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Dynamics of photoexcitations in a poly[3-dodecylthiophene] thin film studied by femtosecond visible–near-infrared absorption spectroscopy
15 May 1998
Chemical Physics Letters 288 Ž1998. 165–170
Dynamics of photoexcitations in a polyw3-dodecylthiophenex thin film studied by femtosecond visible–near-infrared absorption spectroscopy A. Matsuse a , S. Takeuchi b
a,1
, K. Yoshino b, T. Kobayashi
a
a Department of Physics, UniÕersity of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan Department of Electronic Engineering, Osaka UniÕersity, Yamada-Oka 2-1, Suita, Osaka 565, Japan
Received 28 November 1997; in final form 19 February 1998
Abstract Femtosecond transient absorption of a polyŽ3-dodecylthiophene. film is measured in a wide spectral range from visible to near-infrared. Our results show that the transient absorption consists of four decay components. The two faster components are assigned to self-trapping and the subsequent relaxation process of photogenerated excitons. The relaxation time constant of the self-trapped exciton is evaluated as 680 " 50 fs. The excited species corresponding to the third slow component is identified as a pair of intrachain, oppositely-charged polarons on the basis of its power-law decay and its spectral peak around 1.4 eV. A long-lived transient absorption due to interchain polarons is also observed. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In recent years, progress in the field of high-speed optoelectronics and information processing has inspired the search for new materials with large thirdorder nonlinear optical susceptibilities Žsee, e.g., w1x.. One-dimensional conjugated polymers are one of the promising candidates for future practical nonlinear optical devices. As a consequence of their often large nonlinear optical susceptibilities, ultrafast responses, and quasi-one-dimensional structures, the interest of many scientists has been directed to their electrical and optical properties in order to study the relevant energy band structures and the mechanisms of optical nonlinearity. 1 Present address: Department of VUV Photoscience, Institute for Molecular Science, Myodaiji, Okazaki 444, Japan.
There have been many studies reported for transpolyacetylene and its derivatives, which have the simplest backbone structure with alternating carbon–carbon single and double bonds w2–5x. It is well established that solitons are formed as localized excitations in such polymers with a degenerate ground state w6–9x. On the other hand, polythiophene ŽPT. and its derivatives have a structure which is similar to that of cis-polyacetylene and is stabilized by the sulfur atom interacting only weakly with the p-electron system of the backbone. Due to their non-degenerate ground state, the soliton cannot exist but polarons and bipolarons are formed after photoexcitation w1,10x. The absorption bands associated with the polarons and bipolarons appear in the near-infrared region w7,10x. Therefore, it is necessary to investigate the absorption behavior of these polymers in the near-in-
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 2 6 1 - 9
166
A. Matsuse et al.r Chemical Physics Letters 288 (1998) 165–170
frared region in order to determine the photoexcited species formed in polymers. In this Letter, we report the first results of femtosecond pump–probe experiments over a wide spectral range from visible to near-infrared Ž500–1600 n m . o n a th in film o f wp o ly Ž3 dodecylthiophene.xŽP3DT., which has a side group of R s ŽCH 2 .11CH 3 .
2. Experimental The experimental apparatus for pump–probe transient absorption measurement in the visible–near-infrared region has been reported previously w11x. The light source is a Ti:sapphire oscillatorramplifier system w12x. It produces 0.45 mJ amplified pulses at 800 nm Ž1.55 eV. with a typical duration of 200 fs. The amplified pulse is divided into two parts. One is frequency-doubled in a 3 mm thick LiB 3 O5 crystal and the generated second harmonic at 400 nm Ž3.10 eV. is used as a pump pulse. The other is focused into a cell containing carbon tetrachloride to generate a white-light continuum pulse by self-phase modulation. It is used as a probe and a reference, both being detected with a polychromatorrmultichannel-camera system. The delay time between the pump and the probe pulses is changed by using a mechanical stage driven by a stepping motor. The time resolution of the pump-probe measurements ranges from 240 to 300 fs depending on the probe-photon energy. A P3DT thin film was electrochemically polymerized on a CaF2 substrate w13x. The film sample was stable in air at room temperature. The stationary absorption spectrum of the film was recorded on a commercial spectrometer.
Photoinduced difference absorption spectra of a P3DT thin film measured at room temperature are shown in Fig. 1 for several delay times between the pump and probe pulses. The pump and probe polarizations are parallel to each other, and the excitation density is ; 1 = 10 16 photonsrcm2 at 3.10 eV. A broad photoinduced absorption ŽPIA. band is observed in the energy region below the stationary absorption edge instantaneously with photoexcitation Ž0 ps.. A strong bleaching of the ground-state absorption is also seen above 2.0 eV. The PIA band changes from this initial flat spectrum to a doublypeaked spectrum in 1 ps: two PIA peaks at 1.2–1.3 and 1.8–1.9 eV become noticeable because the PIA signals at - 1 and 1.4–1.5 eV exhibit a rapid decay as compared to the two PIA peaks. Of these two peaks, the PIA peak at 1.8–1.9 eV decays faster than the other. Thus, only one broad PIA peak around 1.3 eV is dominant at delay times longer than 10 ps.
3. Results and discussion The lowest-energy electronic absorption band of P3DT rises at ; 1.9 eV and has a peak at 2.4 eV. This band is assigned mainly to a p–p ) transition along the one-dimensional polymer chain. Upon photoexcitation of this band at 3.1 eV, which is 0.7 eV higher than the peak energy, electron–hole pairs are initially produced via the predominant intrachain excitation.
Fig. 1. Photoinduced difference absorption spectra of a P3DT thin film obtained at several delay times. The signals in the spectral region of 1.5–1.6 eV are not shown, because they are distorted owing to spiky structures on the probe and reference spectra. Stationary absorption spectrum of the P3DT film ŽAbs.. is also shown on the top.
A. Matsuse et al.r Chemical Physics Letters 288 (1998) 165–170
These observed features of the PIA spectra do not depend on the excitation density. In our previous paper w14x, the femtosecond transient absorption spectra of P3DT with photoexcitation at 1.97 eV were reported for a probe-photon energy region of 1.3–2.6 eV. The PIA peak at 1.9 eV was observed as well as the broad PIA band at early delay times, but no clear indication of the PIA peak around 1.3 eV was made because of their limited probe-photon energy region. The wider probe-photon energy region in the present study Ž0.78–2.6 eV. has enabled us to find the PIA peak around 1.3 eV. The time-dependent shape of the PIA spectra in Fig. 1 suggests that two or more decay components contribute to the observed PIA signals. In order to examine these decay components, we plot the time dependence of the PIA signals at four probe-photon energies in Fig. 2. At 1.0 eV, the PIA signal exhibits an initial rapid decay in 1 ps, being similar to the cross-correlation trace at the probe-photon energy. This indicates that the decay component Ž‘ultrafast’ component. almost vanishes within a delay time shorter than the resolution time of the measurement. It is obvious from the plot at 1.9 eV that there exists
Fig. 2. Time dependence of the absorbance change at four probephoton energies. The solid circles are experimental data and the solid curves are best-fitted curves Žsee text..
167
Fig. 3. Logarithmic plot of the time dependence of the absorbance change at 1.4 eV after subtraction of the long-lived component. The straight line represents the best-fitted power-law function. The power constant evaluated by the slope of the line is ns 0.56 "0.05.
another component which decays exponentially within a few ps Ž‘fast’ component.. The decay time constant of this fast component is determined as texp s 680 " 50 fs by a fitting analysis taking account of the finite time resolution. In addition to these ultrafast and fast components which characterize the PIA signals at early delay times, a much slower decay component and a long-lived component are also seen clearly for the decays at 1.2 and 1.4 eV. The former ‘slow’ component does not decay exponentially but rather follows a power-law decay Žf tyn . as shown by a logarithmic plot of the decay at 1.40 eV up to 100 ps ŽFig. 3.. The power constant is evaluated as n s 0.56 " 0.05 from the slope of this plot, and it is constant in the probe-photon energy region of 1.3–1.5 eV. The long-lived component is seen as an almost constant signal within the measured time region. Thus, it is concluded from the time dependence in Fig. 2 that the observed PIA signal of the P3DT thin film consists of four decay components in total. Relaxation of photoexcitations in polythiophene derivatives has been discussed in terms of a selftrapping process of an exciton w14x. A photogenerated free exciton or electron–hole pair quickly induces bond-order distortions around itself via strong coupling between electronic excitations and lattice vibrations of the polymer main chain. Since there is no energy barrier for the self-trapping process of the
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one-dimensional exciton, its initial coupling to the C–C stretching vibrations of the polymer chains is expected to take place within a few vibrational periods of 10–20 fs Žnonthermal self-trapped exciton ŽSTE. w15x.. The nonthermal STE subsequently decays toward the bottom of the STE potential Žquasithermal STE w15x. by emitting phonons, i.e. vibrational energy dissipation from the strongly coupled C–C modes to other low-frequency modes. The quasi-thermal STE is the exciton before full thermalization including lattice vibration but after the vibrational energy equilibration among intrachain vibrational modes w15x. The time constant for this phonon emission process was reported as 100 " 50 and 70 " 50 fs for P3DT and polyŽ3-methylthiophene. ŽP3MT., respectively w14,16x. These time constants are shorter than the time resolution of the present study. Thus, because of the short lifetime and the resolutionlimited rise of the ultrafast component observed in the present study, the component can be assigned to both the free exciton and the nonthermal STE which decay by the self-trapping and phonon emission processes, respectively. These two processes cannot be resolved in the present time-resolved measurement. As for the quasi-thermal STE, its decay time constant was so far evaluated as 0.3–0.5 and 0.8 " 0.1 ps for P3DT and P3MT, respectively w14,16x. Since the time constant of the fast component Ž680 fs. is similar to these reported values in the sub-picosecond region, the fast component is ascribable to the quasi-thermal STE. The long-lived component observed in a P3DT thin film was already assigned to a pair of polarons generated on different polymer chains via interchain excitation Žinterchain polaron. on the basis of its pump-polarization dependence and pump-photon density dependence w14x. The time dependence of the PIA signal is well reproduced by the following decay function:
times longer than terf . The parameters for this term, n s 0.56 " 0.05 and terf s 0.1 " 0.05 ps, are evaluated at 1.40 eV, where the PIA signal most likely decays with the power-law behavior as shown in Fig. 3. The fitting is performed using the least-squares method by adjusting the four coefficients. The cross-correlation trace at each probe-photon energy is convoluted with Eq. Ž1. to take into account the energy-dependent time resolution. As shown by solid curves in Fig. 2, the time dependence of the observed PIA signal is well reproduced by this fitting at every probe-photon energy. Spectra of the ultrafast, fast, slow and long-lived components are given respectively by the coefficients AŽ Õ ., B Ž Õ ., C Ž Õ . and DŽ Õ . obtained from the fitting analysis ŽFig. 4.. The ultrafast component exhibits a broad spectrum extending in the energy region below 1.7 eV, while the fast component has a peak around 1.8 eV. This drastic spectral change between the ultrafast and the fast components corresponds to the relaxation from the free exciton Žor electron–hole pair. to the quasi-thermal STE by the self-trapping and subsequent phonon emission processes. The peak around 1.8 eV might be assigned to a transition from the quasi-thermal STE to a biexcitonic state w11,16x. The spectrum of the slow component found in the present study C Ž Õ ., exhibits a broad peak around 1.4
D A Ž t , Õ . s A Ž Õ . d Ž t . q B Ž Õ . exp Ž ytrtexp . qC Ž Õ . erf Ž trterf .
yn
q DŽ Õ. ,
Ž 1.
where erfw x is the error function. The first term corresponds to the ultrafast component. It is described by the delta function d Ž t . because of its short life time. The third term represents the slow component which follows the power-law decay. This term is approximated as C Ž Õ .Ž trterf .yn for delay
Fig. 4. Wavelength dependence of the coefficients of the four components in Eq. Ž1.. v: ultrafast component AŽ Õ ., ^: fast component B Ž Õ ., B: slow component C Ž Õ ., e: long-lived component DŽ Õ .. The data points in the spectral region of 2.0–2.2 eV and above 2.3 eV are not shown, because absorbance change and probe-pulse intensity in these regions are too small to make a reasonable analysis.
A. Matsuse et al.r Chemical Physics Letters 288 (1998) 165–170
eV. It is similar to the spectrum of the long-lived component DŽ Õ ., which was already assigned to the interchain polaron generated by interchain photoexcitation w14x. This spectral similarity suggests that the slow component also corresponds to polaronic excitation. Since the slow component follows the powerlaw decay which is characteristic of a geminate recombination process of two excitations in a one-dimensional polymer chain w17x, we assign it to a pair of oppositely-charged polarons which are generated on the same polymer chain by intrachain photoexcitaion Žintrachain polaron pair.. This is clearly distinguished from the interchain polaron also by its lifetime: the interchain polaron decays in a micro- to mili-second time region by interchain hopping process w10x, while the intrachain polaron vanishes much faster due to its confined nature in the one-dimensional polymer chain. It is noteworthy that the excited species in doped-polythiophene is not a bipolaron but is recently reassigned as a polaron on the basis of the Raman data obtained from the radical cation and dication of a thiophene oligomer w18x. It is expected that a polaron is also generated in photoexcited polythiophene w19x. Since one of the three intragap electronic transitions associated with the polaron is symmetrically forbidden, only two major PIA peaks are observed w19,20x; the two peak energies were reported as 0.65 and 1.5 eV for the doping-induced polaron, and 0.5 and 1.35 eV for the photoinduced polaron w20x. The higher peak energy for the photoinduced polaron Ž1.35 eV. agrees well with the peak energy of the slow component Ž1.4 eV.. This agreement of the two peak energies strongly supports the assignment of the slow component to a pair of intrachain polarons although detailed information about the intrachain polaron is still scarce. The lowenergy peak expected to appear around 0.5 eV will give further information on the formation and relaxation dynamics of the intrachain polaron. Finally, we note that the intrachain polaron can also be generated by relaxation from the STE state Ž‘indirect’ pathway., in addition to its direct excitation from the ground state Ž‘direct’ pathway. described in Eq. Ž1.. Although these two pathways are not completely distinguishable only by the transient absorption data in Fig. 1, the following analysis of the PIA signal at 1.4 eV Žpeak energy of the intra-
169
chain polaron. indicates that the direct pathway is still a major process. Here, we consider only the fast and the slow components corresponding to the STE and the intrachain polaron, respectively, after subtraction of the other two components, i.e. the ultrafast and long-lived components, from the obtained PIA signal. This is because the last two components do not have a chance to mix up with the first two components owing to the large difference in their time dependence. We have made a fitting analysis of the PIA signal after the subtraction by using the following function, searching for the best B and C values for certain k value:
D A Ž t . s B exp Ž ytrtexp . q C k erf Ž trterf .
½
qŽ 1 y k .
1
texp
t
X
H0 exp Žyt rt
=erf Ž t y t X . rterf
exp
yn
4 d tX 5 ,
yn
. Ž 2.
where the parameter k Ž0 F k F 1. denotes the fraction of the direct pathway. The results of this fitting analysis are shown in Fig. 5 for several different k values. As seen in this figure, the calculated curve starts to deviate from the data points as the k value becomes smaller than 1.0. The k value between 0.8 to 1.0 seems to give satisfactory results. Thus, the contribution of the indirect pathway for generation of the intrachain polaron is - 20% and is concluded to be a minor process.
Fig. 5. Photoinduced absorption signal at 1.4 eV after subtraction of the ultrafast and long-lived components Žsee text.. The calculated curves for three k values are also shown; solid line: k s1.0, broken line: k s 0.8, dotted line: k s 0.6.
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Acknowledgements The authors thank Dr. S. Hughes for his careful reading of the manuscript. This work is partly supported by a Proposal-Based Advanced Industrial Technology R & D Program.
References w1x T. Kobayashi ŽEd.., Relaxation in Polymers, World Scientific, Singapore, 1993. w2x C.V. Shank, R. Yen, R.L. Fork, J. Orenstein, G.L. Baker, Phys. Rev. Lett. 22 Ž1982. 1660. w3x G.B. Blanchet, C.R. Fincher, T.C. Chung, A.J. Heeger, Phys. Rev. Lett. 50 Ž1983. 1938. w4x Z. Vardeny, J. Orentein, G.L. Baker, Phys. Rev. Lett. 50 Ž1983. 2032. w5x Z. Vardeny, E. Ehrenfreund, O. Brafman, B. Horovitz, Phys. Rev. Lett. 51 Ž1983. 2326. w6x H. Takayama, Y.R. Lin-Liu, K. Maki, Phys. Rev. B 21 Ž1980. 2388.
w7x W.P. Su, J.R. Schrieffer, A.J. Heeger, Phys. Rev. B 22 Ž1980. 2099. w8x B. Horovitz, Phys. Rev. B 22 Ž1980. 1101. w9x A.J. Heeger, S. Kivelson, J.R. Schrieffer, W.P. Su, Rev. Mod. Phys. 60 Ž1988. 781. w10x G.S. Kanner, X. Wei, B.C. Hess, L.R. Chen, Z.V. Vardeny, Phys. Rev. Lett. 69 Ž1992. 538. w11x S. Takeuchi, T. Masuda, T. Kobayashi, J. Chem. Phys. 105 Ž1996. 2859. w12x S. Takeuchi, T. Kobayashi, Opt. Commun. 109 Ž1994. 518. w13x K. Kaneto, Y. Kohno, K. Yoshino, Solid State Commun. 51 Ž1984. 267. w14x M. Yoshizawa, A. Yasuda, T. Kobayashi, Appl. Phys. B 53 Ž1991. 296. w15x M. Yoshizawa, K. Nishiyama, M. Fujihira, T. Kobayashi, Chem. Phys. Lett. 207 Ž1993. 461. w16x U. Stamm, M. Taiji, M. Yoshizawa, K. Yoshino, T. Kobayashi, Mol. Cryst. Liq. Cryst. 182A Ž1990. 147. w17x I.V. Zozulenko, Solid State Commun. 76 Ž1990. 1035. w18x N. Yokonuma, Y. Furukawa, M. Tasumi, M. Kuroda, J. Nakayama, Chem. Phys. Lett. 255 Ž1996. 431. w19x P.A. Lane, Z. Wei, Z.V. Vardeny, Phys. Rev. Lett. 77 Ž1996. 1544. w20x Y. Furukawa, J. Phys. Chem. 100 Ž1996. 15644.
Chemical Physics Letters 288 Ž1998. 165–170
Dynamics of photoexcitations in a polyw3-dodecylthiophenex thin film studied by femtosecond visible–near-infrared absorption spectroscopy A. Matsuse a , S. Takeuchi b
a,1
, K. Yoshino b, T. Kobayashi
a
a Department of Physics, UniÕersity of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan Department of Electronic Engineering, Osaka UniÕersity, Yamada-Oka 2-1, Suita, Osaka 565, Japan
Received 28 November 1997; in final form 19 February 1998
Abstract Femtosecond transient absorption of a polyŽ3-dodecylthiophene. film is measured in a wide spectral range from visible to near-infrared. Our results show that the transient absorption consists of four decay components. The two faster components are assigned to self-trapping and the subsequent relaxation process of photogenerated excitons. The relaxation time constant of the self-trapped exciton is evaluated as 680 " 50 fs. The excited species corresponding to the third slow component is identified as a pair of intrachain, oppositely-charged polarons on the basis of its power-law decay and its spectral peak around 1.4 eV. A long-lived transient absorption due to interchain polarons is also observed. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction In recent years, progress in the field of high-speed optoelectronics and information processing has inspired the search for new materials with large thirdorder nonlinear optical susceptibilities Žsee, e.g., w1x.. One-dimensional conjugated polymers are one of the promising candidates for future practical nonlinear optical devices. As a consequence of their often large nonlinear optical susceptibilities, ultrafast responses, and quasi-one-dimensional structures, the interest of many scientists has been directed to their electrical and optical properties in order to study the relevant energy band structures and the mechanisms of optical nonlinearity. 1 Present address: Department of VUV Photoscience, Institute for Molecular Science, Myodaiji, Okazaki 444, Japan.
There have been many studies reported for transpolyacetylene and its derivatives, which have the simplest backbone structure with alternating carbon–carbon single and double bonds w2–5x. It is well established that solitons are formed as localized excitations in such polymers with a degenerate ground state w6–9x. On the other hand, polythiophene ŽPT. and its derivatives have a structure which is similar to that of cis-polyacetylene and is stabilized by the sulfur atom interacting only weakly with the p-electron system of the backbone. Due to their non-degenerate ground state, the soliton cannot exist but polarons and bipolarons are formed after photoexcitation w1,10x. The absorption bands associated with the polarons and bipolarons appear in the near-infrared region w7,10x. Therefore, it is necessary to investigate the absorption behavior of these polymers in the near-in-
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 2 6 1 - 9
166
A. Matsuse et al.r Chemical Physics Letters 288 (1998) 165–170
frared region in order to determine the photoexcited species formed in polymers. In this Letter, we report the first results of femtosecond pump–probe experiments over a wide spectral range from visible to near-infrared Ž500–1600 n m . o n a th in film o f wp o ly Ž3 dodecylthiophene.xŽP3DT., which has a side group of R s ŽCH 2 .11CH 3 .
2. Experimental The experimental apparatus for pump–probe transient absorption measurement in the visible–near-infrared region has been reported previously w11x. The light source is a Ti:sapphire oscillatorramplifier system w12x. It produces 0.45 mJ amplified pulses at 800 nm Ž1.55 eV. with a typical duration of 200 fs. The amplified pulse is divided into two parts. One is frequency-doubled in a 3 mm thick LiB 3 O5 crystal and the generated second harmonic at 400 nm Ž3.10 eV. is used as a pump pulse. The other is focused into a cell containing carbon tetrachloride to generate a white-light continuum pulse by self-phase modulation. It is used as a probe and a reference, both being detected with a polychromatorrmultichannel-camera system. The delay time between the pump and the probe pulses is changed by using a mechanical stage driven by a stepping motor. The time resolution of the pump-probe measurements ranges from 240 to 300 fs depending on the probe-photon energy. A P3DT thin film was electrochemically polymerized on a CaF2 substrate w13x. The film sample was stable in air at room temperature. The stationary absorption spectrum of the film was recorded on a commercial spectrometer.
Photoinduced difference absorption spectra of a P3DT thin film measured at room temperature are shown in Fig. 1 for several delay times between the pump and probe pulses. The pump and probe polarizations are parallel to each other, and the excitation density is ; 1 = 10 16 photonsrcm2 at 3.10 eV. A broad photoinduced absorption ŽPIA. band is observed in the energy region below the stationary absorption edge instantaneously with photoexcitation Ž0 ps.. A strong bleaching of the ground-state absorption is also seen above 2.0 eV. The PIA band changes from this initial flat spectrum to a doublypeaked spectrum in 1 ps: two PIA peaks at 1.2–1.3 and 1.8–1.9 eV become noticeable because the PIA signals at - 1 and 1.4–1.5 eV exhibit a rapid decay as compared to the two PIA peaks. Of these two peaks, the PIA peak at 1.8–1.9 eV decays faster than the other. Thus, only one broad PIA peak around 1.3 eV is dominant at delay times longer than 10 ps.
3. Results and discussion The lowest-energy electronic absorption band of P3DT rises at ; 1.9 eV and has a peak at 2.4 eV. This band is assigned mainly to a p–p ) transition along the one-dimensional polymer chain. Upon photoexcitation of this band at 3.1 eV, which is 0.7 eV higher than the peak energy, electron–hole pairs are initially produced via the predominant intrachain excitation.
Fig. 1. Photoinduced difference absorption spectra of a P3DT thin film obtained at several delay times. The signals in the spectral region of 1.5–1.6 eV are not shown, because they are distorted owing to spiky structures on the probe and reference spectra. Stationary absorption spectrum of the P3DT film ŽAbs.. is also shown on the top.
A. Matsuse et al.r Chemical Physics Letters 288 (1998) 165–170
These observed features of the PIA spectra do not depend on the excitation density. In our previous paper w14x, the femtosecond transient absorption spectra of P3DT with photoexcitation at 1.97 eV were reported for a probe-photon energy region of 1.3–2.6 eV. The PIA peak at 1.9 eV was observed as well as the broad PIA band at early delay times, but no clear indication of the PIA peak around 1.3 eV was made because of their limited probe-photon energy region. The wider probe-photon energy region in the present study Ž0.78–2.6 eV. has enabled us to find the PIA peak around 1.3 eV. The time-dependent shape of the PIA spectra in Fig. 1 suggests that two or more decay components contribute to the observed PIA signals. In order to examine these decay components, we plot the time dependence of the PIA signals at four probe-photon energies in Fig. 2. At 1.0 eV, the PIA signal exhibits an initial rapid decay in 1 ps, being similar to the cross-correlation trace at the probe-photon energy. This indicates that the decay component Ž‘ultrafast’ component. almost vanishes within a delay time shorter than the resolution time of the measurement. It is obvious from the plot at 1.9 eV that there exists
Fig. 2. Time dependence of the absorbance change at four probephoton energies. The solid circles are experimental data and the solid curves are best-fitted curves Žsee text..
167
Fig. 3. Logarithmic plot of the time dependence of the absorbance change at 1.4 eV after subtraction of the long-lived component. The straight line represents the best-fitted power-law function. The power constant evaluated by the slope of the line is ns 0.56 "0.05.
another component which decays exponentially within a few ps Ž‘fast’ component.. The decay time constant of this fast component is determined as texp s 680 " 50 fs by a fitting analysis taking account of the finite time resolution. In addition to these ultrafast and fast components which characterize the PIA signals at early delay times, a much slower decay component and a long-lived component are also seen clearly for the decays at 1.2 and 1.4 eV. The former ‘slow’ component does not decay exponentially but rather follows a power-law decay Žf tyn . as shown by a logarithmic plot of the decay at 1.40 eV up to 100 ps ŽFig. 3.. The power constant is evaluated as n s 0.56 " 0.05 from the slope of this plot, and it is constant in the probe-photon energy region of 1.3–1.5 eV. The long-lived component is seen as an almost constant signal within the measured time region. Thus, it is concluded from the time dependence in Fig. 2 that the observed PIA signal of the P3DT thin film consists of four decay components in total. Relaxation of photoexcitations in polythiophene derivatives has been discussed in terms of a selftrapping process of an exciton w14x. A photogenerated free exciton or electron–hole pair quickly induces bond-order distortions around itself via strong coupling between electronic excitations and lattice vibrations of the polymer main chain. Since there is no energy barrier for the self-trapping process of the
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one-dimensional exciton, its initial coupling to the C–C stretching vibrations of the polymer chains is expected to take place within a few vibrational periods of 10–20 fs Žnonthermal self-trapped exciton ŽSTE. w15x.. The nonthermal STE subsequently decays toward the bottom of the STE potential Žquasithermal STE w15x. by emitting phonons, i.e. vibrational energy dissipation from the strongly coupled C–C modes to other low-frequency modes. The quasi-thermal STE is the exciton before full thermalization including lattice vibration but after the vibrational energy equilibration among intrachain vibrational modes w15x. The time constant for this phonon emission process was reported as 100 " 50 and 70 " 50 fs for P3DT and polyŽ3-methylthiophene. ŽP3MT., respectively w14,16x. These time constants are shorter than the time resolution of the present study. Thus, because of the short lifetime and the resolutionlimited rise of the ultrafast component observed in the present study, the component can be assigned to both the free exciton and the nonthermal STE which decay by the self-trapping and phonon emission processes, respectively. These two processes cannot be resolved in the present time-resolved measurement. As for the quasi-thermal STE, its decay time constant was so far evaluated as 0.3–0.5 and 0.8 " 0.1 ps for P3DT and P3MT, respectively w14,16x. Since the time constant of the fast component Ž680 fs. is similar to these reported values in the sub-picosecond region, the fast component is ascribable to the quasi-thermal STE. The long-lived component observed in a P3DT thin film was already assigned to a pair of polarons generated on different polymer chains via interchain excitation Žinterchain polaron. on the basis of its pump-polarization dependence and pump-photon density dependence w14x. The time dependence of the PIA signal is well reproduced by the following decay function:
times longer than terf . The parameters for this term, n s 0.56 " 0.05 and terf s 0.1 " 0.05 ps, are evaluated at 1.40 eV, where the PIA signal most likely decays with the power-law behavior as shown in Fig. 3. The fitting is performed using the least-squares method by adjusting the four coefficients. The cross-correlation trace at each probe-photon energy is convoluted with Eq. Ž1. to take into account the energy-dependent time resolution. As shown by solid curves in Fig. 2, the time dependence of the observed PIA signal is well reproduced by this fitting at every probe-photon energy. Spectra of the ultrafast, fast, slow and long-lived components are given respectively by the coefficients AŽ Õ ., B Ž Õ ., C Ž Õ . and DŽ Õ . obtained from the fitting analysis ŽFig. 4.. The ultrafast component exhibits a broad spectrum extending in the energy region below 1.7 eV, while the fast component has a peak around 1.8 eV. This drastic spectral change between the ultrafast and the fast components corresponds to the relaxation from the free exciton Žor electron–hole pair. to the quasi-thermal STE by the self-trapping and subsequent phonon emission processes. The peak around 1.8 eV might be assigned to a transition from the quasi-thermal STE to a biexcitonic state w11,16x. The spectrum of the slow component found in the present study C Ž Õ ., exhibits a broad peak around 1.4
D A Ž t , Õ . s A Ž Õ . d Ž t . q B Ž Õ . exp Ž ytrtexp . qC Ž Õ . erf Ž trterf .
yn
q DŽ Õ. ,
Ž 1.
where erfw x is the error function. The first term corresponds to the ultrafast component. It is described by the delta function d Ž t . because of its short life time. The third term represents the slow component which follows the power-law decay. This term is approximated as C Ž Õ .Ž trterf .yn for delay
Fig. 4. Wavelength dependence of the coefficients of the four components in Eq. Ž1.. v: ultrafast component AŽ Õ ., ^: fast component B Ž Õ ., B: slow component C Ž Õ ., e: long-lived component DŽ Õ .. The data points in the spectral region of 2.0–2.2 eV and above 2.3 eV are not shown, because absorbance change and probe-pulse intensity in these regions are too small to make a reasonable analysis.
A. Matsuse et al.r Chemical Physics Letters 288 (1998) 165–170
eV. It is similar to the spectrum of the long-lived component DŽ Õ ., which was already assigned to the interchain polaron generated by interchain photoexcitation w14x. This spectral similarity suggests that the slow component also corresponds to polaronic excitation. Since the slow component follows the powerlaw decay which is characteristic of a geminate recombination process of two excitations in a one-dimensional polymer chain w17x, we assign it to a pair of oppositely-charged polarons which are generated on the same polymer chain by intrachain photoexcitaion Žintrachain polaron pair.. This is clearly distinguished from the interchain polaron also by its lifetime: the interchain polaron decays in a micro- to mili-second time region by interchain hopping process w10x, while the intrachain polaron vanishes much faster due to its confined nature in the one-dimensional polymer chain. It is noteworthy that the excited species in doped-polythiophene is not a bipolaron but is recently reassigned as a polaron on the basis of the Raman data obtained from the radical cation and dication of a thiophene oligomer w18x. It is expected that a polaron is also generated in photoexcited polythiophene w19x. Since one of the three intragap electronic transitions associated with the polaron is symmetrically forbidden, only two major PIA peaks are observed w19,20x; the two peak energies were reported as 0.65 and 1.5 eV for the doping-induced polaron, and 0.5 and 1.35 eV for the photoinduced polaron w20x. The higher peak energy for the photoinduced polaron Ž1.35 eV. agrees well with the peak energy of the slow component Ž1.4 eV.. This agreement of the two peak energies strongly supports the assignment of the slow component to a pair of intrachain polarons although detailed information about the intrachain polaron is still scarce. The lowenergy peak expected to appear around 0.5 eV will give further information on the formation and relaxation dynamics of the intrachain polaron. Finally, we note that the intrachain polaron can also be generated by relaxation from the STE state Ž‘indirect’ pathway., in addition to its direct excitation from the ground state Ž‘direct’ pathway. described in Eq. Ž1.. Although these two pathways are not completely distinguishable only by the transient absorption data in Fig. 1, the following analysis of the PIA signal at 1.4 eV Žpeak energy of the intra-
169
chain polaron. indicates that the direct pathway is still a major process. Here, we consider only the fast and the slow components corresponding to the STE and the intrachain polaron, respectively, after subtraction of the other two components, i.e. the ultrafast and long-lived components, from the obtained PIA signal. This is because the last two components do not have a chance to mix up with the first two components owing to the large difference in their time dependence. We have made a fitting analysis of the PIA signal after the subtraction by using the following function, searching for the best B and C values for certain k value:
D A Ž t . s B exp Ž ytrtexp . q C k erf Ž trterf .
½
qŽ 1 y k .
1
texp
t
X
H0 exp Žyt rt
=erf Ž t y t X . rterf
exp
yn
4 d tX 5 ,
yn
. Ž 2.
where the parameter k Ž0 F k F 1. denotes the fraction of the direct pathway. The results of this fitting analysis are shown in Fig. 5 for several different k values. As seen in this figure, the calculated curve starts to deviate from the data points as the k value becomes smaller than 1.0. The k value between 0.8 to 1.0 seems to give satisfactory results. Thus, the contribution of the indirect pathway for generation of the intrachain polaron is - 20% and is concluded to be a minor process.
Fig. 5. Photoinduced absorption signal at 1.4 eV after subtraction of the ultrafast and long-lived components Žsee text.. The calculated curves for three k values are also shown; solid line: k s1.0, broken line: k s 0.8, dotted line: k s 0.6.
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Acknowledgements The authors thank Dr. S. Hughes for his careful reading of the manuscript. This work is partly supported by a Proposal-Based Advanced Industrial Technology R & D Program.
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