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Electrical characterization of the ITO/NiPc/PEDOT : PSS junction diode

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2010 J. Phys. D: Appl. Phys. 43 405104

(http://iopscience.iop.org/0022-3727/43/40/405104)

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 43 (2010) 405104 (5pp) doi:10.1088/0022-3727/43/40/405104

Electrical characterization of theITO/NiPc/PEDOT : PSS junction diodeMutabar Shah1,3, M H Sayyad1, Kh S Karimov1,2 and Fazal Wahab1

1 Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, District Swabi, KhyberPakhtunkhwa 23640, Pakistan2 Physical Technical Institute of Academy of Sciences, Rudaki Ave. 33, Dushanbe 734025, Tajikistan

E-mail: mutabar shah@hotmail.com and mutabarshah@gmail.com

Received 27 June 2010, in final form 20 August 2010Published 21 September 2010Online at stacks.iop.org/JPhysD/43/405104

AbstractThis paper reports on the fabrication and characterization of an ITO/NiPc/PEDOT : PSSjunction diode. A thin film of nickel phthalocyanine (NiPc) was deposited by the thermalvacuum deposition method on indium tin oxide (ITO) used as a substrate. The current–voltagecharacteristics of the diode were measured at room temperature under dark condition andshowed rectifying behaviour. The values of several electrical parameters such as idealityfactor, barrier height, conductivity, and series and shunt resistances were calculated.

1. Introduction

Indium tin oxide (ITO) has been widely used as the anodematerial in different optoelectronic and electronic devices suchas photovoltaic cells, organic light emitting diodes and organicfield effect transistors [1]. The high electrical conductivitysimilar to metallic materials, low electrical resistivity, excellentsubstrate adherence and easy processability of ITO havebeen reported [2]. Its contacts with some of the organicsemiconducting materials have shown promising results forapplications in electronic devices [3, 4]. It is known thatelectronic devices are fabricated and commercialized on thebasis of conventional inorganic semiconductors that are widelyused in electronics and instrumentation [5–7]. Organicsemiconductor devices attract considerable interest due totheir processing advantages over traditional inorganic devices.For the fabrication of organic semiconductor devices manytechniques have been used such as lithography, spin-coating,thermal evaporation and printing [8–10].

The interesting properties of organic semiconductorsinclude a large variety of molecular and crystal structures,highly anisotropic structures, electronic and optical properties,the presence of quasi-one-dimensional conduction, metal–insulator transitions such as Anderson transition, Peierlstransition and Hubbard–Mott transitions, photo-inducedcharge-transfer phenomenon, etc [11, 12]. Concerningelectronic devices organic semiconductors are attractive due

3 Author to whom any correspondence should be addressed.

to their potential cheapness of materials and simplicity oftechnology, which allow one to deposit thin films fromsolutions by spin-coating, drop-casting, pulverization or byphysical vacuum deposition at relatively low temperatures(400–600 C).

As we know that phthalocyanine (Pc) and metallo-phthalocyanines (M-Pc) are thermally and chemically stable,their thin films can be deposited by thermal evaporation withoutdissociations [13, 14]. It has been found that these compoundsare usually light stable and absorb in the red and visibleregion of the solar spectrum [15]. The electronic structure ofM-Pc can be characterized by the delocalization of electronswith the highest occupied molecular orbital (HOMO)–lowestunoccupied molecular orbital (LUMO) transitions in thevisible region and are important semiconducting materialsfor electronics and optoelectronics devices. The electricalproperties of surface-type Schottky diodes prepared by athermal evaporator have been studied [9, 16] and variouselectrical parameters of metal/nickel phthalocyanine (NiPc)junctions from current–voltage (I–V ), capacitance–voltage(C–V ) and capacitance–frequency (C–F ) characteristics havebeen determined. NiPc is one of the metal phthalocyanineswhich appears to be a more promising material for applicationin active devices due to its relative high mobility [17].

The aim of this study is to fabricate a sandwich-typeITO/NiPc/PEDOT : PSS structure and to determine its junctionparameters such as series resistance, ideality factor and barrierheight using Cheung’s and Norde’s functions via current–voltage (I–V ) characteristics.

0022-3727/10/405104+05$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 43 (2010) 405104 M Shah et al

Figure 1. Cross sectional view of the ITO/NiPc/PEDOT : PSSjunction diode.

2. Experimental

The NiPc powder and PEDOT : PSS aqueous solution (1.3 wt%dispersed in H2O) used in this experiment were purchased fromSigma Aldrich. The substrate was cleaned in an ultrasonic bathfor 10 min and then dried. A thin film of 100 nm thicknessof NiPc was deposited by a thermal vacuum evaporator onthe cleaned ITO-coated glass substrate. The thin film wasgrown with a deposition rate of 0.2 nm s−1. During depositionthe pressure remained constant at 5.5 × 10−5 mbar. The thinfilm of NiPc, after deposition, was annealed at 150 C for 1 h.The top electrode PEDOT : PSS was deposited by the drop-casting technique on the thin film of NiPc to fabricate theITO/NiPc/PEDOT : PSS junction diode. Then the device wasdried at 50 C for 2 h after the deposition of PEDOT : PSS.The conductive polymer PEDOT : PSS plays the role of anelectrode. The cross sectional view of the fabricated structureis shown in figure 1. Functional testing was made on a KarlSuss PM 5 probe station. The electrical characteristics weremeasured using a current–voltage source, Keithley-228A andKeithley-196 system DMM.

3. Results and discussion

The forward and reverse bias current–voltage (I–V )characteristics of the ITO/NiPc/PEDOT : PSS junction diode isshown in figure 2. It is observed from the current–voltage curvethat the device shows rectifying behaviour with a potentialbarrier at the PEDOT : PSS/NiPc interface. The rectificationratio (the ratio of forward to reverse bias currents) is equalto 4, 7 and 10 at the applied voltages of ±1 V, ±1.5 V and±2 V, respectively. The turn on voltage of the device is 0.5 V.According to the thermionic emission theory, the current in aSchottky barrier diode is given by:

I = Io

[exp

(qV

nkT

)− 1

], (1)

where the saturation current Io is expressed as follows:

Io = AA∗T 2 exp

(−qφb

kT

), (2)

where q is the electron charge, φb is the barrier height, V is theapplied voltage, A∗ is the effective Richardson constant equalto 1.3×105 A cm−2 K−2, A is the effective diode area, T is the

-4 -3 -2 -1 0 1 2 3 4-10

0

10

20

30

40

50

60

-4 -3 -2 -1 0 1 2 3 4

-2

-1

0

1

2

3

4

5

Cur

rent

(µA

)

Voltage (V)

lg (

I)

V

Figure 2. I–V curve for the sandwich-type ITO/NiPc/PEDOT : PSSstructure.

absolute temperature, k is the Boltzmann constant and n is theideality factor. The value of n can be determined through therelation

n = q

kT

dV

d(ln I )(3)

and

φb = kT

qln

(AA∗T 2

Io

). (4)

The value of the ideality factor can be calculated usingequation (3) from the slope of the linear region of the current–voltage measurements. The linear region of the voltage liesbetween 0.5 and 2.5 V. For an ideal diode the value of n is one,but usually it is greater than unity. The higher values of n maybe due to the oxide layer on the surface of metal electrodes,series resistance or interfacial states. The experimentallyobtained value of the ideality factor shows how much thebehaviour of the fabricated diode is comparable to an idealdiode. The value of the ideality factor was found to be 19for the sandwich-type ITO/NiPc/PEDOT : PSS junction diode.The value of barrier height is obtained from equation (4) andis equal to 0.81 eV.

To calculate the series and shunt resistance of the diodes,the junction resistance (Rj) was plotted against the externalapplied voltage, where Rj = ∂V/∂I . Figure 3 shows thegraph of the applied voltage versus junction resistance ofthe ITO/NiPc/PEDOT : PSS junction diode. The junctionresistance approaches a minimum value at a high forwardbias voltage which is series resistance RS having a value of0.024 M, while in the reverse bias the maximum value of theresistance is shunt resistance Rsh and is found to be 2.23 M.

To study the transport mechanism of the sandwich-typeITO/NiPc/PEDOT : PSS structure, double logarithmic (log I

versus log V ) graph is plotted in the forward bias as shownin figure 4. This figure shows the power law behaviour ofcurrent I ∝ V m with different values of m. As observed infigure 4, the double logarithmic graph of the sandwich-typeITO/NiPc/PEDOT : PSS structure diode shows three distinct

2

J. Phys. D: Appl. Phys. 43 (2010) 405104 M Shah et al

-4 -3 -2 -1 0 1 2 3 4

0.0

0.5

1.0

1.5

2.0

2.5

Junc

tion

Res

ista

nce

(RJ)

Voltage (V)

Rsh = 2.23 ΜΩ

Rs = 0.024 ΜΩ

Figure 3. Junction resistance–applied voltage for theITO/NiPc/PEDOT : PSS structure diode.

-0.4 -0.2 0.0 0.2 0.4 0.6-0.5

0.0

0.5

1.0

1.5

2.0

Log

(I)

Log (V)

2.2

3

1

Figure 4. Double logarithmic plot of I–V curve ofITO/NiPc/PEDOT : PSS.

linear regions. The first region is the ohmic region havinga slope of about one and it obeys Ohm’s law. This regionshows the thermionic emission region. At higher voltagessquare power law (I ∝ V m) is obtained as a second region (II)followed by a third region of slope m = 3. These resultsshow that the transport mechanism is due to a space chargelimited current (SCLC) dominated by a discrete trapping levelin region II and a trapped charge limited current (TCLC) withan exponential distribution of traps in region III.

A semi-log plot of reverse current plotted against V 1/2

for the ITO/NiPc/PEDOT : PSS diode is shown in figure 5at room temperature. As seen from this figure, the currentis characterized by two linear segments with different slopeswhich may be interpreted in terms of the two field-loweringmechanisms, which are Poole–Frenkel [18] and Richardson–Schottky mechanisms [19]. Under reverse bias conditionsthe inset of figure 2 shows a lower current as compared withforward bias, indicating the existence of a current limitation

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Ln (

I)

V1/2

Figure 5. Semi-logarithmic plot of current versus square root ofvoltage for ITO/NiPc/PEDOT : PSS.

mechanism, induced either by the Schottky or the Poole–Frenkel effect. The current–voltage expressions for bothmechanisms are given as [20]

I = AA∗T 2 exp

(−φb

kT

)exp

(βRSV

1/2

kT d1/2

)(5)

for the Schottky effect and

I = Io exp

(βPFV

1/2

kT d1/2

)(6)

for the Poole–Frenkel effect.In the above equations, φb is the Schottky barrier height at

the injecting electrode interface, Io the low field current, andβRS and βPF are the Schottky and Poole–Frenkel field loweringcoefficients, respectively. The theoretical values for βRS andβPF were given by

2βRS = βPF =(

q3

πεoεr

)1/2

. (7)

Using the relative permittivity of NiPc (εr = 4), the theoreticalvalues of βRS and βPF were calculated from equation (7)and were found to be 3.8 × 10−5 eV m1/2 V−1/2 and 7.6 ×10−5 eV m1/2 V−1/2, respectively. From the slopes of thelow and high voltage regions, the values of β were 2.1 ×10−5 eV m1/2 V−1/2 and 9 × 10−5 eV m1/2 V−1/2, respectively.The experimentally calculated value of β for the low fieldregion is about 0.55 times the theoretical value of βRS and0.28 times the theoretical value of βPF. The calculated valueof β for the high voltage region is 0.24 times the theoreticalvalue of βRS and 0.12 times the theoretical value of βPF. Theexperimentally obtained value of β for the high voltage regionis close to the theoretical value calculated for the Richardson–Schottky effect. This effect suggests a bulk limited conductionprocess.

An alternate method derived by Cheung and Cheung [21]can also be used to calculate the barrier height, ideality factor

3

J. Phys. D: Appl. Phys. 43 (2010) 405104 M Shah et al

0.00 1.50x10-5 3.00x10-5 4.50x10-5 6.00x10-50.4

0.6

0.8

1.0

1.2

1.4

dV/d

lnI (

V)

I(A)

Rs= 0.014 MΩ

n=19.3

Figure 6. Plot of dV/d ln I versus I for the ITO/NiPc/PEDOT : PSSdiode.

and series resistance. Series resistance RS is an importantparameter that influences the electrical characteristics of adiode, so considering the series resistance RS, the current in aSchottky diode is expressed as

I = Io exp

[q

(V − IRS

nkT

)], (8)

where IRS is the voltage drop across the series resistance ofthe device. From the above equation, Cheung’s functions areexpressed as

dV

d ln(I )= n

kT

q+ IRS, (9)

H(I) = V − nkT

qln

(Io

AA∗T 2

)(10)

andH(I) = IRS + nφb. (11)

In figures 6 and 7, the experimental values of (dV /d ln I )–I andH(I)–I are plotted for the ITO/NiPc/PEDOT : PSS junctiondiode. The RS and n values were determined from the slopeand y-axis intercept of the dV /d ln I–I plot and were found tobe 0.0144 M and 19.3, respectively. The graph of H(I)–I

is obtained using equation (10). The barrier height φb and RS

values were calculated from the H(I)–I plot and were equalto 0.71 eV and 0.02 M, respectively.

Furthermore, an alternate method for the determinationof series resistance and barrier height was proposed by Norde[22]. The Norde function is given as

F(V ) = V

γ− kT

qln

(I

AA∗T 2

). (12)

The effective barrier height is given by

φb = F(Vo) +Vo

γ− kT

q(13)

and

RS = kT (γ − n)

qIo, (14)

0.00 1.50x10-5 3.00x10-5 4.50x10-5 6.00x10-5

13.6

14.0

14.4

14.8

15.2

H(I

) (V

)

I(A)

Rs=0.02 MΩ

ϕ=0.71 ev

Figure 7. H(I) versus I plot for the ITO/NiPc/PEDOT : PSS diode.

1.5 2.0 2.50.702

0.703

0.704

0.705

0.706

0.707

0.708

0.709ϕ= 0.76 evRs= 0.016 MΩ

F(V

)

Voltage (V)

Figure 8. F(V ) versus V plot for the ITO/NiPc/PEDOT : PSS diode.

where F(Vo) is the minimum value of F(V ), Vo is thecorresponding voltage and Io is the corresponding current atV = Vo and γ is a dimensionless integer greater than theideality factor. Figure 8 shows the plot of F(V ) versus V forthe ITO/NiPc/PEDOT : PSS junction diode. From the graph φb

and RS values were calculated to be 0.76 eV and 0.016 M,respectively. The value of the ideality factor can also bedetermined by the method proposed by Bohlin [23]. Using thismethod the value of the ideality factor was found to be 19.42.The values of Io1, Io2, Vo1 and Vo2 were 19.794 × 10−6 A,54.53 × 10−6 A, 1.5 V and 2 V.

The electronic parameters obtained using differentmethods are shown in table 1. All these results showed goodagreement with each other.

The charges flow over a potential barrier with sufficientthermal energy in thermionic emission [6]. In this process,unlike a p–n junction where current is determined as thediffusion of minority carriers, the current is produced dueto majority carriers over a potential barrier, and this processusually occurs in a Schottky barrier diode [6]. As known, themobility of charge carriers in NiPc is below 1 cm2 V−1 s−1,

4

J. Phys. D: Appl. Phys. 43 (2010) 405104 M Shah et al

Table 1. Comparison of the electronic parameters obtained usingdifferent methods.

Ideality Series resistance Barrier heightfactor (M) (eV)

ln I–V 19 0.024 0.81dV /d ln I 19.3 0.014H(I) 0.020 0.71Norde’s function 19.42 0.016 0.76

therefore, according to the Ioffe–Regel criterion [12], thehopping mechanism of conduction from one localized stateto another by a thermally activated process takes place in thissemiconductor.

At the NiPc and PEDOT : PSS interface, there is probablya sufficiently large concentration of defects, traps, etc,which make the conduction process non-ideal. Therefore,a low rectification ratio and, accordingly, a low forward toreverse current ratio were observed in this diode. Therefore,the thermionic emission approach can be used for theITO/NiPc/PEDOT : PSS diode in the first approximation only.

4. Conclusions

We have investigated and fabricated a sandwich-typeITO/NiPc/PEDOT : PSS junction diode. The junctionparameters such as ideality factor, barrier height and seriesresistance of the diode were calculated by three differentmethods. All these parameters obtained using differentmethods are in close agreement with other.

Acknowledgments

The authors are grateful to the Ghulam Ishaq Khan Instituteof Engineering Sciences and Technology (www.giki.edu.pk)for its support during this work. They are pleased to thank theHigher Education Commission (HEC), Pakistan, for providingPhD scholarship to Mr Mutabar Shah and Mr Fazal Wahab.

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