Optimizing Electrical Conductivity of PEDOT:PSS Films: A Systematic Study of Individual and Combined Enhancement Techniques

doi:10.21203/rs.3.rs-4014120/v1

This study focuses on crafting organic conductive electrodes for flexible substrate-based organic electronics, exploring techniques to enhance PEDOT:PSS film conductivity, including acid treatments, noble and non-noble metal nanoparticle doping, multiple layer depositions, and mono-layer graphene and graphene nanoplatelets insertion. These methods are systematically investigated, both individually and in combination, to optimize enhancement and provide engineering flexibility for various applications.

The optimized process prioritizes PEDOT:PSS multilayer depositions and nitric acid treatment, proving more effective and cost-efficient. The research details a process of rendering PEDOT:PSS polymer films highly conductive, highlighting their patterning abilities and robust adhesion to oxidized Si wafers and flexible substrates. Adhesion is accomplished with oxygen plasma treatment, and patterning involves an Ag sacrificial layer, PEDOT:PSS etching, and Ag island removal. Enhanced electrical conductivity is achieved through multiple PEDOT:PSS depositions without significant thickness increase. The paper explores doping PEDOT:PSS with metal nanoparticles like Cu and Ag, in both bulk and topical applications. Compared to a single-layer PEDOT:PSS of the same thickness, the optimized multilayer polymer stack, treated with nitric acid, reduces sheet resistance from 1 MΩ/sq to 6.7 Ω/sq. The study also addresses film aging and ways to mitigate reliability effects induced by the ambient environment.

Poly 3,4-ethylene dioxythiophene: polystyrene sulphonate (PEDOT:PSS) [1] is one of the most promising options for organic highly conductive films, along with polyacetylene, polypyrrole (PPy), polyaniline (PANI), and P3HT. It has garnered a lot of interest in a variety of application areas. The benefits of PEDOT:PSS are as follows: it is light weight, easily processed and deposited by spin coating, spray coating, and ink-jet printing; it has good mechanical flexibility and integrity; it is good thermally stable; it can increase conductivity by several orders of magnitude (research is ongoing); and its work function, which is between 4.5 and 5.2 eV and roughly comparable to Cu's work function of 4.7 eV, makes it suitable for use in hybrid crystalline silicon solar cells [2].

PEDOT:PSS has been successfully applied in energy devices, next-generation photovoltaics [3], optoelectronic devices as a transparent and flexible electrode [4–9], and functional packaging layers [10–12]. Indium-tin oxide (ITO) films have been effectively replaced by PEDOT:PSS conductive layers [13–15]. PEDOT:PSS has been employed as an efficient buffer layer in charge extraction and injection devices [16–18]. Energy storage cells use PEDOT composites as a redox-active component [19, 20]. Furthermore, PEDOT:PSS has been utilized in thermoelectric materials [72,21] and bioelectronic devices [22, 23]. There have been reports of highly conductive PEDOT:PSS sheets for ITO-free liquid crystal displays [24].

The electrical conductivity of pristine PEDOT:PSS films is approximately 0.1 S cm^− 1, which is much smaller than that of metal oxides like TaO_x (10 − 6 S cm^− 1), but still significantly lower than that of metals like Ta, which has an approximate conductivity of (7 x 10⁴ S cm^− 1). This is despite all of its advantages. Nonetheless, a variety of techniques have been found to improve PEDOT:PSS's electrical conductivity [24]. In order to explain potential doping strategies, a closer examination of the morphology and properties of the PEDOT:PSS films is necessary before providing a quick summary of the enhancing procedures [1].

PEDOT:PSS is made up of two parts: the insulating PSS, whose oligomers entangle the PEDOT oligomers, and the conductive PEDOT. The hydrophilic PSS chains' hydrogen sulfate groups, which adhere to the core surface and create a micelle structure, envelop the hydrophobic and water-insoluble PEDOT grains, allowing them to remain inside the core. The grain border has a thickness of 30A–40Å [25].

A PSS polyanion is electrostatically linked to the positively charged PEDOT core. Because PEDOT is hydrophobic, it prefers to avoid water during deposition and settles near the bottom of the film, leaving the top sections of the film occupied by the hydrophilic and hygroscopic PSS threads that are drawn to water. [75,26]. Consequently, a phase separation between the PSS-rich top and the PEDOT-rich bottom happens in wet films.

After that, thermal annealing at 90°C to 130°C improves electrical conductivity because the PEDOT granules' increased density and decreased absorption of water are inadequate to mix the two phases. The PEDOT:PSS films have two distinct lateral and perpendicular conductivities as a result of the phase separation. The perpendicular conductivity, which is around three orders of magnitude smaller, is dominated by space charge effects, whereas the lateral electric conductivity is produced by the hopping of charge carriers. PEDOT:PSS is made up of horizontal layers of flattened PEDOT grains divided from one another by nearly continuous barriers made of PSS ribbons following the spin coating deposition.

These characteristics of PEDOT:PSS films indicate that there are three main approaches to improving their electrical conductivity: a) increasing the concentration of PEDOT grains while lowering the concentration of PSS; b) blocking the electrostatic attraction between PEDOT and PSS to allow electrons to move more easily between p bonds across the carbon skeleton; and c) using nanoparticles or nanocomposites to fill the space between PEDOT islands within the bulk of PSS. Sarifuddin group has provided a thorough evaluation of such approaches [1].

The patterning of conductive polymers is the last stage in the process integration to create organic electrodes. Organic polymer films have been patterned using a variety of techniques, including ink-jet printing, vapor deposition using shadow masks, soft and hard imprint lithography, and traditional photolithography.[27, 28]. For patterning polymeric materials, ink-jet printing is the method of choice due to its superior roll-to-roll process capabilities. Its resolution is constrained to 10–20 mm, though.[28, 29]. Although shadow-mask deposition is a widely used method for small-molecule patterning, it has resolution issues in the 25–30 mm region [30].

Moreover, a high vacuum chamber is needed for shadow-mask deposition. The imprint lithography technique has demonstrated the smallest feature resolution down to 10 nm [31, 32]. Nevertheless, this method is exceedingly costly and accessible only in extremely specialized labs. Moreover, problematic registration of related patterns on different photomasks is a tricky problem in all of the previously stated approaches, which makes the manufacturing of multilayer devices challenging. Since photolithography is still the most widely used patterning technology in the silicon-based semiconductor industry today, it is still the most appealing way for patterning inorganic electronic materials. Because PEDOT:PSS is not chemically compatible, photolithography has not been very successful in patterning PEDOT:PSS until recently.

When active organic materials in traditional photolithography are exposed to lithography process solvents, they deteriorate chemically [82,33]. Using improved photolithography methods, Ouynag et al. [83,34] have examined recent PEDOT:PSS patterning approaches. The deterioration of PEDOT:PSS can be prevented through the etch procedures, insertion of sacrificial protective layers, and appropriate material selection. A method for depositing and patterning photoresist on top of a sacrificial silver layer that is covering the PEDOT:PSS layer has been demonstrated by Ouyang et al. [34]. Next, the silver interlayer is selectively removed using an appropriate silver etchant, like nitric acid, exposing PEDOT:PSS segments that are ready to be etched by oxygen plasma.

PEDOT:PSS patterns can be produced on Si wafer substrates by etching the remaining silver islands and removing the photoresist. Additionally, Taylor et al. [35] have demonstrated that even nanoscale PEDOT:PSS designs may be achieved by the use of matching photoresist and orthogonal solvents, as well as with the introduction of a novel set of safe procedures including newly designed photopolymers. Ultimately, it has been shown that PEDOT:PSS can be successfully patterned using a picosecond laser direct ablation using pulses at 355 nm and 1064 nm wavelengths [36–38].

To sum up, the primary obstacles to employing PEDOT:PSS for highly conductive films are still adherence to the substrate, patterning, material compatibility, and achieving conductivity above 10³ S cm^− 1.

In our earlier paper [39], we introduced a unique approach for manufacturing conductive organic electrodes utilizing doped PEDOT-PSS polymer sheets. Our results demonstrated the remarkable potential of these electrodes by demonstrating their strong adherence to a variety of substrates, including flexible materials such as Mylar and oxidized silicon wafers. One easy stage in the process was cleaning with oxygen plasma to obtain effective substrate adherence. In order to shield PEDOT:PSS from the common solvents used in the photolithography process, we patterned the films using a sacrificial silver metal layer. Using successive PEDOT-PSS depositions, it is possible to achieve a substantial improvement in electrical conductivity (by over two orders of magnitude) without significantly increasing film thickness, according to reference [39].

Understanding the roles of PEDOT (the conductive component) and PSS (the non-conductive component) within the deposited PEDOT:PSS material is furthered by the observation of an exponential relationship between sheet resistance and the number of sequential PEDOT-PSS coatings. Further PEDOT:PSS coatings beyond certain number of layers n_max (6 for multilayer deposition only) result in diminishing benefits when it comes to reducing the sheet resistance without the use of further enhancing techniques. To make up for the material lost as a result of acid treatment, the number of coatings, n_max at which only insignificant gains are made rises to roughly 9–10 coatings when combined with the acid treatments examined in this study.

Moreover, we investigated the topical application of Cu nanoparticles (Cu NPs) as a doping agent to soft-baked PEDOT-PSS films in our earlier work [39], which demonstrated an additional noteworthy two-order-of-magnitude increase in electrical conductivity. The two approaches of conductivity increase, numerous PEDOT-PSS coatings, and Cu NP doping, however, did not add up as expected. Instead, we observed a significant counteractive impact. Said another way, there were no cumulative gains from combining these two approaches.

In this paper, we propose new methods that build upon and supplement our earlier work, as we continue to investigate strategies to improve the electrical conductivity of PEDOT:PSS layers. We specifically look at the impact of adding mono- and multi-layer graphene, noble metal nanoparticles, and treatments with nitric, phosphoric, and sulfuric acids. When we looked at the topical application of Cu NPs in our previous study, we found that bulk doping of Cu was unproductive because of oxidation in aqueous solutions. We use silver (Ag) noble metal nanoparticles in our recent study to help with the advantageous bulk doping of the PEDOT:PSS layers.

Among the acid therapies (H₂SO₄, H₃PO₃, HNO₃), nitric acid treatment performs the best overall, and our work is focused on optimizing it. In this ongoing effort, we also aim to evaluate the suitability of different approaches for enhancing conductivity and investigate possible areas of overlap. Whether used topically or as bulk doping, the dispersion of silver nanoparticles shows encouraging results; nonetheless, it's noteworthy to note that their combination with acid treatment does not produce any additional significant improvements.

The incorporation of metal nanoparticles into PEDOT:PSS has garnered significant attention in previous research. For instance, X. Zhang et al. [40] successfully used inkjet printing to create PEDOT:PSS thin films doped with silver nanoparticles, which led to excellent electrical and optical capabilities. Similarly, doping PEDOT:PSS with silver nanoparticles boosted its conductivity, according to Patil et al. [41]. By presenting a streamlined synthesis process for their production, R.-C. Zhang et al. [42] illustrated the efficacy of gold nanoparticle-PEDOT:PSS nanocomposites as catalysts in alkaline direct ethanol fuel cells. Furthermore, O. Ghazy et al. [43] enhanced PEDOT:PSS conductivity, particularly in organic solar cells, by employing silver particles generated by gamma radiation.

Lastly, L. Pham et al. [44] provided compelling evidence of considerable increases in polymer conductivity upon the application of Cu NPs as dopants [6, 45].

As with metal nanoparticles, introducing graphene (in monolayer and triple-layer variants) has benefits of its own, but unlike silver nanoparticles, it does not function in tandem with nitric acid treatment. Film aging and ambient-induced and ambient-independent effects cause conductivity to deteriorate, which is addressed by optimizing the nitric acid treatment. The effectiveness of the simple method, which combines many PEDOT:PSS depositions with optimal nitric acid treatment while taking the effects of acid concentration and acid treatment time into account, is a noteworthy finding in this work. This strategy offers a more straightforward and economical way to increase PEDOT:PSS conductivity than the intricate techniques utilizing metal nanoparticles and graphene layers.

The optimized multilayer PEDOT:PSS treated with optimized nitric acid reduces sheet resistance from 1 MΩ/sq to 6.7 Ω/sq, which is ten times better than the lowest sheet resistance of 62Ω/sq achieved with six PEDOT:PSS layers and topical dispersion of Cu nanoparticle doping reported in our previous work [39]. This improvement is in comparison to a single-layer PEDOT:PSS of the same thickness.

In one investigation, Meng et al. [45] treated individual PEDOT:PSS films with phosphoric acid for 0.5 and 10 minutes. According to their research, phosphoric acid can effectively remove the PSS component, which enhances electron extraction and boosts the efficiency of solar cell devices [45]. They concluded that the devices' inability to collect electrons was caused by an excessive amount of PSS.

Our results support this assessment and suggest that adding nitric acid to solar cells could increase their efficiency even more. In their research, F. Zhang's group [46] also showed how to produce semitransparent polymer solar cells (PCS) with a visual transmittance of 24.6% and a power conversion efficiency of 9.40% [46]. They also adjusted PCS to have a 15.6% power conversion rate, which resulted in strong transmittance in the visible light range and low transmittance in the near-infrared spectrum [47]. Furthermore, Z. Xie’s group [40] has shown how to create highly conductive PEDOT:PSS transparent electrodes for polymer solar cells via a post-spin-rinsing method.

In this paper, we also investigate the synergies and combinations of these methods, proving their usefulness and creating room for a wide range of engineering applications.

The structure of this document is as follows: We start the section on electrode fabrication by providing a quick overview of the basic manufacturing procedures for depositing and patterning PEDOT:PSS layers. We also go over how to improve the electrical conductivity of these films, which we previously discussed in our study [39]. After that, we turn our attention to giving a thorough explanation of the acid treatment procedure as well as the creation, use, and insertion of mono- and multilayer graphene as well as Ag NP dispersions.

The effects of sulfuric, phosphoric, and nitric acids individually as well as the effects of topical and bulk doping PEDOT:PSS with silver nanoparticles are first discussed in the Results section. In this first section, we focus on the individual technique results. We then explore the synergistic benefits that arise from combining these approaches with the multi-layer PEDOT:PSS deposition procedure in the latter portion of the Results section. Lastly, the Concluding Remarks section contains a summary of the paper's most important findings.

Fabrication of the Polymer Electrode

This article focuses on new approaches aimed at improving the electrical conductivity of PEDOT:PSS. The detailed fabrication process for the device has been fully explained in our previous research [39]. Specifically, we explore the use of acid treatment involving nitric, sulfuric, and phosphoric acids, as well as the creation of silver nanoparticles dispersions, both topical and bulk. Additionally, we examine how environmental conditions affect the organic electrodes by storing them in standard laboratory conditions and vacuum chambers for up to 10 days. Before discussing these new enhancement methods, let's provide a brief summary of the electrode fabrication process, which was extensively covered in reference [39].

The process of creating organic electrodes began by using a commercial PEDOT:PSS dispersion in water. This dispersion was applied to clean substrates, such as oxidized Si wafers and flexible Mylar substrates, to form organic films. We employed the spin-coating method for film deposition, varying the spin speed from 500 rpm to 3000 rpm. This selection of spin speed was particularly important for creating multilayer PEDOT:PSS stacks, where the speed of each layer, including speed ramps, had to be carefully chosen to achieve the desired film thickness and minimize sheet resistance. Subsequently, applying oxygen plasma cleaning to the oxidized Si wafer greatly improved the uniformity of these films.

After completing the spin-coating and soft-baking steps, we measured the thickness of the film using both a Dektak profilometer and an atomic force microscope (AFM). Interestingly, both methods yielded measurements that were within a range of +/-1 nm of each other, confirming the accuracy of our measurements. Additionally, we found that the uniformity of the PEDOT:PSS film, whether on oxidized Si wafers or flexible Mylar substrates, remained consistent within a tolerance of +/- 2 nm.

To create patterns on the PEDOT:PSS film, we utilized protective layers, specifically thin films of PVD-deposited silver (Ag). These layers played a crucial role in photolithography by shielding the film from exposure to UV light and chemical reagents. The photolithography process involved exposing the desired pattern and then developing the photoresist AZ 5214E-IR. Subsequently, the exposed regions of silver underwent etching using a solution of HNO₃:H₂O. This was followed by rinsing and removing the PEDOT:PSS layer through treatment with oxygen plasma. Essentially, the previous research [39] covered essential steps in the fabrication process of the PEDOT:PSS film, including deposition, uniformity assessment, and precise photolithography-based patterning, with a particular focus on achieving enhanced electrical conductivity.

In our new efforts to enhance acid treatment methods for PEDOT:PSS samples, we conducted a comparative analysis involving three types of acids: phosphoric, sulfuric, and nitric, each at concentrations ranging from 0–100%. We varied the duration of acid treatment from 1 second to 60 seconds. Our methodology included using each acid at full concentration (100%) for a brief 2-second treatment period, resulting in a notable decrease in the sheet resistance of PEDOT:PSS. Initially, we exclusively applied the acid treatment to a single layer of the PEDOT:PSS material to ascertain the most effective kind of acid along with its optimal concentration.

The acid treatment procedure begins with precisely measuring the desired concentration of acid, followed by immersing PEDOT:PSS Si-wafer samples in the acid solution for the specified time. Subsequently, the samples are promptly rinsed in DI water to remove any residual acid, ensuring the accuracy of the results. As outlined in more detail elsewhere [39], during the spin deposition of PEDOT:PSS, a vertical phase separation phenomenon occurs, leading to the segregation of PEDOT and PSS components. This results in the accumulation of conductive PEDOT strips at the base of the wet film and a PSS-rich solution at the upper segment.

The transition between PEDOT and PSS phases is gradual rather than abrupt, with the highest concentration of PSS at the top and the lowest at the bottom, exhibiting the opposite behavior for PEDOT. Whether the acid treatment targets a single or multiple layers of PEDOT:PSS, its fundamental action involves removing PSS from the upper layer. The residue consists mainly of PEDOT, occasionally containing residual PSS inclusions.

We conducted experiments to apply silver nanoparticles (Ag NPs) to multilayer PEDOT:PSS using both topical and bulk doping methods. For bulk doping, we combined 5 mg of Ag NPs with 10 ml of PEDOT:PSS solution to create a concentration of 0.5 mg/ml. The mixture was stirred for at least 1 hour and then sonicated for an additional hour. In contrast, for surface doping, we prepared a dispersion solution of Ag NPs in ethanol with the same 0.5 mg/ml concentration, followed by the same stirring and sonication process as used in the bulk doping method. The dispersion of Ag nanoparticles in ethanol was then spin-coated onto the surface of the PEDOT:PSS layer at 1500 rpm.

We chose a silver nanoparticle concentration of 0.5 mg/ml based on our previous research involving copper nanoparticles [39]. In a prior study, we tested different concentrations of Cu NPs, specifically 0.2 mg/ml and 0.5 mg/ml, and found that the latter (0.5 mg/ml) produced the best results. To maintain consistency, we selected the same concentration (0.5 mg/ml) when working with silver nanoparticles.

Regarding the silver nanoparticles, we obtained two powders from Sky Springs Nanomaterials, Inc., one with nanoparticles ranging in size from 20–30 nm and another with nanoparticles ranging from 50–60 nm. Larger sizes were observed to yield higher conductivity in PEDOT:PSS, especially noticeable with multiple PEDOT:PSS coatings, as elaborated in the subsequent section.

Graphene has been utilized in various forms in the past to augment the electrical conductivity of PEDOT:PSS films, leading to notable enhancements. In a study referenced as [48], graphene composites were incorporated into PEDOT:PSS films for use in energy harvesting systems. These composites were dispersed in a PSS solution, and their concentration was adjusted to achieve optimal electrical conductivity. This approach resulted in a remarkable 41% increase in conductivity compared to undoped PEDOT:PSS. Consequently, the improved conductivity led to a 93% higher power factor compared to devices based on pristine PEDOT:PSS.

A similar doping strategy involving graphene composites and treatment with concentrated H₂SO₄ has been employed by M. Zhang, as cited in reference [49], to enhance the electrocatalytic activity for the oxygen reduction reaction at the cathodes of fuel cells and metal-air batteries. The resulting PEDOT:PSS/graphene composites exhibited synergistically enhanced electrocatalytic activity, increased tolerance to the methanol crossover effect and CO poisoning, and enhanced durability compared to a Pt/C electrode.

P.C. Mahakul et al., as mentioned in reference [50], utilized doping of PEDOT:PSS with reduced graphene oxide-carbon nanotubes to enhance the conductivity of polymer films for transparent electrode applications. They observed a maximum conductivity of 3804 S/cm, which is comparable to that of indium oxide (4000 S/cm). In another study referenced as [51], PEDOT:PSS films were doped with graphene and graphene quantum dots, revealing that both materials improve the conductivity of PEDOT:PSS films with only a slight decrease in transparency (13–14%).

D. Liu et al., as described in reference [52], achieved stable and highly conductive polymer films by doping PEDOT:PSS with graphene nanocomposites for biosensor applications. They deposited graphene nanoplatelet (GNP) composites on fluoride tin oxide (FTO) via the electrospray technique from a mixture solution of PEDOT:PSS and GNPs, followed by treatment with H₂SO₄ acid. The enhanced FTO electrode exhibited very high catalytic activity for detecting dopamine, effectively distinguishing the electrochemical oxidation signals of ascorbic and uric acids.

S.H. Ko et al., as outlined in reference [53], presented a flexible sensor utilizing graphene oxide/PEDOT:PSS composites for the voltammetric determination of selective low levels of dopamine. The sensor demonstrated a dopamine detection limit of 0.008 µM and a sensitivity of 69.3 µA/µMcm².

F-P. Du et al. [54] attained high conductivity polymer films by incorporating graphene quantum dots (GQD) into PEDOT:PSS. They observed an electrical conductivity of 7172 S/m, representing a 31% increase compared to pristine PEDOT:PSS. Furthermore, the thermal conductivity exhibited an even greater enhancement of 113% compared to pristine PEDOT:PSS. The PEDOT:PSS/GQDs were synthesized using a straightforward casting method. The strong π-π bonding between GQDs and PSS chains resulted in the decoupling and phase separation of PEDOT:PSS and PSS chains, leading to improved electrical and thermal conductivities.

M.A. Badri et al. [55] combined PEDOT:PSS with exfoliated graphene as an alternative to indium tin oxide in optoelectronic devices. The graphene flakes were produced via the graphite exfoliation technique. They achieved a favorable balance, with the conductivity and optical transparency reaching as high as 4.2x10³ S/cm and 94%, respectively.

In another study referenced as [56], an electrochromic device was successfully fabricated using PEDOT:PSS and graphene as active, flexible conductive electrode films. S.K. Nemani [56] utilized the wrinkling instability of graphene to impart hydrophobic properties to the electrode. This approach resulted in a wide range of color contrast, flexibility, and anti-wetting characteristics of the device.

G.J. Adekoya et al. [57] provided a comprehensive review of various doping techniques of PEDOT:PSS with graphene and its derivatives, including graphene oxide, reduced graphene oxide, and graphene quantum dots. They discussed recent advancements in the integration of PEDOT:PSS with graphene derivatives for applications in energy storage devices, such as supercapacitors, in detail [58].

In this manuscript, we explore a novel approach to enhance the performance of PEDOT:PSS films by introducing mono- and trilayer graphene layers between the oxidized Si wafer and the PEDOT:PSS films.

The transfer process for the graphene monolayer involved several sequential steps:

First, polyethylene terephthalate (PET) and the graphene-on-copper sheet were cut into 1cm x 1cm squares, and the graphene-on-copper sheet was affixed to the PET using tape.

Next, a polymethyl methacrylate (PMMA) solution was spin-coated onto the graphene at two different speeds: 500 RPM for 5 seconds followed by 2500 RPM for 45 seconds. The PMMA solution, comprising 10 mL of PMMA solution mixed with 10 mL of anisole solvent, resulted in a final concentration of 4.5% PMMA. Approximately 4 to 5 drops of this PMMA solution were dispensed onto the graphene and spin-coated. An annealing step followed, with options including 50°C for 2 minutes.

Following this, the PET was removed, and the sample underwent oxygen plasma exposure at 30W of power, with a gas flow rate of 10 SCCM of oxygen gas, for 1 minute.

The copper foil was then etched away by immersing it in a copper etchant (FeCl3) for approximately 10 minutes until most of the copper was dissolved.

The film was transferred into deionized (DI) water using a glass slide, left in the DI water for about 10 minutes, and this process was repeated two more times with fresh DI water each time.

After the third DI water bath, the graphene film was lifted onto the oxidized Si wafer sample, facilitating the transfer of graphene onto the wafer.

Subsequently, the sample was air-dried for 4 hours and placed in a vacuum chamber for 12–24 hours.

The PMMA was then removed by initially baking the wafer at 85°C for 5 minutes, followed by a bake at 140°C for 15 minutes, and then immersing the sample in warm acetone at 55°C for 1 hour.

Finally, the sample was cleaned with isopropyl alcohol cleaning solvent (IPA) for 30 minutes and dried to obtain graphene on the oxidized Si wafer.

This comprehensive procedure ensured the successful transfer of the graphene monolayer onto the wafer. Additionally, the graphene trilayer is provided as a distinct sample on 1 cm × 1 cm sections of oxidized Si wafers by the company Graphenea [59]. This trilayer consists of three graphene monolayers stacked directly upon each other.

The sheet resistance, represented as R_sq, or the conductivity, denoted as σ, of the PEDOT:PSS layers, has been determined using the four-probe measurement technique. The electrical properties, including conductivity and sheet resistance, are interconnected and can be expressed by Eq. (1):

$$\sigma =\frac{1}{{R}_{sq}\times t} eq.\left(1\right)$$

Take a pristine PEDOT:PSS film, for instance, that was deposited at 2000 rpm, yielding a film thickness (t) of 56 nm and a sheet resistance of R_sq, or 1 MΩ/sq. This sheet resistance value correlates to a conductivity (σ) value of 0.18 S/cm, as per Eq. (1).

We then compared the outcomes with topical application of copper nanoparticles (Cu NP), a strategy we used in our prior investigation [39], given the inefficiency of bulk doping with Cu NP. In the topical doping method, both Ag and Cu NP yielded comparable sheet resistance values, ranging from 121–260 Ω/sq for 3×PEDOT:PSS layers and 173–256 Ω/sq, respectively.

Additionally, with 3 PEDOT:PSS layers treated with 60% HNO₃ acid, both types of nanoparticles had comparable effects on sheet resistance. Consequently, it can be said that doping with Cu nanoparticles presents an economically viable option to doping with silver particles. Both Ag and Au nanoparticles, when used in bulk doping, produce equal degrees of conductivity improvement in PEDOT:PSS films, as reported in the literature [41, 42]. Thus, our previous conclusion about bulk doping's reduced efficacy in comparison to topical doping probably holds true for Au NPs as well.

The key findings of this investigation are outlined in Table 1, detailing the sheet resistance of single and multiple layers of PEDOT:PSS with and without acid treatment, and with and without Ag and Cu nanoparticles. Notably, the spinning speed significantly affects the sheet resistance. For instance, a single layer of PEDOT:PSS spun at 2000 rpm exhibits a sheet resistance of 1MW (sample 1), whereas at 1500 rpm, the sheet resistance decreases to 230 kΩ (sample 2). Additionally, multilayer deposition consistently reduces the sheet resistance: from 230 kΩ for sample 2 to 85kΩ for sample 4, 60 kΩ for sample 5, 5.8kΩ for sample 10, 2.3 kΩ for sample 11, 0.98 kΩ for sample 12, and 212 Ω for sample 19, for 2, 3, 4, 5, 6, and 9 PEDOT:PSS coatings, respectively. This trend of sheet resistance as a function of multiple PEDOT:PSS layers is illustrated in Fig. 1, showing an exponential decrease in sheet resistance with the number of layers. Roughly, the sheet resistance trend can be described by the following equation:

${\varvec{R}}_{\varvec{s}\varvec{q}}\left(\varvec{n}\right)=827\varvec{k}\varvec{\Omega }\times \mathbf{e}\mathbf{x}\mathbf{p}\left(-\frac{\varvec{n}}{0.78}\right)$ Eq. (2)

The multilayer PEDOT:PSS stack underwent acid treatment using three acids: HNO₃, H₂SO₄, and H₃PO₃. We have found that among these three acids, the most significant conductivity enhancement occurred with HNO₃ acid. Table 2 provides an illustration of the reduction in sheet resistance achieved for the three acids at 100% concentration, applied for 2 seconds on a 3-layer PEDOT:PSS.

Table 1

Samples with various processing steps and the resulting sheet resistance. The applied electrical enhancement process steps are discussed in the text.
Sample	Manufacturing Process	Sheet resistance [Ω/sq]
2,1	PEDOT:PSS 1 layer (2000 rpm)	1M
2	PEDOT:PSS 1 layer (1500 rpm)	230K
3	PEDOT:PSS 1 layer (2000 rpm) + 60% HNO₃	182
4,4	PEDOT:PSS 2 layers (3000 rpm)	85k
6,5	PEDOT:PSS 3 layers (1500 rpm)	60k
5.6	PEDOT:PSS 2 layers (1500 rpm)	80k
6.7	PEDOT:PSS 3 layers (1500 rpm)	60k
8,8	PEDOT:PSS 3 layers (1500, 1500, 3000 rpm)	12k
8,9	PEDOT:PSS 3 layers (1500, 1500, 3000 rpm) + 60% HNO₃	51
22,10	PEDOT:PSS 4 layers (1500, 2000, 3000, 3000 rpm)	4.8k
23,11	PEDOT:PSS 5 layers (1500, 2000, 3000, 3000, 3000 rpm)	2.3k
24,12	PEDOT:PSS 6 layers (1500, 2000, 3000, 3000, 3000, 3000 rpm)	0.98k
11,13	PEDOT:PSS 1 layer + Cu(25nm)0.2mg/ml 2000 rpm	1.2k
12,14	PEDOT:PSS 2 layers + Cu(25nm) 0.2 mg/ml @ 2000 rpm	467
16,15	PEDOT:PSS 3 layers + Cu(60nm) 0.2mg/ml 1500 rpm	173
25,16	PEDOT:PSS 6 layers (1500, 2000, 3000, 3000, 3000, 3000 rpm) + Cu(60nm) 0.2mg/ml	62
16,17	PEDOT:PSS 6 layers (1500, 2000, 3000, 3000, 3000, 3000 rpm) + 60% HNO₃	22
21,18	PEDOT:PSS 3 layers (1500, 2000, 3000 rpm) + Cu(60nm) 0.5 mg/ml	256
19	PEDOT:PSS 9 layers (1500, 2000, 3000 rpm)	212
20	PEDOT:PSS 9 layers (1500, 2000, 3000 rpm) + 60% HNO₃	17.5

Table 2: Impact of treatment by HNO₃, H3PO₄, and H₂SO₄ on R_sq of a single layer PEDOT:PSS. The concentration of all acids was 100%.

Acid	No acid	HNO₃	H₃PO₄	H₂SO₄
Sheet Res. [W/sq]	18k	51	65	356

Similar reductions in sheet resistance have been achieved for other multilayer PEDOT:PSS stacks, using acid concentrations ranging from 20–100%, and acid treatment times spanning from 2 seconds to 1 minute. As the most favorable outcomes were observed with nitric acid, further comparisons are presented specifically for HNO₃. The treatment process for a multi-layer stack of PEDOT:PSS is illustrated in Fig. 2. Initially, as subsequent PEDOT:PSS layers are deposited, the PEDOT phase migrates towards the bottom, while the water-soluble PSS phase forms the upper segment of each layer. As the next PEDOT:PSS layer is applied, the PEDOT component of the second coating infiltrates through the soluble PSS of the first coating, thereby thickening and compacting the combined PEDOT phase at the bottom. Subsequently, when acid treatment is administered, the upper segment of the layer comprising the nonconductive PSS phase is removed. It's important to note that during the acid treatment, a significant portion of PSS inclusions within the PEDOT phase is likely to be removed as well, consequently enhancing the conductivity of the resulting layer. In Fig. 3, the influence of HNO₃ treatment on the multilayer stack is illustrated on a logarithmic scale, showcasing the sheet resistance against the number of PEDOT:PSS coatings. This is juxtaposed with similar results for samples that underwent no acid treatment, as depicted in Fig. 1. It is evident that acid treatment consistently leads to a reduction in sheet resistance across all multilayer stacks.

We also explored the influence of acid concentration on the sheet resistance of a single PEDOT:PSS layer. Table 3 displays the results for a single PEDOT:PSS layer subjected to HNO₃ acid treatment for 30, 60, and 90 sec minute, using five different concentrations.

Table 3

Effect of treatment with HNO₃ at various concentration levels on the sheet resistance (R_sq) of a single-layer PEDOT:PSS for three acid treatment durations: 30 s, 60 s, and 90 s.
HNO₃ conc.	100%	80%	60%	40%	20%
R_sq [W/sq] for 30 s	135	179	185	221	1977
R_sq [W/sq] for 60 s	130	171	181	201	1823
R_sq [W/sq] for 90 s	127	169	177	197	1801

From Table 3, it is evident that an acid concentration of 40% or higher is necessary to significantly impact the sheet resistance. While the greatest reduction in R_sq is observed at the highest concentration of 100%, the differences in reduction factors across acid concentrations are relatively small, ranging between 130 and 201 Ω/sq. The etch durations of 30 s, 60 s, and 90 s have only a marginal effect on the sheet resistance.

We also examined the stability of acid treatment over time. Samples were left to stand for several days, with sheet resistance monitored periodically. A slight degradation in sheet resistance was observed over a few days, with the most significant change occurring after the first day. This degradation stabilized to a constant value after ca. 3–4 days. The degradation was lowest for PEDOT:PSS treated at 20%, amounting to approximately 80%, and highest for 100% HNO₃ concentration, approximately DEG = 160%, as illustrated in Table 4. Here, the degradation of sheet resistance is defined as DEG=(R_sq(n = 0)-R_sq(n))/R_sq(n = 0), where R_sq(n) represents the sheet resistance measured on the n-th day after manufacturing.

Table 4

Degradation of R_sq after 8-day exposure to laboratory ambient conditions for a single layer of PEDOT:PSS treated with nitric acid at various concentration levels.
HNO₃ conc.	100%	80%	60%	40%	20%
DEG [%] after 8 days	160	138	110	103	82

In Table 5 the sheet resistance of 1xPEDOT:PSS layer treated in 60% nitrid acid for three durations of 1s, 2s, and 2 s, is shown. It is apparent that varying the etch duration from 1s to 3s has a negligible effect on the sheet resistance.

Table 5

Effect of treatment with HNO₃ at 60% concentration on the sheet resistance (R_sq) of a single-layer PEDOT:PSS for three acid treatment durations: 1 s, 2 s, and 3 s.
Duration HNO₃ 60% [s]	1	2	3
Sheet Res. [W/sq]	189	189	187

In the scenario depicted in Fig. 3, the HNO₃ treatment of multilayer PEDOT:PSS coatings indicates that the HNO₃ treatment was applied only once, upon the deposition of the respective final PEDOT:PSS coating. This raised the question of what would occur if the HNO₃ treatment were applied after each individual PEDOT:PSS coating. To investigate this, we manufactured a sample with six coatings (6×PEDOT:PSS), with each coating being exposed to the same HNO₃ treatment before the subsequent PEDOT:PSS coating was applied. The resulting sheet resistance is shown in Table 6 and is compared to the sheet resistance of the sample with six PEDOT:PSS coatings treated with HNO₃ once upon the application of the sixth PEDOT:PSS coating. Table 6 indicates that subjecting each PEDOT:PSS layer of the 6-layer stack to nitric acid treatment results in a moderate decrease in resistance, from 22 Ω with a single HNO₃ etch after the final PEDOT:PSS layer deposition to 17 Ω. This modest reduction in sheet resistance does not seem to warrant the implementation of five additional etch treatment steps.

Table 6

The comparison between the impact of HNO3 treatment applied after the deposition of each PEDOT:PSS coating versus a single HNO3 treatment after the deposition of the 6th PEDOT:PSS layer.
HNO₃ 60% 30s	Applied only after 6th PEDOT:PSS layer deposition	Applied after each PEDOT:PSS layer deposition
Sheet Res. [W/sq]	22	17

We now turn our focus to exploring the potential of silver nanoparticles (Ag NP) in enhancing the electrical conductivity of PEDOT:PSS. Given silver's inert characteristics, we employed two methods of introduction: bulk doping, where Ag NP dispersion was blended with the PEDOT:PSS solution, and topical application following a soft bake of the PEDOT:PSS. Two different sizes of Ag NP, ranging from 20–30 nm and 50–60 nm, were utilized. Our findings indicate that larger Ag nanoparticles consistently result in higher conductivity values. Table 7 provides a comparison between bulk and topical applications of Ag NP for both 1×PEDOT:PSS and 3×PEDOT:PSS configurations.

Table 7

Influence of topical and bulk doping employing Ag NP with a size of 50–60 nm at a concentration of 0.5 mg/ml on the sheet resistance (R_sq) of a PEDOT:PSS monolayer (1xPEDOT:PSS) and a triple PEDOT:PSS layer stack (3xPEDOT:PSS).
Ag NP 0.5mg/ml NP 5–60 nm	Topical doping 1 × PEDOT:PSS	Bulk Doping 1 × PEDOT:PSS	Topical doping 3× PEDOT:PSS	Bulk Doping 3 × PEDOT:PSS
R_sq [W/sq]	464	629k	121	30k

Topical application of Ag NP leads to a significantly lower sheet resistance compared to the bulk method, showcasing a three-order-of-magnitude enhancement. In our prior research [39], we observed similar outcomes regarding topical doping using Cu NPs. However, bulk doping with Cu NPs is not feasible due to their strong oxidation in the aqueous PEDOT:PSS solution, as confirmed in experimental tests [39]. Hence, the method of topical doping with a high concentration of Ag or Cu NPs near the surface may be favored, particularly in applications like ReRAM cells, where the presence of Cu or Ag is immensely beneficial [60–62]. The corresponding sheet resistances for Cu NP doping are presented in Table 8.

Table 8

Influence of topical and bulk doping employing Cu NP with a size of 60 nm at a concentration of 0.5 mg/ml on the sheet resistance (R_sq) of a monolayer (1$\times$PEDOT:PSS), double (2$\times$PEDOT:PSS) and a triple (3$\times$PEDOT:PSS) PEDOT:PSS layer stack.
Cu NP 0.5mg/ml NP 60 nm	Topical doping 1 × PEDOT:PSS	Topical Doping 2 × PEDOT:PSS	Topical doping 3× PEDOT:PSS
R_sq [W/sq]	756	467	184

This implies that even with noble metal nanoparticles, bulk doping proves ineffective due to potential issues like inadequate incorporation of Ag NP within PEDOT:PSS and potential insulating coating of Ag NP by PSS. To our knowledge, there are no existing reports on the topical application of metal nanoparticles in PEDOT:PSS. A comparison of Tables 7 and 8 clearly indicates that topical doping, whether with Cu or Ag, is more effective than bulk doping. Furthermore, topical doping with Cu NP demonstrates comparable effectiveness to doping with Ag NP. Therefore, considering its economic advantage, Cu emerges as the preferred doping technique for PEDOT:PSS films.

We then compared the outcomes with topical application of copper nanoparticles (Cu NP), a strategy we used in our prior investigation, given the inefficiency of bulk doping with Ag NP. Ag and Cu NP produced equivalent sheet resistance values of 121–260 Ω/sq and 173–256 Ω/sq for 3 PEDOT:PSS layers, respectively. Additionally, with 3 PEDOT:PSS layers treated with 60% HNO₃ acid, both types of nanoparticles had comparable effects on sheet resistance. Consequently, it can be said that doping with Cu nanoparticles presents an economically viable option to doping with silver particles. Both Ag and Au nanoparticles, when used in bulk doping, produce equal degrees of conductivity improvement in PEDOT:PSS films, as reported in the literature [41, 42]. Thus, our previous conclusion about bulk doping's reduced efficacy in comparison to topical doping probably holds true for Au NPs as well.

The third method to enhance the conductivity of PEDOT:PSS included depositing graphene on an oxidized Si wafer prior to PEDOT:PSS layer deposition in an effort to lessen PEDOT:PSS sheet resistance. As mentioned in the previous section, two different forms of graphene were tested: trilayer graphene and a single big graphene monolayer. The graphene monolayer alone exhibited a sheet resistance of 20 kΩ/sq, whereas the trilayer graphene showed a significantly reduced resistance of 50 Ω/sq when deposited on oxidized Si wafers and flexible Mylar substrates.

However, adhesion problems with PEDOT:PSS were observed utilizing trilayer graphene. In order to overcome this difficulty, we plasma-treated the trilayer graphene, which greatly increased PEDOT:PSS adhesion to the trilayer graphene. Regrettably, the trilayer graphene's sheet resistance significantly declined as a result of the improvement in adhesion. Additionally, the range of R_sq values from 14 Ω/sq to 4 kΩ/sq showed a very non-uniform distribution of sheet resistance. This significant heterogeneity led to the general abandonment of subsequent trilayer graphene attempts.

However, we managed to synthesize a single sample of 6xPEDOT:PSS on a trilayer graphene that had been plasma-treated, yielding a sheet resistance of 1350 Ω/sq. The absence of sheet resistance nonuniformity throughout the wafer is one positive consequence of this conclusion. Conversely, a graphene monolayer in combination with nine PEDOT:PSS layers produced a decent 36.4 Ω/sq sheet resistance. The sheet resistance of 9xPEDOT:PSS treated with 60% nitric acid, which produced R_sq = 7.7 Ω/sq, is still exceeded, though. Notably, results as low as 14 Ω/sq at the lower end of the sheet resistance spectrum for trilayer graphene treated with plasma provide some hope for possible future investigations. It remains viable to effectively address the issue of non-uniformity, particularly considering that working with trilayer graphene is more manageable compared to the intricate process of transferring a graphene monolayer onto a substrate.

Let's now investigate the synergistic impact of several enhancing techniques. The combined impacts of several augmentation techniques have not yet been documented, as far as we are aware. First, we investigate the combination of silver nanoparticle doping and nitric acid treatment. The HNO₃ treatment alone for 3xPEDOT:PSS produces a sheet resistance of 50 Ω/sq; topical application of Ag NP also produces a similar result.

Therefore, it can be concluded that the additional inclusion of metal nanoparticles alongside acid treatment has not demonstrated significant improvement.

Applying nitric acid treatment and graphene monolayer insertion at the same time is an interesting combination of enhancing techniques. Even while each technique dramatically lowers sheet resistance on its own, their combined impact seems insignificant. For example, nitric acid treatment of a 9xPEDOT:PSS layer results in a sheet resistance of 7.7 Ω/sq; the addition of a graphene monolayer only significantly reduces the sheet resistance to 6.7 Ω/sq. This study indicates that the most effective approach for enhancing PEDOT:PSS conductivity involves combining nitric acid treatment with a multi-layer coating technique. Other combinations with existing enhancement methods do not yield additional advantages.

We have concluded our extensive study, initially introduced in reference [3], aimed at reducing the sheet resistance of a PEDOT:PSS layer. In this investigation, we explored additional enhancement techniques, including acid treatment using three different acids, HNO₃, H₂SO₄, and H₃PO₄, at various concentrations and treatment durations, as well as topical and bulk doping with metal nanoparticles such as Ag and Cu. Additionally, we examined the use of graphene in two forms: as a single layer and as a trilayer. We assessed the impact of these enhancement techniques on sheet resistance reduction both individually and in combination. Our findings indicate that most combined enhancement techniques offer only marginal improvements that may not justify their economic feasibility. However, the incorporation of graphene, whether as a monolayer or trilayer, significantly reduces the sheet resistance of PEDOT:PSS. Among the various methods studied, the combination of nitric acid treatment with multiple PEDOT:PSS coatings emerges as the most cost-effective and technologically straightforward option when compared to graphene insertion and metal nanoparticle doping.

Author Contribution

A.D. and A.C. suggested many and performed all of the experiments described in the paper. M.O. planned, conceptualized the research and has written the paper.

Acknowledgments:

We extend our appreciation to Dr. Don Leber and Prof. Scot Ransbottom for their invaluable support in facilitating this research and offering guidance on navigating clean room challenges.

Nur’Aishah Ahmad Shahrim, a Zuraida Ahmad,*a Amelia Wong Azman, Yose Fachmi Buysc and Norshahida Sarifuddina, “Mechanisms for doped PEDOT:PSS electrical conductivity improvement”, Mater. Adv., 2021, 2, 7118–7138
S. Mahato, L. G. Gerling, C. Voz, R. Alcubilla, J. Puigdollers, “PEDOT:PSS as an Alternative Hole Selective Contact for ITO-Free Hybrid Crystalline Silicon Solar Cell”, IEEE J. Photovolt., v6(4), 934, 2016
D. Alemu, H. Y. Wei, K. C. Ho, and C. W. Chu, “Highly conductive PEDOT:PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells,” Energy Environ. Sci., vol. 5, pp. 9662–9671, 2012.
Ouyang, J.; Chu, C.W.; Chen, F.C.; Xu, Q.; Yang, Y. High-Conductivity Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices”, Adv. Funct. Mater. 2005, 15, 203–208
Hsiao, Y.-S.; Whang, W.-T.; Chen, C.-P.; Chen, Y.-C. High-conductivity poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film for use in ITO-free polymer solar cells. J. Mater. Chem. 2008, 18, 5948–5955
Kim, N.; Kee, S.; Lee, S.H.; Lee, B.H.; Kahng, Y.H.; Jo, Y.-R.; Kim, B.-J.; Lee, K. Highly Conductive PEDOT:PSS Nanofibrils Induced by Solution-Processed Crystallization. Adv. Mater. 2014, 26, 2268–2272
Kim, S.; Sanyoto, B.; Park, W.-T.; Kim, S.; Mandal, S.; Lim, J.-C.; Noh, Y.-Y.; Kim, J.-H. Purification of PEDOT:PSS by Ultrafiltration for Highly Conductive Transparent Electrode of All-Printed Organic Devices. Adv. Mater. 2016, 28, 10149–10154.
Jang, H.; Kim, M.S.; Jang, W.; Son, H.; Wang, D.H.; Kim, F.S. Highly conductive PEDOT:PSS electrode obtained via post-treatment with alcoholic solvent for ITO-free organic solar cells. J. Ind. Eng. Chem. 2020, 86, 205–210.
Jeong, S.-H.; Ahn, S.; Lee, T.-W. Strategies to Improve Electrical and Electronic Properties of PEDOT:PSS for Organic and Perovskite Optoelectronic Devices. Macromol. Res. 2019, 27, 2–9
Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J.R. Poly(3,4-ethylenedioxythiophene) and Its Derivatives: Past, Present, and Future. Adv. Mater. 2000, 12, 481–494
Kirchmeyer, S.; Reuter, K. Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophene). J. Mater. Chem. 2005, 15, 2077–2088.
Elschner, A.; Lövenich, W. Solution-deposited PEDOT for transparent conductive applications. MRS Bull. 2011, 36, 794–798.
D.S. Hecht, L. Hu, G. Irvin, Emerging transparent electrodes based on thin filmsof carbon nanotubes, graphene, and metallic nanostructures, Adv. Mater. 23(2011) 1482–1513
S.-I. Na, S.-S. Kim, J. Jo, D.-Y. Kim, Efficient and flexible ITO-free organicsolar cells using highly conductive polymer anodes, Adv. Mater. 20 (2008)2061–2067.
A. Ramizy, Z. Hassan, K. Omar, Y. Al-Douri, New optical features to enhance solar cell performance based on porous silicon surfaces, Appl. Surf. Sci. 257(2011) 6112–6117
Earmme, T.; Jenekhe, S.A. Solution-Processed, Alkali Metal-Salt-Doped, Electron-Transport Layers for High-Performance Phosphorescent Organic Light-Emitting Diodes. Adv. Funct. Mater. 2012, 22, 5126–5136.
Cha, H.; Li, J.; Li, Y.; Kim, S.-O.; Kim, Y.-H.; Kwon, S.-K. Effects of Bulk Heterojunction Morphology Control via Thermal Annealing on the Fill Factor of Anthracene-based Polymer Solar Cells. Macromol. Res. 2020, 28, 820–825.
Wang, L.; Park, J.S.; Lee, H.G.; Kim, G.-U.; Kim, D.; Kim, C.; Lee, S.; Kim, F.S.; Kim, B.J. Impact of Chlorination Patterns of Naphthalenediimide-Based Polymers on Aggregated Structure, Crystallinity, and Device Performance of All-Polymer Solar Cells and Organic Transistors. ACS Appl. Mater. Interfaces 2020, 12, 56240–56250.
Österholm, A.M.; Shen, D.E.; Dyer, A.L.; Reynolds, J.R. Optimization of PEDOT Films in Ionic Liquid Supercapacitors: Demonstration As a Power Source for Polymer Electrochromic Devices. ACS Appl. Mater. Interfaces 2013, 5, 13432–13440
Gulercan, D.; Commandeur, D.; Chen, Q.; Sarac, A.S. A Ternary PEDOT-TiO2-Reduced Graphene Oxide Nanocomposite for Supercapacitor Applications. Macromol. Res. 2019, 27, 867–875. [CrossRef]
Bernards, D.A.; Malliaras, G.G. Steady-State and Transient Behavior of Organic Electrochemical Transistors. Adv. Funct. Mater. 2007, 17, 3538–3544.
Rivnay, J.; Leleux, P.; Ferro, M.; Sessolo, M.; Williamson, A.; Koutsouras, D.A.; Khodagholy, D.; Ramuz, M.; Strakosas, X.; Owens, R.M.; et al. High-performance transistors for bioelectronics through tuning of channel thickness”, Sci. Adv. 2015, 1, e1400251.
Harman, D.G.; Gorkin Iii, R.; Stevens, L.; Thompson, B.; Wagner, K.; Weng, B.; Chung, J.H.Y.; in het Panhuis, M.; Wallace, G.G. “Poly(3,4-ethylenedioxythiophene):dextran sulfate (PEDOT:DS)–A highly processable conductive organic biopolymer” Acta Biomater. 2015, 14, 33–42.
Tsu-Ruey Chou,a Szu-Hua Chen,a Yen-Te Chiang,b Yi-Ting Lina, Chih-Yu Chao,” Highly conductive PEDOT:PSS films by posttreatment with dimethyl sulfoxide for ITO-free liquid crystal display”, J. Mater. Chem. C, 2015, 3, 3760
A. M. Nardes, M, Kemerink, R. A. J. Janssen, J. A. M. Bastiaansen, N. M. M. Kiggen, B. M. W. Langeveld, A. J. J. M. van Breemen, M. M. de Kok, “Microscopic Understanding of the Anisotropic Conductivity of PEDOT:PSS Thin Films”, Adv. Mater. 2007, 19, 1196–1200, DOI: 10.1002/adma.200602575
F. Zabihi, Y. Xie, S. Gao, M. Eslamian, „Morphology, conductivity, and wetting characteristics of PEDOT:PSSthin films deposited by spin and spray coating”, Appl/ Surf. Sci., 338 (2015), 163–177
E. Menard, M. A. Meitl, Y. G. Sun, J. U. Park, D. J. L. Shir, Y. S. Nam, S. Jeon, J. A. Rogers, Chem. Rev. 2007, 107, 1117.
J. R. Sheats, J. Mater. Res. 2004, 19
B. J. de Gans, P. C. Duineveld, U. S. Schubert, Adv. Mater. 2004, 16, 203
M. M. Ling, Z. N. Bao, Chem. Mater. 2004, 16, 4824.
M. D. Stewart, S. C. Johnson, S. V. Sreenivasan, D. J. Resnick, C. G. Willson, J. Microlith. Microfab. Microsys. 2005, 4
D. Y. Khang, H. Kang, T. Kim, H. H. Lee, Nano Lett. 2004, 4, 633.
J. Huang, R. Xia, Y. Kim, X. Wang, J. Dane, O. Hofmann, A. Mosley, A. J. de Mello, J. C. de Mello, D. D. C. Bradley, J. Mater. Chem. 2007, 17, 1043
S. Ouyang, Y. Xie, D. Wang, D. Zhu, X. Xu, Te Tan, H. H. Fong, Surface Patterning of PEDOT:PSS by Photolithography for Organic Electronic Devices”, J. Nanomaterials v. 2015, # 603148
P. G. Taylor, Jin-Kyun Lee, Alexander A. Zakhidov, Margarita Chatzichristidi, Hon Hang Fong, John A. DeFranco, George, G. Malliaras, and Christopher K. Ober, “Orthogonal Patterning of PEDOT:PSS for Organic Electronics using Hydrofluoroether Solvents”, Adv. Mater. 2009, 21, 2314–2317
D. Son, J.-H. Kim, D. Park, W. K. Choi, F. Li, J. Hun Ham, T. W. Kim, Nanotechnology, 2008, 19, 055204.
K. Chondroudis and D. B. Mitzi, Chem. Mater., 1999, 11, 3028.
Z. Xiao, Y. Yuan, Y. Shao, Q. Wang, Q. Dong, C. Bi, P. Sharma, A. Gruverman and J. Huang, Nat. Mater., 2014, 4, 193.
A. Chakraborty, D. Herrera, Payton Fallen,. Hall, N. Bampton, T. Olivero & M. Orlowski, “Conductive organic electrodes for flexible electronic devices”, Scient. Rep. (2023) 13: 412
X. Zhang, J. Wu, J. Wang, J. Zhang, Q. Yang, Y. Fu, Z. Xie, „Highly conductive PEDOT:PSS transparent electrode prepared by a post-spin-rinsing method for efficient ITO-free polymer solar cells” Solar Energ. Mat. & Solar Cells, 144(2016), 143–149
D. S. Patil, S. A. Pawar, J. Hwang, J. H. Kim, P. S. Patil, J. C. Shin, Silver incorporated PEDOT: PSS for enhanced electrochemical performance, J. Ind. Eng. Chem., 2016, 42, 113–120.
R.-C Zhang, D. Sun, R. Zhang, W-F Lin, M. Macias-Montero, J. Patel, S. Askari, C. McDonald, D. Mariotti, P. Maguire, “Gold nanoparticle-polymer nanocomposites synthesized by room temperature plasma and their potential for fuel cell electrocatalytic application”, Scientific Reports 7: 46682 (2017)
O. A. Ghazy, M. M. Ibrahim, F. I. Abou-Elfadl, H. M. Hosni, E. M. Shehata, N. M. Dehiedy, M. R. Balboul, “PEDOT:PSS incorporated silver nanoparticles prepared by gamma radiation for the application in organic solar cells” J. Rad. Res. & Appl. Science, 8 (2015) 166–172
L. Q. Pham, J. H. Sohn, C. W. Kim, J. H. Park, H. S. Kang, B. C. Lee and Y. S. Kang, “Copper nanoparticles incorporated with conducting polymer: Effects of copper concentration and surfactants on the stability and conductivity”, J. Colloid Interface Sci., 2012, 365, 103–109
W. Meng, R. Ge, Z. Li, J. Tong, T. Liu, Q. Zhao, S. Xiong, F. Jiang, L. Mao, Y Zhou, “Conductivity Enhancement of PEDOT:PSS Films via Phosphoric Acid Treatment for Flexible All-Plastic Solar Cells”, ACS Appl. Mater. Interfaces 2015, 7, 14089–14094
Z. Hu, J. Wang, Z. Wang, W. Gao, Q. An, M. Zhang, X. Ma, J. Wang, J. Miao, C. Yang, F. Zhang, „Semitransparent ternary nonfullerene polymer solar cells exhibiting 9.40% efficiency and 24.6% average visible transmittance” Nano Energy, 55 (2019) 424–432
Z. Hu, Z. Wang, Q. An, and F. Zhang, „Semitransparent polymer solar cells with 12.37% efficiency and 18.6% average visible transmittance, Science Bulletin, 65(2) (2020), 131–137
D. Yoo, J. Kim, J H Kim, „Direct synthesis of highly conductive PEDOT:PSS/graphene composites and their applications in energy harvesting systems”, Nano Research 2014, 7(5):717:730
M. Zhang, W. Yuan, B. Yao, C. Li, G. Shi, “Solution-Processed PEDOT:PSS/Graphene Composites as the Electrocatalyst for Oxygen Reduction Reaction”, ACS Appl. Mat. & Interf., 2014,6, 3587–3593
P.C. Mahakul, K, Sa, B. Das, B.V. Subramaniam, S. Saha, B. Moharana, J. Raiguru, S. Dash, J. Mukherjee, P. Mahanadia, “ Preparation and characterization of PEDOT:PSS/reduced graphene oxide-carbon nanotubes hybrid composites for transparent electrode applications”, j. Mater. Sci (2017) 52: 5696–5707
D. P. Kepic, Z. M. Markovic, S. P. Jovanovic, D. B. Peruško, M. D. Budimir, I. D. Holclajtner-Antunovic, V. B. Pavlovic, B. M. Todorovic-Markovic, “Preparation of PEDOT:PSS thin films doped with graphene and graphene quantum dots”, Synthet. Metals, (2014) 198: 150–154
D. Liu, M. M. Rahman, C. Ge, J. Kima, J-J Lee, „Highly stable and conductive PEDOT:PSS/graphene nanocomposites for biosensor applications in aqueous medium”, New J. Chem. (2017) 41: 15458–15465
S.H. Ko, S. W. Kim, Y. J. Lee, „ Flexible sensor with electrophoretic polymerized graphene oxide/PEDOT:PSS composite for voltametric determination of dopamine concentration”, Scientific Reports (2021) 11:21101
F.P. Du, N-N Cao, Y-F Zhang, P. Fu, Y-G Wu, Z-D Lin, R. Shi, A. Amini, C. Cheng, “PEDOT:PSS/graphene quantum dots with enhanced thermoelectric properties via strong interfacial interactions and phase separation”, Scientific Reports, (2018) 8:6441
M. A. S. Badri, N. F.Noor, A. R. Zain, M. MatSalleh, T. H. Tengku Aziz, “Exfoliated graphene-alkaline lignin-PEDOT:PSS composite as a transparent conductive electrode”, Nanomat. & Nanotech. (2021) 11: 1–8
S. K Nemani, D. Chen, M. H. Mohamed, H. Sojoudi, “ Stretchable and Hydrophobic Electrochromic Devices Using Wrinkled Graphene and PEDOT:PSS’, j Nanomat. (2018), 3230293
G. J. Adekoya, R. E.l Sadiku, S. S. Ray, “Nanocomposites of PEDOT:PSS with Graphene and its Derivatives for Flexible Electronic Applications: A Review”, Macromol. Mater. Eng., (2021), 306: 2000716
O. Faruk, B. Adak, “Recent advances in PEDOT:PSS integrated graphene and MXene-based composites for electrochemical supercapacitor applications”, Synthet. Metals, (2023) 297: 117384
https://www.graphenea.com/products/graphene-on-your-substrate
T. Liu, Y. Kang, M. Verma, M. Orlowski, IEEE Electron Dev. Lett. 33, 429 (2012)
Y. Kang, H. Ruan, R. O. Claus, J. Heremans, and M. Orlowski, Nanoscale Research Lett. 11:179 (2016)
T. Liu, Y. Kang, S. El-Helw, T. Potnis, M. Orlowski, Jpn, J. Appl. Phys. 52, 084202 (2013)

No competing interests reported.

Optimizing Electrical Conductivity of PEDOT:PSS Films: A Systematic Study of Individual and Combined Enhancement Techniques

Status:

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Abstract

Figures

Introduction

Fabrication of the Polymer Electrode

Results and their Analysis

Conclusion

Declarations

Author Contribution

Acknowledgments:

References

Additional Declarations

Status:

Version 1