- Email: [email protected]
Ultrafast charge photogeneration in conjugated polymer thin films
Synthetic Metals 116 (2001) 9±13
Ultrafast charge photogeneration in conjugated polymer thin ®lms Carlos Silvaa, Mark A. Stevensa, David M. Russella, Sepas Setayeshb, Klaus MuÈllenb, Richard H. Frienda,* a
Optoelectronics Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
b
Abstract We report femtosecond transient absorption spectroscopy on thin ®lms of blue-emitting derivatives of polyindeno¯uorene (PIF). Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped 1Bu exciton and charge pair absorption spectra. We ®nd that charge pairs are produced within the instrument resolution (100 fs) followed by exciton±exciton bimolecular annihilation on a picosecond timescale. Two possible mechanisms for ultrafast charge generation are considered: direct charge separation of nascent 1Bu excitons by static quenching at intrinsic defects and sequential transitions to produce highly energetic excitons that dissociate ef®ciently. Photophysical modelling of intensity-dependent data reveals that sequential absorption followed by charge separation is the most likely mechanism for ultrafast charge pair generation, with the ratio of excited-state to ground-state absorption cross section times the effective yield of charge pairs 0:1. These observations are consistent with previous photocurrent measurements and quantum chemical calculations assigning highly excited states to greatly delocalised excitons [KoÈhler et al., Nature 392 (1998) 903]. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Femtosecond transient absorption; Conjugated polymers; Excitations; Polyindeno¯uorene
1. Introduction Owing to their promise as active materials for optoelectronic applications, luminescent conjugated polymers have been the focus of intensive research for over a decade. Investigation of the photophysics of these materials is important not only from the viewpoint of the technological development of organic devices, but also from a fundamental perspective. The basic understanding of primary photoexcitations in low-dimensional electronic structures provides the basis for theoretical modelling of this class of materials. In a recent publication [1] we reported femtosecond transient absorption measurements on thin ®lms of two derivatives of polyindeno¯uorene (PIF), a processable `step-ladder' analogue of poly-p-phenylene (see inset of Fig. 1). Detailed excitation intensity-dependence measurements allowed the unravelling of 1Bu absorption spectrum from absorption of charged photoexcitations. The two absorption bands are broadly overlapped and it was found * Corresponding author. Tel.: 44-1223-337218; fax: 44-1223-353397. E-mail address: [email protected] (R.H. Friend).
that charge generation occurred on an ultrafast timescale < 100 fs), too early for it to be uniquely a result of exciton±exciton bimolecular annihilation. Two possible mechanisms can account for such prompt charge generation. In the ®rst, nascent excitons in close proximity to intrinsic defects may be directly separated into a charge pair by static quenching. This process requires a rather high concentration of defects or suf®ciently large delocalisation of the lowest excitonic wave function accompanied by a low binding energy [2]. Alternatively, an exciton may be promoted to a higher lying state resonant with the pump laser during the duration of the pump pulse. This highly excited state may dissociate ef®ciently producing charge pairs. Both of these processes would produce charge pairs on a shorter time scale than expected for exciton±exciton annihilation. In this paper we focus on the ultrafast charge generation mechanism by employing a photophysical model to distinguish between these two alternatives. Sequential electronic transitions to a highly excited state and subsequent exciton dissociation is found to be consistent with the observed excitation intensity dependence of the initial transient absorption signal in the spectral region attributed to absorption of charges.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 0 4 - X
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C. Silva et al. / Synthetic Metals 116 (2001) 9±13
Fig. 1. Absorption (dashed lines), photoluminescence (solid lines) and early time transient absorption (solid circles) spectra of PIF derivatives. The absorption and PL spectra corresponding to PIFTO have thin point size, while those corresponding to PIFTEH have thicker point size. The transient absorption spectrum is for PIFTO at a pump fluence of 366 mJ/cm2. The inset shows the structure of PIF, with R n-octyl (PIFTO) or 2-ethylhexyl (PIFTEH).
2. Experimental The synthesis of poly-6,60 ,12,120 -tetraalkyl indeno¯uorene, with either n-octyl (PIFTO) or 2-ethylhexyl (PIFTEH) side groups, is described elsewhere [3]. Films of 100 nm thickness were spin-coated onto Spectrosil substrates from anhydrous p-xylene solutions. Films were prepared and handled in a nitrogen-®lled glove box and were kept in a dynamic vacuum of 10ÿ5 mbar during experiments. The femtosecond transient absorption apparatus is described in detail elsewhere [4]. Brie¯y, femtosecond pulses at a 1 kHz repetition rate were derived from a home-built dye-ampli®ed Ti:sapphire laser system. The pump beam at 390 nm (3.18 eV) was focused on the sample to a 125 mm spot. The weaker probe beam, consisting of a single-®lament white-light continuum, was focused to 50 mm in the same region of the sample after passing through a computercontrolled variable optical delay. Both pump and probe beams were horizontally linearly polarised. Spectrally resolved measurements of the fractional change in probe transmission due to the pump pulse (DT/T) were performed with a 0.25-m spectrometer and a Peltier-cooled CCD camera. The chirp across the white-light continuum was numerically corrected using an empirical determination of the dispersion. Alternatively, detecting a narrow portion of the spectrally dispersed probe beam with silicon photodiodes and lockin techniques allowed single-wavelength measurements to be made. 3. Results and discussion The transient absorption spectrum for PIFTO, 1 ps after excitation, is shown as solid circles in Fig. 1. The positive
Fig. 2. Absorption transients for PIFTO at a pump fluence of (A) 380 mJ/cm2 and (B) 5 mJ/cm2. The probe photon energies are indicated in panel A. The inset shows the same data over a 5 ps window.
signal in the region of PL is probe-induced stimulated emission (SE). To the red of 2.45 eV we observe two broad photoinduced absorption (PA) features peaked at 2.1 eV and at 1.5 eV. Fig. 2 displays the time evolution of SE at 2.64 eV (open circles), PA at 1.46 eV (solid triangles) and 2.14 eV (open double triangles) in two different excitation ¯uence regimes. At a moderately high pump ¯uence (Fig. 2A), the SE dynamics display a subpicosecond component and a slower, 50 ps component. An additional nanosecond component is evident in both PA regions. Ampli®ed spontaneous emission (ASE) assisted by waveguiding in the plane of the ®lm rapidly depletes the singlet exciton population in this excitation regime [5]. Exciton±exciton bimolecular annihilation is also signi®cant and has a similar effect. As the pump ¯uence is lowered (Fig. 2B), the relative importance of the subpicosecond component diminishes. In addition, the nanosecond component of the PA spectral feature at 2.14 eV is suppressed such that the dynamics closely match those at SE. In contrast, the dynamics at 1.46 eV display a nanosecond decay component throughout all of the pump ¯uence range investigated herein. On the basis of this data, we assign the PA features to 1Bu absorption, with an additional contribution from charge pair absorption centred in the near-IR region (1.5 eV). It is important to note that these absorption features are broad and overlapped. We observe signi®cant charge pair
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
11
densities are given respectively by
Fig. 3. Excitation fluence dependence of the initial transient absorption amplitude at the probe photon energies displayed in Fig. 2. These amplitudes were obtained from a fit to multiexponential functions convoluted with the instrument response function. The solid and dashed lines are the result of the photophysical model described by Eqs. (1)±(3) and (5) (see text).
absorption as far blue as 2.1 eV at high ¯uence. We also point out that no signi®cant build-up of the charge pair absorption kinetic component is observed on a picosecond timescale in Fig. 2A. Instead, the transient absorption data appear as a superposition of the 1Bu kinetic signature and an instrument-limited build-up of the charge pair absorption. Furthermore, we do not observe signi®cant difference in the dynamics of PIFTO and PIFTEH [1]. The normalised initial amplitude of the three spectral components is shown in Fig. 3 as a function of excitation ¯uence. This amplitude is determined from multiexponential ®ts to the data. The signals at 2.64 and 2.14 eV, assigned to 1Bu transitions, vary linearly with pump ¯uence up to 30 mJ/cm2 and saturate at higher ¯uences. In contrast, the signal at 1.46 eV, assigned to a combination of 1Bu and charge pair absorption, has signi®cantly higher saturation ¯uence. From the evidence in Figs. 2 and 3 we conclude that an ultrafast charge pair generation mechanism operates by static quenching of neutral excitons. We envision two possibilities of such a mechanism. In the ®rst, 1Bu states can directly dissociate into charge pairs promptly after photoexcitation due to interaction with defects. Alternatively, the 1Bu states can be re-excited to a state with Ag symmetry that is resonant with the pump photon energy and promptly dissociate into charge pairs from the higher-lying state. While both mechanisms result in charge pairs, the 1Bu saturated photophysical behaviour is expected to be quite different. This issue of ultrafast charge generation is important because it has profound implications on the nature of low-lying singlet excitons. In order to address this issue we use two kinetic models that distinguish between the alternative mechanisms. In either case the 1Bu (nex) and charge pair (nch) population
dnex nex R t ÿ ÿ bn2ex ÿ G tnex dt tex
(1)
dnch nch b 2 ÿ n G tnex dt tch 2 ex
(2)
where tex and tch are the exciton and charge unimolecular time constants, respectively and b is the exciton±exciton bimolecular annihilation rate constant. The factor of 1/2 in Eq. (2) arises from the expectation that half of the selfannihilated excitons produce charge pairs, with the rest producing ground states [6]. The rate of pumping of 1Bu, R(t), is given by " # sg Ing t2 exp ÿ 2 (3) R t p 2gpump 2pgpump where sg is the ground-state absorption cross section in cm2, I the intensity in photons/cm2, ng the ground-state chromophore density and gpump is related to the pump pulsewidth. The static quenching term, G(t), is dependent on which static quenching mechanism described above is significant. For the case of direct static quenching of 1Bu excitons, it is given by " # fsq t2 G t p exp ÿ 2 (4) 2gpump 2pgpump where fsq is the yield of static quenching. That is, excitons can dissociate into charge pairs only during the pumping process (i.e. as they are being created). On the other hand, in the case of sequential excitation, the static quenching term is given by G t
fsq R t ng
(5)
where in this case the term fsq is effectively the excited-state to ground-state absorption cross section ratio times the yield of static quenching. The model including Eq. (5) assumes that either return to the lowest excited electronic state is fast compared to gpump or that the yield of highly pumped exciton dissociation is close to one. The difference between the alternative models is simply that Eq. (5) depends on pump intensity. Results from the kinetic models are presented in Fig. 4 and the parameters used in the simulation are presented in the inset of part B. Part A plots the early time (i.e. immediately after the pump process) nex (®lled circles) and nch (open squares) as a function of pump ¯uence using Eqs. (1)± (4), while part B plots the same using Eqs. (1)±(3) and (5). In either case the initial exciton density is saturated above 30 mJ/cm2 pump ¯uence. However, the initial charge density varies differently in the two models. If the initial 1Bu population density is saturated, then that of charge pairs produced by direct dissociation of this species is consequently saturated (Fig. 4A). If, on the other hand, the charge pairs are generated via two sequential electronic transitions,
12
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
Fig. 4. Initial (i.e. immediately after pumping) 1Bu exciton (left axes) and charge (right axis) population densities as a function of pump fluence from the photophysical models: (A) Charges are produced by static quenching from the 1Bu state, (B) Charges are produced from a higher state produced by two sequential transitions. The inset of part B shows the input parameters used in both simulations.
the overall saturation behaviour is governed by the relative values of the absorption cross sections. In a situation where the excited-state to ground-state cross section ratio times the effective yield of charges is small compared to the groundstate cross section the initial charge population density varies quadratically with pump ¯uence at suf®ciently low ¯uence. There is a regime where the ground-state transition is saturated but the excited-state transition is not. In this case the initial charge density varies linearly with pump ¯uence. Finally, at suf®ciently large pump ¯uence (just beyond the ¯uence range in Fig. 4), both transitions saturate and the initial charge density becomes independent of ¯uence. This trend is observed in Fig. 4B and is consistent with our measured saturation behaviour. This is demonstrated in Fig. 3, in which the results of the kinetic modelling are superimposed on the experimental data for comparison. The solid curve is simply the initial exciton density shown in Fig. 4B. In order to reproduce the saturation curve at 1.46 eV, we combined 2.2 times the charge pair saturation curve with the exciton saturation curve in Fig. 4B. It is clear from Fig. 3 that sequential excitation to a highly excited state followed by dissociation is consistent with our measurements. It is worth noting that within this model the ratio of excited-state to ground-state absorption cross sections times the effective yield of charge pairs is 0:1. It is interesting to note that in a previous report by our groups the steady-state photocurrent in a MEH-PPV photo-
cell was observed to be signi®cantly higher at high photon energies compared with the p±p transition energy [7]. This observation was rationalised with quantum chemical calculations in which the hole was ®xed in the centre of a PPV oligomer composed of 11 monomer units and the probability of ®nding the electron at all other sites was computed. It was determined that while the 1Bu state is con®ned to 5 monomer units around the site of the hole, there are higher-lying states that have signi®cant probability of ®nding the electron throughout the whole oligomer. It was argued that the high photocurrent at high photon energies is the result of populating such delocalised states, which can ®nd charge transfer defects with ease compared to the localised 1Bu state. Of course, these are states with Bu symmetry, as opposed to the Ag states likely accessed in our experiment. However, the conclusion that high-lying delocalised states are responsible for exciton dissociation is consistent with our conclusion herein. Recent multiple pulse experiments have suggested that a higher lying kAg state, coupled with the 1Bu state, produces weakly bound polaron pairs [8]. Earlier ultrafast work concluded that biexcitons are responsible for generating interchain electron±hole pairs [9]. Our conclusions are also in agreement with recent photocurrent cross-correlation measurements [10] and with measurements that probe photoinduced polarons via infrared-active vibrational modes (IRAV) in picosecond [11] resolved experiments. IRAVprobe experiments are slightly cleaner than our experiments because they directly probe charge density without the overlap from exciton absorption as in our near-IR probe experiments. However, we are encouraged that this ultrafast charge generation mechanism appears to be general for a broad range of conjugated polymers. Agreement for this charge generation mechanism, however, is not universal. Recent ultrafast work by Moses et al., which probes the IRAV modes, has been interpreted to support direct ultrafast generation of charge pairs. The principal arguments used to reach this conclusion are the lack of correlation between the ultrafast nature of charge generation and the exciton lifetime and a linear dependence on the initial polaron signal [2]. However, we emphasise that these observations are also possible if charge pairs are generated indirectly via sequential one-photon transitions and exciton dissociation. Quadratic intensity dependence is exclusively expected only for a single true two-photon transition involving virtual states. In order to distinguish between the two mechanisms it is important to measure the initial polaron densities at a broad pump ¯uence range and to monitor the extent of saturation of the 1Bu state throughout the entire ¯uence range. 4. Conclusions We have identi®ed the absorption region of the lowest excited 1Bu state in ®lms of soluble derivatives of PIF.
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
Superimposed on this absorption, we observe absorption of charge pairs in the near-IR part of the spectrum. Charge pairs are generated within the pumping process (i:e: < 100 fs) and are generated by sequential excitation to highly excited states followed by ef®cient exciton dissociation. Acknowledgements This work was supported by the EPSRC (RG 25535) and by the European Commission (BRITE EURAM contract number BRPR-CT-97-0469, `OSCA'). References [1] C. Silva, D.M. Russell, M.A. Stevens, J.D. Mackenzie, S. Setayesh, K. MuÈllen, R.H. Friend, Chem. Phys. Lett. 319 (2000) 494.
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[2] D. Moses, A. Dogariu, A.J. Heeger, Chem. Phys. Lett. 316 (2000) 356. [3] S. Setayesh, D. Marsitzky, K. MuÈllen, Macromolecules 33 (2000) 2016. [4] M.A. Stevens, C. Silva, D.M. Russell, R.H. Friend, submitted to Phys. Rev. B, `in preparation'. [5] R.G. Kepler, V.S. Valencia, S.J. Jacobs, J.J. McNamara, Synth. Met. 78 (1996) 227. [6] M. Pope, C.E. Swenberg (Eds.), Electronic Processes in Organic Crystals, Oxford University Press, Oxford, 1982. [7] A. KoÈhler, D.A. dos Santos, D. Beljonne, Z. Shuai, J.-L. BreÂdas, A.B. Holmes, A. Kraus, K. MuÈllen, R.H. Friend, Nature 392 (1998) 903. [8] S.V. Frolov, Z. Bao, Z.V. Vardeny, Synth. Met. 116 (2001) 5±7. [9] V.I. Klimov, D.W. McBranch, N. Barashkov, J. Ferraris, Phys. Rev. B 58 (1998) 7654. [10] C. Zenz, G. Lanzani, G. Cerullo, W. Graupner, G. Leising, U. Scherf, S.D. Silvestri, Z. Christian, Synth. Met. 116 (2001) 27±30. [11] U. Mizrahi, I. Shtrichman, E. Ehrenfreund, D. Gershoni, Z.V. Vardeny, Synth. Met. 116 (2001) (this issue).
Ultrafast charge photogeneration in conjugated polymer thin ®lms Carlos Silvaa, Mark A. Stevensa, David M. Russella, Sepas Setayeshb, Klaus MuÈllenb, Richard H. Frienda,* a
Optoelectronics Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
b
Abstract We report femtosecond transient absorption spectroscopy on thin ®lms of blue-emitting derivatives of polyindeno¯uorene (PIF). Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped 1Bu exciton and charge pair absorption spectra. We ®nd that charge pairs are produced within the instrument resolution (100 fs) followed by exciton±exciton bimolecular annihilation on a picosecond timescale. Two possible mechanisms for ultrafast charge generation are considered: direct charge separation of nascent 1Bu excitons by static quenching at intrinsic defects and sequential transitions to produce highly energetic excitons that dissociate ef®ciently. Photophysical modelling of intensity-dependent data reveals that sequential absorption followed by charge separation is the most likely mechanism for ultrafast charge pair generation, with the ratio of excited-state to ground-state absorption cross section times the effective yield of charge pairs 0:1. These observations are consistent with previous photocurrent measurements and quantum chemical calculations assigning highly excited states to greatly delocalised excitons [KoÈhler et al., Nature 392 (1998) 903]. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Femtosecond transient absorption; Conjugated polymers; Excitations; Polyindeno¯uorene
1. Introduction Owing to their promise as active materials for optoelectronic applications, luminescent conjugated polymers have been the focus of intensive research for over a decade. Investigation of the photophysics of these materials is important not only from the viewpoint of the technological development of organic devices, but also from a fundamental perspective. The basic understanding of primary photoexcitations in low-dimensional electronic structures provides the basis for theoretical modelling of this class of materials. In a recent publication [1] we reported femtosecond transient absorption measurements on thin ®lms of two derivatives of polyindeno¯uorene (PIF), a processable `step-ladder' analogue of poly-p-phenylene (see inset of Fig. 1). Detailed excitation intensity-dependence measurements allowed the unravelling of 1Bu absorption spectrum from absorption of charged photoexcitations. The two absorption bands are broadly overlapped and it was found * Corresponding author. Tel.: 44-1223-337218; fax: 44-1223-353397. E-mail address: [email protected] (R.H. Friend).
that charge generation occurred on an ultrafast timescale < 100 fs), too early for it to be uniquely a result of exciton±exciton bimolecular annihilation. Two possible mechanisms can account for such prompt charge generation. In the ®rst, nascent excitons in close proximity to intrinsic defects may be directly separated into a charge pair by static quenching. This process requires a rather high concentration of defects or suf®ciently large delocalisation of the lowest excitonic wave function accompanied by a low binding energy [2]. Alternatively, an exciton may be promoted to a higher lying state resonant with the pump laser during the duration of the pump pulse. This highly excited state may dissociate ef®ciently producing charge pairs. Both of these processes would produce charge pairs on a shorter time scale than expected for exciton±exciton annihilation. In this paper we focus on the ultrafast charge generation mechanism by employing a photophysical model to distinguish between these two alternatives. Sequential electronic transitions to a highly excited state and subsequent exciton dissociation is found to be consistent with the observed excitation intensity dependence of the initial transient absorption signal in the spectral region attributed to absorption of charges.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 0 4 - X
10
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
Fig. 1. Absorption (dashed lines), photoluminescence (solid lines) and early time transient absorption (solid circles) spectra of PIF derivatives. The absorption and PL spectra corresponding to PIFTO have thin point size, while those corresponding to PIFTEH have thicker point size. The transient absorption spectrum is for PIFTO at a pump fluence of 366 mJ/cm2. The inset shows the structure of PIF, with R n-octyl (PIFTO) or 2-ethylhexyl (PIFTEH).
2. Experimental The synthesis of poly-6,60 ,12,120 -tetraalkyl indeno¯uorene, with either n-octyl (PIFTO) or 2-ethylhexyl (PIFTEH) side groups, is described elsewhere [3]. Films of 100 nm thickness were spin-coated onto Spectrosil substrates from anhydrous p-xylene solutions. Films were prepared and handled in a nitrogen-®lled glove box and were kept in a dynamic vacuum of 10ÿ5 mbar during experiments. The femtosecond transient absorption apparatus is described in detail elsewhere [4]. Brie¯y, femtosecond pulses at a 1 kHz repetition rate were derived from a home-built dye-ampli®ed Ti:sapphire laser system. The pump beam at 390 nm (3.18 eV) was focused on the sample to a 125 mm spot. The weaker probe beam, consisting of a single-®lament white-light continuum, was focused to 50 mm in the same region of the sample after passing through a computercontrolled variable optical delay. Both pump and probe beams were horizontally linearly polarised. Spectrally resolved measurements of the fractional change in probe transmission due to the pump pulse (DT/T) were performed with a 0.25-m spectrometer and a Peltier-cooled CCD camera. The chirp across the white-light continuum was numerically corrected using an empirical determination of the dispersion. Alternatively, detecting a narrow portion of the spectrally dispersed probe beam with silicon photodiodes and lockin techniques allowed single-wavelength measurements to be made. 3. Results and discussion The transient absorption spectrum for PIFTO, 1 ps after excitation, is shown as solid circles in Fig. 1. The positive
Fig. 2. Absorption transients for PIFTO at a pump fluence of (A) 380 mJ/cm2 and (B) 5 mJ/cm2. The probe photon energies are indicated in panel A. The inset shows the same data over a 5 ps window.
signal in the region of PL is probe-induced stimulated emission (SE). To the red of 2.45 eV we observe two broad photoinduced absorption (PA) features peaked at 2.1 eV and at 1.5 eV. Fig. 2 displays the time evolution of SE at 2.64 eV (open circles), PA at 1.46 eV (solid triangles) and 2.14 eV (open double triangles) in two different excitation ¯uence regimes. At a moderately high pump ¯uence (Fig. 2A), the SE dynamics display a subpicosecond component and a slower, 50 ps component. An additional nanosecond component is evident in both PA regions. Ampli®ed spontaneous emission (ASE) assisted by waveguiding in the plane of the ®lm rapidly depletes the singlet exciton population in this excitation regime [5]. Exciton±exciton bimolecular annihilation is also signi®cant and has a similar effect. As the pump ¯uence is lowered (Fig. 2B), the relative importance of the subpicosecond component diminishes. In addition, the nanosecond component of the PA spectral feature at 2.14 eV is suppressed such that the dynamics closely match those at SE. In contrast, the dynamics at 1.46 eV display a nanosecond decay component throughout all of the pump ¯uence range investigated herein. On the basis of this data, we assign the PA features to 1Bu absorption, with an additional contribution from charge pair absorption centred in the near-IR region (1.5 eV). It is important to note that these absorption features are broad and overlapped. We observe signi®cant charge pair
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
11
densities are given respectively by
Fig. 3. Excitation fluence dependence of the initial transient absorption amplitude at the probe photon energies displayed in Fig. 2. These amplitudes were obtained from a fit to multiexponential functions convoluted with the instrument response function. The solid and dashed lines are the result of the photophysical model described by Eqs. (1)±(3) and (5) (see text).
absorption as far blue as 2.1 eV at high ¯uence. We also point out that no signi®cant build-up of the charge pair absorption kinetic component is observed on a picosecond timescale in Fig. 2A. Instead, the transient absorption data appear as a superposition of the 1Bu kinetic signature and an instrument-limited build-up of the charge pair absorption. Furthermore, we do not observe signi®cant difference in the dynamics of PIFTO and PIFTEH [1]. The normalised initial amplitude of the three spectral components is shown in Fig. 3 as a function of excitation ¯uence. This amplitude is determined from multiexponential ®ts to the data. The signals at 2.64 and 2.14 eV, assigned to 1Bu transitions, vary linearly with pump ¯uence up to 30 mJ/cm2 and saturate at higher ¯uences. In contrast, the signal at 1.46 eV, assigned to a combination of 1Bu and charge pair absorption, has signi®cantly higher saturation ¯uence. From the evidence in Figs. 2 and 3 we conclude that an ultrafast charge pair generation mechanism operates by static quenching of neutral excitons. We envision two possibilities of such a mechanism. In the ®rst, 1Bu states can directly dissociate into charge pairs promptly after photoexcitation due to interaction with defects. Alternatively, the 1Bu states can be re-excited to a state with Ag symmetry that is resonant with the pump photon energy and promptly dissociate into charge pairs from the higher-lying state. While both mechanisms result in charge pairs, the 1Bu saturated photophysical behaviour is expected to be quite different. This issue of ultrafast charge generation is important because it has profound implications on the nature of low-lying singlet excitons. In order to address this issue we use two kinetic models that distinguish between the alternative mechanisms. In either case the 1Bu (nex) and charge pair (nch) population
dnex nex R t ÿ ÿ bn2ex ÿ G tnex dt tex
(1)
dnch nch b 2 ÿ n G tnex dt tch 2 ex
(2)
where tex and tch are the exciton and charge unimolecular time constants, respectively and b is the exciton±exciton bimolecular annihilation rate constant. The factor of 1/2 in Eq. (2) arises from the expectation that half of the selfannihilated excitons produce charge pairs, with the rest producing ground states [6]. The rate of pumping of 1Bu, R(t), is given by " # sg Ing t2 exp ÿ 2 (3) R t p 2gpump 2pgpump where sg is the ground-state absorption cross section in cm2, I the intensity in photons/cm2, ng the ground-state chromophore density and gpump is related to the pump pulsewidth. The static quenching term, G(t), is dependent on which static quenching mechanism described above is significant. For the case of direct static quenching of 1Bu excitons, it is given by " # fsq t2 G t p exp ÿ 2 (4) 2gpump 2pgpump where fsq is the yield of static quenching. That is, excitons can dissociate into charge pairs only during the pumping process (i.e. as they are being created). On the other hand, in the case of sequential excitation, the static quenching term is given by G t
fsq R t ng
(5)
where in this case the term fsq is effectively the excited-state to ground-state absorption cross section ratio times the yield of static quenching. The model including Eq. (5) assumes that either return to the lowest excited electronic state is fast compared to gpump or that the yield of highly pumped exciton dissociation is close to one. The difference between the alternative models is simply that Eq. (5) depends on pump intensity. Results from the kinetic models are presented in Fig. 4 and the parameters used in the simulation are presented in the inset of part B. Part A plots the early time (i.e. immediately after the pump process) nex (®lled circles) and nch (open squares) as a function of pump ¯uence using Eqs. (1)± (4), while part B plots the same using Eqs. (1)±(3) and (5). In either case the initial exciton density is saturated above 30 mJ/cm2 pump ¯uence. However, the initial charge density varies differently in the two models. If the initial 1Bu population density is saturated, then that of charge pairs produced by direct dissociation of this species is consequently saturated (Fig. 4A). If, on the other hand, the charge pairs are generated via two sequential electronic transitions,
12
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
Fig. 4. Initial (i.e. immediately after pumping) 1Bu exciton (left axes) and charge (right axis) population densities as a function of pump fluence from the photophysical models: (A) Charges are produced by static quenching from the 1Bu state, (B) Charges are produced from a higher state produced by two sequential transitions. The inset of part B shows the input parameters used in both simulations.
the overall saturation behaviour is governed by the relative values of the absorption cross sections. In a situation where the excited-state to ground-state cross section ratio times the effective yield of charges is small compared to the groundstate cross section the initial charge population density varies quadratically with pump ¯uence at suf®ciently low ¯uence. There is a regime where the ground-state transition is saturated but the excited-state transition is not. In this case the initial charge density varies linearly with pump ¯uence. Finally, at suf®ciently large pump ¯uence (just beyond the ¯uence range in Fig. 4), both transitions saturate and the initial charge density becomes independent of ¯uence. This trend is observed in Fig. 4B and is consistent with our measured saturation behaviour. This is demonstrated in Fig. 3, in which the results of the kinetic modelling are superimposed on the experimental data for comparison. The solid curve is simply the initial exciton density shown in Fig. 4B. In order to reproduce the saturation curve at 1.46 eV, we combined 2.2 times the charge pair saturation curve with the exciton saturation curve in Fig. 4B. It is clear from Fig. 3 that sequential excitation to a highly excited state followed by dissociation is consistent with our measurements. It is worth noting that within this model the ratio of excited-state to ground-state absorption cross sections times the effective yield of charge pairs is 0:1. It is interesting to note that in a previous report by our groups the steady-state photocurrent in a MEH-PPV photo-
cell was observed to be signi®cantly higher at high photon energies compared with the p±p transition energy [7]. This observation was rationalised with quantum chemical calculations in which the hole was ®xed in the centre of a PPV oligomer composed of 11 monomer units and the probability of ®nding the electron at all other sites was computed. It was determined that while the 1Bu state is con®ned to 5 monomer units around the site of the hole, there are higher-lying states that have signi®cant probability of ®nding the electron throughout the whole oligomer. It was argued that the high photocurrent at high photon energies is the result of populating such delocalised states, which can ®nd charge transfer defects with ease compared to the localised 1Bu state. Of course, these are states with Bu symmetry, as opposed to the Ag states likely accessed in our experiment. However, the conclusion that high-lying delocalised states are responsible for exciton dissociation is consistent with our conclusion herein. Recent multiple pulse experiments have suggested that a higher lying kAg state, coupled with the 1Bu state, produces weakly bound polaron pairs [8]. Earlier ultrafast work concluded that biexcitons are responsible for generating interchain electron±hole pairs [9]. Our conclusions are also in agreement with recent photocurrent cross-correlation measurements [10] and with measurements that probe photoinduced polarons via infrared-active vibrational modes (IRAV) in picosecond [11] resolved experiments. IRAVprobe experiments are slightly cleaner than our experiments because they directly probe charge density without the overlap from exciton absorption as in our near-IR probe experiments. However, we are encouraged that this ultrafast charge generation mechanism appears to be general for a broad range of conjugated polymers. Agreement for this charge generation mechanism, however, is not universal. Recent ultrafast work by Moses et al., which probes the IRAV modes, has been interpreted to support direct ultrafast generation of charge pairs. The principal arguments used to reach this conclusion are the lack of correlation between the ultrafast nature of charge generation and the exciton lifetime and a linear dependence on the initial polaron signal [2]. However, we emphasise that these observations are also possible if charge pairs are generated indirectly via sequential one-photon transitions and exciton dissociation. Quadratic intensity dependence is exclusively expected only for a single true two-photon transition involving virtual states. In order to distinguish between the two mechanisms it is important to measure the initial polaron densities at a broad pump ¯uence range and to monitor the extent of saturation of the 1Bu state throughout the entire ¯uence range. 4. Conclusions We have identi®ed the absorption region of the lowest excited 1Bu state in ®lms of soluble derivatives of PIF.
C. Silva et al. / Synthetic Metals 116 (2001) 9±13
Superimposed on this absorption, we observe absorption of charge pairs in the near-IR part of the spectrum. Charge pairs are generated within the pumping process (i:e: < 100 fs) and are generated by sequential excitation to highly excited states followed by ef®cient exciton dissociation. Acknowledgements This work was supported by the EPSRC (RG 25535) and by the European Commission (BRITE EURAM contract number BRPR-CT-97-0469, `OSCA'). References [1] C. Silva, D.M. Russell, M.A. Stevens, J.D. Mackenzie, S. Setayesh, K. MuÈllen, R.H. Friend, Chem. Phys. Lett. 319 (2000) 494.
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[2] D. Moses, A. Dogariu, A.J. Heeger, Chem. Phys. Lett. 316 (2000) 356. [3] S. Setayesh, D. Marsitzky, K. MuÈllen, Macromolecules 33 (2000) 2016. [4] M.A. Stevens, C. Silva, D.M. Russell, R.H. Friend, submitted to Phys. Rev. B, `in preparation'. [5] R.G. Kepler, V.S. Valencia, S.J. Jacobs, J.J. McNamara, Synth. Met. 78 (1996) 227. [6] M. Pope, C.E. Swenberg (Eds.), Electronic Processes in Organic Crystals, Oxford University Press, Oxford, 1982. [7] A. KoÈhler, D.A. dos Santos, D. Beljonne, Z. Shuai, J.-L. BreÂdas, A.B. Holmes, A. Kraus, K. MuÈllen, R.H. Friend, Nature 392 (1998) 903. [8] S.V. Frolov, Z. Bao, Z.V. Vardeny, Synth. Met. 116 (2001) 5±7. [9] V.I. Klimov, D.W. McBranch, N. Barashkov, J. Ferraris, Phys. Rev. B 58 (1998) 7654. [10] C. Zenz, G. Lanzani, G. Cerullo, W. Graupner, G. Leising, U. Scherf, S.D. Silvestri, Z. Christian, Synth. Met. 116 (2001) 27±30. [11] U. Mizrahi, I. Shtrichman, E. Ehrenfreund, D. Gershoni, Z.V. Vardeny, Synth. Met. 116 (2001) (this issue).