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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.3 Concepción sep. 2005

http://dx.doi.org/10.4067/S0717-97072005000300013 

 

J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 597-602

 

ELECTROPOLYMERIZATION OF HETEROARYLENE AZOMETHINES

 

FRANCISCO BROVELLI1, BERNABÉ L. RIVAS1*, JEAN CHRISTIAN BERNÈDE2

1Department of Polymers, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile. e-mail: brivas@udec.cl; FAX: 56+41+245974
2LAMP, Facultè de Sciences et des Tecniques, Université de Nantes, 2 Rue de la Houssinière 44322 BP 92208 Nantes Cedex 3, France


ABSTRACT

Organic diodes were obtained using a thiophene and furan azomethines derivative polymer electrochemically synthesized in nonaqueous media. The polymers were deposited on indium tin oxide (ITO) coated glass substrates. The films obtained were physicochemically characterized by electron probe microanalysis and X-ray photoelectron spectroscopy. The current­voltage (I­V) characteristics exhibit a turn-on voltage at about 2.5-3.5 V depending on the heterocycle nature. The electroluminescence­voltage (EL­V) curve has a similar shape but a shift of 1-2 V toward higher voltage. The charge injection mechanism is related with Fowler-Nordheim tunneling effect.

Key words: diodes; electrochemistry; ESCA/XPS.


INTRODUCTION

Since the first report of electroluminescence from organic conjugate polymers [1], efforts have been devoted to the design of light-emitting devices. Organic light-emitting diodes (OLEDs) are composed of three or more layers [2]. The first one is a transparent conductive oxide (TCO). The second layer is an organic active film, which can be a multilayer, in order to improve the electroluminescence efficiency [2­6]. The third layer is an aluminum film because of its relatively low work function and its good stability compared to that of metal with a lower work function such as calcium.

Polymers with thiophene units have often been used in OLEDs [7]. The modification of the properties of these structures with different substitution on the thiophene nucleus has been studied. It has been shown that the transport properties, the emitted light color, and the stability of the devices can be tailored by such process.

In this paper, we report a study of the charge carrier injection mechanism of electrochemically synthesized polymers. A discussion is developed on the current­voltage (I­V) curves measured at room temperature. For the active layer, we used a polymer based on a heteroarylene azomethines (see Figure 1). The principal characteristic of these polymers is that they have electrons, however there is high grade of rotation of the aromatic ring, which induces a limited electronic delocalization to the new polymers. Such limited delocalization should decrease the carrier leakage toward electrodes.


Figure 1. Structure of electrochemically polymerized monomers. (X,X) = (O,O): N,N'-bis(2-furylmethylene)-1,4-diaminobenzene and (S,S): N,N'-bis(2-thienylmethylene)-1,4-diaminobenzene.

EXPERIMENTAL PART

Synthesis and characterization of monomers

N,N'-bis(2-furylmethylene)-1,4-diaminobenzene (M1) and N,N'-bis(2-thienylmethylene)-1,4-diaminobenzene (M2) were synthesized from 2-furan and 2-thiophene carboxaldehyde and 1,4-phenylenediamine, respectively [8,9], following the scheme 1. The monomers were purified by dissolving in hot 10% ethanol aqueous solution, and it was extracted with CH2Cl2.

Scheme 1

Elemental analysis of M1. Found: C, 73.26; H, 4.14; N, 10.84; O, 11.76. Calculated: C, 72.72; H, 4.58; N, 10.60; O, 12.11. 1H-NMR Bruker 200 MHz (CDCl3) [ppm]: 7.14 (dd, 1H, 3.8, 3.8 Hz), 7.27 (s, 2H), 7.50 (dd, 2H, 5.2, 3.8 Hz), 8.62 (s, 1H). 13C-NMR Bruker 50 MHz (CDCl3) [ppm]: 152.2 (C2), 149.5 (C7), 147.0 (C6), 145.7 (C5), 122 (C8), 116.4 (C4), 112.2 (C3). FTIR pellets KBr [cm-1] nC-H 3150, nC=N 1635, dCHoop (2-sust) 890, 790, 735.

Elemental analysis of M2. Found: C, 64.53; H, 3.88; N, 9.39; S, 22.96. Calculated: C, 64.83; H, 4.08; N, 9.45; S, 21.64. 1H-NMR Bruker 200 MHz (CDCl3) [ppm]: 6.55 (dd, 1H, 1.75, 1.76 Hz) 6.95 (d, 1H, 3.35 Hz), 7.29 (s, 2H), 7.61 (d, 1H, 1.49 Hz), 8.32 (s, 1H). 13C-NMR Bruker 50 MHz (CDCl3) [ppm]: 152.4 (C6), 149.4 (C7), 143.0 (C2), 132.2 (C3), 130.3 (C4), 127.8 (C5), 122 (C8). FTIR pellets KBr [cm-1] nC-H 3100, nC=N 1630, dCHoop (2-sust) 860, 780, 727.

Electrochemical setup

The transparent conductive oxide (TCO) was a commercial ITO glass electrodes (Delta Technologies, USA). The whole glass substrate was fully covered; therefore, some ITO had to be removed. After masking a line 2 mm broad, the ITO was etched using Zn-HCl as an etchant [10]. Then, the substrates were cleaned using a H2O2 treatment following a process described by Osada et al [11,12]. The substrates were treated with a 80°C H2O-H2O2-NH4 OH solution (5:1:1 volume parts) for 20 min, followed by rinsing with boiling distilled water for 5 min [13].

The setup for cyclic voltammetry was described previously [14]. An ITO glass electrode (geometrical area : 0.4 cm2) and Ag/AgCl in a tetramethylammoniun chloride solution (Me4NCl) [15] were used as working and reference electrodes, respectively. The potential was adjusted with respect to the saturated calomel electrode (SCE). Platinum gauze was used as a counter-electrode. Acetonitrile (Aldrich, anhydrous) was stored in an atmosphere of dry argon and over molecular sieves (4Å). The supporting electrolyte was tetrabutylammonium tetrafluorborate (Bu4NBF4) supplied by Aldrich and was dried under vacuum at 60°C. The electropolymerization of monomers was carried out from solutions containing 1 mM of monomer and 0.1 M Bu4NBF4. Polymerization was achieved by cycling between -0.5 to 2.5 V vs SCE at 100 mVs-1.

Surface characterization

An X-ray source was used for ESCA measurements. High-resolution scans with a good signal/noise ratio were obtained in the C1s, S2p, O1s, and N1s regions of the spectrum. The quantitative studies were based on the determination of the C1s, S2p, O1s, and N1s peaks areas: 0.2, 0.44, 0.6, and 0.36 respectively, as sensitive factors. The deconvolution of the XPS peaks into different contributions and the quantitative interpretation were made after the subtraction of the background using the Shirley method [16]. The developed curve-fitting programs permit the variation of parameters, such as the Gaussian/Lorentzian ratio, the full width at half-maximum (FWHM), and the position and the intensity of the contributions.

I-V and El-V curves

Organic diodes were prepared by deposition of Al vaporized under vacuum (10-7 atm) onto the top of the polymeric film deposited onto a ITO glass substrate. The deposition rate was 10 nms-1, and the film thickness was about 100 nm. The curves I-V were measured by a Keithley 617 programmable electrometer, a Keithley 2000 multimeter, and a Lambda IEEE-488 programmable power supply Model LLS6060-GPIB interfaced to an IBM PC computer. Electroluminescene (EL) was detected through the transparent ITO electrode. The light output was detected using a silicon photodiode and a Keithley 617 electrometer.

RESULTS AND DISCUSSION

Electrochemical polymerization

The electrochemical polymerization of monomers has been performed in agreement with the reported in literature [17-19]. As it is observed in the figure 2a, two anodic peaks are shown that would correspond to the oxidation of the heterocyclic ring and the imine bond respectively. Unlike the previous case, for M2 a single peak is only observed, which would indicate that the reactivities of the heterocyclic ring and the imine bond are similar (see figure 2b). In both cases, by means of successive scans it is possible to obtain thin films on the electrodic surface. In both cases, the voltammetric profiles are irreversible, which gives account of the lost of the electronic delocalization, as a result of the oxidation of imine bonds [9]. However, it is possible diminish this effect with a precise control of the anodic potential limit. On the other hand, the difference between the potentials of anodic peaks, is a clear indication, that both species have different reactivities, which is measured of the energy of level HOMO of each molecule [17,18].


Figure 2. Anodic voltammetric profile of monomers. Interfase ITO/Monomer 1mM + Bu4NBF4 0.1M in acetonitrile, scan rate 100 mV/s. a) M1 and b) M2

Surface caracterization

Electron probe microanalysis (EPMA) has been used for qualitative analysis (see Table I). The composition of the polymeric thin films found are in agreement with the theoretical values.


Table I. Experimental and theoretical composition values obtained from EPMA measurements.

The surface composition was estimated by XPS. In both cases, there is some oxygen contamination (see Table II). These extra atoms can be related to air moisture contamination on the polymer surface. In the case of the polymer 2 also, there is some sulfur deficiency, confirming film contamination. The deconvolution of the XPS peaks is shown in Figure 3. The C1s spectra of the polymers showed asymmetric tailing. This is a consequence of considerable reorganization of valence electrons in the form of shake-up and/ shake off phenomena, in addition with the removal of a shielding electron [20]. That is indicative of long-range disordered conjugation [21,22]. The deconvolution of the C1s peak (see Fig. 3(a)) shows that four contributions are necessary to obtain a good fit between the experimental and theoretical curves. The first contribution, placed at 285.3 eV, can be attributed to the C-C bond. This C-C binding energy was taken as a reference, as is often the case [23]. The second one at 286.7 corresponds to C-N. However, it cannot be attributed to C-N only; therefore, it should be assigned to a mixture of C-N and C-OH. The third contribution at 288.5 eV corresponds to the C=O bonds. The fourth peaks (290.7 eV) can be assigned to COOH surface contamination. The O1s peak (see Fig. 3(b)) can be deconvoluted in two contributions: the first one, situated at about 532.1 eV, corresponds C-O-C bonds; the second one, at 534.4 eV, can be assigned to some absorbed H2O [23]. The nitrogen contribution (see Fig. 3(c) can be deconvoluted in two, which can be assigned to imine (399.5 eV) and amine (400.7eV) [8], that is the XPS analysis, confirms the partial oxidation of imine moieties in this polymer.


Table II. Experimental and theoretical composition values obtained from XPS measurements.


Figure 3. XPS peak deconvolution of polymer 1: (a) C1s; (b) O1s; (c) N1s level.

The XPS spectrum of polymer 2 indicates that the deconvolution of the C1s peak (see Fig. 4(a)) present four contributions. The first contribution, located at 285.2 eV, can be attributed to the C-C bond [23]. The second peak at 286.8 eV is difficult to assign. Its binding energy is too high to be attributed to C-S, but it is situated between the binding energy values corresponding to C-N and C-O. Therefore, it should be attributed to a mixture of C-N, and C-O, while C-S cannot be discriminated from the C-C contribution. The third contribution placed at 288.4 eV corresponds to the C=O bonds. The fourth peak (290.7 eV) can be assigned to COOH surface contamination.


Figure 4. XPS peak deconvolution of polymer 2: (a) C1s; (b) O1s; (c) S2p; (d) N1s level

The O1s peak (see Fig. 4(b)) can be splited into two contributions: The first one, situated at about 532.8 eV, corresponds to the C=O bonds; the second one, at 534.8 eV, can be assigned to absorbed H2O [23]. It is worth noting that the S2p line corresponds to a doublet (see Fig. 4(c)). The contributions at 164.8 eV and 166.0 eV, correspond to C-S bonds of the polymer and there is no presence of sulfur oxidized. The nitrogen contribution (see Fig. 4(d)) can be deconvoluted in two, which can be assigned to imine (399.2 eV) and amine (400.6 eV) [8]. The XPS confirm also the partial oxidation of imine moieties in the polymer 2.

I-V and EL-V mesurements

The figure 5 shows I-V and El-V curves obtained from device with the ITO/polymer/Al configuration. Current intensity increases rapidly with bias above 2.5 and 3.0 V for polymer 1 and polymer 2 (see figure 5a), respectively. As can be seen from the I-V curve, the threshold voltage is considerably low compared with PPV polymer [24] and very close to another polymers derived from thiophene [25]. The two curves have the same shape. It can be seen that the electroluminescence (EL) (see figure 5b) appears after the increase of the current in the forward direction.


Figure 5. (a) I-V curve and (b) corresponding EL-V, for the ITO/polymer/Al device under bias direct. Polymer 1 (close square) and polymer 2 (close triangle)

To interpret the I-V behaviour of the devices, the structure can be assimilated to classical Schottky model for an M/SC (metal/p-type semiconductor) contact. According to the thermoionic emission theory [26] the following equation is obtained:

with A* the Richardson constant; fB, the barrier height; and n, an ideality factor. It can be seen in Figure 6 that a good fit can be obtained in the turn-on voltage range. However, values of 3-6 are found for the ideality factor n, indicating that the model of a standard Schottky contact, for which values of n = 1­2 are typical, [25,26] is not applicable.


Figure 6. Semilogarithmic plot of I-V curves. a) ITO/polymer 1/Al device; (b) ITO/polymer 2/Al device.

Then Fowler-Nordhiem model has been found to be applicable in the high field domain (see Fig. 6), which presumes tunneling of charge carriers directly into the bands of semiconductor in that field domain.

The tunneling process is described by the Fowler­Nordheim theory [27]:

where F is the electric field

and

where h is the barrier height and m* is the effective mass of the holes in the polymer. Assuming that the electric field is constant across the device and that the holes effective mass is equals the free electron mass [28], the barrier height can be estimated. Figure 7 shows a plot of ln (I/F2) versus 1/F of a ITO/Polymer/Al device. As predicted by the theory, above the turn-on voltage, the plot is linear. Therefore, the barrier height can be estimated. These data shows a constant barrier height of 0.35 and 0.39 eV for ITO/polymer 1/Al and ITO/polymer 2/Al devices respectively.


Figure 7. Fowler-Nordheim plots for ITO: a) ITO/polymer 1/Al; b) ITO/polymer 2/Al.

The polymers used here, as other thiophene derivatives, are hole transporting layer (HTL) and, therefore, the I-V characteristics can be attributed to positive charge, since the smaller barrier height should be situated at the ITO/polymer interface. The HOMO and LUMO of the polymers have been estimated, the ITO electron affinity is about 4.7 eV, while the work function of Al is 4.3 eV. Therefore, the energy barrier at the ITO polymer interface is 0.6 eV for M1 and 0.65 eV for M2, while the electron energy barrier at the polymer/Al interface is around 0.75 eV, which confirms the attribution of the current to hole. Moreover, if we compare the theoretical barrier height values (0.35 eV and 0.39 eV) to the experimental values (0.60 eV and 0.65 eV) it can be seen that they are not good values. The difference between experimental and theoretical values has been already encountered. It should be noted that the barrier height calculated corresponds to a rough estimation since m* values have been taken equal to the free electron mass.

CONCLUSIONS

Polymers based on heteroarylene azomethines were electrochemically synthesized. The structures ITO/Polymer /Al based on these polymers exhibit rectifying behavior. When the current is enough high, there is also light emission. The charge injection mechanism can be atributted to Fowler-Nordheim tunneling effect

ACKNOWLEDGEMENTS

The authors thank to FONDECYT (Grant No 3020010) for the financial support.

 

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