Electrical properties of poly(meta/para)phenylene

S. Besbes a,*, A. Bouazizi a, H. Ben Ouada a, H. Maaref b, A. Haj Said c, F. Matoussi c

aUR Physico-Chimie des Interfaces et Ingenierie Textile, Departement de Physique, Faculte des Sciences de Monastir, 5000 Monastir, TunisiabLaboratoire de Physique des Semi-conducteurs, Departement de Physique, Faculte des Sciences de Monastir, 5000 Monastir, Tunisia

cUR Electrochimie des Materiaux Organiques et Membranes, Departement de Chimie, Faculte des Sciences de Monastir, 5000 Monastir, Tunisia

Abstract

In this study, we investigate the ac electrical response of the ITO/PMPP/Al devices for medium- and short-chain lengths. From the

conducting mechanisms and the dielectric behavior of ITO/PMPP/Al structures using current–voltage measurements, the transport

characteristics of electrons and holes have been analyzed. It was found that the electron and hole currents are space-charge limited with traps.

Moreover, carrier transport in PPMT has been studied as a function of frequency and dc bias and the devices are modeled with the equivalent

circuits for short- and medium-length chains. Based on the results of the impedance analysis, conducting mechanism and dielectric behaviors

are discussed.

D 2002 Published by Elsevier Science B.V.

Keywords: Conjugated polymer; SCLC; Electrochemical synthesis

1. Introduction

Conjugated polymers like polythiophene (PT), poly[ p-

phenylene vinylene] (PPV) and derivatives, attract consid-

erable attention nowadays for their potential use in thin-film,

microelectronic andoptoelectronic devices. The synthesis and

electroluminescent properties of these materials have been

extensively studied [1,2]. The aim of this paper is to study the

transport properties of electrons and holes in poly(meta/

para)phenylene. For this purpose, current–voltage (I–V)

measurements were carried out on ITO/polymer/Al sand-

wiched structures. The analysis of experimental data provides

further information to better understand the electrical behav-

ior of devices based on this polymer system.Moreover, carrier

transport in PPMThas been studied as a function of frequency

and dc bias and the devices are modeled with the equivalent

circuits for short- and medium-length chains. Based on the

results of the impedance analysis, conducting mechanism and

dielectric behaviors are discussed.

2. Experiment

The devices studied here consist of ITO/PMPP (po-

ly(meta/para) substituted phenylene)/Al structures prepared

on a glass substrate. PMPP was synthesized by anodic

oxidation of p-methoxytoluene with an ultrasonic stirring

as described in more detail elsewhere [3].

The FTIR, 1H and 13C NMR indicate that the polymer

under investigation is a substituted polyphenylene which

has a random structure containing both meta- and para-

phenylene motives (Fig. 1).

The deposition of the films was carried out by heating the

evaporation cell into which the powder of PMPP was

loaded, under a pressure of 10� 6 Torr. The growth rate of

about 15 A/s, monitored by a quartz oscillator, was con-

trolled by the heater temperature (t = 300 jC). The thicknessof polymeric layers was fixed at 1000 A.

Polymer films were deposited onto the substrates with an

indium tin oxide (ITO) electrode, which were precleaned by

successive ultrasonic treatment for 1 h in acetone and

isopropyl alcohol and followed by drying with nitrogen

gas and then drying in a vacuum oven for several hours.

Indium tin oxide (ITO) thin films have been studied exten-

sively for good efficiency for hole injection into organic

materials. They have also been widely utilized as the anode

contact for organic light-emitting diode (OLED) [4]. Alumi-

num was vapor deposited as the cathode at a working

pressure below 10� 6 Torr, yielding an active size of 5 mm

diameter [5]. The current–voltage characteristics of the ITO/

PMPP/Al devices were measured from an applied bias of

� 20 to 20 V for medium chain and from an applied bias of

0928-4931/02/$ - see front matter D 2002 Published by Elsevier Science B.V.

PII: S0928 -4931 (02 )00079 -6

* Corresponding author.

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Materials Science and Engineering C 21 (2002) 273–276

� 10 to 10 V for short chain using a Keithley 236 source

measure unit.

3. Current–voltage measurements

The transport of charges have been investigated using

current–voltage measurements in indium tin oxide (ITO)/

poly(meta/para)phenylene (PMPP)/Al structures for me-

dium- and short-chain lengths. Fig. 2 shows the current–

voltage characteristics for medium- and short-chain lengths.

The I–V curve for medium chain show a typical diode

behavior with a low threshold bias (Vs = 6.5 V), and the

current is only observed in the forward bias. For a short

chain, the same diode behaviour was shown with a low

threshold bias (Vs = 4.2 V) but the current was observed in

both forward and backward bias voltage.

Fig. 3 shows that in the increasing voltage portion of the

I–V curve three segments for ITO/PMPP (medium chain)/

Al and two segments for ITO/PMPP (short chain)/Al can be

identified. The voltage dependence of current appears to

follow the power law J~Vm. At low voltage, corresponding

to an ohmic region ( J~V) (m = 1), [a], which then becomes

space-charge limited with a single discrete set of shallow

traps ( J~V2) (m = 2), [b], and a final trap-filled region, [c],

with a further increase of the voltage ( J~V 4) (m = 4 > 2).

These regions can be well explained using standard space-

charge limited current (SCLC) theory [6]. Charge transport

through a thin polymer film may be an electrode limited

Fig. 1. Motives of PMPP.

Fig. 2. Current –voltage characteristics of 1000-A thick ITO/PMPP

(medium chain)/Al (M) and ITO/PMPP (short chain)/Al (C) in linear plots.

Fig. 3. Current–voltage characteristics of 1000-A thick ITO/PMPP (medium

chain)/Al (M) and ITO/PMPP (short chain)/Al (C) in log– log plots.

Fig. 4. (a) The variation of the conductivity versus frequency at different

applied bias for ITO/PPMT (medium chain)/Al device (d= 1000 A) (5: 0 V,

o: 3 V, D: 5 V). (b) The variation of the conductivity versus frequency at

different applied bias for ITO/PPMT (short chain)/Al device (d= 1000 A)

(5: 0 V, o: 1 V, D: 2 V).

S. Besbes et al. / Materials Science and Engineering C 21 (2002) 273–276274

process or a bulk limited process. In the second case, a

SCLC process, in the absence of traps in the polymer, the

current density can be written as [7].

J ¼ 9

8

V 2

d3ðelÞ ð1Þ

where e is the permittivity of the polymer (PMPP) (e= 3.2), dis the polymer film thickness (1000 A), l is the charge carrier

mobility andV is the applied voltage. If traps are present in the

polymer, the SCLC may be decreased by several orders of

magnitude. Rose [8] argued that neither the space-charge

density nor the field distribution should be altered by trap-

ping, but the equation relating current to voltage should be

modified by a trap-limiting parameter h relating the propor-

tion of trapped charges (Pt) to free charges (P) and J is now

written [9]:

J ¼ 9

8

V 2

d3ðelhÞ ð2Þ

where h is given by

h ¼ p

pþ ptð3Þ

Thus, for the trap-free case, Pt = 0, therefore h= 1; withtraps present h is always less than unity.

We believe that the asymmetry of the I–V characteristics

observed in the case of the ITO/PMPP (medium chain)/Al

arises from the two following essential causes: (i) the

presence of traps in the used organic materials; (ii) chemical

reaction between aluminum and oxygen in the interface

PMPP (medium chain)/Al [10]. This latter chain has a high

density of traps compared to that of the short chain. The

traps are probably due to the pendant oxygen in the metal

organic interface. The decomposition of the I–V character-

istics in three regions for medium chain and two regions for

short chain is also justified.

4. Impedance measurements

The measured conductivity, r of ITO/PPMT (medium

chain)/Al and ITO/PPMT (short chain)/Al devices versus

frequency at different applied bias V is depicted in Fig. 4a

and b. The plots show that the conductivity initially is

almost frequency independent with a slight increase on the

higher frequency side for the different bias voltages. In

Fig. 5. (a) The impedance Cole–Cole plot of the ITO/PPMT (medium

chain)/Al device (d= 1000 A) for V= 0 V (D), 3 V (o), 5 V (5). (b) The

impedance Cole–Cole plot of the ITO/PPMT (short chain)/Al device at

several bias voltage (d= 1000 A) (5: 0 V, o: 2 V).

Fig. 6. (a) The impedance Cole–Cole plot of the ITO/PPMT (medium

chain)/Al device at several bias voltage in log– log plot (d= 1000 A). (b) The

impedance Cole–Cole plot of the ITO/PPMT (short chain)/Al device at

several bias voltage in log– log plot (d= 1000 A).

S. Besbes et al. / Materials Science and Engineering C 21 (2002) 273–276 275

general, the trend of the conductivity for such compounds is

found to be possible understood from the law [11]:

rðwÞ ¼ r0 þ ws

where w is the angular frequency, r(w) and r0 are the ‘‘ac’’and ‘‘dc’’ conductivities, respectively, and s is the critical

exponent (0 < s < 1). Values of s for ITO/PPMT (medium

chain)/Al are found to be 0.33, 0.17 and 0.06 for V= 0, 3

and 5 V, respectively. For ITO/PPMT (short chain)/Al, this

parameter is, however, equal to 0.02 and 0.003 for V= 0 and

2 V. As can be seen, the critical exponent s tends to decrease

as the applied bias increases. The frequency-dependent

conductivity shows unambiguously that the transport of

charge carriers is induced by hopping conduction mecha-

nism [12].

Fig. 5a shows the impedance Cole–Cole plots of the ITO/

PPMT (medium chain)/Al device for different dc bias V= 0,

3 and 5 V. This plot shows a single semicircle, which means

that the equivalent circuits for these applied voltages are

designed as a single parallel resistor Rp and capacitor Cp

network with a series resistance Rs [13].

Fig. 5b shows the impedance Cole–Cole plots of ITO/

PPMT (short chain)/Al device for different dc bias voltages.

These measurement show the same behavior like ITO/PPMT

(medium chain)/Al and the same equivalent circuit.

Fig. 6a and b show the Cole–Cole plot of the ITO/PPMT

(medium chain)/Al and ITO/PPMT (short chain)/Al devices

at several dc bias voltages. We plot the data in log–log scale

because the size of the curvature decreased abruptly with

increasing bias voltage. The slopes are in the order of 0.44

and 0.37 for medium- and short-chain lengths, respectively,

which means that the curvature is semicircle [13]. The

minimum of Re Z value (Re Z at the highest frequency)

shows that there is a parallel resistance to the capacitor, and

it is about 167 and 138 V for medium- and short-chain,

respectively, for 0 V. The maximum of Re Z value (Re Z at

the lowest frequency) represents the addition of a series

resistance and a parallel resistance to the capacitance, which

is 96 and 112 V for medium- and short-chain, respectively,

when the bias voltage was zero [13]. Therefore, the equiv-

alent circuit for the device can be designed as a single

parallel resistor Rp and capacitor Cp network with a series

resistance Rs. Rs is probably due to the ohmic contact at the

hole injecting ITO/PPMT interface [10].

5. Conclusion

In this work, we have investigated the transport proper-

ties of charges using current–voltage measurements in thin

layers of poly(para/meta)phenylene (PMPP) sandwiched

between ITO and aluminum electrodes. The current–volt-

age characteristics in linear plots show a typical diode

behavior with a low threshold bias voltages (Vs) of 6.5

and 4.2 V for medium- and short-chain lengths, respectively.

On the other hand, the current–voltage characteristics in

log– log plots have demonstrated that the transport of

charges is well described by space-charged limited current.

The conductance on frequency is investigated and modeled

the devices with the equivalent circuit can be designed as a

single parallel resistor RP and capacitor CP network with a

series resistance RS and our experimental results are in good

agreement with existing theoretical models.

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