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. 2025 Apr 9;10(15):14738–14744. doi: 10.1021/acsomega.4c08898

Biaxially Prestrained Stretchable Electrodes Based on Ag/Organic Composite Film

Jeong Eun Kang 1, Myeong Hoon Jeong 1, Kyoung Jin Choi 1,*
PMCID: PMC12019521  PMID: 40290918

Abstract

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With the growing interest in wearable or stretchable electronics, research on stretchable electrodes has also been active. Generally, metal electrodes have high conductivity but very low stretchability, while organic electrodes have high stretchability but lower conductivity than metals. In this paper, metal/organic hybrid electrodes were fabricated on elastic poly(dimethylsiloxane) (PDMS) substrates to take advantage of the high conductivity of metals and the high stretchability of organic semiconductors. Additionally, by prestraining the PDMS substrate, the stretchability of the electrodes was further increased. However, the uniaxially prestrained electrodes produced compressive stresses in the direction perpendicular to the stretching direction due to Poisson’s effect, resulting in many cracks. To solve this problem, biaxial prestrain was introduced to the PDMS substrate. The electrodes fabricated with uniaxial prestrain exhibited a structure with wrinkles aligned in one dimension, whereas the electrodes with biaxial prestrain displayed highly ordered, two-dimensional wrinkle patterns arranged on the electrode surface. The electrodes with biaxial prestrain maintained stable electrical performance even after 200 cycles of stretching at a strain of 50%, withstanding up to 130% strain. Furthermore, of all the poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS)-based electrodes reported so far, our electrode showed the lowest sheet resistance of 0.91 Ω/sq. The strategy of our study offers promising opportunities for integrated wearable devices.

Introduction

Stretchable electronics are gaining more attention due to their potential applications in wearable devices,1−3 soft robotics,4 and biomedical implants.5 These electronics offer unique advantages such as stretchability, conformability, and durability, making them ideal for integration with the human body and other dynamic environments. There are two primary approaches to fabricating stretchable electronics. The first approach involves using stretchable materials for all components, ensuring uniform elasticity throughout the device.6,7 The second approach utilizes rigid components for critical parts, while employing stretchable materials for electrodes and interconnects.8,9 This hybrid method combines the reliability of rigid components with the flexibility of stretchable materials, providing a versatile solution for various applications. Regardless of the approach taken, the implementation of stretchable electrode technology is essential to achieve the desired performance and functionality.

Stretchable electrodes can be fabricated using various materials such as metals, and conductive organic materials. By introducing metal thin films onto elastic substrates, stretchable electrodes with very high conductivity were achieved but with very low tensile strain, typically less than 10–20%.10 At tensile strains above 10%, the resistance value of the metal electrode increases rapidly. Gray et al. fabricated serpentine structured Ag thin film stretchable electrodes, achieving a maximum tensile strain of 50%.11 Au nanoparticle base wrinkled multilayer electrodes on PDMS surface was stretched up to 100%.12 However, all the above electrodes have limitations in mechanical stability. To increase mechanical stability, metal nanowires (NWs) were introduced on stretchable substrate. Ag NWs on an elastic substrate produce ultrastretchable electrodes with a maximum tensile strain of 35–460%13−15 but exhibited significantly higher resistance than metal thin film electrodes. Stretchability and Transmittance of Cu NWs electrodes were 250% and over 90%.16 So, metal nanowires can apply transparent and stretchable electrodes.

Stretchable electrodes created using conductive polymers typically exhibit lower conductivity, but higher stretchability compared to metals.17 As a result, many researchers aim to enhance the electrical properties of these electrodes by utilizing conductive polymers in stretchable electrode fabrication, either through additive treatments or by combining them with metals or organic materials. Lee et al. printed a mixed solution of PEDOT:PSS and Ag NWs onto an elastic substrate to fabricate stretchable electrodes capable of withstanding a tensile strain of 40%.18 PEDOT:PSS + Ag nanowire-based electrodes19 demonstrated a transmittance of 80–90%, making them suitable for use as transparent electrodes.

Utilizing carbon nanotubes (CNTs) or graphene for electrode fabrication offers relatively higher electrical performance compared to conductive polymers, along with superior stretchability compared to metal electrodes. Cho et al. introduced stretchable carbon nanotube patterns onto an elastic conductor, achieving the maximum tensile strain of 100%.20 In addition, electrodes combining Ag NWs and CNTs maintain stable electrical properties at tensile strains exceeding 200%.21 Considering the various research results, there is a trade-off between conductivity and stretchability in the fabrication of stretchable electrodes.

This study aims to overcome the drawbacks of existing stretchable electrodes by fabricating metal/organic hybrid electrodes. We fabricated hybrid stretchable electrodes that combine the advantages of PEDOT:PSS electrode and metal thin film to achieve excellent stretchability and electrical performance. The electrical and mechanical properties of PEDOT:PSS were investigated with different types of additives. Also, we introduced prestrain to create a wrinkled metal/organic hybrid electrode, and we discuss both extrinsic and intrinsic characteristics of these hybrid electrodes. The extrinsic stretchability of stretchable electrodes refers to the region where the electrical resistance remains low until the wrinkles become flattened by stretching. While the intrinsic stretchability refers to the region where the flattened electrode stretches further by its own elasticity, resulting in a slight increase in electrical resistance. And biaxial and uniaxial prestrains were introduced to PDMS substrates, and the electrical properties and mechanical stretchability were compared. The hybrid electrode with biaxial prestrain exhibited superior mechanical stability compared to the electrode with uniaxial prestrain, experiencing less influence from Poisson’s effect. In addition, to further improve the long-term stability of the hybrid stretchable electrode, an Au thin film layer was introduced at the interface between the PEDOT:PSS and Ag thin films to prevent oxidation of the PEDOT:PSS.

Results and Discussion

Experimental Section

To fabricate PDMS substrates, PDMS prepolymer and curing agents (Sylgard 184) were mixed in a ratio of 10:1. The PDMS layer was prepared by spin-coating and curing at 70 °C for 2 h on a dummy Si wafer and peeled-off from the Si wafer. The peeled-off PDMS layers were stretched biaxially or uniaxially. UVO (Ultraviolet Ozone) was treated on prestrained PDMS layers for 30, 40, and 50 min. 0.1 wt % Triton X-100 and 4 wt % additives (Poly(ethylene glycol) (PEG), Dimethyl sulfoxide (DMSO), Ethylene glycol (EG), Sorbitol, Maltitol, Xylitol) were added to the PEDOT:PSS (PH1000, Clevios) solution and the mixtures were stirred for at least 4 h. PEDOTPSS mixtures were spin-coated onto the prestrained PDMS substrates and annealed at 130 °C for 10 min. Metal thin films were deposited using an electron beam evaporator. Finally, PDMS substrates were released. Sheet resistances of the electrode were measured with a four-point probe system. Mechanical stretching tests of the electrode were conducted using a manual tensile tester and a multimeter. The surface morphology of the stretchable electrodes was investigated using an optical microscope (OM) and scanning electron microscope (SEM).

Scheme 1 provides an overview of the fabrication process for the metal/organic hybrid electrode. Initially, a mixed solution of prepolymer PDMS and curing agent, in a 10:1 ratio, was spin-coated onto a Si substrate and cured at 70 °C for 2 h to create the PDMS stretchable substrate. After peeling the PDMS off the Si, it was stretched uniaxially or biaxially and fixed using custom-made jigs, followed by UVO treatment. This treatment was formed a hydrophilic silica hardening layer on the PDMS surface. Subsequently, a PEDOT:PSS solution was spin-coated and annealed. Finally, an Ag thin film was deposited using an electron beam evaporator, and the PDMS substrate was released from its prestrained state.

Scheme 1. Schematic Illustration of the Fabrication Process for Metal/Organic Hybrid Electrodes.

Scheme 1

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The structure of pristine PEDOT:PSS consists of a core–shell configuration, where conductive PEDOT molecules are encased by insulating PSS shells.22 This core–shell arrangement results in the pristine PEDOT:PSS having relatively low conductivity, around ∼1 S/cm and the stretchability, which is the main focus of this research, is poor. Commonly, with additive treatments, the structure of PEDOT:PSS changes from a core–shell structure to a linear structure, with a mixture of conductive PEDOT and insulating PSS, exposing the PEDOT portion and improving the conductivity.22Table S1. indicates that various additives such as DMSO, EG, PEG, xylitol, and others enhance the electrical performance of PEDOT:PSS thin films.23−26 It was observed that various additives influence not only conductivity but also affect the stretchability of the electrode.

We conducted tensile tests on stretchable electrodes formed by spin-coating PEDOT:PSS thin films, treated with various additives such as DMSO, xylitol, and PEG, onto flat PDMS substrates without prestrain to observe the intrinsic stretchability of PEDOT:PSS electrodes. During tensile testing, cracks and rips were observed in the PEDOT:PSS thin films. Here, “cracks” refer to formations resembling fissures or fractures, while “rips” describes tearing or splitting. For the PEDOT:PSS electrode treated with DMSO, cracks appeared at a 5% strain, with an increase in both the number and size of cracks at a 10% strain (Figure 1a,b). In the case of the PEDOT:PSS electrode treated with xylitol, rips occurred at a 40% strain, with numerous rips of various sizes observed at 70% strain (Figure 1c,d). However, for the PEDOT:PSS electrode treated with PEG, minimal changes in morphology were observed at a 40% strain, with slight rips occurring at a 70% strain (Figure 1e,f). As observed in the OM images, the electrical properties are maintained to some extent until full-scale cracking was observed, after which the electrical resistance increases sharply. The tensile test results (Figure 1g) show that the PEG-treated PEDOT:PSS stretchable electrode exhibited the highest intrinsic stretchability and retained its electrical performance at 50% strain, unlike the electrodes treated with DMSO and xylitol.

Figure 1.

Figure 1

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OM images of PEDOT:PSS treated with different additives under various strain conditions: (a, b) DMSO-treated PEDOT:PSS, (c, d) xylitol-treated PEDOT:PSS, (e, f) PEG-treated PEDOT:PSS. (g) Normalized resistance measurements under strain. (Scale bar: 200 μm).

The stretchable electrodes exhibit different wrinkle patterns depending on the UVO treatment time. In our hybrid-type stretchable electrodes, it is crucial to form structurally well-aligned wrinkles in order to enhance mechanical stability. When UVO treatment was applied to prestrained PDMS films, wrinkles formed on the surface, as shown in Figure S1. This phenomenon occurs due to the oxidation of the PDMS surface during UVO treatment, resulting in the formation of a thin silica hardening layer.27,28 The elasticity mismatch between this silica layer and the underlying elastomeric PDMS layer leads to the formation of wrinkles upon the removal of prestrain, as depicted in Figure S1. By adjusting the UVO treatment time for each prestrained PDMS film, different wrinkle patterns appeared on the surface. Notably, a 40 min UVO treatment produced the most well-aligned wrinkles (Figure S1b). The PDMS treated for 30 min exhibited disordered wrinkles (Figure S1a), while samples treated for 50 min displayed longer wrinkles along the y-axis compared to the x-axis (Figure S1C). Consequently, all stretchable electrodes were fabricated using the 40 min UVO treatment, which produced well-aligned wrinkles. Additionally, PEDOT:PSS is hydrophilic, which makes it difficult to coat onto hydrophobic PDMS surfaces. However, UVO treatment generates OH groups on the PDMS surface, making it hydrophilic.29 As a result, PEDOT:PSS can be effectively coated onto UVO-treated PDMS surfaces. This plays an important role not only in wrinkle formation on the PDMS surface but also in the uniform coating of PEDOT:PSS.

The thickness of each layer was optimized prior to electrode fabrication. Clearly, the electrical conductivity increases as the thickness of PEDOT:PSS increases. However, to fabricate stretchable electrodes that are both highly conductive and mechanically stretchable, it is essential to form well-aligned two-dimensional (2D) wrinkle structures. Figure S2. shows the morphology of wrinkles as a function of PEDOT:PSS thickness. At a thickness of 200 nm, well-aligned 2D wrinkles were formed. At 300 nm, the 2D wrinkles became somewhat disorganized, and at 400 nm, the wrinkle structure transformed from 2D to one-dimensional (1D). Similarly, the metal/PEDOT:PSS hybrid electrodes exhibited the best-aligned 2D patterns in the Ag (100 nm)/PEDOT:PSS (100 nm) electrode (Figure S3). In contrast, Ag (200 nm)/PEDOT:PSS (100 nm) or Ag (100 nm)/PEDOT:PSS (200 nm) electrodes displayed disordered 2D wrinkles. Therefore, we fabricated hybrid electrodes with the thickness of each layer below 200 nm for electrical and mechanical analysis.

In Figure 2, OM images illustrate the morphology of the metal/organic hybrid stretchable electrode and the stretchable substrate. When uniaxial prestrain was applied in the x-direction (Figure 2a–c), the hybrid stretchable electrode with one-dimensional wrinkles was produced. This morphology aligns with findings from other studies, such as Zhou et al.27 The thickness of PEDOT:PSS was approximately 100 nm, and the metal thin film was about 60 nm thick. The spacing of the one-dimensional wrinkles was approximately 120 μm for bare PDMS, 100 μm for PEDOT:PSS/PDMS, and 60 μm for Ag/PEDOT:PSS/PDMS, with denser patterns forming as the thin film thickness on the substrates increases. The denser pattern geometry was due to the increased difference in the coefficient of expansion between the soft PDMS layer and thin stiff layer of amorphous silica on the PDMS surface as the thin silica layer becomes thicker.28 Similarly, when biaxial prestrain was applied in both the x and y directions (Figure 2d–f), a two-dimensional wrinkled stretchable electrode was created. Similar shapes of biaxial prestrained electrodes have been produced by groups like Li et al.30 The spacings of the two-dimensional wrinkles were approximately 80 μm for bare PDMS, 60 μm for PEDOT:PSS/PDMS, and 40 μm for Ag/PEDOT:PSS/PDMS, with denser patterns observed as the thin film thickness on the substrates increases. The uniformity of our 2D stretchable electrode can be explained by the uniformity of the 2D wrinkle structure generated by the prestretching process. Therefore, we verified the wrinkle structure by optical photography of the entire device. The OM images in Figure S4 show that the 9 different positions have similar geometries. Also, the sheet resistances were measured by cutting a 2 × 2 cm2 electrode into 9 pieces showed similar values ranging from 0.91 to 0.94 Ω/sq. Therefore, the uniformity of the electrode was good.

Figure 2.

Figure 2

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Wrinkle formation on PDMS substrate after UVO treatment and subsequent coating processes: (a) uniaxial 80% prestrain, (b) after PEDOT:PSS coating, (c) after PEDOT:PSS coating and Ag deposition, (d) biaxial 80% prestrain, (e) after PEDOT:PSS coating, (f) after PEDOT:PSS coating and Ag deposition.

To evaluate the mechanical properties of the metal/organic hybrid stretchable electrode, tensile tests and cyclic stretching tests were conducted. R0 values of hybrid stretchable electrodes range from 1.1 to 1.2 Ω/cm. Figure 3a,c show the elongation test results for uniaxially and biaxially prestrained hybrid electrodes, respectively. For uniaxially prestrained electrodes, the resistance gradually increases under stretching, with a 30% increase at 80% tensile strain and a sharp rise beyond 90% tensile strain. Biaxially prestrained electrodes maintained nearly identical electrical performance in both directions up to 80% elongation. However, beyond 100% elongation, there was a sharp increase in resistance. Tensile test results for biaxially prestrained samples show similar outcomes in the x and y directions. We introduced 80% of prestrain for the wrinkle-structured electrode to have high extrinsic stretchability with good mechanical stability. Beyond 80% of stretching, the stretchable electrodes were intrinsically stretchable. We have indicated the extrinsic region with little change in electrical resistance, the intrinsic region with a gradual increase in resistance beyond the extrinsic region, and the failure region with a sharp increase in resistance outside the intrinsic region. Figure 3b,d presents the results of 200 cycles of cyclic stretching tests at 50% stretching for uniaxially and biaxially prestrained hybrid electrodes, respectively. The uniaxial 80% prestrain electrode was fabricated with prestrain in the x-axis direction. Therefore, when stretched in the x-axis direction, the resistance change in the x-axis direction is relatively minor due to the presence of 1D wrinkles on the electrode. However, when stretched in the x-axis direction, Poisson’s effect acts in the y-axis direction, perpendicular to the stretching direction causing multiple cracks to form. In other words, Poisson’s ratio is the ratio of relative contraction to relative expansion and can be calculated using eq (1) in Supporting Information. As a result, as observed in the OM image in Figure S6(c), not only cracks perpendicular to the direction of strain (x-axis) but also parallel cracks (x-axis crack) occur due to this effect. However, for biaxially prestrained electrodes, prestrained in the xy-directions, stress caused by PDMS expansion upon release was reduced, preventing the occurrence of parallel cracks (x-axis crack). Consequently, when measuring the resistance changes after 200 cycles of stretching, minimal changes are observed in both the x and y directions. Furthermore, as seen in the OM and SEM images (Figure S6(a,b)), only a few cracks perpendicular to the direction of stretching (y-axis crack) were observed. Based on the images in Figure S6, the crack density of the uniaxially and biaxially prestrained hybrid electrodes was evaluated by summing the total length of the cracks per unit area. This analysis, supported by literature,31,32 reveals that the uniaxially prestrained hybrid electrode exhibits a crack density of 0.0512 μm/μm2, which is more than four times higher than that of the biaxially prestrained hybrid electrode (0.0123 μm/μm2) (Table S2). This result provides evidence that the mechanical stability of biaxially prestrained metal/organic hybrid electrodes is superior to that of uniaxially prestrained ones.

Figure 3.

Figure 3

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Performance tests of hybrid electrodes under strain: (a) maximum elongation test of uniaxial prestrained hybrid electrode, (b) normalized resistance measurements during 50% strain cycle test of uniaxial prestrained hybrid electrode, (c) maximum elongation test of biaxial prestrained hybrid electrode, (d) normalized resistance measurements during 50% strain cycle test of biaxial prestrained hybrid electrode. (All samples were strained in the x-axis direction for testing).

Some literature suggests that PEDOT:PSS is vulnerable to long-term stability issues due to its hygroscopic nature. To address this, tests were conducted on metal/PEDOT:PSS hybrid electrodes. Initially, PEDOT:PSS electrodes treated with PEG additive show rapid degradation in electrical performance within 18 h under laboratory conditions. However, samples with Ag thin film deposited on PEDOT:PSS exhibited a resistance increase of 1.48 times after 100 h under the same conditions (Figure 4). According to Suh et al., the acidic components of PEDOT:PSS corrode the grain boundary junctions of Ag, leading to the migration of Ag+ ions to form Ag nanoparticles on other surfaces, including the surface of the Ag nanoparticles themselves.33 Based on this rationale, an interfacial layer between PEDOT:PSS and Ag electrodes was deemed necessary. Therefore, Au, known for its low chemical reactivity among metals, was introduced as the interfacial layer. Consequently, the Ag70 nm/Au30 nm/PEDOT:PSS hybrid electrode shows a 1.23 times increase in resistance after 100 h under laboratory conditions, which is a significantly lower value of resistance compared to the Ag/PEDOT:PSS electrode.

Figure 4.

Figure 4

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Long-term stability test results for PEDOT:PSS electrodes, Ag/PEDOT:PSS electrodes, and Ag/Au/PEDOT:PSS electrodes under room conditions.

To compare and analyze the properties of stretchable electrodes, we conducted a comprehensive literature review (Table S3 and Figure 5). Most flexible electrodes exhibit a trade-off between flexibility and electrical conductivity. Particularly, electrodes based on PEDOT:PSS show significantly low conductivity at high levels of stretchability, with a maximum strain of 80%. Metal nanomesh-based flexible electrodes demonstrate relatively higher conductivity compared to PEDOT:PSS electrodes, but lower stretchability. Stretchable electrodes based on Ag NWs exhibit remarkable stretchability at 100% strain. Serpentine structure metal thin films, formed by structural deformation of metal films, can achieve 100% strain, but exhibit lower electrical conductivity. However, our metal/PEDOT:PSS hybrid electrode had the lowest sheet resistance of 0.91 Ω/sq among the compared electrodes with a tensile strength of 130%. This demonstrates our hybrid electrodes not only superior conductivity but also exceptional stretchability.

Figure 5.

Figure 5

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Comparative analysis of maximum elongation and conductance of reported stretchable electrodes versus the electrodes developed in this study.

Conclusions

Metal/PEDOT:PSS hybrid stretchable electrodes were fabricated by introducing uniaxial or biaxial prestrain on a PDMS substrate to combine the good conductivity of a metal electrode with the stretchability of an organic electrode. Uniaxial prestrain electrodes were easy to crack due to compressive stresses generated in the perpendicular direction to the tensile direction due to the Poisson effect during stretching. In contrast, biaxial prestrain electrodes were not affected by compressive stresses due to the formation of two-dimensional wrinkles on the surface, resulting in excellent stretchability and mechanical stability. Additionally, the introduction of an Au interfacial layer between the PEDOT:PSS and Ag thin films significantly enhanced long-term stability. This innovative metal/PEDOT:PSS hybrid stretchable electrode demonstrates superior performance, maintaining stable electrical properties even after 200 cycles of stretching at a strain of 50% and withstanding up to 130% strain. This combination of high conductivity and stretchability presents great promise for future stretchable display applications, especially if the metal thin film portion is produced as a thin transparent film or grid.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) granted funded by Korea government (MSIT) (grant no. 2021M3H4A1A02051234). The authors thank UNIST Common Research Facility for helping with the equipment.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c08898.

  • Electrical properties of stretchable electrodes – SEM analysis of PDMS substrate and metal/organic hybrid electrodes (PDF)

Author Contributions

† J.E.K. and M.H.J. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao4c08898_si_001.pdf (1.1MB, pdf)

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