Analysis of mobile ionic impurities in polyvinylalcohol thin films by thermal discharge current and dielectric impedance spectroscopy Analysis of mobile ionic impurities in polyvinylalcohol thin films[.]
Analysis of mobile ionic impurities in polyvinylalcohol thin films by thermal discharge current and dielectric impedance spectroscopy , M Egginger and R Schwödiauer Citation: AIP Advances 2, 042152 (2012); doi: 10.1063/1.4768805 View online: http://dx.doi.org/10.1063/1.4768805 View Table of Contents: http://aip.scitation.org/toc/adv/2/4 Published by the American Institute of Physics AIP ADVANCES 2, 042152 (2012) Analysis of mobile ionic impurities in polyvinylalcohol thin films by thermal discharge current and dielectric impedance spectroscopy 1,a M Egginger1,2 and R Schwodiauer ă Department of Soft Matter Physics, Johannes Kepler University, Altenbergerstraße 69, 4040 Linz, Austria isiQiri interface technologies GmbH, Softwarepark 37, 4232 Hagenberg, Austria (Received 15 August 2012; accepted November 2012; published online 21 November 2012) Polyvinylalcohol (PVA) is a water soluble polymer frequently applied in the field of organic electronics for insulating thin film layers By-products of PVA synthesis are sodium acetate ions which contaminate the polymer material and can impinge on the electronic performance when applied as interlayer dielectrics in thin film transistors Uncontrollable voltage instabilities and unwanted hysteresis effects are regularly reported with PVA devices An understanding of these effects require knowledge about the electronic dynamics of the ionic impurities and their influence on the dielectric properties of PVA Respective data, which are largely unknown, are being presented in this work Experimental investigations were performed from room temperature to 125◦ C on drop-cast PVA films of three different quality grades Data from thermal discharge current (TDC) measurements, polarization experiments, and dielectric impedance spectroscopy concurrently show evidence of mobile ionic carriers Results from TDC measurements indicate the existence of an intrinsic, build-in electric field of pristine PVA films The field is caused by asymmetric ionic double layer formation at the two different film-interfaces (substrate/PVA and PVA/air) The mobile ions cause strong electrode polarization effects which dominate dielectric impedance spectra From a quantitative electrode polarization analysis of isothermal impedance spectra temperature dependent values for the concentration, the mobility and conductivity together with characteristic relaxation times of the mobile carriers are given Also shown are temperature dependent results for the dc-permittivity and the electronic resistivity The obtained results demonstrate the feasibility to partly remove contaminants from a PVA solution by dialysis cleaning Such a cleaning procedure reduces the values of ion concentration, conductivity and relaxation frequency Copyright 2012 Author(s) This article is distributed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4768805] I INTRODUCTION Polyvinylalcohol (PVA) is an attractive dielectric polymer in the field of organic electronics Since the first successful application of PVA as a gate dielectric for an all-organic field effect transistor (OFET), in the year 1990,1 the material became increasingly popular over the years By 2006, PVA was considered as “ one of the most widely used polymer dielectrics for organic electronic applications”.2 Today, PVA is still wide spread and a commonly used polymer dielectric in the research field of organic electronics.3–6 Much of this attraction is due to easy processing in combination with desirable dielectric characteristics: Very good insulating properties and notably a fairly high dielectric permittivity qualified PVA as a favorable organic material for interlayer dielectrics The widespread use of PVA however, revealed also a serious disadvantage as the material a Electronic mail: reinhard.schwoediauer@jku.at 2158-3226/2012/2(4)/042152/15 2, 042152-1 C Author(s) 2012 042152-2 M Egginger and R Schwodiauer ă AIP Advances 2, 042152 (2012) was found to be responsible for uncontrollable voltage instabilities in OFETs Those instabilities, which cause a hysteresis in the device’s transfer characteristic, have been frequently reported for PVA and other dielectrics alike (see7, and the references within), but a generally accepted explanation for the cause has not yet surfaced As the device problems are obviously related to the interlayer dielectric it appears to be necessary to dedicate further research work towards a better characterization of the dielectric materials involved The electrical nature of many insulating materials, including PVA, is only partially understood Only the most common electrical properties are well known Standard parameters like the dielectric permittivity and the dielectric loss factor, conductivity and dielectric strength, though important, often give insufficient information when it comes to electronic applications Equally important, but more subtle, are properties related to charge injection, charge trapping and charge storage Also the effect of impurities, especially the presence of mobile ionic carriers, often have a crucial impact on the electronic performance Thus, knowledge about related parameters, like temperature dependent concentration and mobilities of ionic impurities, are most relevant For pure PVA - a material which is predominately produced and used for non-electronic applications - the respective data are largely unknown This work is a first step towards a more comprehensive characterization of PVA to gain a better understanding of the electrical properties beyond already tabulated standard parameters The paper presents a series of experiments with three different grades of PVA: A standard off-the-shelf product, a special-purpose PVA material for electronic applications, and a dialysis cleaned special-purpose material are being discussed Thermal discharge current (TDC) measurements with all three grades, and additional polarization experiments demonstrate the presence of mobile ionic carriers Pronounced effects of electrode polarization appear already in pristine PVA films The electrode polarization of all samples has been investigated in detail by dielectric impedance spectroscopy (DIS) A quantitative fit of the measured spectra allow to deduce the temperature dependent concentration of the mobile ionic carriers and the related mobility, conductivity and relaxation time Further parameters like the dc-permittivity and the dc-resistivity are also given with respect to temperature The results reveal that the dielectric properties of PVA are generally affected by the ionic impurities Dialysis cleaning of the polymer solution can reduce the concentration of impurities which lead to a lower ion conductivity and to a longer ion relaxation time II EXPERIMENT A Materials and sample preparation A 7% solution of “normal grade” PVA was prepared with 1.75 g of PVA from Sigma Aldrich R 40–88 (average molecular weight MW ∼ 205000 g/mol) This off-thewith tradename Mowiol shelf polymer was submerged and dissolved at 80◦ C in 25 ml ultra pure, 18 M-cm water from a Millipore Ultra Pure Water System In the same way a similar 7% solution of “electronic grade” PVA R 40–88 special-purpose material (for electronic application), received was prepared from a Mowiol directly from the manufacturer: “Kuraray Specialties Europe GmbH” From this 25 ml solution approximately 2×5 ml were filled in two dialysis tubings (Sigma–Aldrich, D0530) and submerged into a 500 ml beaker with ultra pure, 18 M-cm water Dialysis proceeded under constant stirring for more than 24 hours at room temperature The final obtained “dialysis grade” PVA solution was poured into a volumetric flask and stored for further use at 4◦ C; the same happened also with the “normal grade” and the “electronic grade” solutions Small amounts of the solutions were further passed through a 0.45 μm PE filter and stored at 4◦ C as well The filtered solutions were taken for PVA film casting The quantitative analysis of experimental data require samples of a certain minimum thickness Films with thicknesses between 20 and 70 μm were fabricated from all three PVA grades via dropcasting on a specially prepared PEEK substrate A schematic of the substrate is depicted in Fig 1(a) The mm thick disc with a diameter of 30 mm has a polished surface with a shallow circular groove of 15 mm in diameter The groove serves two purposes: First it marks and defines the area which is to be filled with a predefined volume between 150 and 600 μl of PVA solution, and second the 042152-3 M Egginger and R Schwodiauer ă AIP Advances 2, 042152 (2012) FIG PEEK substrate for PVA drop-cast films with sub-surface contact of the bottom gold electrode (a) Cross sectional view of the substrate, the drop-cast PVA film, and the gold coated stainless steel top electrode (b) Geometric capacitor model of a drop-cast PVA film with an air gap between the non-uniform film surface and the top electrode (c) groove works as a kind of “drop stop” which hinders the PVA-water solution to form a spherical drop under the strong force of surface tension As a result the coated area remains coated during the slow drying process, and, as sketched in Fig 1(b), the final film dries into a plano-concave shape with maximal thickness at the rim, and a thinner central area with only a moderate thickness variation Within this area the films are electrically contacted by a small, gold coated stainless steel disk of mm diameter and mm height The electrical counter-contact at the bottom is realized with a 13 mm diameter gold electrode evaporated on top of the PEEK substrate Prior to evaporation a 0.5 mm interconnect-hole, perpendicular through the substrate, and positioned mm off the center, is filled with silver paste to contact the open end of an insulated wire which is fed through a second bore parallel to the surface This embedded contact wire is designed to avoid any unwanted surface leakage-currents between top- and bottom electrode as they may occur in high-voltage polarization experiments Samples were drop-casted from filtered PVA solutions with a variable volume pipette Slow drying was carried out at room temperature beneath a protective dome under constant flow of nitrogen The dried PVA films are hygroscopic and were thus stored in an evacuated desiccator for a number of days before being installed in the experimental setup B Experimental setup and procedure A custom made sample chamber has been designed and used for all measurements A scheme of the sample chamber together with the installed PVA sample and the instrumentation is presented in Fig The aluminum chamber can be evacuated and/or filled with inert gas The PEEK substrate is sticked to a Linkam THMSB heating element via a thin layer of heat conductive paste On the substrate is the PVA film with the top electrode centrally positioned and firmly held in place by a thermally and electrically insulated clamping bar Both, the top- and the bottom electrodes are wired to BNC feed-through connectors The connected instrument for TDC measurements is a Keithley 6514 System Electrometer, an additional (Rhode & Schwarz NGK 280) voltage source in series is used for polarization experiments, and DIS is performed with a Novocontrol measurement system consisting of an Alpha-A mainframe and a ZG4 test interface Together with a Linkam TSM 90 temperature controller the instruments are connected to a personal computer for instrument control and data storage Reference data of the temperature gradient across the PEEK substrate were recorded from room temperature to 170◦ C with a 100 μm thin NiCr-Ni thermocouple embedded in a ∼150 μm thin epoxy-film on a dummy-substrate A sample was removed from the desiccator and installed instantly in the chamber The bottom electrode-wire was soldered to the BNC ground and the top-electrode was positioned and held by the attached rigid signal wire at some distance above the PVA film The gap between film and top-electrode remained during a following 24 hour film-drying procedure where a reduced chamber- 042152-4 M Egginger and R Schwodiauer ă AIP Advances 2, 042152 (2012) FIG Experimental setup for thermal discharge current measurements and dielectric impedance spectroscopy on drop-cast PVA films pressure of 10 μm) were found to produce higher currents, but also much thinner spin-casted films (1011 , and both impedance components show a decrease which is almost proportional to inverse frequency This frequency behavior follows an empirical impedance function,√known as “constant phase element” (CPE) which takes the form ZCPE = Z0 (i ωτ )−β , where i = −1 represents the imaginary unit, ω = 2π f denotes the angular frequency, and Z0 , τ and β ∈ (0, 1] are temperature dependent parameters A CPE appears in the majority of experimental data on solid and liquid electrolytes.18 The CPEs of drop-cast PVA samples at room temperature are reasonably well described with a common exponent of β ≈ 0.98 √ With increasing temperatures the impedance, |Z | = Z 2 + Z 2 , decreases and deviate from the CPE response A maximum in the imaginary part of the impedance, near 30 mHz, is already clearly visible at about 50◦ C for the “normal grade” PVA For the “electronic-” and “dialysis grade” PVA the maximum appears at ca 60◦ C The maxima shift to higher frequencies with further elevated temperatures, and the impedance in the lower frequency range decrease substantially At the final film temperature the low frequency impedance values have dropped by five orders of magnitude and the maxima have shifted into the kHz range In general, the high temperature impedance data over the whole frequency range resemble very much spectra that are dominated by 042152-8 M Egginger and R Schwodiauer ă AIP Advances 2, 042152 (2012) strong electrode polarization effects Such spectra have been frequently observed with ion-blocking electrodes connected to electrolyte systems.19–23 The ac-characteristic of electrode polarization (EP) had been investigated and analyzed by Chang and Jaffe.24 The work was continued by MacDonald25, 26 and also by Coelho.27, 28 The general theory of EP was simplified for less complex situations by Klein et al.21 Klein’s description of EP for single ion conductors has been used for this work to fit the PVA impedance data Examples of the fitted data are given in the third and the fourth row of Fig with a spectra at maximum temperature and a second spectra at 70◦ C All spectra together with the related fit functions are presented in the supplemental material.29 D Data Analysis The fitting model for data analysis describes a PVA sample, according to Fig 1(c), as parallel plate capacitor with a total area A, and an overall thickness L, together with a thin air-gap located underneath the electrode and defined by an effective gap-area, Ag , and an effective gap-thickness, Lg Both air-gap parameters were estimated experimentally The dielectric material response is based on an elementary impedance description with an ohmic, temperature dependent PVA film resistance, ˜ and a dispersive capacitor in parallel The corresponding impedance is R, Z (ω) = R˜ β + i ω R˜ C0 εEP (ω) (1) ˜ L˜ expresses the geometric capacitance defined by a PVA surface area A, ˜ and a where C0 = ε0 A/ ˜ corresponding film thickness L; ε0 denotes the vacuum permittivity The temperature dependent exponent, β, is related to the CPE which describes the power law characteristics of the spectra The dielectric permittivity, εEP (ω), is assumed to be determined by electrode polarization which can be modeled with a Debye-like permittivity:25 εEP (ω) = εs + (εs, EP − εs )/(1 + i ωτ EP ); εs and εs, EP are the temperature dependent high- and low frequency limits, and τ EP is the temperature dependent relaxation (build up and decay-) time for the EP due to mobile ionic carriers The applied fitting model uses a modified formula based on the dielectric function proposed by Cole and Cole30 εEP (ω) = εs + εs,EP − εs + (i ω τEP )α (2) The temperature dependent α-parameter allows for an improved fitting of the Z extrema It was found close to a Debye characteristic (α = 1), ranging between 0.9 < α < 1, and it can be associated with a distribution of relaxation times.18 The EP relaxation time, τ EP , is related to the transit-time of ˜ A distribution of ions from one interface to the other and it scales directly with the drift length, L τ EP can be expected due to the different and inhomogeneous interface regions and related trapping mechanisms, and also because of the irregular thickness of the drop-cast film The drift length and its relation to the Debye length is crucial in the theory of EP The relation ˜ D This parameter was introduced and defined by Macdonald25 denoted with the symbol M ≡ 12 L/L is related to almost all dielectric properties and it also relates the high frequency permittivity, εs , with the low-frequency permittivity21 εs, EP = (M − 1) εs (3) The parameter M can therefore be obtained, in principle, from experimental data via the high- and low frequency limits (M εs, EP /εs 1) of the real permittivity, ε = Re(εEP ) Alternatively, the parameter M can be obtained also via the frequencies f ε and fδ associated with the maximum of the imaginary permittivity, ε = Im(εEP ), and the maximum of the loss angle, tan δ = ε /ε, respectively; the relation is M = ( f δ / f ε )2 21 However, the spectra of PVA in permittivity representation, ε(ω) = ε(ω) − i ε (ω), not show any evident saturation with high- and low frequency ε −limits, and the maxima of ε are not clearly expressed either, but superimposed by conduction processes Important characteristic values can be extracted more easily from the impedance representation, Z(ω) = Z(ω) + i Z (ω), as displayed in Fig The mid-frequency minimum and the high frequency maximum of Z (ω) are 042152-9 M Egginger and R Schwodiauer ă AIP Advances 2, 042152 (2012) clearly expressed and separated Thus, in a good approximation, the associated frequencies, fm and fh , can be directly related to f ε ≈ f m2 / f h , and fδ ≈ fm These two frequency values allow to estimate the parameter M ≈ ( f h / f m )2 (4) with a better accuracy The Z (ω) data also allow to determine directly the temperature dependent β-parameter via simple power-law fits, since at sufficient distance from the extrema Z (ω)∝ω−β For solid and liquid electrolytes the CPE related β-parameter is generally known not to be constant over the entire frequency range.18 For PVA at elevated temperatures, β was also found to be slightly higher in the high frequency regime, f > fh , compared to the mid- to low frequency range, fl < f < fm All PVA Z(ω) spectra were fitted with the high frequency β values throughout the full frequency range Of course, in the low frequency range, and especially at lower temperatures, the data are less accurately described But fits with low frequency β values produce even stronger deviations in the higher frequency- and higher temperature range as shown with dashed lines in Fig (a, iii) and (a, iv) Nonetheless, fits with low frequency β values are helpful in case of a low frequency Z -maximum Such a maximum could be confirmed for the Z (ω) spectrum of “normal grade” PVA at the highest experimental temperature The apparent saturation of Z (ω) towards the lowest frequencies constitutes one branch of the low-frequency maximum, at fl 10 mHz, which can be directly related to an effective sample resistance R A simple analysis of Eq (1) at fl yields Z (2π f l ) sin (πβ/2) R + cos (πβ/2) (5) The knowledge of R for a spectra at a specific temperature determines the corresponding fit function to a large extent The best-fit parameters for that spectra can thus be taken as a reliable reference for all successive fits of data without a fl -maximum at lower temperature levels Unfortunately, lowfrequency maxima could not be measured for “electronic grade” and “dialysis grade” PVA samples Measurements at even lower frequencies and/or higher temperatures were not evaluated because the additional thermal load accelerates permanent changes of the material due various effects such as degassing of water vapor and molecular fragments,10 increased crystallization and oxidation and softening effects So, any distinct values of R are not available for “electronic-” and “dialysis grade” PVA, and because of the product R˜ εEP (ω) in Eq (1) the fit parameters εs and εs, EP cannot be determined without ambiguity Therefore, impedance data fitting of “electronic-” and “dialysis grade” PVA started with an initial educated guess of parameters which were chosen reasonably similar to the “normal grade” parameters E Fit Results R Data fitting has been performed manually with routines and tools encoded in Mathematica Much effort was made to find all fit parameters close to sufficiently smooth functions with reasonable physical values Following the work of Klein et al.21 the fit parameters can be directly converted (see Appendix) into characteristic material properties The basic non-ionic dielectric properties are given with Fig showing for all three PVA grades the temperature dependent electronic resistivity, ρ, and the the static permittivity, εs , of an ion-free PVA material The “electronic-” and “dialysis grade” spectra below 70◦ C cannot be fitted reliably because of disappearing Z (ω)-minima, and hence related data are not given Results for the fitted spectra suggest a higher electronic resistivity for the special-purpose “electronic grade” PVA compared to the standard “normal grade” PVA The highest resistivity shows the “dialysis grade” PVA which is on average by a factor of 1.9 above the “electronic grade” PVA The static dielectric permittivities of all grades are very similar and show practically no difference within the fitting accuracy The amounts of mobile ionic impurities are displayed in Fig 7, and numerical values at selected temperatures are given in Table I At temperatures above 85◦ C all grades of PVA show a concentration, p, of mobile Na+ ions in the order of 1024 m−3 with a somewhat higher concentration close to 1025 m−3 in the “electronic grade” PVA This matches with the measured Na2 O ash content of 042152-10 M Egginger and R Schwodiauer ă 10 AIP Advances 2, 042152 (2012) 11 (a) ρ (Ωm) 10 10 10 10 10 40 (b) εs 30 20 10 40 60 80 100 temperature T (°C) 120 FIG Fit results for electronic resistivity (a) and static permittivity (b) of “normal grade” , “electronic grade” , and “dialysis grade” PVA films 127 p (1/m ) 10 10 temperature T (°C) 97 84 72 60 50 3.1 25 24 10 10 112 23 22 2.5 2.6 2.7 2.8 2.9 -1 1000/T (10 K ) FIG Fit results for mobile ionic impurity concentrations in “normal grade” , “electronic grade” , and “dialysis grade” PVA films about 0.14% which corresponds to ca × 1024 m−3 , and which is indicated by the dashed line in Fig Dialysis of “electronic grade” PVA reduces the mobile ion concentration by a factor of ∼5, and both, “dialysis grade” and “normal grade” PVA show a similar mobile ion concentration of about 1.4 × 1024 m−3 at high temperatures The concentrations of mobile ions exhibit an unusual temperature dependence which neither follows an an Arrhenius-type behavior, ln p ∝ − 1/T, nor is it described satisfactory with a similar expression from ionic dissociation theory, ln p ∝ − 1/(ε(T) T).31 Such a behavior has also been observed and presented elsewhere32 but no explanation was given In the case of PVA mainly one reason account for the particular temperature dependence: Pristine PVA films are not stable over temperature, and thus all dielectric properties change irreversibly at first heating M Egginger and R Schwodiauer ă 10 10 fh (Hz) 10 10 10 10 10 10 AIP Advances 2, 042152 (2012) fm (Hz) 042152-11 -1 -2 10 -3 50 60 70 80 90 100 110 120 130 temperature T (°C) FIG Frequency position fm at minimum, and fh at maximum of Z for “normal grade” PVA measured in the 1st run (), the 2nd run ( × ) and the 3rd run ( ) TABLE I Numeric parameters of mobile Na+ ions in “normal grade” (ng), “electronic grade” (eg), and “dialysis grade” (dg) PVA drop-cast films at various temperatures T(◦ C) 125 110 95 80 73 65 58 ng p (1021 m−3 ) eg dg 1462.6 1333.3 1143.3 781.85 443.22 191.72 67.10 6611.8 6737.8 6317.5 4504.1 1238.8 n.a n.a 1631.7 1580.5 1449.9 331.78 47.73 n.a n.a μ(10−12 V/(ms)) ng eg dg 1082.1 347.6 97.85 19.28 7.29 1.59 0.15 10.715 3.473 0.591 0.080 0.029 n.a n.a 41.77 8.726 1.300 0.247 0.155 n.a n.a ng σ (10−9 S/m) eg dg 253218 74043.3 17899.7 2412.0 517.25 48.91 1.63 11334.9 3744.1 597.11 57.52 5.83 n.a n.a 10905.9 2206.63 301.48 13.13 1.18 n.a n.a ng τ (ms) eg dg 0.0014 0.0046 0.0188 0.1302 0.5442 4.8882 119.30 0.0435 0.1331 0.8303 8.1227 62.12 n.a n.a 0.0341 0.1690 1.2252 21.119 159.51 n.a n.a This is documented with Fig showing the frequency positions, fm and fh , for the Z -extrema of three successive measurements of a “normal grade” PVA sample After the first measurement run, up to the highest experimental temperature of 125◦ C, the extrema of Z in the second run have clearly shifted irreversibly to lower frequencies and the third run cause a further, however much smaller down shift As already mentioned, the change of dielectric properties upon heating must be attributed to an increase of crystallinity and to the degassing of small volatile compounds The degassing of water vapor is presumably the most relevant process Besides concentration, data fitting also reveal values for mobility, μ, and conductivity, σ , of the mobile ions The temperature dependent mobility for all PVA grades is shown in Fig 9(a) Two dashed lines are depicted in addition Based on early measurements,33–35 the lines illustrate the temperature dependent mobility region of Na+ in thermally grown SiO2 films The mobility of Na+ in SiO2 has been investigated since mobile alkali ions in the oxide were found to be a major cause for electrical instabilities in metal-oxide-silicon structures Sodium ions are hard to avoid and thus the most prominent impurities in oxides Depending on different vendors, peak concentrations of 1023 m−3 to 1025 m−3 can be found, but normally much lower values well below 1017 m−3 are achievable.36 In PVA Na+ ions are integral components of the polymer with fairly high concentrations (>1020 m−3 @ room temperature) and with mobility values at least comparable to those in SiO2 Consequently, the high sodium concentration, p, and the pronounced mobility, μ, (numerical values at selected temperatures are given in Table I) result in a high ionic conductivity, σ ∝ pμ, as depicted in Fig 9(b) Both diagrams in Fig show the “normal grade” PVA with highest values for mobility and conductivity In comparison to the “electronic grade” data the mobility values are more than two orders of magnitude higher, and the respective conductivity is higher by more than one order of magnitude A significantly smaller but still evident difference also exists between the “electronic grade” and the “dialysis grade” PVA In agreement to expectation dialysis removes small impurities and can therefore increase the mobility; the “dialysis grade” mobility is, on average, by a factor of 042152-12 M Egginger and R Schwodiauer ă 127 112 AIP Advances 2, 042152 (2012) temperature T (°C) 97 84 60 72 50 10 -10 μ (m /Vs) (a) 10 10 -12 -14 σ0 (S/m) 10 10 10 (b) -4 -8 -12 2.5 2.6 2.7 2.8 2.9 -3 -1 1000/T (10 K ) 3.1 FIG Fit results for mobility (a) and conductivity (b) of mobile ionic impurities in “normal grade” , “electronic grade” , and dialysis grade” PVA films The region between the dashed lines in (a) illustrate the mobility of Na+ in thermally grown SiO2 films for comparison higher compared to the “electronic grade” mobility Dialysis also lowers the conductivity due to the noticeable reduction of mobile carrier concentration (see Fig 7) The difference in conductivity between the “electronic-” and the “dialysis grade” PVA increases with decreasing temperature; at 73◦ C the dialysis grade conductivity has dropped already by a factor of The stark difference between “normal-” and “electronic grade” PVA is already apparent in the dielectric impedance spectra (see Fig and the supplemental material29 ) The spectra of “normal grade” samples show significantly higher relaxation frequencies The relaxation frequencies are directly related to conductivity and relaxation time, τ ≡ ε0 εs /σ = τ EP /M, of the mobile ions The corresponding data for all grades are displayed in Fig 10, and numerical values at selected temperatures are given in Table I The relaxation time for ions in “electronic grade” PVA is 60 times longer, on average, than in “normal grade” PVA, and similar to the “dialysis grade” values at higher temperatures The relaxation time in the “dialysis grade” PVA, however, increases stronger with decreasing temperature, and at 73◦ C the temporal distance to the “electronic grade” relaxation time has grown by a factor of 2.5 All data presented in Fig and Fig 10 illustrate a reduced ion dynamics in the “electronic grade” PVA compared to the “normal grade” material This difference indicates a different chemical composition of the two polymers It is believed that ion conduction is primarily affected by segmental motion of chains which allow ion migration by hopping between neighboring sites A second mechanism is related to diffusion of sufficiently short polymer chains But further discussion would require better product-knowledge about the detailed chemistry (branched chains, tacticity, volatile content, molar weight distribution, etc .) of the two materials Unfortunately, such information is not yet available at present 042152-13 M Egginger and R Schwodiauer ă 10 10 τ (s) 10 10 127 112 2.5 2.6 temperature T (°C) 97 84 72 60 50 3.1 -1 -2 10 10 AIP Advances 2, 042152 (2012) -3 -4 10 10 -5 -6 2.7 2.8 2.9 -1 1000/T (10 K ) FIG 10 Fit results for relaxation times of mobile ionic impurities in “normal grade” , “electronic grade” , and “dialysis grade” PVA films IV SUMMARY AND CONCLUSION Dissociated ionic Na+ /Ac− impurities in PVA, which are a general byproduct of VA-polymer synthesis, have significant effects on the electrical behavior of the material The presented experimental results from thermal discharge current measurements, polarization experiments and dielectric impedance spectroscopy at elevated temperatures demonstrate conclusively the presence of mobile ionic carriers in PVA thin films The presence of mobile Na+ cations in large amounts must be held responsible for a variety of measurable electronic effects Thermal discharge currents from pristine drop-cast PVA films under short circuit condition arise from internal electric fields due to asymmetric ionic double layer formation at the substrate/PVA/air interfaces As asymmetric interface conditions are always present with thin film preparation, a resulting intrinsic electric field and a polarization of polar molecules has to be expected for all ion containing polymer solutions External polarization of PVA at high fields result in characteristic transient currents with retarded ion current peaks Dielectric impedance spectra of drop-cast PVA films with ion-blocking Au-electrodes show pronounced effects of electrode polarization which can be described very accurately with an electrode polarization model for single-ion conductors Quantitative analysis of isothermal dielectric impedance spectra reveal rather high values for concentration, mobility and conductivity of mobile ions at temperatures above ca 60◦ C – 70◦ C At temperatures above the assumed glass transition temperature of about ∼60◦ C, mobility values of Na+ ions, in samples with standard off-the-shelf PVA, are already comparable to values found in poly(ethylene oxide) based ionomers.21 Mobility values at lower temperatures are in a range comparable to measurements of Na+ in thermally grown SiO2 films where they still cause instabilities in MOS-devices - though present in much lower concentrations The concentration of mobile ionic carriers can be reduced by dialysis cleaning of the polymer solution A 24 hour dialysis was found to reduce the ion concentration by a factor of This reduction lead to a measurable decrease in conductivity and also to an increase of relaxation times In conclusion, mobile Na+ impurities must be considered to cause measurable effects in PVA at electric fields of sufficient strength even at room temperature A contribution of mobile Na+ impurities to uncontrollable instabilities of OFETs with PVA gate dielectrics shall not be ignored Application of clean materials with a low level of contaminants and by-products can be expected to result in an improved stability of organic electronic devices ACKNOWLEDGMENTS We want to thank Prof Dr S Bauer for many helpful comments and suggestions and Prof Dr N.S Sariciftci for providing technical support and facility excess Valuable measurements 042152-14 M Egginger and R Schwodiauer ă AIP Advances 2, 042152 (2012) performed by Kuraray are much appreciated Financial support by the Austrian Science Foundation, FWF project P20772-N20 is gratefully acknowledged APPENDIX: The basic relations of the electrode polarization theory is summerized here for completeness and clarity The application of an electric potential to ion blocking electrodes of a parallel plate capacitor with an ionic dielectric causes the accumulation of free cations next to the negative electrode The cations there, create a positive charge distribution and leave a negative charge distribution at the oposite electrode The distribution width is characterized by the Debye length εs ε0 kB T (A1) LD = q p where q and p represents the ion charge and concentration respectively, and kB denotes the Boltzmann constant The ration between twice the Debye length and the thickness of the dielectric is definefined ˜ D An applied ac potential of sufficiently low frequency reverses the with the symbol M ≡ 12 L/L polarity of the ionic charge distribution The related ion relaxation time is expressed with εs ε0 (A2) τ= σ which is inversly proportional to the ion conductivity σ = q pμ (A3) The mobility, μ, can be expressed with μ= ˜ ( 21 L/M) kB T q τ (A4) The ratio M can be estimated with experimental data according to Eq (4), and all quantities can be calculated from thereon X Peng, G Horowitz, D Fichou, and F Garnier, Appl Phys Lett 57, 2013 (1990) Maliakal, in Organic Field-Effect Transistors, Optical Science and Engineering, Vol 128, edited by Z Bao, and J Locklin (CRC Press, 2007) Chap 3.2, pp 229–251 Wei Wang, Dongge Ma, Su Pan, and Yudan Yang, “Hysteresis mechanism in low-voltage and high mobility pentacene thin-film transistors with polyvinyl alcohol dielectric,” Applied Physics Letters, 101(3), 033303 (2012) Soojin Lim, Boseok Kang, Donghoon Kwak, Wi Hyoung Lee, Jung Ah Lim, and Kilwon Cho, “Inkjet-printed reduced graphene oxide/poly(vinyl alcohol) 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(2012) Analysis of mobile ionic impurities in polyvinylalcohol thin films by thermal discharge current and dielectric impedance spectroscopy 1,a M Egginger1,2 and R Schwodiauer ă Department of Soft... experiments and dielectric impedance spectroscopy at elevated temperatures demonstrate conclusively the presence of mobile ionic carriers in PVA thin films The presence of mobile Na+ cations in large... exponent of n 11 for rising currents (driven by rampedtemperature) and n −0.9, for decreasing currents during the first period of isothermal discharge This first period ends as the discharge currents