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

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

    G. Safoula a, K. Napo a, J.C. Bernede a,*, S. Touihri b, K. Alimi c

    a Groupe de Physique des Solides pour lElectronique, Universite de Nantes, Equipe Couches Minces et Materiaux Nouveaux, FSTN,BP 92208, 2, Rue de la Houssiniere, 44072 Nantes Cedex 03, France

    b Department de Physique, Fac. Sciences Sfax, 3038 Sfax, Tunisiac 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 currentvoltage

    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

    PooleFrenkel 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. 2001 Elsevier Science Ltd. All rights reserved.

    Keywords: Poly(N-vinylcarbazole); Thin films; PooleFrenkel; 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 [16]

    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 [1013]. 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 [1417] and recently physical pro-

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

    ported in the literature.

    European Polymer Journal 37 (2001) 843849

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

    51-12-55-28.

    E-mail address: jean-christian.bernede@physique.univ-nan-

    tes.fr (J.C. Berne`de).

    0014-3057/01/$ - see front matter 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

    [2125] to better understand CT process between PVK

    and halogen as well as to compare the electrical prop-

    erties of PVK thin films obtained by evaporation to that

    of films obtained more classically by spin coating.

    2. Materials and methods

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

    ductivity of thin films obtained by using two dierent

    techniques. In the first one [15] thin films are obtained

    using vacuum thermal evaporation of PVK powder pre-

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

    consists of thin films deposited from pure PVK powder

    and then doped in an iodine atmosphere. Thin films

    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 films obtained through these

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

    shown that the post-doped films were more homogeneous

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

    these films. The doping level of the dierent 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. voltagecurrent (IV )

    were accomplished in a wide voltage range (11000 V) at

    room temperature (T 270 K). The d.c. voltage wasfurnished 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 cuivreconstantan 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. Thethickness of PVK iodine doped thin films 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 dierent components

    and the quantitative interpretation, were made after

    subtraction of the background using the Shirley method

    [26]. The developed curve-fitting 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-fitting program, in order to ob-

    tain the best fit. The binding energy positions were

    corrected to annul the charging eects. 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 profile 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 5:1 104 Pa with a10 mA emission current and a 3 kV ion beam energy. In

    such conditions all the surface of the films was sputtered.

    3. Results and discussion

    Room temperature conductivity of the dierent

    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 dierences in con-

    ductivity at room temperature between pre- and post-

    doped PVK thin films are probably indicated by their

    structure, stability and iodine concentration.

    Before anymore electrical characterisation the eect

    of the halogen doping on the films have been checked by

    XPS measurement.

    Since XPS spectra of PVK and iodine doped PVK

    have been discussed elsewhere [2123] only the most

    significant 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 film Pure PVK Iodine doped Iodine post-doped Chlorine doped Chlorine post-doped

    r (X cm)1 6 1010 108 106 6 1010 10106r6 108

    844 G. Safoula et al. / European Polymer Journal 37 (2001) 843849

  • In the case of chlorine doped sample the C1s peak

    shape is strongly modified, which induces a very dierent

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

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

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

    CN [27] while the third one can be attributed to some

    contamination COH. 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 thirdpeak 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 dierent chlo-

    rinated compounds. This high reactivity of chlorine with

    PVK explains the instability of the measured conduc-

    tivity and the small eect 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 films, the iodine depth

    profiles 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 films, there is only 0.3 at.% in the films ob-

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

    films).

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

    rent (J ) as a function of voltage (V ) for dierent thin

    films. The logarithm transform of these two variables

    ( lnJ , lnV ) portrays a linear relationship.In the case of pure PVK thin films (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 films iodine

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

    it presents three dierent rates of change, allowing to

    subdivide the plots in three sections according to the

    electrical field E.

    The rate of change observed in lower electrical field

    (E < 7 104 Vm1) 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 ( ) dierent components.

    Fig. 2. XPS iodine profile in PVK thin films: (a) pre-doped film;

    (b) post-doped film.

    G. Safoula et al. / European Polymer Journal 37 (2001) 843849 845

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

    which indicates space charge limited conduction.

    In the high electrical field domain (E > 2:5105 Vm1), disparate results are obtained. PVK thinfilms iodine post-doped, in particular, have a non-ohmic

    behaviour as portrayed in Fig. 3c (slope 1:4). Pre-doped films presented also a non-ohmic conduction at

    the second measure (slope 1:6, Fig. 3b).Lastly, in section with intermediate electrical field,

    7 104 < E < 105 V m1, the behaviour of PVK thinfilms 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 films. However, for pre-doped thin films, the shift that occurs under high elec-

    trical field is not reversible.

    It should be reminded that dierent conduction

    mechanisms of polymeric materials have been suggested

    [2834], and these are:

    SchottkyRichardson emission consisting of an injec-tion of carriers from electrodes,

    PooleFrenkel eect which is related to the trap ioni-sation applying an electrical field,

    Space charge eect.

    In this study specifically, the electrical conductivity in

    relation to an applied electrical field, showed up three

    sections of conduction:

    At low electrical field (E < 7 104 Vm1) the elec-trical conductivity is governed by space charge as de-

    scribed by J / E2 law.At intermediate electrical field (7 104 < E< 2:5

    105 Vm1), after stabilisation, the conductivity is ohmic.At high electrical field (E > 2:5 105 Vm1) the

    conductivity regime is dominated by electrical field ef-

    fects.

    In the following paragraph, we will discuss these

    dierent regimes as a function of the mentioned con-

    duction mechanisms.

    The eect of space charge is due to the dierence 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 a U2

    d3 a

    dE2 1

    where a 9=8ee0l in trapless insulator or a 9=8ee0lh 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 forPVK), l: mobility of carriers and d: films thickness (interelectrode distance in a longitudinal structure).

    Fig. 4 represents the variation of the current density J

    versus E2 of PVK thin films iodine doped, in the regionof low electrical field. Estimated mobility of the car-

    riers from the line slopes, using trapless insulator, is

    1:1 106 cm2 V1 s1. This value was found in severalstudies [3538]. In these studies the PVK films have been

    obtained by spin-coating which means that the PVK has

    a high molecular weight. In the present work, the films

    been obtained by evaporation, which means that, as

    discussed in others papers [24,3942] 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. Voltagecurrent characteristics lnJ lnV of PVK thinfilms obtained from: (a) pure PVK powder, (b) PVK obtained

    from iodine doped powder (3 at.%) first measure (), secondmeasure (s) and (c) PVK film obtained from post-doping

    technique.

    846 G. Safoula et al. / European Polymer Journal 37 (2001) 843849

  • the PVK films, whatever their origin, confirms 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

    dierent molecular weight.

    In high electrical field regimes, the experimental re-

    sults are often interpreted by either the Schottky eect or

    the PooleFrenkel eect. These two theories although,

    dierent in their principles, result in expressions of the

    same form: ln J / E1=2. These two theories are essen-tially dierent from each other by the following points:

    Schottky eect is limited to the surface of electrodes, PooleFrenkel eect is a volume eect. 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 field. The dropping DE0 of the barrier is ofthe form:

    DE0 e3E

    pe0e

    1=2 bPFE1=2 2

    where bPF is a PooleFrenkel constant and e an ele-mentary charge.

    The corresponding current density J is described by

    the following relationship:

    J A exp bPFE1=2 EakT

    3

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

    plied electrical fieldV/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 eect corresponds to the modification of the

    potential barrier of the metalinsulator interface.

    Emission of carriers is analogous to thermoionic emis-

    sion, the applied field contributes to decrease extraction

    work of metalinsulator.

    The corresponding current density is described in this

    form:

    J AT 2 exp bSE1=2 UkT

    4

    where A: SchottkyRichardson constant (A 120Acm2), U: work function of metal and bS: Schottkyconstant. It is calculated from this expression:

    bS e3

    4pee0

    1=25

    PooleFrenkel constant is twice as Schottkys. Thisratio is explained by the fact that in the first 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 4:38 105 eV m1=2 V1=2.In practice, it is often very dicult to distinguish

    between these two mechanisms of conduction solely, on

    the basis of these curves lnJ f lnE2. A solution tothis problem consists, in using PooleFrenkel conduc-

    tivity as a function of electrical field.

    In high electrical field regimes, our structures were of

    longitudinal type. Therefore PooleFrenkel eect should

    be more dominating as compared, to the surface eect

    (Schottky eect). In this case, the electrical conductivity

    under high electrical field must be pursuing Frenkels law.

    rPF r0 exp bPFE1=2 EakT

    7

    where r0: electrical conductivity corresponding to lowelectrical field.

    Fig. 4. Density of current versus electrical field J f E2 of PVK thin films iodine doped obtained by: (a) pre-doping technique (3at.%. iodine in pure PVK powder) and (b) post-doping technique.

    G. Safoula et al. / European Polymer Journal 37 (2001) 843849 847

  • The electrical conductivity of our samples was de-

    duced from the following relationship:

    rPF JE 8

    where J: through film current density (Am2) and E:applied electrical field (V m1).

    Fig. 5 represents the result obtained lnr f E1=2 .From this figure, two regions of distinguished conduc-

    tion can be seen (lower and higher electrical field).

    In region of high electrical field, the value of Poole

    Frenkel coecients calculated from the line slopes is

    bPF 4:74 105 eVm1=2 V1=2 (Fig. 5a) and bPF 4:53 105 eVm1=2 V1=2 (Fig. 5b) respectively for PVKthin film iodine pre and post-doped. These values con-

    form to the theoretical value.

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

    PVK films [1,34,43], PooleFrenkel eect is the most

    dominating conduction mechanism at high electrical

    field for PVK films iodine doped with either technique,

    however, in the case of thin films obtained from pre-

    doped method, an application of intense electrical field is

    necessary before the stabilisation mechanism, comes up.

    Intense electrical field induced probably ionisation of

    iodine, which provokes the behaviour observed. Subse-

    quently, modification of the electrical state of iodine,

    following the application of the electrical field has been

    already reported [5].

    The dierence noticed in structure and behaviour

    between these two types of iodine thin films doped is

    probably related to the dierence of iodine applied

    quantity in these films. Post-doped films contained more

    iodine and thus more ionised iodine in comparison to

    the pre-doped films. We have already showed that a

    ratio of I3 =I2 80% [22] exists in our doped films.

    The dierence of the electrical behaviour can be ex-

    plained by the variation in morphology and structure of

    these films, as post-doped films have a better surface

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

    (T 370 K) have a reticular structure [22]. Similarlypre-doped thin films, which are electrically unstable,

    evolve probably to reticular structure, following the

    application of an intense electrical field.

    Therefore, after stabilisation of the iodine doping,

    whatever the sample is, the electrical behaviour of the

    evaporated PVK films is similar to that of spin-coated

    films. 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 confirms that the

    carbazole group are not destroyed during the evapora-

    tion process, which corroborates earlier optical charac-

    terisation of these films [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 films and

    only iodine doped PVK samples could be extensively

    studied by electrical measurements.

    Fig. 5. Electrical conductivity versus electrical field ln r f E1=2 of PVK thin films iodine doped obtained by: (a) pre-dopingtechnique (3 at.% iodine in pure PVK powder) and (b) post-doping technique.

    848 G. Safoula et al. / European Polymer Journal 37 (2001) 843849

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

    dine concentration present in the films depends on the

    doping process. The iodine concentration in the post-

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

    higher than that present in the films obtained by evap-

    oration of a pre-doped powder.

    Therefore this study concerns mainly the electrical

    behaviour of PVK thin films iodine pre and post-doped

    under an electrical field. Post-doped PVK thin films

    seemed to be more stable than those obtained from pre-

    doped powder. The latter required an intense electrical

    field treatment before stabilisation.

    PVK thin films iodine doped present a conduction,

    which is controlled either by space charge or by ohmic

    conduction in both low and moderate field. Whereas the

    conduction mechanism seems dominated by Poole

    Frenkel eect.

    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 films. 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 films are similar to that of spin-

    coated PVK films. The main dierence between these

    two families of samples is the molecular weight of the

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

    than in the case of evaporated films. 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 confirms that

    the carbazole group is preserved during the evaporation

    process.

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