PEDOT:PSS
{{Short description|Polymer}}
File:Polythiophenes Pedotpss.png
File:Conducting-Polymer-Electrodes-for-Gel-Electrophoresis-pone.0089416.s001.ogv switching in two PEDOT:PSS electrodes connected by a piece of PhastGel SDS buffer strips. The electrodes were reversibly and repeatedly oxidized and reduced by switching the polarity of an applied 1 V potential. This was observed by a color change between dark (reduced PEDOT) and light (oxidized PEDOT) blue within the electrodes, demonstrating the transport of ions between and into the electrodes.{{Cite journal
|vauthors=Bengtsson K, Nilsson S, Robinson N | title = Conducting Polymer Electrodes for Gel Electrophoresis
| doi = 10.1371/journal.pone.0089416
| pmid = 24586761
| journal = PLOS ONE
| volume = 9
| issue = 2
| pages = e89416
| year = 2014
| pmc = 3929695
| doi-access = free
| bibcode = 2014PLoSO...989416B
}}]]
Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a composite material where PEDOT (the conductive polymer) provides electrical conductivity, and PSS (polystyrene sulfonate) acts as a counter-ion to balance the charge and improve the water solubility and processability of PEDOT. Polystyrene sulfonate is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt.{{Cite journal | doi = 10.1002/(SICI)1521-4095(200004)12:7<481::AID-ADMA481>3.0.CO;2-C| title = Poly(3,4-ethylenedioxythiophene) and Its Derivatives: Past, Present, and Future| journal = Advanced Materials| volume = 12| issue = 7| pages = 481–494| year = 2000| last1 = Groenendaal | first1 = L.| last2 = Jonas | first2 = F.| last3 = Freitag | first3 = D.| last4 = Pielartzik | first4 = H.| last5 = Reynolds | first5 = J. R.| bibcode = 2000AdM....12..481G}}
Synthesis
PEDOT:PSS can be prepared by mixing an aqueous solution of PSS with EDOT monomer, and to the resulting mixture, a solution of sodium persulfate and ferric sulfate.{{cite book|last1=Geoghegan|first1=Mark|last2=Hadziioannou|first2=Georges|title=Polymer electronics|date=2013|publisher=Oxford University Press|location=Oxford|isbn=9780199533824|page=125|edition=First|url=https://books.google.com/books?id=aE8ovTeazrUC&pg=PA125}}{{cite journal|last1=Yoo|first1=Dohyuk|last2=Kim|first2=Jeonghun|last3=Kim|first3=Jung Hyun|title=Direct synthesis of highly conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/graphene composites and their applications in energy harvesting systems|journal=Nano Research|date= 2014|volume=7|issue=5|pages=717–730|doi=10.1007/s12274-014-0433-z|s2cid=95642579|url=http://www.thenanoresearch.com/upload/justpdf/0433.pdf|accessdate=31 August 2017}}
The addition of these reagents initiates the oxidative chemical polymerization of EDOT in water to form PEDOT.{{cite journal |last1=Sakunpongpitiporn |first1=Phimchanok |last2=Phasuksom |first2=Katesara |last3=Paradee |first3=Nophawan |last4=Sirivat |first4=Anuvat |title=Facile synthesis of highly conductive PEDOT:PSS via surfactant templates |journal=RSC Advances |date=2019 |volume=9 |issue=11 |pages=6363–6378 |doi=10.1039/C8RA08801B|pmid=35517248 |pmc=9060941 |bibcode=2019RSCAd...9.6363S }} The stabilizing PSS forms a shell around a core of PEDOT in a nano-sized structure. The negatively charged sulfonic acid ions help stabilize the positively charged PEDOT ions.{{cite web |title=PEDOT and PEDOT:PSS Synthesis |url=https://www.ossila.com/pages/pedot-synthesis#PEDOT:PSS-synthesis |website=Ossila |language=en}}
Applications
PEDOT:PSS has the highest efficiency among conductive organic thermoelectric materials (ZT~0.42) and thus can be used in flexible thermoelectric generators.{{cite journal|doi=10.1080/14686996.2018.1487239|title=Organic π-type thermoelectric module supported by photolithographic mold: A working hypothesis of sticky thermoelectric materials|journal=Science and Technology of Advanced Materials|volume=19|pages=517–525|year=2018|last1=Satoh|first1=Norifusa|last2=Otsuka|first2=Masaji|last3=Ohki|first3=Tomoko|last4=Ohi|first4=Akihiko|last5=Sakurai|first5=Yasuaki|last6=Yamashita|first6=Yukihiko|last7=Mori|first7=Takao|issue=1|pmc=6052422|pmid=30034560|bibcode=2018STAdM..19..517S }} Yet its largest application is as a transparent, conductive polymer with high ductility. For example, AGFA coats 200 million photographic films per year{{citation needed|reason=This information may be out of date|date=November 2015}} with a thin, extensively-stretched layer of virtually transparent and colorless PEDOT:PSS as an antistatic agent to prevent electrostatic discharges during production and normal film use, independent of humidity conditions, and as electrolyte in polymer electrolytic capacitors.{{clarify|reason=Confusing compound sentence -- it's not clear how the second clause "as electrolyte in polymer electrolytic capacitors" connects to the remainder of the sentence.|date=December 2022}}
If organic compounds, including high boiling solvents like methylpyrrolidone, dimethyl sulfoxide, sorbitol, ionic liquids and surfactants, are added conductivity increases by many orders of magnitude.{{Cite journal|last1=Kim|first1=Yong Hyun|last2=Sachse|first2=Christoph|last3=Machala|first3=Michael L.|last4=May|first4=Christian|last5=Müller-Meskamp|first5=Lars|last6=Leo|first6=Karl|date=2011-03-22|title=Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells|journal=Advanced Functional Materials|volume=21|issue=6|pages=1076–1081|doi=10.1002/adfm.201002290|s2cid=136583700 }}{{Cite journal | doi = 10.1016/S0379-6779(01)00576-8| title = Enhancement of electrical conductivity of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) by a change of solvents| journal = Synthetic Metals| volume = 126| issue = 2–3| pages = 311–316| year = 2002| last1 = Kim | first1 = J. Y. | last2 = Jung | first2 = J. H. | last3 = Lee | first3 = D. E. | last4 = Joo | first4 = J.}}{{Cite journal | doi = 10.1016/j.polymer.2004.10.001| title = On the mechanism of conductivity enhancement in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film through solvent treatment| journal = Polymer| volume = 45| issue = 25| pages = 8443–8450| year = 2004| last1 = Ouyang | first1 = J. | last2 = Xu | first2 = Q. | last3 = Chu | first3 = C. W. | last4 = Yang | first4 = Y. | last5 = Li | first5 = G. | last6 = Shinar | first6 = J. }}{{Cite journal | doi = 10.1021/cm070398z| title = Influence of Ionic Liquids on the Electrical Conductivity and Morphology of PEDOT:PSS Films| journal = Chemistry of Materials| volume = 19| issue = 9| pages = 2147–2149| year = 2007| last1 = Döbbelin | first1 = M. | last2 = Marcilla | first2 = R. | last3 = Salsamendi | first3 = M. | last4 = Pozo-Gonzalo | first4 = C. | last5 = Carrasco | first5 = P. M. | last6 = Pomposo | first6 = J. A. | last7 = Mecerreyes | first7 = D. }}{{Cite journal
| pmid = 20356194
| year = 2010
| last1 = Xia
| first1 = Y
| title = Significant conductivity enhancement of conductive poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) films through a treatment with organic carboxylic acids and inorganic acids
| journal = ACS Applied Materials & Interfaces
| volume = 2
| issue = 2
| pages = 474–83
| last2 = Ouyang
| first2 = J
| doi = 10.1021/am900708x
}} This makes it also suitable as a transparent electrode, for example in touchscreens, organic light-emitting diodes,{{Cite journal|last1=Kim|first1=Yong Hyun|last2=Lee|first2=Jonghee|last3=Hofmann|first3=Simone|last4=Gather|first4=Malte C.|last5=Müller-Meskamp|first5=Lars|last6=Leo|first6=Karl|date=2013|title=Achieving High Efficiency and Improved Stability in ITO-Free Transparent Organic Light-Emitting Diodes with Conductive Polymer Electrodes|journal=Advanced Functional Materials|volume=23|issue=30|pages=3763–3769|doi=10.1002/adfm.201203449|s2cid=137196552 }} flexible organic solar cells{{Cite journal|last1=Park|first1=Yoonseok|last2=Berger|first2=Jana|last3=Tang|first3=Zheng|last4=Müller-Meskamp|first4=Lars|last5=Lasagni|first5=Andrés Fabián|last6=Vandewal|first6=Koen|last7=Leo|first7=Karl|date=2016|title=Flexible, light trapping substrates for organic photovoltaics|journal=Applied Physics Letters|volume=109|issue=9|pages=093301|doi=10.1063/1.4962206|bibcode=2016ApPhL.109i3301P|doi-access=free}}{{Cite journal|last1=Park|first1=Yoonseok|last2=Nehm|first2=Frederik|last3=Müller-Meskamp|first3=Lars|last4=Vandewal|first4=Koen|last5=Leo|first5=Karl|date=2016|title=Optical display film as flexible and light trapping substrate for organic photovoltaics|url=https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-10-A974|journal=Optics Express|volume=24|issue=10|pages=A974-80|doi=10.1364/OE.24.00A974|pmid=27409970|bibcode=2016OExpr..24A.974P|doi-access=free}} and electronic paper to replace the traditionally used indium tin oxide (ITO). Owing to the high conductivity (up to 4600 S/cm),{{Cite journal|title = Ultrahigh electrical conductivity in solution-sheared polymeric transparent films|journal = Proceedings of the National Academy of Sciences|date = 2015-11-17|pmc = 4655535|pmid = 26515096|pages = 14138–14143|volume = 112|issue = 46|doi = 10.1073/pnas.1509958112|first1 = Brian J.|last1 = Worfolk|first2 = Sean C.|last2 = Andrews|first3 = Steve|last3 = Park|first4 = Julia|last4 = Reinspach|first5 = Nan|last5 = Liu|first6 = Michael F.|last6 = Toney|first7 = Stefan C. B.|last7 = Mannsfeld|first8 = Zhenan|last8 = Bao|bibcode = 2015PNAS..11214138W|doi-access = free}} it can be used as a cathode material in capacitors replacing manganese dioxide or liquid electrolytes. It is also used in organic electrochemical transistors.
The conductivity of PEDOT:PSS can also be significantly improved by a post-treatment with various compounds, such as ethylene glycol, dimethyl sulfoxide (DMSO), salts, zwitterions, cosolvents, acids, alcohols, phenol, geminal diols and amphiphilic fluoro-compounds.{{Cite journal|last1=Bießmann|first1=Lorenz|last2=Kreuzer|first2=Lucas Philipp|last3=Widmann|first3=Tobias|last4=Hohn|first4=Nuri|last5=Moulin|first5=Jean-François|last6=Müller-Buschbaum|first6=Peter|date=2018-03-21|title=Monitoring the Swelling Behavior of PEDOT:PSS Electrodes under High Humidity Conditions|url=https://pubs.acs.org/doi/10.1021/acsami.8b00446|journal=ACS Applied Materials & Interfaces|language=en|volume=10|issue=11|pages=9865–9872|doi=10.1021/acsami.8b00446|pmid=29484879 |issn=1944-8244}}{{Cite journal | doi = 10.1002/adfm.200400016| title = High-Conductivity Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices| journal = Advanced Functional Materials| volume = 15| issue = 2| pages = 203–208| year = 2005| last1 = Ouyang | first1 = J.| last2 = Chu | first2 = C. -W. | last3 = Chen | first3 = F. -C. | last4 = Xu | first4 = Q.| last5 = Yang | first5 = Y.| s2cid = 95522337| doi-access = free}}{{cite journal|last1=Saghaei|first1=Jaber|last2=Fallahzadeh|first2=Ali|last3=Saghaei|first3=Tayebeh|title=ITO-free organic solar cells using highly conductive phenol-treated PEDOT:PSS anodes|journal=Organic Electronics|date= 2015|volume=24|pages=188–194|doi=10.1016/j.orgel.2015.06.002}}{{cite journal|last1=Fallahzadeh|first1=Ali|last2=Saghaei|first2=Jaber|last3=Yousefi|first3=Mohammad Hassan|title=Effect of alcohol vapor treatment on electrical and optical properties of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) films for indium tin oxide-free organic light-emitting diodes|journal=Applied Surface Science|date=2014|volume=320|pages=895–900|doi=10.1016/j.apsusc.2014.09.143|bibcode=2014ApSS..320..895F }}{{cite journal|last1=Saghaei|first1=Jaber|last2=Fallahzadeh|first2=Ali|last3=Yousefi|first3=Mohammad Hassan|title=Improvement of electrical conductivity of PEDOT:PSS films by 2-Methylimidazole post treatment|journal=Organic Electronics|date= 2015|volume=19|pages=70–75|doi=10.1016/j.orgel.2015.01.026}} This conductivity is comparable to that of ITO, the popular transparent electrode material, and it can triple that of ITO after a network of carbon nanotubes and silver nanowires is embedded into PEDOT:PSS{{Cite journal | doi = 10.1088/1468-6996/16/2/025002| pmid = 27877771| title = Planar silver nanowire, carbon nanotube and PEDOT:PSS nanocomposite transparent electrodes|pmc=5036479| journal = Science and Technology of Advanced Materials| volume = 16| issue = 2| pages = 025002| year = 2015| last1 = Stapleton | first1 = A. J. | last2 = Yambem | first2 = S. D. | last3 = Johns | first3 = A. H. | last4 = Afre | first4 = R. A. | last5 = Ellis | first5 = A. V. | last6 = Shapter | first6 = J. G. | last7 = Andersson | first7 = G. G. | last8 = Quinton | first8 = J. S. | last9 = Burn | first9 = P. L. | last10 = Meredith | first10 = P. | last11 = Lewis | first11 = D. A. | bibcode = 2015STAdM..16b5002S}} and used for flexible organic devices.{{Cite journal|doi=10.1080/14686996.2019.1568750|title=Simultaneously enhanced optical, electrical, and mechanical properties of highly stretchable transparent silver nanowire electrodes using organic surface modifier|journal=Science and Technology of Advanced Materials|volume=20|pages=116–123|year=2019|last1=Entifar|first1=Siti Aisyah Nurmaulia|last2=Han|first2=Joo Won|last3=Lee|first3=Dong Jin|last4=Ramadhan|first4=Zeno Rizqi|last5=Hong|first5=Juhee|last6=Kang|first6=Moon Hee|last7=Kim|first7=Soyeon|last8=Lim|first8=Dongchan|last9=Yun|first9=Changhun|last10=Kim|first10=Yong Hyun|issue=1|pmc=6383608|pmid=30815043|bibcode=2019STAdM..20..116E }}
PEDOT:PSS is generally applied as a dispersion of gelled particles in water. A conductive layer on glass is obtained by spreading a layer of the dispersion on the surface usually by spin coating and driving out the water by heat. Special PEDOT:PSS inks and formulations were developed for different coating and printing processes. Water-based PEDOT:PSS inks are mainly used in slot die coating, flexography, rotogravure and inkjet printing. If a high viscous paste and slow drying is required like in screen-printing processes PEDOT:PSS can also be supplied in high boiling solvents like propanediol. Dry PEDOT:PSS pellets can be produced with a freeze drying method which are redispersable in water and different solvents, for example ethanol to increase drying speed during printing. Finally, to overcome degradation to ultraviolet light and high temperature or humidity conditions PEDOT:PSS UV-stabilizers are available.
Linköping University claim to have made a "wooden transistor" by replacing the lignin from balsawood with PEDOT:PSS[https://liu.se/en/news-item/varldens-forsta-tratransistor LU News:The world’s first wood transistor28 April 2023]
Mechanical Properties
Since PEDOT:PSS is most frequently used in thin film architectures, several methods have been developed to accurately probe its mechanical properties; for example, water-supported tensile testing, four-point bend tests to measure adhesive and cohesive fracture energy, buckling tests to measure modulus, and bending tests on PDMS and polyethylene supports to probe the crack onset strain.{{Cite journal|last1=Lipomi|first1=Darren J.|last2=Bao|first2=Zhenan|date=2017-02-02|title=Stretchable and ultraflexible organic electronics|journal=MRS Bulletin|volume=42|issue=2|pages=93–97|doi=10.1557/mrs.2016.325|issn=0883-7694|doi-access=free|bibcode=2017MRSBu..42...93L }} Though PEDOT:PSS has a lower electrical mobility than silicon, which can also be incorporated into flexible electronics through the incorporation of stress-relief structures, sufficiently flexible PEDOT:PSS can enable lower cost-processing, such as roll-to-roll processing.{{Cite journal|last1=Root|first1=Samuel E.|last2=Savagatrup|first2=Suchol|last3=Printz|first3=Adam D.|last4=Rodriquez|first4=Daniel|last5=Lipomi|first5=Darren J.|date=2017-03-25|title=Mechanical Properties of Organic Semiconductors for Stretchable, Highly Flexible, and Mechanically Robust Electronics|journal=Chemical Reviews|volume=117|issue=9|pages=6467–6499|doi=10.1021/acs.chemrev.7b00003|pmid=28343389|issn=0009-2665|doi-access=free}} The most important characteristics for an organic semiconductor used in thin-film architectures are low modulus in the elastic regime and high stretchability prior to fracture. These properties have been found to be highly correlated to relative humidity.{{Cite journal|last1=Lang|first1=Udo|last2=Naujoks|first2=Nicola|last3=Dual|first3=Jurg|date=2008-12-30|title=Mechanical characterization of PEDOT:PSS thin films|url=http://dx.doi.org/10.1016/j.synthmet.2008.11.005|journal=Synthetic Metals|volume=159|issue=5–6|pages=473–479|doi=10.1016/j.synthmet.2008.11.005|issn=0379-6779}} At high relative humidity (>40%) hydrogen bonds are weakened in the PSS due to the uptake of water which leads to higher strain before fracture and lower elastic modulus. At low relative humidity (<23%) the presence of strong bonding between PSS grains leads to higher modulus and lower strain before fracture. Films at higher relative humidity are presumed to fail by intergranular fracture, whereas lower relative humidity leads to transgranular fracture. Additives like 3-glycidoxypropyltrimethoxysilane (GOPS) can drastically improve the mechanical stability in aqueous media even at low concentrations of 1 wt% without significantly impeding the electrical properties.{{Cite journal|last1=ElMahmoudy|first1=Mohammed|last2=Inal|first2=Sahika|last3=Charrier|first3=Anne|last4=Uguz|first4=Ilke|last5=Malliaras|first5=George G.|last6=Sanaur|first6=Sébastien|date=2017-02-20|title=Tailoring the Electrochemical and Mechanical Properties of PEDOT:PSS Films for Bioelectronics|url=http://dx.doi.org/10.1002/mame.201600497|journal=Macromolecular Materials and Engineering|volume=302|issue=5|pages=1600497|doi=10.1002/mame.201600497|issn=1438-7492|hdl=10754/623061|s2cid=136269465 |hdl-access=free}}
PEDOT:PSS can also show self-healing properties if submerged in water after sustaining mechanical damage.{{Cite journal|last1=Zhang|first1=Shiming|last2=Cicoira|first2=Fabio|date=2017-08-28|title=Self-Healing: Water-Enabled Healing of Conducting Polymer Films (Adv. Mater. 40/2017)|journal=Advanced Materials|volume=29|issue=40|doi=10.1002/adma.201770291|issn=0935-9648|doi-access=free}} This self-healing capability is proposed to be enabled by the hygroscopic property of PSS−.{{Cite journal|last1=Xin|first1=Xing|last2=Xue|first2=Zexu|last3=Gao|first3=Nan|last4=Yu|first4=Jiarui|last5=Liu|first5=Hongtao|last6=Zhang|first6=Wenna|last7=Xu|first7=Jingkun|last8=Chen|first8=Shuai|date=October 2020|title=Effects of conductivity-enhancement reagents on self-healing properties of PEDOT:PSS films|url=http://dx.doi.org/10.1016/j.synthmet.2020.116503|journal=Synthetic Metals|volume=268|pages=116503|doi=10.1016/j.synthmet.2020.116503|s2cid=224922736|issn=0379-6779}} Common PEDOT:PSS additives that improve the electrical conductivity have varying effects on self-healing. While ethylene glycol improves electrical and mechanical self-healing, sulfuric acid reduces the former but improves the latter, presumably because it undergoes autoprotolysis. Polyethylene glycol improves the electrical and thermoelectric self-healing, but reduces the mechanical self-healing.
PEDOT:PSS is also attractive for conductive textile applications. Though it results in inferior thermoelectric properties, wet-spinning has been shown to result in high conductivity and stiff fibers due to preferential alignment of polymer chains during fiber drawing.{{Cite journal|last1=Sarabia-Riquelme|first1=Ruben|last2=Shahi|first2=Maryam|last3=Brill|first3=Joseph W.|last4=Weisenberger|first4=Matthew C.|date=2019-07-08|title=Effect of Drawing on the Electrical, Thermoelectrical, and Mechanical Properties of Wet-Spun PEDOT:PSS Fibers|url=http://dx.doi.org/10.1021/acsapm.9b00425|journal=ACS Applied Polymer Materials|volume=1|issue=8|pages=2157–2167|doi=10.1021/acsapm.9b00425|s2cid=199176952 |issn=2637-6105}}