Current - Voltage characteristics of polymethylthiophene/tio2 nanocomposite cells

751 Journal of Chemistry, Vol. 45 (6), P. 751 - 756, 2007 CURRENT - VOLTAGE CHARACTERISTICS OF POLYMETHYLTHIOPHENE/TiO2 NANOCOMPOSITE CELLS Received 8 November 2006 Vu Quoc Trung1, Thai Doan Tinh1, Tran Vinh Dieu2, Jiri Pfleger3 1Faculty of Chemistry, Hanoi University of Education 2Centre of Polymer Research, Hanoi University of Technology 3Institute of Macromolecular Chemistry, Czech Academy of Sciences SUMMARY Titanium oxide nanoparticles (TiO2) were combined with polymethylthiophene (PMT), giving nanocomposites in core-shell structure via chemical polymerization. In this work, the current - voltage characteristics of sandwich cells composed from such nanocomposite have been studied. For these investigations films of nanocomposites were prepared by electrophoretic deposition (EPD). I - INTRODUCTION Conducting polymer nanocomposites are rather prepared with interesting properties and application potentials [1 - 4]. Recently, such nanocomposites have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and photocurrent measurements [2 - 4]. In last paper, photo- electrochemical properties of PMT/TiO2 nanocomposite layers were reported [5]. The photocurrent spectra measured in electrolyte was performed at several bias potentials, discovering the separate photoelectrochemical signals from TiO2 (n-type semiconductor) and from PMT (p-type semiconductor). In sandwich cells, the photocurrent spectra showed the signals of p-n junction at 450 nm. In this work, current - voltage characteristics of such films are presented in order to study photoelectrical and electrical properties the cells fabricated from the PMT/TiO2 nanocomposites. II - EXPERIMENTAL 1. Preparation of PMT/TiO2 nanocomposites and sandwich cells Nanocomposites were prepared as described in Refs. [2 - 4]. The amount of PMT (5.3%) in the composites was calculated from the data of the thermogravimetrical measurements (performed with a thermobalance, METTLER TG 50). For photoelectrical measurements in solid state, sandwich cells of ITO/composite/aluminium were prepared. Composite layer was electrophoretically deposited on ITO and then a top electrode of gold was deposited using vacuum deposition. For photocurrent measurements in electrolyte (containing a redox couple I3 -/I- in CH3CN with a Pt counter electrode), layers of PMT/TiO2 nanocomposite were electrophoretically deposited on ITO plates. 2. Material characterization Photoelectrical measurements were made with an electrometer Keithley 236 and a Xenon lamp (Müller-Elektronik, P = 150 W). 752 -4 -3 -2 -1 0 1 2 3 4 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 C ur re nt de ns ity /l g( A ) Voltage / V -4 -3 -2 -1 0 1 2 3 4 -1.0x10-4 0.0 1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4 6.0x10-4 7.0x10-4 8.0x10-4 C ur re nt de ns ity /A Voltage / V III - RERULTS AND DISCUSSION 1. Current - voltage curves of solid cell Current - voltage curves (in the dark) of a solid Au//PMT/TiO2//Al cell in a range of ±4 V are shown in Fig. 1. The typical Schottky diode behaviour is explained by the fact that gold formed an Ohmic contact while aluminium formed a blocking contact with the composite, similar to [6]. Inset of Fig. 1 shows I/V characteristics on a semi-logarithmic scale. The rectification ratio of 102 (at ±4 V) was found. According to [7], the rectification in a p/n junction can be explained: An n-type semiconductor contains mobile negative charges (electrons) and an equal concentration of fixed positive charges (ionized donors). Meanwhile, a p-type semiconductor contains mobile positive charges (holes) and fixed negative charges (ionized acceptors). With the two regions in contact, the mobile electrons and mobile holes can flow across the heterojunction and recombine together, resulting in the n-type region with a net positive charge and in the p- type region with a net negative charge. Thus, a field that was established has a direction in opposes with the further flow and brings the Fermi level in the n and p region to the same level. The band bending connected with the charging of the interface is shown in Fig. 2 (I). If a positive potential is applied to the p region and a negative potential to the n region, the effect is as shown in Fig. 2 (II). The potential barrier between two regions is lowered and the forward currents of both holes and electrons are greatly increased. The current arising from the generation of minority carriers remains the same and so there is a net flow of current across the junction with contributions from both holes and electrons. If the p region is made negative with respect to the n region, the potential barrier becomes much higher and the forward flow drops to a very low value for both kinds of carriers Fig. 2 (III). From plotting in the semi-logarithmic scale under forward bias, four different regions can be distinguished as indicated in Fig 3. First, there is the ideal diode region (0.3 - 0.75 V) where the diode junction quality is quantified using its ideal factor, n. The ideal factor can be obtained from the equation (1) [8]: slopeTB qn 1. ..303.2 = (1) Where the slope of the current – voltage curve Figure 1: Current-voltage curve of the Au//PMT/TiO2//Al cell (Inset: on a semi-logarithmic scale) 753 0.0 0.3 0.6 0.9 1.2 1E-7 1E-6 1E-5 1E-4 C ur re nt /A Voltage / V Recombination Ideal diode High injection Series resistance (I) (II) (III) p n p n p n Figure 2: p/n junction: I) equilibrium condition II) forward bias and III) reverse bias [7] in the semi-logarithmic scale is in units of V/decade; q is the electronic charge; B is the Boltzmann’s constant and T is the temperature (in degree Kelvin). In the study, n of 4.4 is calculated, quite different from the ideal junction (n = 1, [8]). This is due to the presence of non-ideal effects. It can be explained by the soft structure of the deposited composite layer as well as by the bad contact between the composite particles and the electrode surface. On the left of the ideal diode region, there is a region where the current is dominated by the trap-assisted recombination in the depletion region. On the right of the ideal diode region, the current becomes limited by high injection effects and by the series resistance. High injection occurs in a forward biased p-n diode when the injected minority carrier density exceeds the doping density. High injection will therefore occur first in the lowest doped region of the diode since that region has the highest minority carrier density. For higher forward bias voltages, the current is not exponentially longer increase with voltage. However, it still increased linearly due to either the contact resistance between the metal and the semiconductor or the resistively of the semiconductor, or the series resistance of the connecting wires, or both. These four regions can be observed in almost p- n diodes although the high-injection region rarely occurs, as the series resistance tends to limit the current first. Figure 3: Current-voltage characteristic of a Au//PMT/TiO2//Al cell under forward bias (data from Fig. 1) 754 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 C ur re nt de ns ity /µ A .c m -2 Potential / V a) b) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.00015 -0.00010 -0.00005 0.00000 0.00005 under white light illumination in the dark I(A ) U(V) Current-voltage characteristics of a solid state ITO//PMT/TiO2//Al cell tested both in the dark and under illumination through ITO using a Xenon lamp (150 W) are shown in Fig.4. In this case, the short circuit current (JSC) and the open circuit voltage (VOC) were 0.87 µA.cm -2 and 0.2 V, respectively. 2. Current - voltage curves of cell in electrolyte Current-voltage characteristics of a photoelectrochemical PMT/TiO2 cell in the I2/I3 - electrolyte system recorded both in the dark and under light illumination through ITO are shown in Fig. 5. In the dark, the current remains relatively constant in a range of (-0.4 V) - (+ 0.8 V). During illumination, a cathodic photocurrent is observed at cathodic potentials. It indicates that the neutral PMT behaves as a p-type semiconductor. The short circuit current (JSC) and the open circuit voltage (VOC) are Figure 4: Current-voltage characteristic of the solid ITO//PMT/TiO2//Al cell: a) in the dark and b) under illumination Figure 5: I/V characteristic of photoelectrochemical PMT/TiO2 (5.3%) cell deposited on ITO in the I2/I3 - electrolyte 755 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 C ur re nt de ns ity /m A. cm -2 Potential / VSCE 1st cycle 10th cycle determined to be 0.06 mAcm-2 and 0.45 V, respectively. The fill factor (FF) for the cell, which is a measure of the squareness of the out put characteristic, is calculated to be 0.5 [9]. Obviously, these values are higher than in the case discussed above (solid cell). This indicates that the high resistance of the solid cells was leading to low photocurrents. This result also explained why the ideal junction of p-n diode calculated above was high (n = 4.4). 3. Cyclic voltammetry Fig. 6 shows the CV curves of PMT/TiO2 nanocomposite layers deposited on platinum at 20 V. The onset potential of oxidation was 0.4 VSCE, the observed anodic peak in vicinity of potential of PMT was + 0.7 VSCE, both in agreement with values published in the literature [10]. The value of onset oxidation potential was very near with that of open circuit voltage showed in Fig. 5. The anodic peak current- density decreased from 0.65 µA.cm-2 in the first cycle to 0.6 µA.cm-2 in the 10 th cycle. The reduction potential of PMT was + 0.65 VSCE. As in the case of the polythiophene/TiO2 composites [2 - 4], the TiO2 core showed no electrochemical activity. Figure 6: Cyclovoltammogram of PMT/TiO2 composite layer prepared by EPD on platinum IV - CONCLUSIONs Thin films of polymethylthiophene/titanium oxide nanocomposites were prepared by electrophoretic deposition. Obtained films were applicable for the investigation of photoelec- trochemical and electrical properties of the nanocomposite. The electrochemical behaviour of the composite layers was investigated by cyclic voltammetry. The reversible and stable redox behaviour of polymethylthiophene was obtained. The oxidation potential of polymethyl- thiophene (+ 0.4 VSCE) was determined by cyclic voltammetry. A sandwich cell composed of polymethyl- thiophene/titanium dioxide nanocomposite layer between ITO and Au electrodes was developed. The behaviour of a p/n diode was investigated. The ideality factor of the diode was 4.4. 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