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A.c. impedance of frozen junction polymer light-emitting electrochemical cells
SYnli'lUllTIIC I I|TRLS ELSEVIER
Synthetic Metals 97 (1998) 191-194
A.c. impedance of frozen junction polymer light-emitting electrochemical cells Yongfang Li a,,, Jun Gao a, Deli Wang a, Gang Yu b, Yong Cao b, Alan J. Heeger a,b Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106-5090, USA b UNIAX, 6780 Cortona Drive, Santa Barbara, CA 93117-3022, USA Received 6 July 1998; accepted 24 July 1998
Abstract
The structure of the frozen p--i-n junction in polymer light-emitting electrochemical cells (LECs) is studied by measurements of the a.c. impedance of LEC devices. Impedance plots of frozen junction LECs show characteristics that are typical of polymer light-emitting diodes. The data demonstrate the 'freeze-out' of ionic mobility. The frequency-independent and voltage-independent capacitance of the frozenjunction supports the conclusion that the insulating 'i' region in the p-i-n junction occupies most of the thickness of the LEC polymer layer. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Impedance; Electrochemical cells; Light-emitting devices; Frozen junctions
1. Introduction
Polymer light-emitting electrochemical cells (LECs) provide a novel approach for achieving light emission from electroluminescent polymers [ 1-10]. LECs are composed of a polymer blend containing luminescent polymer and solid electrolyte sandwiched between an indium-tin oxide (ITO) electrode and a metal electrode. Application of a voltage greater than the energy gap of the luminescent polymer results in electrochemical p-doping near the anode and n-doping near the cathode; a p-i-n junction is created in situ. Light emission occurs in the insulating region between the p- and n-doped layers. Electrochemical doping and the formation of a dynamic p-i-n junction have been confirmed by optical beam induced current (OBIC) measurements carded out on surface LECs [11]. Ionic motion in the polymer blend plays a key role in the electrochemical doping and the formation of the p-i-n junction. After the junction is formed, however, ionic mobility is undesirable. Degradation of the luminescent polymer can occur by over-oxidation or over-reduction when applying bias voltages sufficient for high brightness displays. The frozen p-i-n junction approach provides a method for freezing out ionic motion after formation of the p-i-n junction [ 12]. * Corresponding author.
Frozen junction LECs have been demonstrated with fast response and high brightness [ 12,13]. A.c. impedance data have been utilized to characterize the p-i-n junction in LECs [ 14]. In comparison with the ideal semicircle for LEDs, the Z"-Z' plots obtained from LECs show flattened semicircles. The deformation of the semicircle may be caused by the frequency dependence of the ionic contribution to the polarizability at low frequencies [ 15], and/or could result from the roughness of the interface in the p-i-n junction [ 14]. The frequency-independent capacitance observed at high frequencies (the electronic contribution) can be analyzed to determine the structure of the p--i-n junction. We report here the results of measurements of the a.c. impedance of frozen junction LECs. The results show that, in frozen junction LECs, the a.c. impedance versus frequency is in all respects like that of LEDs. We note, in particular, the ideal semicircular shape of Z" versus Z' plots and the perfect overlap of the Z" versusfcurves in the high frequency region. Thus, the measurements prove that there is no significant ionic motion when operating LECs in the frozen-junction mode. The frequency-independent capacitance in the frozen junction regime agrees with the values calculated from the a.c. impedance in the high frequency region for the same LECs at room temperature [ 14]. The results confirm that the insulating 'i' region occupies most of the thickness of the polymer film [ 14].
0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 0 3 7 9 - 6 7 7 9 ( 98 ) 0 0 1 2 4 - 6
E L i et al. / Synthetic Metals 97 (1998) 191-194
192
2. Experimental
'
The LECs studied had the following structure: I T O / M E H PPV:PEO(LiCF3SO3)/A1 (abbreviated as LEC(PEO) in the following) and ITO/MEH-PPV:crown ether(Liimide)/A1 (abbreviated as LEC(CE) in the following); where MEH-PPV is poly[5-(2'-ethylhexyloxy)-2-methoxy- 1,4-phenylene vinylene ], PEO is poly (ethylene oxide), and the crown ether is 2,3,11,12-dicyclohexano-l,4,7,10, 13,16-hexaoxacyclooctadecane (DCH-18Cr6). The weight ratio was MEH-PPV:PEO(or crown ether):Li salt= 12:5:1. The polymer blend films were spin-cast onto ITO substrates from cyclohexanone solution. The film thickness was about 200-300 nm. Details on the device fabrication and lightemission will be published elsewhere [ 16]. The a.c. impedance was measured using a HP 4192A impedance analyzer in the frequency region from 100 Hz to 2 M Hz with an a.c. drive voltage of 30 mV. The voltage bias was applied with the ITO electrode as anode (positive) and the A1 electrode as cathode (negative). Test devices for the frozen junction experiment were mounted in a cryostat and cooled by liquid nitrogen.
3. Results
'
'
I
. . . .
i
,
,
,
,
I
,
,
,
,
[
,
,
,
4.0V A
A
/3
10
A
/3 A A A
A
A
0
\
6.0 V
, , , i .... 0
~'~- . . . , . . . .
5
10
~ . ~.
15
20
z' (k~) Fig. 1. Imaginary part (Z") vs. real part (Z') plots of the a.c. impedance of a frozen junction LEC(PEO) device at various bias voltages at 120 K: 0.0V ( © ) , 2 . 0 V ( x ) , 4 . 0 V ( A ) a n d 6 . 0 V ( 0 ) .
Z"= l/toC= l/2'rrfC
(2)
Thus, the plot of log Z" versus log f should show a straight line with the slope of - 1 in the high frequency region. When wRC= 1, Z" has a maximum value equal to R/2 (at the top point on the semicircle of the Z" versus Z' plot). When the frequency is low enough that to2R:C: << 1, then Eq. (1) simplifies to Z " = t o R 2 C = 2"rrfR2 C
Fig. 1 shows the Z" versus Z' plots for the LEC(PEO) devices with frozen p - i - n junction at 120 K; i.e. well below the glass transition temperature of PEO (Tg ~ 208 K). The frozen junction was created by first applying a 3.0 V bias voltage at room temperature for 20 min, then decreasing the temperature to 100 K while leaving the device under bias [ 12]. Ideal semicircles were obtained for the Z" versus Z' plots at bias voltages of 4.0 and 6.0 V, indicating that the device can be represented by a parallel RC circuit in which the capacitance (C) and the resistance (R) are frequency independent, as in polymer LEDs [ 14,17,18]. This result confirms the freeze-out of ionic motion in the frozen p - i - n junction. In order to compare the behavior of the dynamic p - i - n junction, the Z" versus Z' plot was also measured at room temperature for the same LEC device, and a flattened semicircle is obtained, in agreement with previous observations [ 14]. The fact that the semicircle for the p - i - n junction is ideal in the frozen junction mode makes it clear that the deformation of the semicircle at room temperature is caused by the ionic response at low frequencies. For a parallel RC circuit, the frequency dependences of Z' and Z" are as follows [ 14] : Z'-
'
0.0 V 15 -¢~ 2.0v
R 1+to2R2C2'
Z"=
toR2C I+toERZC2
(1)
where to is the angular frequency, to= 21rf When the frequency is sufficiently high that to2R2C2 >> 1, Eq. ( 1 ) simplifies to
(3)
Thus, a plot of log Z" versus logfshould show a straight line with the slope of -4- 1 at low frequencies. Fig. 2 shows the log Z" versus logfcurves for a series of bias voltages (0.0, 2.0, 4.0 and 6.0 V). In agreement with Eq. (2), a plot of log Z" versus log fyields a straight line with the slope of - 1 above 30 kHz, similar to the behavior of polymer LEDs. In the low frequency region, the curves show a straight line with the slope of + 1, as expected from Eq. (1). The shift of the maximum in Z" to higher frequency at higher bias voltages results from the increased LEC current (lower resistance) of the device at higher bias voltages. The capacitance can be calculated from the slope of the straight line (at frequencies above 30 kHz) using Eq. (2): 10s
. . . . .
11,1~
........
O.OV ~*~ 2.0 V m 104
I
........
i
........
i
........
®%
.-"'a°.
,,_., 103
g~
~,.0V**
** lit
10
2
tm
"6.0V
t i
lO
1
........
102
I
103
........
I
........
104
I
105
........
I
106
......
107
f (Hz) Fig. 2. Frequency dependence of the imaginary part of the a.c. impedance of the frozen junction LEC (PEO) device at various bias voltages at 120 K: 0.0V ( O ) , 2.0V ( X ) , 4 . 0 V ( A ) a n d 6 . 0 V ( 0 ) .
Y.Li et al./ SyntheticMetals97(1998)191-194 C = 1.61 nF at 120 K and is voltage independent. The temperature dependence of the capacitance was measured in 20 K intervals, as shown in Fig. 3. The capacitance increases with increasing temperature to 1.93 nF at 260 K; the slight increase results from the increase in the dielectric constant in this temperature range [ 19 ]. At room temperature, the capacitance of the device at zero bias is 1.54 nF and that at 3.0 V is 1.95 nF (both calculated from the high frequency values of Z" using Eq. ( 2 ) ) . The frequency-independent capacitances in the frozen junction regime and the weakly temperature-dependent capacitance in Fig. 3 indicate that the capacitance values obtained from the high frequency slope of log Z" versus log f a r e reliable for the frozen junction and for the dynamic p - i - n junction at room temperature. Thus, the interpretation of the capacitance as an indicator of the thickness of the 'i' layer in the p - i - n junction is consistent with all the data [ 14]. The quality of the Z" versus Z' semicircle can be analyzed from the ratio of its height (h) to its width (w). The ratio ( r = h/w) should be 0.5 for an ideal semicircle. In order to determine the temperature at which ionic motion begins, r(7") was measured from 100 to 260 K for the frozen p - i - n junction L E C ( P E O ) biased at 4.0 V; the data are shown in Fig. 4. The semicircle is perfect with r = 0.5 for temperatures lower than 140 K. At higher temperatures, r(T) begins to decrease, i.e. the semicircle begins to flatten, dropping to r = 0.4 at 260 K. This indicates that the ions begin to respond to the a.c. drive voltage at T = 150 K. It is well known that the temperature dependence of the ionic conductivity of polymer electrolyte obeys the Vogel, Tamman and Fulcher (VTF) equation
[20]:
o'=o'o exPI R(~_BTo) l
(4)
where To is a reference temperature of the polymer, which is approximately 50 K lower than the glass transition temperature, Tg [20]. At temperatures below To, the ionic conductivity will be close to zero. For PEO, Tg = 208 K, so To = 158 K
2.00
. . . .
I
. . . .
I
. . . .
I
. . . .
I
' ' ' ' I
. . . .
I
. . . .
I
. . . .
I ' ' ' '
0.50 0.45 0.40 0.35 0.30 . . . . 50
' .... ' .... ' .... ' .... 100 150 200 250 300 Temperature (K) Fig. 4. Temperature dependence of the ratio of height (h) to width (w) of the semicircle in the Z" vs. Z' plots of the frozenjunction of LEC(PEO) at 4.0V.
which is quite close to the temperature where the deformation of the semicircle of Z" versus Z' plots begins. The same procedure was carded out for frozen junction L E C ( C E ) devices. The results were completely consistent with the data obtained from the L E C ( P E O ) devices. Thus, the impedance data presented in detail above for the frozen junction L E C ( P E O ) devices appear to be general features of LECs.
4. Conclusions In summary, we have measured the a.c. impedance of LECs in the dynamic junction regime and in the frozen junction regime. In the frozen junction regime, LED-like impedance behavior was observed, i.e. Z" versus Z' plots are ideal semicircles, and the capacitance is frequency independent. Evidence of ionic motion was observed at temperatures above To of the polymer electrolyte, where To is approximately 50 K lower than Tg. The measured capacitance in the frozen junction LEC is consistent with that expected for a parallelplate capacitor in which the thickness of the dielectric layer is determined by the 'i' layer within the p - i - n junction.
. . . .
~.)'-'~ 1.90
1.80 . ....~
0.55
193
Acknowledgements This work was supported by the National Science Foundation under Grant No. ECE-9528204.
. 1.70
References (") 1.60 1.50 . . . . 50
I .... I .... , .... , .... 100 150 200 250 300 Temperature (K) Fig. 3. Temperature dependence of the capacitance of the frozen junction LEC(PEO) at 4.0 V.
[ 1] Q. Pei, F. Klavetter, US Patent No. 5 682 043 (1997). [2] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 268 (1995) 1086. [3] Q. Pei, Y. Yang, G. Yu, C. Zhang, A.J. Heeger, J. Am. Chem. Soc. 118 (1996) 3922. [4] Y. Cao, G. Yu, C.Y. Yang, A.J. Heeger, Appl. Phys. Lett. 68 (1996) 3218.
194 [5] [6] [7] [8] [9] [10] [11] [12] [ 13] [ 14]
Y. Li et al. / Synthetic Metals 97 (1998) 191-194
Q. Pei, Y. Yang, J. Am. Chem. Soc. 118 (1996) 7416. G. Yu, Q. Pei, A.J. Heeger, Appl. Phys. Lett. 70 (1997) 934. G. Yu, US Patent No. 5 677 546 (1997). Y. Yang, Q. Pei, J. Appl. Phys. 81 (1997) 3294. Y. Yang, Q. Pei, Appl. Phys. Lett. 68 (1996) 2708. Y. Cao, Q. Pei, M.R. Andersson, G. Yu, A.J. Heeger, J. Electrochem. Soc. 144 (1997) L317. D.J. Dick, A.J. Heeger, Y. Yang, Q. Pei, Adv. Mater. 8 (1996) 985. J. Gao, G. Yu, A.J. Heeger, Appl. Phys. Lett. 71 (1997) 1293. G. Yu, Y. Cao, M.R. Andersson, J. Gao, AJ. Heeger, Adv. Mater. 10 (1998) 385. Y.F. Li, J. Gao, G. Yu, Y. Cao, A.J. Heeger, Chem. Phys. Lett. 287 (1998) 83.
[15] G. Yu, Y. Cao, C. Zhang, Y.F. Li, J. Gao, A.J. Heeger, Appl. Phys. Lea., in press. [ 16] J. Gao, Y.F. Li, G. Yu, A.J. Heeger, to be published. [17] A.J. Campbell, D.D.C. Bradley, D.G. Lidzey, J. Appl. Phys. 82 (1997) 6326. [ 18] I.H. Campbell, D.L. Smith, J.P. Ferraris, Appl. Phys. Lett. 66 (1995) 3030. [ 19] M.C. WintersgiU, J.J. Fontanella, in J.R. MacCallum, C.A. Vincent (eds.), Polymer Electrolyte Reviews 2, Elsevier Applied Science, Barking, UK, 1989, pp. 43-59. [ 20 ] P.G. Bruce, F.M. Gray, in P.G. Bruce (ed.), Solid State Electrochemistry, Cambridge University Press, New York, 1995, pp. 130-138.
Synthetic Metals 97 (1998) 191-194
A.c. impedance of frozen junction polymer light-emitting electrochemical cells Yongfang Li a,,, Jun Gao a, Deli Wang a, Gang Yu b, Yong Cao b, Alan J. Heeger a,b Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA 93106-5090, USA b UNIAX, 6780 Cortona Drive, Santa Barbara, CA 93117-3022, USA Received 6 July 1998; accepted 24 July 1998
Abstract
The structure of the frozen p--i-n junction in polymer light-emitting electrochemical cells (LECs) is studied by measurements of the a.c. impedance of LEC devices. Impedance plots of frozen junction LECs show characteristics that are typical of polymer light-emitting diodes. The data demonstrate the 'freeze-out' of ionic mobility. The frequency-independent and voltage-independent capacitance of the frozenjunction supports the conclusion that the insulating 'i' region in the p-i-n junction occupies most of the thickness of the LEC polymer layer. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Impedance; Electrochemical cells; Light-emitting devices; Frozen junctions
1. Introduction
Polymer light-emitting electrochemical cells (LECs) provide a novel approach for achieving light emission from electroluminescent polymers [ 1-10]. LECs are composed of a polymer blend containing luminescent polymer and solid electrolyte sandwiched between an indium-tin oxide (ITO) electrode and a metal electrode. Application of a voltage greater than the energy gap of the luminescent polymer results in electrochemical p-doping near the anode and n-doping near the cathode; a p-i-n junction is created in situ. Light emission occurs in the insulating region between the p- and n-doped layers. Electrochemical doping and the formation of a dynamic p-i-n junction have been confirmed by optical beam induced current (OBIC) measurements carded out on surface LECs [11]. Ionic motion in the polymer blend plays a key role in the electrochemical doping and the formation of the p-i-n junction. After the junction is formed, however, ionic mobility is undesirable. Degradation of the luminescent polymer can occur by over-oxidation or over-reduction when applying bias voltages sufficient for high brightness displays. The frozen p-i-n junction approach provides a method for freezing out ionic motion after formation of the p-i-n junction [ 12]. * Corresponding author.
Frozen junction LECs have been demonstrated with fast response and high brightness [ 12,13]. A.c. impedance data have been utilized to characterize the p-i-n junction in LECs [ 14]. In comparison with the ideal semicircle for LEDs, the Z"-Z' plots obtained from LECs show flattened semicircles. The deformation of the semicircle may be caused by the frequency dependence of the ionic contribution to the polarizability at low frequencies [ 15], and/or could result from the roughness of the interface in the p-i-n junction [ 14]. The frequency-independent capacitance observed at high frequencies (the electronic contribution) can be analyzed to determine the structure of the p--i-n junction. We report here the results of measurements of the a.c. impedance of frozen junction LECs. The results show that, in frozen junction LECs, the a.c. impedance versus frequency is in all respects like that of LEDs. We note, in particular, the ideal semicircular shape of Z" versus Z' plots and the perfect overlap of the Z" versusfcurves in the high frequency region. Thus, the measurements prove that there is no significant ionic motion when operating LECs in the frozen-junction mode. The frequency-independent capacitance in the frozen junction regime agrees with the values calculated from the a.c. impedance in the high frequency region for the same LECs at room temperature [ 14]. The results confirm that the insulating 'i' region occupies most of the thickness of the polymer film [ 14].
0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 0 3 7 9 - 6 7 7 9 ( 98 ) 0 0 1 2 4 - 6
E L i et al. / Synthetic Metals 97 (1998) 191-194
192
2. Experimental
'
The LECs studied had the following structure: I T O / M E H PPV:PEO(LiCF3SO3)/A1 (abbreviated as LEC(PEO) in the following) and ITO/MEH-PPV:crown ether(Liimide)/A1 (abbreviated as LEC(CE) in the following); where MEH-PPV is poly[5-(2'-ethylhexyloxy)-2-methoxy- 1,4-phenylene vinylene ], PEO is poly (ethylene oxide), and the crown ether is 2,3,11,12-dicyclohexano-l,4,7,10, 13,16-hexaoxacyclooctadecane (DCH-18Cr6). The weight ratio was MEH-PPV:PEO(or crown ether):Li salt= 12:5:1. The polymer blend films were spin-cast onto ITO substrates from cyclohexanone solution. The film thickness was about 200-300 nm. Details on the device fabrication and lightemission will be published elsewhere [ 16]. The a.c. impedance was measured using a HP 4192A impedance analyzer in the frequency region from 100 Hz to 2 M Hz with an a.c. drive voltage of 30 mV. The voltage bias was applied with the ITO electrode as anode (positive) and the A1 electrode as cathode (negative). Test devices for the frozen junction experiment were mounted in a cryostat and cooled by liquid nitrogen.
3. Results
'
'
I
. . . .
i
,
,
,
,
I
,
,
,
,
[
,
,
,
4.0V A
A
/3
10
A
/3 A A A
A
A
0
\
6.0 V
, , , i .... 0
~'~- . . . , . . . .
5
10
~ . ~.
15
20
z' (k~) Fig. 1. Imaginary part (Z") vs. real part (Z') plots of the a.c. impedance of a frozen junction LEC(PEO) device at various bias voltages at 120 K: 0.0V ( © ) , 2 . 0 V ( x ) , 4 . 0 V ( A ) a n d 6 . 0 V ( 0 ) .
Z"= l/toC= l/2'rrfC
(2)
Thus, the plot of log Z" versus log f should show a straight line with the slope of - 1 in the high frequency region. When wRC= 1, Z" has a maximum value equal to R/2 (at the top point on the semicircle of the Z" versus Z' plot). When the frequency is low enough that to2R:C: << 1, then Eq. (1) simplifies to Z " = t o R 2 C = 2"rrfR2 C
Fig. 1 shows the Z" versus Z' plots for the LEC(PEO) devices with frozen p - i - n junction at 120 K; i.e. well below the glass transition temperature of PEO (Tg ~ 208 K). The frozen junction was created by first applying a 3.0 V bias voltage at room temperature for 20 min, then decreasing the temperature to 100 K while leaving the device under bias [ 12]. Ideal semicircles were obtained for the Z" versus Z' plots at bias voltages of 4.0 and 6.0 V, indicating that the device can be represented by a parallel RC circuit in which the capacitance (C) and the resistance (R) are frequency independent, as in polymer LEDs [ 14,17,18]. This result confirms the freeze-out of ionic motion in the frozen p - i - n junction. In order to compare the behavior of the dynamic p - i - n junction, the Z" versus Z' plot was also measured at room temperature for the same LEC device, and a flattened semicircle is obtained, in agreement with previous observations [ 14]. The fact that the semicircle for the p - i - n junction is ideal in the frozen junction mode makes it clear that the deformation of the semicircle at room temperature is caused by the ionic response at low frequencies. For a parallel RC circuit, the frequency dependences of Z' and Z" are as follows [ 14] : Z'-
'
0.0 V 15 -¢~ 2.0v
R 1+to2R2C2'
Z"=
toR2C I+toERZC2
(1)
where to is the angular frequency, to= 21rf When the frequency is sufficiently high that to2R2C2 >> 1, Eq. ( 1 ) simplifies to
(3)
Thus, a plot of log Z" versus logfshould show a straight line with the slope of -4- 1 at low frequencies. Fig. 2 shows the log Z" versus logfcurves for a series of bias voltages (0.0, 2.0, 4.0 and 6.0 V). In agreement with Eq. (2), a plot of log Z" versus log fyields a straight line with the slope of - 1 above 30 kHz, similar to the behavior of polymer LEDs. In the low frequency region, the curves show a straight line with the slope of + 1, as expected from Eq. (1). The shift of the maximum in Z" to higher frequency at higher bias voltages results from the increased LEC current (lower resistance) of the device at higher bias voltages. The capacitance can be calculated from the slope of the straight line (at frequencies above 30 kHz) using Eq. (2): 10s
. . . . .
11,1~
........
O.OV ~*~ 2.0 V m 104
I
........
i
........
i
........
®%
.-"'a°.
,,_., 103
g~
~,.0V**
** lit
10
2
tm
"6.0V
t i
lO
1
........
102
I
103
........
I
........
104
I
105
........
I
106
......
107
f (Hz) Fig. 2. Frequency dependence of the imaginary part of the a.c. impedance of the frozen junction LEC (PEO) device at various bias voltages at 120 K: 0.0V ( O ) , 2.0V ( X ) , 4 . 0 V ( A ) a n d 6 . 0 V ( 0 ) .
Y.Li et al./ SyntheticMetals97(1998)191-194 C = 1.61 nF at 120 K and is voltage independent. The temperature dependence of the capacitance was measured in 20 K intervals, as shown in Fig. 3. The capacitance increases with increasing temperature to 1.93 nF at 260 K; the slight increase results from the increase in the dielectric constant in this temperature range [ 19 ]. At room temperature, the capacitance of the device at zero bias is 1.54 nF and that at 3.0 V is 1.95 nF (both calculated from the high frequency values of Z" using Eq. ( 2 ) ) . The frequency-independent capacitances in the frozen junction regime and the weakly temperature-dependent capacitance in Fig. 3 indicate that the capacitance values obtained from the high frequency slope of log Z" versus log f a r e reliable for the frozen junction and for the dynamic p - i - n junction at room temperature. Thus, the interpretation of the capacitance as an indicator of the thickness of the 'i' layer in the p - i - n junction is consistent with all the data [ 14]. The quality of the Z" versus Z' semicircle can be analyzed from the ratio of its height (h) to its width (w). The ratio ( r = h/w) should be 0.5 for an ideal semicircle. In order to determine the temperature at which ionic motion begins, r(7") was measured from 100 to 260 K for the frozen p - i - n junction L E C ( P E O ) biased at 4.0 V; the data are shown in Fig. 4. The semicircle is perfect with r = 0.5 for temperatures lower than 140 K. At higher temperatures, r(T) begins to decrease, i.e. the semicircle begins to flatten, dropping to r = 0.4 at 260 K. This indicates that the ions begin to respond to the a.c. drive voltage at T = 150 K. It is well known that the temperature dependence of the ionic conductivity of polymer electrolyte obeys the Vogel, Tamman and Fulcher (VTF) equation
[20]:
o'=o'o exPI R(~_BTo) l
(4)
where To is a reference temperature of the polymer, which is approximately 50 K lower than the glass transition temperature, Tg [20]. At temperatures below To, the ionic conductivity will be close to zero. For PEO, Tg = 208 K, so To = 158 K
2.00
. . . .
I
. . . .
I
. . . .
I
. . . .
I
' ' ' ' I
. . . .
I
. . . .
I
. . . .
I ' ' ' '
0.50 0.45 0.40 0.35 0.30 . . . . 50
' .... ' .... ' .... ' .... 100 150 200 250 300 Temperature (K) Fig. 4. Temperature dependence of the ratio of height (h) to width (w) of the semicircle in the Z" vs. Z' plots of the frozenjunction of LEC(PEO) at 4.0V.
which is quite close to the temperature where the deformation of the semicircle of Z" versus Z' plots begins. The same procedure was carded out for frozen junction L E C ( C E ) devices. The results were completely consistent with the data obtained from the L E C ( P E O ) devices. Thus, the impedance data presented in detail above for the frozen junction L E C ( P E O ) devices appear to be general features of LECs.
4. Conclusions In summary, we have measured the a.c. impedance of LECs in the dynamic junction regime and in the frozen junction regime. In the frozen junction regime, LED-like impedance behavior was observed, i.e. Z" versus Z' plots are ideal semicircles, and the capacitance is frequency independent. Evidence of ionic motion was observed at temperatures above To of the polymer electrolyte, where To is approximately 50 K lower than Tg. The measured capacitance in the frozen junction LEC is consistent with that expected for a parallelplate capacitor in which the thickness of the dielectric layer is determined by the 'i' layer within the p - i - n junction.
. . . .
~.)'-'~ 1.90
1.80 . ....~
0.55
193
Acknowledgements This work was supported by the National Science Foundation under Grant No. ECE-9528204.
. 1.70
References (") 1.60 1.50 . . . . 50
I .... I .... , .... , .... 100 150 200 250 300 Temperature (K) Fig. 3. Temperature dependence of the capacitance of the frozen junction LEC(PEO) at 4.0 V.
[ 1] Q. Pei, F. Klavetter, US Patent No. 5 682 043 (1997). [2] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 268 (1995) 1086. [3] Q. Pei, Y. Yang, G. Yu, C. Zhang, A.J. Heeger, J. Am. Chem. Soc. 118 (1996) 3922. [4] Y. Cao, G. Yu, C.Y. Yang, A.J. Heeger, Appl. Phys. Lett. 68 (1996) 3218.
194 [5] [6] [7] [8] [9] [10] [11] [12] [ 13] [ 14]
Y. Li et al. / Synthetic Metals 97 (1998) 191-194
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