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Organic semiconductor orange dye was dissolved in distilled water at room temperature and solutions were prepared with concentrations from 0.019 to 0.155 mol dm-3. The electrical conductivity of these aqueous solutions were investigated in a temperature range of 23 - 62oC, at a frequency of 5 - 1000 Hz , and a voltage ranging from 0 - 2.3 V. The length, width and height of the dip type conductance cell for measurement of the resistance were equal to 2, 1 and 1 cm respectively. It was found that the electrical conductivity of the OD solution increase with temperature, frequency and the applied voltage. Conductivity - concentration relationship showed a maximum conductivity at 0.05 mol dm-3. The conductivity mechanism can be explained on the basis of relaxation, electrophoretic effects and double layer formation at the electrode-solution interface. An equivalent circuit of the cell was developed and simulation studies of its impedance were carried out.
Key words: concentration; conductivity; frequency; orange dye solution; organic semiconductor; temperature; voltage.
A large number of academic papers have reported studies of organic photo-electrochemical and electro-chemical cells. This is mainly due to their low cost, easy device fabrication and interesting electrical and optical properties. The conversion of light to electricity by cis-X2 Bis (2,2'-bipyridyl-4,4'-dicarboxylate) ruthenium complexes on nanocrystalline TiO2 electrodes have been reported by Nazeeruddin et al. (1993). A solar-to-electric energy conversion efficiency of 10 % was achieved by these authors. The photo-electrochemistry of single crystal C60 and fullerene photo-electrochemical solar cells properties were studied by Sinke et al. (1998). The properties of dye-TiO2 organic solar cell were investigated by Sinke et al. (1998). This study had shown a record efficiency of 11% and 6.5% for a 0.25 and 1.6 cm2 cell areas respectively. Photoelectric behavior of n-GaAs and n-AlxGa1-xAs in CH3CH has been investigated by Casagrande et al. (2000), who reported an open-circuit voltage of 0.83 V, a short-circuit current density of 20 mA cm-2 and an energy conversion efficiency of > 10 % at 88 mW cm-2 of simulated AM 1.5 solar illumination. Historical background, present status and development prospects for the new generation of photo-electrochemical cells, including dye-sensitized nanocrystaline TiO2 film, was reviewed by M.Gratzel, 2001.
Using orange dye (OD) a number of devices have been fabricated: Electrochemical Zn/Orange Dye Aqueous Solution / Carbon cell (Karimov et al. 2006), orange dye thin film electrochemical hygrometers (Karimov et al. 2005) and photoelectric n-Si and p-Si/Orange dye / Conductive glass cells (Sayyad et al. 2005) have been investigated. In other studies the high conductivity of inorganic-organic polymer electrolyte (insertion of poly (ethylene) oxide into LiV3O8) (Yang et al. 2005) have been reported and the voltammetry curves of organic electrolytes with Li ions were investigated (Lee et al. 2002). To optimize the properties of devices based on OD aqueous solutions it would be desirable to investigate their electrical conductivity. This paper presents studies of the electrical properties of OD aqueous solution.
Distilled water and commercially produced organic semiconductor orange dye (OD), C17H17N5O2 (Figure 1) with molecular weight of 323 g/mole was used for preparation of solution in concentration range from 0.019 to 0.155 mol dm-3 at room temperature. The electrical conductivity of orange dye aqueous solution was investigated in a temperature range of 23 - 62oC, a frequency range of 5 - 1000 Hz , and in a voltage range of 0 - 2.3 V. The length, width and height of the dip type conductance cell with aluminum electrodes (aluminum was stable in the solution) were equal to 2, 1 and 1 cm respectively. Conventional digital instruments were used to measure the electrical conductivity of the sample.
Figure 1: Molecular structure of orange dye (OD).
RESULTS AND DISCUSSION
Figure 2 shows the conductivity - concentration relationship at T=25 oC at a frequency and voltage of 10 Hz and 1 V respectively. It is seen that conductivity is maximum at concentrations from 0.038 to 0.077 mol dm-3.
Figure 2: Orange dye aqueous solution conductivity - concentration relationship at T=25 oC and applied frequency and voltage of 10 Hz and 1 V respectively.
As the solutions concentration is increased from a lower value, the number of ions increases which increases the conductivity but when the ion concentration becomes large (above 0.077 mol dm-3) their velocity decreases and thus a decrease in conductivity is observed (it may be due to strengthening of electrophoretic effect) that is seen from the following expression (Krasnov 1982):
Ïƒ = 10-3 Î± c F (vc + va), (1)
Where F is a constant, Î± is dissociation constant, c is concentration of the solution (in mol dm-3), vc and va are velocity of cations and anions respectively. Figure 3 shows the conductivity - temperature relationships for 0.019, 0.038, 0.077 and 0.155 mol dm -3 at 10 Hz and 1 V.
Figure 3: Orange dye aqueous solution conductivity - temperature relationships at concentration of 0.019 mol dm-3 (1), 0.038 mol dm-3 (2), 0,077 mol dm-3 (3) and 0.155 mol dm-3 (4), and applied frequency and voltage of 10 Hz and 1 V respectively.
It is seen that conductivity increases with temperature linearly. Usually the increase in conductivity is due to increase in ions velocity (Eq.1) and this behavior is described with the following expression (Krasnov 1982):
Ïƒ2 = Ïƒ1 [1 + A (T2-T1)], (2)
Where Ïƒ1 and Ïƒ2 are the conductivity values at T1 and T2 respectively, A is temperature conductance coefficient, that is equal to 0.0141, 0.0111, 0.0081 and 0.111/oC for the concentrations of 0.019, 0.038, 0.077 and 0.155 mol dm-3 respectively. These temperature conductance coefficients are close to the values obtained for acids, alkalis and salts (Krasnov 1982).
Fig.4 shows conductivity - frequency relationships of the solution at a concentration of 0.019, 0.038, 0.077 and 0.155 mol dm-3 at 25oC and 1 V. It is seen that conductivity increases in each case and the initial rate also depends on concentration. Usually, electrolytes conductivity enhances at high frequency electric field (above 1 MHz) when ionic atmosphere cannot be re-established (Hibbert 1993).
Figure 4: Orange dye aqueous solution conductivity - frequency relationships at concentration of 0.019 mol dm-3 (1), 0.038 mol dm-3 (2), 0,077 mol dm-3 (3) and 0.155 mol dm-3 (4), and T=25 o C, and applied voltage of 1 V.
The increase in conductivity in this case may be due to decrease in the double layer capacitance, impedance and resistance of the electrolyte due to weakening of the relaxation effect (Hibbert 1993) and increase in the ions velocity (Eq.1) with frequency. In the well-known equivalent circuit of the electrochemical cell (Figure 5, Hibbert 1993 & Christensen & Hamnett 1994) an electrode-solution junction capacitance (Cj) may be added due to the presence of Al2O3 film on the surface of aluminum electrode that may also contribute to the frequency dependence of the conductivity.
Figure 5: An equivalent electrical circuit to an electrochemical cell: RSol is resistance of the solution, Cj and CD is the electrode-solution junction capacitance and double layer capacitance respectively, RCT is charge-transfer resistance , Zw is the Warburg impedance. (Hibbert 1993 & Christensen & Hamnett 1994).
Figure 6 shows the current - voltage characteristics of the OD solutions at concentration of 0.019, 0.038, 0.077 and 0.155 mol dm-3, at T=25 oC and applied frequency of 10 Hz. The curves are super linear meaning that the conductivity increases with applied voltage or electric field.
Figure 6: Orange dye aqueous solution current - voltage characteristics at concentration of 0.019 mol dm-3 (1), 0.038 mol dm-3 (2), 0,077 mol dm-3 (3) and 0.155 mol dm-3 (4), and T=25 o C, and applied frequency of 10 Hz.
It is well-known that high electric field (around of 1-10kV/cm) affects conductivity (Hibbert 1993), first, due to the retarding effect of the ionic atmosphere on ions motion and increase in the ions velocity, it is the first Wein effect that is seen mostly in strong electrolytes, second, due to enhanced dissociation or increase in the ion concentration, it is the second Wein effect that has been observed in weak electrolytes. Actually, the effect of the electric field depends on nature of ions and ionic atmosphere. From physical point of view, it may be assumed that ions are in potential wells (Neamen 1992) and the decreases in the potential barriers height may increase the conductivity as the ions velocity increases. On the other hand, injection of charges from electrodes may increase due to electric field, like the space charge limited current phenomenon in OD films (Moiz et al. 2005), that in turn increase the conductivity due to increase in ions concentration.
Figure 7 shows that the magnitude of solution's impedance (Z) decreases with frequency. It means that the capacitive reactance dominates over the solution's resistance (Rsol), but is much lower than the charge-transfer resistance (RCT), and Zw the Warburg impedance.
Figure 7: Orange dye aqueous solution impedance - frequency relationship at concentration of 0.038 mol dm-3, at room temperature and applied voltage of 1 V: 1 - experimental, 2- simulation.
Therefore we can, as a first approximation, simplify the equivalent circuit (Figure 5) to a series connection of the solution resistance and effective capacitance of the junction and double layer capacitances. At low frequency we may expect that the impedance is equal to capacitive reactance and at high frequency to the solution's resistance instead. Using these approximations a simulated data was obtained (Figure 7) for comparison with experimental data. Both curves show similar behavior with frequency.
Currently, for aluminum electrolytic capacitors, which can provide high values of capacitance in a small volume, a moistened borax paste electrolyte is used (Irwin & Wu 1999). Study of electrical properties of orange dye aqueous solution made in the current research shows that OD may also be used. This may be considered as one of the new application of this material. Identification of the nature, concentration and velocity of the ions is a special task that may be carried out in further investigations.
It was found that the electrical conductivity of orange dye aqueous solution increases with temperature, applied frequency and voltage. The conductivity is maximum at a concentration of 0.038 mol dm-3. Electrical conductivity mechanism of the OD solutions is explained on the basis of relaxation and electrophoretic effects in the solution, junction and double layers capacitances in the electrode-solution interface. The equivalent circuit of the cell was developed and simulation of the electrical behavior of OD solution was carried out. As efficiency of a number of photo- and electro-chemical cells, depends on conductivity of the electrolyte, the current data obtained allows to optimize parameters of the orange dye aqueous solution with respect to concentration, applied voltage, frequency and temperature.