Sequential Solution Polymerization of Poly(34-ethylenedioxythiophene) Using V2O5 as Oxidant for Flexible Touch Sensors

摘要

class="head no_bottom_margin" id="sec1title">IntroductionPoly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer with many attractive properties, such as good conductivity, high optical transparency in visible range, excellent flexibility, and good chemical stability. These properties render it a key component for transparent electrodes, electrochromic devices, electromagnetic shielding, etc.(, , , , ) In situ polymerization of 3,4-ethylenedioxythiophene (EDOT) monomers is an approach to obtain PEDOT films. There are generally three methods, namely, electropolymerization (EP) (), oxidative chemical vapor deposition (OCVD), and vapor phase polymerization (VPP) (, ). In the EP process, the EDOT monomer is dissolved in electrolyte and then polymerized on an electrode under electrical bias (, ). A conductive substrate is required in such a process, thus limiting the application of this method. In the OCVD process, the monomer and oxidant are delivered in vapor phase at the same time, making it possible to synthesize, deposit, and dope the conjugated polymer in a single step (, ). In contrast, VPP needs two steps: (1) forming an oxidant layer on the substrate by solution means, e.g., spin coating, dip coating, blade coating, etc. and (2) exposing the oxidant-covered substrate to monomer vapor (). In recent years, the VPP method has gained popularity in EDOT polymerization. Kim et al. first polymerized PEDOT film via VPP; they used FeCl3·6H2O as oxidant and the obtained PEDOT film exhibited a low conductivity of 1 S/cm at thickness ranging from 20 to 100 nm (). Then Winther-Jensen et al. changed the oxidant to iron (III) p-toluenesulfonate, and the conductivity exceeded 1,000 S/cm with the addition of pyridine (). After that, different weak bases such as pyridine, imidazole, and glycol-based block copolymers such as poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) and poly(ethylene glycol)-ran-poly(propylene glycol) (PEG-ran-PPG) were added to the oxidant solution to adjust the polymerization rate and to inhibit side reactions, leading to highly conductive PEDOT films (href="#bib39" rid="bib39" class=" bibr popnode">Metsik et al., 2014, href="#bib21" rid="bib21" class=" bibr popnode">Huang and Chu, 2011, href="#bib71" rid="bib71" class=" bibr popnode">Zuber et al., 2008, href="#bib13" rid="bib13" class=" bibr popnode">Fabretto et al., 2009, href="#bib14" rid="bib14" class=" bibr popnode">Fabretto et al., 2012, href="#bib43" rid="bib43" class=" bibr popnode">Ouyang et al., 2018). Notably, Sung et al. obtained single crystalline PEDOT nanowires in nanoscale channels of a mold covered with FeCl3, the average conductivity reached 7,629 S/cm, and the highest value was up to 8,797 S/cm (href="#bib9" rid="bib9" class=" bibr popnode">Cho et al., 2014). Although extremely high conductivity of PEDOT is demonstrated, the deposition rate and film homogeneity still cannot meet the industrial requirement. Therefore a faster and more controllable polymerization technique is highly desirable.In most of the redox reactions of EDOT polymerization, Fe³⁺ acts as the electron acceptor. However, common oxidants like FeCl3 and Fe(III) tosylate can crystallize easily, resulting in structural defects in the polymerized film (href="#bib13" rid="bib13" class=" bibr popnode">Fabretto et al., 2009, href="#bib51" rid="bib51" class=" bibr popnode">Shi et al., 2017). Vanadium pentoxide (V2O5) is known for its strong oxidizing property and high activity toward EDOT monomer (href="#bib40" rid="bib40" class=" bibr popnode">Murugan et al., 2001, href="#bib18" rid="bib18" class=" bibr popnode">Guo et al., 2015b). Zhang et al. synthesized PEDOT nanofibers using V2O5 as the oxidant at room temperature in a single step, and the conductivity was 15 S/cm, higher than that of pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film (href="#bib69" rid="bib69" class=" bibr popnode">Zhang et al., 2005). V2O5 could change the morphology of PEDOT chains from granular to nanofibrillar for better electrical connectivity. Guo et al. synthesized layered V2O5/PEDOT nanowires in large scale by stirring the aqueous mixture of V2O5 powder and EDOT at room temperature (href="#bib17" rid="bib17" class=" bibr popnode">Guo et al., 2015a). V2O5 acted as both an oxidant and a template for EDOT polymerization. Therefore, V₂O₅ could be a good candidate for the polymerization of EDOT owing to its dual function of oxidizing and seeding template.In this work, a novel and facile method, sequential solution polymerization (SSP), is introduced to fabricate highly conductive PEDOT films in situ by solution means in high throughput. As shown in href="/pmc/articles/PMC6352564/figure/sch1/" target="figure" class="fig-table-link figpopup" rid-figpopup="sch1" rid-ob="ob-sch1" co-legend-rid="lgnd_sch1">Scheme 1, the process involves sequential deposition of a methanesulfonic acid (MSA) solution of V₂O₅ and 2,6-di-tert-butylpyridine (DTBP) as well as an acetonitrile (MeCN) solution of EDOT monomers (href="#bib4" rid="bib4" class=" bibr popnode">Benoit et al., 1988, href="#bib3" rid="bib3" class=" bibr popnode">Bashir et al., 2013). The whole process can be completed within a minute. The electrical conductivity of the PEDOT film can reach 1,420 S/cm, with an average value around 1,333 S/cm. Compared with the widely used commercial PEDOT:PSS (PH1000, Heraeus GmbH), the SSP PEDOT film has better crystallinity and higher doping level. Characterizations suggest that the SSP PEDOT films have comparable charge carrier mobility as PH1000, but three orders of magnitude higher carrier concentrations than PH1000. In addition, this process is compatible with large-scale printing techniques. A large-area (15 × 12 cm) PEDOT film is deposited successfully on polyethylene terephthalate (PET) film, and a low sheet resistance of 81 Ω/sq was obtained. The flexible SSP PEDOT film was applied to capacitive touch sensor, which showed favorable touch function and still worked well after folding it with a bending radius less than 1 mm. This new polymerization route paves the way to scalable deposition of conductive and homogeneous PEDOT films on flexible substrate.href="/pmc/articles/PMC6352564/figure/sch1/" target="figure" rid-figpopup="sch1" rid-ob="ob-sch1">class="inline_block ts_canvas" href="/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6352564_sc1.jpg" target="tileshopwindow">target="object" href="/pmc/articles/PMC6352564/figure/sch1/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC6352564/figure/sch1/" target="figure" rid-figpopup="sch1" rid-ob="ob-sch1">Scheme 1Synthesis of PEDOT Film via Sequential Solution Polymerization (SSP)Schematic diagram of the synthesis procedure of PEDOT film and chemical structure of methanesulfonic acid (MSA), vanadium pentoxide (V₂O₅), 2,6-di-tert-butylpyridine (DTBP), 3,4-ethylenedioxythiophene (EDOT), and its polymer PEDOT.