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poly(3-hexylthiophene) contacts
Synthetic Metals, 41-43 (1991) 499-502
499
RECTIFYING METAL/POLY(3-HEXYLTHIOPHENE) CONTACTS
G. GUSTAFSSON, O. INGAN.~S, M. SUNDBERG AND C. SVENSSON Department of Physics (IFM), Link6ping University, S- 581 83 Link6ping, Sweden
ABSTRACT Rectifying contacts between poly(3-hexylthiophene) and Indium-Tin oxide or Aluminum have been studied by means of current-voltage measurements and capacitance-voltage measurements. The results show that the current transport through these contacts is not well described by conventional thermionic emission theory. Furthermore, the dopant distribution is not homogeneous in the polymer. Dopants accumulate near the polymer/metal interface. This accumulation can be enhanced by means of an electric field applied across the structure.
INTRODUCTION The benefit of using polymers as the active material in conventional electronic devices is the low cost and the processability. The processability may considerably simplify the fabrication of devices. This has recently been demonstrated through the use of the melt and solution processable poly(3-alkylthiopenes) (P3ATs) in field effect transistors and diodes [1-7]. Especially, the melt processing [6,7] is very promising in this context. However, to be able to produce high performance polymer devices, the fundamental physics of the charge transfer processes in this class of polymers in general and in these devices in particular must be understood. In this paper we report the results of a study of rectifying contacts between poly(3-hexylthiophene) (P3HT) and Indium-Tin Oxide or Aluminum.
EXPERIMENTAL The synthesis and the characterization of the P3HT has been reported elsewhere [8]. Thin films (15004000/10 of P3HT were prepared by spinning a solution (20 g/l) of P3HT in chloroform on a glass substrate coated with either Indium-Tin Oxide (ITO) or Gold. ITO/P3HT/Au structures were made by evaporating Au contacts on top of the polymer. AI/P3HT/Au structures were made by evaporating or sputtering Aluminum on top of the polymer. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
5OO
The current-voltage measurements were done with a BAS 100A electrochemical analyzer with a voltage scan rate of typically 100 mV/s. A Hewlett-Packard 4192A LF impedance analyzer was used to measure capacitance at I kHz RESULTS AND DISCUSSION Current-volta~,e measurements Fig. 1 shows the current-voltage characteristics of the ITO/P3HT and A1/P3HT junctions. As can be seen hoth junctions show rectifying behavior. The rectification ratio at +-IV is around 100, but values around 1000 have been obtained in some structures. The rectifying behavior of these contacts can be rationalized by a simple comparison of work functions. The work function of lightly p-doped P3HT can be calculated to be around 5.1-5.2 eV [5] while the work function of AI and ITO is around 4.2 eV. The combination of P3HT with either A1 or ITO should thus give a rectifying junction. Even if this comparison is rather naive it nevertheless predicts the correct behavior in this case. According to the thermionic emission theory, the current density through a Schottky barrier can be described by the following equation [9]: J = J 0 [ e (qV/nkT) - 1]
(1)
where J0 = A*T 2e(-~bb'/kT)
(2)
A* is the effective Richardson constant, ~ is the barrier height and n is the diode quality factor. By applying equation (1) to the linear part of the current-voltage characteristics of the A1/P3HT/Au diode shown in Hg. 1 the quality factor, n, can be calculated to a value of 3.9. This value deviates significantly from 1 which is the value expected for an ideal Schottky diode. This shows that the charge transfer processes of Metal/P3HT junctions are not well described by the thermionic emission model. This is also very clear in some data published by Tomozawa et.al. [4] which give the temperature dependence of the forward current voltage characteristic of In/P3HT diodes. Taking values of the saturation current at temperatures of 200 and 300 K from their data, and estimating the Richardson constant A* from these numbers, we obtain a value of A*=10-13 Acm-2K-2 which is an absurd value when compared to the free electron value A*= 120 Acm-2K -2. Caoacitance-volta~e measurements The capacitance per unit area of the depletion layer at the interface of a Schottky diode is given by C - e% Xd
(3)
where Xd is the depletion layer width. If the dopant distribution, N a is constant through the depletion region we obtain the following relation between the barrier capacitance and the reverse bias voltage: 1 _ 2(Vint -V) C 2 - q~oNa where
(4)
501 ITO/P3HT/Au
ITO/P3HT/Au
zoo;,,
165
,"
"-::.
1¢ -0.5
0 BIAS (V)
BIAS(%/)
AI/P3HT/Au
.='
~*'*N..
5OOO
.
~'.,rd,d,'
,. " *
lo"0!
AI/P3HT/Au
"o =0oo
10.1 ~ I
,,=~,~
1000
-01~
d
BtAS (%)
o's
11o
Z~
BIAS(V)
Fig. 1 Current-voltage characteristics of an ITO/P3HT/Au device (top) and an AI/P3HT/Au device (bottom). The diode area is around l x l 0 -~ cm 2. Fig. 2 Capacitance-voltage data of an ITO/P3HT/Au device (top) and an A]/P3HT/Au device (bottom). The data is plotted as C 2 vs V. The diode area is around 2x10"2 cm 2.
lff
(5)
Vint = vbi- ~
is the intercept voltage, and Vbi is the built-in voltage. A plot of 1/(22 versus applied voltage should thus give a straight line. As can be seen in the capacitance data for the AI/P3HT/Au and ITO/P3HT/Au devices (Fig. 2) them is a deviation from a straight line close to zero voltage. This indicates an
inhomogeneousdopingof the
polymer. From the capacitance data and eq. 3 and 4 the dopant distribution profile can be calculated. As can be seen in Fig. 3 there is an accumulation of dopants near the interface. This dopant distribution profile can be changed by means of electrical polarization. This is clearly shown in Fig. 3 where the dashed line shows the dopant distribution profile of the ITO/P3HT/Au device al~er being polarized at back bias (-2V) for 3,5 h. Current transnort mechanism The current-voltage measurements presented above show that the charge transport through the barrier is not well described by conventional thermionic emission theory. One probable cause of this behavior may be that tunneling through the barrier makes a major contribution to the charge transport. The accumulation of dopants near the interface decreases the thickness of the barrier and thus increases the probability for tunneling. However, the tunneling process must be more complex in these systems than in conventional
502
ITO/P3HT/Au
10
E
£
=~o
4~o
6~o
s~o
x (AJ
Fig. 3 Doping profile of an ITO/F3HT/Au device before (solid line) and afi~r (dashedline) it has been polarized at back bias (-2V) for 3.5 h. semiconductors since the charge carriers in conducting polymers consists of polarons and bipolarons and not electrons and holes. An injection of holes into the polymer must therefore be followed by an immediate relaxation to localized polarons or bipolarons, the energy of which is different from the valence band energy. Furthermore, the transfer of charge from the polymer to the metal may occur by direct tunneling from a polaron or bipolaron state. CONCLUSIONS The charge transfer process in metal/P3HT junctions cannot be described by ordinary thermionic emission theory. Furthermore, the doping is not homogeneous throughout the polymer film; the dopants are accumulated near the rectifying interface. This doping profile can be altered by the applied voltage. ACKNOWLEDGEMENTS We are grateful to Dr J. E. 0sterhohn and Dr. J. Laakso at Neste Oy for preparing the P3HT. This work was supported by Neste Oy and Nordic Fund for Industrial and Technological Development. REFERENCES I. 2.
A.Assadi, C. Svensson, M. WiUander and O. Ingan/ts, Appl. Phys. Lea.. 53 (1988) 195. J. Paloheimo, E. Punkka, H. Stubb and P. Kuivalainen, to be published in R.M. Metzger (Ed.), Proc. of NATO ASI "Lower Dimensional Systems and Molecular Electronics". Snetses. Greece.1223 June 1989. Plenum Press.
3.
H. Tomozawa, D. Braun, S. Phillips, A.J. Heeger and H. Kroemer, Svnth. Met. 22 (1987) 63.
4.
H. Tomozawa, D. Braun, S. Phillips, R. Worland A.J. Heeger and K. Kroemer, Synth. Met. 28,
5.
G. Gustafsson, M. Sundberg, O. lngan/ts and C. Svensson, L Mol. Electron..6 (1990) 105.
6.
M. Sundberg, G. Gustafsson and O. Inganas, to be published in Ap.pJ,_P_IIy.~s~.,~L
7.
O. Inganas, G. Gustafsson and C. Svensson, these procedings.
8.
J.-E. Osterholm, J. Laakso, P. Nyholm, H. Isotalo, H. Stubb, O. lngangs and W. R. Salaneck,
9.
Svnth. Met.. 28 (1989) C435. S.M. Sze, Physics of Semiconductor Devices. Wiley-Interscience, New York, 2rid edn. 1981.
C687 (1989).
499
RECTIFYING METAL/POLY(3-HEXYLTHIOPHENE) CONTACTS
G. GUSTAFSSON, O. INGAN.~S, M. SUNDBERG AND C. SVENSSON Department of Physics (IFM), Link6ping University, S- 581 83 Link6ping, Sweden
ABSTRACT Rectifying contacts between poly(3-hexylthiophene) and Indium-Tin oxide or Aluminum have been studied by means of current-voltage measurements and capacitance-voltage measurements. The results show that the current transport through these contacts is not well described by conventional thermionic emission theory. Furthermore, the dopant distribution is not homogeneous in the polymer. Dopants accumulate near the polymer/metal interface. This accumulation can be enhanced by means of an electric field applied across the structure.
INTRODUCTION The benefit of using polymers as the active material in conventional electronic devices is the low cost and the processability. The processability may considerably simplify the fabrication of devices. This has recently been demonstrated through the use of the melt and solution processable poly(3-alkylthiopenes) (P3ATs) in field effect transistors and diodes [1-7]. Especially, the melt processing [6,7] is very promising in this context. However, to be able to produce high performance polymer devices, the fundamental physics of the charge transfer processes in this class of polymers in general and in these devices in particular must be understood. In this paper we report the results of a study of rectifying contacts between poly(3-hexylthiophene) (P3HT) and Indium-Tin Oxide or Aluminum.
EXPERIMENTAL The synthesis and the characterization of the P3HT has been reported elsewhere [8]. Thin films (15004000/10 of P3HT were prepared by spinning a solution (20 g/l) of P3HT in chloroform on a glass substrate coated with either Indium-Tin Oxide (ITO) or Gold. ITO/P3HT/Au structures were made by evaporating Au contacts on top of the polymer. AI/P3HT/Au structures were made by evaporating or sputtering Aluminum on top of the polymer. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
5OO
The current-voltage measurements were done with a BAS 100A electrochemical analyzer with a voltage scan rate of typically 100 mV/s. A Hewlett-Packard 4192A LF impedance analyzer was used to measure capacitance at I kHz RESULTS AND DISCUSSION Current-volta~,e measurements Fig. 1 shows the current-voltage characteristics of the ITO/P3HT and A1/P3HT junctions. As can be seen hoth junctions show rectifying behavior. The rectification ratio at +-IV is around 100, but values around 1000 have been obtained in some structures. The rectifying behavior of these contacts can be rationalized by a simple comparison of work functions. The work function of lightly p-doped P3HT can be calculated to be around 5.1-5.2 eV [5] while the work function of AI and ITO is around 4.2 eV. The combination of P3HT with either A1 or ITO should thus give a rectifying junction. Even if this comparison is rather naive it nevertheless predicts the correct behavior in this case. According to the thermionic emission theory, the current density through a Schottky barrier can be described by the following equation [9]: J = J 0 [ e (qV/nkT) - 1]
(1)
where J0 = A*T 2e(-~bb'/kT)
(2)
A* is the effective Richardson constant, ~ is the barrier height and n is the diode quality factor. By applying equation (1) to the linear part of the current-voltage characteristics of the A1/P3HT/Au diode shown in Hg. 1 the quality factor, n, can be calculated to a value of 3.9. This value deviates significantly from 1 which is the value expected for an ideal Schottky diode. This shows that the charge transfer processes of Metal/P3HT junctions are not well described by the thermionic emission model. This is also very clear in some data published by Tomozawa et.al. [4] which give the temperature dependence of the forward current voltage characteristic of In/P3HT diodes. Taking values of the saturation current at temperatures of 200 and 300 K from their data, and estimating the Richardson constant A* from these numbers, we obtain a value of A*=10-13 Acm-2K-2 which is an absurd value when compared to the free electron value A*= 120 Acm-2K -2. Caoacitance-volta~e measurements The capacitance per unit area of the depletion layer at the interface of a Schottky diode is given by C - e% Xd
(3)
where Xd is the depletion layer width. If the dopant distribution, N a is constant through the depletion region we obtain the following relation between the barrier capacitance and the reverse bias voltage: 1 _ 2(Vint -V) C 2 - q~oNa where
(4)
501 ITO/P3HT/Au
ITO/P3HT/Au
zoo;,,
165
,"
"-::.
1¢ -0.5
0 BIAS (V)
BIAS(%/)
AI/P3HT/Au
.='
~*'*N..
5OOO
.
~'.,rd,d,'
,. " *
lo"0!
AI/P3HT/Au
"o =0oo
10.1 ~ I
,,=~,~
1000
-01~
d
BtAS (%)
o's
11o
Z~
BIAS(V)
Fig. 1 Current-voltage characteristics of an ITO/P3HT/Au device (top) and an AI/P3HT/Au device (bottom). The diode area is around l x l 0 -~ cm 2. Fig. 2 Capacitance-voltage data of an ITO/P3HT/Au device (top) and an A]/P3HT/Au device (bottom). The data is plotted as C 2 vs V. The diode area is around 2x10"2 cm 2.
lff
(5)
Vint = vbi- ~
is the intercept voltage, and Vbi is the built-in voltage. A plot of 1/(22 versus applied voltage should thus give a straight line. As can be seen in the capacitance data for the AI/P3HT/Au and ITO/P3HT/Au devices (Fig. 2) them is a deviation from a straight line close to zero voltage. This indicates an
inhomogeneousdopingof the
polymer. From the capacitance data and eq. 3 and 4 the dopant distribution profile can be calculated. As can be seen in Fig. 3 there is an accumulation of dopants near the interface. This dopant distribution profile can be changed by means of electrical polarization. This is clearly shown in Fig. 3 where the dashed line shows the dopant distribution profile of the ITO/P3HT/Au device al~er being polarized at back bias (-2V) for 3,5 h. Current transnort mechanism The current-voltage measurements presented above show that the charge transport through the barrier is not well described by conventional thermionic emission theory. One probable cause of this behavior may be that tunneling through the barrier makes a major contribution to the charge transport. The accumulation of dopants near the interface decreases the thickness of the barrier and thus increases the probability for tunneling. However, the tunneling process must be more complex in these systems than in conventional
502
ITO/P3HT/Au
10
E
£
=~o
4~o
6~o
s~o
x (AJ
Fig. 3 Doping profile of an ITO/F3HT/Au device before (solid line) and afi~r (dashedline) it has been polarized at back bias (-2V) for 3.5 h. semiconductors since the charge carriers in conducting polymers consists of polarons and bipolarons and not electrons and holes. An injection of holes into the polymer must therefore be followed by an immediate relaxation to localized polarons or bipolarons, the energy of which is different from the valence band energy. Furthermore, the transfer of charge from the polymer to the metal may occur by direct tunneling from a polaron or bipolaron state. CONCLUSIONS The charge transfer process in metal/P3HT junctions cannot be described by ordinary thermionic emission theory. Furthermore, the doping is not homogeneous throughout the polymer film; the dopants are accumulated near the rectifying interface. This doping profile can be altered by the applied voltage. ACKNOWLEDGEMENTS We are grateful to Dr J. E. 0sterhohn and Dr. J. Laakso at Neste Oy for preparing the P3HT. This work was supported by Neste Oy and Nordic Fund for Industrial and Technological Development. REFERENCES I. 2.
A.Assadi, C. Svensson, M. WiUander and O. Ingan/ts, Appl. Phys. Lea.. 53 (1988) 195. J. Paloheimo, E. Punkka, H. Stubb and P. Kuivalainen, to be published in R.M. Metzger (Ed.), Proc. of NATO ASI "Lower Dimensional Systems and Molecular Electronics". Snetses. Greece.1223 June 1989. Plenum Press.
3.
H. Tomozawa, D. Braun, S. Phillips, A.J. Heeger and H. Kroemer, Svnth. Met. 22 (1987) 63.
4.
H. Tomozawa, D. Braun, S. Phillips, R. Worland A.J. Heeger and K. Kroemer, Synth. Met. 28,
5.
G. Gustafsson, M. Sundberg, O. lngan/ts and C. Svensson, L Mol. Electron..6 (1990) 105.
6.
M. Sundberg, G. Gustafsson and O. Inganas, to be published in Ap.pJ,_P_IIy.~s~.,~L
7.
O. Inganas, G. Gustafsson and C. Svensson, these procedings.
8.
J.-E. Osterholm, J. Laakso, P. Nyholm, H. Isotalo, H. Stubb, O. lngangs and W. R. Salaneck,
9.
Svnth. Met.. 28 (1989) C435. S.M. Sze, Physics of Semiconductor Devices. Wiley-Interscience, New York, 2rid edn. 1981.
C687 (1989).