Electrospun transparent conductive film

Conductive and transparent film has attracted significant attention and interests due to widespread use of touch screens control panel on electrical devices. Conventional conductive ITO glasses have high transparency but it is constrained by high costs and rigidity. An advantage of polymers is they generally costs lower and is flexible. Electrospinning has been shown to be able to create a thin transparent film comprising of nanofibers. This has attracted interests in using it to form conductive, flexible and transparent film.

Several studies have also shown that sintered, inorganic nanofibrous membrane is optically transparent without a matrix material surrounding it. The resultant material is also flexible and may be used in a variety of applications from solar cell to flexible electronic screens. Indium tin oxide (ITO) is a transparent conducting oxide that has been used in many applications. Electrospinning of its precursors, indium chloride tetrahydrate, tin chloride pentahydrate and poly(vinyl pyrrolidone) mixture has yielded nanofiber with diameter of about 220 nm. Although the resultant electrospun membrane is white, calcination at temperature above 400 °C forms pure ITO nanofiber membrane which is transparent. The transparent sheet is made of fibers with diameter of about 100 nm. Higher calcination temperature was shown to improve the optical transmittance through the membrane up to 92%. However, the membrane thickness needs to be very small with deposition duration as low as 20s to obtain the highest transmittance. When deposition duration was increased to 120s, the optical transmittance drops from 91% to 75% [Munir et al 2008]. While ITO on its own is transparent, other non-transparent inorganic material has been made transparent when the nanofibrous membrane is sufficiently thin.

Conductive and transparent film consisting of copper nanofiber network has been constructed on a glass substrate. Generally, precursor of copper is blended with a carrier polymer for electrospinning. Sintering was next carried out to remove the organic component and reduce the nanofiber to CuO. Lastly, the CuO nanofibers are reduced to form Cu nanofiber. Kim et al (2015) compared the optical transmission and electrical conductivity of randomly oriented copper nanofiber prepared using polyvinyl alcohol (PVA) and polyvinyl butyral (PVB) as carrier polymers for electrospinning of its precursor. Their studies showed that the optical transmittance was influenced by the amount of copper nanofibers and not the carrier polymers used but the electrical resistance were affected by the carrier polymers (See table 1). The poorer electrical conductivity of Cu nanofiber derived from PVA carrier was attributed to high reduction temperature used compared to PVB and this led to discontinuity of the nanofibers on the substrate.

Optical and electrical properties of the Cu nanofibers fabricated with different amounts of Cu source. [Kim et al 2015. Journal of Nanomaterials 2015; 2015: 518589. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Wu et al (2010) used copper acetate/polyvinyl acetate solution for electrospinning to give nanofibers. With a thin nanofiber layer, the optical transmittance is excellent in the visible and near-infrared ranges. Aligned Cu nanofibers was able to show 90% transmittance at sheet resistance of 25 ohm/sq[Wu et al 2010]. Transparent and flexible electrodes have been constructed by transferring the Cu nanofiber network to poly(dimethylsiloxane) (PDMS) substrate [Wu et al 2010]. To improve the thermal and chemical resistance of copper nanofiber membrane, Hsu et al (2012) used atomic layer deposition to coat a passivation layer of aluminum-doped zinc oxide (AZO) and aluminum oxide on copper nanofibers. Deterioration in the conductivity of the copper membrane is investigated by measuring its resistance. Unprotected copper nanofibers quickly become insulating when it undergo thermal oxidation at 160 °C in dry air and 80 °C in humid air (80% relative humidity). However, Cu nanofiber with AZO/Al₂O₃ layer showed an increased sheet resistance of only 10% after baking the fibers at 160 °C for 8 h. When bare copper nanofibers were coated and baked with an acidic layer of PEDOT:PSS, its sheet resistance increased by 6 order of magnitude while protected copper nanofiber showed an 18% increase demonstrating the superior protection offered by AZO/Al₂O₃ layer. With the coating, optical transmittance over the visible light wavelength range showed less than 1% decrease with no effect on the electrical conductance of the Cu nanofibers.

An alternative method of fabricating metal nanowire on a transparent substrate using electrospun nanofibers is to load reducing agent in the fiber followed by immersion in a salt solution for reduction of the ions form a continuous coating over the nanofiber. This method is demonstrated by Hsu et al (2014) to form Ag or Cu nanowire. In their process, they electrospun polyvinyl butyral (PVB) loaded with tin (II) chloride (SnCl₂) fibers onto a glass substrate. The PVB/ SnCl₂ fibers were immersed in silver nitrate solution for reduction of Ag⁺ to Ag. At 70 nm metal deposition thickness, the metallization coverage and optical transmittance was good. Resistance and transmittance for Ag nanowire was 8.5 Ω/sq and 90% respectively.

Stretching the concept of using electrospun nanofibers in the construction of conductive linkages, conductive nanotroughs have been fabricated using electrospun nanofibers as a template material. A variety of methods are available for coating a conductive layer on electrospun nanofibers. Metallic coatings such as chromium, gold, copper, silver and aluminum can be applied to electrospun nanofibers using thermal evaporation [Wu et al 2013]. Platinum and nickel has been coated on electrospun nanofibers using electron-beam evaporation and silicon and ITO using a.c. magnetosputtering [Wu et al 2013]. The coated nanofiber sheet can be transferred to a silicon substrate using dropcast. With a coating thickness of 100 nm on one side, the nanotrough layer is sufficiently strong to be self-supporting after the nanofiber templates have been removed [Wu et al 2013]. A single gold nanotrough was found to have an electric conductivity of 2.2 x 10⁵ S/cm which is slightly less than its polycrystalline bulk [Wu et al 2013]. The nanotrough network exhibit greater transparency than flat nanostrips due to its concave shape which reduces its electromagnetic cross-section with transmittance of more than 90% for Cu and Au nanotrough materials across the visible wavelengths. It is also highly bendable, stretchable and foldable without significant deterioration in electrical conductivity [Wu et al 2013].

Schematic of nanotroughs on substrate.

Electrosplating may also be used for coating electrospun fibers around its full surface. This method was used by An et al (2016) to manufacture a copper nanofiber transparent and flexible conducting film. Electrospun polyacrylonitrile nanofiber was used as the template material for electroplating copper onto it. To achieve optical transmittance of more than 70%, the fiber deposition duration needs to be less than 60s and unplated support materials removed. The presence of unplated support fibers needs to be removed as they scatter a significant amount of light. Sheet resistance of the membrane is able to reach 0.3 Ω/sq while maintaining good flexibility. This resistance value is much lower than earlier reported conductive and transparent membrane using electrospun fibers. Electrospun fibers may be used as a seed layer to grow copper network [Kim et al 2018]. This copper network was formed by first electrospinning a polymer solution containing palladium ions. The electrospun layer was subsequently decomposed and calcinated at high temperature to form a seed layer. This seed layer was used for copper electroless deposition. The resultant electrode showed transparency of over 90% over the entire visible light range and a sheet resistance of 4.9 ohms/sq. Kim et al (2018) showed that the electrode may be used as a heater. Increasing the voltage supplied to the electrode showed a corresponding increase in the temperature. Temperature of up to 150 °C can be maintained. However, when the applied voltage was more than 20V, the electrode was destroyed as the temperature drops from a maximum localised temperature of 210°C. DiGregorio et al (2022) constructed a transparent conducting electrode (TCE) using a combination of metallization and plating on electrospun fibers. Active silver ink was first blended with water soluble polymers for electrospinning into fibers. The deposited fibers were calcined at 300 °C to give rise to silver nanowires. An UV-ozone treatment was used to activate the silver nanowires followed by electrodeless copper deposition so that the intersecting junctions between fibers were covered to form a continuous conductive surface. At the initial plating, the electrical resistance of the sheet reduces significantly. However, as all the junctions are fused by the copper plating, further increase in plating duration only brings modest drop in resistance. The TCE with optimum plating showed a sheet resistances of 0.33 Ω sq^-1 and visible light transmittance of 86% with a Haacke figure of merit of 652x10^-3Ω^-1. The effect of bending the TCE on its sheet resistance was carried out by transferring the TCE onto a flexible clear polypropylene tape and subjected it to 2.5 mm radius compound bending cycles.There was a small 2.5% increase in sheet resistance, from 734 to 753 mΩ sq^-1 after 1000 bending cycles which demonstrated its bending tolerance.

Where the conductivity requirement is relatively low, a simple method of imparting electrical conduction property is to use sputtering. Gold, platinum and silver are routinely coated on electrospun fibers using sputtering prior to viewing under scanning electron microscope. This is to give it sufficient conduction for greater clarity. Similar coating method has been used on electrospun fibers to create a thermochromic membrane. Busuioc et al (2016) used electrospinning to produce a thin layer poly(methyl methacrylate) (PMMA) nanofibers membrane such that it exhibits a high level of transparency. Note that the thinner the layer, the greater the transparency. Sputtering was used to coat a layer of silver and gold on its surface for electrical conduction. Thermal treatment was used to melt the PMMA template such that the resultant metallic shell adheres to the base substrate. Note that the thinner the fibrous membrane, the greater its resistivity and the electrical energy is converted to heat. Thermochromic ink was painted over the membrane using a small brush. When a voltage is applied, the membrane heats up and the thermochromic ink changes color accordingly.

Electrospun conductive nanofibers network with high transparency has been used as an antenna in a soft, smart contact lens for real-time detection of the cortisol concentration in tears [Ku et al 2020]. This antenna needs to occupy a large area over the soft contact lens and exhibit a low sheet resistance for wireless operation of standardized NFC chips. To construct such an antenna, a suspension of Ag nanoparticle ink in ethylene glycol was electrospun to form a network of continuous Ag nanofibers. Thermal annealing was carried out to form a conductive Ag nanofibers network. Finer Ag nanowires were subsequently electrosprayed over the Ag nanofibers network to improve the conductivity of the antenna. The constructed Ag nanofibers/ Ag nanowire antenna has an average sheet resistance(Rs) of 0.3 ohm per square and a transparency of 71% at 550 nm. To maintain conductivity of the antenna over a substantial length of time, passivation by coating the antenna with a layer of parylene elastomeric cover may be used to retard Ag oxidation.

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