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Characteristics of buckminsterfullerene doped conducting polymer
Synthetic Metals, 55-57 (1993) 2991-2996
2 9 9 "1
CHARACTERISTICS OF BUCKMINSTERFULLERENE DOPED CONDUCTING POLYMER
Katsumi YOSHINO, Shigenori MORITA, Tsuyoshi KAWAI, Hisashi ARAKI, Xiao Hong YIN and Anvar A. ZAKHIDOV*
Department of Electronic Engineering, Faculty of Engineering, Osaka University 2-1 Yamada-Oka, Suita, Osaka (JAPAN)
ABSTRACT Conducting polymers such as poly(3-alkylthiophene) are effectively doped with buckminsterfullerene
(C60). Change of absorption spectrum and drastic quenching of
photoluminescence have been observed upon C60 doping. On the other hand, the slight enhancement of electrical conductivity and the change of ESR by C60 doping is not so remarkable compared with conventional strong dopants. Photoconductivity of poly(3-alkylthiophene) is enhanced and the response time becomes shorter by C60 doping, which suggests that C60 is a weak dopant, providing mainly photoioduced charge transfers between C60 and the polymer. These results are explained by taking electronic energy states of poly(3-alkylthiophene) and C60 into consideration at account of polaronic effec~.~ in both C60 and polymer. Small enhancement of electrical conductivity and significant quenching of photoluminescence have been also observed upon C70 doping, but change of optical absorption is less remarkable than the case of C60 doping. Keywords: Buckminsterfullerene, conducting polymer, C60, doping, photoconduction, conductivity, poly(3-alkylthiophene), photoinduced charge transfer INTRODUCTION Poly(3-alkylthiophene) exhibits unique properties such as fusibility[l], solubility[2-4], thermochromism[5], sotvatochromism[6], gel chromism[7] and anomalous luminescence[8]. It is also interesting, because inter-chain distance is relatively long depending on the substituted alkyl chain length. On the other hand, C60 named Buckminsterfullerene, has attracted much interest as a new type of carbon. It exhibits superconducting and ferromagnetic behavior in the alkali metal doped state and in TPAE doped state, respectively [9,10].
We have also reported electroluminescence in a C60
diode[ 11 ]. Permanent address: Department of Thermophysics, Uzbek Academy of Science, Katartal 28, Chilanzar, C.Tashkent, 700135 Uzbekistan, CIS. 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
2992
So far most studies have been focused on doping of donors such as alkali metals (Na, K, Rb etc.) into C60. That is, C60 has been considered to be a host for dopants. On the contrary, we have noticed that C60 can also be used as a dopant for conducting polymer[12,13]. In this paper, we report the small changes of electrical, optical and magnetic properties of poly(3alkylthiophene) contrasted by drastic changes of photoluminescence and photoconductivity upon C60 doping and the origin of these unique effects is discussed, in terms of weak type doping and photoinduced charge transfer. EXPERIMENTAL Poly(3-alkylthiophene)s were prepared from 3-alkylthiophene monomers utilizing FeCI3 as a catalyst. The detailed preparation method has already been reported in our previous papers[4]. Here, mainly poly(3-hexylthiophene)(PAT-6) and poly(3-octadecylthiophene)(PAT-18) are discussed as example, however, other poly(3-alkylthiophene)s with different alkyl chain length exhibited similar characteristics. C60 prepared by an arc discharge from graphite, washing with toluene, provided by Science Laboratories Co. Ltd., was used in this study. Both poly(3-alkylthiophene) and C60 were dissolved in toluene and then the solution was cast onto a glass substrate. Electrical conductivity was measured by two- and four-probe methods. Photoconductivity was studied by irradiating Xe arc lamp light passing through a monochrometer on the sample sandwiched between ITO (Indium-Tinoxide) and gold electrodes. Optical absorption and electron spin iesonance (ESR) measurements were carried out utilizing a Hitachi 330 spectrophotometer and a Bruker SP300, respectively. C70 doped poly(3-alkylthiophene)s prepared by similar procedure were also used for comparison. RESULTS AND DISCUSSION As evident in Fig.l, the electrical conductivity increases with increasing concentration of C60 in poly(3-alkylthiophene). However, contrary to conventional strong dopant such as iodine, CIO4-. BF,~ etc., the enhancement of conductivity was at most up to 10-6 S/cm at around 5 mol% and at higher dopant concentration conductivity again decreased. As shown in Fig.2, the interband optical absorption of PAT-6 (peaked at 520nm) was suppressed and the new absorption peak evolved at a lower energy range than the original band gap of PAT-b. These spectral change clearly support the doping effect of C60 and may be interpreted in terms of polaron and/or bi-polaron models, at account of large disorder effect also. The temperature dependence of electrical conductivity of C60-doped PAT-6 is also unique. That is, the electrical conductivity of PAT-6 doped with relatively high concentration of C60 increases with increasing temperature but after reaching the highest conductivity it again decreases at higher temperature.
2993 10 5
1.C
oO/~°\o
E 10.6
0.5
> 10-7
APoly(3-hexylt hiophene)
y,l__ .
/0
8
.
.
.
.
/
.
Poly(3-hexylthiophene) 10
I
0.1
1 t0 100 Concentrationof C60 (mol%)
Fig. 1. Dependence of electrical conductivity of poly(3-hexylthiophene) on concentration of C60.
0
!
500 1000 1500 Wavelength (nm)
2000
Fig. 2. Absorption spectra of poly(3hexylthiophene) as a function of C60 concentration.
The relatively low conductivity and the unique temperature dependence of conductivity suggest that the doping effect of C60 is slightly weak compared with that of conventional dopants. These characteristics should originate in the electronic energy states of C60 and poly(3alkylthiophene). C60 is considered to play a role as a weak acceptor to poly(3-alkylthiophene), because the top of the valence band of poly(3-alkylthiophene) is slightly lower in energy than the LUMO of C60. Therefore, the electron-transfer from poly(3-alkylthiophene) to C60 seems to require a small amount of energy of 0.1 ~0.2eV. This energy can be gained by several mechanisms. (1)
Polaronic effects both in poly(3-alkylthiophene) and in C60 (polarization and Jahn-Teller
distortion) play roles due to self-trapping energy gains (~0.1 --0.2eV). (2)
The Coulombic attraction between P- (polaron) of C60 and P+ (polaron) of poly(3-
alkylthiophene) can give additional energy lowering (--0.3~0.5eV). (3)
Large size of C60 can influence the conformation ofalkyl side chain and poly(3-alkylthiophene)
main chain, resulting in the modification of band scheme of poly(3-alkylthiophene) (Raising of valence band top [14] ). Effects of C60 dopant on poly(3-alkylthiophene) were found to depend also on length of alkyl side chain. The change of electronic band scheme, especially top of valence band by the length of alkyl side chain may be the origin of this difference. ESR of C60 doped poly(3-alkylthiophene) were slightly different from those with conventional strong dopants as shown in Fig.3. The narrowing of ESR line width and enhancement of spin density upon C60 doping were much modest than the case of conventional dopants. Therefore, they can not be simply explained by the formation of bi-polarons in this case. The possibility that C60contributes to the observed ESR signal can not be ruled out at this stage. It should also be stressed that photoconductivity of poly(3-alkylthiophene) is found to be remarkably enhanced upon C60 doping. That is, photoconductive response of poly(3-alkylthiophene)
2994
Poly(3-hexylthiophene) 1.0
el)
o\
o/
Poly(3-octadecylthiophene) Cso 10 mol%
i'-o
x
(5 -r
(D C) =_ O. O0
--0.5
o 0
........
0.1
i
t
i
i .....
i
.......
1 10 Concentration of C60 (mol%)
0
100
Fig. 3. Dependence of line width and spin density of poly(3-hexylthiophene) on the concentration of C60.
o0
~
;
4
Energy (eV) Fig. 4. Spectra of photoconductivity of poly(3-octadecylthiophene) as a function of C60 concentration.
is relatively small. However in the C60 doped poly(3-alkylthiophene) photoconductivity was enhanced by many orders of magnitude, especially in the spectral ranges around 600~700nm and around 400nm as shown in Fig.4. With increasing C60 concentration, the signal was remarkably enhanced. The response in the long wavelength region, should be induced by the excitation of electron from the valence band of poly(3-alkylthiophene) to conduction band at the initial stage. Holes in poly(3-alkylthiophene) will relax to positive polaron P+. On the other hand electrons in the conduction band and the P- (negative polaron in poly(3-alkylthiophene)) or the exciton levels in poly(3-alkylthiophene) will be transferred to higher excited state of C60, which will relax to negative polaron P- in C60 as shown in Fig.5. In this case, P+ in poly(3-alkylthiophene) can contribute to photoconduction. Because of this effective separation of positive and negative charges, the initial geminate recombination should be much suppressed, resulting in the enhancement of photoconductivity. This is in agreement with the theoretical expectation that the photoconductivity can be enhanced in the weak charge-transfer complex[ 15] and similar enhancement of interaction between polymer and dopant upon excitation was also pointed out by us[16]. For the photoresponse around 40Ohm, the electron excitation from the ground state of C60 to a high excited state may contribute as an initial process. In this case electron may be relaxed to negative polaron P- in C60 and hole in C60 will be transferred to valence band of poly(3-alkylthiophene), which also relaxes to P+ in poly(3-alkylthiophene). Therefore in this case also effective photoinduced charge separation occurs and P+ in poly(3-alkylthiophene) contributes to conduction, resulting in the enhancement of photoconduction. It should also be mentioned that the decay time of photoconductivity becomes much shorter upon C60 doping. In the case of C60 doped poly(3-alkylthiophene), when P+ in poly(3-alkylthiophene)
2995
/hi U )2)_
_ _
P~ ~).~ 7_-
PAT
06o
- A +''~'''~
PAT
-t-rC6o
Fig. 5. Energy band scheme of poly(3-alkylthiophene) and C60: a) photoinduced charge transfer, 1) excitation at around 650nm, 2) excitation at around 350rim, shows charge transfer between PAT and C60, shows the recombination decay, b) weak CT in the ground state stabilized by Coulombic attraction. migrates near to P- in C60, they can recombine. This is relatively high probability because of short distance between poly(3-alkylthiophene) and C60 as compared to inter chain decay of photoconductivity in undoped conducting polymers, resulting in the fast decay of photoconduction. Recently Wang et al reported photoconduction in C60-PVK (polyvinylcarbazole) system[17]. However in their case, photo excitation of PVK is neglected because of large band gap contrary to poly(3-alkylthiophene) with small band gap. In our case, both photoexcitations in C60 and poly(3alkylthiophene) contributed to photoconduction. C70 was also doped in poly(3-alkylthiophene) and photoluminescence was quenched. However, the change of absorption spectrum upon doping was not so remarkable.
The enhancement of
electrical conductivity upon C70 doping was also not so remarkable compared with C60 doping as shown in Fig.6. This less doping effect of C70 may be originated in smaller Jahn-Teller distnrdon and polaronic effect in C70. SUMMARY The present experimental study can be summarized as follows. (l) Electrical conductivity of poly(3-alkylthiophene) was enhanced upon C60 doping. However this effect is not so remarkable as the case of conventional strong dopant. (2) Absorption spectrum remarkably changed upon C60 doping. (3) Photoluminescence was quenched dramatically by C60 doping. (4) ESR line width becomes narrower and spin density is slightly enhanced upon C60 doping. However it is not so remarkable as the case of conventional strong dopants. (5) Photoconducting of poly(3-alkylthiophene) was enhanced remarkably and decay line became shorter upon C60 doping. These unique doping characteristics were explained by taking electronic energy diagram of both poly(3-alkylthiophene) and C60 into consideration showing that C60 is a weak dopant,
2996
lx10 -6 Poly(3-hexylthiophene) E lx10.7
/
to
0
Fig.6. Dependence of electrical conductivity of poly(3-hexylthiophene) on the concentration of C7o.
o
lx10-8 =
~
0.1
.....
I
1
i
i
i
i i iiii
10
Concentration of C7o (tool%)
characterized by small charge transfer in ground state, but efficient photoinduced charge transfer in the excited state. (6) Most conducting polymers should exhibit similar effects upon C60 doping. That is, either photoinduced charge transfer or small doping effect depending on the energy of valence band top. (7) C70 was also doped in poly(3-alkylthiophene) but the effect was less compared with the case of C6O doping. REFERENCES 1 K.Yoshino, S.Nakajima and R.Sugimoto, J~,J,Appl.Phys, 26 (1987) L1038. 2 M.Sato, S.Tanaka and K.Kaeriyama, J.Chem.Soc.Chem.Commun. (1986) 873. 3 D.L.Elsenbaumer, K.Y.Jen and R.Oboodi, ~ (1986)169. 4 R.Sugimoto, S.Takeda, H.B.Gu and K.Yoshino, Chem.Expressl (1986) 635. 5 K.Yoshino, D.H.Park, B.K.Park, M.Onoda and R.Sugimoto, ,lpn.J.AppI.Phys, 27 (1988) L1612. 6 K.Yoshino, P.Love, M.Onoda and R.Sugimoto, _Jpn.J.Appl.Phy~. 27 (1988) L2388. 7 K.Yoshino, K.Nakao and R.Sugimoto, ~n.J.Appl,Pb s.ys~828(1989) L490. 8 K.Yoshino, S.Nakajima, D.H.Park and R.Sugimoto, J p n . J . A p p ~ . 27 (1988) L716. 9 A.F.Herbard, M.J.Rosseinsky, R.C.Haddon, D.W.Murphy, S.H.Glarum, T.T.M.Palstra, A.P.Ramirez and A.R.Kortan, ~ t u r e 350 (1991) 600. 10 P.-M.Allemand, K.C.Khemani, A.Koch, F.Wudl, K.Holczer, S.Donovan, G.Gruner and J.D.Thompson, Science 253 (1991) 301. 11 M.Uchida, Y.Ohmori and K.Yoshino, Jpn.J.Appl.Phys. 30 (1991) L2104. 12 S.Morita, A.A.Zakhidov and K.Yoshino, Solid Stale Commun. 82 (1992) 249. 13 S.Morita, A.A.Zakhidov, T.Kawai, H.Araki and K.Yoshino,/p_n.J.Appl.Phys. 31 (1992) L890. 14 M.Onoda, Y.Manda, S.Morita and K.Yoshino, PJ ~ s . S o c . ~ . 58 (1989) 1895. 15 A.A.Zakhidov, Proc. of ELORMA'87. lnt Conf.,Moscow, 1988p. 196, Synth~Met.41-43 (1991) 3393. 16 K.Yoshino, S.Hayashi, G.Ishii and Y.Inuishi, Jpn.J.Appl.Phys, 2;2 (1983) L376. 17 Y.Wang, Nature 356 (1992) 585.
2 9 9 "1
CHARACTERISTICS OF BUCKMINSTERFULLERENE DOPED CONDUCTING POLYMER
Katsumi YOSHINO, Shigenori MORITA, Tsuyoshi KAWAI, Hisashi ARAKI, Xiao Hong YIN and Anvar A. ZAKHIDOV*
Department of Electronic Engineering, Faculty of Engineering, Osaka University 2-1 Yamada-Oka, Suita, Osaka (JAPAN)
ABSTRACT Conducting polymers such as poly(3-alkylthiophene) are effectively doped with buckminsterfullerene
(C60). Change of absorption spectrum and drastic quenching of
photoluminescence have been observed upon C60 doping. On the other hand, the slight enhancement of electrical conductivity and the change of ESR by C60 doping is not so remarkable compared with conventional strong dopants. Photoconductivity of poly(3-alkylthiophene) is enhanced and the response time becomes shorter by C60 doping, which suggests that C60 is a weak dopant, providing mainly photoioduced charge transfers between C60 and the polymer. These results are explained by taking electronic energy states of poly(3-alkylthiophene) and C60 into consideration at account of polaronic effec~.~ in both C60 and polymer. Small enhancement of electrical conductivity and significant quenching of photoluminescence have been also observed upon C70 doping, but change of optical absorption is less remarkable than the case of C60 doping. Keywords: Buckminsterfullerene, conducting polymer, C60, doping, photoconduction, conductivity, poly(3-alkylthiophene), photoinduced charge transfer INTRODUCTION Poly(3-alkylthiophene) exhibits unique properties such as fusibility[l], solubility[2-4], thermochromism[5], sotvatochromism[6], gel chromism[7] and anomalous luminescence[8]. It is also interesting, because inter-chain distance is relatively long depending on the substituted alkyl chain length. On the other hand, C60 named Buckminsterfullerene, has attracted much interest as a new type of carbon. It exhibits superconducting and ferromagnetic behavior in the alkali metal doped state and in TPAE doped state, respectively [9,10].
We have also reported electroluminescence in a C60
diode[ 11 ]. Permanent address: Department of Thermophysics, Uzbek Academy of Science, Katartal 28, Chilanzar, C.Tashkent, 700135 Uzbekistan, CIS. 0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
2992
So far most studies have been focused on doping of donors such as alkali metals (Na, K, Rb etc.) into C60. That is, C60 has been considered to be a host for dopants. On the contrary, we have noticed that C60 can also be used as a dopant for conducting polymer[12,13]. In this paper, we report the small changes of electrical, optical and magnetic properties of poly(3alkylthiophene) contrasted by drastic changes of photoluminescence and photoconductivity upon C60 doping and the origin of these unique effects is discussed, in terms of weak type doping and photoinduced charge transfer. EXPERIMENTAL Poly(3-alkylthiophene)s were prepared from 3-alkylthiophene monomers utilizing FeCI3 as a catalyst. The detailed preparation method has already been reported in our previous papers[4]. Here, mainly poly(3-hexylthiophene)(PAT-6) and poly(3-octadecylthiophene)(PAT-18) are discussed as example, however, other poly(3-alkylthiophene)s with different alkyl chain length exhibited similar characteristics. C60 prepared by an arc discharge from graphite, washing with toluene, provided by Science Laboratories Co. Ltd., was used in this study. Both poly(3-alkylthiophene) and C60 were dissolved in toluene and then the solution was cast onto a glass substrate. Electrical conductivity was measured by two- and four-probe methods. Photoconductivity was studied by irradiating Xe arc lamp light passing through a monochrometer on the sample sandwiched between ITO (Indium-Tinoxide) and gold electrodes. Optical absorption and electron spin iesonance (ESR) measurements were carried out utilizing a Hitachi 330 spectrophotometer and a Bruker SP300, respectively. C70 doped poly(3-alkylthiophene)s prepared by similar procedure were also used for comparison. RESULTS AND DISCUSSION As evident in Fig.l, the electrical conductivity increases with increasing concentration of C60 in poly(3-alkylthiophene). However, contrary to conventional strong dopant such as iodine, CIO4-. BF,~ etc., the enhancement of conductivity was at most up to 10-6 S/cm at around 5 mol% and at higher dopant concentration conductivity again decreased. As shown in Fig.2, the interband optical absorption of PAT-6 (peaked at 520nm) was suppressed and the new absorption peak evolved at a lower energy range than the original band gap of PAT-b. These spectral change clearly support the doping effect of C60 and may be interpreted in terms of polaron and/or bi-polaron models, at account of large disorder effect also. The temperature dependence of electrical conductivity of C60-doped PAT-6 is also unique. That is, the electrical conductivity of PAT-6 doped with relatively high concentration of C60 increases with increasing temperature but after reaching the highest conductivity it again decreases at higher temperature.
2993 10 5
1.C
oO/~°\o
E 10.6
0.5
> 10-7
APoly(3-hexylt hiophene)
y,l__ .
/0
8
.
.
.
.
/
.
Poly(3-hexylthiophene) 10
I
0.1
1 t0 100 Concentrationof C60 (mol%)
Fig. 1. Dependence of electrical conductivity of poly(3-hexylthiophene) on concentration of C60.
0
!
500 1000 1500 Wavelength (nm)
2000
Fig. 2. Absorption spectra of poly(3hexylthiophene) as a function of C60 concentration.
The relatively low conductivity and the unique temperature dependence of conductivity suggest that the doping effect of C60 is slightly weak compared with that of conventional dopants. These characteristics should originate in the electronic energy states of C60 and poly(3alkylthiophene). C60 is considered to play a role as a weak acceptor to poly(3-alkylthiophene), because the top of the valence band of poly(3-alkylthiophene) is slightly lower in energy than the LUMO of C60. Therefore, the electron-transfer from poly(3-alkylthiophene) to C60 seems to require a small amount of energy of 0.1 ~0.2eV. This energy can be gained by several mechanisms. (1)
Polaronic effects both in poly(3-alkylthiophene) and in C60 (polarization and Jahn-Teller
distortion) play roles due to self-trapping energy gains (~0.1 --0.2eV). (2)
The Coulombic attraction between P- (polaron) of C60 and P+ (polaron) of poly(3-
alkylthiophene) can give additional energy lowering (--0.3~0.5eV). (3)
Large size of C60 can influence the conformation ofalkyl side chain and poly(3-alkylthiophene)
main chain, resulting in the modification of band scheme of poly(3-alkylthiophene) (Raising of valence band top [14] ). Effects of C60 dopant on poly(3-alkylthiophene) were found to depend also on length of alkyl side chain. The change of electronic band scheme, especially top of valence band by the length of alkyl side chain may be the origin of this difference. ESR of C60 doped poly(3-alkylthiophene) were slightly different from those with conventional strong dopants as shown in Fig.3. The narrowing of ESR line width and enhancement of spin density upon C60 doping were much modest than the case of conventional dopants. Therefore, they can not be simply explained by the formation of bi-polarons in this case. The possibility that C60contributes to the observed ESR signal can not be ruled out at this stage. It should also be stressed that photoconductivity of poly(3-alkylthiophene) is found to be remarkably enhanced upon C60 doping. That is, photoconductive response of poly(3-alkylthiophene)
2994
Poly(3-hexylthiophene) 1.0
el)
o\
o/
Poly(3-octadecylthiophene) Cso 10 mol%
i'-o
x
(5 -r
(D C) =_ O. O0
--0.5
o 0
........
0.1
i
t
i
i .....
i
.......
1 10 Concentration of C60 (mol%)
0
100
Fig. 3. Dependence of line width and spin density of poly(3-hexylthiophene) on the concentration of C60.
o0
~
;
4
Energy (eV) Fig. 4. Spectra of photoconductivity of poly(3-octadecylthiophene) as a function of C60 concentration.
is relatively small. However in the C60 doped poly(3-alkylthiophene) photoconductivity was enhanced by many orders of magnitude, especially in the spectral ranges around 600~700nm and around 400nm as shown in Fig.4. With increasing C60 concentration, the signal was remarkably enhanced. The response in the long wavelength region, should be induced by the excitation of electron from the valence band of poly(3-alkylthiophene) to conduction band at the initial stage. Holes in poly(3-alkylthiophene) will relax to positive polaron P+. On the other hand electrons in the conduction band and the P- (negative polaron in poly(3-alkylthiophene)) or the exciton levels in poly(3-alkylthiophene) will be transferred to higher excited state of C60, which will relax to negative polaron P- in C60 as shown in Fig.5. In this case, P+ in poly(3-alkylthiophene) can contribute to photoconduction. Because of this effective separation of positive and negative charges, the initial geminate recombination should be much suppressed, resulting in the enhancement of photoconductivity. This is in agreement with the theoretical expectation that the photoconductivity can be enhanced in the weak charge-transfer complex[ 15] and similar enhancement of interaction between polymer and dopant upon excitation was also pointed out by us[16]. For the photoresponse around 40Ohm, the electron excitation from the ground state of C60 to a high excited state may contribute as an initial process. In this case electron may be relaxed to negative polaron P- in C60 and hole in C60 will be transferred to valence band of poly(3-alkylthiophene), which also relaxes to P+ in poly(3-alkylthiophene). Therefore in this case also effective photoinduced charge separation occurs and P+ in poly(3-alkylthiophene) contributes to conduction, resulting in the enhancement of photoconduction. It should also be mentioned that the decay time of photoconductivity becomes much shorter upon C60 doping. In the case of C60 doped poly(3-alkylthiophene), when P+ in poly(3-alkylthiophene)
2995
/hi U )2)_
_ _
P~ ~).~ 7_-
PAT
06o
- A +''~'''~
PAT
-t-rC6o
Fig. 5. Energy band scheme of poly(3-alkylthiophene) and C60: a) photoinduced charge transfer, 1) excitation at around 650nm, 2) excitation at around 350rim, shows charge transfer between PAT and C60, shows the recombination decay, b) weak CT in the ground state stabilized by Coulombic attraction. migrates near to P- in C60, they can recombine. This is relatively high probability because of short distance between poly(3-alkylthiophene) and C60 as compared to inter chain decay of photoconductivity in undoped conducting polymers, resulting in the fast decay of photoconduction. Recently Wang et al reported photoconduction in C60-PVK (polyvinylcarbazole) system[17]. However in their case, photo excitation of PVK is neglected because of large band gap contrary to poly(3-alkylthiophene) with small band gap. In our case, both photoexcitations in C60 and poly(3alkylthiophene) contributed to photoconduction. C70 was also doped in poly(3-alkylthiophene) and photoluminescence was quenched. However, the change of absorption spectrum upon doping was not so remarkable.
The enhancement of
electrical conductivity upon C70 doping was also not so remarkable compared with C60 doping as shown in Fig.6. This less doping effect of C70 may be originated in smaller Jahn-Teller distnrdon and polaronic effect in C70. SUMMARY The present experimental study can be summarized as follows. (l) Electrical conductivity of poly(3-alkylthiophene) was enhanced upon C60 doping. However this effect is not so remarkable as the case of conventional strong dopant. (2) Absorption spectrum remarkably changed upon C60 doping. (3) Photoluminescence was quenched dramatically by C60 doping. (4) ESR line width becomes narrower and spin density is slightly enhanced upon C60 doping. However it is not so remarkable as the case of conventional strong dopants. (5) Photoconducting of poly(3-alkylthiophene) was enhanced remarkably and decay line became shorter upon C60 doping. These unique doping characteristics were explained by taking electronic energy diagram of both poly(3-alkylthiophene) and C60 into consideration showing that C60 is a weak dopant,
2996
lx10 -6 Poly(3-hexylthiophene) E lx10.7
/
to
0
Fig.6. Dependence of electrical conductivity of poly(3-hexylthiophene) on the concentration of C7o.
o
lx10-8 =
~
0.1
.....
I
1
i
i
i
i i iiii
10
Concentration of C7o (tool%)
characterized by small charge transfer in ground state, but efficient photoinduced charge transfer in the excited state. (6) Most conducting polymers should exhibit similar effects upon C60 doping. That is, either photoinduced charge transfer or small doping effect depending on the energy of valence band top. (7) C70 was also doped in poly(3-alkylthiophene) but the effect was less compared with the case of C6O doping. REFERENCES 1 K.Yoshino, S.Nakajima and R.Sugimoto, J~,J,Appl.Phys, 26 (1987) L1038. 2 M.Sato, S.Tanaka and K.Kaeriyama, J.Chem.Soc.Chem.Commun. (1986) 873. 3 D.L.Elsenbaumer, K.Y.Jen and R.Oboodi, ~ (1986)169. 4 R.Sugimoto, S.Takeda, H.B.Gu and K.Yoshino, Chem.Expressl (1986) 635. 5 K.Yoshino, D.H.Park, B.K.Park, M.Onoda and R.Sugimoto, ,lpn.J.AppI.Phys, 27 (1988) L1612. 6 K.Yoshino, P.Love, M.Onoda and R.Sugimoto, _Jpn.J.Appl.Phy~. 27 (1988) L2388. 7 K.Yoshino, K.Nakao and R.Sugimoto, ~n.J.Appl,Pb s.ys~828(1989) L490. 8 K.Yoshino, S.Nakajima, D.H.Park and R.Sugimoto, J p n . J . A p p ~ . 27 (1988) L716. 9 A.F.Herbard, M.J.Rosseinsky, R.C.Haddon, D.W.Murphy, S.H.Glarum, T.T.M.Palstra, A.P.Ramirez and A.R.Kortan, ~ t u r e 350 (1991) 600. 10 P.-M.Allemand, K.C.Khemani, A.Koch, F.Wudl, K.Holczer, S.Donovan, G.Gruner and J.D.Thompson, Science 253 (1991) 301. 11 M.Uchida, Y.Ohmori and K.Yoshino, Jpn.J.Appl.Phys. 30 (1991) L2104. 12 S.Morita, A.A.Zakhidov and K.Yoshino, Solid Stale Commun. 82 (1992) 249. 13 S.Morita, A.A.Zakhidov, T.Kawai, H.Araki and K.Yoshino,/p_n.J.Appl.Phys. 31 (1992) L890. 14 M.Onoda, Y.Manda, S.Morita and K.Yoshino, PJ ~ s . S o c . ~ . 58 (1989) 1895. 15 A.A.Zakhidov, Proc. of ELORMA'87. lnt Conf.,Moscow, 1988p. 196, Synth~Met.41-43 (1991) 3393. 16 K.Yoshino, S.Hayashi, G.Ishii and Y.Inuishi, Jpn.J.Appl.Phys, 2;2 (1983) L376. 17 Y.Wang, Nature 356 (1992) 585.