- Email: [email protected]
polymer electrolyte junctions prepared by electrochemical polymerization of pyrrole in polymer electrolyte
Synthetic Metals, 32 (1989) 201 - 208
201
ELECTRICAL PROPERTIES OF POLYPYRROLE/POLYMER ELECTROLYTE JUNCTIONS PREPARED BY ELECTROCHEMICAL POLYMERIZATION OF PYRROLE IN POLYMER ELECTROLYTE AKIRA WATANABE,* KUNIO MORI, AKEMI TAKAHASHI and YOSHIRO NAKAMURA
Department of Applied Chemistry, Faculty of Engineering, lwate University, Ueda, Morioka 020 (Japan) (Received March 15, 1989; accepted April 12, 1989)
Abstract Polypyrrole/PEO-LiC104 junctions were prepared in situ by the electrochemical polymerization of pyrrole in the polymer electrolyte PEO-LiC104. The junction prepared in situ by the potential sweep method exhibited rectifying behavior in the current-voltage curves. The capacitance-voltage properties of the junction showed an energy barrier at the interface between polypyrrole and PEO-LiC104. On the contrary, polypyrrole/PEO-LiC104 junctions prepared in situ by galvanostatic and potentiostatic methods did not show rectifying behavior. These results are explained on the basis of the properties of the interface between p-type polypyrrole and the polymer electrolyte.
1. Introduction The discovery of highly conductive polyacetylene induced considerable attention on conducting polymers [1]. These have been investigated extensively in terms of technological applications and the new conducting mechanism. Of recent interest is the electrical properties of the interfaces between a conducting polymer and another material. Rectifying junctions with semiconductors and metals have been reported on polyacetylene [2 - 4] and polypyrrole [5 - 7]. The surface modifications of semiconductor electrodes using conductive polymers also have been investigated to inhibit photodegradation in photoelectrochemical cells (PEC) [8- 13]. Skotheim [14] reported the electrochemical synthesis of interfaces between electronically and ionically conducting polymers in situ and described the application of the interface to the solid-state PEC using an n-Si electrode. The preparation of polypyrrole/polymer electrolyte bilayer composites by the *Present address: Chemical Research Institute of Non-aqueous Solutions, Tohuku University, Katahira, Sendai 980, Japan. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
202 in situ electrochemical polymerization of pyrrole in the polymer electrolyte has also been reported by Watanabe e t al. [15]. In this paper we report the electrical properties of the interfaces between polypyrrole and PEO-LiC1On prepared in situ by various electrolytic methods. The p-type semiconductor characteristics of polypyrrole are known [16] and the photocurrent generation at the liquid-junction interface between polypyrrole and the electrolyte has been reported [7]. We will discuss the electrical properties of the junctions between polypyrrole and the polymer electrolyte PEO-LiCIO4.
2. Experimental Materials
Pyrrole was purified by vacuum distillation. PEO (Mw = 20 000, Nakarai Chem. Ltd.) complexed with dehydrated LiC1On was used as a polymer electrolyte. Pyrrole was electrochemically polymerized in PEO-LiC1On; pyrrole, PEO and LiC104 were mixed in the wt. ratio of 1:1:1, and gels were formed by mixture. The mixture was sandwiched between two Pt-coated glass electrodes (4.0 X 2.5 cm) with a rubber separator (2.5 X 2.5 cm) of thickness ~0.17 cm. The rubber separator held the sample in its square hole (1.5 X 1.0 cm). The mixture was electrolyzed in the cell by three different methods (potential sweep, galvanostatic and potentiostatic method) and the junctions between polypyrrole and PEO-LiC10 n were prepared in situ. Different kinds of junctions between polypyrrole and the polymer electrolyte were formed as follows. Polypyrrole films were prepared on Pt-coated glass electrode in a 0.1 M pyrrole/0.05 M (n-Bu)4NC104 acetonitrile solution by the potential sweep method between 0 and 2.0 V for 12 h or by the galvanostatic method at 0.4 mA for 20 h. The polypyrrole-coated electrode was joined with electrolytes consisting of PEO (M~ 20 000), ethylene glycol and LiC104 in the wt.ratio of 1:1:1. Measurements
Cyclic voltammograms and current-voltage curves were measured by a potentiostat/galvanostat (Hokuto HA-201) and a function generator (Hokuto HB-103). The capacitance-voltage characteristics were measured by an LCR meter (Delika Mini bridge D1S) at 1 kHz. 3. Results Figure 1 shows the cyclic voltammograms for the electrochemical polymerization of pyrrole in PEO-LiC104 by the potential sweep method. In the measurements of cyclic voltammograms and current-voltage curves, the voltage is the cell voltage without a reference electrode, so we expressed it as 'applied voltage' in all the Figures. On increasing the number of cycles, the anodic current at 1.3 V decreased and oxidation and reduction peaks near 0.9 V increased. The former is the oxidation of pyrrole and the latter is the
203
redox reaction of polypyrrole deposited on the anode. The absorption spectrum of the polypyrrole/PEO-LiC104 composite showed an absorption peak at ~400 nm and a broad band at the near i.r. region, which are typical of C104--doped polypyrrole [16]. It was observed that polypyrrole exists between the anode and the polymer electrolyte and the polypyrrole/PEOLiC104 junction is formed. The cyclic voltammogram of the polypyrrole/ PEO-LiC104 junction prepared in situ by the potential sweep method is shown in Fig. 2. The cyclic voltammogram is quite asymmetric. In the range of 0 to --8 V, the redox current scarcely appears, in contrast to the range of 0 to 2 V. In spite of the residue of pyrrole monomers, the deposition of polypyrrole on the counter electrode was not observed even at --8 V because of the rectifying behavior. Figure 3 shows the current-voltage curve of the polypyrrole/PEO-LiC104 junction, where the potential was swept from 0 V to positive voltages and then swept from 0 V to negative voltages. The current-voltage curve also suggests the rectifying behavior of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method. The rectifying behavior, however, was not observed for polypyrrole/ PEO-LiCIO4 junctions prepared in situ by galvanostatic or potentiostatic methods. Figure 4 shows cyclic voltammograms of polypyrrole/PEO-LiC104 junctions prepared by galvanostatic or potentiostatic methods; these are almost symmetrical. Current-voltage curves are also symmetrical, as shown in Fig. 5. In this case, the deposition of polypyrrole on both electrodes was observed in the measurements of cyclic voltammograms and current-voltage curves. The junctions between polypyrrole and PEO-LiC104 were prepared by different ways. Polypyrrole films were electrochemically polymerized on an electrode in a 0.1 M pyrrole/0.05 M (n-Bu)4NC104 acetonitrile solution by potential sweep or galvanostatic methods. The polypyrrole-coated electrode was joined with the polymer electrolyte consisting of PEO (Mw 20 000), 0.3
0.3
g
--
-o.~
0.1
h_ ~
-o.~ i
Appl
led voltage (V)
"-=4 Applied
i
voltage (V)
Fig. 1. Cyclic voltammograms for the electrochemical polymerization of pyrrole in PEOLiC104. Sweep rate, 100 mV/s. Numbers i - 4 indicate the number of cycles. Fig. 2. Cyclic voltammogram of the polypyrrole/PEO-LiClO4 junction prepared in situ by the potential sweep method between 0 and 2.0 V for 30 rain. Sweep rate, 100 mV/s.
204 0.3
.
~
~ 0.04
0.2
~
~
cm 0.1
fi
b
0.02
~
_.:..==....
-~ -0.02 u
-0.1
.~ -0.04
-'2
"1
'
-'1
Applied voltage (V)
'
'
^pplied voltage (V)
Fig. 3. Current-voltage curve of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method between 0 and 2.0 V for 30 min. Sweep rate, 100 mV/s. Fig. 4. Cyclic voltammograms of the polypyrrole/PEO-LiC104 junctions: (a) prepared in situ by the galvanostatic method at 0.01 mA for 100 rain; (b) prepared in situ by the potentiostatic method at 4 V for 30 rain. Sweep rate, 100 mV/s. 0.04 0.04
~
~
/s /
0.02
0,[12
,
o
o
-0.02
-0.02
-0.04
/
u _12
11
I
o
I
1
Appl led voltage (V)
I
2
-0.04 Applied voltage (V)
Fig. 5. Current-voltage curves of the polypyrrole/PEO-LiCIO4 junctions: (a) prepared in situ by the galvanostatic method at 0.01 mA for 100 min; (b) prepared in situ by the potentiostatic method at 4 V for 30 rain. Sweep rate, 100 mV/s. Fig. 6. Current-voltage curves of the junctions between PEO-LiC104 and polypyrrolecoated electrode. Polypyrrole films were prepared in a 0.1 M pyrrole/0.05 M (n-Bu)4NC104 acetonitrile solution by (a) the potential sweep method between 0 and 2.0 V for 12 h and by (b) the galvanostatic method at 0.04 mA/cm 2 on a Pt-coated electrode.
ethylene glycol and LiC104 in the wt. ratio of 1:1:1. The current-voltage curves of the polypyrrole/PEO-LiC1Oa junctions are shown in Fig. 6. Figure 6(a) shows the current-voltage curve of the polypyrrole/PEO-LiC1Oa junction using a polypyrrole-coated electrode prepared by the potential sweep m e t h o d between 0 and 2.0 V. However, the rectifying behavior as shown in Fig. 3 does not appear in Fig. 6. The junction between PEO-LiC104 and the polypyrrole-coated electrode prepared by the galvanostatic m e t h o d also did not show rectifying behavior, as shown in Fig. 6(b). The capacitance-voltage measurement is useful to characterize the interface of a semiconductor [17, 18]. In the plots of 1/C 2 against reverse applied voltage, the potential where the line intersects the potential axis yields the value of the diffusion potential V D. In the space-charge layer, an
205
electric potential difference VD is caused by two layers which are charged oppositely to each other. The diffusion potential VD expresses the magnitude of the energy barrier for carrier transport. Figure 7 shows the C - V characteristics of the polypyrrole/PEO-LiC104 junction prepared by the potential sweep method. On increasing the reverse applied voltage, the thickness of the space-charge layer increases and hence the capacitance decreases [17, 18]. The VD value of 1.21 eV is determined by the intersect of the potential in Fig. 7. Polypyrrole/PEO-LiCIO4 junctions prepared by galvanostatic and potentiostatic methods, however, did not show the energy barrier at the interfaces. Figure 8 shows the C - V characteristics of the polypyrrole/PEOLiC104 junctions prepared in situ by the galvanostatic method. The capacitance is almost constant both at reverse and forward applied voltage. This means that there is no energy barrier at the interface between polypyrrole and PEO-LiC104. This result is consistent with the current-voltage characteristics, which do not show rectifying behavior. The polypyrrole/PEOLiCIO4 junctions prepared in situ by the potentiostatic method (Fig. 9) also show the same tendency.
4. Discussion There are two possible junctions as the rectifying junction. One is the junction between polypyrrole and the electrode, and the other is the junction between polypyrrole and PEO-LiC104. It is known that polypyrrole has the characteristics of a p-type semiconductor [7, 16]. Schottky contact between polypyrrole and a metal with a low work function has been reported [7]. However, the contact between p-type polypyrrole and a metal with a high work function such as Pt can be considered as an ohmic contact. On the other hand, the semiconductor/electrolyte junctions have been B
10
•
• o•
C
e•
•
4
• o O •
OoO
o ×
o
b
x o•
~2
o 9oC~Go.--o,......~o
° 8
.o ? o ~ p o 9 -6
-4
-2
0
2
4
Reverse epp] ied voltage (V)
6
0
_~ _~ _~
~
~
~
Reverse applied voltage (V)
Fig. 7. Capacitance-voltage characteristics of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method between 0 and 2.0 V for 30 rain. Fig. 8. Capacitance-voltage characteristics of polypyrrole/PEO-LiC104 junctions prepared in situ by the galvanostatic method at 0.01 mA. Electrolysis time: (a) 100 min; (b) 230 min; (c) 760 min.
206 tO
T~ × •
o
o
u
.
o
•
o
a
.
.
~
b
m n
0
o
8
Reverse epplied voltage (V)
Fig. 9. Capacitance-voltage characteristics of polypyrrole/PEO-LiC104 junctions prepared in situ by the potentiostatic method at (a) 2V for 30 rain and at (b) 4V for 30 rain.
utilized for photoelectrochemical cells (PEC) [19 - 21]. It has been reported that photocurrent is generated at polypyrrole/electrolyte and polythiophene/ electrolyte interfaces [7, 22]. From analogy with the semiconductor/electrolyte junctions, it is inferred that the rectification as shown in Figs. 2 and 3 is caused by the junction between polypyrrole and the polymer electrolyte. At the semiconductor/electrolyte interface, a depletion layer within the semiconductor and an oppositely charged layer within the electrolyte are caused by the equalization of the electrochemical potential across the built-in potential barrier. At equilibrium, Fermi levels are adjusted by the formation of an electric double layer between the semiconductor and the electrolyte. The magnitude of the potential barrier is determined by the bulk properties of the semiconductor and the interface properties at the junction [ 19 - 21]. There is a possibility that the bulk properties of the p-type polypyrrole influence the I - V characteristics of the polypyrrole/PEO-LiC104 junctions. However, the junction between PEO-LiC104 and the polypyrrole-coated electrode prepared by the potential sweep method did not show rectifying behavior, as shown in Fig. 6(a), whereas the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method showed rectifying behavior clearly. Another possibility is that the interface properties at the junction influence the electrical properties of the junction. By the in situ electrochemical oxidation of pyrrole, polypyrrole deposits on the anode and C104- ions are incorporated into polypyrrole as dopants. These dopants are rather fixed in polypyrrole because of the low diffusibility of ions in polymer electrolyte. Actually, the absorption spectra of the polypyrrole/ PEO-LiC104 composite sandwiched between the Pt-coated transparent electrodes were scarcely changed by the negative potential sweep, and show the typical spectra of C104--doped polypyrrole. This suggests the difficulty of dedoping in a high resistivity environment such as polymer electrolyte. By the consumption of C104- ions as dopants, C104- ions are deficient and Li ÷ ions are in excess at the interface between polypyrrole and PEO-LiC104. Under galvanostatic and potentiostatic conditions, the excess Li ÷ ions diffuse to the counter electrode by the applied voltage. In this case, the interface between C104--doped polypyrrole and the oppositely charged layer caused
207
by the excess Li + is not definite. Under potential sweep conditions, the cathodic sweep causes the absorption of excess Li ÷ ions on the C104--doped polypyrrole. The formation of an electronic double layer between p-type polypyrrole and PEO-LiC104 causes the electric field in the space-charge layer, thus a depletion layer within the p-type polypyrrole is formed. The capacitance-voltage characteristics of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method clearly confirms the existence of the depletion layer. The diffusion potential, VD, is determined as 1.21 eV in Fig. 7. The polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method has large capacitance in comparison with the junctions produced by the other methods. This also suggests that the polypyrrole/PEO-LiC104 interface produced in situ by the potential sweep method is definite compared to the interface produced by the other methods. In Figs. 8 and 9, the capacitance of the junction decreases with increasing the electrolysis time, i.e., the thickness of the polypyrrole layer. This must be due to the formation of the undefined interface between the polypyrrole and PEO-LiC104 on increasing the thickness of the polypyrrole layer. The junctions between PEO-LiC104 and polypyrrole-coated electrode do not show rectifying behavior (Fig. 6). This must be due to the homogeneous distribution of Li ÷ and C104- at the interface because of the low diffusibility of ions in polymer electrolyte. Contrary to this, an electronic double layer is easily formed between a semiconductor and an electrolyte [19 - 21]. The difference between the usual electrolyte and a polymer electrolyte can be explained by the mobility of ionic species in the media and the properties of the electric double layer at the interface.
5. Conclusion The rectifying junction between polypyrrole and PEO-LiC104 was prepared in situ by electrochemical polymerization of pyrrole in PEOLiC104 under potential sweep conditions. The rectifying behavior of the junction was explained by the depletion layer which was formed between p-type polypyrrole and the oppositely charged layer caused by excess Li ÷ ions.
References 1 H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C.-K. Chiang and A. J. Heeger, J. Chem. Soc. Chem. Commun., (1977) 578. 2 M. Ozaki, D. L. Peebles, B. R. Weinberger, C. K. Chiang, S. C. Gau, A. J. Heeger and A. G. MacDiarmid, Appl. Phys. Left., 35 (1979) 83. 3 P. M. Grant, T. Tani, W. D. Gill, M. Krounbi and T. C. Clarke, J. Appl. Phys., 52 (1981) 869. 4 J. Kanicki, Mol. Cryst. Liq. Cryst., 105 (1984) 203.
208
5 0 . Ingan~is, T. Skotheim and I. Lundstrbm, J. Appl. Phys., 54 (1983) 3636. 6 O. Ingan~is and I. LundstrSm, Synth. Met., 10 (1984/85) 5. 7 M. Kaneko, K. Okuzumi and A. Yamada, J. Electroanal. Chem., 183 (1985) 407. 8 R. Noufi, D. Tench and L. F. Warren, J. Electrochem. Soc., 127 (1980) 2310. 9 R. Noufi, A. J. Frank and A. J. Nozik, J. A m . Chem. Soc., 103 (1981) 1849. 10 R. Noufi, A. J. Nozik, J. White and L. F. Warren, J. Electrochem. Soc., 129 (1982) 2261. 11 T. Skotheim, L.-G. Petersson, O. Ingan~/s and I. LundstrSm, J. Electrochem. Soc., 129 (1982) 1737. 12 T. Skotheim and I. Lundstrbm, J. Electrochem. Soc., 129 (1982) 894. 13 T. A. Skotheim and O. Ingan~is, J. Electrochem. Soc., 132 (1985) 2116. 14 T. A. Skotheim, Synth. Met., 14 (1986) 31. 15 M. Watanabe, K. Tadano, K. Sanui and N. Ogata, Chem. Lett., (1987) 1239. 16 K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant, G. B. Street, G. P. Gardini and J. F. Kwak, Synth. Met., 1 (1979/80) 329. 17 A. J. Bard and L. F. Faukner, Electrochemical Methods, Wiley, New York, 1980, Ch. 14. 18 S. M. Sze, Physics o f Semiconductor Devices, Wiley, New York, 2nd edn., 1981. 19 A. L. Fahrenbruch and R. H. Bube, Fundamentals o f Solar Cells, Academic Press, New York, 1983, Ch. 12. 20 K. L. Chopra and S. R. Das, Thin Film Solar Cells, Plenum Press, New York/London, 1983, Ch. 11. 21 S. J. Fonash, Solar Cell Device Physics, Academic Press, New York, 1981, Ch. 6. 22 T. Kenmochi, E. Tsuchida, M. Kaneko and A. Yamada, Electrochim. Acta, 30 (1985) 1405.
201
ELECTRICAL PROPERTIES OF POLYPYRROLE/POLYMER ELECTROLYTE JUNCTIONS PREPARED BY ELECTROCHEMICAL POLYMERIZATION OF PYRROLE IN POLYMER ELECTROLYTE AKIRA WATANABE,* KUNIO MORI, AKEMI TAKAHASHI and YOSHIRO NAKAMURA
Department of Applied Chemistry, Faculty of Engineering, lwate University, Ueda, Morioka 020 (Japan) (Received March 15, 1989; accepted April 12, 1989)
Abstract Polypyrrole/PEO-LiC104 junctions were prepared in situ by the electrochemical polymerization of pyrrole in the polymer electrolyte PEO-LiC104. The junction prepared in situ by the potential sweep method exhibited rectifying behavior in the current-voltage curves. The capacitance-voltage properties of the junction showed an energy barrier at the interface between polypyrrole and PEO-LiC104. On the contrary, polypyrrole/PEO-LiC104 junctions prepared in situ by galvanostatic and potentiostatic methods did not show rectifying behavior. These results are explained on the basis of the properties of the interface between p-type polypyrrole and the polymer electrolyte.
1. Introduction The discovery of highly conductive polyacetylene induced considerable attention on conducting polymers [1]. These have been investigated extensively in terms of technological applications and the new conducting mechanism. Of recent interest is the electrical properties of the interfaces between a conducting polymer and another material. Rectifying junctions with semiconductors and metals have been reported on polyacetylene [2 - 4] and polypyrrole [5 - 7]. The surface modifications of semiconductor electrodes using conductive polymers also have been investigated to inhibit photodegradation in photoelectrochemical cells (PEC) [8- 13]. Skotheim [14] reported the electrochemical synthesis of interfaces between electronically and ionically conducting polymers in situ and described the application of the interface to the solid-state PEC using an n-Si electrode. The preparation of polypyrrole/polymer electrolyte bilayer composites by the *Present address: Chemical Research Institute of Non-aqueous Solutions, Tohuku University, Katahira, Sendai 980, Japan. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
202 in situ electrochemical polymerization of pyrrole in the polymer electrolyte has also been reported by Watanabe e t al. [15]. In this paper we report the electrical properties of the interfaces between polypyrrole and PEO-LiC1On prepared in situ by various electrolytic methods. The p-type semiconductor characteristics of polypyrrole are known [16] and the photocurrent generation at the liquid-junction interface between polypyrrole and the electrolyte has been reported [7]. We will discuss the electrical properties of the junctions between polypyrrole and the polymer electrolyte PEO-LiCIO4.
2. Experimental Materials
Pyrrole was purified by vacuum distillation. PEO (Mw = 20 000, Nakarai Chem. Ltd.) complexed with dehydrated LiC1On was used as a polymer electrolyte. Pyrrole was electrochemically polymerized in PEO-LiC1On; pyrrole, PEO and LiC104 were mixed in the wt. ratio of 1:1:1, and gels were formed by mixture. The mixture was sandwiched between two Pt-coated glass electrodes (4.0 X 2.5 cm) with a rubber separator (2.5 X 2.5 cm) of thickness ~0.17 cm. The rubber separator held the sample in its square hole (1.5 X 1.0 cm). The mixture was electrolyzed in the cell by three different methods (potential sweep, galvanostatic and potentiostatic method) and the junctions between polypyrrole and PEO-LiC10 n were prepared in situ. Different kinds of junctions between polypyrrole and the polymer electrolyte were formed as follows. Polypyrrole films were prepared on Pt-coated glass electrode in a 0.1 M pyrrole/0.05 M (n-Bu)4NC104 acetonitrile solution by the potential sweep method between 0 and 2.0 V for 12 h or by the galvanostatic method at 0.4 mA for 20 h. The polypyrrole-coated electrode was joined with electrolytes consisting of PEO (M~ 20 000), ethylene glycol and LiC104 in the wt.ratio of 1:1:1. Measurements
Cyclic voltammograms and current-voltage curves were measured by a potentiostat/galvanostat (Hokuto HA-201) and a function generator (Hokuto HB-103). The capacitance-voltage characteristics were measured by an LCR meter (Delika Mini bridge D1S) at 1 kHz. 3. Results Figure 1 shows the cyclic voltammograms for the electrochemical polymerization of pyrrole in PEO-LiC104 by the potential sweep method. In the measurements of cyclic voltammograms and current-voltage curves, the voltage is the cell voltage without a reference electrode, so we expressed it as 'applied voltage' in all the Figures. On increasing the number of cycles, the anodic current at 1.3 V decreased and oxidation and reduction peaks near 0.9 V increased. The former is the oxidation of pyrrole and the latter is the
203
redox reaction of polypyrrole deposited on the anode. The absorption spectrum of the polypyrrole/PEO-LiC104 composite showed an absorption peak at ~400 nm and a broad band at the near i.r. region, which are typical of C104--doped polypyrrole [16]. It was observed that polypyrrole exists between the anode and the polymer electrolyte and the polypyrrole/PEOLiC104 junction is formed. The cyclic voltammogram of the polypyrrole/ PEO-LiC104 junction prepared in situ by the potential sweep method is shown in Fig. 2. The cyclic voltammogram is quite asymmetric. In the range of 0 to --8 V, the redox current scarcely appears, in contrast to the range of 0 to 2 V. In spite of the residue of pyrrole monomers, the deposition of polypyrrole on the counter electrode was not observed even at --8 V because of the rectifying behavior. Figure 3 shows the current-voltage curve of the polypyrrole/PEO-LiC104 junction, where the potential was swept from 0 V to positive voltages and then swept from 0 V to negative voltages. The current-voltage curve also suggests the rectifying behavior of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method. The rectifying behavior, however, was not observed for polypyrrole/ PEO-LiCIO4 junctions prepared in situ by galvanostatic or potentiostatic methods. Figure 4 shows cyclic voltammograms of polypyrrole/PEO-LiC104 junctions prepared by galvanostatic or potentiostatic methods; these are almost symmetrical. Current-voltage curves are also symmetrical, as shown in Fig. 5. In this case, the deposition of polypyrrole on both electrodes was observed in the measurements of cyclic voltammograms and current-voltage curves. The junctions between polypyrrole and PEO-LiC104 were prepared by different ways. Polypyrrole films were electrochemically polymerized on an electrode in a 0.1 M pyrrole/0.05 M (n-Bu)4NC104 acetonitrile solution by potential sweep or galvanostatic methods. The polypyrrole-coated electrode was joined with the polymer electrolyte consisting of PEO (Mw 20 000), 0.3
0.3
g
--
-o.~
0.1
h_ ~
-o.~ i
Appl
led voltage (V)
"-=4 Applied
i
voltage (V)
Fig. 1. Cyclic voltammograms for the electrochemical polymerization of pyrrole in PEOLiC104. Sweep rate, 100 mV/s. Numbers i - 4 indicate the number of cycles. Fig. 2. Cyclic voltammogram of the polypyrrole/PEO-LiClO4 junction prepared in situ by the potential sweep method between 0 and 2.0 V for 30 rain. Sweep rate, 100 mV/s.
204 0.3
.
~
~ 0.04
0.2
~
~
cm 0.1
fi
b
0.02
~
_.:..==....
-~ -0.02 u
-0.1
.~ -0.04
-'2
"1
'
-'1
Applied voltage (V)
'
'
^pplied voltage (V)
Fig. 3. Current-voltage curve of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method between 0 and 2.0 V for 30 min. Sweep rate, 100 mV/s. Fig. 4. Cyclic voltammograms of the polypyrrole/PEO-LiC104 junctions: (a) prepared in situ by the galvanostatic method at 0.01 mA for 100 rain; (b) prepared in situ by the potentiostatic method at 4 V for 30 rain. Sweep rate, 100 mV/s. 0.04 0.04
~
~
/s /
0.02
0,[12
,
o
o
-0.02
-0.02
-0.04
/
u _12
11
I
o
I
1
Appl led voltage (V)
I
2
-0.04 Applied voltage (V)
Fig. 5. Current-voltage curves of the polypyrrole/PEO-LiCIO4 junctions: (a) prepared in situ by the galvanostatic method at 0.01 mA for 100 min; (b) prepared in situ by the potentiostatic method at 4 V for 30 rain. Sweep rate, 100 mV/s. Fig. 6. Current-voltage curves of the junctions between PEO-LiC104 and polypyrrolecoated electrode. Polypyrrole films were prepared in a 0.1 M pyrrole/0.05 M (n-Bu)4NC104 acetonitrile solution by (a) the potential sweep method between 0 and 2.0 V for 12 h and by (b) the galvanostatic method at 0.04 mA/cm 2 on a Pt-coated electrode.
ethylene glycol and LiC104 in the wt. ratio of 1:1:1. The current-voltage curves of the polypyrrole/PEO-LiC1Oa junctions are shown in Fig. 6. Figure 6(a) shows the current-voltage curve of the polypyrrole/PEO-LiC1Oa junction using a polypyrrole-coated electrode prepared by the potential sweep m e t h o d between 0 and 2.0 V. However, the rectifying behavior as shown in Fig. 3 does not appear in Fig. 6. The junction between PEO-LiC104 and the polypyrrole-coated electrode prepared by the galvanostatic m e t h o d also did not show rectifying behavior, as shown in Fig. 6(b). The capacitance-voltage measurement is useful to characterize the interface of a semiconductor [17, 18]. In the plots of 1/C 2 against reverse applied voltage, the potential where the line intersects the potential axis yields the value of the diffusion potential V D. In the space-charge layer, an
205
electric potential difference VD is caused by two layers which are charged oppositely to each other. The diffusion potential VD expresses the magnitude of the energy barrier for carrier transport. Figure 7 shows the C - V characteristics of the polypyrrole/PEO-LiC104 junction prepared by the potential sweep method. On increasing the reverse applied voltage, the thickness of the space-charge layer increases and hence the capacitance decreases [17, 18]. The VD value of 1.21 eV is determined by the intersect of the potential in Fig. 7. Polypyrrole/PEO-LiCIO4 junctions prepared by galvanostatic and potentiostatic methods, however, did not show the energy barrier at the interfaces. Figure 8 shows the C - V characteristics of the polypyrrole/PEOLiC104 junctions prepared in situ by the galvanostatic method. The capacitance is almost constant both at reverse and forward applied voltage. This means that there is no energy barrier at the interface between polypyrrole and PEO-LiC104. This result is consistent with the current-voltage characteristics, which do not show rectifying behavior. The polypyrrole/PEOLiCIO4 junctions prepared in situ by the potentiostatic method (Fig. 9) also show the same tendency.
4. Discussion There are two possible junctions as the rectifying junction. One is the junction between polypyrrole and the electrode, and the other is the junction between polypyrrole and PEO-LiC104. It is known that polypyrrole has the characteristics of a p-type semiconductor [7, 16]. Schottky contact between polypyrrole and a metal with a low work function has been reported [7]. However, the contact between p-type polypyrrole and a metal with a high work function such as Pt can be considered as an ohmic contact. On the other hand, the semiconductor/electrolyte junctions have been B
10
•
• o•
C
e•
•
4
• o O •
OoO
o ×
o
b
x o•
~2
o 9oC~Go.--o,......~o
° 8
.o ? o ~ p o 9 -6
-4
-2
0
2
4
Reverse epp] ied voltage (V)
6
0
_~ _~ _~
~
~
~
Reverse applied voltage (V)
Fig. 7. Capacitance-voltage characteristics of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method between 0 and 2.0 V for 30 rain. Fig. 8. Capacitance-voltage characteristics of polypyrrole/PEO-LiC104 junctions prepared in situ by the galvanostatic method at 0.01 mA. Electrolysis time: (a) 100 min; (b) 230 min; (c) 760 min.
206 tO
T~ × •
o
o
u
.
o
•
o
a
.
.
~
b
m n
0
o
8
Reverse epplied voltage (V)
Fig. 9. Capacitance-voltage characteristics of polypyrrole/PEO-LiC104 junctions prepared in situ by the potentiostatic method at (a) 2V for 30 rain and at (b) 4V for 30 rain.
utilized for photoelectrochemical cells (PEC) [19 - 21]. It has been reported that photocurrent is generated at polypyrrole/electrolyte and polythiophene/ electrolyte interfaces [7, 22]. From analogy with the semiconductor/electrolyte junctions, it is inferred that the rectification as shown in Figs. 2 and 3 is caused by the junction between polypyrrole and the polymer electrolyte. At the semiconductor/electrolyte interface, a depletion layer within the semiconductor and an oppositely charged layer within the electrolyte are caused by the equalization of the electrochemical potential across the built-in potential barrier. At equilibrium, Fermi levels are adjusted by the formation of an electric double layer between the semiconductor and the electrolyte. The magnitude of the potential barrier is determined by the bulk properties of the semiconductor and the interface properties at the junction [ 19 - 21]. There is a possibility that the bulk properties of the p-type polypyrrole influence the I - V characteristics of the polypyrrole/PEO-LiC104 junctions. However, the junction between PEO-LiC104 and the polypyrrole-coated electrode prepared by the potential sweep method did not show rectifying behavior, as shown in Fig. 6(a), whereas the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method showed rectifying behavior clearly. Another possibility is that the interface properties at the junction influence the electrical properties of the junction. By the in situ electrochemical oxidation of pyrrole, polypyrrole deposits on the anode and C104- ions are incorporated into polypyrrole as dopants. These dopants are rather fixed in polypyrrole because of the low diffusibility of ions in polymer electrolyte. Actually, the absorption spectra of the polypyrrole/ PEO-LiC104 composite sandwiched between the Pt-coated transparent electrodes were scarcely changed by the negative potential sweep, and show the typical spectra of C104--doped polypyrrole. This suggests the difficulty of dedoping in a high resistivity environment such as polymer electrolyte. By the consumption of C104- ions as dopants, C104- ions are deficient and Li ÷ ions are in excess at the interface between polypyrrole and PEO-LiC104. Under galvanostatic and potentiostatic conditions, the excess Li ÷ ions diffuse to the counter electrode by the applied voltage. In this case, the interface between C104--doped polypyrrole and the oppositely charged layer caused
207
by the excess Li + is not definite. Under potential sweep conditions, the cathodic sweep causes the absorption of excess Li ÷ ions on the C104--doped polypyrrole. The formation of an electronic double layer between p-type polypyrrole and PEO-LiC104 causes the electric field in the space-charge layer, thus a depletion layer within the p-type polypyrrole is formed. The capacitance-voltage characteristics of the polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method clearly confirms the existence of the depletion layer. The diffusion potential, VD, is determined as 1.21 eV in Fig. 7. The polypyrrole/PEO-LiC104 junction prepared in situ by the potential sweep method has large capacitance in comparison with the junctions produced by the other methods. This also suggests that the polypyrrole/PEO-LiC104 interface produced in situ by the potential sweep method is definite compared to the interface produced by the other methods. In Figs. 8 and 9, the capacitance of the junction decreases with increasing the electrolysis time, i.e., the thickness of the polypyrrole layer. This must be due to the formation of the undefined interface between the polypyrrole and PEO-LiC104 on increasing the thickness of the polypyrrole layer. The junctions between PEO-LiC104 and polypyrrole-coated electrode do not show rectifying behavior (Fig. 6). This must be due to the homogeneous distribution of Li ÷ and C104- at the interface because of the low diffusibility of ions in polymer electrolyte. Contrary to this, an electronic double layer is easily formed between a semiconductor and an electrolyte [19 - 21]. The difference between the usual electrolyte and a polymer electrolyte can be explained by the mobility of ionic species in the media and the properties of the electric double layer at the interface.
5. Conclusion The rectifying junction between polypyrrole and PEO-LiC104 was prepared in situ by electrochemical polymerization of pyrrole in PEOLiC104 under potential sweep conditions. The rectifying behavior of the junction was explained by the depletion layer which was formed between p-type polypyrrole and the oppositely charged layer caused by excess Li ÷ ions.
References 1 H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C.-K. Chiang and A. J. Heeger, J. Chem. Soc. Chem. Commun., (1977) 578. 2 M. Ozaki, D. L. Peebles, B. R. Weinberger, C. K. Chiang, S. C. Gau, A. J. Heeger and A. G. MacDiarmid, Appl. Phys. Left., 35 (1979) 83. 3 P. M. Grant, T. Tani, W. D. Gill, M. Krounbi and T. C. Clarke, J. Appl. Phys., 52 (1981) 869. 4 J. Kanicki, Mol. Cryst. Liq. Cryst., 105 (1984) 203.
208
5 0 . Ingan~is, T. Skotheim and I. Lundstrbm, J. Appl. Phys., 54 (1983) 3636. 6 O. Ingan~is and I. LundstrSm, Synth. Met., 10 (1984/85) 5. 7 M. Kaneko, K. Okuzumi and A. Yamada, J. Electroanal. Chem., 183 (1985) 407. 8 R. Noufi, D. Tench and L. F. Warren, J. Electrochem. Soc., 127 (1980) 2310. 9 R. Noufi, A. J. Frank and A. J. Nozik, J. A m . Chem. Soc., 103 (1981) 1849. 10 R. Noufi, A. J. Nozik, J. White and L. F. Warren, J. Electrochem. Soc., 129 (1982) 2261. 11 T. Skotheim, L.-G. Petersson, O. Ingan~/s and I. LundstrSm, J. Electrochem. Soc., 129 (1982) 1737. 12 T. Skotheim and I. Lundstrbm, J. Electrochem. Soc., 129 (1982) 894. 13 T. A. Skotheim and O. Ingan~is, J. Electrochem. Soc., 132 (1985) 2116. 14 T. A. Skotheim, Synth. Met., 14 (1986) 31. 15 M. Watanabe, K. Tadano, K. Sanui and N. Ogata, Chem. Lett., (1987) 1239. 16 K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant, G. B. Street, G. P. Gardini and J. F. Kwak, Synth. Met., 1 (1979/80) 329. 17 A. J. Bard and L. F. Faukner, Electrochemical Methods, Wiley, New York, 1980, Ch. 14. 18 S. M. Sze, Physics o f Semiconductor Devices, Wiley, New York, 2nd edn., 1981. 19 A. L. Fahrenbruch and R. H. Bube, Fundamentals o f Solar Cells, Academic Press, New York, 1983, Ch. 12. 20 K. L. Chopra and S. R. Das, Thin Film Solar Cells, Plenum Press, New York/London, 1983, Ch. 11. 21 S. J. Fonash, Solar Cell Device Physics, Academic Press, New York, 1981, Ch. 6. 22 T. Kenmochi, E. Tsuchida, M. Kaneko and A. Yamada, Electrochim. Acta, 30 (1985) 1405.