polaron-formation ) process. This is basically due to the fact that the mean hopping time of an excitation is strongly dependent on its energy [ 111. We therefore can define a demarcation energy .F,, sep arating states where radiative recombination is more
06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
243
Volume 209, number 3
CHEMICAL PHYSICS LETTERS
2 July 1993
probable than hopping to sites with lower energy. For excitation with photon energy considerably larger than the demarcation energy, a transient red-shift of the luminescence spectra is expected due to energy relaxation. This red-shift should be absent for excitation with photon energy close to or smaller than EIOC-
Experiments were performed on 10 urn thick solution cast films of a fluorescent conjugated polymer, poly-(pphenyl-phenylenevinylene) (PPPV). The material was prepared via the Heck reaction. For comparison, we have also investigated a homogeneous polymer blend consisting of II PPPV in polycarbonate (PC). A tunable coumarin 102 dye laser, synchronously pumped by a frequency-tripled modelocked Nd: YLF laser providing 7 ps pulses was used for the excitation. To avoid nonlinear annihilation effects the output power was reduced to 1 mW (pulse rate was 76 MHz) and focused onto a spotsize of 1.5 mm diameter. The fluorescence was passed through a 0.64 m monochromator and detected by a streak camera with 20 ps time resolution.
. ‘;
PPPV
2. Experimental
1.5
2.0
2.5
3.0
Energy
(eV)
3.5
4.0
Fig. 1. Low-temperature (IO K) luminescence and absorption spectra of PPPV. E,, denotes the demarcation energy (see text)
astaken from site-selectivefluorescenceexperiments.
3. Results and discussion Absorption and cw photoluminescence spectra of PPPV at low temperature (T= 10 K) are depicted in fig. 1. The broad absorption band contains the fundamental S, cS,-, electronic transition as well as different vibronic bands at higher energies, which are not resolved due to the inhomogeneous and homogeneous broadening. The emission at lower photon energies reveals the fundamental electronic transition at the high-energy side as well as several different vibronic sidebands at lower energies. The demarcation energy E,, as determined from siteselective fluorescence experiments [ 121 is also indicated in fig. 1. The time dependence of the photoluminescence after excitation with two different photon energies is summarized in fig. 2, where normalized luminescence spectra of the high-energy emission band are shown for different delay times after pulsed excitation. For excitation with photon 244
T=lOK
2.4
2.5
2.6 Energy
-
2.7
(eV)
Fig. 2. Time-resolved spectra of the high-energy luminescence band of PPPV after pulsed excitation with different photon encrgies E,.
energy considerably larger than E,, (upper part) the data reveal a clear red-shift of the photoluminescence with time while this red-shift is absent for excitation close to E,, (lower part). The spectral shift for excitation with high photon energy (E,,>E,,,) is also clearly reflected in the PL decay monitored at different emission energies as displayed in fig. 3 for an excitation energy of E,,= 2.66 eV. The PL decay is fast at high energy close to the excitation energy
-500
0
2 July 1993
CHEMICAL PHYSICS LETTERS
Volume 209, number 3
500 Time
1000
1500
(ps)
Fig. 3. Decay of the luminescence in PPPY for different luminacence energies after excitation with a photon energy of 2.66 eV.
and becomes gradually slower at lower energies. Decay constants decreasing from 330 ps for a photon energy of 2.4 eV to 55 ps at 2.6 eV can be deduced. These results can be interpreted consistently in terms of a random walk concept of neutral excitations. It predicts for an excitation created close to the center of the DOS (which is in our case at even higher energy) rapid energy relaxation due to downward hopping. At lower energies, well in the tail of the DOS, energy relaxation is slowed down due to the decrease of the density of states and the corresponding decrease of the number of nearest neighbours with lower energies. At these energies radiative decay becomes the most probable process. Observing a temporal shift of the PL in PPPV if E,,,> Eloc yet not if E,,, - E,, represents clear evidence for the occurrence of energy-dependent hopping of the optical excitations. In the presence of extrinsic traps, which may act as recombination centers, nonradiative recombination may occur in addition to the relaxation. This nonradiative recombination, however, will be essential only for states above E,=, where the excitations are mobile and thus can migrate to trap states [ 9 1. Our results also show that the Stokes shift between the purely electronic absorption and emission bands has to be essentially attributed to energy relaxation. We would like to point out that the relaxation behavior observed here is similar to the energy relaxation in amorphous semiconductors [ 131. Yet, different to amorphous semiconductors, in the case of PPPV E,,
does not correspond to a mobility edge as seen from electroabsorption measurements indicating that in PPPV absorbing states are localized both below and above E,, [ 141. The experimental results for the polymer blend ( 1%PPPV in PC) are depicted in fig, 4. For the excitation 2 ps laser pulses at Eat 3.6 eV were used. Already on a time scale comparable to the temporal resolution of the experimental apparatus a broad spectrum evolved. This shows that intra-chain relaxation relaxation processes play the dominant role for the initial relaxation. The subsequent evolution of the luminescence spectrum in the polymer blend, however, is different from that in the homopolymer. Due to the hampered inter-chain transport in the diluted system, even for excitation at high photon energies no significant red-shift of the luminescence spectra can be observed after the ultrafast intra-chain relaxation. The relaxation into energetically deeper states as well as the trapping process is strongly suppressed as compared with pure PPPV. In the polymer blend the luminescence decay is almost independent of the detection energy. Due to the suppression of the nonradiative recombination channel the excitations decay predominantly radiatively. A decay time of 630 ps is deduced from the luminescence trace at 2.6 eV, which is shown in the
r,
c
z
: 5
‘\
P+ -
1.. 2.6
1..
2.7
.
1..
2.8
Energy
.
-
5oops ooops 4ops
1 *. 2.9
* 3.0
(eV)
Fig. 4. Time-resolved spectra of the high-energy side of the luminescence of the polymer blend ( 1% PPPV in PC) after pulsed excitation. The excitation energy is 3.6 eV. The inset shows the transient luminescence at 2.6 eV.
245
Volume209,number3
CHEMICALPHYSICSLETTERS
inset. This is more than one order of magnitude larger than in the homopolymer at the same detection energy, giving direct evidence for a reduction of energy relaxation as well as nonradiative recombination, In summary, we have demonstrated that the dynamics of optical excitations in PPPV can be qualitatively described by a random walk of the excitations within an inhomogenously broadened distribution of states assigned to subunits of the polymer backbone. Comparison of the excitation dynamics of pure PPPV and of a polymer blend consisting of 1% PPPV in PC demonstrates that interchain relaxation dominates on the picosecond time scale in the homopolymer. We also confirm our previous conclusion that the essential contribution to the Stokes shift between the purely electronic absorption and emission band comes from energy relaxation.
Acknowledgement We gratefully acknowledge H. Martelock for preparing the polymer samples. We are indebted to the Deutsche Forschungsgemeinschaft and the Stiftung Volkswagenwerk for financial support.
246
2 July 1993
References [ 1] H.G. Kiess, ed., Conjugated conducting polymers (Springer, Berlin, 1992). [2] T. Kobayashi, ed., Nonlinear optics of organics and semiconductors (Springer, Berlin, 1989). [3) W.P. Su, J.R. Schrieffer and A.J. Heeger, Phys. Rev. Letters 42 (1979) 1698. 14) N.F. Colaneri, D.D.C. Bradley, R.H. Friend, P.L. Bum, A.B. Holmes and C.W. Spangler, Phys. Rev. B 42 ( 1990) 11670. [51K. Fesser, A.R. Bishop and D.K. Campbell, Phys. Rev. B 27 (1983) 4804. [6] S. Mukamel and H.X. Wang, Phys. Rev. Letters 69 (1992) [7] Ei. Mahrt, J. Yang, A. Greiner, H. Blssler and D.D.C. Bradley, Macromol. Chem. Rapid Commun. 11 ( 1990) 415. [ 81U. Rauscher, H. Bissler, D.D.C. Bradley and M. Hennecke, Phys. Rev. B 42 (1990) 9830. [9] U. Lemmer, R.F. Mahrt, Y. Wada, A. Greiner, H. BIssler and E.O. G&l, Appl. Phys. Letters 62 (1993), in press. [lo] I.D.W. Samuel, B. Crystall, G. Rumbles, P.L. Burn, A.B. Holmes and R.H. Friend, Synth. Metals 54 ( 1993) 281. [ 111 B. Movaghar, M. Griinewald, 9. Ries, H. B%sler and D. Wiinz, Phys. Rev. B 33 (1986) 5545. [ 121 S. Heun, R.F. Mahrt, A. Greiner, U. Lemmer, H. Bjissler, D.A. Hall&y, D.D.C. Bradley, P.L. Bum and A.B. Holmes, J. Phys. C 5 (1993) 247. [ 131 R. Fischer, E.O. GBbel, G. Noll, P. Thomas and A. Weller, in: Hopping and related phenomena, Vol. 2, ed. H. Fritzsche (World Scientific, Singapore, 1990) p. 403. [ 141 A. Horvath, H. BLsler and G. Weiser, Pbys. Stat. Sol. (a) 173 (1992) 755.