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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
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
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
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
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
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
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.
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