Synthetic Metals 146 (2004) 187–196

Doping of polyaniline by transition metal salts: current–voltagecharacteristics of the ITO/polymer film/metal heterostructures

P.S. Smertenkoa, O.P. Dimitrieva,∗, S. Schraderb, L. Brehmerb

a Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Pr. Nauki 45, Kiev 03028, Ukraineb Institute of Physics, Potsdam University, Am Neuen Palais 10, 14469 Potsdam, Germany

Received 29 April 2004; received in revised form 29 June 2004; accepted 1 July 2004Available online 12 August 2004

Abstract

Films of emeraldine base of polyaniline (PAni) doped by various transition metal salts have been prepared, and current–voltage characteristicsof the indium-tin oxide (ITO)/PAni film/metal electrode heterostructures were investigated. It was found that the electrical characteristicsof the heterostructures are greatly affected by the dopant used and the metal electrode used. Different dopants resulted in different currenta the currentb ound at lowa de regimew ntial in thefi y increasedo©

K

1

bitdfismt

cbc

ted inin-

lmshtly,ainsr for

-

lms.pre-oachtheive a

withthe

ular

0d

nomalies with asymmetric current–voltage characteristics. Depending on the dopant used, the exponential and power law ofehavior can be distinguished. Depending on the metal electrode used, two different regimes of current passing have been fpplied voltages, namely, a nearly ohmic regime for the indium electrode, and a diode regime for the aluminum electrode. The dioas found to accompany by a positive charge accumulation in the film near the film/metal interface, which creates a built-in potelm. The amount of positive charges accumulated at the interface and therefore the value of the built-in potential can be reversiblr reduced by successive runs of the applied voltage in the forward or reverse direction, respectively.2004 Elsevier B.V. All rights reserved.

eywords:Polyaniline; Transition metal salt; Doping; Current–voltage characteristics

. Introduction

Last years, organic film/inorganic metal heterojunctionsecome widely used in organic electronic devices, especially

n organic light-emitting diodes (OLEDs) and field-effectransistors (OFETs). Although extensive work studying theevice properties has been performed, and suitable organiclm-inorganic metal pairs have been selected for their con-truction, there are still some problems to understand theechanisms of charge transport through such a heterojunc-

ion.Polyaniline (PAni) is frequently used as an element for

onstruction of OLEDs or OFETs. Its insulating emeraldinease (EB) form can be doped by protonic acid, leading to theonducting emeraldine salt (ES) form. Properties of some of

∗ Corresponding author. Fax: +380 44 2655530.E-mail address:dimitr@isp.kiev.ua (O.P. Dimitriev).

the ES/inorganic metal heterojunctions have been reporthe literature. It was found for example, that copper anddium electrodes form excellent ohmic contacts to PAni fi[1], while aluminum electrode leads to non-ohmic, sligrectifying current–voltage (IV) characteristics[2]. Howeverthe underlying physics of such a different behavior rembeyond the frames of good understanding. It is not cleaexample, why Al with its work function (Φ = 4.2 eV) being intermediate between that of In (Φ = 3.9 eV) and Cu (Φ= 4.47 eV) does not create an ohmic contact to PAni fiStrictly speaking, an amorphous polymer film cannot besented in terms of the band theory. Therefore an apprbased on the relationship of electron work functions ofcontacting materials seems to be not acceptable to grecipe for creating ohmic or rectifying heterojunctions.

Recently, we have shown that EB can interact directlyvarious metal ions that induces doping of the polymer onmolecular level and leads to the formation of macromolec

379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2004.07.001

188 P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196

complexes in solutions[3] and to the increase of conductivityin polymer films[4]. So, chemical interactions of PAni anda metal electrode at the film/metal interface can be antici-pated, which affect charge transport through the heterojunc-tion substantially. In addition, doping of PAni by transitionmetal ions induces various changes in the film morphology,and these changes correlate well with the changes in the filmconductivity[4,5]. In this work, we investigate the electricalcharacteristics of PAni films doped by transition metal saltsin more detail. Our goal is to study how the change of thedopant used as well as the change of the top electrode affectsIV characteristics of the ITO/PAni-based film/metal electrodeassembly.

2. Experimental

2.1. Sample preparation

Emeraldine base of polyaniline was dissolved in N-methylpyrrolidinone (NMP) to prepare a stock solution withconcentration of 1 wt.%, followed by treatment of the so-lution in an ultrasonic bath and filtering of the solutionto remove undissolved particles. Salts of rare-earth metals(EuCl3, Eu(NO3)3, Sm(NO3)3, Nd(NO3)3) and transitionm asd solu-t mera dinga rdert ) tom ixeds 0%)a

nce∼ d ind B-i erep theI mmi oughm efi thet thee NMPf theo by,f ughs

2

darda wasa n oft spec

tively. All characteristics were measured at room tempera-ture. The differential approach was used for analysis ofIVcharacteristics. The differential method of the analysis con-sists in determination of the dimensionless parameters�(V)and�(V),

α(V ) = d(lgI)

d(lgV )= V

I

(dI

dV

)(1)

γ(V ) = d(lgα)

d(lgV )= V

α

(dα

dV

), (2)

whereV is the applied bias andI is the current passing throughthe structure. While valueα is the exponent in the power lawdependenceI(V) ∼ Vα, γ is the exponent in the exponentiallaw is I(V) ∼ exp(Vγ ) [6,7]. Singular points and certain re-gions (minima, maxima, constancy range, inflection, etc.) ofα(V) andγ(V) dependencies can be determined, and thesepoints correspond to the certain charge flow and recombina-tion mechanisms[7–13].

3. Results and discussions

3.1. DC electrical conductivity: general behavior inITO/doped EB/metal electrode heterostructures

3nal

d Thee sev-e sitiveb thep

oesn olutev ther herv rentw f thea viorua cteris-t y thep=a

lopebi ge,α lowedb ias,s ion= opeα argec asa nters

etals (NiCl2, ZnCl2, FeSO4, LaCl3) have been usedopants. The salts were dissolved in NMP to prepare

ions with desirable concentrations. A mixture of the polynd inorganic salt solutions was prepared normally by adfew drops of the salt solution to the polymer one in o

o maintain a ratio of the EB (calculated in tetramer unitsetal ions as 1:1. Concentration of the polymer in the m

olution was only slightly reduced (by value less than 2s compared with that of the stock solution.

Glass substrates covered by ITO (with resista60�/square) were cut into 20 mm squares, cleaneistilled ethanol and dried in dust-free air. Complex E

norganic salt films with thickness of 10–15 microns wrepared by drop-casting from the solution mixture onto

TO substrates. Top electrodes (aluminum or indium) 3n diameter and 100 nm in thickness were deposited thr

asks in vacuum at 10−4 Torr within several days from thlm deposition. Additional pressure contact was made toop electrode by an indium pad. The delayed step withlectrode deposition was necessary to remove excess of

rom the film because of slow solvent evaporation. Onther hand, a rapid removal of solvent from the films

or example, annealing was found to result in a very rourface morphology, so it was not employed.

.2. Electrical measurement and characterization

Quasi-DC measurements were performed by a stanutomated tester 14 TKS-100 (Russia). Voltage sweeppplied to the heterostructure step by step with duratio

he each step 250 ms and measurement time 90 ms, re

-

.1.1. Diode-like behavior in HCl-doped samplesTheIV characteristics of EB films doped by a conventio

opant, i.e. by hydrochloric acid, were investigated first.lectrical behavior of the ITO/ES/Al heterostructures hasral main features. Forward bias is defined here as a poias on the bottom ITO electrode, i.e. a positive bias on-type organic layer relative to the top contact.

Under forward bias, in the range of up to 8 V, current dot increase monotonously. In many samples the absalue of current was found to rapidly decrease to 0 inange of 0.2–1.0 volt followed by its rapid increase at higoltage (Fig. 1a). Meanwhile under a reverse bias, the curas found to monotonously increase in the whole range opplied voltage. In order to compare the electrical behander forward and reverse biases,IV characteristics inFig. 1are presented in the same quarter. The electrical chara

ics of the HCl-doped samples can be modeled here bower law, i.e.I(V) ∼ Vα, whereα can be fitted by valuesα1.49 andα = 1.22 in the region of∼2–10 V for the forwardnd reverse bias, respectively.

The differential method finds a fine structure of the sehaviour of theIV characteristics. It can be found thatα = 1.5

n the region of 0.8–2 V under forward bias. At higher voltadecreases to 1.25 and then increases again to 1.5 foly its further increase to more than 2.5. Under reverse bαhows an increase from 0.3 to 1.5, with the extended regα1.25 within applied voltages of 3–7 V. As known, the sl= 1.5 corresponds to bimolecular recombination of ch

arriers[8–10]. The model of bimolecular recombinationpplied to inorganic semiconductors implies that deep ce

P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196 189

Fig. 1. (a)IV characteristics of the ITO/(EB:HCl)/Al heterostructure. Circles: forward bias; stars: reverse bias; (b) and (c)α for the forward and reverseIVcharacteristics, respectively.

or traps are initially filled by electrons mostly[9]. All theinjected holes will recombine then with the electrons at thedeep centers. That means that there is no restriction for thehole recombination. As to our case, we shall see that traps inthe polymer film are filled initially by the positive charges orholes.

3.1.2. Effect of the dopant usedChange of the dopant used was found to affect conduc-

tivity of PAni films and to induce further changes inIVcharacteristics of the ITO/complex EB film/Al heterostruc-tures. It was found that one group of dopants, namely LaCl3,EuCl3, NiCl2, results in highly asymmetricalIV characteris-tics (Fig. 2), whereas the other group of dopants that includesZnCl2, FeSO4, nitrates of lanthanide metals, results in moresymmetricalIV characteristics (Fig. 3). These results are con-sistent with our recent findings[5] that the above two groupsof dopants lead to different changes in the film morphology.Dopants of the first group lead to a highly grained structureof the polymer film, while dopants of the second group donot lead to the formation of grains, but retain a large amountof solvent in films cast from a solution, which plasticizes thepolymer films.

The polymer films doped by transition metal salts showa more complex electrical behavior as compared with theH s. Int neari h thei ationom

by a small decrease and further increase of current at highervoltages.

The differential analysis ofIV characteristics of the sam-ples concludes on principally different electrical behavior offilms doped by the first and second group of salts, respec-tively. Electrical characteristics of films doped by the secondgroup of salts in the region without current anomalies, i.e. inthe range of∼0.3–2 V, can be fitted by the power law, that issimilar to the HCl-doped samples described above, withα be-ing in the range of 1.25–2.00 for the forward bias (Fig. 3c–e)and in the range of 1.0–1.5 for the reverse bias (Fig. 3g and f).The electrical behavior of samples doped by the first groupof salts in the same region, i.e. in the region without currentanomalies, cannot be fitted by the power law, becauseα isincreasing above 3.0 (Fig. 2b), so that theIV characteristicsunder forward bias can be approximated by the exponentiallaw, I(V) ∼ exp(V�), with γ = 0.3 (Fig. 2c).

3.1.3. Current instability and irreproducibilityIn addition to the complex behavior ofIV characteristics,

it was found that current can vary at a constant applied volt-age, i.e. it is unstable. The current changes occur by leapsaround an average value, therefore those have a character ofnoise. The current leaps were observed both under forwardand reverse biases (see, for example,Fig. 2). As was men-t llyw alsoc ity isq amec ionsc or-

Cl-doped samples, especially that under forward biahe region of 1–3 V we observed a more rapid, superlincrease of current under forward bias as compared witncrease of current under reverse bias, followed by saturf the superlinear increase (Figs. 2 and 3). A distinct maxi-um in the range of 3–8 V can be observed (Fig. 2), followed

ioned in the previous work[5], these leaps occur normaithin 10% of the average value, however, higher leapsan be observed. It was also noted that the film conductivuite sensitive to preparation conditions of films of the somposition; small variations in the preparation conditan result in variations of the film conductivity within 1–2

190 P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196

Fig. 2. (a) TypicalIV characteristics of the ITO/(EB:EuCl3)/Al heterostructure. Circles: forward bias; stars: reverse bias; (b) and (c)α andγ for the forwardIVcharacteristics, respectively.

ders of magnitude. Therefore, if the film is inhomogeneous,small variations in the current pathways can result in largecurrent leaps and thus the above effect should have an intrin-sic origin. It is interesting that current instability was foundto be higher for samples doped by salts of the first group. Thisresult is consistent with a more rough, grained structure ofthese samples as compared with films doped by the secondgroup of salts.

In addition, it was found that current values are often ir-reproducible. As an illustration of this irreproducibility, afilm doped by ZnCl2 demonstrated a gradual change of theIV characteristics under successive runs of both forward andreverse bias, respectively (Fig. 3). Samples doped by otherdopants also showed irreproducibleIV characteristics, how-ever, the character of the current behavior there was morecomplex and dependent on the dopant used. As will beshown below, the irreproducibility effect is not of stochas-tic origin, but related to the trap charging and dischargingphenomena.

3.1.4. Effect of the top electrodeIt was found that the top electrode affectsIV characteris-

tics significantly. Change of the aluminum electrode for theindium one resulted in more linear and symmetricIV charac-teristics, with the reduced or completely suppressed currentanomalies. These results were especially typical for samplesd

It was found that samples doped by nitrates of rare-earthelements and with In electrode, show almost ohmic behavior,with excellent reproducibility ofIV characteristics (Fig. 5).The differential analysis has shown that the electrical behav-ior of these samples can be fitted by the ohmic law only atlow applied voltages, becauseα is smoothly increasing fromα = 1.0 toα ∼ 2.0 at higher voltages (Fig. 5b–d). However,IV characteristics for these cases are completely symmetri-cal, andα follows the same behavior for both forward andreverse biases.

3.2. Anomalous behavior of DC electrical conductivity

3.2.1. Trap charging and discharging phenomenaThe fact that the current is equal to 0 only at a nonzero

forward bias applied to the film, i.e. in the range of 0.2–1.5 V(Figs. 1a and 2a), points out the presence of a built-in poten-tial in the heterostructure. Such a built-in potential is commonfor heterostructures with different top and bottom electrodes,and it is usually assigned to the contact-potential differenceof the two electrodes[14]. However, we argue that this built-in potential can be of the different origin, because it can bereversibly changed for the same heterostructure, as will beshown below. We suppose that this potential is created bycharged traps in the polymer film and a barrier against move-ment of these charges to electrodes under short circuit. Thep ces

oped by the second group of dopants (Fig. 4). hysical significance of the built-in potential is that it redu

P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196 191

Fig. 3. IV characteristics of the ITO/(EB:ZnCl2)/Al heterostructure. (a) Circles: first; squares: second; triangles: third run under forward bias; (b) crosses: first;bars: second run under reverse bias; (c); (d); (e)α for theIV characteristics in (a); (f); (g)α for theIV characteristics in (b), respectively.

192 P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196

Fig. 4. IV characteristics of the ITO/(EB:ZnCl2)/In heterostructure. Circles:first, squares: second run under forward bias; crosses: first, bars: second rununder reverse bias.

the applied voltage such that a net drift current in forward biasdirection can only be achieved if the applied voltage exceedsthe built-in one. Because the built-in bias can be equalizedonly by the applied voltage in a forward bias direction, hence

Fu

the built-in bias is due to the trapped charge distribution whichcreates a positive potential near the film/metal interface rel-ative to the ITO electrode, and a negative potential near thefilm/ITO interface relative to the top contact. Such a distribu-tion can be created either by positively charged traps or holesnear the metal/polymer interface or negatively charged trapsnear the ITO/polymer interface. A noticeable built-in poten-tial was revealed only in heterostructures with Al electrode;in heterostructures with In electrode it was too negligible, ifany, although these metals have close values of their workfunctions. Therefore, one can conclude that the phenomenonis affected by the top electrode, and that the positive chargesare located near the metal/film interface. This conclusion isconsistent with the fact that PAni is ap-type semiconductorand serves as a hole-injection layer in OLEDs[15]. It is in-teresting that Constantino et al. reported that the potential ofPAni films is always positive in respect to the contacting metalelectrode[16], that is in good agreement with our results.

The presence of positive charges or holes in the film allowsto easy remove these charges through the film/ITO interfaceunder applied reverse bias; in the case of applied forward biasthese will be accumulated at the film/metal interface due tothe barrier at this interface. The presence of this barrier canbe evidenced by the increase of the built-in potential as a re-sult of successive runs of the applied voltage in the forward

ig. 5. (a)IV characteristics of the ITO/(EB:Eu(NO3)3)/In heterostructure. Circlender reverse bias; (b); (c); (d)α for the first, second and third run of the forwar

s: first, squares: second, bars: third run under forward bias; crosses: first rund bias, respectively.

P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196 193

Fig. 6. IV characteristics of the ITO/(EB:EuCl3)/Al heterostructure undersuccessive runs of the applied forward bias.

bias direction (Fig. 6). In this process, the positive chargeaccumulation occurs near the barrier at the film/metal inter-face which gives rise to the built-in potential increase. Onthe other hand, a reverse bias does not affectIV characteris-tics significantly (Fig. 7). On the contrary, this bias reducesthe built-in potential, evidently, due to the trap discharging.Moreover, after several runs of the applied reverse bias theIV characteristics become almost symmetrical and withoutanomalies (Fig. 8, solid curve).

The important conclusion from the above results is that thediode-like behavior in the metal/PAni/ITO heterostructures iscontrolled mainly by the trap charging/discharging phenom-ena, but not by the relationship of work functions of the metalelectrode and the polymer film. Indeed, it is known that un-

F rs osses:s

Fig. 8. Effect of recovering the forwardIV characteristics inITO/(EB:EuCl3)/Al heterostructure. Solid circles: after several runsof the applied forward bias; open circles: after 30 min recovering withoutapplied voltage; dashed curve: after one run of the applied reverse bias;solid curve: after several runs of the applied reverse bias.

doped and acid-doped polyanilines have work functions inbetween those of Al (Φ = 4.2 eV) and Au (Φ = 5.1 eV)[2].Therefore, both Al and In (Φ = 3.9 eV) electrodes would re-sult in similarIV characteristics. However, In electrode doesnot yield trap charging and results in almost symmetrical(Fig. 4) or completely symmetrical (Fig. 5a) IV character-istics, whereas Al electrode causes trap charging and resultsin highly asymmetrical diode-like characteristics (Figs. 1–3).On the other hand, trap discharging leads to dramatic changesof the diode-like characteristics, making them almost sym-metrical even in heterostructures with Al electrode.

3.2.2. Self-healed current anomaliesIt should be noted that although many samples possessed

current anomalies, in some cases we observed the effectof self-healing, i.e. reduction or complete disappearance ofthese anomalies under successive runs of the applied bias.

As was mentioned, one of these anomalies is current insta-bility. We observed that a noisy switching behavior of currentin samples doped by the second group of salts can be sub-stantially reduced upon subsequent runs of the applied bias(Fig. 7). The mechanism of this phenomenon is unknown.We suppose that current can induce some effect of thermalannealing leading to removal of some defects and/or stabi-lization of current pathways. Another current anomaly causedby trap charging and a barrier near the top electrode, can bea volt-a ts oft

3fi

fac-t The

ig. 7. IV characteristics of the ITO/(EB:EuCl3)/Al heterostructure undeuccessive runs of the applied reverse bias. Solid curve: first run; crecond run; bars: fifth run.

lso removed in some cases by subsequent applying age in the forward direction, for samples doped by sal

he second group, for example, ZnCl2 (Fig. 3a).

.3. Mechanisms of conductivity in metal/PAni-basedlm/ITO heterostructures

Based on the above data, we emphasize two mainors influencing charge transport in the PAni-based films.

194 P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196

first one is the dopant used. In addition to the conventionalchemical interaction of the dopant with PAni chains, the in-organic salt affects morphology of the PAni film[4,5], thuscreating different structural defects, which can serve ascharge traps.

The second factor is the metal electrode used. The roleof the electrode is dramatic and can be additionally clarifiedby comparingIV characteristics of the HCl-doped PAni filmswith evaporated In electrode, In pad contact made by pres-sure, and evaporated Al electrode. In this series,α changesfrom nearly 1.0 (ohmic regime) for samples with evaporatedIn electrode to∼1.5 and higher for samples with In pad con-tact and Al electrode. So, chemical interaction of the metaland the polymer at the interface and/or oxidation of the elec-trode is expected to be responsible for deviation ofIV char-acteristics from the ordinary behavior. Otherwise, both Aland In electrodes would result in similarIV characteristics,because their work functions have close values. Al3+ ionsare known to be able to coordinate to PAni chains result-ing in formation of the EB:Al complex which precipitates inthe NMP solution[5]. So, formation of an interfacial layerwith different properties than the bulk materials as a result ofthe metal-polymer interaction is expected to be highly pos-sible. This conclusion is consistent with the results of Tan etal. [17], where it was proved by X-ray photoelectron spec-t anta then thec ulk-a sitiont h asA

ist itede asesc fp ded ygent r ex-p ectlya troded . Sucha lim-i ida-t elec-t hs rvedf alsord iffu-s Ref.[

dei fac-t odei cal

Fig. 9. Comparison ofIV characteristics of the ITO/(EB:EuCl3)/Al (circles:forward bias; bars: reverse bias) and ITO/(EB:EuCl3)/TCNQ/Al (squares:forward bias; crosses: reverse bias) heterostructures.

of PAni films doped by EuCl3, NiCl2, appears to promotedeeper penetration of metal species into the film. PAni filmsdoped by ZnCl2 and lanthanide nitrates are more highlyplasticized by the entrapped solvent, so that such a struc-ture does not allow for deep penetration of metal into thefilm.

In order to model properties of the interfacial layer whichcan be formed as a result of the metal–electrode interaction,we deposited a thin buffer layer of TCNQ in between thecomplex polymer film, namely EuCl3-doped EB, and Al elec-trode. TCNQ is known as a good electron acceptor in respectto EB [21], so it should create a certain amount of positivecharge or holes near the polymer/TCNQ interface as a re-sult of the electron transfer from the EB to TCNQ.IV char-acteristics of the ITO/polymer film/TCNQ/Al heterostruc-ture showed high values of the built-in potential and currentanomalies, which were even higher than in assemblies with-out TCNQ (Fig. 9). So we concluded that an interfacial layerformed in between the polymer and the top electrode pos-sesses similar properties, i.e. it has electron-accepting prop-erties. Thus, the polymer-metal interfacial layer as an elec-tron acceptor can be a source of the positive charge on thepolymer traps and, additionally, it creates an electrostaticbarrier for holes injected from the ITO electrode againsttheir motion to the cathode under forward bias. This is alsot suchh ardb sot otherh for-w ects.T se oft ap-pi film

roscopy that Al atoms interact preferentially with the dopnions in the case of the protonated PAni films, and withitrogen atoms of the conjugated polymer backbone inase of the neutral PAni film, and that the migration of bdsorbed oxygen to the surface in response to Al depo

akes place, with the formation of aluminum oxides, sucl2O3.Adsorption of atmospheric gases by the polymer film

he other problem that influences quality of the deposlectrode. It is well known that adsorbed atmospheric gontaminate sample surfaces and affectIV characteristics oolymer films[18,19]. The delayed step with the electroeposition in our experiments could be a reason for ox

o adsorb on the film surface. It was found in some of oueriments that the Al electrode faded spontaneously dirfter the deposition, as compared with a reference eleceposited onto glass surface under the same conditionschange was noted mainly for samples, which were pre

narily stored in open air, and it therefore signals on oxion of the metal at the polymer surface. This changedrode yielded highest anomalies inIV characteristics, witteep changes inα. The analogous phenomenon we obseor the copper electrode. Copper cations are known toeadily form complex with PAni chains[5], and PAni oxi-izes copper atoms at the Cu/PAni interface, with the dion of copper cations into the film, as was shown in20].

In addition to chemical reactivity of the metal electron respect to PAni, film morphology can be the otheror influencing penetration of metal ions of the electrnto the film. A grained, highly porous structure typi

he reason for the appearance of a built-in potential ineterostructures. A threshold value of the applied forwias is needed to overcome the built-in bias of the film

hat the injected holes can reach the cathode. On theand, when holes are injected into the film under appliedard bias, many of them can be trapped by the film defhe trap-charging phenomenon is the reason for increa

he built-in bias in the film upon successive runs of thelied voltage in forward bias direction (Fig. 6). On apply-

ng a reverse bias, holes become withdrawn from the

P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196 195

Fig. 10. Energy diagram of the anomalous electrical behavior in ITO/(EB:EuCl3)/Al heterostructures.

through the ITO electrode, thus providing trap-dischargingphenomenon.

Anomalous behavior ofIV characteristics can be consid-ered in terms of the energy diagram (Fig. 10). On short circuitof the ITO/PAni/Al heterostructure (U = 0 in Fig. 10) the ex-cess of holes trapped near the metal/polymer interface driftsto the ITO electrode, driven by the built-in potential equal toU0. This creates an opposite current in respect to the directionof the applied forward bias. As the forward bias increases itbecomes equal to the built-in potential (U=U0 > 0 inFig. 10).Since this moment free holes become injected from the ITOelectrode giving rise to a rapid increase of current with the ap-plied forward bias, in the regionU1 +U0 >U >U0 (Fig. 10).However, there is a barrier for holes at the Al/polymer inter-face, so that the holes should tunnel through this barrier. Atunneling character of the charge carrier transport at moder-ate values of the forward bias gives rise to the maximum inIV characteristics atU =U0 +U1 (Fig. 10). At higher forwardbiases (U > U0 + U1 in Fig. 10), the voltage becomes com-parable with the barrier height and charge transport changesfrom tunneling to the drift behavior.

4. Conclusions

ITO/complex PAni film/metal heterojunctions show dif-ferentIV characteristics depending on the dopant and metalelectrode used. One group of dopants, i.e. EuCl3, NiCl3,LaCl3, leads to very complex and asymmetricIV characteris-tics, with the differential slope being normally more than 3.0under forward bias. This electrical behavior can be modeledby exponential distribution of traps in the polymer film. Cur-rent anomalies observed in the above samples are consistentwith this model. The other group of dopants, i.e. nitrates ofrare-earth metals and ZnCl2, results in more symmetricalIVcharacteristics, with the reduced or completely suppressedcurrent anomalies, and with the differential slope normallybeing in the range of 1.0–2.0 under forward bias. This elec-trical behavior can be approximated by the power law.

Unusual electrical behavior in ITO/complex PAnifilm/metal heterostructures was found in reply to the deposi-tion of aluminum as the top electrode. The anomalous behav-ior was explained by trap charging/discharging phenomenanear the polymer/metal interface. Contamination of the poly-

196 P.S. Smertenko et al. / Synthetic Metals 146 (2004) 187–196

mer surface by atmospheric gases and chemical reaction ofmetal electrode with the polymer surface are probably themain factors contributing to the above processes. Creation ofa barrier and accumulation of positive charges near the poly-mer/metal interface dramatically changeIV characteristicsof the heterostructures from a nearly ohmic to the diode-likeregime.

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