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Effect of oxidant on the performance of conductive polymer films prepared by vacuum vapor phase polymerization for smart window applications
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Smart Mater. Struct. 24 035016 (http://iopscience.iop.org/0964-1726/24/3/035016) View the table of contents for this issue, or go to the journal homepage for more
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Smart Materials and Structures Smart Mater. Struct. 24 (2015) 035016 (9pp)
doi:10.1088/0964-1726/24/3/035016
Effect of oxidant on the performance of conductive polymer films prepared by vacuum vapor phase polymerization for smart window applications Robert Brooke, Manrico Fabretto, Nastasja Vucaj, Kamil Zuber, Eliza Switalska, Lachlan Reeks, Peter Murphy and Drew Evans Thin Film Coatings Group, Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia E-mail:
[email protected] Received 11 December 2014 Accepted for publication 12 January 2015 Published 10 February 2015 Abstract
Conductive polymers synthesized by vacuum vapour phase polymerization (VPP) were investigated and optimized by changing the oxidant solution and VPP chamber parameters for their incorporation into ‘smart window’ electrochromic devices. Additionally, the interaction of two oxidant solutions with typical electrode materials (aluminium and indium tin oxide) were examined with respect to material etching, device cosmetics and long term device degradation (over 10 000 switch cycles). Devices made with conducting polymers synthesized with the oxidant Fe(Tos)3 rather than FeCl3 produced superior device performance with respect to optical switching range (%T), switch speed and optical relaxation. S Online supplementary data available from stacks.iop.org/SMS/24/035016/mmedia Keywords: electrochromic, PEDOT, polypyrrole, vapour phase polymerization, electroactive, smart window (Some figures may appear in colour only in the online journal) Introduction
input to alter the transmitted heat/visible light [3]. While there are a wide variety of materials and technology that can be employed [3], electrochromic devices incorporating electroactive polymers can provide an alternative to inorganic smart windows, some of which use scarce materials with limited supply. Common examples of inorganic electrochromic materials include tungsten oxide and other transition metal oxides [4]. Devices utilizing such materials, however, require high grade purity for acceptable performance which often requires expensive manufacturing processes [4, 5] whereas the organic counterparts, in general, do not. Traditionally, the synthesis of conducting polymers (CPs) has been dominated by wet chemical and electrochemical approaches [6, 7]. These techniques can be limited by several factors including issues with scale up (solution/ink stability and dispensing), homogenous deposition and
The design and implementation of energy efficient buildings, vehicles, infrastructure etc is one of the major issues needing attention and is the impetus for a sustainable energy-use society. This requires the modification or creation of new materials which will contribute to the generation and/or reduction in energy usage. In vehicles and dwellings one of the main sources of heat ingress/egress occurs though ‘heat bridges’, of which the largest contributor is (automotive or architectural) glazing. A number of passive barriers have been developed including double and triple glazed windows [1], and infrared reflective coatings [2]. A complementary strategy involves the use of ‘smart windows’ which have the ability to alter their optical transmittance in response to changes in the surrounding environment by means of an applied electrical 0964-1726/15/035016+09$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
Smart Mater. Struct. 24 (2015) 035016
R Brooke et al
cosmetic appearance, and in the case of electrochemically synthesized polymers the need for conductive substrates [8, 9]. An alternate technique is vapour phase polymerization (VPP) in which the polymer is synthesized in situ on the substrate and is largely limitation-free with respect to coverage area. VPP involves the deposition of an oxidant solution by such means as; spray, spin, dip, knife, slot-die coating etc, followed by exposure to a monomer vapour within a sealed chamber, after which the film is washed to remove consumed oxidant and any un-reacted monomer [10–12]. The technique is amenable to any monomer that is able to sublime or evaporate into a gaseous state and coverage area is largely limited by the volume of the chamber itself. The end result is a thin film CP ready for incorporation into an opto-electronic device. VPP synthesized CPs have been employed in many applications ranging from biosensors [13–16] to organic photovoltaic devices [17, 18], with many such devices employing two of the more commonly available polymers, namely, polypyrrole (PPy) [19–25] and poly(3,4-ethylenedioxythiophene) (PEDOT) [26–32]. Some devices and applications, however, have suffered from less than acceptable performance due to poor material performance, inhomogeneous deposition, and short device life-time due to unintended chemical reactions under switching potentials. These are important issues needing attention if the intended device must meet some minimum performance specification or be cosmetically acceptable to the end-user. The synthesis of CPs using VPP has been primarily conducted using two commonly available oxidant species, namely FeCl3 [20, 24, 33] and Fe(Tos)3 [10, 28, 34], although other oxidants [9, 35–37] have been used as well. While it has been reported that the choice of oxidant (and its concentration) affects the properties of the synthesized CP [9], little work has been conducted on the manner by which the oxidant may interact with common electrode materials (i.e. aluminium (Al) and indium tin oxide (ITO)) used in such devices. Previous reports have demonstrated that acidic oxidant solutions degrade inkjet printer heads [34, 38] and that FeCl3 solutions are used as etchants for circuit board [39] manufacture. Given this, there has surprisingly been little consideration given to the potential for deleterious interactions between the oxidant and the conductive substrate when using VPP to synthesis the polymers. For their application in electrochromic devices, CPs are required to possess certain performance characteristics such as; sufficient service-life, acceptable optical transmission, fast switching times, and long-lived optical memory for energy efficient operation. For practical applications electrochromic devices should be expected to withstand greater than ten thousand switches with little to no degradation in performance [40], have a maximum switching time of a few seconds [41] and, for certain devices have an optical memory of over an hour [42]. In this paper, the two commonly used oxidant species of FeCl3 and Fe(Tos)3 were investigated in the VPP of pyrrole and 3,4-ethylenedioxythiophene (EDOT). The oxidant interaction with Al and ITO electrode materials was investigated over the same duration that typical VPP processes are conducted. Analysis using x-ray photoelectron
spectroscopy (XPS) was employed to elucidate any electrode etching leading to metallic content within the resulting PEDOT film. Furthermore optimization of the VPP of pyrrole using the oxidant Fe(Tos)3 was undertaken in an effort to produce high quality conductive films that were sufficiently mechanically robust so as to be suitable for use in electrochromic devices. The performance and operational degradation of the PEDOT–PPy electrochromic device, made using FeCl3 and Fe(Tos)3, was investigated with respect to; optical memory, optical switch, switching time, and device degradation over more than ten thousand switching cycles.
Experimental Iron (III) chloride hexahydrate (FeCl3), pyrrole, hexanol, butanol, ethanol, tri-block copolymer poly(ethylene glycol– propylene glycol–ethylene glycol) (PEG–PPG–PEG), Mw = 5800 g.mol, P-123 were obtained from Aldrich. Iron (III) tosylate (Fe(Tos)3) as a 40 wt% solution in butanol (CB40 V2) and EDOT were purchased from Heraeus. The ionic liquid (IL), 1-butyl-1-methylpyrrolidinium bis-(trifluoromethylsulfonyl)imide, was obtained from Merck and desiccated under vacuum to remove trace water (
