The mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic) acid using linear-diol additives: Its effect on electrochromic performance

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Thin Solid Films 516 (2008) 7828 – 7835 www.elsevier.com/locate/tsf

The mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic) acid using linear-diol additives: Its effect on electrochromic performance M. Fabretto a,⁎, C. Hall a , T. Vaithianathan c , P.C. Innis b , J. Mazurkiewicz b , G.G. Wallace b , P. Murphy a,c b

a Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA, 5095, Australia Intelligent Polymer Research Institute, University of Wollongong, Wollongong, NSW, 2522, Australia c Mawson Institute, University of South Australia, Mawson Lakes, SA, 5095, Australia

Received 27 November 2007; received in revised form 13 March 2008; accepted 18 April 2008 Available online 2 May 2008

Abstract The conductivity of poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic) acid (PEDOT–PSS) thin-films was increased by up to 900 times with the addition of linear-diols. Three series of linear-diols were investigated. It is proposed that three mechanisms contribute to improved conductivity; conformational changes in the polymer from coil to expanded, spatial separation of PEDOT–PSS grains and charge screening. The variation in conductivity with temperature indicates that diols affect both intra- and inter-chain charge transports. Electrochromic devices were constructed using standard and enhanced conductivity PEDOT–PSS. Switching times from bleached to darkened states were reduced while darkened to bleached states remained unaltered. Total percent transmission change (%ΔTx) remained constant indicating that enhanced conductivity PEDOT–PSS improves switching but does not lead to additional electrochromic redox sites being accessed. © 2008 Elsevier B.V. All rights reserved. Keywords: PEDOT–PSS; Thin-film; Linear-diol; Electrical conductivity; Electrochromic device

1. Introduction Of the many conducting polymers that have been developed, poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic) acid or PEDOT–PSS (see Fig. 1) has probably been one of the most commercially successful since its development by Bayer in the late 1980s [1–3]. This stems from the fact that the polymer has many positive attributes such as thermal stability, adequate conductivity and good optical transmission. Moreover the addition of the PSS polyanion has made PEDOT water-soluble, giving it good film forming properties which makes it easy to utilise in various applications [4]. These attributes have generated a high level of interest in the aqueous based polymer for applications in electronics and related fields. The down side in having achieved this high level of processability is that ⁎ Corresponding author. Tel.: +61 8 83023675; fax: +61 8 83023683. E-mail address: [email protected] (M. Fabretto). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.099

conductivity has suffered and excess PSS has been shown to cause corrosion [5]. In thin-film form the polymer has been used in organic light emitting diodes as the hole-injecting layer [6], source/drain electrodes in thin-film transistors [7], anodes in photovoltaic cells [8], as anti-static coatings for photographic films [9,10], and as an electrode material in capacitors [11]. Due to its chromic properties it has also found widespread use in electrochromic devices such as display panels and so called “smart-glass” [12–15]. A number of research groups have focused on improving the conductivity of PEDOT–PSS without sacrificing the polymer's optical transmission properties with the addition of non-ionic additives or so called secondary dopants [16-20]. However there is no information on how the improved conductivity influences the operation of electrochromic devices where the electroactive polymer is PEDOT–PSS. The additives are typically added post polymerisation and prior to film formation. They include polyalcohols such as

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2. Experimental details

Fig. 1. The structural formula of PEDOT–PSS complex.

sorbitol or glycerol and polar solvents such as dimethyl sulfoxide (DMSO), tetrahydrofuran (THF) or dimethyl formamide (DMF). The exact mechanism(s) involved in the conductivity improvement in PEDOT–PSS with the addition of secondary dopants is still open to debate [21,22]. Inganas et al. [16,17] observed an increase in conductivity with the addition of sorbitol and suggested that it performs the function of a plasticiser, decreasing inter-chain interactions and helping reorientate the polymer chains in a more favourable manner. Jonsson et al. [23] also added sorbitol but proposed that excess PSS is washed away from the surface of the PEDOT–PSS solution during film formation creating better connectivity between individual polymer grains near the surface and hence forming better conductivity pathways. Kim and co-workers [24] added polar solvents such as, DMSO, DMF and THF, and observed an increase in the conductivity of PEDOT–PSS films. They found a correlation between the dielectric constant of the solvent and conductivity. It was suggested that some of the polar solvent may remain in the PEDOT–PSS film, thus providing a screening effect between the positively charged PEDOT chains and the negatively charged PSS chains. The reduction in Coulomb interaction between the two polymer chains facilitating easier charge movement along the PEDOT backbone. Ouyang et al. [21,25] noted that the largest increase in conductivity occurred when the additives had two or more polar groups. They proposed that the driving force for conformational change of PEDOT was the interaction between the dipole of the additive and the positive charge (or dipole) on the PEDOT chains. This led to an expanded conformation, promoting greater inter-chain interactions as well as increasing charge delocalisation along the PEDOT chain. In this work the conductivity of PEDOT–PSS thin-films was systematically examined with the addition of three different series of linear end-terminated diol additives. The effects of additive concentration and chain length were investigated for the three different diol series. In addition, PEDOT polymers showing the most pronounced conductivity improvements were then utilised in the construction of complementary electrochromic devices using enhanced PEDOT and polyaniline (PANI). Switching rates and optical transmission changes were measured and compared to a standard PEDOT–PANI device.

The PEDOT–PSS thin-films were made using commercially available aqueous PEDOT–PSS from Aldrich, Cat. No. 483095. PANI–PSS was synthesised according to a method reported by Li and Kaner [26], purified by dialysis and used without further treatment. The linear-diol additives were 98% purity or higher from Aldrich and were grouped into three series: Alkane diol — 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol and 1,5-pentanediol; Ethylene diol — diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol (TeEG) and; Thiol diol — 2,2'2′thiodiethanol, 2,2′-dithiodiethanol. The structural formulas of the diols are given in Table 1. Diol concentrations ranging from 0.5 × 10− 4 to 4.0 × 10− 4 mol g− 1 of PEDOT–PSS were added to 6 mL of PEDOT–PSS solution and the mixtures were placed on a tumble-wheel (Model RSM6, Ratex, Australia) and spun at low speed for 24 h. To enhance wetting 1.2 mg of surfactant (BYK346, Chemie, Germany) was added to PEDOT–PSS and PANI–PSS solutions prior to spin or spray deposition. Thin-films of PEDOT–PSS solution were spin-coated (Model P6700, Specialty Coating System, USA) at various speeds from 1250–3000 rpm onto glass microscope slides to establish an optimum spin speed. Edge defects were observed for samples spun at speeds below 1750 rpm and a final spin speed of 2000 rpm was selected. The microscope slides were scored on the backside, snapped and mounted “edge-on” so that thickness measurements could be made using a high resolutionscanning electron microscope (HR-SEM) (Philips XL30 FEGSEM with Oxford CT1500HF Cryo-stage). Film thicknesses ranged from 50 to 100 nm for the different spin speeds and an example is given in Fig. 2. Prior to use all glass slides were cleaned with ethanol in an ultra-sonic bath (Model 160T, Soniclean, Australia) and rinsed with ultra-pure water, resistivity > 18 MΩ·cm (Model UHQPS, Elga, England). The plates were then air plasma cleaned (Model PDC-32G, Harrick, USA) for 1 min and two chromium pads 25 mm apart were direct current (DC) sputter-coated onto the plates using an in-house built DC sputter system. Pad surface resistance was less than 10 Ω/□ which was deemed insignificant compared to the typical values (N2 kΩ/□)of PEDOT–PSS thin-films produced in this study. Samples were spin cast, oven dried at 125 °C for 1 h, before conductivity measurements were carried out at room temperature (21–23 °C). All conductivity measurements were performed in accordance with ASTM D4496-87 [27]. Table 1 Chemical name, structural formula and chain length for each linear-diol arranged into three series, namely, Alkane, Ethylene and Thiol Series name Chemical name

Structural formula

Alkane

HOCH2CH2OH 5.1 HOCH2CH2CH2OH 6.4 7.7 HOCH2(CH2)2CH2OH HOCH2(CH2)3CH2OH 9.0 HOCH2CH2OCH2CH2OH 8.7 HOCH2CH2(OCH2CH2)2OH 12.2 HOCH2CH2(OCH2CH2)3OH 15.8 HOCH2CH2SCH2CH2OH 9.2 HOCH2CH2SSCH2CH2OH 10.7

Ethylene

Thiol

1,2-ethanediol 1,3-propanediol 1,4-butanediol 1,5-pentanediol diethylene glycol triethylene glycol tetraethylene glycol 2,2′-thiodiethanol 2,2′-dithiodiethanol

Length (Å)

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Fig. 2. High resolution-scanning electron microscopy image of PEDOT–PSS thin-film casts onto glass substrate at a spin-coat speed of 2000 rpm. The substrate was back-scored, snapped and edge mounted. Four discrete measurements (nanometres) of the PEDOT polymer thickness are shown along the length of the substrate.

Conductivity data are the average of a minimum of four individual measurements. The average error for conductivity measurement data across all samples was ±5.0% relative error. Electrochromic devices were constructed by spray depositing PEDOT–PSS and PANI–PSS onto 50 × 50 mm indium tin oxide (ITO) glass substrates (resistivity b 15 Ω/□, Yih Dar, Taiwan) and then oven cured for 1 h. Polymer deposition thickness was monitored via percent change in optical transmission using a Hunterlab ColorQuestXE (USA). Fig. 3 shows the construction of the device. The device's ITO plates are offset to allow flying leads to be attached. A 125 μm polymer spacer was used to ensure consistent spacing and the perimeter was sealed with two part epoxy glue leaving a small opening for vacuum back filling. The electrolyte utilised in the device was an ionic liquid synthesised in-house. After out-gassing the vent port was sealed. Changes in optical transmission were recorded using a Hunterlab ColorQuestXE. 3. Results and discussion It has been proposed [17,21,25] that one of the polar groups of the additive may interact with the dipole or positive charge on the PEDOT polymer with the other polar group forming a hydrogen bond with the PSS polyanion. Thus the action of the additive may be to form a pseudo-scaffolding network between PEDOT–PSS grains, increasing the distance between positive charges on the PEDOT and negative charges on the PSS. This effect has been referred to as plasticising [17,28]. In general, a polymer that has discrete positive (or negative) charge islands distributed along its length will tend to adopt an expanded structure due to electrostatic repulsion. However for the PEDOT–PSS complex, strong ionic coupling between the two polymers tends to inhibit any electrostatic repulsion between similar charges along each polymer chain, promoting a more compact structure. With the incorporation of diol additives, ionic coupling may be significantly reduced. As a result the increase in electrostatic repulsion

between ions of the same sign along the PEDOT and PSS polymers will now favour an expanded structure as first proposed by MacDiarmid [29] and recently confirmed by Ouyang and coworkers [21,25,30]. It is understood that an expanded conformation promotes the delocalisation of charge along the PEDOT chain, leading to an increase in charge mobility and hence on a macroscopic scale an increase in conductivity [21,25,31]. Chain lengths for the various diols were calculated using Gaussian 03W software (Gaussian Inc, USA), and all were shown to be linear. The linear lengths of the diols varied from 5.1 to 15.8 Angstroms and are given in Table 1. Fig. 4 shows the electrical conductivity (S cm− 1) of PEDOT–PSS thin-films as a function of diol concentration for the three different series, namely, alkane, ethylene and thiol. Note that the pristine PEDOT–PSS without any additive had a film resistance of 1850 kΩ/□ corresponding to a conductivity of only 77.2 mS cm− 1. For the ethylene and thiol series, one can generally say that the maximum conductivity was recorded at a diol concentration of 3 × 10− 4 mol g− 1. Concentrations higher than this resulted in decreased conductivity. For the alkane series, the conductivity was still trending upwards for the two shortest diols with increasing concentration (and is the subject of further investigation). However as for the ethylene and thiol series, the conductivity for the two longer alkane diols had similarly decreased after a concentration of 3 × 10− 4 mol g− 1. The increase in conductivity, due to diol addition, is due to an expanded polymer structure being formed which leads to better intra-conductivity (i.e. along individual polymer chains). The trade off is that individual polymer grains become physically separated leading to poorer inter-conductivity (i.e. across individual polymer chains). Obviously a maximum conductivity is achieved when the increase in intra-conductivity equals the decrease in inter-conductivity. Close examination of the maximum conductivity for the alkane diol series is significantly lower than for the ethylene and thiol series. Such a result begs the question as to whether the increase in conductivity is purely related to linear chain length or whether there is an additional effect, such as Coulomb charge screening, as the pseudo-scaffolding structure is established. Fig. 5 shows PEDOT conductivity plotted against chain length for each of the diols, at low (1.0 × 10− 4 mol g− 1), intermediate (2.0 × 10− 4 mol g− 1) and high (4.0 × 10− 4 mol g− 1) concentrations. If the only or dominant mechanism for an

Fig. 3. Electrochromic device with complementary PEDOT–PSS and PANI– PSS deposited onto opposite ITO glass substrates. The glass substrates are offset to allow electrodes to be connected to the ITO surfaces. Spacer/seal separation is nominally 150 μm. (Note: Figure is not to scale.)

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diol series are linear and have identical terminal groups. Ideally one would expect that a certain chain length will generate the optimum conformational change from compact to expanded while at the same time minimally sacrificing inter-molecular packing of the PEDOT–PSS complex. Examining the low diol concentration results (Fig. 5A) one could argue that this was indeed the case. All the data from the different diol series fall essentially onto a common master curve with a steep increase in

Fig. 4. PEDOT–PSS conductivity (S cm− 1) as a function of linear-diol concentration (×10− 4 mol g− 1 of PEDOT–PSS). A) Alkane series, B) Ethylene series, C) Thiol series.

increase in conductivity was due to a conformational change from compact to expanded then it is not unreasonable to expect that the results should be independent of the diol series and based on the chain length of the additive. That is to say, all data points from the three diol series should fall onto a common master-line. Such an assumption is reasonable given all three

Fig. 5. PEDOT–PSS conductivity (S cm− 1) as a function of linear-diol chain length (Å) for three concentration loadings: A) Low, 1 × 10− 4 mol g− 1; B) Intermediate 2 × 10− 4mol g− 1 and; C) High, 4 × 10− 4mol g− 1.

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conductivity occurring at a chain length of approximately 9 Å. This result tends to suggest that short chained diols are unable to form an effective pseudo-scaffolding structure at low concentrations, that is, they are unable to effect a significant conformational change in the polymer. There is however a notable exception, namely 2,2'2′-dithiodiethanol, indicating the possibility of a mechanism other than chain length alone being responsible for the improvement in conductivity. Examining the intermediate concentration results (Fig. 5B) the data still essentially falls onto a master curve, the exception now being the diol DEG. Once again a sharp increase in conductivity is noted at a chain length of 9 Å. This tends to further support the notion that short chained diols are unable to form effective scaffolding structures, especially at low concentrations. At high diol concentrations (Fig. 5C) a marked departure from that seen at the two lower concentration levels is observed. The data from the three diol series have separated and there is no longer a common master-line. The ethylene series producing the highest conductivity, the alkane series the lowest and the thiol series intermediate between the two. If Coulomb charge screening also plays an active role in increased conductivity as proposed by Kim and co-workers [24], then given a constant chain length one should record differing results for the three diol series due to their markedly different electron shell distribution. The closest three diol additives with respect to chain length from the different series are diethylene glycol = 8.7 Å, 1,5pentanediol = 9.0 Å and 2,2'2′-thiodiethanol = 9.2 Å. The conductivity versus concentration for these diols is given in Fig. 6. At low diol concentrations, 0.5–1.0 × 10− 4 mol g− 1, there is a small to modest increase in PEDOT conductivity with no appreciable difference between the diols. However as the diol concentration is increased the conductivity of PEDOT differs appreciably, a strong indication that an additional mechanism and not chain length alone is responsible for increased conductivity. A possible explanation could be that at low concentrations the pseudo-scaffolding structure formed by the diols promotes an expanded conformation as previously

Fig. 6. PEDOT–PSS conductivity (S cm− 1) as a function of linear-diol concentration for similar diol chain length additives from three different series, namely, Ethylene — diethylene glycol (8.7 Å), Alkane — 1,5-pentanediol (9.0 Å) and Thiol — 2,2′-thiodiethanol (9.2 Å).

noted [20,21,25], with concomitant separation of the PEDOT– PSS grains [17,28]. However, this pseudo-scaffolding structure is incomplete at these low concentration levels. Therefore one can postulate that the dominant factor influencing an increase in PEDOT conductivity at low diol concentrations is chain length. Simply, the larger separation between polymers grains favours a more expanded polymer structure, with greater interconnectivity, as well as inducing some ionic decoupling. The end result is an increase in conductivity. At high concentrations when the pseudo-scaffolding structure is complete interconnectivity of polymer grains remains high but the effective Coulombic screening produced by the diols becomes an additional factor which further increases PEDOT conductivity. Therefore, if two diols have the same chain length the diol that produces a greater effective Coulombic screening will affect a greater increase in conductivity. As the diol concentration is further increased the conductivity begins to fall and this can be explained in terms of an excess in additive decreasing the packing density of the PEDOT grains resulting in less interconnectivity. It is accepted that conductivity in conjugated polymers is both a function of intra- and inter-chain charge transport [25,28,29]. Intra-chain charge transport follows the polaron-model first proposed by Heeger et al. [32]. The hallmark of polaron-like behaviour is that conductivity increases with temperature. Essentially the electronic charges along the polymer chain are strongly localised due to the ionic coupling with the dopant polyanion. The semi-ordered sites along the dopant polyanion provide potential wells that confine the charge carriers, thus hindering charge movement [33]. Thermal excitation helps to overcome this coupling so that charge can more easily move from one localised site to the next. As a result conductivity increases with temperature. Inter-chain charge transport is metallic-like in behaviour. A metallic response is marked by an increase in conductivity with decreasing temperature. The electrons that carry electrical current within a metal travel as free particles, and the frequency with which these electrons are scattered determines the inherent conductivity of the metal. As the dominant source of scattering is caused by thermal vibrations of the atoms, lowering the temperature has the effect of freezing these vibrations. Timpanaro et al. [28] examined PEDOT–PSS using scanning tunnelling microscopy and was able to show PEDOT cigarshaped clusters in a sea of PSS, however no quantitative measurements for the phase segregated regions were given. Using conductive-mode atomic force microscopy, Ionescu-Zanetti et al. [34] were able to estimate that the conductivity normal to the surface was four orders of magnitude smaller than the macroscopic in-plane conductivity, indicating a lamellar PEDOT–PSS structure. However intra- and inter-chain conductivities were not possible. Given that the task of separating intra- and inter-chain charge transport phenomena and assigning quantitative values is difficult, the following intuitive picture is developed to explore the two phenomena with respect to the addition of diols to PEDOT–PSS. Intra-chain charge transport should increase as the charge carriers are decoupled from their respective ionic partners. This decoupling should increase as the separation between adjacent PEDOT and PSS grains increases and as the polymer adopts an expanded conformation. If this premise is correct, one

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Fig. 7. PEDOT–PSS conductivity (linear-diol concentration 3 × 10− 4 mol g− 1) as a function of temperature for the Ethylene series: DEG, TEG and TeEG. The conductivity of each sample was within ±1% of each other at room temperature (20 °C).

would expect to see the smallest improvement in conductivity with increasing PEDOT–PSS temperature when the longest diol additive was used. The reason being that the longest diol additive will have decoupled the ionic pair (between adjacent PEDOT and PSS grains) by the greatest margin, thus producing the highest intra-chain charge mobility. If this is true, thermal excitation will provide the smallest increase in intra-charge transport to the polymer complex containing the longest diol additive. The trade off for increased intra-chain transport is that inter-chain charge transport may well be reduced as a result of the decrease in packing density between individual PEDOT–PSS grains. The ethylene series of diol additives, at a concentration of 3 × 10− 4 mol g− 1, were used to examine the proposed scenario. Three samples containing the diols, DEG, TEG and TeEG were prepared so that they had essentially the same conductivity at room temperature (i.e. 60.7 ± 0.5 S cm− 1 at 20 °C). Examination of Fig. 7 reveals that the increase in conductivity with temperature for the PEDOT–PSS thin-films increased by the greatest amount for the shortest diol DEG, followed by TEG and finally TeEG. The result is consistent with the notion that the longest diol additive, TeEG, would have affected the greatest ionic decoupling between adjacent PEDOT–PSS complexes. Hence the benefit of thermal excitation to intra-chain charge transport would have seen TeEG improving by the least amount compared to the two shorter ethylene diols. Given that all three PEDOT–PSS films had the same conductivities at room temperature (20 °C) one can assume that the sum of the intra- and inter-chain charge transport must have been the same. Thus one can conclude that the increase in intrachain charge transport for the TeEG sample, compared to DEG and TEG samples, must have been offset by an approximately equal decrease in inter-chain charge transport. The most likely explanation being that the longer chained TeEG diol reduces the packing density and hence interconnectivity between individual PEDOT–PSS grains. To examine whether enhanced conductivity PEDOT–PSS thin-films translates into improved electrochromic device perfor-

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mance three devices were constructed, a standard PEDOT–PANI device, where both polymers were untreated and two devices utilising triethylene glycol and 2,2'2′-dithiodiethanol enhanced conductivity PEDOT. PANI was kept constant throughout. The results are summarised in Table 2. Within experimental error no significant difference was noted in the total percent change in optical transmission, Δ%T, during device switching with all three devices recording values between 46 to 47%. The result tends to suggest that no additional PEDOT material is accessed during redox switching when enhanced conductivity PEDOT is used. The result is not unexpected as diol addition enhances polymer conductivity and interconnectivity but an increase in Δ%T would indicate that extra chromic sites had been generated. Or at the very least that chromic sites that were originally isolated had now been connected into the polymer matrix. Clearly the results do not support these notions. The minimum and maximum transmission values however were shifted downwards slightly for the two conductivity enhanced PEDOT devices. Whether this is due to slight variations in deposition thicknesses or indeed a result of the diols is unknown at this stage and subject to further investigation. Of significant note is the greatly reduced switching times going from a bleached to darkened state when the two enhanced conductivity PEDOT samples are used in the devices. For comparison purposes Fig. 8A,B highlights the difference between the standard device and the TEG-enhanced PEDOT device. For the standard PEDOT–PANI device the time taken to go from a totally bleached to a totally darkened state was 60 s, the thiol-enhanced PEDOT–PANI device taking 20 s, with the ethylene-enhanced PEDOT–PANI device taking 16 s. The time taken for a 95% switch was 18, 4 and 4 s for the standard, thiol-enhanced and ethylene-enhanced PEDOT PANI devices. PEDOT in its bleached state (p-doped) is in its most conductive form and the conductivities for the three PEDOT samples in their neutral form were 77 mS cm− 1, 58 S cm− 1 and 70 S cm− 1 for standard, thiol-enhanced, and ethylene-enhanced PEDOT. PEDOT undergoes reduction when electrically switched from the bleached to darkened state, and so the rate of electron ingress from the ITO substrate into the polymer is enhanced if the initial conductivity is higher. Warren and Madden [35] utilised a variable resistance transmission line concept to model the behaviour of the polymer during a switch. In this model the polymer is described as

Table 2 Total change in optical transmission, Δ%Tx, optical switching range, %Tx, and switching response times for 100% and 95% of full switch both in the darkening direction TD100%, TD95% and the bleaching direction TB100%, TB95%, for Standard, triethylene glycol (Ethylene series) and 2,2′-dithioldiethanol (Thiol series) enhanced electrochromic devices Optical parameters

Standard PEDOT/PANI

Thiol — PEDOT/PANI

Ethylene — PEDOT/PANI

Δ%Tx %Tx range TD100% (s) TD95% (s) TB100% (s) TB95% (s)

47% 13–60% 60 18 12 4

46% 11–57% 20 4 12 4

46% 10–56% 16 4 12 4

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switching, electron egress at the PEDOT–ITO interface quickly changes this region into a highly conductive state, further enhancing electron egress, which causes the next layer to undergo the same reaction and so on. This leads to a runaway situation with the result that switching speed is largely unaffected by improvements in the conductivity of PEDOT. The limiting factor is electron egress through the low conductivity portion of the polymer. 4. Summary and conclusion

Fig. 8. Switching response times for PEDOT–PANI electrochromic device. A) Standard PEDOT, B) Ethylene-enhanced (TEG) PEDOT. 95% of the full optical switch is highlighted by upper and lower dashed lines.

a transmission line composed of resistive elements for both ionic and electronic mobility and capacitive elements for the charge capacity of the polymer film. If the polymer's electronic conductivity is initially high and progressively goes low in a sigmoidal manner [35,36] with electron ingress, one would intuitively expect switching responses to be quicker with higher conductivity PEDOT and the results tend to support this. As PEDOT is reduced the conductivity progressively reduces through the PEDOT layers leading to a darkening transition that is initially fast but slows appreciably as each successive PEDOT layer transitions from high to low conductivity. This point is highlighted by the fact that the time taken to get to 95% of a full switch is 18, 4 and 4 s (standard, thiol-enhanced and ethyleneenhanced PEDOT, refer Table 2), with the last 5% of the switch taking an additional 42, 16 and 12 s for the three respective devices. Interestingly the switching times, within experimental error, going from a darkened to bleached state (electron egress/PEDOT oxidisation) remained essentially the same for all three devices. The time taken to reach the maximum bleached state was 12 s for all three devices with 95% of maximum taking 4 s for each device. In the reduced state the conductivity of PEDOT is low. On

Thin-films of PEDOT–PSS were cast onto glass substrates and diol additives from three different series were used namely, alkane, ethylene and thiol, in varying concentrations. All lineardiols tested increased the conductivity of PEDOT–PSS thinfilms with the best diol increasing the conductivity by a factor of up to 900 times. At low and intermediate concentrations all three diol additive types produced improvements in conductivity that were related to diol chain length. At higher concentrations a departure from a common master-fit was noted with the ethylene series producing the best conductivity. To explain this phenomenon it was proposed that in addition to increased separation and conformational changes in the PEDOT–PSS complex, Coulomb screening between polymer grains may also play a role in conductivity improvement. The conductivity temperature dependence for ethylene enhanced PEDOT–PSS films was investigated in an attempt to elucidate between intra- and inter-chain charge mobility. Three different samples within the ethylene series were produced so that they had the same conductivity at room temperature, indicating that the sum of intra- and inter-chain charge mobility was the same for each sample. At elevated temperatures the largest improvement in conductivity occurred with DEG (shortest chain length), the least with TeEG (longest chain length). This suggests that DEG had the highest interchain charge mobility but at the expense of intra-change charge mobility. Conversely it indicated that TeEG had the highest intra-chain charge mobility but that this was at the expense of reduced inter-chain charge mobility. Electrochromic devices were constructed with conductivity enhanced PEDOT using ethylene diol — TEG and thiol diol — 2,2'2′-Dithiodiethanol additives and compared to a standard PEDOT–PANI device. Both enhanced conductivity PEDOT– PANI devices produced faster switching times going from bleached to darkened states, indicating an increase in the rate of electron ingress when the PEDOT is initially in its most conductive state. Switching times going from darkened to bleached states did not improve over the standard PEDOT– PANI device, indicating that the rate of electron egress remained essentially unaltered when the PEDOT was initially in its least conductive state. Acknowledgement The authors acknowledge the financial support of the Australian Research Council in the form of grant and a Federation Fellowship.

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