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Transient occurrence of polarons in electrochemically doped poly(3-methylthiophene) via a discrete electronic level
Solid State Communications, Printed in Great Britain.
Vo1.62,No.7,
pp.483-486,
0038-1098/87 $3.00 + .OO 01987 Pergamon Journals Ltd.
1987.
TRANSIENT OCCURRENCE OF POLARONS IN ELECTROCHEMICALLY DOPED POLY(3-METHYLTHIOPHENE) VIA A DISCRETE ELECTRONIC LEVEL K. Murao* Advanced
(Received
15 December
and K. Suzuki
Research Laboratory, Hitachi, Kokubunji, Tokyo 185, Japan 1986, in revised
Ltd.,
form 2 February
1987 by W. Sasaki)
at Pt/poly-l/PMeT [poly-1: Time-resolved in situ reflection spectra -PMeT: poly(3-methylthiophene)l ruthenium(I1) complex polymer; electrodes have been measured during the electrochemical doping of PMeT via a discrete electronic level of poly-1 layer. Transient occurrence of polarons has been detected in PMeTtogether with the anomalously rapid relaxations of the charged states.
of electrochemical 'narrow windows' through levels. in a previous letterlo), we reported on the deposition of a thin PMeT tpoly(3-methylthiophene)) layer (t100 nm) onto electrically insulating but electrochemically active metalloorganic thin films, poly[tris(4-vinyl-*'-methyl2,2'-bipyridine)ruthenium (II) perchloratel There is (hereafter abbreviated as poly-1). strong indication that quasi-equilibria for charge redistributions in PMeT are established within the PMeT layer very rapidly upon electrochemical charge injections based on this bilayer This feature would reduce, to configuration. the complexity arising from the some extent, concurrent twofold effects, i.e., charge diffusion and reactions among quasi particles In this communication, we describe (polarons). our findings on the anomalous time-resolved, & situ optical spectra of the bilayer film upon --The spectra electrochemical charge injections. taken in this scheme also evidence the transient occurrence of polarons in the cource of relaxations in the doped PMeT. Platinum disks (4 mm diameter) embedded in An glass were used as the working substrates. Ag/Ag+ quasi reference electrode was used which showed 290 mV vs. SCE (saturated calomel elecwas purified by distilling trode). Acetonitrile over P2O5. Electrochemical measurements were performed by immersing electrodes in 0.1 M TBAP (tetrabutylammonium perchlorate)/acetonitrile electrolytic solutions in one-compartment cells. All of the electrochemical experiments were carried out under argon-purged conditions. Timeresolved, in situ reflection spectra at the --electrode surfaces in electrochemical cells were measured with a normally incident light beam led by fiber optics using an MCPD-100 Multi-Channel Photo-Detector (Ohtsuka Denshi Co.). Each spectrum is based on the accumulated signals obtained by eight repeated wavelength sweeps with a gate time of 12 ms each. The bilayer electrodes, expressed as Pt/poly-l/PMeT, were prepared as follows, according to the method described in a previous letter-lo) The poly-1 inner layer was first deposited by electropolymerization of its mono-
Electrochemistry of conducting polyhetero(PPyrr) and polythiocycles, e.g., polypyrrole phene (PT) has been the subject of extensive Their electrochemical doping-undoping studies. processes have been found to be equivalent to the reversible generation of the most stable, non-degenerate mid-gap states, i.e., bipolarons, based on the electronically unequivalent Structures in the extensively delocalized pi-electron Occursystems of the polymer backbones. l-3) rence of polarons in PPyrr 4.5) and PT6-g) chains has been proved by optical spectroscopy and other physical measurements basically under static conditions at very low doping levels, except for transient ESR studies of PPyrr under dynamic conditions,*.') Thus, the studies of electrochemically induced changes in PPyrr and PT have provided understanding of charged states in non-degenerate ground-state systems. However, such studies so far reported have been exclusively confined to a configuration where the films of polyheterocycles are attached to conducting substrates from which charge injections apparently suffers occur. This configuration from a problem that the electronic states of the surface films are ever since affected by the Fermi levels of the substrates. This disadvantage becomes prominent when one attempts to observe electronic relaxations occurring within the conducting films by applying potentials to the substrates. This point apparently limits the effectiveness of such studies under dynamic conditions which are expected to provide insights into the relaxation processes taking place upon charge injections. it should be interIn the above context, esting to study doping-induced changes where electrochemical charge injections into conducting polymer films are performed via discrete electronic levels of the adjacent electrochemically active but electrically insulating layers. Charge injections in this scheme would generate energetically less diversified charged states. Moreover, the subsequent electronic interactions between the conducting polymer layer and the substrate are possible only *
To whom correspondence
should be addressed. 483
484
POLARONS IN ELECTROCHEMICALLYDOPED POLY(3-METHYLTHIOPHENE)
mer precursor at the platinum substrate.ll) The coverage of the poly-1 layer is typically 2.2 x 10-g mol cm -2 , which has a thickness of approximately 36 nm determined by ellipsometry. PMeT was then deposited at the Pt/poly-1 electrode by electropolymerization which was initiated at the platinum electrode surface. The growth of PMeT occurs within the existing poly-1 layer and further proceeds outside the poly-1 layer. The phase separation of both polymer components, poly-1 and PMeT, was subsequently caused by repeated electrochemical potential sweeps in 0.1 and the bilayer structure, M TBAP/acetonitrile, was eventually obtained. The Pt/poly-l/PMeT, anodic charge densities passed during the deposition of PMeT are typically 33.4 mC cme2. Any detectable loss of the deposited PMeT was not found during the phase separation process. The cyclic voltammograms of the bilayer electrode taken in 0.1 M TBAP/acetonitrile indicate that the oxidation and reduction waves for PMeT occur at the oxidation and reduction potentials of the inner layer, +0.87 V and -1.75 V vs. Ag/Ag+, These electrochemical levels are respectively. Ru(I1) ion and ligand centered, respectively, tris(4-vinyl-4'-methyl-2,2'-bipyriin dine)ruthenium(II) complex centers in poly-l. While the poly-1 layer is an electrical insuwithin the poly-1 lator, the charge transport is possible in an electrochemical layer mechanism (successive electron self-exchange reactions among active centers).12) Although the possibility cannot be ruled out that some overlapping of both layers remains even after the the phase-separating potential sweeps, finally achieved electrochemistry unambiguously evidences the totally mediated oxidation (doping) and reduction (undoping) reactions of the PMeT layer via the electrochemical levels of While the platinum subthe inner layer.lO) strate and the PMeT layer are electrically short circuited in the PMeT-as-grown electrodes, they become highly resistive upon the electrochemical potential sweeps which lead to the mediated These observations are conelectrochemistry. sistent with those for the PPyrr bilayer electrodes.11r13) Thus, the electrochemical charge inlections into the PMeT layer via discrete electronic levels have been achieved by the above bilayer configuration. The energy configuration of the Pt/poly-l/PMeT electrode is depicted in Fig. 1. The positions of the conduction and valence band edges of PMeT were estimated based on optica16) and electrochemicallO) data, and are consistent to the reported values.14) The reduction of the upper (i) and oxidation of the lower (ii) levels The of poly-1 can be reversibly effected. positive (doping) and negative (undoping) charges are reversibly injected into the PMeT layer via the lower (ii) and upper (1) electrochemical levels of the poly-1 layer by raising and lowering the applied potentials of the The dopingplatinum substrate, respectively. -, be monitored by both cyclic undoping cycles voltammograms and optical switchings. in situ reflection spectra Fig. 2 shows -__ taken during the doping process in neutral PMeT in Pt/poly-l/PMeT by stepping the potential from whole spectra The -1.0 to +0.68 V vs. Ag/Ag+. cover the events for ca. 70 s after the onset of The band maxima at 780 nm in the application.
Vol. 62, No. 7
+ zQ, --2.0 2 g
--1.0
c w-o 5 t. s t0 - 1.0 n.
Pt
poly-1
PMeT
(a> H3C
neutral state
bipolaron state (b)
poly(3-methylthiophene)
(a) Schematic representation of the Fig. 1. energy diagram for Pt/poly-l/poly(3-methylthiophene) bilayer electrode. A, B: applied potential levels for negative and positive charge injectlons Into the outer layer, respectively. PMeT: poly(3-methylthiophene). For poly-1 see (b) Chemical structures of poly(3text. in neutral and blpolaron methylthiophene) states.
The 2 are due to bipolarons.2e6-8) Fig. absorption changes due to the oxidation of the inner layer are small for kc600 nm for the employed coverages while for 600 nmchno change by the separate experiments on is detected, A striking feature is Pt/poly-1 electrodes. that a well-defined isosbestic point is formed in Fig. 2, indicating that each spectrum represents a state close to quasi equilibrium including P+ + P+ + BP2+ (P+: polaron, BP2+: bipolaron) reactions in the shortinterFormation of the isosbestic points was vals. also observed upon dopinq in various modes. e.g., by linear sweeps of the potential in the
Vol. 62, No. 7
485
POLARONS IN ELECTROCHEMICALLYDOPED POLY(3-METHYLTHIOPHENE)
0.6
L I
400
initial
1
1
600
800
I
1000
Wavelength/nm In situ reflection spectra at Pt/polyFig. 2. -l/poly(3-methylthiophene) in 0.1 M tetrabutylammonium perchlorate/acetonitrile solution obtained in potential step from -1.Oto +0.68 V Initial 10 spectra were taken every vs. Ag/Ag+. 1 s, and the subsequent 6, 5 and 2 spectra were taken every 2, 5 and 10 s, respectively, following the potential step.
It should be noted that no positive direction. isosbestic behavior was observed without the in this time intervening layer, i.e., Pt/PMeT, scale, even when PMeT films with the similar thickness (~100 nm) were used. This implies that the isosbestic behavior should be attributed to the present specific scheme of charge inlections based on the bilayer configuration, rather than to the relatively thin PMeT layer. This consideration is consistent with the fact that several tens of minutes have been employed in preceding studies under static conditions in order to achieve quasi equilibria in PT.2r8) When the positive charges are injected more rapidly in Pt/poly-l/PMeT, significant spectral changes were observed as follows. Positive charges were first injected by stepping the potential from -1.0 to +0.85 V vs. Ag/Ag+. Fig. 3 shows the changes in difference spectra for 4 s following the potential step. The difference spectra are based on five original spectra taken every 1 s including the initial neutral state. Although the isosbestic point is not formed here relatively high doping rate, two due to the bands are detected in the first stage (spectrum 1) at 670 nm and 940 nm, which are gradually replaced by the evolution of a band eventually centered at 750 nm (spectrum 4). These characteristic changes are strikingly consistent with those reported by Kaneto et a1.6t8) and Hattori et a1.7) based on very lightly doped PT under static conditions. They ascribed the former two bands and the latter band to polaronic and Harbeke bipolaronic transitions, respectively.
0
-0.1
600
800
1000
Wavelength/nm Fig. 3. Difference reflection spectra taken in situ at Pt/poly-l/poly(3-methylthiophene) in Oy M tetrabutylammonium perchlorate/acetonitrile in potential step from -1.0 to CO.85 V vs. Ag/Ag+. These spectra are based on five original spectra taken every 1 s, including the initial neutral state.
486
POLARONS
IN ELECTROCHEMICALLY
et
on similar spectral a1.9) also reported changes of PMeT due to polarons at very low doping levels in equilibria, while they did not detect a band corresponding to that at 670 "m of Hence, we conclude that the the present study. spectral changes in Fig. 3 represent the dynamic process of the reaction, P+ + P++ BP2+ in the course of achieving equilibrium. The absorption bands at 940 nm and at 750 "m are ascribed to transitions ~2 and 03 for polarons Aland bipolarons in Fig. 1, respectively. though the origin of the band at 670 "m is still we assume it is due to an excitonic uncertain, transition that becomes prominent in the presence of polarons, i.e., 'polaron-assisted exciwhile Vardeny et al. assigned tonic transition', a sharp peak at 1.95 eV in their IRAV (infraredactive vibrational) excitation profile of PT as The spectral being due to a" excito".15) 3 were obtained I" changes similar to Fig. various modes of potential applications, i.e., linear sweep and potential step with and without subsequent open-circuiting during the relaxation. In a separate experiment, we found that the spectral change in the last stage of the relaxations to the final, most stable blpolaron states involves the increase in intensity of This observation suggests interband transition. the relaxations of bipolarons with higher energies to the most stable, equilibrated form. This process is identical to reduction in the of thiophene bipolaron size, i.e., the number rings between a pair of positive charges of bipolarons (equivalent to the reduction of the n number in Fig. l(b)).
DOPED
POLY(3+ETHYLTHIOPHENE)
Vol.
62, No,
The occurrence of the relatively rapid, BP2+ polaron-quenching reactlon, Pf + P+ + may imply that the observed polarons are intrinsic and should not necessarily be related to the structural imperfections in the polymer be stressed that, without the chains. It should intervening layer with discrete electrochemical levels, i.e., Pt/PMeT, the detection of the polarons I" PMeT was unsuccessful transient because the spectral changes observed under the dynamic conditions were ambiguous and were much less systematic, probably reflecting the manifold relaxation processes occurring within the PMeT layer. The present study suggests that the positive charge injections via discrete electronic levels give rise to energetically much less dlversifled charged states, which allows relaxations in pathways simpler than those occurring at metallic surfaces. Polarons were not observed for undoping potential steps or sweeps starting from the doped (oxidized) state where bipolarons were prevailing. In conclusion, we have found, by means of time-resolved in ____ situ reflectlo" spectroscopy, __ that PMeT achieves quasi equilibria anomalously rapldly upon electrochemical dopings via a dlsCrete electronic level of the adlacent electrically Insulating layer in a bllayer configuration. This remarkable feature enabled us to detect transiently occurrIng polarons in PMeT during the electrochemical doping in this scheme. Further efforts along this line should provide insights into the dynamic properties of highly phonon-coupled charged states I" the conducting polymers.
REFERENCES 1) J. C. Scott, P. Pfluger, M. T. Krounbi. and G. B. Street, Phys. Rev. BG, 2140 (1983). A. J. Heeger, 2) T. -C. Chung, J. H. Kaufman, and F. Wudl, Phys. Rev. B30, 702 (1984). 3) J. C. Scott, J. L. Bredas,?. Yakushi. P. Pfluger, and G. B. Street, Synth. Metals, 2, 165 (1984). 4) J. H. Kaufman, N. Colaneri, J. C. Scott, and G. B. Street, Phys. Rev. Lett., 53, 1005 (1984). 5) F. Genoud, M. Guglielml, M. Nechtschein, E. Genies, and M. Salmon, Phys. Rev. Lett., 55, 118 (1985). 6) K. Kaneto, Y. Kohno, and K. Yoshino. Solid State Commun., 5J, 267 (1984). w. Hayes, K. Wong, K. Kaneto, 7) T. Hattori, J. Phys. CE, L803 (1984). and K. Yoshino, 8) K. Kaneto, S. Hayashi, S. Ura, and K. Yoshino, J. Phys. Sot. Jpn., 54, 1146 (1985).
E. Meier, W. Kobel, M. Egli, 9) G. Harbeke, H. Kiess, and E. Tosattl, Solld State Commun., 55, 419 (1985). 10) K. Murao and K. Suzuki, Chem. Lett., 1986, 2101. 11) K. Murao and K. Suzuki, J. Chem. Sot., Chem. Comm., 1984, 238. 12) See for example: F. B.Kaufman, A. H. Schroeder, E. M. Engler, S. R. Kramer, and J. Q. Chambers, J. Am. Chem. Sot., 102, 483 (1980). 13) K. Murao and K. Suzuki, Appl. Phys. Lett., 47, 724 (1985). and F. Garnler, 14) s. GlenIs, G. Tourillon, Thin Solid Films, 122, 9 (1984). Vardeny, E. Ehrenfreund, 0. Brafman, 15) z. M. Nowak, H. Schaffer, A. J. Heeger, and F. Wudl, Phys. Rev. Lett., 56, 671 (1986).
7
Vo1.62,No.7,
pp.483-486,
0038-1098/87 $3.00 + .OO 01987 Pergamon Journals Ltd.
1987.
TRANSIENT OCCURRENCE OF POLARONS IN ELECTROCHEMICALLY DOPED POLY(3-METHYLTHIOPHENE) VIA A DISCRETE ELECTRONIC LEVEL K. Murao* Advanced
(Received
15 December
and K. Suzuki
Research Laboratory, Hitachi, Kokubunji, Tokyo 185, Japan 1986, in revised
Ltd.,
form 2 February
1987 by W. Sasaki)
at Pt/poly-l/PMeT [poly-1: Time-resolved in situ reflection spectra -PMeT: poly(3-methylthiophene)l ruthenium(I1) complex polymer; electrodes have been measured during the electrochemical doping of PMeT via a discrete electronic level of poly-1 layer. Transient occurrence of polarons has been detected in PMeTtogether with the anomalously rapid relaxations of the charged states.
of electrochemical 'narrow windows' through levels. in a previous letterlo), we reported on the deposition of a thin PMeT tpoly(3-methylthiophene)) layer (t100 nm) onto electrically insulating but electrochemically active metalloorganic thin films, poly[tris(4-vinyl-*'-methyl2,2'-bipyridine)ruthenium (II) perchloratel There is (hereafter abbreviated as poly-1). strong indication that quasi-equilibria for charge redistributions in PMeT are established within the PMeT layer very rapidly upon electrochemical charge injections based on this bilayer This feature would reduce, to configuration. the complexity arising from the some extent, concurrent twofold effects, i.e., charge diffusion and reactions among quasi particles In this communication, we describe (polarons). our findings on the anomalous time-resolved, & situ optical spectra of the bilayer film upon --The spectra electrochemical charge injections. taken in this scheme also evidence the transient occurrence of polarons in the cource of relaxations in the doped PMeT. Platinum disks (4 mm diameter) embedded in An glass were used as the working substrates. Ag/Ag+ quasi reference electrode was used which showed 290 mV vs. SCE (saturated calomel elecwas purified by distilling trode). Acetonitrile over P2O5. Electrochemical measurements were performed by immersing electrodes in 0.1 M TBAP (tetrabutylammonium perchlorate)/acetonitrile electrolytic solutions in one-compartment cells. All of the electrochemical experiments were carried out under argon-purged conditions. Timeresolved, in situ reflection spectra at the --electrode surfaces in electrochemical cells were measured with a normally incident light beam led by fiber optics using an MCPD-100 Multi-Channel Photo-Detector (Ohtsuka Denshi Co.). Each spectrum is based on the accumulated signals obtained by eight repeated wavelength sweeps with a gate time of 12 ms each. The bilayer electrodes, expressed as Pt/poly-l/PMeT, were prepared as follows, according to the method described in a previous letter-lo) The poly-1 inner layer was first deposited by electropolymerization of its mono-
Electrochemistry of conducting polyhetero(PPyrr) and polythiocycles, e.g., polypyrrole phene (PT) has been the subject of extensive Their electrochemical doping-undoping studies. processes have been found to be equivalent to the reversible generation of the most stable, non-degenerate mid-gap states, i.e., bipolarons, based on the electronically unequivalent Structures in the extensively delocalized pi-electron Occursystems of the polymer backbones. l-3) rence of polarons in PPyrr 4.5) and PT6-g) chains has been proved by optical spectroscopy and other physical measurements basically under static conditions at very low doping levels, except for transient ESR studies of PPyrr under dynamic conditions,*.') Thus, the studies of electrochemically induced changes in PPyrr and PT have provided understanding of charged states in non-degenerate ground-state systems. However, such studies so far reported have been exclusively confined to a configuration where the films of polyheterocycles are attached to conducting substrates from which charge injections apparently suffers occur. This configuration from a problem that the electronic states of the surface films are ever since affected by the Fermi levels of the substrates. This disadvantage becomes prominent when one attempts to observe electronic relaxations occurring within the conducting films by applying potentials to the substrates. This point apparently limits the effectiveness of such studies under dynamic conditions which are expected to provide insights into the relaxation processes taking place upon charge injections. it should be interIn the above context, esting to study doping-induced changes where electrochemical charge injections into conducting polymer films are performed via discrete electronic levels of the adjacent electrochemically active but electrically insulating layers. Charge injections in this scheme would generate energetically less diversified charged states. Moreover, the subsequent electronic interactions between the conducting polymer layer and the substrate are possible only *
To whom correspondence
should be addressed. 483
484
POLARONS IN ELECTROCHEMICALLYDOPED POLY(3-METHYLTHIOPHENE)
mer precursor at the platinum substrate.ll) The coverage of the poly-1 layer is typically 2.2 x 10-g mol cm -2 , which has a thickness of approximately 36 nm determined by ellipsometry. PMeT was then deposited at the Pt/poly-1 electrode by electropolymerization which was initiated at the platinum electrode surface. The growth of PMeT occurs within the existing poly-1 layer and further proceeds outside the poly-1 layer. The phase separation of both polymer components, poly-1 and PMeT, was subsequently caused by repeated electrochemical potential sweeps in 0.1 and the bilayer structure, M TBAP/acetonitrile, was eventually obtained. The Pt/poly-l/PMeT, anodic charge densities passed during the deposition of PMeT are typically 33.4 mC cme2. Any detectable loss of the deposited PMeT was not found during the phase separation process. The cyclic voltammograms of the bilayer electrode taken in 0.1 M TBAP/acetonitrile indicate that the oxidation and reduction waves for PMeT occur at the oxidation and reduction potentials of the inner layer, +0.87 V and -1.75 V vs. Ag/Ag+, These electrochemical levels are respectively. Ru(I1) ion and ligand centered, respectively, tris(4-vinyl-4'-methyl-2,2'-bipyriin dine)ruthenium(II) complex centers in poly-l. While the poly-1 layer is an electrical insuwithin the poly-1 lator, the charge transport is possible in an electrochemical layer mechanism (successive electron self-exchange reactions among active centers).12) Although the possibility cannot be ruled out that some overlapping of both layers remains even after the the phase-separating potential sweeps, finally achieved electrochemistry unambiguously evidences the totally mediated oxidation (doping) and reduction (undoping) reactions of the PMeT layer via the electrochemical levels of While the platinum subthe inner layer.lO) strate and the PMeT layer are electrically short circuited in the PMeT-as-grown electrodes, they become highly resistive upon the electrochemical potential sweeps which lead to the mediated These observations are conelectrochemistry. sistent with those for the PPyrr bilayer electrodes.11r13) Thus, the electrochemical charge inlections into the PMeT layer via discrete electronic levels have been achieved by the above bilayer configuration. The energy configuration of the Pt/poly-l/PMeT electrode is depicted in Fig. 1. The positions of the conduction and valence band edges of PMeT were estimated based on optica16) and electrochemicallO) data, and are consistent to the reported values.14) The reduction of the upper (i) and oxidation of the lower (ii) levels The of poly-1 can be reversibly effected. positive (doping) and negative (undoping) charges are reversibly injected into the PMeT layer via the lower (ii) and upper (1) electrochemical levels of the poly-1 layer by raising and lowering the applied potentials of the The dopingplatinum substrate, respectively. -, be monitored by both cyclic undoping cycles voltammograms and optical switchings. in situ reflection spectra Fig. 2 shows -__ taken during the doping process in neutral PMeT in Pt/poly-l/PMeT by stepping the potential from whole spectra The -1.0 to +0.68 V vs. Ag/Ag+. cover the events for ca. 70 s after the onset of The band maxima at 780 nm in the application.
Vol. 62, No. 7
+ zQ, --2.0 2 g
--1.0
c w-o 5 t. s t0 - 1.0 n.
Pt
poly-1
PMeT
(a> H3C
neutral state
bipolaron state (b)
poly(3-methylthiophene)
(a) Schematic representation of the Fig. 1. energy diagram for Pt/poly-l/poly(3-methylthiophene) bilayer electrode. A, B: applied potential levels for negative and positive charge injectlons Into the outer layer, respectively. PMeT: poly(3-methylthiophene). For poly-1 see (b) Chemical structures of poly(3text. in neutral and blpolaron methylthiophene) states.
The 2 are due to bipolarons.2e6-8) Fig. absorption changes due to the oxidation of the inner layer are small for kc600 nm for the employed coverages while for 600 nmchno change by the separate experiments on is detected, A striking feature is Pt/poly-1 electrodes. that a well-defined isosbestic point is formed in Fig. 2, indicating that each spectrum represents a state close to quasi equilibrium including P+ + P+ + BP2+ (P+: polaron, BP2+: bipolaron) reactions in the shortinterFormation of the isosbestic points was vals. also observed upon dopinq in various modes. e.g., by linear sweeps of the potential in the
Vol. 62, No. 7
485
POLARONS IN ELECTROCHEMICALLYDOPED POLY(3-METHYLTHIOPHENE)
0.6
L I
400
initial
1
1
600
800
I
1000
Wavelength/nm In situ reflection spectra at Pt/polyFig. 2. -l/poly(3-methylthiophene) in 0.1 M tetrabutylammonium perchlorate/acetonitrile solution obtained in potential step from -1.Oto +0.68 V Initial 10 spectra were taken every vs. Ag/Ag+. 1 s, and the subsequent 6, 5 and 2 spectra were taken every 2, 5 and 10 s, respectively, following the potential step.
It should be noted that no positive direction. isosbestic behavior was observed without the in this time intervening layer, i.e., Pt/PMeT, scale, even when PMeT films with the similar thickness (~100 nm) were used. This implies that the isosbestic behavior should be attributed to the present specific scheme of charge inlections based on the bilayer configuration, rather than to the relatively thin PMeT layer. This consideration is consistent with the fact that several tens of minutes have been employed in preceding studies under static conditions in order to achieve quasi equilibria in PT.2r8) When the positive charges are injected more rapidly in Pt/poly-l/PMeT, significant spectral changes were observed as follows. Positive charges were first injected by stepping the potential from -1.0 to +0.85 V vs. Ag/Ag+. Fig. 3 shows the changes in difference spectra for 4 s following the potential step. The difference spectra are based on five original spectra taken every 1 s including the initial neutral state. Although the isosbestic point is not formed here relatively high doping rate, two due to the bands are detected in the first stage (spectrum 1) at 670 nm and 940 nm, which are gradually replaced by the evolution of a band eventually centered at 750 nm (spectrum 4). These characteristic changes are strikingly consistent with those reported by Kaneto et a1.6t8) and Hattori et a1.7) based on very lightly doped PT under static conditions. They ascribed the former two bands and the latter band to polaronic and Harbeke bipolaronic transitions, respectively.
0
-0.1
600
800
1000
Wavelength/nm Fig. 3. Difference reflection spectra taken in situ at Pt/poly-l/poly(3-methylthiophene) in Oy M tetrabutylammonium perchlorate/acetonitrile in potential step from -1.0 to CO.85 V vs. Ag/Ag+. These spectra are based on five original spectra taken every 1 s, including the initial neutral state.
486
POLARONS
IN ELECTROCHEMICALLY
et
on similar spectral a1.9) also reported changes of PMeT due to polarons at very low doping levels in equilibria, while they did not detect a band corresponding to that at 670 "m of Hence, we conclude that the the present study. spectral changes in Fig. 3 represent the dynamic process of the reaction, P+ + P++ BP2+ in the course of achieving equilibrium. The absorption bands at 940 nm and at 750 "m are ascribed to transitions ~2 and 03 for polarons Aland bipolarons in Fig. 1, respectively. though the origin of the band at 670 "m is still we assume it is due to an excitonic uncertain, transition that becomes prominent in the presence of polarons, i.e., 'polaron-assisted exciwhile Vardeny et al. assigned tonic transition', a sharp peak at 1.95 eV in their IRAV (infraredactive vibrational) excitation profile of PT as The spectral being due to a" excito".15) 3 were obtained I" changes similar to Fig. various modes of potential applications, i.e., linear sweep and potential step with and without subsequent open-circuiting during the relaxation. In a separate experiment, we found that the spectral change in the last stage of the relaxations to the final, most stable blpolaron states involves the increase in intensity of This observation suggests interband transition. the relaxations of bipolarons with higher energies to the most stable, equilibrated form. This process is identical to reduction in the of thiophene bipolaron size, i.e., the number rings between a pair of positive charges of bipolarons (equivalent to the reduction of the n number in Fig. l(b)).
DOPED
POLY(3+ETHYLTHIOPHENE)
Vol.
62, No,
The occurrence of the relatively rapid, BP2+ polaron-quenching reactlon, Pf + P+ + may imply that the observed polarons are intrinsic and should not necessarily be related to the structural imperfections in the polymer be stressed that, without the chains. It should intervening layer with discrete electrochemical levels, i.e., Pt/PMeT, the detection of the polarons I" PMeT was unsuccessful transient because the spectral changes observed under the dynamic conditions were ambiguous and were much less systematic, probably reflecting the manifold relaxation processes occurring within the PMeT layer. The present study suggests that the positive charge injections via discrete electronic levels give rise to energetically much less dlversifled charged states, which allows relaxations in pathways simpler than those occurring at metallic surfaces. Polarons were not observed for undoping potential steps or sweeps starting from the doped (oxidized) state where bipolarons were prevailing. In conclusion, we have found, by means of time-resolved in ____ situ reflectlo" spectroscopy, __ that PMeT achieves quasi equilibria anomalously rapldly upon electrochemical dopings via a dlsCrete electronic level of the adlacent electrically Insulating layer in a bllayer configuration. This remarkable feature enabled us to detect transiently occurrIng polarons in PMeT during the electrochemical doping in this scheme. Further efforts along this line should provide insights into the dynamic properties of highly phonon-coupled charged states I" the conducting polymers.
REFERENCES 1) J. C. Scott, P. Pfluger, M. T. Krounbi. and G. B. Street, Phys. Rev. BG, 2140 (1983). A. J. Heeger, 2) T. -C. Chung, J. H. Kaufman, and F. Wudl, Phys. Rev. B30, 702 (1984). 3) J. C. Scott, J. L. Bredas,?. Yakushi. P. Pfluger, and G. B. Street, Synth. Metals, 2, 165 (1984). 4) J. H. Kaufman, N. Colaneri, J. C. Scott, and G. B. Street, Phys. Rev. Lett., 53, 1005 (1984). 5) F. Genoud, M. Guglielml, M. Nechtschein, E. Genies, and M. Salmon, Phys. Rev. Lett., 55, 118 (1985). 6) K. Kaneto, Y. Kohno, and K. Yoshino. Solid State Commun., 5J, 267 (1984). w. Hayes, K. Wong, K. Kaneto, 7) T. Hattori, J. Phys. CE, L803 (1984). and K. Yoshino, 8) K. Kaneto, S. Hayashi, S. Ura, and K. Yoshino, J. Phys. Sot. Jpn., 54, 1146 (1985).
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