Fabrication of PEDOT:PSS/silver nanowire based films for the development of transparent heating systems

University of Minho School of Engineering Sara Coelho Ribeiro Fabrication of PEDOT:PSS/silver nanowire based films for the development of transparent heating systems december 2023 University of Minho School of Engineering Sara Coelho Ribeiro Fabrication of PEDOT:PSS/silver nanowire based films for the development of transparent heating systems Masters Dissertation Master’s in Engineering Physics Devices, Microsystems and Nanotechnologies Dissertation supervised by MSc Ricardo Martins Campos PhD Maria de Fátima Guimarães Cerqueira december 2023 Copyright and Terms of Use for Third Party Work This dissertation reports on academic work that can be used by third parties as long as the internationally accepted standards and good practices are respected concerning copyright and related rights. This work can thereafter be used under the terms established in the license below. Readers needing authorization conditions not provided for in the indicated licensing should contact the author through the RepositóriUM of the University of Minho. License granted to users of this work: CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/ i Acknowledgements/Agradecimentos Um muito obrigado ao Ricardo, em primeiro lugar pela oportunidade, mas também pela orientação, acompanhamento e ajuda ao longo da tese, e acima de tudo pela disponibilidade, simpatia e compreensão. Obrigado pela confiança e motivação, especialmente na fase final, que foram um incentivo para conseguir concluir esta tese num período de tempo excepcionalmente pequeno. Obrigado à professora Fátima, pela disponibilidade, orientação e atenção ao longo desta tese. Um muito obrigado à Liliana, pela presença desde o primeiro dia, pela disponibilidade para ajudar, pela simpatia e pelo acompanhamento e motivação ao longo de toda a tese. Aos meus amigos, Magda e Leander, obrigado pela amizade ao longo destes anos, especialmente nos momentos mais difíceis. Obrigado pelo companheirismo, pelo carinho, pelos bons momentos, mas também pela colaboração nos trabalhos que fizemos em conjunto. Obrigado pela força quando decidi desistir do primeiro projeto de tese, e pela confiança e ânimo quando iniciei esta tese e me propus a terminá-la no prazo normal. Obrigado por tornarem estes anos melhores. Aos meus pais, um obrigado pelo esforço que fizeram para que eu e a minha irmã pudéssemos frequentar a universidade. Obrigado pelos conselhos, pelo apoio e confiança que depositaram em mim. À minha irmã Tânia, o agradecimento mais especial, por termos vivido esta fase em conjunto, pelo apoio incondicional, pelos conselhos, por me ajudar a ser mais determinada e decisiva. ii Statement of Integrity I hereby declare having conducted this academic work with integrity. I confirm that I have not used plagiarism or any form of undue use of information or falsification of results along the process leading to its elaboration. I further declare that I have fully acknowledged the Code of Ethical Conduct of the University of Minho. University of Minho, Braga, december 2023 Sara Coelho Ribeiro iii Abstract Transparent and flexible heating systems have become increasingly important for applications such as in defrosting windows, sensors, or heating displays. PEDOT:PSS and silver nanowire (AgNW) films offer the possibility of high transparency, flexibility, and conductivity, making them promising substitutes for ITO, the most used material in these systems. In this work, these films were studied with the aim of utilizing screen-printing as a reproducible method to produce effective transparent and flexible heating systems with low input voltages. With that purpose, AgNWs were synthesized and the effect of different factors on the films’ resistances was studied: the films’ thickness; PEDOT:PSS modifications with water, PEG, glycerol, methanol, or DMSO; post-treatments with methanol, DMSO, CTAB or sodium borohydride, and the utilization of sintered AgNWs, an ionic liquid, and PVA. The results showed that depositing a layer of AgNWs on PEDOT:PSS decreased its resistance slightly, while another layer of PEDOT:PSS decreased it considerably. Modifying PEDOT:PSS and post-treating it with methanol had an insignificant effect on the resistances, while post-treatment with DMSO generally lowered them. Utilizing sintered AgNWs, an ionic liquid, PVA, or post-treating the films in CTAB or sodium borohydride was ineffective. As such, it was concluded that the resistance depended mainly on the thickness of the films. When characterized thermally with an input voltage of 12 V, most films showed an insignificant increase in their temperature. Nevertheless, films with two manually deposited layers of PEDOT:PSS post-treated with methanol with a layer of AgNWs, and films with screen-printed PEDOT:PSS with a big mesh size and a layer of AgNWs were able to increase their temperature values by almost 5 °C. In conclusion, different methods were studied to improve the conductivity of films for transparent and flexible heating systems, which showed that the films’ thickness is a key factor in their resistance and, consequently, in their heating abilities. Keywords Transparent flexible heating systems, PEDOT:PSS, silver nanowires, solvents, post-treatments, films deposition, screen-printing, resistance iv Resumo Sistemas de aquecimento transparentes e flexíveis têm ganhado importância em aplicações como o desembaciamento de janelas, em sensores ou no aquecimento de displays . Os filmes de PEDOT:PSS e nanofios de prata (AgNWs) podem atingir alta transparência, flexibilidade e condutividade, tornando-os substitutos promissores do ITO, o material mais usado nestes sistemas. Neste trabalho, estes filmes foram estudados com o objetivo de utilizar a serigrafia como um método reprodutível para produzir sistemas de aquecimento transparentes e flexíveis eficazes com baixas voltagens de entrada. Assim, AgNWs foram sintetizados e foi estudado o efeito de vários fatores nas resistências dos filmes: espessura dos filmes; modificações do PEDOT:PSS com água, PEG, glicerol, metanol ou DMSO; pós-tratamentos com metanol, DMSO, CTAB ou borohidreto de sódio, e utilização de AgNWs sinterizados, de um líquido iónico, e de PVA. Os resultados mostraram que a deposição de uma camada de AgNWs sobre PEDOT:PSS diminuiu ligeiramente a resistência, enquanto outra camada de PEDOT:PSS a diminuiu consideravelmente. As modificações do PEDOT:PSS e o pós-tratamento com metanol tiveram um efeito insignificante nas resistências, enquanto o pós-tratamento com DMSO geralmente teve um efeito redutor. A utilização de AgNWs sinterizados, líquido iónico, PVA, ou o pós-tratamento dos filmes com CTAB ou borohidreto de sódio foram ineficazes. Como tal, concluiu-se que a resistência dependia principalmente da espessura dos filmes. Quando caracterizados termicamente com uma voltagem de entrada de 12 V, a maioria dos filmes teve um aumento insignificante na sua temperatura. No entanto, filmes com duas camadas de PEDOT:PSS depositadas manualmente e pós-tratadas com metanol e com uma camada de AgNWs, e filmes com PEDOT:PSS serigrafado com um grande tamanho de malha e uma camada de AgNWs, aumentaram a sua temperatura cerca de 5 °C. Em conclusão, foram estudados diferentes métodos para melhorar a condutividade de filmes para sistemas de aquecimento transparentes e flexíveis, tendo-se observado que a espessura dos filmes é um fator chave na sua resistência e, consequentemente, no seu aquecimento. Palavras-chave Sistemas de aquecimento transparentes e flexíveis, PEDOT:PSS, nanofios de prata, solventes, pós-tratamentos, deposição de filmes, serigrafia, resistência v Contents List of figures xi List of Tables xii Acronyms xiii 1 Introduction and state of the art 1 1.1 PEDOT:PSS ...................................... 2 1.2 Silver nanoparticles and nanowires .......................... 6 1.2.1 Synthesis of silver nanowires - electrochemical synthesis ........... 7 1.2.2 Synthesis of silver nanowires - polyol method ................. 9 1.2.3 Cleaning/Welding of silver nanowires ..................... 11 2 Films for heating systems 15 3 Materials 17 4 Experimental methods 18 4.1 Synthesis of silver nanowires ............................. 18 4.2 Film deposition/printing ................................ 19 4.2.1 Deposition/printing methods ......................... 19 4.2.2 Silver nanowire films ............................. 20 4.2.3 PEDOT:PSS and AgNW films ......................... 20 4.2.4 Treatment of films with methanol or DMSO .................. 22 4.2.5 Films with cleaned AgNWs .......................... 24 4.2.6 Treatment of films with CTAB or sodium borohydride ............. 25 4.2.7 Ionic liquid and PVA ............................. 26 4.3 Characterization of the films .............................. 27 vi Acronyms [EMIM][BF4]1-ethyl-3-methylimidazolium tetrafluoroborate EDOT 3,4-ethylenedioxythiophene GBL γ-butyrolactone CTAB Cetyltrimethylammonium bromide CTAC Cetyltrimethylammonium chloride DMF Dimethylformamide DMSO Dimethyl sulfoxide EDA Ethylenediamine EDTA Ethylenediamine tetraacetic acid EG Ethylene glycol GLY Glycerol ITO Indium tin oxide IPA Isopropyl alcohol LPDP Liquid phase depositional polymerization MeOH Methanol MaBr Methylammonium bromide MAI Methylammonium iodide NMP N-methyl-2-pyrrolidone OTAC Octadecyltrimethylammonium chloride xiii ΩOhm o-CVD Oxidative chemical vapor deposition PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate PC Polycarbonate PEG Polyethylene glycol PEN Polyethylene naphthalate PET Polyethylene terephthalate PMMA Poly(methacrylic acid) PVA Polyvinyl alcohol PVP Polyvinylpyrrolidone RPM Rotations per minute SCE Saturated calomel electrode AgCl Silver chloride AgNPs Silver nanoparticles AgNWs Silver nanowires AgNO3Silver nitrate NaBH4Sodium borohydride NaCl Sodium chloride NaOH Sodium hydroxide NaClO4Sodium perchlorate TBAC Tetrabutylammonium chloride VPP Vapor phase polymerization xiv Chapter 1 Introduction and state of the art Transparent heating systems are devices that appear transparent to the human eye due to their high transmittance of visible light. Despite being transparent and often flexible, these devices are able to generate heat utilizing an electric input. The maximum temperature they can reach can be controlled through the variation of the input voltage [1]. Transparent flexible heating systems have been gaining importance due to their possible applications in numerous areas, such as in the defrosting of windows, in thermochromic displays (that change their color according to their temperature), in sensors, or in heating outdoor displays [1,2]. The most used material for transparent heaters is indium tin oxide (ITO) due to its high transmittance of visible light (90%) while maintaining a low sheet resistance of around 10 Ω/sq, which corresponds to a high electrical conductivity. Nevertheless, ITO’s high cost of manufacturing, high brittleness, low flexibility, and slow thermal response are detrimental to its use at a large scale [2,3]. As a consequence of the disadvantages associated with ITO, there have been many attempts at creating transparent flexible heating systems with other materials. Carbon nanotubes, graphene, nanowires, and various conductive polymers have been used for this purpose [1]. One promising approach to this problem is the use of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a polymer that can achieve great thermal stability and transparency, which are important characteristics for this application, although its electrical conductivity often needs to be improved [1,3,4]. Silver nanowires (AgNWs) have also been used to substitute ITO on transparent flexible heating systems due to their high electrical and thermal conductivity and high transmittance of visible light. Films obtained through the deposition of AgNWs have been reported to have similar sheet resistance and transmittance of visible light to ITO films (10 Ω/sq and 90%, respectively) [2]. Furthermore, both approaches have been adopted simultaneously in films containing PEDOT:PSS and silver nanowires. The PEDOT:PSS solution aids in the even dispersion of the AgNWs on the films, which 1 can decrease temperature variations throughout the film [5]. These films can be deposited, for instance, by micro gravure or slot-die [3], spray-coating [5,6,7], doctor blading [5], or screen-printing [1]. There have been reports of films with sheet resistances higher (15 Ω/sq [3], 19.4 Ω/sq [6]), similar (8 Ω/sq [6], 10.76 Ω/sq [7]), or lower (3 Ω/sq [5]) than ITO films, while maintaining high transmittance in the visible range. The films’ sheet resistance (and, consequently, the electrical conductivity) and the transmittance of visible light depend on several factors, namely the concentration of the reagents used, the thickness of the films, the added solvents or even the post-treatments of the films. These factors, which will be discussed in the sections below, are the justification for the differences in sheet resistance reported. Here, films of PEDOT:PSS and AgNWs have been studied with the objective of obtaining and optimizing flexible transparent conducting films for application in heating systems. 1.1 PEDOT:PSS Poly(3,4-ethylenedioxythiophene), or PEDOT, is a conducting polymer that was first reported in 1988 by the German company Bayer AG, and it is composed of the monomer EDOT (3,4-ethylenedioxythiophene) [8]. PEDOT can be obtained mainly through electrochemical polymerization and oxidative polymerization, although in 1999 Yamamoto & Abla reported the production of PEDOT through a transition metal-mediated polymerization [8,9]. The electropolymerization of EDOT is carried out with three electrodes (working, counter, and reference) in an electrolyte, for example, lithium perchlorate (LiClO4). A PEDOT film is formed on the working electrode and its characteristics, such as the thickness and the electrical conductivity, are dependent on the voltages and/or currents applied and on the time of the reaction. In 2007, for instance, Yang et al. reported producing a fibrillar PEDOT:PSS coating of 7 µm in an electropolymerization carried out at a current density of 5 A/m2for 30 minutes [10]. In 2010, Poverenov et al. deposited PEDOT films of around 1 µm at a constant potential of 1.15 – 1.20 V and a charge of 50 mC, or in cyclic voltammetry with voltages between -1.0 and 1.4 V, or -1.0 and 1.6 V, at a scan rate of 100 mV/s for 10 to 15 cycles [11]. In the same year, Lee et al. reported the electropolymerization of EDOT at 1.20 V for 15 seconds, which resulted in a PEDOT film of thickness 300 nm [12]. Furthermore, in 2019, Seki et al. showed that increasing the electropolymerization time resulted in films with higher thickness and lower conductivity [13]. The oxidative polymerization of PEDOT resorts to the use of an oxidizing agent and can be divided into various methods. This process was utilized in 1988 by Bayer AG and consisted of mixing an oxidizing agent, 2 EDOT, and a solvent, which resulted in insoluble PEDOT [8]. Additionally, PEDOT can be produced through vapor phase polymerization (VPP), in which a substrate with a deposited oxidant is subjected to EDOT vapors. In this case, the velocity of the reaction influences the resulting PEDOT chains. Consequently, it has been reported the use of a solvent that can slow the reaction, forming longer polymer chains, and increasing the conductivity of the PEDOT film [14]. In 2004, Winther-Jensen & West utilized pyridine with this objective, while simultaneously reducing additional acidic reactions, which happened as a result of the low pH value due to the presence of iron(III) (Fe3+) in the oxidant, originating unwanted short PEDOT chains [15]. Furthermore, the oxidative polymerization of PEDOT can be achieved through oxidative chemical vapor deposition (o-CVD), in which vapors of the oxidant and the monomer intersect at the substrate. In this method, the oxidant must have high enough volatility for the reaction to occur, but not too high as to not cause damage to the system. The characteristics of the resulting PEDOT depend on the conditions of the reaction such as the pressure or the temperature of the substrate [14]. In 2018, for instance, Wang et al. reported utilizing a pressure of around 1 mTorr, temperature values between 190 and 300°C, and reaction times of 20 minutes to 2 hours to obtain films with thicknesses between tens and hundreds of nanometers [16]. In 2020, Gharahcheshmeh et al. used a pressure of 1 Torr and temperature values between 80 and 140°C to deposit PEDOT films of thicknesses varying between similar values [17]. PEDOT has also been fabricated by liquid phase depositional polymerization (LPDP), a process in which a substrate with an oxidant is immersed in an EDOT solution. The reaction occurs at the substrate, where a film is produced. This method allows better control of the reaction’s conditions in comparison to other methods, especially VPP and o-CVD, making it easily reproducible and, consequently, more attractive for mass production [18,19,20]. Furthermore, Bayer AG introduced, in 1990, the oxidative polymerization of EDOT in the presence of PSS, a water-soluble substance. PSS’s negative charge balances PEDOT’s positive charge, forming a stable and highly conductive polymer, PEDOT:PSS [8,21]. PEDOT:PSS is an aqueous dispersion which structure is theorized to consist of PEDOT chains linked to sections of the longer PSS chains by Coulombic forces, in a random coil format, due to the repellence between unbound PSS molecules (Figure 1) [4,6]. 3 Figure 1: PEDOT:PSS’s structure. Adapted from [22,23]. PEDOT:PSS’s conductivity can be increased using UV light or thermal and chemical treatments [4]. In 2007, for example, Lin et al. reported an increment of the conductivity in PEDOT:PSS films subjected to UV light for 30 minutes, which was attributed to an alteration of the PEDOT:PSS structure from coil to extended coil or linear structure [24]. Furthermore, PEDOT:PSS films can become more conductive through thermal treatments, as reported in 2003 by Huang et al. In that work, films of this polymer were treated with temperature values ranging from 100 to 250°C, always resulting in an increase in the conductivity, although higher temperature values or excessive times of treatment could lead to the damage of the films and consequently the increase of their resistivity [25]. In 2004, for example, Nguyen et al. reported small decreases in the conductivity of PEDOT:PSS films subjected to temperature values from 120 to 250°C for two hours, with higher temperature values resulting in even lower conductivities [26]. Additionally, PEDOT:PSS’s conductivity can be increased by solvents such as dimethyl sulfoxide (DMSO), methanol, glycerol, or ethylene glycol (EG) [4]. In 2005, for instance, Snaith et al. reported an increase of three orders of magnitude in the conductivity of PEDOT:PSS films through the addition of 8% (V/V) of glycerol [27]. Alemu et al. studied, in 2012, the effects of methanol on the conductivity of PEDOT:PSS films and determined that using a drop of methanol on the films, immersing them in it, or a combination of both methods, resulted in an increase in the conductivity. According to the authors, this increase was due to the removal of PSS mainly from the films’ surface, which also increased their stability [28]. Furthermore, Jikei et al. reported, in 2014, the enhancement of PEDOT:PSS films’ conductivity using vapor treatments of various solvents, from which the highest conductivities were obtained with methanol, DMSO, dimethylformamide (DMF), EG, and N-methyl-2-pyrrolidone (NMP) [29]. In 2019, Lingstedt et al. 4 studied three different methods that utilized DMSO to decrease the resistivity of PEDOT:PSS films through the separation of PEDOT:PSS and PSS abundant zones – mixing DMSO with PEDOT:PSS, subjecting PEDOT:PSS films to DMSO vapor and immersing PEDOT:PSS films in DMSO – and found that the most effective method was the immersion of the films [30]. Other solvents such as polyethylene glycol (PEG) have also been used. In 2005, Wang et al. reported the effect of different molecular weights and concentrations of PEG added to PEDOT:PSS in the conductivity of the resulting films. In the studied solutions of PEDOT:PSS with PEG (with molecular weights (g/mol) of 400, 800, 1500, or 2000) the increase of PEG’s concentration caused an increase in the conductivity until a certain concentration, from which the conductivity decreased. The highest conductivity of 17.7 S/cm was obtained for PEDOT:PSS with 40.6 mM of PEG-400, which was 177 higher than that of the PEDOT:PSS film without any PEG addition, which was only 0.1 S/cm. The maximum conductivities for films with PEG-800, PEG-1500 and PEG-2000 were 14.2, 9.4, and 6.5 S/cm, respectively, indicating that the highest conductivities achieved decreased with the increase of PEG’s molecular weight [31]. In 2013, Mengistie et al. observed the same trend for PEG with various molecular weights, although they were able to enhance the conductivity of their films by a factor of more than two thousand times by adding 2% (V/V) of PEG with molecular weights (g/mol) of 200, 300 or 400 to PEDOT:PSS, while keeping the transmittance of visible light of the film above 90%. Further treatments, either by dropping methanol or dipping the films in it, resulted in an increment of the conductivity for films with PEG-200 and up to 2% (V/V) of PEG-300 [32]. In 2020, Li et al. reported reaching an even higher conductivity of 1399 S/cm in a film of PEDOT:PSS with 4% (V/V) of PEG-400 that was soaked in methanol for 20 minutes [33]. The addition of PEG to PEDOT:PSS increases the conductivity of films by distancing PEDOT:PSS and PSS-rich zones, but from a certain concentration its accumulation on the surface of the films begins to cause an isolating effect, which decreases the conductivity [31,34]. Moreover, ionic liquids have also been used to reduce the resistivity of PEDOT:PSS films. In 2020, Yemata et al. reported using ionic liquids, as well as nitric acid, as post-treatments to reduce the resistivity of PEDOT:PSS films, concluding that the acid treatment was more effective, although the ionic liquids 1-butyl-3-methylimidazolium with trifluoromethanesulfonate (([BMIM][OTF]-OTf) or with tetrafluoroborate ([BMIM][BF4]) also increased the conductivity of the films [35]. In 2022, Imae et al. utilized the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB]) to increase the conductivity of PEDOT:PSS films through the removal of PSS, which is replaced by the TCB anion [36]. Furthermore, other acids and bases have also been utilized to optimize PEDOT:PSS’s films conductivity. In 2017, Fan et al. studied the effects of a post-treatment with a sulfuric acid (H2SO4) solution followed 5 by a sodium hydroxide (NaOH) solution on the conductivity of PEDOT:PSS films and observed that the first solution caused an increase in the conductivity, while the second caused the opposite [37]. Wu et al. reported, in the same year, mixing chloroplatinic acid (H2PtCl6·6H2O) to PEDOT:PSS, obtaining films with an enhancement in conductivity by a factor of a thousand. According to the authors, this was due to the oxidation of PEDOT and the reshaping of PEDOT:PSS’s structure from coil to linear or extended coil structure [38]. PEDOT:PSS films can have their conductivity increased by salts as well. In 2016, Yu et al. studied the conductivity of PEDOT:PSS films with methylammonium iodide (MAI) and methylammonium bromide (MABr) solutions in different organic solvents such as water, ethanol, methanol, acetone, DMSO, EG, dimethylformamide (DMF) or γ-butyrolactone (GBL). For a concentration of 0.1 M, for instance, the conductivity always increased, in most cases by four orders of magnitude. The highest conductivity, more than nine thousand times that of a PEFOT:PSS film, was registered for a film of PEDOT:PSS with 0.1 M of MAI in GBL. Furthermore, it was concluded that MAI is a better conductivity enhancer than MABr in DMF and GBL solutions, due to its stronger interaction with PEDOT allied with the effect of the solvent [39]. Li et al. studied, in 2020, the effect of different-sized cations on PEDOT:PSS films doped with salts, concluding that smaller cations enhance more the conductivity likely due to their faster mobility, which allows them to easily bond to PSS, leaving anions to bond to PEDOT, thus breaking many PEDOT’s bonds to PSS and increasing the conductivity [40,41]. In that work, the conductivity of a PEDOT:PSS film was increased more than five hundred times through the addition of 0.05 mmol/g of bis(trifluoromethanesulfonyl)imide lithium salt to the solution [40]. 1.2 Silver nanoparticles and nanowires Nanoparticles, such as silver nanoparticles (AgNPs), have particular properties that are different than the bulk materials they are composed of due to their high surface-to-volume ratio. These properties vary with the nanoparticles’ dimensions and shapes, which has prompted their application and investigation in many areas [42]. In 2012, for instance, Melendez et al. reported that silver nanoparticles (AgNPs) with bigger diameters caused an increase in the conductivity of films of PEDOT:PSS [43]. Silver nanoparticles can be produced through many methods. One of them is the physical deposition, which utilizes evaporation and condensation of source materials. In 2006, for instance, Jung et al. reported the synthesis of spherical silver nanoparticles using this method, by heating silver with a ceramic heater. The diameter of the nanoparticles ranged between 6.2 and 21.5 nm, with the size increasing with the 6 increase of the temperature of the heater [44]. Nanoparticles can also be produced by using laser ablation, as was reported, for example, in 2019, by Alhamid et al., to produce spherical AgNPs with diameters of tens of nanometers [45]. Silver nanoparticles can be synthesized through chemical methods as well, which consist of the reduction of the silver cation (Ag+) and the further aggregation of the silver atoms with the use of a stabilizer such as polyvinylpyrrolidone (PVP) or poly(methacrylic acid) (PMMA) [42]. Electrochemical synthesis and the polyol method are some of the most widely used methods to chemically fabricate AgNPs. The electrochemical synthesis utilizes two or three electrodes. The flowing current oxidizes the anode, freeing electrodes that are then used in the reduction of silver ions from the electrolyte solution at the cathode/working electrode, forming the nanoparticles at its surface. This is a process with high yield [46]. The polyol method consists of using a polyol such as ethylene glycol (EG), glycerol, or polyethylene glycol (PEG) as the solvent and the reducer simultaneously [47], alongside a stabilizer such as PVP. Furthermore, silver nanowires have been produced with the aid of templates, such as membranes [48,49]. In contrast to AgNPs, AgNWs are a promising approach to obtaining flexible and transparent electrodes to use in heating systems due to their high thermal and electrical conductivity, high transmittance of visible light, and straightforward methods of fabrication [50]. In 2022, for example, Barbic et al. reported the use of a polycarbonate membrane with pores to synthesize AgNWs. A gold layer was deposited on one side of the membrane and a commercial silver enhancement solution was made to reach this layer only through the pores. The reduction of the silver ions started automatically at the gold layer, which also functioned as a catalyst. The AgNWs were obtained after subjecting the structure to ultrasounds, which removed the gold layer, and using chloroform for 1 hour with agitation to dissolve the membrane. The resulting silver nanowires had diameters of (30 ± 5) or (70 ± 15) nm for membranes with pores of diameter of 15 or 30 nm, respectively [49]. 1.2.1 Synthesis of silver nanowires - electrochemical synthesis Zhu et al. utilized this technique, in 2002, to produce silver nanowires by two different approaches. Both syntheses took place in an ultrasonic bath of 50 Hz and 100 W, under a nitrogen atmosphere, at a temperature of 30°C, and with an aqueous solution of 50 mL of distilled water with 0.10 g of silver nitrate (AgNO3) and 1.00 g of ethylenediamine tetraacetic acid (EDTA) as the electrolyte. Following the reactions, the deposits were centrifugated, cleaned with distilled water and ethanol multiple times, and dried in a vacuum. The authors reported that the ultrasounds might help in the formation of AgNWs due to the partial removal of silver nanoparticles from the working electrode, promoting the growth in a wire shape through the aggregation of more silver nanoparticles. Furthermore, it was determined that EDTA in concentrations 7 over 20 g/L facilitated the growth of AgNWs by slowing the reaction through a decrease in the number of silver ions, due to the creation of silver-EDTA complexes. In the first approach, AgNWs were generated with a two-electrode configuration with a 5 x 5 mm platinum sheet as the working electrode and a platinum wire as the counter electrode. The reaction occurred at a constant current of 10 mA for 30 minutes and resulted in nanowires of diameter 40 nm and length 6 µm. The second approach used a three-electrode configuration with the same electrodes but with the addition of a saturated calomel reference electrode (SCE). The synthesis was carried out at a constant potential of -0.3 V for 45 minutes and produced bent silver nanowires with diameters of 80 nm and lengths over 15 µm [51]. In the following year, Tian et al. used a polycarbonate membrane with pores (10 nm in diameter, 6 µm in thickness, 6 x 108pores per cm2), as a template for the growth of AgNWs. The electrochemical reaction utilized a 200 nm gold film evaporated on one side of the membrane as the working electrode, a platinum wire as the counter electrode, and a SCE as the reference. The electrolyte consisted of a solution of 20 mL of a commercial silver bath with the addition of around 2% (w/w) of gelatin and further dilution in water until it reached a volume of 40 mL. The reaction occurred at a temperature of 40°C and a high negative voltage for 1 to 2 minutes, followed by a constant deposition potential lower than -0.9 V. Subsequently, the membrane was dissolved in dichloromethane and the silver nanowires were precipitated through centrifugation. The resultant AgNWs had 40 nm of diameter and 3 to 5 µm of length [48]. Lin & Wang reported, in 2005, the electrochemical synthesis of AgNWs in a solution of 20 mM of AgNO3, 0.1 M of potassium nitrate (KNO3), and 1.0 g/L of PMMA in distilled water. A bare glassy carbon disk with 4 mm of diameter, a platinum wire, and a SCE were used as the working, counter, and reference electrodes, respectively. The working electrode was treated electrochemically at a constant potential of 1.8 V for 2 minutes in a distilled water solution of 1 M of sodium hydroxide (NaOH) prior to the production of the silver nanowires, which took place at a voltage of -0.2 V for 5 minutes and at around 22°C. This process resulted in AgNWs of a diameter of 30 to 40 nm and length of 3 µm with some silver nanoparticles connected. Furthermore, the lack of PMMA in the electrolyte or the increase of its concentration to 3.0 g/L yielded spherical nanoparticles or dendritic structures, respectively [52]. In 2007, Zheng et al. utilized two electrolytes for the electrochemical growth of AgNWs. Highly oriented pyrolytic graphite or an iron plate previously mechanically polished and washed with distilled water, and a platinum electrode were employed as working and counter electrodes, respectively, in the production of silver nanowires, which was carried out in a constant potential mode with a current density of 1 mA/cm2 for 2 hours. For one of the electrolytes, composed of 500 mL of an aqueous solution of 1.0 mM of AgNO3, the resulting nanowires had diameters of 10 to 50 nm and lengths of several µm. For the other electrolyte, 8 Chapter 2 Films for heating systems Heating systems, especially flexible and transparent ones, have gained importance due to their many potential applications, including in defrosting windows, thermochromic displays, and sensors [2]. Conductive thin films have been widely studied for this purpose because they can be deposited on transparent substrates such as glass and on flexible transparent materials like polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). Additionally, thin films offer the advantage of adjustable resistance and transmittance through various methods, such as solvent addition, post-treatments, or modifications in thickness or concentration. For instance, the addition of silver nanowires to PEDOT:PSS can enhance film conductivity while maintaining the transmittances at high values. The increase in temperature of films when used as heaters is attributed to the Joule effect, in which the heat (H) produced by a flowing current is proportional to the square of the applied voltage (V) and the time (t), and inversely proportional to the resistance of the films (R) (2.1) [2]. Therefore, films with lower resistances show better performance, making it the main objective of many studies to decrease the resistance of the films as much as possible in order to maintain the input voltage at adequate values for further applications. H=V2t R(2.1) In this context, PEDOT:PSS films with silver nanowires (PEDOT:PSS/AgNW films) have been used to fabricate conductive, transparent, and flexible electrodes. In 2011, for instance, Gaynor et al. utilized AgNWs deposited on PEDOT:PSS to produce transparent PET and glass electrodes. Compared with similar ITO electrodes, these showed lower transmittances but also lower resistances and better stability. PEDOT:PSS/AgNW and ITO films on PET and glass were further applied in a photovoltaic cell and showed the same efficiency [68]. In the following year, Choi et al. reported the fabrication of transparent films with better performance than ITO’s in terms of resistance and transmittance. PEDOT:PSS treated with DMSO was spray-coated on a previously spray-coated AgNW film and further laminated, which flattened the silver nanowires, resulting in a less rough film and in a decrease of 28% in the resistance. Annealing 15 of the AgNW films at 140°C for 20 minutes before PEDOT:PSS deposition was able to further decrease the resistance of the PEDOT:PSS/AgNW film by almost 28% [7]. The applicability of transparent and flexible PEDOT:PSS/AgNW electrodes on heating systems has been studied as well. In 2014, for instance, Ji et al. tested the heating of PEDOT:PSS/AgNW films on PET produced through doctor blading of AgNWs and further doctor blading or spin coating of PEDOT:PSS diluted in isopropanol. The films showed transparencies around 90% and good stability, and reached uniform temperature values above 100°C for an input voltage of 6 V. The PEDOT:PSS/AgNW films were deposited on glass and compared to ITO films on the same substrate, showing a shorter response time and needing less input voltage to reach the same temperature. Further tests showed that this film was able to defog a window in 40 seconds [5]. In 2017, He et al. studied the effects of different concentrations of silver nanowires added to PEDOT:PSS on the heating of the resulting films, which were deposited through screen printing on PET, an easily reproducible technique. The temperature could be homogeneously increased up to 99°C with the application of 40 V for films with a AgNW concentration of 12 mg/mL and transmittances around 74% [1]. In 2021, Zheng et al. reported the fabrication of a PEDOT:PSS/AgNW film through the spray-coating of a solution of PVP-free silver nanowires, DMSO and PEDOT:PSS on PEN. Both additives to PEDOT:PSS enhanced the conductivity of the resulting films enough to achieve an optimal temperature of almost 115°C throughout the film with an input of 7 V [6]. In 2023, Rezvani et al. reported the heating of PEDOT:PSS/AgNW films on PET treated with an intense pulsed light, which increased the conductivity of the films through the removal of PSS from the PEDOT:PSS, the removal of PVP from the nanowires, and the welding of AgNW junctions. The treated samples were able to increase their temperature by up to 50% [69]. Furthermore, other films with silver nanowires, such as films of AgNWs and poly(p-phenylene vinylene) [70], carbon nanotubes [2], PEDOT:PSS and ITO [71], cobalt nanoparticles [55], polyimide [72], polyvinyl alcohol (PVA) [73,74], or polymer composites [75,76] have also been studied for possible applications in heating systems, showing a wide range of maximum temperature values between around 75 and 350°C, depending on the film and the applied voltage. In this work, the main goal is the production of a heating system, produced by the screen printing technique, that operates at low values of tensions applied (low power consumption) with a great temperature gradient and transmittance of visible light. 16 Chapter 3 Materials Commercial PEDOT:PSS ink was aquired from Saralon. PVP, silver nitrate, glycerol, sodium chloride, methanol, sodium borohydride, PVA, CTAB, and PEG of molecular weight 2000 g/mol were obtained from Merck/Sigma Aldrich. DMSO was acquired from Thermo Fisher. Commercial silver nanowire ink was obtained from Dycotec Materials Ltd, and the ionic liquid 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was acquired from Iolitec-Ionic Liquids Technologies GmbH. The substrates used - polyethylene terephthalate (PET), polyimide (Kapton) and polycarbonate (PC) - were obtained from Mitsubishi Polyester Film GmbH, DuPont, and AGI, respectively. 17 Chapter 4 Experimental methods 4.1 Synthesis of silver nanowires Silver nanowires were produced according to the polyol method. For that, AgCl was first synthesized. An aqueous solution of 1 M of sodium chloride and an aqueous solution of 0.5 M of silver nitrate were prepared. 5 mL of the NaCl solution were added to the same volume of the AgNO3solution under magnetic agitation for 1 minute and protected from light. The mixture resulted in the formation of silver chloride (AgCl) and the supernatant was removed. To produce the silver nanowires, 0.34 g of PVP were dissolved in 20 mL of glycerol and heated to 160°C in a closed vessel. While maintaining the temperature and agitation, 25 mg of AgCl were added to the solution. After 3 minutes, 0.11 g of AgNO3were also added, turning the solution yellow due to the formation of silver nanoparticles. The solution was left to react for 15 minutes, developing a dark gray green color, which indicates the formation of the silver nanowires. This process is schematized on Figure 3, while Figure 4shows the setup (left) and the AgNW ink obtained after the synthesis (right). Figure 3: Synthesis of silver nanowires. 18 Figure 4: Laboratory setup for the synthesis of AgNWs (left) and the final result of the reaction (right). The obtained silver nanowires in glycerol were used without any modification or diluted with 40 or 55% (w/w) of methanol in an attempt to increase their adhesion to PEDOT:PSS, while potentially improving the conductivity of the films. 4.2 Film deposition/printing 4.2.1 Deposition/printing methods In this work, three methods were utilized to produce films: screen-printing, doctor blading, and spin coating (Figure 5). Screen-printing is a reproducible method that results in uniform films [1], as such, it was used to print films of the best approaches throughout the experiments. In this process, the ink is first spread on a screen with a certain mesh size and with a predefined pattern. Then, the screen contacts the substrate and the ink is transferred to it through the passage of a squeegee that pushes the ink through the mesh [77]. The films are formed after curing. Doctor blading is a manual technique for the deposition of films in which a blade or squeegee is used to spread a solution in a substrate, occasionally in the presence of a mask. This results in a thin layer of the solution on the substrate, which forms the film after the evaporation of the solvents. Finally, spin coating is a simple method of depositing films in which a solution is placed on the substrate, which is spun at high velocities for a short period of time, spreading the solution. The thickness of the film obtained after evaporation of the solvents depends on the velocity of the deposition and on the viscosity of the solution utilized [78]. In this work, the use of these diverse deposition methods resulted in films with varying thicknesses, namely in a range between 0.0030 and 0.0110 mm. 19 Figure 5: Methods of deposition: Screen-printing schematic and machine used in this work (top), doctor blading and spin coating schematics (bottom). Adapted from [1,79]. 4.2.2 Silver nanowire films Initially, films of just silver nanowires were prepared. AgNWs in glycerol with 55% (w/w) of methanol were spin coated at 1000 RPM for 40 seconds on PET and PC but showed no effective adhesion to the substrates. Trying to improve the adhesion, the same solution was spin coated on PET, Kapton and PC previously subjected to a 40 W/cm UV treatment mat for two consecutive times, at velocity 0.022 m/s. However, that process did not increase the adhesion of the films to the substrates. Consequently, two layers of commercial AgNW ink were deposited on PET and Kapton using the doctor blade method and dried at 110°C in an oven. Furthermore, as Kapton is more resistant to higher temperature values than PET, two films of this substrate were thermally annealed at 225°C for 1 hour. 4.2.3 PEDOT:PSS and AgNW films Films of PEDOT:PSS, modified PEDOT:PSS, and PEDOT:PSS with silver nanowires were fabricated on substrates of PET, Kapton, and PC using the three previously mentioned techniques. Each layer was cured at 110°C, and occasionally overnight at 60°C or at room temperature, until the film was visibly dry. Furthermore, several films containing silver nanowires were thermally annealed at 225°C for 1 hour and at 180°C for 5 hours. Table 1shows the PEDOT:PSS solutions that were used to produce films and the 20 corresponding deposition methods, the characteristics of the silver nanowire solutions that were utilized to deposit a AgNW film on top of the PEDOT:PSS, unless otherwise specified, and additional information, namely the annealing of some of the samples. Table 1: Films of PEDOT:PSS, modified PEDOT:PSS and PEDOT:PSS with AgNW deposited through doctor blading, screen-printing or spin coating. 21 4.2.4 Treatment of films with methanol or DMSO Initially, films with just one layer of PEDOT:PSS on Kapton were immersed in methanol and DMSO for 5, 20, 40 and 60 minutes to evaluate the effect of different treatment times on the conductivity. Furthermore, films with PEDOT:PSS on PET, Kapton and PC were also treated with methanol or DMSO after the deposition and curing of each PEDOT:PSS layer. Treatments with methanol were done following two different methods: dropping a methanol drop on the films followed by its evaporation at 110°C in an oven or in a hotplate at 140°C, or immersion in methanol, followed by the same curing processes. Treatments in DMSO consisted in simply immersing the films in it and drying them at 110°C in an oven. Afterwards, a AgNW film was deposited on top of each the treated films, except for the films fabricated with silver nanowires already in solution with PEDOT:PSS. Table 2and Table 3detail the characteristics of the films treated with methanol and DMSO, respectively, specifying the PEDOT:PSS solutions used, their deposition method, the type of treatment, the silver nanowire solutions used, their method of deposition, and additional information, namely the annealing and/or other immersion treatments of some of the samples. 22 Table 2: Characteristics of films with PEDOT:PSS treated in methanol. 23 Table 3: Characteristics of films with PEDOT:PSS treated in DMSO. 4.2.5 Films with cleaned AgNWs Silver nanowires obtained from the synthesis were cleaned through centrifugations in ethanol and water, three times each, at 6000 RPM for 10 minutes. The resulting AgNWs powder was added to PEDOT:PSS 24 resistance. Additionally, another layer of AgNWs was deposited with the same method, which increased the conductivity of the film further. Figure 9demonstrates the resistance’s variations with the addition of each layer. Figure 9: Resistance for films with subsequential layers of PEDOT:PSS and AgNWs. The effect of the number of PEDOT:PSS layers and their thickness was further studied. Films with two layers of PEDOT:PSS and one layer of the same solution of AgNWs deposited through doctor blading and spin coating, respectively, demonstrated similar final resistances to the previous approach (Figure 10), although the second layer of PEDOT:PSS caused a higher decrease in the resistance than the first AgNW layer in the previous film. Additionally, the thermal annealing of the films in Kapton resulted in a decrease in their conductivity. It seems that the effect of the one layer of AgNWs deposited in one layer of PEDOT:PSS is explained by an increment of thickness. Figure 10: Resistance variations for films with two layers of PEDOT:PSS and one layer of AgNWs in glycerol with 55% (w/w) methanol. The same solutions were used to produce films on PET through the screen-printing of the two PE31 DOT:PSS layers, while the AgNWs solution was deposited through spin coating. Figure 11 shows the resistance associated with the addition of each layer to the film. Even though screen-printing is a more reproducible technique, the resulting films have higher resistance due to the thinner PEDOT:PSS layers. In comparison to the average (0.0050 ± 0.0009) mm thickness obtained by the doctor blading method, screen-printed layers had an average thickness of (0.0030 ± 0.0002) mm. Figure 11: Resistance variations for films with two layers of screen-printed PEDOT:PSS and one layer of AgNWs. Figures 12 and 13 show photographs of samples obtained by this approach without and with the AgNWs layer, respectively, on the left, and their corresponding SEM images on the right. Figure 13 reveals the even dispersion of the as-synthesized AgNWs across the PEDOT:PSS film. It demonstrates the quality of the AgNW synthesis process given that the AgNWs had consistent dimensions, and that the nanowires stayed on PEDOT:PSS’ surface and did not penetrate it, which can be attributed to the film’s homogeneity and compactness. Figure 12: Photograph (left) and SEM image (right) of two layers of screen-printed PEDOT:PSS. 32 Figure 13: Photograph (left) and SEM image (right) of a film with two screen-printed layers of PEDOT:PSS and one layer of AgNWs. To further test the influence of the thickness of the layers, this process was exceptionally repeated with a screen-printing mesh of bigger size, which resulted in the passage of more ink onto the PET substrate and consequently in thicker films, of average (0.0110 ± 0.0006) mm. Figure 14 shows the resistances of the last PEDOT:PSS layer and of the final film (with AgNWs). Even though the resistances of the PEDOT:PSS layers were lower than in all of the prior films, the addition of the silver nanowires did not decrease them significantly. This approach showed the highest increase in temperature among all films throughout this work, likely due to also having the highest thickness. Figure 14: Resistance variations for films with two layers of PEDOT:PSS and one layer of AgNWs. The screen-printing of the PEDOT:PSS layers was carried out with a bigger mesh size, which resulted in thicker films. Figure 15 shows a photograph of a film obtained with this approach, as well as its SEM image, in which it is noticeable the non-uniform distribution of the AgNWs, which could be due to the difference in 33 the roughness of the film. Furthermore, the scarce dispersion of the nanowires does not allow for the creation of a AgNW network, which could result in higher conductivities. This, alongside the fact that the AgNWs are only on the film’s surface, corroborates the presumption that the lower resistances of the films were mainly due to their higher thickness and not due to different interactions between the nanowires and the underlying PEDOT:PSS. Figure 15: Film with two layers of screen-printed PEDOT:PSS with bigger mesh size and one layer of AgNWs and its corresponding SEM image. Additionally, Figure 16 shows two infrared pictures of films obtained with this approach at the highest temperature they could reach, which corresponded to average increases in the temperature values of (4.8 ± 0.1) and (2.0 ± 0.1)°C for the 1st and 3rd designs, respectively. Figure 16: Infrared pictures of heated films of two screen-printed layers of PEDOT:PSS with a bigger mesh size and one layer of AgNWs. Furthermore, films with three layers of PEDOT:PSS were screen-printed on PET, and a layer of the same AgNWs solution was spin coated on top. Figure 17 displays the variation in resistance caused by the addition of each layer. Although the layer of silver nanowires did not significantly reduce the resistance, the third layer of PEDOT:PSS decreased it considerably. With an average thickness of (0.0050 ± 0.0005) mm, 34 these films showed conductivities similar to the films with two layers of doctor bladed PEDOT:PSS, likely due to the identical thicknesses, but lower than the films with two screen-printed layers of PEDOT:PSS with bigger mesh size, which were thicker. Figure 17: Variation of the resistance for films with three layers of screen-printed PEDOT:PSS and one layer of spin-coated AgNWs. A photograph of a film obtained with this approach is shown in Figure 18, along with its SEM image. Similar to other films, the AgNWs had an even dispersion, even though it was scarce, which could explain the lack of effect of the AgNW layer on the resistance of the film. Figure 18: Photograph of a film with three layers of screen-printed PEDOT:PSS and one layer of AgNWs, and its SEM image. Finally, films with intercalated silver nanowires and PEDOT:PSS were produced. Figure 19 shows the variation of the resistance for PEDOT:PSS deposited through doctor blading with intercalated spin-coated as-synthesized AgNWs in glycerol and 55% (w/w) methanol on Kapton, and for the same approach but with 35 screen-printed PEDOT:PSS on PET. These films had average thicknesses of (0.0050 ± 0.0020) and (0.0040 ± 0.0004) mm, respectively. The similar thicknesses could explain the similar conductivities between films of screen-printed PEDOT:PSS with intercalated and non-intercalated AgNWs shown previously. Figure 19: Resistance fluctuation for films with a AgNW layer intercalated in PEDOT:PSS layers deposited through doctor blading (left) and screen-printing (right). In essence, these results are in accordance with the previously mentioned works developed by He et al. [1], Ji et al. [5], and Rezvani et al. [69], in which the resistance of films of PEDOT:PSS and AgNWs was lower than films of just one of the substances. The thickness of the films seemed to be the main factor in their final resistance, while the addition of a layer with silver nanowires usually decreased it only slightly. A future study using different AgNWs concentrations should be performed. Table 5shows the temperature variation of these films when subjected to an input voltage of 12 V. Most of the films increased their temperature insignificantly, except for the film with screen-printed PEDOT:PSS with the bigger mesh size which, due to having the lowest resistance, increased its temperature the most, by almost 5°C. 36 Table 5: Resistances and increase in temperature for films with PEDOT:PSS and AgNWs. Furthermore, Figure 20 shows how the average temperature of the films increased approximately linearly with the inverse of their resistance. Thus, the lower the resistance of the films the higher the increase in their temperature. Figure 20: Average temperature increase (∆T) for the inverse of the resistances (1/R) of the films discussed in this section. 5.4.2 Films with modified PEDOT:PSS In an effort to obtain films with better conductivities, different solvents were added to PEDOT:PSS. According to the aforementioned reports by Wang et al. [31], Mengistie et al. [32], Li et al. [33], and 37 Snaith et al. [27], the addition of PEG, glycerol, or both, to PEDOT:PSS decreases the resistance of the resuting films. Furthermore, in the previously mentioned works by Alemu et al. [28], Jikei et al. [29], and Lingstedt et al. [30], for instance, the combination of methanol or DMSO with PEDOT:PSS can also enhance the conductivity of films. As such, all of these approaches were taken in this work, and the results are displayed henceforth. Initially, films of PEDOT:PSS with two layers were doctor bladed onto Kapton and PET in the preliminary design with the aim of having another benchmark for the following results. Films with one layer had an average resistance of (0.365 ± 0.005) kΩ, while the addition of the second layer lowered the resistance to an average of (0.185 ± 0.005) kΩ. Figures 21 and 22 show the variation of the resistance for films with two doctor bladed layers of PEDOT:PSS with 30 or 50% (w/w) of water, as well as an additional spin-coated layer of as-synthesized AgNWs in glycerol and 55% (w/w) methanol. Films with more water had PEDOT:PSS layers with higher resistances, likely due to its dilution, which was reflected in their final resistance. These results demonstrate that water could break the interaction between PEDOT and PSS monomers, leading to a decrease in condutivity. The silver nanowires layer decreased the resistance only slightly, which could be explained by the weak dispersion and alignment of the nanowires between PEDOT:PSS chains. Furthermore, the thermal annealing of random samples of both approaches, at 225°C for 1 hour and at 180°C for 5 hours, resulted in a decrease of the conductivity of the films. Figure 21: Resistance for each layer of films with PEDOT:PSS with 30% (w/w) of water. Films in the 1st design had lower resistances probably due to their smaller area. 38 Figure 22: Resistance variation for films with PEDOT:PSS with 50% (w/w) of water. PEDOT:PSS modified with PEG was also utilized for the production of films. For that, an aqueous solution of PEDOT:PSS with PEG and an additional layer of as-synthesized AgNWs in glycerol were deposited through doctor blading on Kapton. However, for both of these layers the resistances were higher than 600 MΩ. Moreover, the same method was used to deposit a solution of the same concentrations but with an additional 2.5% (V/V) of as-synthesized AgNWs in glycerol. Neverthless, this also resulted in films with resistances of millions of ohms, which were not applicable in the objective of this work. Furthermore, films of PEDOT:PSS treated with PEG and glycerol were produced on Kapton substrate. Figure 23 displays the variation of the resistances with the addition of each layer, for films with one or two layers of PEDOT:PSS with PEG and glycerol and another layer of as-synthesized AgNWs, all deposited through doctor blading. AgNWs in glycerol (GLY) or in glycerol with 40 or 55% (w/w) of methanol (MeOH) were used due to potential interactions between methanol, glycerol and PEG, as they are all polyols (all have an OH group). The results demonstrate a lack of uniformity, as the resistances of the first PEDOT:PSS layer were significantly different, even though they were deposited by the same method, and the addition of the same AgNWs solutions did not have similar effects in both types of films. 39 Figure 23: Resistance of films with one or two layers of PEDOT:PSS with PEG and glycerol, and one layer of AgNWs. Moreover, glycerol solutions with PEDOT:PSS and methanol were also used to fabricate films. First, a PEDOT:PSS glycerol solution of concentration 0.2 g/mL with 10% (V/V) of methanol was deposited on Kapton, as well as an additional layer of as-synthesized AgNWs in glycerol, or in glycerol with 40 or 55% (w/w) of methanol, both through the doctor blading method. In this approach, the resistances were inconsistent for both layers, ranging from hundreds of thousands to hundreds of millions of ohms, which represented a too low conductivity for the application studied in this work. Additionally, the same methods were used for the fabrication of films of a PEDOT:PSS glycerol solution of concentration 0.4 g/mL with 5% (V/V) of methanol, with further deposition of similar AgNWs layers, in the same substrate. Although the average resistance did not variate consistently, as seen in Figure 24, for individual samples the application of a layer of AgNWs in glycerol with 55% (w/w) of methanol did result in a slight decrease of the resistance. The same solvents systems in PEDOT:PSS and AgNWs seemed to work better. Figure 24: Resistance variation of films with one layer of a PEDOT:PSS glycerol solution of concentration 0.4 g/mL with 5% (V/V) of methanol and another layer of AgNWs. 40 layers of PEDOT:PSS were treated with this method, followed by the deposition of as-synthesized AgNWs in glycerol with 55% (w/w) of methanol. Figures 32 and 33 show the resistances associated with each step of the process for films with doctor bladed or screen-printed PEDOT:PSS, respectively, and spin-coated AgNWs, on PET. In both cases the treatment resulted in practically no variation in the resistance, especially when compared to the effect of the addition of the second layer of PEDOT:PSS. As in previous approaches, the screen-printing technique originated films with lower conductivities, presumably due to their smaller thickness, (0.0040 ± 0.0005) mm, when compared to the thickness of the doctor bladed films, (0.0050 ± 0.0006) mm. Figure 32: Resistances for films with two layers of doctor bladed PEDOT:PSS immersed in methanol and one layer of AgNWs. 47 Figure 33: Variation of the resistance for films with two layers of screen-printed PEDOT:PSS immersed in methanol and one layer of AgNWs. Despite the inefficiency of the treatment, the films with the two doctor bladed layers of PEDOT:PSS immersed in methanol and another layer of AgNWs showed the best results among all films deposited by that method throughout this work. Figure 34 shows a film obtained with this approach, as well as its corresponding SEM image. Like in previous films, the AgNWs were evenly dispersed, albeit sparsely, which could be the reason for their minimal impact on the resistance of the films. Figure 34: Film with two doctor bladed layers of PEDOT:PSS immersed in methanol and one layer of AgNWs, and its SEM image. Furthermore, films with one or two layers of PEDOT:PSS modified with PEG were also soaked in methanol. Figure 35 shows the resistance variation for one or two layers of PEDOT:PSS with PEG immersed in methanol with an additional layer of as-synthesized AgNWs in glycerol or in glycerol with 40 or 55% (w/w) of methanol. The films were deposited through doctor blading on Kapton. As was the case 48 in previous approaches, films with two layers of PEDOT:PSS had lower resistances, while those with one layer showed a high uncertainty associated with the deposition of AgNWs in glycerol. Figure 35: Resistances for films with one or two layers of PEDOT:PSS with PEG and another layer of AgNWs in glycerol with different methanol concentrations. Following these results, two layers of PEDOT:PSS with PEG were screen-printed and as-synthesized AgNWs in glycerol and 55% (w/w) of methanol were deposited through spin coating. As seen on Figure 36, the treatment of the first PEDOT:PSS layer had an inconsistent effect on the resistance or even increased it. On the contrary, the immersion of the film with the double layer of the polymer resulted in similar or slightly lower resistances. In comparison to the doctor bladed films, of average thickness (0.0070 ± 0.0010) mm, the screen-printed films were less conductive, likely due to its lower thickness, of average (0.0040 ± 0.0010) mm. Additionally, these films had higher resistances than non-modified PEDOT:PSS films deposited by the same method. Furthermore, some samples were annealed at 225°C for 1 hour and at 180°C for 5 hours, which increased the resistance of the films by one or two orders of magnitude. 49 Figure 36: Resistance variations for screen-printed PEDOT:PSS with PEG and spin-coated AgNWs in glycerol and 55% (w/w). Furthermore, PEDOT:PSS modified in the same way was used to produce films with intercalated AgNWs in glycerol with 55% (w/w) of methanol, in which all layers were deposited by doctor blading on Kapton. Figure 37 displays how the treatment was able to decrease the resistance of both PEDOT:PSS layers. The films obtained with this approach had a slightly lower average resistance than the non-intercalated films deposited by the same method. Figure 37: Resistances for films with two treated layers of PEDOT:PSS and one intercalated layer of AgNWs. Moreover, films with one or two layers of doctor bladed PEDOT:PSS with glycerol on Kapton were also 50 treated with methanol. Additionally, as-synthesized silver nanowires in glycerol with different concentrations of methanol were deposited through the same method. Figure 38 shows the resistances for films of one or two layers treated with a drop of methanol, in which the effect of the second layer is noticeable. The uncertainties and differences in the resistance of the first layer may be attributed to the inconsistency of the deposition method. Figure 38: Resistance fluctuation for films with one or two layers of PEDOT:PSS with glycerol treated with a drop of methanol and one layer of AgNWs. Similar films were also treated with an additional immersion in methanol. Figure 39 displays the results obtained with this approach. In this case, there was also a considerable uncertainty in the resistance of the first layer of PEDOT:PSS, and the final resistances were similar to the ones obtained by the previous drop treatment. Figure 39: Resistance for films with one or two layers of PEDOT:PSS with glycerol treated with a drop and immersion in methanol and one layer of AgNWs. Finally, a solution of PEDOT:PSS with AgNWs in glycerol with 55% (w/w) methanol was used to produce films by doctor blading. The results of their treatment in methanol are presented in Figure 40, which shows 51 that the immersion was not effective in this case. Figure 40: Resistance variation for a film of PEDOT:PSS with AgNWs in glycerol and methanol treated with immersion in methanol. Table 7summarily presents the results for films treated with methanol. In comparison with nonmodified PEDOT:PSS films, most of the treatments did not result in films with lower resistances. Nevertheless, films with two doctor bladed or screen-printed layers of PEDOT:PSS treated with immersion in methanol had slightly lower or similar resistances to simple PEDOT:PSS films, respectively. Films of PEDOT:PSS with PEG and an intercalated layer of AgNWs seemed to be the best approach for utilizing methanol immersion as a treatment. Furthermore, the screen-printed films were characterized in terms of the increase in temperature when a voltage of 12 V was applied. Additionally, the films with two layers of PEDOT:PSS deposited through doctor blading and imersed in methanol, as they had the lowest resistance for the films with the “final” designs, were also characterized. Finally, the film with two layers of PEDOT:PSS with an intercalated layer of silver nanowires, all deposited by doctor blading, was also characterized given the results obtained. All films showed a lower increase in the temperature than the films with the double doctor bladed PEDOT:PSS layer in the 1st and 2nd designs, which could elevate their temperature on average more than 4 and 3°C, respectively. 52 Table 7: Summary of the results obtained for films treated with methanol. 53 The first films treated with DMSO were films of one or two layers of PEDOT:PSS. Figure 41 shows the resistance associated with the doctor blading and treatment of each layer of PEDOT:PSS and the further deposition of as-synthesized AgNWs in glycerol or in glycerol with 40 or 55% (w/w) of methanol on a Kapton 54 substrate. Films with the double layer of the conducting polymer originated films with higher conductivities, especially those with silver nanowires with methanol. Figure 41: Resistances for films with one or two layers of PEDOT:PSS treated with DMSO and one layer of AgNWs. Following these observations, films with two layers of PEDOT:PSS were screen-printed on Kapton and PET and soaked in DMSO. However, as a consequence of this treatment, films on PET lost their adherence to the substrate. As such, Figure 42 presents the results for films on Kapton with an additional layer of as-synthesized AgNWs in glycerol and 55% (w/w) of methanol deposited through spin coating and doctor blading, in which it is observable that the DMSO treatment increased the resistance of the films, even if only slightly. Once more, the screen-printing method resulted in higher resistances than the doctor blading, even though in this case the average thicknesses measured were identical: (0.0040 ± 0.0003) mm. Additionally, a few samples of this approach were annealed at 225°C for 1 hour and at 180°C for 5 hours, all of which had their conductivity decreased. Figure 42: Resistances for screen-printed PEDOT:PSS layers treated in DMSO and one layer of AgNWs. 55 Furthermore, this treatment was applied in PEDOT:PSS layers of films with an intercalated layer of as-synthesized AgNWs in glycerol and 55% (w/w) of methanol, all doctor bladed on Kapton. Figure 43 displays the resistance fluctuations in this process, in which it is possible to notice effective treatment of the first layer of PEDOT:PSS. Nevertheless, the final resistance obtained with this approach was the same as the one with the non-intercalated layers deposited by the same method. Figure 43: Variation in the resistance for films with PEDOT:PSS treated with DMSO and an intercalated layer of AgNWs. Following these results, the same approach was taken without treatment of the last layer of PEDOT:PSS. The conductive polymer was doctor bladed on Kapton and the same silver nanowire solution was deposited through spin coating. As shown in Figure 44, the resistance followed the same trend as in Figure 43. Figure 44: Resistance for films with one layer of treated PEDOT:PSS, one intercalated layer of AgNWs, and one untreated layer of PEDOT:PSS. Moreover, PEDOT:PSS modified with PEG was doctor bladed on Kapton and immersed in DMSO. An additional layer of as-synthesized silver nanowires in glycerol and 55% (w/w) of methanol was deposited 56 Figure 51 shows the changes on the resistance caused by this treatment. For all films the immersion in CTAB followed by rinsing with water and drying at room temperature increased their resistance substantially, while rinsing the films with ethanol resulted in similar resistances for all of them, which were still higher than the initial resistances. Figure 51: Effects of CTAB immersion on films with PEDOT:PSS. Furthermore, prompted by the work reported by Ge et al. [58], the following previously mentioned films were immersed in NaBH4: (A) Two layers of doctor bladed PEDOT:PSS and one layer of spin-coated AgNWs in glycerol and 55% (w/w) of methanol. (B) Two layers of PEDOT:PSS with an intercalated layer of AgNWs in glycerol and 55% (w/w) of methanol, both doctor bladed. (C) Two layers of PEDOT:PSS immersed in methanol and a layer of AgNWs in glycerol and 55% (w/w) of methanol, both doctor bladed. For films treated with NaBH4, which results are shown in Figure 52, the treatment decreased the conductivity in all cases, contrary to the reported by Ge et al. [58]. This could be due to reactions between the sodium borohydride and the remaining glycerol and methanol from the AgNW solution, which formed hydrogen bubbles [81,82] that damaged the films, as seen in Figure 53. 63 Figure 52: Variation in the resistance of films with PEDOT:PSS immersed in sodium borohydride Figure 53: Photographs of the damaged films after immersion in NaBH4. 5.4.6 Ionic liquid and PVA Given the high conductivity associated with ionic liquids and PVA’s very high transparency and flexibility, and while drawing inspiration on the previously mentioned works by Yemata et al. [35], Imae et al. [36], Lan et al. [73], and Kaikanov et al. [74], these substances were used in an attempt to obtain films with low resistances. While the ionic liquid may increase the conductivity of the films through the formation of PEDOT and PSS-rich zones, while substituting PSS for other anions [35,83], PVA might improve films’ flexibility and mechanical stability while maintaining their transparency [73]. The following previously doctor bladed films were modified with ionic liquid: (A) Two layers of PEDOT:PSS immersed in methanol and a layer of AgNWs in glycerol and 55% (w/w) of methanol. (B) Two layers of PEDOT:PSS with an intercalated layer of AgNWs in glycerol and 55% (w/w) of methanol. (C) One layer of PEDOT:PSS. 64 Figure 54 shows the effects of spin coating a layer of ionic liquid on the films (left) or immersing them in it (right). For both treatments the resistances did not decrease, which rendered these treatments ineffective. Figure 54: Variation in the resistance of films modified with ionic liquid by spin coating (left) or immersion (right). Furthermore, solutions of ionic liquid and PVA in a ratio of 0.8:1 were used. The deposition of this solution originated a film with resistance 36.5 MΩ, while adding 5 mM or 10 mM of silver nitrate resulted in films of 800 kΩor 61.2 MΩ, respectively. Figure 55 shows the final aspect of the films, in which the effect of the silver nitrate on the color and transparency is notorious. Figure 55: Films formed on Petri dishes: ionic liquid and PVA in a ratio of 0.8:1 (left), with 5 or 10 mM of AgNO3(center and right, respectively). The PVA, ionic liquid, and 5mM of AgNO3solution was spin coated or added to PEDOT:PSS in concentrations of 20, 50 and 80% (w/w), and the variations of the resistances associated with these processes are shown in Figure 56 and 57, respectively. The presence of PVA lead to an increment of electrical resistance. 65 Figure 56: Resistance for a spin-coated layer of ionic liquid with PVA and AgNO3on doctor bladed PEDOT:PSS. Figure 57: Resistance for different concentrations of the solution of ionic liquid, PVA and AgNO3added to PEDOT:PSS. Moreover, a solution of ionic liquid and PVA in a ratio of 3:1 was used. The deposition of this solution originated a film with 1.26 MΩof resistance, while the addition of 5 mM or 10 mM of silver nitrate produced films with resistances of 1.02 MΩor 3.5 MΩ, respectively. Figure 58 shows the final films. For this ratio, the difference in color and transparency is only observable for the highest concentration of AgNO3. 66 Figure 58: Films deposited on Petri dishes: ionic liquid and PVA in a ratio of 3:1 (left) with 5 or 10 mM of AgNO3(center and right, respectively). Figures 59 and 60 show the resistance of films with a spin-coated layer of the ionic liquid, PVA and 5 mM of AgNO3solution, and of films resultant from adding this solution to PEDOT:PSS in concentrations of 20, 50 or 80% (w/w), respectively. The presence of PVA lead to an increment of electrical resistance. Figure 59: Resistance for films of one doctor bladed layer of PEDOT:PSS and one spin-coated layer of a solution of ionic liquid, PVA and 5 mM of AgNO3. Figure 60: Resistances for films of PEDOT:PSS with different concentrations of a solution of ionic liquid, PVA, and AgNO3. 67 For solutions with both ratios between ionic liquid and PVA, with AgNO3, spin coating a layer on top of PEDOT:PSS films increased their resistance, which was also verified for films in which the solutions were added to PEDOT:PSS. For these films, the lower the concentration of PEDOT:PSS, the lower the conductivities of the films. Finally, a solution of silver nanowires in IPA and acetone with PVA was used to produce a film as well, which had no conductivity. Given that the modifications to PEDOT:PSS films using the ionic liquid and PVA increased the resistances of the films, and that the films produced with these reagents and AgNO3and the film with PVA and AgNWs all showed high resistances, it was concluded that utilizing this ionic liquid and PVA was not an effective method to increase the conductivity of the films. The average results displayed graphically throughout this chapter are further elaborated in the appendices of this work, providing detailed values for a more comprehensive understanding of the results. 68 Chapter 6 Conclusion and future work In this work, films were produced and studied with the objective of using screen-printing as a reproducible technique to produce flexible and transparent heating systems that would operate efficiently with low input voltages. Given that the heating mechanism is inversely proportional to the resistance, the main intention throughout this work was to obtain films with the lowest resistance possible. With that purpose, films of the conducting polymer PEDOT:PSS with silver nanowires were fabricated. Silver nanowires were synthesized through the polyol method and modified with the aim of improving their adhesion and conductivity when utilized with PEDOT:PSS. PEDOT:PSS was modified and its films were post-treated with different solvents with the goal of reducing their resistance. The approaches with the highest conductivities were then screen-printed on different substrates, and the films had their temperature analyzed with an infrared camera when subjected to an input voltage of 12 V. With the obtained results it was possible to conclude that the addition of a layer of as-synthesized silver nanowires, whether in just glycerol, directly from the synthesis, or with different concentrations of methanol, generally had only a slight effect on the resistance of the films. This was likely due to the sparse dispersion of the nanowires on the films, which was observable in SEM images and might be improved by increasing the AgNWs’ concentration. The addition of PEDOT:PSS layers, however, did originate considerable decreases in the resistances of the films, likely due to an increase in their thicknesses. Furthermore, modifying PEDOT:PSS with water, PEG, glycerol, glycerol and PEG, glycerol and methanol, or with DMSO did not result in films with lower resistances. Moreover, films post-treated with methanol overall did not show lower resistances than untreated films. However, films with two doctor bladed layers of PEDOT:PSS immersed in methanol with an additional layer of silver nanowires had slightly lower resistances than simple PEDOT:PSS films and showed the highest temperature increase for films produced by this method. On the contrary, post-treatment with DMSO was generally effective, but the temperature increase was only slight. Additionally, utilizing silver nanowires that had been previously sintered was not an effective approach 69 at increasing the conductivity of films. Furthermore, treating films with immersion in CTAB or sodium borohydride solutions did not decrease their resistance either. Finally, utilizing an ionic liquid and PVA was not an effective approach to obtaining films with high conductivity as well. With these results, it was concluded that the determining factor for reducing the final resistance of the films seemed to be their thickness. In agreement with that assumption, the thicker films produced, with two screen-printed layers of PEDOT:PSS with a bigger mesh size and an additional layer of spin-coated AgNWs, showed the highest average increase in temperature, of almost 5°C. All films produced throughout this work were flexible and had varying levels of transparency. However, the better-performing films were the less transparent ones, since the lower resistances were observed for thicker films. For films with this range of temperature increase, applications can include window defogging or textile implementations, for instance. Future work in this area may include the utilization of other solvents to modify or post-treat PEDOT:PSS and/or silver nanowires with the main objective of obtaining reproducible films with low resistances and simultaneously high flexibility and transmittance of visible light. 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Scientific Reports , 12(1):1–12, 2022. 79 Appendices A Silver nanowire films Tables A.1,A.2, and Ashow the resistance in kΩof films with one and two layers of commercial silver nanowire ink deposited through doctor blading on PET and Kapton. Table A.1: Resistance of films with one and two layers of commercial silver nanowire ink in the 1st design. Substrate Sample 1 layer 2 layers Kapton 1 0.430 2 0.460 3 0.400 250 MΩ 4 0.450 PET 1 0.550 2 0.390 3 0.690 250 MΩ 4 0.610 Table A.2: Resistance of films with one and two layers of commercial silver nanowire ink in the 2nd design. Substrate Sample 1 layer 2 layers Kapton 1 0.400 2 0.490 3 0.530 500 MΩ 4 0.430 PET 1 0.580 2 0.520 3 0.480 500 MΩ 4 0.480 80 Table A.3: Resistance of films with one and two layers of commercial silver nanowire ink in the 3rd design. Substrate Sample 1 layer 2 layers Kapton 1 0.650 2 0.730 3 0.730 600 MΩ 4 0.670 PET 1 0.740 2 0.760 3 0.700 600MΩ 4 0.770 B Effect of number of layers of the PEDOT:PSS films Tables B.1 through B.7 show the values of the resistances in kΩobtained in the study of the effect of the number of layers of PEDOT:PSS on the resistance of the films. The deposited as-synthesized AgNWs were in glycerol with 55% (w/w) methanol. Table B.1: Resistance for films with subsequential layers of doctor bladed PEDOT:PSS and spin-coated AgNWs in the 3rd design on PET. Sample 1x PEDOT:PSS 1x AgNWs 2x AgNWs 1 4.410 3.930 4.150 2 3.310 2.970 3.100 Table B.2: Resistance variations for films with two doctor bladed layers of PEDOT:PSS and one spin-coated layer of AgNWs in the 3rd design. Substrate Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs Kapton 1 4.140 2.030 1.850 2 3.800 1.930 1.840 3 4.870 1.890 1.740 PET 1 3.950 1.890 1.790 81 Table B.3: Resistances for films with two layers of screen-printed PEDOT:PSS and one layer of spin-coated AgNWs in the 1st and 3rd designs, on PET. 1st design Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs 1 7.540 3.020 2.830 2 6.500 2.820 2.660 3 6.000 2.690 2.530 4 6.240 5 7.030 2.900 2.910 3rd design Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs 1 17.90 6.270 5.910 2 13.77 5.300 5.330 3 12.80 5.340 5.080 4 17.75 5.460 5.140 5 15.30 5.690 5.440 Table B.4: Resistances for films with two screen-printed layers of PEDOT:PSS with a bigger mesh size and one spin-coated layer of AgNWs on PET. 1st design Sample 2x PEDOT:PSS 1x AgNWs 1 0.755 0.739 2 0.795 0.770 3 0.875 0.810 4 0.861 0.798 5 0.860 0.795 6 0.783 0.771 3rd design Sample 2x PEDOT:PSS 1x AgNWs 1 1.568 1.460 2 1.550 1.480 3 1.750 1.600 4 1.685 1.570 5 1.686 1.590 6 1.632 1.480 82 Table B.5: Resistance for films with three layers of screen-printed PEDOT:PSS and one layer of spin-coated AgNWs on the 1st and 3rd designs on PET. 1st design Sample 1x PEDOT:PSS 2x PEDOT:PSS 3x PEDOT:PSS 1x AgNWs 1 6.000 2.650 1.670 1.560 2 6.120 2.720 1.700 1.580 3 5.790 2.680 1.700 1.590 4 6.060 2.670 1.660 1.570 5 7.500 3.050 1.790 1.690 3rd design Sample 1x PEDOT:PSS 2x PEDOT:PSS 3x PEDOT:PSS 1x AgNWs 1 14.30 5.530 3.370 3.15 2 13.76 5.390 3.290 3.040 3 16.05 5.830 3.450 3.230 4 16.37 5.790 3.410 3.190 5 17.85 6.300 3.530 3.330 Table B.6: Resistances for films with a spin-coated AgNW layer intercalated in doctor bladed PEDOT:PSS layers in the 1st design, on Kapton. Sample 1x PEDOT:PSS 1x AgNWs 2x PEDOT:PSS 1 2.160 1.950 0.900 2 1.800 1.680 0.895 3 2.230 2.150 0.962 4 2.040 1.930 0.948 5 17.85 6.300 3.530 Table B.7: Resistances for films with a spin-coated AgNW layer intercalated in screen-printed PEDOT:PSS layers, on PET. 1st design Sample 1x PEDOT:PSS 1x AgNWs 2x PEDOT:PSS 1 6.480 5.700 2.580 2 6.730 5.870 2.780 3 6.270 5.520 2.490 4 6.540 5.920 2.750 5 6.080 5.410 2.570 3rd design Sample 1x PEDOT:PSS 1x AgNWs 2x PEDOT:PSS 1 14.14 13.33 5.610 2 16.15 15.53 5.790 3 13.09 12.62 5.340 4 14.15 14.22 5.430 5 14.62 13.14 5.150 83 C Films with modified PEDOT:PSS Tables C.1 through C.7 show the resistances in kΩof films with one or two layers of modified PEDOT:PSS and one layer of as-synthesized AgNWs in glycerol, or glycerol with 40 or 55% (w/w) of methanol, referred to as GLY, 40% or 55%, respectively, on the sample’s section of the tables. Table C.1: Resistance for films with two doctor bladed layers of PEDOT:PSS with 30% (w/w) of water and one spin-coated layer of AgNWs in glycerol and 55% (w/w) methanol. Substrate Design 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs Annealing 5h 180 °C Kapton 1 3.300 1.250 1.120 1.910 2 3.660 1.700 1.550 2.460 3 5.600 2.630 2.400 4.000 PET 1 3.100 1.400 1.200 2 3.820 1.800 1.510 3 5.700 2.600 2.400 Table C.2: Resistance for films with two doctor bladed layers of PEDOT:PSS with 50% (w/w) of water and one spin-coated layer of AgNWs in glycerol and 55% (w/w) methanol in the 2nd design. Substrate Samples 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs Annealing 1h 225 °C Annealing 5h 180 °C Kapton 1 4.590 2.250 1.970 10.30 27.00 2 4.390 2.600 2.120 3.380 PET 1 4.040 2.090 2 4.300 2.160 1.800 Table C.3: Resistance of films with one or two doctor bladed layers of PEDOT:PSS with PEG and glycerol, and one doctor bladed layer of AgNWs. The films were deposited on Kapton in the preliminary design. 1 layer Sample 1x PEDOT:PSS 1x AgNWs 1 8.000 2 (GLY) 3.500 1.200 3 (40%) 7.000 2.500 4 (55%) 7.000 1.500 2 layers Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs 1 2.000 0.700 2 (GLY) 2.000 0.500 8.000 3 (40%) 1.100 0.900 3.000 4 (55%) 1.000 0.800 6.000 84 Table C.4: Resistances for films with one layer of a PEDOT:PSS glycerol solution of concentration 0.4 g/mL with 5% (V/V) of methanol and another layer of AgNWs, both doctor bladed on Kapton in the preliminary design. Sample 1x PEDOT:PSS 1x AgNWs 1 (40%) 1.200 630 2 (40%) 1.500 1.800 3 1.100 4 (55%) 900 790 5 (55%) 1.500 1000 6 400 Table C.5: Resistances for films of PEDOT:PSS aqueous solutions with glycerol and 5 or 10% (V/V) of methanol, and one layer of AgNWs in glycerol with 55% (w/w) methanol. All layers were doctor bladed on Kapton in the preliminary design. 5% (V/V) of methanol Sample 1x PEDOT:PSS 1x AgNWs 1 0.800 0.480 2 1.00 0.260 10% (V/V) of methanol Sample 1x PEDOT:PSS 1x AgNWs 1 345.0 465.0 2 333.0 485.0 3 16.00 38.00 Table C.6: Resistances for films with one or two layers of PEDOT:PSS with 10% (w/w) DMSO and one layer of AgNWs, all doctor bladed on Kapton in the preliminary design. 1 layer Sample 1x PEDOT:PSS 1x AgNWs 1 0.340 2 (GLY) 0.460 0.430 3 (40%) 0.200 0.170 4 (40%) 0.410 0.690 5 (40%) 0.490 0.550 6 (55%) 0.300 0.150 7 (55%) 0.230 0.220 8 (55%) 0.250 0.250 2 layers Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs 1 0.600 0.300 2 0.600 0.320 3 (GLY) 0.260 0.150 0.500 4 (40%) 0.280 0.120 0.450 5 (55%) 0.250 0.080 0.380 85 Table C.7: Resistances for films with one or two layers of PEDOT:PSS with 40% (w/w) DMSO and one layer of AgNWs, all doctor bladed on Kapton in the preliminary design. 1 layer Sample 1x PEDOT:PSS 1x AgNWs 1 1.200 2 0.490 3 (GLY) 0.930 0.870 4 (40%) 0.890 0.700 5 (40%) 1.100 1.000 6 (55%) 0.670 0.620 7 (55%) 0.850 0.900 2 layers Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs 1 1.000 0.600 2 1.300 0.600 3 (GLY) 1.000 0.340 0.430 4 (40%) 1.000 0.330 0.380 5 (55%) 0.800 0.340 0.360 D Treatment of films with methanol or DMSO Tables D.1 through D.20 show the resistances in kΩof films with one or two layers of PEDOT:PSS post-treated with methanol or DMSO. In the samples sections of the tables, the references to GLY, 40% and 55% indicate that the AgNW layer was composed of as-synthesized AgNWs in glycerol, or in glycerol with 40% (w/w) or 55% (w/w) of methanol, respectively. Table D.1: PEDOT:PSS films in Kapton treated with immersion in methanol for different times. Sample 1x PEDOT:PSS After treatment Treatment time (min) 1 1.480 1.510 5 2 1.990 2.070 20 3 1.840 1.910 40 4 1.900 2.010 60 Table D.2: PEDOT:PSS films in Kapton treated with immersion in DMSO for different times. Sample 1x PEDOT:PSS After treatment Treatment time (min) 1 1.710 1.730 5 2 1.770 1.900 20 3 1.750 1.830 40 4 2.100 2.110 60 86 Table D.3: Resistances for films with one or two layers of PEDOT:PSS treated with a drop of methanol and another layer of AgNWs, all deposited through doctor blading in the preliminary design on Kapton. 1 layer Sample 1x PEDOT:PSS after treatment 1x AgNWs 1 0.450 2 0.320 3 (GLY) 1.200 3.500 4 (GLY) 0.650 1.200 5 (40%) 0.500 0.600 6 (40%) 0.450 0.450 7 (55%) 0.500 0.250 8 (55%) 0.420 0.480 2 layers Sample 1x PEDOT:PSS after treatment 2x PEDOT:PSS after treatment 1x AgNWs 1 0.640 0.240 2 0.360 0.180 3 (GLY) 0.430 0.340 0.230 4 (40%) 0.560 0.230 0.175 5 (55%) 0.800 0.180 0.240 Table D.4: resistance for films with one or two layers of PEDOT:PSS treated with a drop of methanol followed by immersion in it and an additional layer of AgNWs. All layers were deposited through doctor blading in the preliminary design on Kapton. 1 layer Sample 1x PEDOT:PSS 1x AgNWs 1 0.290 2 0.420 3 (GLY) 0.420 0.450 4 (GLY) 0.260 0.260 5 (40%) 0.320 0.350 6 (40%) 0.340 0.430 7 (55%) 0.370 0.350 8 (55%) 0.320 0.200 2 layers Sample 1x PEDOT:PSS 2x PEDOT:PSS 1x AgNWs 1 0.450 0.170 2 0.230 0.120 3 (GLY) 0.410 0.160 0.140 4 (40%) 0.370 0.130 0.150 5 (55%) 0.700 0.130 0.150 87 Table D.5: Resistances for films with two layers of doctor bladed PEDOT:PSS immersed in methanol and another spin-coated layer of AgNWs in glycerol with 55% (w/w) of methanol, in the 1st, 2nd and 3rd designs. Substrate Design 1x PEDOT:PSS 1x PEDOT:PSS after treatment 2x PEDOT:PSS 2x PEDOT:PSS after treatment 1x AgNWs Kapton 1st 1.720 1.740 0.820 0.823 0.723 2nd 2.490 2.480 1.090 1.030 0.996 3rd 4.100 4.100 1.690 1.610 1.550 PET 1st 1.900 1.870 0.805 0.820 0.703 2nd 2.900 2.500 1.080 1.150 0.914 3rd 4.000 3.930 1.600 1.780 1.420 Table D.6: Resistances for films with two layers of screen-printed PEDOT:PSS immersed in methanol and another spin-coated layer of AgNWs in glycerol with 55% (w/w) of methanol, in the 1st and 3rd designs on PET. 1st design Sample 1x PEDOT:PSS 1x PEDOT:PSS after treatment 2x PEDOT:PSS 2x PEDOT:PSS after treatment 1x AgNWs 1 6.700 6.830 2.800 2.910 2.650 2 6.840 6.830 3.020 3.080 2.770 3 6.900 7.120 2.750 2.850 2.530 4 7.830 7.330 2.820 2.890 2.530 5 7.000 7.500 2.900 3.070 2.690 3rd design Sample 1x PEDOT:PSS 1x PEDOT:PSS after treatment 2x PEDOT:PSS 2x PEDOT:PSS after treatment 1x AgNWs 1 15.86 16.12 6.170 6.630 5.690 2 16.54 17.28 6.500 6.700 5.860 3 17.36 17.95 6.150 6.380 5.610 4 19.67 18.44 6.020 6.240 5.450 5 16.16 16.90 6.000 6.040 5.500 88 Table D.16: Resistances for films with one layer of PEDOT:PSS immersed in DMSO, one intercalated layer of AgNWs in glycerol and 55% (w/w) of methanol, and one untreated layer of PEDOT:PSS. PEDOT:PSS was doctor bladed, while the AgNWs were spin-coated in the 3rd design on Kapton. Sample 1x PEDOT:PSS 1x PEDOT:PSS after treatment 1x AgNWs 2x PEDOT:PSS 1 4.270 4.160 3.880 2.030 2 4.390 4.250 3.980 1.930 Table D.17: Resistances for films with two layers of PEDOT:PSS with PEG further immersed in DMSO and an additional layer of AgNWs in glycerol and 55% (w/w) of methanol. All layers were doctor bladed in the 1st, 2nd, and 3rd designs on Kapton. Design 1x PEDOT:PSS 1x PEDOT:PSS after treatment 2x PEDOT:PSS 2x PEDOT:PSS after treatment 1x AgNWs 1st 2.900 2.050 1.120 0.950 0.800 2nd 4.500 2.750 1.230 1.150 0.953 3rd 7.000 3.850 2.200 1.790 1.600 Table D.18: Resistances for films with two layers of PEDOT:PSS with PEG further immersed in DMSO and an additional layer of AgNWs in glycerol and 55% (w/w) of methanol. PEDOT:PSS layers were screenprinted, while AgNWs were spin coated in the 1st and 3rd designs on Kapton. 1st design Sample 1x PEDOT:PSS 1x PEDOT:PSS after treatment 2x PEDOT:PSS 2x PEDOT:PSS after treatment 1x AgNWs 1 9.400 8.400 3.620 3.150 2.870 2 9.80 8.980 3.700 3.470 2.980 3 10.10 8.590 3.600 3.380 3.070 4 10.00 8.620 3.750 3.280 2.950 5 9.700 8.850 3.680 3.250 2.930 3rd design Sample 1x PEDOT:PSS 1x PEDOT:PSS after treatment 2x PEDOT:PSS 2x PEDOT:PSS after treatment 1x AgNWs 1 19.90 18.43 7.350 6.390 5.730 2 21.20 18.38 7.500 6.550 6.100 3 21.30 23.61 7.440 7.360 6.370 4 21.40 18.22 7.620 6.690 6.250 5 21.90 18.70 7.590 6.450 6.070 Table D.19: Resistances for films of PEDOT:PSS with PEG and AgNWs in glycerol and 55% (w/w) of methanol doctor bladed in the 2nd design on Kapton and further immersed in DMSO. Sample Before treatment After treatment 1 12.00 4.930 2 11.00 5.580 95 Table D.20: Resistances associated with DMSO treatment of films of doctor bladed PEDOT:PSS followed by immersion in AgNWs in glycerol and 55% (w/w) of methanol. Sample 1x PEDOT:PSS 1x PEDOT:PSS after treatment After immersion in AgNWs solution Immersion time (min) 1 5.050 5.220 5.860 30 2 3.780 3.800 6.320 60 3 5.610 5.680 17.00 90 E Films with cleaned AgNWs Tables E.1 and E.2 show the resistances in kΩassociated with the use of cleaned and sintered assynthesized AgNWs. Table E.1: Resistances for the spin coating of sintered AgNWs in water onto a film of two screen-printed layers of PEDOT:PSS in the 1st and 3rd designs. 1st design Substrate Sample 1x PEDOT:PSS 1x AgNWs PET 1 5.200 4.980 2 5.150 3 5.600 Kapton 4 5.630 5.130 5 6.000 6 5.600 3rd design Substrate Sample 1x PEDOT:PSS 1x AgNWs PET 1 9.210 8.390 2 8.750 3 8.560 Kapton 4 8.950 9.380 5 8.160 6 8.730 96 Table E.2: Resistances for the spin coating of sintered AgNWs in water onto a film of three screen-printed layers of PEDOT:PSS in the 1st and 3rd designs. 1st design Substrate Sample 1x PEDOT:PSS 1x AgNWs PET 1 3.330 2.920 2 3.440 3 3.630 Kapton 4 3.400 3.220 5 3.330 6 3.240 3rd design Substrate Sample 1x PEDOT:PSS 1x AgNWs PET 1 5.510 5.280 2 5.760 3 5.710 Kapton 4 5.180 4.860 5 5.540 6 5.390 F Treatment of films with CTAB or sodium borohydride Tables F.1 and F.2 show the resistances in kΩof the following films treated with immersion in CTAB: (A) Two layers of screen-printed PEDOT:PSS and one layer of spin-coated AgNWs in glycerol and 55% (w/w) of methanol. (B) Two layers of screen-printed PEDOT:PSS with an intercalated spin-coated layer of AgNWs in glycerol and 55% (w/w) of methanol. (C) Two layers of screen-printed PEDOT:PSS immersed in methanol and a layer of spin-coated AgNWs in glycerol and 55% (w/w) of methanol. (D) Two layers of screen-printed PEDOT:PSS with PEG immersed in DMSO and a layer of spin-coated AgNWs in glycerol and 55% (w/w) of methanol. Table F.1: Resistances for films treated with CTAB and rinsed with water. Film Before treatment After treatment A 2.880 14.00 B 2.760 7.400x103 C 2.700 5.200 D 3.090 11.0 97 Table F.2: Resistances for films treated with CTAB and rinsed with ethanol. Film Before treatment After treatment A 2.560 6.000 B 2.810 6.600 C 2.790 6.400 D 3.040 6.430 Table F.3 shows the resistances in kΩof the following films treated with immersion in sodium borohydride (NaBH4): (A) Two layers of doctor bladed PEDOT:PSS and one layer of spin-coated AgNWs in glycerol and 55% (w/w) of methanol. (B) Two layers of PEDOT:PSS with an intercalated layer of AgNWs in glycerol and 55% (w/w) of methanol, both doctor bladed. (C) Two layers of PEDOT:PSS immersed in methanol and a layer of AgNWs in glycerol and 55% (w/w) of methanol, both doctor bladed. Table F.3: Resistances for films treated with NaBH4. Film Before treatment After treatment A 1.800 2.620 B 1.400 1.910 C 0.919 1.110 G Ionic liquid and PVA Tables G.1 through G.5 show the resistances in kΩfor films with ionic liquid and PVA. Table G.1, specifically, shows the effect of modifying the following doctor bladed films with ionic liquid: (A) Two layers of PEDOT:PSS immersed in methanol and a layer of AgNWs in glycerol and 55% (w/w) of methanol. (B) Two layers of PEDOT:PSS with an intercalated layer of AgNWs in glycerol and 55% (w/w) of methanol. (C) One layer of PEDOT:PSS. 98 Table G.1: Resistances for films modified with ionic liquid by spin coating or immersion. Spin coating Film Without ionic liquid With ionic liquid A 0.695 0.900 B 1.430 1.500 C 1.920 1.930 Immersion Film Before immersion After immersion A 0.723 2.300 B 0.900 1.060 Table G.2: Resistances for a spin-coated layer of ionic liquid with PVA (ratio 0.8:1 in weight) and AgNO3 on doctor bladed PEDOT:PSS in the 2nd design. Sample 1x PEDOT:PSS 1x LI+PVA+AgNO3 1 2.000 2.800 2 2.600 3.100 Table G.3: Resistances for different concentrations of the solution of ionic liquid, PVA and AgNO3added to PEDOT:PSS, for a weight ratio between ionic liquid and PVA of 0.8:1, in the 2nd design. Sample Resistance 20 % of IL+PVA+5 mM of AgNO312.60 50 % of IL+PVA+5 mM of AgNO3700.0 92.00 80 % of IL+PVA+5 mM of AgNO33800 Table G.4: Resistances for a spin-coated layer of ionic liquid with PVA (ratio 3:1 in weight) and AgNO3on doctor bladed PEDOT:PSS in the 2nd design. Sample 1x PEDOT:PSS 1x PEDOT:PSS 1 2.300 3.300 2 2.200 3.800 Table G.5: Resistances for different concentrations of the solution of ionic liquid, PVA and AgNO3added to PEDOT:PSS, for a weight ratio between ionic liquid and PVA of 3:1, in the 2nd design. Sample Resistance 20 % of IL+PVA+5 mM of AgNO3 40.0 50 % of IL+PVA+5 mM of AgNO3 440.0 60.00 80 % of IL+PVA+5 mM of AgNO3 4000