Electrode effect on electrical conduction in thin film of polyvinyl pyrrolidone

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 47, October 2009, pp Electrode effect on electrical conduction in thin film of polyvinyl pyrrolidone Ashutosh Verma 1, P K Khare 2 & Rudra Kant Srivastava 1 1 Department of Physics, Bipin Behari P G College, Jhansi Department of PG Studies and Research in Physics, Rani Durgavati University, Jabalpur Received 18 September 2008; revised 7 January 2009; accepted 9 March 2009 Electrical conduction behaviour in ployvinyl pyrrolidone (PVP) has been investigated in both transient and steady state conditions in the temperature range C at different dc electric fields from 05 to 100 kv/cm using similar and dissimilar, both types of electrode systems. The nature of transient currents has mainly been attributed to the dipolar nature of PVP in the lower temperature region where slope value n of the Curie Von Schweidler law is found to vary from 0.43 to 0.62 for similar electrode system and from 0.78 to 0.98 for dissimilar electrode system. The low field steady state conduction is found to be ohmic in nature. The high field non-linear nature of conduction currents is found to be strongly temperature dependent and an attempt has been made to explain these curves taking into account various conduction processes. A strong temperature dependence of the conduction current rules out tunnelling as the possible conduction mechanism. The Poole-Frankel and Richardson-Schottky coefficients estimation shows that Richardson-Schottky conduction mechanism is operative in controlling the transport of charges in PVP foil electrets. This result is, further, confirmed using different electrodes systems of Jonscher and Ansari. Keywords: Polyvinyl pyrrolidone, Transient current, Steady state conduction, Dipolar orientation, Richardson Schottky conduction 1 Introduction Earlier, we have reported the dielectric 1,2 properties of polyvinyl pyrrolidone (PVP) and polyvinylidene fluoride (PVDF) foil electrets and also electrical 3,4 properties of PVDF. However, looking into numerous fascinating applications 5 of PVP, we have extended the study to electrical conduction in PVP to provide an understanding of fundamental electrical properties and charge transport mechanism. The mechanisms generally discussed for various polymer films are tunnelling 6, Poole-Frankel and Richardson-Schottky emission 7-9 and space charge limited conduction 9. If the applied field is high enough and the electrode makes ohmic current with the polymer, charge carriers are injected into the polymer by lowering of the barrier at the metal insulator interface. This effect is referred to as the Richardson-Schottky (RS) emission. In Poole-Frankel (PF), charge carriers get trapped in Coulombic potential well, then it can be detrapped on lowering of the trap depth by an applied electric field. The main difference between RS and PF-models is that former is electrode limited whereas in the later, the conductivity is bulk limited. The elucidation of the charge injection and carrier migration process will become essential for further progress in future practical applications of PVP. In the present paper, an experimental study of the electrical conduction of PVP is reported. 2 Experimental Details Thin PVP films are grown from a suitable solution by the isothermal immersion technique 10. PVP (2.4 g) has been dissolved in 30 ml of chemically pure chloroform at room temperature and continuously stirred for about 60 min by means of a teflon coated magnetic stirrer. Thereafter, it has been stirred and heated at 50 C to yield a homogeneous solution. The PVP films are grown on to a metal coated thoroughly cleaned glass substrates. After films deposition, the glass slide has been taken out and out gassed in air at 40 C for 24 h to remove all the traces of solvent. This is followed by room temperature out gassing at 10 5 Torr for a further period of 24 h to reduce the memory effects and enhance the repeatability of the measured currents 11. Metallic electrodes of area 1.33 cm 2 are vacuum deposited on interfacial layers. The geometry of the sandwich configuration of the electrodes has been the same as reported earlier The thickness of the samples has been estimated by measuring the capacitance of the fabricated sandwiches taking the dielectric constant ε to be 3 at 10 MHz. Both the currents has been recorded with a Kiethlly 600 B electrometer and voltage has been

2 738 INDIAN J PURE & APPL PHYS, VOL 47, OCTOBER 2009 applied from a high voltage unit (EC-HV 4800D). The transient currents have been measured for different electric fields (5-100 kv cm 1 ) at different operating temperatures (40-80 C) for 20 µm thick samples and steady state conduction currents have been measured for samples of 5, 20 and 30 µm thickness for different fields and temperatures. All the measures have been taken in the second run to obtain reproducible results, for similar and dissimilar electrode systems. 3 Results 3.1 Transient currents Figure 1 shows the representative transient currents (charging mode) temporal behaviour of PVP samples at various temperatures for an electric field of 100 kv cm 1 for similar (Al-PVP-A1) and dissimilar (A1-PVP-Ag) electrode systems. It may be noted that the magnitude of the charging current at a prescribed time is thermally activated over the temperature range C. These plots have been found to be characterized by two regions: one short time region and another long time region. The current transient in both the regions are observed to obey the Curie-Von Schweidler law 15. I (t) = A (T) t n where t is the time after application of the filed. A (T) is a temperature dependent factor and n is the decay constant. The currents decay at a faster rate for short period of time and then tends to attain a steady value. For samples, poled with higher fields and also at higher temperatures, the current tends to attain steady value earlier. The magnitude of the transient current is higher for similar electrodes (Al-Al) samples then for dissimilar electrodes (Al-Ag) combinations. The current in short time region decays very fast and in long time region, it decays with slow rate and attains steady value within 4 to 6 min. of the application of the field. The decay rate of the transient currents declines with the increase of temperature. Also the ratio I 0 /I s (I o current observed immediately after applying field, I s the steady state current) is much larger in lower temperature range (below 50 C) and goes on decreasing with increasing operating temperature (above 60 C). Figure 2 shows the transient currents temporal behaviour of PVP samples poled at 50 C with various fields for the same electrodes systems. 3.2 Steady state conduction The curves in Figs 3-5 show the dependence of steady state conduction on electric field for PVP samples of different thicknesses at different temperatures using different electrode configurations Cu-Cu and Al-Cu; Ag-Ag and Al-Ag and Sn-Sn and Al-Sn, respectively, plotted in the form of logi versus logv curves. There is a departure from straight line and knees appear in all the curves. The steady state or isothermal characteristics reveal almost ohmic behaviour initially in a limited field region, which Fig. 1 Temporal behaviour of transient current at various temperatures with constant poling field (100 kvcm 1 ) for PVP samples using Al-PVP-Al and Al-PVP-Ag electrode systems

3 VERMA et al.: ELECTRODE EFFECT ON ELECTRICAL CONDUCTION IN POLYVINYL PYRROLIDONE 739 Fig. 2 Temporal behaviour of transient current with various poling fields at 50 C for PVP samples using Al-PVP-Al and Al-PVP-Ag electrode system Fig. 3 Field dependence of steady state current (logi logv plots) at various temperatures for PVP films using Cu-PVP-Cu and Al-PVP-Cu electrode systems gradually becomes non-ohmic at higher fields above V cm 1. However, the operating temperature seems to play a crucial role in determining non-linear nature of these curves. The increment in the current is approximately the same for whole range of temperatures. The nature of the thermograms is nonlinear but similar for all temperature. Samples of thickness 5 µm exhibit more current than that of thickness 30 µm samples. The magnitude of the current has been found to be lower in dissimilar electrode combinations (Al-Cu; Al-Ag and Al-Sn) than similar electrode (Cu-Cu, Ag-Ag and Sn-Sn) systems. To identify the probable mechanism, out of Richardson-Schottky and Poole-Frankel in present investigation. logi versus E 1/2 characteristics has been

4 740 INDIAN J PURE & APPL PHYS, VOL 47, OCTOBER 2009 Fig. 4 Field dependence of steady state current (logi logv plots) at various temperatures for PVP films using Ag-PVP-Ag and Al-PVP-Ag electrode systems Fig. 5 Field dependence of steady state current (logi logv plots) at various temperatures for PVP films using Sn-PVP-Sn and Al-PVP-Sn electrode system drawn (Figs 6-8) for samples of thickness 5, 20 and 30 µm, respectively using similar electrode configurations (Al-Al, Cu-Cu, Ag-Ag and Sn-Sn) and dissimilar electrodes combinations (Al-Cu, Al-Ag and Al-Sn) by taking Al, Ag, Cu and Sn as upper electrode while the lower electrode has Al metal in each configuraltion. Though straight line character of the log I versus E ½ curves (Figs 6-8) is quite discernible at high fields, noticeable deviations are found in the lower field region.

5 VERMA et al.: ELECTRODE EFFECT ON ELECTRICAL CONDUCTION IN POLYVINYL PYRROLIDONE 741 Fig. 6 Schottky plots (logi versus E ½ ) at various temperatures for PVP films of 5 µm thickness using similar and dissimilar electrode system 4 Discussion Figures 1 and 2 show the representative transient current behaviour of PVP samples at various temperatures and fields, respectively. It can be observed that the current response consists of two components; first one decays very fast lasting few seconds and the second one decays with slow rate and attains steady state within 4 to 8 min of the application of the electric field. From this, it is evident that at least two distinct mechanisms 16,17 shall be responsible for the current decay. One mechanism is operative in the range of short time with a particular value of decay constant, n 1 and the other mechanisms is operative in the range of long times with a higher value of decay constant, n 2. The origin of the transient currents in polymers has been ascribed to number of mechanisms notably dipolar relaxation, charge injection leading to space charges, electrodes polarization and tunnelling to empty traps. In the case of tunnelling to empty traps, it is well established that the tunnelling current should be Fig.7 Schottky plots (logi versus E ½ ) at various temperatures for PVP films of 20 µm thickness using similar and dissimilar electrode system independent of temperature. However, in the present case, charging transient show thermal dependence. Thus, tunnelling as a possible mechanism can be discarded. In the case of electrode polarization, the transient currents are reported to be proportional to t n with the value of decay constant n=0 at short times, and n>1 at longer times. However, in the present case, at short times the values of n are found to vary from 0.43 to 0.62 for similar electrode system and from 0.78 to 0.98 for dissimilar electrode combinations. These observations show that the process of electrode polarization is unlikely to be dominant in the present case. However, the dipolar relaxation seems to be the major contributor to the transient current particularly at lower temperatures as PVP exhibit polar character. This is further confirmed on comparison with the temperature dependence of the transient currents to

6 742 INDIAN J PURE & APPL PHYS, VOL 47, OCTOBER 2009 Fig. 8 Schottky plots (logi versus E ½ ) at various temperatures for PVP films of 30 µm thickness using similar and dissimilar electrode system thermally stimulated discharge current (TSDC) study made on this polymer earlier 21. The β-relaxation peak 90 C in TSDC spectra of PVP attributed to the dipolar relaxation due to presence of carbonyl group in the structure of PVP is shown in Fig. 9. This can be held responsible for much faster decay of transient currents. The decline in the time dependence of transient current with increasing temperature may be attributed to the cumulative effect of rapid decrease in the relaxation time, a characteristic of transient phenomenon and the progressive increase of steady state conduction. Struick 22 has shown that solid like polymers are not in thermodynamic equilibrium at temperatures below their glass transition. For such material, free volume enthalpy and entropy values are greater than they would be in equilibrium state. The gradual approach to equilibrium affects many properties, e.g. the free volume of the polymer may decrease. The decrease in free volume lowers the mobility of chain segments and also charge carriers. The decrease in mobility may be expected to reduce conductivity. At high electric fields, a change in mobility may take place faster than at lower fields and also recombination of charge carries may be more. This may be responsible to make the observed current in the present case to approach a stable value in relatively shorter periods under high fields. The dependence of steady state conduction on electric field for PVP samples at different temperatures is shown in Figs 3-5, plotted in the form of log I - logv curves for Cu-PVP-Cu, Al-PVP-Cu; Ag-PVP-Ag, Al-PVP-Ag and Sn-PVP-Sn, Al-PVP-Sn electrode system, respectively. Most of the curves show usual ohmic behaviour initially for electric fields below about 10 kvcm 1, which becomes nonohmic at higher fields. There is a departure from straight line and knees appear in all the curves. The slope values (n) in lower field region lie between 0.48 to 0.68 and a non-ohmic conduction with the slope between 1.87 to 2.05 in higher field region are observed. The ohmic behaviour can be understood on the basis of the reasonable assumption that at lower fields, the injection of carriers from the contacts is less and the initial current which is governed by the intrinsic free carriers in the material. The current will be ohmic until the injected free carrier density becomes comparable with the thermally created carrier density. However, at sufficiently higher fields the conduction current is mainly due to injected space charge, this is further confirmed by the value of slope 2 in higher field region. Figures 3-5 further reveal that the conduction current is thickness dependent 5 µm thickness samples exhibit more conduction current than 30 m thickness samples. Also, the magnitude of the conduction current has been found to be higher in similar electrode system than dissimilar electrode combinations. However, similar type of behaviour is also seen in transient current studies (Figs 1 and 2). Vinyl derivatives are prominent species among high polymers containing a lot of structural disorders and impurities with them. The slope values 2 are the typical proof for the continuous trap level distribution in the band gap of semi insulating PVP. The defects and impurities can govern the conduction mechanism and also act as trapping centre and get populated by injected charge carriers from the electrodes. Charge carriers from these localized levels are thermally

7 VERMA et al.: ELECTRODE EFFECT ON ELECTRICAL CONDUCTION IN POLYVINYL PYRROLIDONE 743 excited to their respective transport bands using thermally activated ohmic conduction. It will be worthwhile to examine the present results in close relations to electronic processes, mainly Richardson- Schottky and Poole-Frankel type conduction associated with the injection of charge carriers from outside the bulk of the samples. The current density for these processes is expressed, respectively as 20 : i = AT 2 exp [(B RS E ½ φ) / kt] and (1) i= BE ½ exp [(B PF E ½ φ) / kt] (2) Where A and B are constants, E is applied electric field, φ is the work functions, B RS and B PF are the Richardson Schottky and Poole-Frankel coefficients, respectively given as : B RS = 1/kT (e 3 /4πεε ο )½ and B PF = 2 B RS (3) where k is Boltzman constant, T the temperature, e the electronic charge, ε the high frequency dielectric constant and ε ο is the permittivity of vacuum. The Schottky mechanism involves the field assisted thermionic emission of charges from electrodes into samples whereas in the Poole-Frankel effect, the current is considered to be due to field assisted thermal excitation of electrons from traps into conduction band. In Figs 6-8, the data of Figs 3-5 are replotted in the form of log I versus E 1/2 usually known as Schottky plots, for samples of different, thicknesses using different electrode combinations. From 50 to 90 kv/cm, the data are well represented by straight line and above 90 kv/cm, the data again follow a straight line fit with lesser slope. Theoretical values of B RS (Schottky case) and B PF (Poole-Frankel case) calculated from Eq. (3) at different temperatures are compared with their experimental values calculated from the slopes of the plots of logi versus E ½ given in Table 1. The experimental values of B RS in the present case, are found to be close to the theoretically observed values of B RS and based on the premise that Richardson Schottky is the particular mechanism that is operative here in the case of PVP also, as reported earlier 23,24 in case of polymer liquid crystal and cobalt naphthenate, respectively. However, in order to confirm Richardson Schottky mechanism, Jonscher and Ansari 25 method of different electrodes is adopted. In this method, the Table 1 Comparison of calculated and experimental values of B RS and B PF Temp ( C) B RS (Theo) 10 4 B RS (Expt) B PF (Theo) 10 4 B PF (Expt) 10 4 ε (stand) CH 2 CH 2 CH 2 N C effect of different electrode material viz. Al., Cu, Ag and Sn having different work functions 3.38, 4.46, 4.31 and 4.09 ev, respectively, on the logi versus logv characteristics has to be considered for deciding conduction mechanism. The conduction current through the samples differs when the upper electrode aluminium is replaced by copper, silver and tin electrodes. The magnitude of current has been found to be higher in similar electrode Al-Al, Cu-Cu, Ag-Ag and Sn-Sn systems than in dissimilar electrode combination. This shows the effect of the material of the electrodes on the conduction current of the sample sandwiched between them. The values of conduction current seem to be controlled by the effective work function for metal-insulator-metal interfaces. The difference between the work function metal (1) and metal (2) will control the magnitude of current but for similar electrodes, the characteristics of polymer may prevail as the net contribution of current from charge, injected from electrode, would then be zero. The work function of the polymer must be taken to lie above those of metals. So the resultant barrier height between polymer-metal (1) i.e. aluminium, is sufficiently higher than the effective barrier height of polymer-metal (2) i.e. Cu/Ag/Sn interfaces. Their difference will control the magnitude of conduction current. Separate distinct lines are obtained for dissimilar electrode combination. These observations are consistent with the proposed R-S mechanism and O CH CH 2 n Fig. 9 Carbonyl group in the structure of PVP

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