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J. Phys. Chem. C 2007, 111, 1840-1846
Electronic and Thermoelectric Properties of Polyaniline Organic Semiconductor and Electrical Characterization of Al/PANI MIS Diode F. Yakuphanoglu*,† and B. F. S¸ enkal‡ Faculty of Arts and Sciences, Department of Physics, Fırat UniVersity, 23119 Elazig, Turkey, and Faculty of Arts and Sciences, Department of Chemistry, I˙ stanbul Technical UniVersity, Maslak, I˙ stanbul, Turkey ReceiVed: August 16, 2006; In Final Form: NoVember 10, 2006
The electrical conductivity, thermoelectric power, and metal-insulator-semiconductor diode properties of polyaniline prepared in ionic liquid (PANI) have been investigated. The electrical conductivity of the polyaniline increases exponentially with increasing temperature. The electrical conductivity value at 28 °C is 0.21 S/cm. The Seebeck coefficient of the PANI decreases with increasing temperature. The electrical conductivity and thermoelectric power results suggest that the PANI is a p-type semiconductor polymer. The Al/PANI Schottky diode was fabricated and is a metal-insulator-semiconductor type device. The ideality factor n and barrier height φb values of the diode at 298 K were found to be 2.78 and 0.85 eV, respectively. The barrier inhomogeneities are a very important explanation of the higher values of the ideality factor. The Gaussian distribution function was suggested for describing barrier height inhomogeneities. The standard deviation of the barrier height distribution σo indicates the presence of the interface inhomogeneities. The φB value obtained from C-V measurement is higher than that of the φB value obtained I-V measurements.
1. Introduction Polyaniline is one of the most promising conducting materials for applications in optoelectronics and microelectronics devices. The electrical conduction in macromolecular systems is an interesting research field, from both theoretical and experimental points of view. The conjugated polymers exhibit conducting or semiconducting properties. Semiconducting polymers are now attracting considerable attention as promising materials for the development of optoelectronic devices such as light emitting diodes, photovoltaic cells, and nonlinear optical systems.1 To study the charge transport mechanism occurring in organic semiconductors, analysis of both the thermoelectric power and temperature-dependence on direct conductivity is required. The thermoelectric power performance will be one of such interesting properties specific for electroconductive polymers. The thermoelectric material is one of the promising materials, effective for saving energy and for developing advanced materials.2 The fabrication and characterization of the Schottky diode barrier by using organic semiconductors and their derivatives have been carried out in recent years.3,4 Polyaniline (PANI) is a p-type semiconductor.5 The fabrication of polyaniline-based microelectronic devices such as diodes and transistors has been reported. Semiconducting polymers have been used in the fabrication of microelectronic devices including field effect transistor (FETs), Schottky diodes, light emitting diodes (LEDs), etc., due to their unique electrical, optical, and magnetic properties.6 Metal-polyaniline Schottky junctions, where polyaniline was doped with various dopants such as hydrochloric acid, formic acid, iodine, and methylene blue, were studied.7 The metal/organic semiconductor Schottky junction as an alternate to the metal/inorganic semiconductor junction has been developed, which has opened the new possibility of replacing conventional inorganic devices by organic ones.8,9 Among † ‡
Fırat University. I˙ stanbul Technical University.
conducting polymers, PANI has received greater attention due to its advantages over other conducting polymers. The simplicity of its preparation from cheap materials, superior stability to air oxidation, controllable electrical conductivity by doping, and reversible electrochromism10 make it very useful in preparing lightweight batteries,11electrochromic devices,12 sensors,13 and electroluminescent devices.14 Despite the great potential use of PANI its processing has remained a big problem due to its insolubility in common organic solvents. About 1% of the solubility is observed in N-methyl-2-pyrrolidone (NMP), which also acts as a plasticizer.15 Incorporation of alkyl substituents increases solubility; however, electrical conductivity is reduced.16 Preparation conditions, such as electrolytes and solvents, can affect many properties of PANI. To improve the electrical properties of PANI, we have focused on the study of the polymerization of aniline in ionic liquid. Ionic liquids (ILs) are organic salts with a low melting point ( 3kT/q
(4)
where n is the ideality factor, q is the electronic charge, V is the applied voltage, T is the temperature, and Io is the saturation current given by
( )
Io ) AA*T2 exp -
φb kT
(5)
and
φb ) kT ln
( ) AA*T2 Io
(6)
where A is the contact area, A* is the Richardson constant, and φb is the barrier height. The n ideality factor is obtained by the following relationship,
n)
q dV kT d(ln I)
(7)
The n values were obtained from the slope of forward I-V curves at different temperatures, as shown in Figure 4. The
Yakuphanoglu and S¸ enkal
1844 J. Phys. Chem. C, Vol. 111, No. 4, 2007
suggested for describing barrier height inhomogeneities, such as Gaussian and log-normal.53-57 The Gaussian function is expressed as
[(
)]
(φb - φ h b)2 1 exp 2σ2 σx2π
P(φb) )
(8)
where 1/σx2π corresponds to the normalization constant. The total current through the Schottky contact is given by
∫-∞+∞ I(φb,V)P(φb) dφb
I)
(9)
and after eq 9 is integrated, the total current is determined as
[ (
I(V) ) AdA*T2 exp Figure 4. Variation of the ideality factor with temperature of the Al/ PANI Schottky diode.
σ2q -q φ hb kT 2kT
)] ( (
)[ )]
q(V - IRs) 1kT -q(V - IRs) (10) exp kT exp
The Gaussian distribution is affected by the bias and distribution parameters are defined φ hb ) φ h bo + γV and σ ) σo + ξV , where φ h b and σo are the zero-bias mean barrier height and standard deviation, respectively. If eq 10 is rearranged, one obtains
I ) AdA*T2 exp
Figure 5. The plots of φap vs 1/2kT and 1/nap - 1 vs 1/2kT of the Al/ PANI Schottky diode.
obtained ideality factor is higher than unity. The greater than unity ideality factor shows deviation of Schottky diode characteristics from ideal behavior.47 The Al/PANI Schottky diode obeys a MIS configuration rather than an ideal Schottky diode.48 This indicates the existence of an insulating layer between metal and semiconductor, because the surface of Al is exposed to air before the coating with PANI48 and, furthermore, the interface oxide layer can also be formed by water or vapor adsorbed onto the surface of the Al before deposition of the PANI solution on the surface of the Al metal. The obtained ideality factor is higher than that of the ideality factors (1.2 and 1.9) for aluminum contacts to poly(3-octylthiophene) and poly(3-methylthiophene),49 whereas the ideality factor value is lower that that of the ideality factors (2.57-4.48) for rectifying contact between aluminum and electrodeposited poly(3-methylthiophene)50 and Schottky diode with chemically prepared copolymer having hexylthiophene and cyclohexylthiophene units (n ) 4.60, 12.3, and 9.13)51 and indium/polyaniline Schottky devices (n ) 4.41, 5.50, and 9.65).52 The barrier height of the diode was calculated by using Io values obtained from linear portions of Figure 3 at low voltages, which obeys eq 4. The obtained barrier height values increase with temperature, as shown in Figure 5. The barrier inhomogeneities are a very important explanation of the higher values of the ideality factor because the performance and reliability of Schottky diodes especially depend on the formation of the insulator layer between metal and semiconductor interface, inhomogeneities of Schottky barrier contacts and series resistance of diodes. Various types of distribution functions are
( ) (
)[
-qφap q(V - IRs) exp 1kT kTnap -q(V - IRs) exp kT
(
)]
(11)
where Rs is the series resistance and φap and nap are the apparent barrier height and the ideality factor, respectively. The Gaussian distribution of the apparent barrier height and variation of the ideality factor with temperature are expressed by the following relationships58-60
h bo φap ) φ
qσo2 2kT
(12)
and
(
)
1 qξ -1 )γnap 2kT
(13)
where φap is the apparent barrier height, σo is the standard deviation of the barrier height distribution, nap is the apparent ideality factor, and γ and ξ are the voltage deformation of the barrier height distributions. The plots of φap vs 1/2kT and 1/nap - 1 vs 1/2kT are shown in Figure 5. The σo and φ h bo values were determined from the slope and intercept of φap vs 1/2kT and were found to be 0.22 and 1.77 eV. The σo value of 0.22 indicates the presence of the interface inhomogeneities. The values of γ and ξ were determined from the plot of 1/nap - 1 vs 1/2kT and are found to be 0.83 and 0.078 V, respectively. The linearity in the plot of 1/nap - 1 vs 1/2kT indicates the existence of the voltage deformation.1 The modified Richardson plot is also expressed by the following relationship
ln
() ( ) Io
T
2
-
q2σo2
2 2
2k T
)-
φ h bo + ln(A*A) kT
(14)
The plot of ln(Io/T2) - q2σo2/2K2T2 vs 1000/T is shown in Figure 6. The φ h bo value was determined from Figure 6 and was found to be 1.64 eV. This value is almost in agreement with the value obtained from the φap vs 1000/T plot.
Polyaniline Organic Semiconductor
J. Phys. Chem. C, Vol. 111, No. 4, 2007 1845 cm-3 and 0.65 V, respectively. The potential difference between the Fermi level and the top of the valence band of the polyaniline Vp value was calculated from the following relationship45,61
Vp )
()
kT Nv ln q Na
(17)
with
Nv ) (4.82 × 1015)T3/2
Figure 6. Plot of ln(Io/T2) - q2σo2/2K2T2 vs 1000/T of the Al/PANI Schottky diode.
Figure 7. Plot of C-2 vs V of the Al/PANI Schottky diode.
3.3. The Capacitance-Voltage Characteristics and the Interface States Properties of the Al/PANI Schottky Diode. The capacitance-voltage (C-V) curve of the Al/PANI Schottky diode at 1 MHz is shown in Figure 7. Reverse bias capacitance measurement of the diode was performed at 1 MHz so that the interface states cannot respond to the alternating current (AC) signal. The diode parameters such as barrier height and diffusion potential can be obtained from the C-V curve. The diode depletion region capacitance is expressed as45
2(Vd + V) 1 ) 2 2 C A osqNa
(15)
where Vd is the diffusion potential at zero bias, s is the dielectric constant of the semiconductor, and Na is the doping concentration in the semiconductor. The barrier height can be obtained from the following relationship
φB ) V d + V p
(16)
Plot of C-2 vs V is shown in Figure 7. The deviation of the C-V data from that expected for the ideal Schottky diode may be due to the interfacial layer, effective contact area variation, and traps in the depletion layer.46 The linear region of the C-2 vs V plot indicates formation of the Schottky diode between the semiconductor and the metal. This linear region can be used to determine doping concentration, Na, and diffusion potential, Vd, values. The Na and Vd values were determined from the slope and the intercept of Figure 7 and were found to be 1.2 × 1014
( ) mh* mo
3/2
(18)
where Nv is the effective density of states in the polyaniline valence band, mh* is the effective mass of holes, and mo is the remaining mass of the electron. The Nv value was calculated with the use of eq 18. The φB value was calculated by using the obtained Vp (0.31 V) value and was found to be 0.96 eV. The discrepancy between φB(C-V) and φB(I-V) can be explained by the existence of excess capacitance and Schottky barrier height inhomogenity. The direct current across the interface is exponentially dependent on barrier height and the current is sensitive to barrier distribution at the interface, whereas the capacitance is insensitive to potential fluctuations on a length scale of less than the space charge width that the capacitancevoltage method averages over the whole area. Consequently, the φB value obtained from C-V measurement is higher than that of the φB value obtained I-V measurements. The determination of Schottky barrier height from I-V characteristics is more reliable than that from C-V characteristics, if one can be confident that the current is determined by thermionic emission (TE) theory.46 According to TE, the forward current-voltage characteristic gives a good straight line and in turn one obtains a reliable value of barrier height. We can evaluate that the value of the barrier height obtained from C-V measurement for the diode studied is more reliable. 4. Conclusions The electrical conductivity, thermoelectric power, and Schottky diode properties of the polyaniline prepared in ionic liquid have been investigated. The electrical conductivity and thermoelectric power results suggest that the PANI is a p-type semiconductor polymer. The Al/PANI Schottky diode was fabricated and is a metal-insulator-semiconductor type device. The Gaussian distribution function was suggested for describing barrier height inhomogeneities. The standard deviation of the barrier height distribution σo indicates the presence of the interface inhomogeneities. The φB value obtained from C-V measurement is higher than that of the φB value obtained I-V measurements. Acknowledgment. This work was supported by the Turkish Scientific and Technological Research Council of Turkey (Tubitak) (Project No. 105T137). The authors wish to thank The Scientific and Technological Research Council of Turkey. References and Notes (1) Yakuphanoglu, F.; Basaran, E.; S¸ enkal, B. F.; Sezer, E. J. Phys. Chem. B 2006, 110, 16908. (2) Toshima, N. Macromol. Symp. 2002, 186, 81. (3) Willander, M.; Assadi, A.; Svensson, C. Synth. Met. 1993, 55-57, 4099. (4) Lei, J.; Liang, W.; Brumlik, C. J.; Martin, C. R. Synth. Met. 1992, 47, 351. (5) Misra, S. C. K.; Chandra, S. Indian J. Chem. 1994, 33A, 583. (6) Gutman, F.; Lyons, L. E. Organic Semiconductors; Wiley: New York, 1967.
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