Journal of the Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.1 Concepción mar. 2007
J. Chil. Chem. Soc., 52.No. 1 (2007)
Synthesis of polymeric thin films by electrochemical polymerization of 1-furfuryl pyrrole. Characterization and charge injection mechanism.
Francisco Brovelli1, Bernabé L. Rivas2*, Jean Christian Bernède3
1) Laboratorio de Materiales Compuestos (LMC-ACC), Universidad del Bío-Bío, Casilla 5-C, C
2) Departamento de Polímeros, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile. email@example.com
3) LAMP, Facultè de Sciences et des Tecniques, Université de Nantes, 2 rue de la Houssinière 44322 BP 92208 Nantes Cedex 3, France.
The polymeric thin film was synthesized by electrochemical polymerization of 1-furfuryl pyrrole on indium tin oxide (ITO) substrates in aprotic organic media. The cyclic voltammograms and DFT calculations indicate that the electrochemical polymerization process would involve pyrrole rings, although oxidation of the furane ring could also take place. The organic thin films were physicochemically characterized by scanning electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy. The surface morphology has shown that the polymer deposition process has high coverage efficiency. The polymeric layer is very thin with an approximate thickness of 150 nm. When the direct bias is applied to the device, the behaviour is ohmic at low voltage; but once an appropriate value is reached, the current begins to increase exponentially. The current–voltage (I–V) curves exhibit rectifying behaviour with a turn-on voltage at 0.70V. The ideality factor n values found are 9-15, indicating disagreement with the model of a standard Schottky contact.The charge injection mechanism in this device was attributed to the tunnelling effect.
Key words: Charge transport, diodes; electrochemistry, ESCA/XPS, thin films
Polypyrrole (PPy) is one of the most studied conducting polymers. It is well known that the mechanical, physical, and chemical properties of PPy strongly depend on the nature of the dopant anion. PPy layers have been used to build chemical sensors in which the organic semiconductor works both as the active component in the electronic device structure and as the chemical sensing transducer element, for example, polymer field-effect transistors, PPy/Au-Schottky, and PPy/Si-heterojunction diodes.
Organic light-emitting diodes (OLEDs) consist in three or more layers and are used to improve the electroluminescence efficiency [1-5]. The junction parameters of the polymer-based diodes, such as the ideality and the rectification factor, are significantly influenced by the dopant nature in the conducting polymers [6-11]. Rectifying junctions are the basic elements of many electronic components. Since the discovery of conducting polymers, rectifying junctions such as the p–n and Schottky junction have been studied to explore the possible application of conducting polymers. In a Schottky diode, some parameters, such as the ideality factor, barrier height and the series resistance effect, provide useful information concerning the diode’s nature. In the Schottky’s classical model, adiode assumes the junction to be abrupt with a fixed barrier height.
Heterocyclic polymers have often been used in OLEDs . The electrical analysis of the (PPy)/p-Si structure shows that the diode ideality factor and the barrier height are n= 1.78 and fb= 0.69 eV, respectively, using the thermo-ionic emission theory . In the PPy/p-InP structure, the measured capacitance decreases with increasing frequency due to a continuous distribution of the interface states in the frequency range. From the I–V curves, the ideality factor and barrier height (BH) values found are 1.68 and 0.59 eV, respectively. The diode shows non-ideal I–V behaviour with an ideality factor higher than the expected value. This result is attributed to the interface states and the device’s homogeneity barrier . The same behaviour has been found in the PPy/n-Si structure . Additionally, metallic polypyrrole polymer (MPP)/n-InSe(:Er) Schottky barrier diodes show that the metallic polypyrrole film provides a good rectifying contact to the n-InSe(:Er) semiconductor. In this case, the diode shows non-ideal I–V behaviour with an ideality factor > 1.0 .
PPy, electrochemically polymerized using tetraethylammonium toluenesulfonate (TOS), nickel phthalocyanine toluenesulfonate (NiPcTS), or copper phthalocyanine toluenesulfonate (CuPcTS) as dopants, shows that the I–V curves characteristic of these junctions are asymmetrical and non-linear. The junction parameters, such as the ideality and rectification factor, are sharply influenced by the conducting polymer’s dopant .
In the heterojunction diodes fabricated with silicon porous (SP) and soluble (PPy), Au/PPy–PS/Al devices, the diodes’ rectifying characteristics were significantly improved. Moreover, it was found that the heterojunction diodes’ rectifying characteristics were strongly dependent on the doping degree, dopants, and solvents for the doped PPy films [18,19]. PPy films have been also used as semi-transparent anodes and poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) as the luminescent polymer. The hole-injection potential barrier between PPy and MEH-PPV determined by Fowler-Nordheim analysis, was 0.23 eV .
Metallic polypyrrole provides a good rectifying contact to the p-Si semiconductor. The current-voltage (barrier height fb0 = 0,84 eV) and capacitance-voltage (fb0 = 0,94 eV) characteristics of the devices are significantly improved by increasing fb0 and decreasing the ideality factor (n = 1,20). These fb0 values are significantly higher than those of conventional Schottky diodes .
In this type of device, metal-organic layer interface determines the injection and conduction mechanisms. In general, the current–voltage measurements may indicate two conduction mechanisms: a conduction limited by traps at lower forward bias and a space charge limited conduction (SCLC) regime at higher forward voltages. In this device, injection and conduction are dominated principally by holes .
In the studied case, there are few articles in the literature related to electronic devices prepared from polyfurane, and consequently it is interesting to study this type of molecules from a technological point of view [23-25]. Accordingly, we report an investigation of the charge carrier injection mechanism of electrochemically synthesized polymer. We address the discussion on the current–voltage (I–V) curves measured at room temperature. For the active layer, we used an electropolymerised organic material, 1-furfurylpyrrole (1-FP) (see Figure 1).
1-furfurylpyrrole (99%) was supplied by Aldrich (Milwaukee, USA), and purified by distillation under vacuum. Indium tin oxide (ITO) glass (Delta Technologies, USA) was used as working electrode. The electrodic substrate was treated following methodologies described in the literature [26-29].
The cyclic voltammetry (CV) setup has been described previously . Pre-treatedITO glass (geometrical area 0,4 cm2) and Ag/AgCl electrodes were used as working and reference electrodes, respectively. The electrolyte was a tetramethyl ammonium chloride solution (Me4N+Cl-) . The voltage was adjusted with respect to the saturated calomel electrode (SCE). Platinum gauze, separated from the working electrode compartment, was used as counter-electrode. The supporting electrolyte was tetrabutyl ammonium tetrafluorborate (Bu4NBF4) supplied by Aldrich and was dried under vacuum at 40°C. The electrochemical polymerizations were carried out in solutions containing 1 mM of monomer and 0.1 M Bu4NBF4. The solvent, anhydrous acetonitrile, was stored in a dry argon atmosphere and over molecular sieves (4Å). Scans were performed by cycling the voltage between –0.5 to 2.2 V vs SCE at 100 mVs-1.
The ab-initio calculations were performed using MacSpartanPro software . The electron density was determined by Density Functional Theory (DFT) B3LYP. The molecular structure was optimized to a gradient norm of <0.0001 a.u.in gas phase. The initial estimation of the geometrical parameters were obtained by semi-empirical method using Austin Model 1 (AM1) , followed by full optimization of all geometrical variables (bond lengths, bond angles, and dihedral angles) without any geometrical constraint.
The surface morphology and the organic layers cross sections were visualized with a JEOL (Peabody, MA) 6400F field-effect scanning electron microscope and the electron probe microanalysis (EPMA) was used for quantitative analysis.
ESCA was used for X-ray photoelectron spectroscopy (XPS) measurements. High-resolution scans with a good signal-to-noise ratio were obtained in the C1s, O1s, and N1s regions of the spectrum. The quantitative studies were based on the determination of the C1s, O1s, and N1s peak areas with 0.2; 0.44, and 0.6 respectively, as sensitive factors. Deconvolution of the XPS peaks into different contributions and the quantitative interpretation were made after subtracting the background with the Shirley method .
Organic devices were prepared by depositing an aluminium layer under vacuum evaporation onto the top of the polymeric thin film. The deposition rate was 1 nms-1, and the film thickness was about 100 nm. The I–V curves 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.
RESULTS AND DISCUSSION
The cyclic voltammogram is shown in Figure 2a. In the first scan, it is possible to distinguish one oxidation peak at 1.57 V corresponding to pyrrole ring oxidation. However, if the anodic potential limit is increased, a second peak appears, which is attributed to the oxidation of the second ring. This feature can be explained by highoccupied molecular orbital (HOMO) mapped into electron densityand the localization of the single occupied molecular orbital (SOMO) in the radical cation by DFT calculations (Figure 2b). According to the HOMO and SOMO electron densities and the voltammogram, the polymerization of this molecule would involve pyrrole rings, although consecutive oxidation of the furane ring could also be taking place depending on the anodic potential [34,35].
Polymer growth is influenced by the anodic potential limit: when it is 2.0 V, a strong decrease in the oxidation current as well as an anodic shift of the peak is observed. However, when the anodic limit is extended to 2.2 V, a more pronounced decrease in the current is observed. This result implies that the polymerization voltage modifies the organic layer’s conducting properties. Under these circumstances, the polymeric film generated at a more anodic voltage would be less conducting. These features are important in the manufacture of organic devices like diodes since the process of un-doping the active layer is not necessary, consequently avoiding ohmic contact between electrodes in the devices.
The surface morphology shows that the deposition process has a high coverage efficiency (see Figure 3). No pinholes are visible after a few voltammetric cycles, although some topological heterogeneity is visible at the surface. It can be observed that the polymeric deposit is very thin with an approximate thickness of 150 nm (see Figure 3a).
Surface composition has been determined by Electron Probe Microanalysis (EPMA). In this case, there is a little oxygen contamination (see Table I). These extra atoms can be related to air moisture contamination.
The XPS spectra are presented in Figure 4. The C1s spectra of the polymers showed asymmetric tailing, which is a consequence of the considerable reorganization of valence electrons in the form of shake-up and shake-off phenomena as well as the removal of a shielding electron .This result is indicative of long-range disordered conjugation [37,38].Deconvolution of the C1s peak indicates four contributions. The first contribution, situated at 285.1 eV can be attributed to the C-C bond. This C-C binding energy is used as reference as is often the case . The second one, at 286.6 eV, is attributed to C-N. The third contribution, situated at 288.2 eV, corresponds to the C-O bonds. The fourth peak can be assigned to COOH surface contamination.
The O1s peak can be deconvoluted in two contributions. The first one, situated at about 530.3 eV, corresponds to polymeric C-O bonds. The second one, at 532.1 eV, can be assigned to contamination or to some H2O adsorbed .
The decomposition of the N1s peak (see Figure 4c) indicates that two contributions are necessary to obtain a good curve fit. The first contribution, located at 399 eV, corresponds to covalent nitrogen . The second contribution, at 401 eV, can be attributed to positively charged nitrogen from supporting electrolyte occluded in the polymer surface.
Electrical characterization: I–V curves
The device’s electrical behaviour measured under environmental conditions is shown in Fig. 5(a). When forward bias is applied, the current indicates a rectifying behaviour where the turn-on voltage is lower in comparison with devices prepared with other heterocyclic polymers . When the bias is applied, the initial behaviour is ohmic at low voltage, but the current begins to increase exponentially once an appropriate value is reached .
In the I–V curves, it is not possible that the dominant conduction mechanism is ohmic, trap-free, and space-charge-limited, or purely tunnelling injection. The temperature and thickness dependence observed in our previous studies indicate that it must be either thermo-ionic emission or thermally-assisted tunnelling [44,45]. In the case of thermo-ionic emission theory , the ideality factor n values found are 9-15, indicating disagreement with the model of standard Schottky contact.
The Fowler-Nordheim tunnelling model was used to interpret the I–V characteristics. This model has been found to be applicable in the high field domain, which assumes tunnelling of charge carriers directly into the semiconductor bands in that field domain. The tunnelling process is well described by the Fowler–Nordheim theory [47,48]:
where f is the barrier height and m* is the effective mass of the positive charges in the polymer. Assuming that the electric field is constant across the device and that the effective mass equals the free electron mass, the barrier height can be estimated. Figure 5b shows a plot of ln (I/F2) versus 1/F of a ITO/Polymer/Al device. As predicted by the theory, the plot is linear above the turn-on voltage. Therefore, the barrier height was estimated and these data indicate a constant barrier height of 0.46 eV for ITO/polymer /Al device.
The polymer used in this work is a hole-transporting layer (HTL), and therefore the I-V characteristics can be attributed to positive charge transport since the lower barrier height should be situated at the ITO-polymer interface. The ITO electron affinity is about 4.7 eV, while the work function of Al is 4.3 eV. Therefore, the whole energy barrier at the ITO-polymer interface is 0.7 eV, while the electron energy barrier at the polymer-Al interface is around 0.9 eV. This result confirms the attribution of the current to positive charges movement. Moreover, the comparison of theoretical barrier height values (0.46 eV) with the experimental values (0.7 eV) indicates that they are acceptable values. Still, the lack of differences found between experimental and theoretical values can be explained by the fact that the height barrier calculated corresponds to a rough estimation since m* values were assumed to be equal to the free electron mass.
A pyrrole-based polymer was electrochemically synthesized. The cyclic voltammograms and DFT calculations indicated that the electrochemical polymerization process would involve pyrrole rings, although oxidation of the furane ring could also take place. The surface morphology shows that there is a high coverage efficiency of the deposition process. It can be observed that the polymeric deposit is very thin with an approximate thickness of 150 nm. The ITO/polymer/Al structures based on this polymer exhibit rectifying behaviour. I–V characteristics exhibited a turn-on voltage of 0,70 V. The charge injection mechanism in this structure is related to a Fowler-Nordheim tunnelling effect.
The authors thank to FONDECYT (Grant No 3020010) for the financial support.
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(Received: July 7, 2006 - Accepted: November 20,2006)