ISSN 0965�545X, Polymer Science, Ser. A, 2010, Vol. 52, No. 1, pp. 55–59. © Pleiades Publishing, Ltd., 2010.

55

1 INTRODUCTION

Polypyrrole (PPy) has drawn a lot of interest due toits thermal and environmental stability, high conduc�tivity and ease of synthesis [1–4]. Poor solubility ofPPy in common solvents, however, limited its practicalapplications in many areas [5, 6]. The synthesis of sol�uble PPy is reported to carry out by polymerizing pyr�role in the presence of dodecylbenzene sulfonic acid(DBSA) by Kwan, Campos and Aleshin [7–9]. Thesoluble PPy can be applied in the fabrication of Schot�tky diodes and in OFETs in the form of thin films thatcan be easily obtained by spin coating [8]. The effi�ciency of such devices depends upon various factorssuch as preparation parameters, stability of polymersand the metal used. When polymer semiconductorsare brought in contact with metal, readjustment ofcharge takes place to establish thermal equilibrium,and a potential barrier occurs in the interfacial spacecharge region. Fabrication of devices with polymers asan active part made it possible to investigate polymersas semiconductor materials and not only as insulatorsor conductors.

To study Schottky diode characteristics one shouldrealize that three kinds of contacts are possible at thepolymer�metal interface: an ohmic contact, a rectify�ing contact and a blocking contact. The ohmic contactpermits free flow of charges from polymer to metal andvise versa, a rectifying contact allows only unidirec�tional flow of charges, and a blocking contact does notallow any injection or extraction of charges from the

1 The article is published in the original.

polymer. It is well known that metals with low workfunctions such as aluminum (4.1 eV) have been used tobuild rectifying Schottky barriers, whereas ohmic con�tacts are formed with high work function metals suchas gold or indium tin oxide (ITO) (5.0 eV). It is worthmention here that metal deposition on polymer is themost critical and important step in a Schottky diodefabrication process. It is important to maintain highvacuum (P < 5 × 10–5 torr) environment in the deposi�tion chamber to prevent oxidation of Al, which isknown to have an adverse effect on its junction prop�erties. Other parameters, e.g. metal deposition ratealso need to be controlled for superior device proper�ties. Typically, 1 Å/s deposition rate at P = 5 × 10–5 torris recommended to obtain high quality of Al deposi�tion [8].

Hydroquinone has been found to be a good compat�ibility agent for PPy–DBSA system, which not onlyincreased the miscibility of PPy and DBSA but alsoenhanced solubility and conductivity of PPy–DBSA.It has also been reported that the flexibility of the filmscan be improved by adding hydroquinone [10, 11].Although junction properties of PPy–DBSA/metalstructures have been studied by a large number ofresearchers [7, 8, 10], effect of hydroquinone on thejunction properties has not been studied yet.

We present here the results of the study of electricalbehavior of doped PPy films in the presence and in theabsence of hydroquinone sandwiched between an ITOsubstrate, which also acts as an electrode on one side,and vacuum deposited aluminum (Al) electrode onthe other side.

Effect of Hydroquinone on the Electrical Propertiesof Dodecylbenzene Sulfonic Acid Doped Polypyrrole/Aluminum

Schottky Junction1

Abdul Shakoor and Tasneem Zahra RizviDepartment of Physics, Quaid�i�Azam University, Islamabad Pakistan P.O.

e�mail: shakoor_47@yahoo.comReceived February 6, 2009;

Revised Manuscript Received March 4, 2009

Abstract—Electrical properties of Schottky barrier diode fabricated using Aluminum for Schottky contactand indium tin oxide for ohmic contact and containing polypyrrole doped with dodecylbenzene sulfonic acidin the presence and in the absence of a plasticizing agent hydroquinone were studied. Various parameters, e.g.saturation current, ideality factor, built in voltage; carrier concentration and barrier height have been calcu�lated and found to be affected by the presence of hydroquinone in the doped polymer. The electrical behaviorof the systems was found to be in a good agreement with the thermionic emission model for the Schottky bar�rier devices. The interaction of the doped polypyrrole with hydroquinone was explained in terms of changein the barrier height and in the carrier concentration of the diodes.

DOI: 10.1134/S0965545X10010086

COMPOSITES

56

POLYMER SCIENCE Series A Vol. 52 No. 1 2010

ABDUL SHAKOOR, TASNEEM ZAHRA RIZVI

EXPERIMENTAL

Pyrrole was obtained from “Aldrich” and vacuumdistilled before use. Ammonium persulfate (APS) andDBSA were obtained from “Fluka” and were used asobtained.

0.15 mol of DBSA was dispersed in 100 ml distilledwater, 0.03 mol of pyrrole was added and the solutionwas kept on magnetic stirring for 3 h. Then 0.15 mol ofthe oxidant APS dissolved in 200 ml distilled water wasthen added drop wise under vigorous stirring and thereactants were allowed to react for 24 h. Afterwards 1 lof methanol was added to the solution and it was keptat room temperature for 2 days. The suspension wasthen filtered and washed. A black paste of doped PPywas obtained which was dried under vacuum at 70°Cfor 24 h. 5g of the product was then dissolved in 40 mlof chloroform and 0.05 g hydroquinone was added.The solution was ultrasonically agitated for 3 h then itwas filtered and spin coated on glass plate with depos�ited ITO. Free standing doped PPy films for spectro�scopic and other measurements were obtained bypeeling off the deposited polymer layer from the sub�strate. These films along with the films grown on ITOglass were vacuumed dried at 70°C for 24 h. The filmswithout hydroquinone were also prepared for compar�ison. IR�spectra of the doped PPy film were recordedon a Perkin�Elmer FT�IR spectrometer PARAGON100 in KBr disc. Aluminum circular electrode of0.2 cm2 was vacuum (10–5 Torr) deposited on one side,while ITO was used as an electrode on the other side.

The current density–voltage (J–V) characteristicswere recorded by means of Keithley 117 electrometersand a current source electrometer. Capacitance–volt�age (C–V) measurements were performed using Sola�tran frequency response analyzer. All measuringinstruments were connected to a PC for data collec�tion and analysis. Schematic diagram of the Schottkydiode is shown in Fig. 1.

RESULTS AND DISCUSSION

The FT�IR spectrum of the doped PPy is shown onFig. 2. Bands at 3410 and 1200 cm–1 are characteristicbands of PPy attributed to N–H stretching and in theplane N–H bending mode respectively; the band at1090 cm–1 corresponds to symmetric C–H stretchingin the plane mode. The stretching modes of PPyappeared in the region 1600–1400 cm–1. Absorptionobserved at 1550 cm–1 corresponds to C–C stretchingmode of PPy chains [12]. Characteristics bands at1560 cm–1 and 1480 cm–1 are stretching vibrations ofC=C and C–N respectively.

The band at 1394 cm–1 is assigned to pyrrole ringbreathing with contributions from C=C/C–C andC⎯N, whereas bands at 110 cm–1 and 933 cm–1 areassigned to C–H in the plane and out of plane bendingrespectively from the PPy polymer main chains [13, 16].

Both films Al/doped PPy–hydroquinone/ITO andAl/doped PPy/ITO were forward biased with a posi�tive bias voltage applied to Al electrode. A non linear

Contactsto external circuit

Al contactPolypyrrole

Glass ITO

Fig. 1. Schematic diagram of Schottky diode.

Abs

orb

ance

, a.

u.

2500 15003500 500Wavenumber, cm−1

3410

1550

1560

1394

1200

1090

1110937

1480

Fig. 2. FT�IR spectra of PPy/DBSA.

POLYMER SCIENCE Series A Vol. 52 No. 1 2010

EFFECT OF HYDROQUINONE ON THE ELECTRICAL PROPERTIES 57

asymmetric relationship was observed between theelectrical current density and the bias voltage as shownin Fig. 3. Such a non linear (J–V) characteristic curveis generally attributed to mechanism of thermionicemission in a Schottky diode. It may, however, benoted that apart from Schottky emission, many othermechanisms such as tunneling, diffusion and spacecharge limited current (SCLC) give rise to non linear(J–V) characteristics. A detailed analysis of (J–V)variation in terms of existing models can help us tofind out the relevant mechanism for data obtained.

Possibility of SCLC mechanism caused by traps atthe interfacial region between metal and polymer con�tact, require a linear plot of current density Vs V2 as forthis mechanism the current density is known to followEq. (1)

(1)

where A is constant of proportionality. As the plot ofcurrent density versus V2 was not a straight line (Fig. 4)as required by Eq. (1), so we assume that SCLC is nota dominant mechanism for charge transfer. Anotherpossible source of non linear J–V characteristic,namely, the bulk limited Poole�Frenkel emission [15]may also be ruled out, as this mechanism gives astraight line plot of ln(J/V) versus V1/2, which was alsonot observed in our case. In our case the slope calcu�lated from plot of our experimental J–V curves wereasymmetrical with the exponential dependence ofelectrical current on bias voltage in a potential range of0.2 V to 1.5 V for forward applied voltage. It is there�fore possible to assume a carrier transport by thermi�onic emission.

J AV2

=

According to thermionic emission model, currentvoltage relationship for the Schottky barrier devices isgiven by Eq. (2)

(2)

where J0 is the saturation current density, e is the elec�tronic charge, n is ideality factor, k is Boltzman con�stant and T is absolute temperature. From J0 the bar�rier height can also be calculated by Richardson equa�tion 3 [16].

(3)

where A* is effective Richardson constant, whichin considered case is taken as 120 A/cm2 K2 and corre�sponds to Richardson constant for free electron; this

J J0 eV/nkT( )exp=

J0 A*T2

eΘb/kT–( )exp=

0.01

−1.0

Current density J, A/cm2

0.5 1.5

0.05

−0.5 0−0.01

0

0.02

0.03

0.04

0.06

a

b

1.0 2.0V, volts

Fig. 3. Current density as a function of Voltage of (a) a doped PPy and (b) a doped PPy in the presence of hydroquinone.

0

Current density, A/cm2

2 3

0.02

0 1V 2, volts

0.04

0.06ab

Fig. 4. Plot of current density vs V2 of (a) a doped PPy and(b) a doped PPy in the presence of hydroquinone.

58

POLYMER SCIENCE Series A Vol. 52 No. 1 2010

ABDUL SHAKOOR, TASNEEM ZAHRA RIZVI

constant is usually assumed for Schottky diode withp�type organic semiconductor in the calculation ofbarrier height Θb [17]. If we take logarithm of Eq. (2)we get linear relation between log J and V with slopeequal to e/nkT and y�intercept equal to log J0. The plotof logJ versus bias voltage V for our data (Fig. 5) is lin�ear below 0.7 Volts and shows saturation above thisvoltage. Θb was calculated from the value of J0 usingEq. (3) at T = 300 K. The parameters J0, n and Θb forAl/PPy–DBSA/ITO and Al/PPy–DBSA hydro�quinone/ITO structures as calculated using aboveequations at room temperature are given in table. Itmay be noted that the saturation current density J0

increases while the ideality the barrier height Θb

decrease with the addition of hydroquinone in dopedPPy. This shows that the diode characteristics of PPy–DBSA/Al junction are improved with the addition ofhydroquinone in PPy–DBSA.

Figure 6 shows the 1/C2 (C is capacitance) versusvoltage graph of both the samples at room tempera�ture. The linear voltage dependence of 1/C2 in a widerange of reverse bias voltage indicates the formation ofa Schottky junction [15]. The built in voltage (Vi)obtained from the intercept of linear curve with thevoltage axis was 1.5 V for PPy–DBSA and 1.1 V forPPy–DBSA/hydroquinone. It is observed that 1/C2 VsV plots is linear from –2.1 to –0.1 V; beyond this volt�age the graphs are no more linear, which is not shownhere. This nonlinearity may be due to heterogeneity of

acceptor depth profile in bulk layer and sets of traps atdifferent energies in energy gap of PPy⎯DBSA andPPy⎯DBSA/hydroquinone. Width of the depletionregion W can be calculated by using Eq. (4) [18]

(3)

N is density of states, q is electronic charge and εs isrelative permittivity of semiconducting materials. Ncan be calculated by using Eq. (6), which is modifiedform of Eq. (5). By using the slope of 1/C2 versus volt�age bias in Eq. (6) N can be calculated [15].

, (4)

. (5)

The values of N and W were found to be 2.01 ×1021 cm–3 and 2.94 × 10–7 cm for PPy⎯DBSA and 2.7 ×1021 cm–3 and 2.01 × 10–7 cm for PPy⎯DBSA/hydro�quinone. These densities of states are in agreementwith the acceptor densities for highly doped polymerswhich are estimated to be of the order of 1021 cm–3

[19]. It may be noted that the density of states Nincreases while the width of the depletion region Wdecreases when hydroquinone is added. This furtherconfirms that the diode characteristics of dopedPPy/Al junction are improved by the presence of hyd�

W2εs Vi V+( )

qN����������������������

1/2

=

C2–

2 Vi V–( )/qN=

N 2qεs

������⎝ ⎠⎛ ⎞ dV

dC2–

���������=

−3.5

logJ (J, A/cm2)

0.6 0.8

−3.0

0 0.2V, volts

−2.0

−1.5ab

−4.0

−2.5

−1.0

0.4 1.0

Fig. 5. Plot of log(J) vs voltages (V) of (a) a doped PPy and(b) a doped PPy in the presence of hydroquinone.

0.20

C−2, ×1012 F−2 cm4

−0.9 0

0.25

−1.5 −1.2V, volts

0.35

0.40

ab

−0.6 −0.30.15

0.30

0.45

Fig. 6. Plot of C–2 versus verse biased voltage of (a) adoped PPy and (b) a doped PPy in the presence of hydro�quinone.

Saturation current, density of states, ideality factor, barrier height, barrier width and built in voltage

Sample J0, A/cm2 N, cm–3 n Θb, eV W, cm Vi, V

PPy/DBSA 2.01 × 10–4 2.01 × 1021 2.99 0.61 2.94 × 10–7 1.50

PPy/DBSA/hydroquinone 3.02 × 10–3 2.72 × 1021 3.47 0.55 2.01 × 10–7 1.17

POLYMER SCIENCE Series A Vol. 52 No. 1 2010

EFFECT OF HYDROQUINONE ON THE ELECTRICAL PROPERTIES 59

roquinone. A similar approach was adopted by Com�pos et al. [8], who found a strong influence of methanegas on the junction parameters of Au/PPy⎯DBSA/AlSchottky diode.

CONCLUSIONS

Schottky barrier diode of Polypyrrole doped withdodecylbenzene sulfonic acid in the presence andabsence of plasticizing agent hydroquinone was fabri�cated using Aluminum for Schottky contact and ITOfor ohmic contact. Various parameters, e.g. saturationcurrent, ideality factor, built in voltage, carrier con�centration and barrier height have been calculated andfound to be affected by the presence of hydroquinonein the doped polymer. The electrical behaviour ofPPy/DBSA/hydroquinone/Al junction was found tobe in good agreement with the thermionic emissionmodel for the Schottky barrier devices. The interac�tion of PPy/DBSA with hydroquinone was explainedin terms of change in the barrier height and in the car�rier concentration of the diodes, which was confirmedby capacitance�voltage measurements.

ACKNOWLEDGMENTS

Authors are grateful to Professor P.J.S. Foot ofKingston University, UK material research group forextending their laboratory facilities. Technical assis�tance from Hugh Malli and Saif (PhD students atKingston) is also sincerely acknowledged. A.S.acknowledges the Higher Education commission ofPakistan (HEC) for awarding Research fellowship.

REFERENCES

1. L. H. Sperling, Introduction to Physical Polymer Science(Wiley, New York, 2005).

2. T. A. Skoththeim, in Handbook of Conducting Polymers(Marcel Dekker, New York, 1986), p. 293.

3. M. Salmon, K. K. Kanazawa, A. F. Diaz, andM. Krounby, J. Polym. Sci., Part C: Polym Lett. 20, 187(1982).

4. S. A. Chen and C. S. Liao, Macromolecules 26, 2810(1993).

5. S. Chao and M. S. Wrighton, J. Am. Chem. Soc. 109,2197 (1987).

6. M. Merz, A. Haimerl, and A. J. Owen, Synth. Met. 25,89 (1988).

7. K. S. Jang, H. Lee, and B. Moon, Synth. Met. 143, 289(2004).

8. M. Campos, F. R. Simoes, and E. C. Pereira, Sens.Actuators, B 125, 158 (2007).

9. A. N. Aleshin, K. Lee, J. Y. Lee, et al., Synth. Met. 99,27 (1999).

10. H. Morgan and P. J. S. Foot, J. Mater. Sci. 36, 5369(2001).

11. D. A. Makeiff, T. Foster, and K. Foster, Technical Mem�orandum DRDC Atlantic TM 2004�301 (2005).

12. N. Arsalani and K. E. Geckeler, React. Funct. Polym.33, 167 (1997).

13. K. S. Jang, H. Lee, B. Moon, and C. Y. Kim, Synth.Met. 143, 289 (2004).

14. S. Lamprakopoulos, D. Yfantis, A. Yfantis, andD. Schmeisser, Synth. Met. 144, 229 (2004).

15. E. H. Rhoderick and R.H. William, Metal Semiconduc�tor Contacts (Clarendon, Oxford, 1998).

16. S. Angappane, N. R. Kini, T. S. Natarjan, et al., ThinSolid Films 417, 202 (2002).

17. H. Tomozawa, F. Braun, S. Philips, et al., Synth. Met.22, 63 (1987).

18. N. DasGupta and A. DasGupta, in SemiconductorDevices (Prentice�Hall of India, 2004), Chap. 9, p. 217.

19. C. Lee and J. Y. Lee, Synth. Met. 84, 149 (1997).