Glutaraldehyde

Super Stretchable and Durable Electroluminescent Devices Based on Double-Network Ionogels

Hiep Dinh Xuan, Bernard Timothy, Ho-Yeol Park, Tuyet Nhi Lam, Dowan Kim, Yeonjeong Go, Jongyoun Kim, Youngu Lee, Sung Il Ahn, Sung-Ho Jin,* and Jinhwan Yoon*

Abstract

Ionogels are good candidates for flexible electronics owing to their excellent mechanical and electrical properties, including stretchability, high conductivity, and stability. In this study, conducting ionogels comprising a double network (DN) of poly(N-isopropylacrylamide-co-N,N′-diethylacrylamide)/ chitosan which are further reinforced by the ionic and covalent crosslinking of the chitosan network by tripolyphosphate and glutaraldehyde, respectively, are prepared. Based on their excellent mechanical properties and high conductivity, the developed DN ionogels are envisioned as stretchable ionic conductors for extremely stretchable alternating-current electroluminescent (ACEL) devices. The ACEL device fabricated with the developed ionogel exhibits stable working operation under an ultrahigh elongation of over 1200% as well as severe mechanical deformations such as bending, rolling, and twisting. Furthermore, the developed ACEL devices also display stable luminescence over 1000 stretch/release cycles or at temperatures as harsh as 200 °C.and high conductivity are required for conductors in stretchable EL devices. Consequently, the development of highly stretchable conducting layers has been intensively investigated to achieve both tolerance against various deformations and good electrical conductivity. To fabricate stretchable conductors for alternating-current electroluminescent (ACEL) devices, several general approaches have been developed in which conducting materials such as carbon nanotubes (CNTs),[1–3] silver nanowires (AgNWs),[4–10] and graphene[11,12] are combined with soft matrices such as polymers, hydrogels, or elastomers. While the aforementioned soft conductors were effectively operable under modest levels of stretching or defor-

1. Introduction

In recent years, stretchable electroluminescent (EL) devices have attracted attention because of their potential applications in deformable and flexible lighting displays and illuminated wearable devices.[1–19] To render highly stretchable EL devices, all components including the emission layer and electrodes must be durable against mechanical deformation. A stretchable emission layer can be easily achieved by mixing the EL materials in a soft matrix;[1–23] however, both mechanical robustness mation, these systems were limited with respect to durable operation at large levels of elongation or for long-term use owing to the intrinsic character of the materials.[1–12]
Generally, when coated on an elastomer, conducting materials show poor conductance reproducibility and durability, since recovery of the original resistance value is difficult, even after release to the initial elastomer length. For example, AgNWs coated on poly(dimethylsiloxane) (PDMS) exhibited poor stretchability, which is normally below elongation of 150% due to the high modulus of the PDMS.[7–10] Furthermore, since the junctions of the nanowires tended to detach and slide away each other at large deformations, both high resistance and poor durability were observed for the conductor.[7–10]
Dispersing conducting materials such as AgNWs and CNTs in a soft matrix is also a well-documented approach for preparing stretchable soft conductors.[20–23] Owing to the efficient percolation networks formed by the conducting materials at high aspect ratios, these systems enable high conductivity and high transparency in the fabricated stretchable electronic devices. However, elongation above the percolation threshold of the CNTs or AgNWs yields a rapid increase in the resistance of the soft conductors,[20–23] limiting the operation of the EL device due to the low conductivity of the electrode.
In an approach different from combining conducting materials with soft matrices, polymers[13] and their crosslinked networks[14–19] swollen in ionic salt solutions have recently been reported as conductors for EL devices to overcome the disadvantages mentioned above. Highly stretchable EL devices can be achieved with ionic conductors, enabling elongations of 700% based on a poly(methyl methacrylate) conductor containing lithium perchlorate[13] and stretching up to 500% for EL devices assembled with lithium salt based hydrogel electrodes.[14,15] However, the weak mechanical properties of the polymer matrices limited the maximum stretchability to less than several hundred percent,[14–19] and water evaporation from the hydrogel electrode adversely affected the long-term stability of the EL device.[15] Consequently, the development of mechanically reinforced ionic conductors with high conductivity is required for extremely stretchable and deformable EL devices. In addition to excellent mechanical properties, the preservation of the brightness performance of stretchable EL devices under various working conditions, such as intensive repetitive stretch/release cycles and harsh working temperatures, is also important for practical use in various fields.
Herein, we describe stretchable and durable conductive ionogels with a double-network (DN) structure in a highly stable ionic liquid (IL), which were further reinforced by ionically and covalently crosslinking the second network. These mechanically reinforced conducting ionogels were envisioned as flexible conductors for EL devices fabricated with an emitting layer of ZnS:Cu,Cl/Ecoflex composite. Owing to the mechanical robustness and good electrical conductivity against extreme elongation for the developed ionogels, the designed EL device exhibited good durability and high brightness, even with an extremely large mechanical strain of up to 1200%. Furthermore, owing to the superior stability of the ionogel, the developed EL devices demonstrated steady light emission over 1000 stretching cycles and a harsh working temperature of 200 °C.

2. Results and Discussion

Mechanically reinforced conductive ionogels were achieved with a double-network geometry[24–27] comprising covalently crosslinked poly(N-isopropylacrylamide-co-N,N′diethylacrylamide) (PNN) and chitosan (Ch). As illustrated in Figure 1a, interpenetrating crosslinked PNN and Ch networks were prepared by the radical polymerization of an aqueous pre-gel solution containing N-isopropylacrylamide, N,N′-diethylacrylamide, and Ch. Then, for further covalent and ionic crosslinking, the poly(N-isopropylacrylamide-coN,N′-diethylacrylamide)/chitosan (PNN/Ch) hydrogels were immersed in sodium tripolyphosphate (TPP) and glutaraldehyde (GA) solution. As shown in the right side of Figure 1a, multiple ionic bonds are formed between the negatively charged oxygens in TPP and the positively charged amine groups in Ch. Treatment with GA also results in the formation of covalent imine bonds between the GA and Ch. These chemical reactions generate reinforced PNN/crosslinked-Ch (PNN/x-Ch), which was expected to improve the mechanical modulus and stretchability of the DN gel. The further crosslinking of the Ch with TPP and GA was confirmed by Fourier transform infrared spectroscopy. (Figure S1, Supporting Information) After the further crosslinking of the Ch network, the network medium was replaced with an ionic liquid by immersing the DN hydrogel in 1-ethyl-3-methylimidazolium bis(trifluorosulfonyl)imide (EMIM-TFSI) and heating at 100 °C for 2 h. At this temperature, the water in the hydrogel network is fully evaporated,[28] finally resulting in the formation of the PNN/x-Ch ionogels.
To confirm the mechanical reinforcement of the prepared ionogels after Ch crosslinking, tensile stress–strain curves were measured for the PNN/Ch and PNN/x-Ch ionogels. As shown in Figure 1b, the PNN/Ch ionogel could be stretched up to 491% with a fracture stress of 72 kPa, while the fracture strain value of the PNN/x-Ch ionogel was 1200% at a higher fracture stress of 235 kPa, indicating the significant enhancement of mechanical toughness and stretchability after the crosslinking of Ch by combination with TPP and GA. Crosslinking by TPP improves the stretchability and toughness by forming a sacrificial re-zippable ionic bond, whereas that by GA contributes to the increase in the modulus and toughness through covalent bonds[29] as fully investigated in Figures S2–S5, Supporting Information. The PNN/x-Ch ionogels are also found to be highly elastic (Figures S6 and S7, Supporting Information) and mechanically robust against various deformations (Figure S8, Supporting Information).
To investigate the electrical properties of the ionogel depending on the elongation, the resistance of the PNN/x-Ch ionogel was measured with an inductance–capacitance–resistance (LCR) meter during stretching on a universal testing machine (UTM). As plotted in Figure 1c, at an initial length of 1.5 cm, the resistance of PNN/x-Ch was 20.7 kΩ, whereas stretching of the sample to 1200% yielded an increase in the resistance to 1316.4 kΩ, that is, 63.6 times higher than that of the original state. The resistance change was well matched with λ2, where the stretch ratio, λ, is 1 + strain (%)/100, indicating that the resistivity of the ionic conductor is independent of the stretch ratio.[30] The ionic conductivity for the ionogel is calculated as 5.15 × 10–3 S m−1.
Furthermore, the durability of the prepared double-network ionogel was confirmed by repetitive elongation/relaxation cycles at an elongation of 100%. The resistance changes of the PNN/xCh ionogel were fully reversible and reproducible without any significant variation over more than 6000 stretch/release cycles, as shown in Figure 1d. Thus, the designed PNN/x-Ch ionogel is highly robust and reliable for long-term use, which is a necessity for its use as a stretchable conductor.
Based on the soft modulus, high mechanical stretchability, and high conductivity of the PNN/x-Ch ionogels, we fabricated ACEL devices using PNN/x-Ch layers as the stretchable electrodes. As illustrated in Figure 2a, the ACEL devices are composed of an emissive layer that is sandwiched between two symmetrical PNN/x-Ch layers as the ionic conductors and dielectric silicone elastomer (Ecoflex 00-30) layers as encapsulation layers. For the emissive layer, ZnS:Cu,Cl nanoparticles were mixed with Ecoflex and then cured to form a stretchable EL composite layer. ZnS:Cu,Cl is one of the most widely used electroluminescent phosphors owing to its excellent long-life of thousands of hours, electrical stability in high electrical fields, and bright blue emission. A digital photograph of the fabricated ACEL device containing a Cu plate is shown in the right side of Figure 2a.
The emission performance of the ACEL device was measured by the luminance changes of the EL device under various electrical field and frequency conditions. For the ACEL device used in this experiment, the thickness of the emission layer was 100 µm, as selected from the luminance measurements. (Figure S9, Supporting Information) The transmittance of the ionogel electrode is found to be more than 80% at the visible light wavelength range (Figure S10, Supporting Information). As shown in Figure 2e, the higher the applied voltage and frequency, the higher the brightness of the emission. The maximum luminance of 95.4 cd m−2 was achieved at 300 V and 10 kHz, which is a comparable or higher luminance than from devices in previous reports[13–15] that were operated at higher voltages ranging from 900 to 2500 V. (Table S1, Supporting Information)
The rise in the applied voltage increasingly accelerates the electrons that activate the luminescence centers, consequently increasing the number of emitted photons. The variation in emission brightness of the ACEL device at various applied voltages was found to be well fitted with the equation L = L0exp(−β/V1/2),[31] where L is the luminance, V is the applied voltage, and L0 and β are empirical parameters. While the emission brightness increases exponentially with the applied voltage, it moderately increases in the given frequency range.
Next, we investigate the durability of the emission performance of the ACEL device under mechanical deformation. As can be seen in Figure 3a, the ACEL device can maintain its light-emitting function even under various deformations such as bending and twisting. Further bending or twisting does not affect the operation of the ACEL device, indicating that the developed ACEL is robust against mechanical deformation due to its elasticity and the softness of the PNN/x-Ch conductor.
To confirm the excellent mechanical robustness of the ionic conductors for the ACEL device, we tested the stability of the luminescence during stretching of the device with large strains.
As shown in Figure 3b; and Movie S1, Supporting Information, in the original state, the ACEL device exhibits a uniform, bright emission over the entire emission layer. The device retains its uniform light emission until an extremely large strain of 1200%. We note that failure of the device at the stretching of over 1200% results from the fracture of the ionogel electrode. Additionally, the developed EL device can be easily stretched by the hand due to its soft modulus. (Figure S12, Supporting Information) Thus, the developed ACEL device will show excellent sustainable light-emitting performance in an extreme, highly stretched state owing to the mechanical and electrical robustness of the PNN/x-Ch ionogel conductors. Notably, this DN-ionogel-based ACEL device shows the best stretchability compared to previous stretchable EL devices, which afforded 700% stretchability for a polymer conductor containing lithium perchlorate[13] and 500% for lithium-salt-based hydrogel electrodes.[14,15] (Table S1, Supporting Information).
To quantify the light-emitting performance as a function of elongation, the luminance changes in the ACEL devices were measured during stretching (Figure 3c). The ACEL device shows a significant increase in luminance of up to 647 cd m−2 as the strain increases to 800%, which corresponds to a 6.78× increase over the initial luminance. Above a strain of 800%, the luminance starts to decrease, with a value of 199.4 cd m−2 at 1200% strain, which is still 2.09× higher than the initial value.
The variation in luminance depending on the strain can be explained by the change in the thickness of the emission layer. When the elongation is increased to 800%, the thickness of the emissive layer decreases correspondingly, resulting in an increase in the electric field in this layer under the constant applied voltage and thereby increasing the luminance. However, above a critical strain value, the decrease in the phosphor density induced by the stretching of the device reduces the brightness as well as lowers the electrical conductivity of the ionogel layer.
We also found that the stretchable ACEL device showed stable electroluminescence from the emission spectrum centered at 460 nm, independent of the strain change (Figure 3d). As shown in Figure 3e, the developed ACEL device delivers neon-blue emissions with a consistent CIE coordinate of (0.15, 0.18) regardless of stretching, further confirming the stable operation of the device, even at an extremely high elongation of 1200%.
The long-term stability of the light-emitting performance as a function of stretching was confirmed by cycling tests. The luminance of the ACEL device was measured over 1000 stretch/ release cycles of 100% elongation. As shown in Figure 4a, the device exhibits fully reversible luminance changes without significant variation, and the luminance values in the original state and at 100% elongation are maintained during the entire testing process. We also observed the operation of the ACEL device at various working temperatures, from 0 °C to 200 °C. As shown in Figure 4b, the light-emitting performance is excellent at each temperature, even at 0 °C and 200°C, owing to the thermal stability of the PNN/x-Ch ionogel and IL (Figures S13 and S14, Supporting Information). These observations clearly show that the developed ionic conductor is mechanically and thermally stable and promising for wide application in soft electrical devices.

3. Conclusion

By utilizing the excellent mechanical properties of ionic conductors Glutaraldehyde comprising a double-network ionogel, PNN/x-Ch, swollen in EMIF-TFSI, the developed ACEL device preserved its bright light-emission operation with stretching up to 1200%, as well as demonstrating high mechanical, electrical, and thermal stability. Based on these positive attributes, we can conclude that the PNN/x-Ch ionogels should be promising candidates for application in new stretchable types of lighting displays, sensors, and unprecedented future electronics.

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