7
Electrical conductivity of halogen doped poly- (N-vinylcarbazole) thin films G. Safoula a , K. Napo a , J.C. Bern ede a, * , S. Touihri b , K. Alimi c a Groupe de Physique des Solides pour l’Electronique, Universite de Nantes, Equipe Couches Minces et Mat eriaux Nouveaux, FSTN, BP 92208, 2, Rue de la Houssini ere, 44072 Nantes Cedex 03, France b Department de Physique, Fac. Sciences Sfax, 3038 Sfax, Tunisia c Fac. Sciences Monastir, 5000 Monastir, Tunisia Received 3 September 1999; received in revised form 4 February 2000; accepted 23 August 2000 Abstract Thin films obtained by thermal evaporation of poly(N-vinylcarbazole) (PVK) powder have been doped with iodine and chlorine either by using a doped powder or by doping deposited films. The doping level of the thin films has been checked by X-ray photoelectron spectroscopy, then the samples have been electrically characterised by current–voltage and room temperature conductivity measurements. It is shown that chlorine reacts with PVK during doping which induces new compounds formation. Three dierent conductivity domains have been put in evidence, in the case of iodine doping. In the low field range the current is dominated by space charge eect. In intermediary field the current is ohmic. In the high field range the Poole–Frenkel eect is dominating. The evolution of properties of some samples is attributed to iodine ionisation of neutral iodine under high field. These results, obtained on evaporated thin films, are compared to those obtained with spin-coated PVK films. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Poly(N-vinylcarbazole); Thin films; Poole–Frenkel; Space charge eect 1. Introduction Organic polymers are recently intensely studied for their potential characteristics, such as the easy physical manipulation and the great flexibility of chemical structure (extraction). These two characteristics allow processing cost of return to be reasonable. Conductor and semiconductor polymers, particularly have many doping possibilities for specific applications. Poly(N-vinylcarbazole) (PVK) is mostly studied [1–6] because of its remarkable photoconductivity properties. One of the investigation aspects consists of optimising the potential application in xerography field [7,8], which needs an absorption in the visible wavelength by com- plex salt formation [9]. Such absorption in the visible region could also be very interesting in order to achieve solar cells. Solar cells based on organic thin layers is now a current interest of many researchers [10–13]. This is mainly due to low cost, simplicity of fabrication of large area, however many progresses should be done to achieve acceptable e- ciency and device stability. In order to progress in that direction a good knowledge of the conduction processes in the films is necessary. The most important properties studied in this mate- rial is charge generation from photo excitable centres, charge transfer (CT) and the electro-optical eect. Dierent methods to obtain PVK thin films using chemical processes [14–17] and recently physical pro- cesses, sputtering [18] or evaporation [19,20], are re- ported in the literature. European Polymer Journal 37 (2001) 843–849 * Corresponding author. Tel.: +33-2-51-12-55-30; fax: +33-2- 51-12-55-28. E-mail address: [email protected] tes.fr (J.C. Berne `de). 0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0014-3057(00)00185-3

Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

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Page 1: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

Electrical conductivity of halogen doped poly-(N-vinylcarbazole) thin ®lms

G. Safoula a, K. Napo a, J.C. Bern�ede a,*, S. Touihri b, K. Alimi c

a Groupe de Physique des Solides pour l'Electronique, Universite de Nantes, Equipe Couches Minces et Mat�eriaux Nouveaux, FSTN,

BP 92208, 2, Rue de la Houssini�ere, 44072 Nantes Cedex 03, Franceb Department de Physique, Fac. Sciences Sfax, 3038 Sfax, Tunisia

c Fac. Sciences Monastir, 5000 Monastir, Tunisia

Received 3 September 1999; received in revised form 4 February 2000; accepted 23 August 2000

Abstract

Thin ®lms obtained by thermal evaporation of poly(N-vinylcarbazole) (PVK) powder have been doped with iodine

and chlorine either by using a doped powder or by doping deposited ®lms. The doping level of the thin ®lms has been

checked by X-ray photoelectron spectroscopy, then the samples have been electrically characterised by current±voltage

and room temperature conductivity measurements. It is shown that chlorine reacts with PVK during doping which

induces new compounds formation.

Three di�erent conductivity domains have been put in evidence, in the case of iodine doping. In the low ®eld range

the current is dominated by space charge e�ect. In intermediary ®eld the current is ohmic. In the high ®eld range the

Poole±Frenkel e�ect is dominating.

The evolution of properties of some samples is attributed to iodine ionisation of neutral iodine under high ®eld.

These results, obtained on evaporated thin ®lms, are compared to those obtained with spin-coated PVK

®lms. Ó 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Poly(N-vinylcarbazole); Thin ®lms; Poole±Frenkel; Space charge e�ect

1. Introduction

Organic polymers are recently intensely studied for

their potential characteristics, such as the easy physical

manipulation and the great ¯exibility of chemical

structure (extraction). These two characteristics allow

processing cost of return to be reasonable. Conductor

and semiconductor polymers, particularly have many

doping possibilities for speci®c applications.

Poly(N-vinylcarbazole) (PVK) is mostly studied [1±6]

because of its remarkable photoconductivity properties.

One of the investigation aspects consists of optimising

the potential application in xerography ®eld [7,8], which

needs an absorption in the visible wavelength by com-

plex salt formation [9].

Such absorption in the visible region could also be

very interesting in order to achieve solar cells. Solar cells

based on organic thin layers is now a current interest of

many researchers [10±13]. This is mainly due to low cost,

simplicity of fabrication of large area, however many

progresses should be done to achieve acceptable e�-

ciency and device stability. In order to progress in that

direction a good knowledge of the conduction processes

in the ®lms is necessary.

The most important properties studied in this mate-

rial is charge generation from photo excitable centres,

charge transfer (CT) and the electro-optical e�ect.

Di�erent methods to obtain PVK thin ®lms using

chemical processes [14±17] and recently physical pro-

cesses, sputtering [18] or evaporation [19,20], are re-

ported in the literature.

European Polymer Journal 37 (2001) 843±849

* Corresponding author. Tel.: +33-2-51-12-55-30; fax: +33-2-

51-12-55-28.

E-mail address: [email protected]

tes.fr (J.C. BerneÁde).

0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.

PII: S0 01 4 -3 05 7 (00 )0 0 18 5 -3

Page 2: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

With regard to what has been said before, we have

attempted to study PVK doped with iodine or chlorine

[21±25] to better understand CT process between PVK

and halogen as well as to compare the electrical prop-

erties of PVK thin ®lms obtained by evaporation to that

of ®lms obtained more classically by spin coating.

2. Materials and methods

In the case of iodine, we describe the electrical con-

ductivity of thin ®lms obtained by using two di�erent

techniques. In the ®rst one [15] thin ®lms are obtained

using vacuum thermal evaporation of PVK powder pre-

doped to 3 at.% iodine atoms. The second technique

consists of thin ®lms deposited from pure PVK powder

and then doped in an iodine atmosphere. Thin ®lms

obtained in this case are called iodine post-doped.

In order to dope the samples with chlorine, they were

exposed to chlorine at room temperature in a glass tube

under a 0.25 atm. chlorine pressure.

The result of a comparative study of the physico-

chemical properties of thin ®lms obtained through these

two processes was published recently [24]. It has been

shown that the post-doped ®lms were more homogeneous

and, as discussed below, that more iodine was present in

these ®lms. The doping level of the di�erent samples has

been estimated through X-ray photoelectron spectros-

copy (XPS) analysis combined with argon ion etching.

The electrical measurements were carried out in dark

chamber. Several sets of measures were realised on each

sample to allow stability monitoring after electrical ex-

citation. Measurements of d.c. voltage±current (I±V )

were accomplished in a wide voltage range (1±1000 V) at

room temperature (T � 270 K). The d.c. voltage was

furnished by a high voltage generator HP 6110 A d.c.

The current intensity was measured with a KEITHLEY

617 electrometer.

The conductivity measurements were done using the

same electrometer, coupled to a multimeter which is

connected to a cuivre±constantan thermocouple to en-

able reading the temperature.

The structure of samples is longitudinal. Golden elec-

trodes were deposited by vacuum evaporation. The dis-

tance between these two electrodes is d � 1 mm. The

thickness of PVK iodine doped thin ®lms varied from 1 to

5 lm.

As mentioned before, the evolution of the polymer

after doping has also been checked by XPS measure-

ments. XPS measurements were performed with a

magnesium X-ray source (1253.6 eV) operating at 10 kV

and 10 mA. The energy resolution was 0.75 eV at a pass

energy of 50 eV. The quantitative XPS study was based

on the determination of the C1s, N1s, I3d5=2, Cl2p and

O1s peak areas, with 0.2, 0.36, 6.4, 0.58 and 0.61, re-

spectively, as sensitivity factors (the sensitivity factors

were given by the manufacturer (Leybold)). The de-

composition of the XPS peaks into di�erent components

and the quantitative interpretation, were made after

subtraction of the background using the Shirley method

[26]. The developed curve-®tting programs permit the

variation of parameters such as the Gaussian/Lorentzian

ratio, the full width at half maximum, the position and

the intensity of the contribution. These parameters were

optimised by the curve-®tting program, in order to ob-

tain the best ®t. The binding energy positions were

corrected to annul the charging e�ects. The calibration

uses the binding energy of the C1s peak of the hydro-

carbon, the position of which is assumed to be constant

at 285 eV [27].

The iodine depth pro®le in the samples was studied

by recording successive XPS spectra after ion etching for

short periods. Using an ion gun, sputtering was ac-

complished at pressures of less than 5:1� 10ÿ4 Pa with a

10 mA emission current and a 3 kV ion beam energy. In

such conditions all the surface of the ®lms was sputtered.

3. Results and discussion

Room temperature conductivity of the di�erent

samples is reported in Table 1. It can be seen that, while

there is systematically an increase of the conductivity

after iodine doping, after chlorine doping there is no or

uncertain conductivity increase. The di�erences in con-

ductivity at room temperature between pre- and post-

doped PVK thin ®lms are probably indicated by their

structure, stability and iodine concentration.

Before anymore electrical characterisation the e�ect

of the halogen doping on the ®lms have been checked by

XPS measurement.

Since XPS spectra of PVK and iodine doped PVK

have been discussed elsewhere [21±23] only the most

signi®cant results will be discussed here. In Fig. 1 the

C1s spectra of pure PVK (Fig. 1a), iodine doped PVK

(Fig. 1b) and chlorine doped PVK (Fig. 1c) are reported.

It can be seen that the signal obtained for pure PVK and

iodine doped PVK are more or less similar with, after

iodine doping a small relative increase of the two smaller

peaks by comparison with the highest one.

Table 1

Room temperature conductivity

Thin ®lm Pure PVK Iodine doped Iodine post-doped Chlorine doped Chlorine post-doped

r (X cm)ÿ1 6 10ÿ10 10ÿ8 10ÿ6 6 10ÿ10 10ÿ106r6 10ÿ8

844 G. Safoula et al. / European Polymer Journal 37 (2001) 843±849

Page 3: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

In the case of chlorine doped sample the C1s peak

shape is strongly modi®ed, which induces a very di�erent

peak decomposition. In Fig. 1a and b the peak situated

at 285 eV can be attributed to C±C bonds [27], that with

a binding energy of about 1 eV more can be assigned to

C±N [27] while the third one can be attributed to some

contamination C±O±H. When a CT complex salt is

obtained positive radicals situated on some carbons and/

or nitrogens of the polymer chains appear. Therefore the

increase, after doping, of the peak situated at 286 eV can

be attributed to C� formation. The increase of the third

peak corresponds to some oxygen contamination of the

sample during the doping process. In the case of chlorine

the fourth peak are of similar area, which means that

there is a strong oxidation of the carbon chain. The

chlorine reacts with the polymer to give di�erent chlo-

rinated compounds. This high reactivity of chlorine with

PVK explains the instability of the measured conduc-

tivity and the small e�ect of chlorine on the polymer

conductivity, since no stable CT complex salt forms, but

chlorine compounds. Therefore no more electrical study

could be done on chlorine doped sample and only

electrical properties of iodine doped PVK will be dis-

cussed below.

About iodine doped PVK thin ®lms, the iodine depth

pro®les are presented Fig. 2. It can be seen that, after a

small decrease of iodine concentration at the beginning

of the etching, there is a stabilisation of the concentra-

tion in the bulk whatever the sample. However it can

also be seen that, if there is about 2 at.% of iodine in the

post-doped ®lms, there is only 0.3 at.% in the ®lms ob-

tained from the pre-doped powder (iodine pre-doped

®lms).

Fig. 3 shows the results of measured density of cur-

rent (J ) as a function of voltage (V ) for di�erent thin

®lms. The logarithm transform of these two variables

( lnJ , lnV ) portrays a linear relationship.

In the case of pure PVK thin ®lms (Fig. 3a), the slope

of this relationship equals 1, meaning that conduction is

of ohmic type. This pattern does not change when

considering other sets of measurements.

Contrarily, in the case of PVK thin ®lms iodine

doped (Fig. 3b and c), if the relationship remains linear

it presents three di�erent rates of change, allowing to

subdivide the plots in three sections according to the

electrical ®eld E.

The rate of change observed in lower electrical ®eld

(E < 7� 104 Vmÿ1) is of the same magnitude (slope �0:4) independently of the technique used. This value is

Fig. 1. C1s peak of (a) pure PVK (- - -) experimental curve, (b)

iodine post-doped PVK (- - -) theoretical curve and (c) chlorine

post-doped PVK (� � �) di�erent components.

Fig. 2. XPS iodine pro®le in PVK thin ®lms: (a) pre-doped ®lm;

(b) post-doped ®lm.

G. Safoula et al. / European Polymer Journal 37 (2001) 843±849 845

Page 4: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

close to the value (0.5) reported in the literature [28,29],

which indicates space charge limited conduction.

In the high electrical ®eld domain (E > 2:5�105 Vmÿ1), disparate results are obtained. PVK thin

®lms iodine post-doped, in particular, have a non-ohmic

behaviour as portrayed in Fig. 3c (slope � 1:4). Pre-

doped ®lms presented also a non-ohmic conduction at

the second measure (slope � 1:6, Fig. 3b).

Lastly, in section with intermediate electrical ®eld,

7� 104 < E < 105 V mÿ1, the behaviour of PVK thin

®lms iodine doped using these two techniques is as fol-

lows: curve in (Fig. 3a and b) shows a rate of change

equal to 1, while curve in Fig. 3c is characterised by

slope greater than 1 (1,3), meaning, that the pattern is

not of ohmic type.

Measurement replicates for these two treatments did

not show any variation of the evolution of lnJ �f � lnV �, for post-doped thin ®lms. However, for pre-

doped thin ®lms, the shift that occurs under high elec-

trical ®eld is not reversible.

It should be reminded that di�erent conduction

mechanisms of polymeric materials have been suggested

[28±34], and these are:

· Schottky±Richardson emission consisting of an injec-

tion of carriers from electrodes,

· Poole±Frenkel e�ect which is related to the trap ioni-

sation applying an electrical ®eld,

· Space charge e�ect.

In this study speci®cally, the electrical conductivity in

relation to an applied electrical ®eld, showed up three

sections of conduction:

At low electrical ®eld (E < 7� 104 Vmÿ1) the elec-

trical conductivity is governed by space charge as de-

scribed by J / E2 law.

At intermediate electrical ®eld (7� 104 < E< 2:5�105 Vmÿ1), after stabilisation, the conductivity is ohmic.

At high electrical ®eld (E > 2:5� 105 Vmÿ1) the

conductivity regime is dominated by electrical ®eld ef-

fects.

In the following paragraph, we will discuss these

di�erent regimes as a function of the mentioned con-

duction mechanisms.

The e�ect of space charge is due to the di�erence in

work function between the metal (electrode) and the

insulator (polymer). If the insulator does not contain

traps the work functions injected carriers remain free

and thus contributes to space charge current, in the same

way, as a non-connected diode. Whereas, the presence of

traps in the insulator reduces the current by trapping

injected carriers.

In these two cases, the current density follows an

expression described by the Mott and Gurney law [28]:

J � aU 2

d3� a

dE2 �1�

where a � �9=8�ee0l in trapless insulator or a � �9=8�ee0lh with the presence of traps in the insulator, h:

fraction of free carriers reported to trapped carriers, e0:

vacuum permittivity, e: material permittivity (e � 3 for

PVK), l: mobility of carriers and d: ®lms thickness (inter

electrode distance in a longitudinal structure).

Fig. 4 represents the variation of the current density J

versus E2 of PVK thin ®lms iodine doped, in the region

of low electrical ®eld. Estimated mobility of the car-

riers from the line slopes, using trapless insulator, is

1:1� 10ÿ6 cm2 Vÿ1 sÿ1. This value was found in several

studies [35±38]. In these studies the PVK ®lms have been

obtained by spin-coating which means that the PVK has

a high molecular weight. In the present work, the ®lms

been obtained by evaporation, which means that, as

discussed in others papers [24,39±42] the chain length of

the polymer is far smaller. Therefore it can be concluded

from the above similarity of the carrier mobility value in

Fig. 3. Voltage±current characteristics lnJ � lnV of PVK thin

®lms obtained from: (a) pure PVK powder, (b) PVK obtained

from iodine doped powder (3 at.%) ± ®rst measure (�), second

measure (s) and (c) PVK ®lm obtained from post-doping

technique.

846 G. Safoula et al. / European Polymer Journal 37 (2001) 843±849

Page 5: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

the PVK ®lms, whatever their origin, con®rms that the

carriers are moving, not along the polymer chain, but by

hopping between carbazole groups. This explains that

similar mobilities are obtained from samples with very

di�erent molecular weight.

In high electrical ®eld regimes, the experimental re-

sults are often interpreted by either the Schottky e�ect or

the Poole±Frenkel e�ect. These two theories although,

di�erent in their principles, result in expressions of the

same form: ln J / E1=2. These two theories are essen-

tially di�erent from each other by the following points:

· Schottky e�ect is limited to the surface of electrodes,

· Poole±Frenkel e�ect is a volume e�ect. It corre-

sponds to the thermal excitation of most carriers

(holes) of trap level to band valence with dropping

of coulombian barrier under the action of the applied

electrical ®eld. The dropping DE0 of the barrier is of

the form:

DE0 � e3Epe0e

� �1=2

� bPFE1=2 �2�

where bPF is a Poole±Frenkel constant and e an ele-

mentary charge.

The corresponding current density J is described by

the following relationship:

J � A expbPFE1=2 ÿ Ea

kT

� ��3�

where A: analogous constant of density current, E: ap-

plied electrical ®eld�V/d, V: applied voltage, k: Boltz-

mann constant, Ea: energy of activation, d: distance

between electrodes, e0: vacuum permittivity and e: ma-

terial permittivity.

Schottky e�ect corresponds to the modi®cation of the

potential barrier of the metal±insulator interface.

Emission of carriers is analogous to thermoionic emis-

sion, the applied ®eld contributes to decrease extraction

work of metal±insulator.

The corresponding current density is described in this

form:

J � AT 2 expbSE1=2 ÿ U

kT

� ��4�

where A: Schottky±Richardson constant (A � 120

Acmÿ2), U: work function of metal and bS: Schottky

constant. It is calculated from this expression:

bS �e3

4pee0

� �1=2

�5�

Poole±Frenkel constant is twice as SchottkyÕs. This

ratio is explained by the fact that in the ®rst case, the

carrier is trapped, whereas in the second case, the carrier

has its image in the other side, once it crosses the barrier.

bPF � 2bS �6�

In PVK material particularly, the dielectric constant

taken to be equal to 3 [33,34] gave a theoretical value of

bPF � 4:38� 10ÿ5 eV m1=2 Vÿ1=2.

In practice, it is often very di�cult to distinguish

between these two mechanisms of conduction solely, on

the basis of these curves lnJ � f � lnE2�. A solution to

this problem consists, in using Poole±Frenkel conduc-

tivity as a function of electrical ®eld.

In high electrical ®eld regimes, our structures were of

longitudinal type. Therefore Poole±Frenkel e�ect should

be more dominating as compared, to the surface e�ect

(Schottky e�ect). In this case, the electrical conductivity

under high electrical ®eld must be pursuing FrenkelÕs law.

rPF � r0 expbPFE1=2 ÿ Ea

kT

� ��7�

where r0: electrical conductivity corresponding to low

electrical ®eld.

Fig. 4. Density of current versus electrical ®eld J � f �E2� of PVK thin ®lms iodine doped obtained by: (a) pre-doping technique (3

at.%. iodine in pure PVK powder) and (b) post-doping technique.

G. Safoula et al. / European Polymer Journal 37 (2001) 843±849 847

Page 6: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

The electrical conductivity of our samples was de-

duced from the following relationship:

rPF � JE

�8�

where J: through ®lm current density (Amÿ2) and E:

applied electrical ®eld (V mÿ1).

Fig. 5 represents the result obtained lnr � f E1=2ÿ �

.

From this ®gure, two regions of distinguished conduc-

tion can be seen (lower and higher electrical ®eld).

In region of high electrical ®eld, the value of Poole±

Frenkel coe�cients calculated from the line slopes is

bPF � 4:74� 10ÿ5 eVm1=2 Vÿ1=2 (Fig. 5a) and bPF �4:53� 10ÿ5 eVm1=2 Vÿ1=2 (Fig. 5b) respectively for PVK

thin ®lm iodine pre and post-doped. These values con-

form to the theoretical value.

Therefore, here also, as in the case of spin-coated

PVK ®lms [1,34,43], Poole±Frenkel e�ect is the most

dominating conduction mechanism at high electrical

®eld for PVK ®lms iodine doped with either technique,

however, in the case of thin ®lms obtained from pre-

doped method, an application of intense electrical ®eld is

necessary before the stabilisation mechanism, comes up.

Intense electrical ®eld induced probably ionisation of

iodine, which provokes the behaviour observed. Subse-

quently, modi®cation of the electrical state of iodine,

following the application of the electrical ®eld has been

already reported [5].

The di�erence noticed in structure and behaviour

between these two types of iodine thin ®lms doped is

probably related to the di�erence of iodine applied

quantity in these ®lms. Post-doped ®lms contained more

iodine and thus more ionised iodine in comparison to

the pre-doped ®lms. We have already showed that a

ratio of Iÿ3 =I2 � 80% [22] exists in our doped ®lms.

The di�erence of the electrical behaviour can be ex-

plained by the variation in morphology and structure of

these ®lms, as post-doped ®lms have a better surface

state compared to the others [17]. Post-doped thin ®lms

(T � 370 K) have a reticular structure [22]. Similarly

pre-doped thin ®lms, which are electrically unstable,

evolve probably to reticular structure, following the

application of an intense electrical ®eld.

Therefore, after stabilisation of the iodine doping,

whatever the sample is, the electrical behaviour of the

evaporated PVK ®lms is similar to that of spin-coated

®lms. Since the molecular weight of the latter is far

smaller than that of the former, it can be concluded that

this parameter is not predominant in the case of PVK.

This result should be attributed to the fact that PVK is a

saturated polymer. In that case the conductivity process

does not take place along the polymer chains, but by

hopping from one chain to another one. More precisely

in the case of PVK from one carbazole group to another

one. The good agreement between the present electrical

study and that of spin-coated samples con®rms that the

carbazole group are not destroyed during the evapora-

tion process, which corroborates earlier optical charac-

terisation of these ®lms [19].

4. Conclusion

In the case of chlorine, its high electronegativity in-

duces a reaction between the dopant and the polymer

chain. Therefore no stable CT complex between chlorine

and PVK can be obtained, but chlorine compounds

form. The chlorine attacks systematically PVK ®lms and

only iodine doped PVK samples could be extensively

studied by electrical measurements.

Fig. 5. Electrical conductivity versus electrical ®eld ln r � f �E1=2� of PVK thin ®lms iodine doped obtained by: (a) pre-doping

technique (3 at.% iodine in pure PVK powder) and (b) post-doping technique.

848 G. Safoula et al. / European Polymer Journal 37 (2001) 843±849

Page 7: Electrical conductivity of halogen doped poly(N-vinylcarbazole) thin films

In the case of iodine doping it is shown that the io-

dine concentration present in the ®lms depends on the

doping process. The iodine concentration in the post-

doped ®lms (2 at.%) is nearly one order of magnitude

higher than that present in the ®lms obtained by evap-

oration of a pre-doped powder.

Therefore this study concerns mainly the electrical

behaviour of PVK thin ®lms iodine pre and post-doped

under an electrical ®eld. Post-doped PVK thin ®lms

seemed to be more stable than those obtained from pre-

doped powder. The latter required an intense electrical

®eld treatment before stabilisation.

PVK thin ®lms iodine doped present a conduction,

which is controlled either by space charge or by ohmic

conduction in both low and moderate ®eld. Whereas the

conduction mechanism seems dominated by Poole±

Frenkel e�ect.

The formation of a charge transfer complex salt

between PVK and iodine contributes to increase of

conductivity by two and four order of magnitude re-

spectively for pre- and post-doped thin ®lms. The in-

crease of conductivity is limited by saturated PVK

chains and probably by the localisation of carriers cre-

ated from atoms of carbazole group.

The present work shows that the electrical properties

of evaporated PVK ®lms are similar to that of spin-

coated PVK ®lms. The main di�erence between these

two families of samples is the molecular weight of the

polymer. It is far higher in the case of spin-coated ®lms

than in the case of evaporated ®lms. This is explained by

the fact that conductivity processes in PVK are related

to the carbazole groups and not to the chain length of

the polymer. Therefore the present study con®rms that

the carbazole group is preserved during the evaporation

process.

References

[1] Pearson JM, Stolka M. Poly(N-vinylcarbazole) polymer

monographs, vol. 6. New York: Gordon and Breach, 1985.

[2] Chapoy LI, Munck DK. J Phys C 1983;3:644.

[3] Pai DM. J Chem Phys 1970;52:2285.

[4] Mort J. Phys Rev B 1972;5:3329.

[5] Nakasawa Y, Ito N, Hoshino K, Hanna JI, Kokado H.

Jpn Appl Phys 1991;30:1002.

[6] Nakasawa Y, Ito N, Hoshino K, Hanna JI, Kokado H.

Jpn Appl Phys 1992;31:305.

[7] Rencroft PJ, Takahashi K. J Non-Cryst Solids 1975;17:71.

[8] Chen C, Mort J. J Appl Phys 1972;43:1164.

[9] Hanna JI, Inoue E. Photo Sci Engng 1983;27:51.

[10] Al-Mohamad A, Soukieh M. Thin Solid Films 1995;

271:132.

[11] Sicot L, Fiorini C, Nunzi JM, Raimond P, Sentein C.

Synth Metals 1999;102:991.

[12] Brabec CJ, Padinger F, Hummelen JC, Janssen RAJ,

Sariciftci NS. Synth Metals 1999;102:861.

[13] Brabec CJ, Padinger F, Sariciftci NS, Hummelen JC.

J Appl Phys 1999;85:6866.

[14] Gill WD. J Appl Phys 1992;43:50.

[15] P®ster G, Gri�tits CH. Phys Rev Lett 1978;40:659.

[16] Ikida M. J Phys Soc Jpn 1991;60:2021.

[17] Papez V. J Electro Anal Chem 1990;282:389.

[18] Stulik P, Biederman H, Slavinski D, Fejfar A, Chudacek I,

Mackus P. J Electron 1991;70(3):509.

[19] Napo K, Chand S, Bernede JC, Safoula G, Alimi K.

J Mater Sci 1992;27:6222.

[20] Bernede JC, Taoudi H, DÕAlmeida K, Delvalle MA, Diaz

F. Polymer thin ®lms obtained from evaporated monomers

recent research developments in polymer science. Trans-

world Research Network, Trivandrum, India, 1998.

[21] Bernede JC, Alimi K, Safoula G. Polym Degrad Stab

1994;46:269.

[22] Safoula G, Alimi K, Touihri S, Bernede JC, Napo K.

J Chem Phys 1995;92:146.

[23] Alimi K, Safoula G, Bernede JC, Rabiller C. J Polym Sci

Part B: Polym Phys 1996;92:845.

[24] Touihri S, Safoula G, Bernede JC, Le Ny R, Alimi K. Thin

Solid Films 1997;304:16.

[25] Safoula G, Touihri S, Postic M, Bernede JC, Molinie Ph.

J Chem Phys 1997;94:1602.

[26] Shirley DA. Phys Rev B 1972;5:6219.

[27] Beanson G, Briggs M. High resolution XPS of organic

polymers ± the Scienta ESCA 300 Database. Chichester,

UK: Wiley, 1992.

[28] Mott NF, Gurney RW. Electronic processes in ionic

crystal. New York: Dover Publications, 1940.

[29] Mott NF, Davis EA. Electronic processes in non crystal-

line materials. Oxford: Clarendon Press, 1971.

[30] Chand S, Radhakrishanan S, Mechdru C. J Phys D: Appl

Phys 1982;15:249.

[31] Reuter R, Franke H. J Appl Phys B 1989;45:219.

[32] Ahmad A, Collins RA. Thin Solid Films 1992;21:75.

[33] Ikida M. J Phys Soc Jpn 1991;60:2031.

[34] Enachescu M, Dima I, Motorga V, Botila T, Ciurea ML,

Korony CH. Rev Roum Phys 1991;36:65.

[35] Regensburger JP. Photobiology 1968;8:429.

[36] Pearson JM. Pure Appl Chem 1977;49:463.

[37] Reimer B, Bassler H. Phys Stat Sol (a) 1979;51:445.

[38] Safoula G, Bernede JC, Alimi K, Molinie P, Touihri S.

J Appl Polym Sci 1996;64:1733.

[39] Blanchet GB, Fincher Jr CR. Adv Mater 1994;6:881.

[40] Ivanov VF, Nekrasov AA, Gribkova OL, Vannikov AV.

Synth Metals 1996;83:249.

[41] Tamada M, Asano M, Yoshida M, Kumakura M. Polymer

1991;32:2064.

[42] Tamada M, Omichi H, Okui N. Thin Solid Films 1995;

268:18.

[43] Drajeva II, Klein-Szymanska B. Comptes Rendus de

lÕAcad�emie Bulgare des Sciences 1984;37:35.

G. Safoula et al. / European Polymer Journal 37 (2001) 843±849 849