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Nov 04, 2024

Resilient, environment tolerant and biocompatible electroluminescent devices with enhanced luminance based on compliant and self-adhesive electrodes | npj Flexible Electronics

npj Flexible Electronics volume 8, Article number: 37 (2024) Cite this article 2163 Accesses 7 Citations 1 Altmetric Metrics details Electroluminescent (EL) devices are of great significance for

npj Flexible Electronics volume 8, Article number: 37 (2024) Cite this article

2163 Accesses

7 Citations

1 Altmetric

Metrics details

Electroluminescent (EL) devices are of great significance for expanding the application range of optoelectronics. However, the realization of EL devices with environment-tolerance, stretchability, mechanical cycling stability, self-adhesion, biocompatibility, and high dielectric constant still remains a challenge. Herein, a type of EL device with enhanced comprehensive performances composing of a chlorinated barium titanate/phosphor/polydimethylsiloxane (Cl-BT/phosphor/PDMS) luminescent layer sandwiched between two silver nanowire-cellulose nanocrystal with II crystalline allomorph/Triton X-100 modified polydimethylsiloxane (AgNW-CNC II/TX-PDMS) electrodes fabricated through a full solution-processing strategy is proposed. Environmentally-friendly CNC II with high transmittance acts as an antioxidant, dispersant and film-former for AgNWs. The hydrophilic modification of TX to PDMS imparts the electrodes with self-adhesion, high stretchability, as well as strong interfacial bonding between TX-PDMS and AgNW-CNC II. The electrodes achieve skin-like modulus by adjusting TX content, endowing the EL devices with a high compliance (186 kPa of Young’s modulus). The luminescent layer with Cl-BT exhibits a high dielectric constant (19) and luminance (up to 72 cd m−2). The assembled EL device with excellent cyclic stability (luminance retention 85% after 400 cycles), durability (luminance retention >94% after 400 min) and stretchability (88% luminance at 200% strain) can work properly at broad temperatures (−20 ~ 70 °C) and underwater. This biocompatible and self-adhesive EL device demonstrates great potential for implantable biomedical devices and wearable displays under harsh environments.

Alternating current (AC) electroluminescent (EL) devices have attracted widespread interest due to their simple structure and potential applications in optoelectronic displays and lighting1,2,3. With the rise of wearable electronics, human-machine interaction and flexible electronics, stretchable EL devices need to be developed to meet the requirements4,5,6. To this end, both the two electrode layers and the luminescent layers of the devices should be stretchable. Moreover, EL devices should have excellent mechanical properties and fatigue resistance to endure different mechanical deformations, such as bending, folding, twisting and stretching, thus improving the service life7. In recent years, ZnS phosphor-based EL devices have been widely studied by dispersing inorganic active materials in an insulating polymer matrix as their luminescent layer. However, sulfide-based phosphor is sensitive to moisture and difficult to remain stable under high humidity conditions, so they cannot be applied in aqueous environments.

Polydimethylsiloxane (PDMS), a hydrophobic elastomer, is widely used as the substrate for flexible electronic devices due to its elasticity, transparency, biocompatibility and stretchability, etc8,9,10,11. Namkoong et al. fabricated a wearable sensor using a PDMS mold with microchannels that exhibited negligible electromechanical stability under bending and stretching12. Veeramuthu et al. developed a sandwich-structured pressure sensor through electrospun nanofibrous water-etched PDMS substrate13. The sensor exhibited strain-insensitivity and excellent mechanical elasticity. For EL devices, PDMS can be used as both the substrate of the electrode and the dielectric material of the luminescent layer, thus providing high mechanical strength, fatigue resistance and waterproofness for the devices. Wang et al. developed a transparent and stretchable EL device by sandwiching ZnS:Cu/PDMS with two single-walled carbon nanotubes/PDMS electrodes. The device maintained bright and constant electroluminescence under various deformation process14. Silver nanowires (AgNWs) are considered as promising candidates for the preparation of stretchable transparent conductive electrodes due to their high conductivity, transparency and large aspect ratio15. To solve the problem of high surface roughness of electrodes caused by local aggregation of unevenly dispersed AgNWs, cellulose nanocrystal with II crystalline allomorph (CNC II) with excellent dispersion is introduced. Compared with cellulose nanofibers (CNFs) and cellulose nanocrystals with I crystalline allomorphs (CNC I), CNC II has a smaller size, which facilitates the fabrication of electrodes with high transparence16,17. To meet the requirements of wearable electronics so that the devices can deform synchronously with human skin, the modulus of the polymer matrix needs to be less than that of human skin (140 ~ 600 kPa). However, PDMS with high modulus cannot mimic the mechanical properties of human skin. In addition, it is difficult to bind AgNWs to the surface of hydrophobic PDMS to prepare uniform conductive electrodes. This is because the hydrophobicity of PDMS prevents AgNWs from wetting the surface of PDMS and causes AgNWs to gather around the water droplet, forming a discontinuous and non-uniform conductive network under the combined action of droplet evaporation and surface tension. Moreover, EL devices based on PDMS electrodes lack self-adhesion, which greatly limits their further applications. Self-adhesive materials usually have a sticky surface, which can adhere to the surface of other materials without the help of other adhesives. The lack of self-adhesion of the electrode substrate requires the use of tapes such as 3 M tape to attach the device to other materials, which affects the luminous performance. At the same time, traditional tape placed on the skin may create biocompatibility issues.

The use of surfactants is the most effective and simplest way to overcome hydrophobicity of PDMS compared to more complex schemes requiring expensive equipment18. Surfactants can be cationic, anionic or non-ionic based on their chemical structures19. Traditional surfactants are amphiphilic molecules with low molecular weight that can self-assemble into various structures in aqueous solution, such as micelles or liposomes. The hydrophobic tail of amphiphilic surfactant molecules is physically adsorbed to the PDMS surface, while the hydrophilic head protrudes into the buffer, thus changing the PDMS surface properties. In this way, surface modification can be accomplished more quickly, cheaply and simply20. Triton X-100 (TX) is a classic non-ionic surfactant. Compared with some other surfactants, TX exhibits non-toxicity, solubility and stability (not easily affected by factors such as pH value and temperature)21,22. Moreover, TX is a good wetting agent, which can reduce the surface tension of PDMS and thus endow PDMS with hydrophilicity23. To improve the hydrophobicity of PDMS, Hojjat Madadi et al. added nonionic surfactants TX to reduce the contact angle of PDMS by 23°24. In addition, TX can inhibit PDMS crosslinking reactions, resulting in the formation of heterogeneous cross-linked polymer networks. This structure can lead to changes in mechanical properties to form a soft and viscous PDMS, thus endowing PDMS substrate self-adhesion and low Young’s modulus.

For AC driven EL devices, high voltage and frequency are usually required, resulting in limited application. Barium titanate (BT) with good chemical stability, insulation (conductivity <10−12 ohm−1 cm−1) and high dielectric constant (1500 ~ 6000) is widely used in luminescent layer25,26,27, which is prepared by dispersing phosphor particles in the BT-PDMS composite. However, the high dielectric constant of BT particles is greatly reduced due to the low dielectric constant PDMS matrix. The three-phase composite film consisted of BT particles, conductive filler and polymer is one of the best methods to improve the dielectric constant of BT-polymer composites28,29. Metal nanoparticles, graphene and carbon nanotubes are widely used as conductive fillers to prepare polymer composites with high dielectric properties30,31,32. However, the existence of conductive fillers tends to lead to relatively high dielectric loss, and the volume fraction of conductive fillers must be close to the permeability threshold to obtain a high dielectric constant, resulting in a low breakdown field. Doping chlorine (Cl) on the surface of BT is one of the ways to improve the dielectric constant of the luminescent layer without conductive fillers. This is because Cl or Cl-containing functional groups can increase the cumulative density of charge carriers at the interface between the Cl-doped filler and the polymer, resulting in increased interface polarization33. Therefore, treatment by chlorinated luminescent layer is a simple and effective method to improve the polarizability, which has become an advantage to explore the optimization of EL device performance.

In this work, a sandwich-structured EL device with high stretchability, environment-tolerance, mechanical stability, self-adhesion and improved dielectric constant was developed through a full solution-processing strategy. The optical properties, self-adhesion, mechanical properties, microscopic morphologies, temperature resistance and waterproofness of the devices were investigated in detail. Compared with classical AgNW-PDMS electrodes, the introduction of surfactant TX could improve the wettability of PDMS, as well as endow the device with a self-adhesion, high stretchability and surface compliance. The TX-PDMS electrode substrate with a hydrophilic surface was conducive to the tight binding of conductive layer. To obtain a uniform conductive network on TX-PDMS matrix, sustainable CNC II was introduced as a dispersing agent and film forming agent to disperse the AgNWs. To increase dielectric constant of the luminescent layer, BT was introduced, and the interfacial compatibility of BT particles with phosphor and PDMS was improved by chlorination process. Due to the water resistance of PDMS, as well as the temperature resistance of PDMS, AgNWs, CNC II and phosphor, the EL devices could be operated normally at −20 ~ 70 °C and underwater. The assembled biocompatible EL devices were expected to be used for human-machine interaction, stretchable lighting, deformable display under various harsh environments.

The preparation process and mechanism of electroluminescent (EL) devices with a sandwich structure are shown in Fig. 1. AgNW-CNC II nanocomplex was uniformly deposited on the TX-PDMS surface to prepare electrodes (Fig. 1a). PDMS were highly hydrophobic with extremely low surface energy (~20 mN m−1)34. To achieve uniform and robust conductive network on PDMS films, stronger bonds needed to be formed between AgNWs and PDMS. Non-ionic surfactants TX were amphiphilic substances with alkyl groups at one end and poly(ethylene glycol) (PEG) groups at the other. The PEG groups were chemically coupled to the surface of AgNWs through O-Ag bonds, thus improving the binding strength between PDMS and AgNWs35. In addition, TX-PDMS exhibited a strong self-adhesion (664 N m−2 to human skin), high stretchability (689% strain) and skin-like modulus (186 kPa) because TX reduced the cross-linking density of PDMS polymer networks. TX could also enhance the wettability of PDMS and thus endowed TX-PDMS electrode substrate with a hydrophilic surface, which facilitated the adhesion of AgNW-CNC II conductive layer with PDMS electrode substrate. Moreover, CNC II with a large number of hydroxyl groups could form hydrogen bonds with TX, further imparting enhanced interface bonding between the conductive layer and the electrode substrate.

a Schematic diagram for the preparation of the AgNW-CNC II/TX-PDMS electrodes. b Schematic of the fabrication process of high-dielectric light emitting layer slurry. c Assembly of EL devices.

In general, highly conductive AgNWs tended to agglomerate, resulting in uneven conductive layers on TX-PDMS surfaces. The biomass-derived CNC II with a high transmittance was used as an antioxidant, green dispersant and film-forming agent to improve the oxidation resistance of electrodes, promote the formation of uniform conductive film, and enhance the interface bonding between AgNW-CNC II conductive layer and TX-PDMS electrode substrate. To increase the light emission, BT with large dielectric constant was introduced into the luminescent layer (Fig. 1b). BT could lead to a strong polarization effect in the electric field, so that the charge was easy to gather at the interface between phosphor and BT36. The presence of highly electronegative Cl atoms in CB further enhanced the differential charge accumulation between Cl-BT particles and phosphor powder, resulting in a sharp increase in light emission intensity29. The dielectric breakdown could be effectively avoided by completely wrapping Cl-BT and phosphor into PDMS as a luminescent layer. The EL devices, readily fabricated by sandwiching a luminescent layer between two AgNW-CNC II/TX-PDMS electrodes (Fig. 1c), could withstand various deformations such as stretching, bending and twisting without leaving any creases and cracks, demonstrating a high mechanical stretchability and flexibility.

The polymer matrix of electrodes with high transmittance, self-adhesion and stretchability was composed of TX and PDMS (Supplementary Fig. 1). The curing mechanism of pure PDMS was silico hydrogen addition reaction of vinyl (-CH = CH2) from hydrosilo (SiH) under Pt catalysts. Pt catalysts were coordinatively unsaturated and readily form complexes with polar functional groups in TX37. The hydrophilic parts of TX were depleted by the Pt catalyst to reduce or inhibit cross-linking, resulting in the formation of heterogeneous TX-PDMS network (Fig. 2a). The decrease in active Pt catalyst concentration led to a decrease in the degree of crosslinking of the TX-PDMS network. The reduction of crosslinking degree could help to improve the stretchability, surface compliance and adhesion of electrodes. Therefore, the addition of TX could make the TX-PDMS softer and stickier than pure PDMS without changing the original chemical structure of PDMS (Supplementary Fig. 2 and Supplementary Note 2). The transmittance of pure PDMS at 550 nm was about 94.6% (Fig. 2b, c). As the TX content increased, the transmittances of 0.2% TX-PDMS, 0.4% TX-PDMS, 0.6% TX-PDMS and 0.8% TX-PDMS at 550 nm decreased to 92.5%, 91.2%, 88.4% and 79.5%, respectively. Considering the transmittance of the electrode, the transmittance of the electrode substrates should be greater than 80%38. Therefore, 0.2% TX-PDMS, 0.4% TX-PDMS and 0.6% TX-PDMS were selected for further analysis. The decrease of transmittance after TX addition was attributed to the formation of micelle structure in PDMS matrix, leading to a light scattering effect. Thanks to the high hydrophobicity of the PDMS, the alkyl groups on the TX chains formed shells and the corresponding PEG formed the center of the micelle structure (Supplementary Fig. 3). When the concentration of TX was above critical micelle concentration (CMC, 106-160 mg L−1), the micelle structures were formed in PDMS23. When the amount of TX added was 0.2 wt%, the concentration of TX was about 2000 mg L−1, which exceeded the CMC. Thus, TX micelles were formed in PDMS. With the increase of TX content, the light scattering intensity increased due to the increase of micelle size31. The larger size of the micelles, stronger steric repulsion was introduced. Therefore, higher concentrations of surfactants TX could more effectively expand the internal network structure of the PDMS matrix39. Compared with pure PDMS, the introduction of TX into PDMS had little effect on the surface morphology of TX-PDMS (Fig. 2d). The surface roughness (Ra) and root mean square roughness (Rq) of the TX-PDMS film increased from 0.320 nm to 0.676 nm and from 0.405 nm to 0.889 nm, respectively, which was attributed to the formation of a non-uniform heterogeneous cross-linking network. It could be seen from XPS that the ratio between C1s and O1s was 1.89 for pure PDMS films, while the ratio between C1s and O1s was 1.99 for TX-PDMS films (Fig. 2e, f). For TX-PDMS, a C1s peak (286.7 eV) next to the original C1s peak (284.8 eV) was considered to be the C-O, indicating the presence of the characteristic signals of hydrophilic-terminated PEG chains of TX on the of TX-PDMS surface40,41. Hence, TX was uniformly introduced into the PDMS and TX-PDMS could maintain a high transmittance for use in transparent electrodes.

a Crosslinking network structure of the PDMS and TX-PDMS. b Transmittance and c photographs of TX-PDMS films. d AFM images of PDMS and TX-PDMS. e XPS wide spectra of PDMS and TX-PDMS. f High resolution spectra of C1s core-levels. g Adhesion force of TX-PDMS films on steel, glass, plastic and human skin. h Photographs of 0.6% TX-PDMS films adhered to various materials.

The adhesive strength increased with the increasing TX content for four types of common materials (Fig. 2g). The adhesive strength of 0.6% TX-PDMS film adhered to human skin (664 N m−2) was about 12 times higher than that of pure PDMS to human skin (56 N m−2), which was conducive to the application of wearable light-emitting devices. In addition, the adhesive strength between the TX-PDMS substrate film and glass was the highest compared with the other three types of material, showing that TX-PDMS had a stronger interface adhesion with smooth surface. The adhesive strength of 0.6% TX-PDMS film adhered to glass (1224 N m−2) was 1.8 times higher than that of human skin. More importantly, the TX-PDMS exhibited repeatable adhesion. After 5 cycles of adhesive/peeling tests, the film remained sufficient adhesion (448 N m−2 to steel plate and 288 N m−2 to human skin) (Supplementary Fig. 4). In addition to steel, glass, plastic and human skin, the TX-PDMS could be tightly adhered to the surface of other common materials, such as rubber, paper and wood (Fig. 2h). The 0.6% TX-PDMS adhered with 40 g weight could be tightly adhered to the index finger without falling off, corroborating the high adhesive strength of TX-PDMS films with human skin. To reveal the effect of TX content on the adhesion of PDMS, the swelling rate and gel fraction of the sample were measured using toluene as solvent (Supplementary Fig. 5). PDMS was a highly hydrophobic elastomer, it could be insoluble in water and ethanol but soluble in organic solvents such as toluene42,43. Therefore, toluene was used as solvent. PDMS with high crosslinking degree had low swelling rate (2.55) and high gel fraction (0.947). With the addition of TX, the swelling rate gradually increased and the gel fraction gradually decreased. The swelling rate and gel fraction of 0.6% TX-PDMS were 1.4 times (3.50) and 0.9 times (0.876) of pure PDMS, respectively. This was because the introduction of TX reduced the cross-linking degree of PDMS, which was conducive to the immersion of toluene solvent. Therefore, 0.6% TX-PDMS with a high transmittance and strong self-adhesion was selected for the substrate to assemble the composite electrode.

In general, the modulus of electrode matrix should be close to that of human skin (140 ~ 600 kPa), so as to meet the requirements of synchronous deformation of EL devices and human skin. However, the modulus of pure PDMS (762 kPa) was greater than that of human skin. Hence, TX was used to adjust the modulus of PDMS. This was because of the formation of a strong complex between the Pt catalyst in PDMS and TX, which inhibited the cross-linking of the PDMS internal network. With the increase of TX content, the elongation at break gradually increased and the tensile strength gradually decreased (Fig. 3a, b and Supplementary Table 1). The Young’s modulus of 0.6% TX-PDMS (186 kPa) was close to that of human skin (Fig. 3c), which meant that the excellent compliance of TX-PDMS could make it fit well to human skin under different deformations44. TX-PDMS with high resilience could be stretched up to ~400% strain without breaking and return to their original length (Fig. 3d, e), indicating good promise for its use as a transparent, stretchable and elastic polymer matrix for epidermal electronic components. Therefore, the mechanical properties of TX-PDMS could be tuned by adjusting the content of TX, so that the TX-PDMS electrode substrates demonstrated high stretchability, recoverability and compliance with the Young’s modulus closing to that of human skin.

a Tensile stress-strain curves of TX-PDMS films with various TX content, and b the corresponding tensile strength and elongation at break. c Tensile elastic modulus derived from (a), insert: stress-strain curves under 0-60% strains from (a). d Tensile loading-unloading curves under 100% strains. e Photographs of 0.6% TX-PDMS withstanding 400% strain. f Curves of the storage modulus G’ and loss modulus G “ as a function of frequency f. g Phase angle δ versus f. h Surface contact angle of PDMS and 0.6% TX-PDMS as a function of time. SEM images and schematic diagram of the wettability of i PDMS and (j) 0.6% TX-PDMS after 10 min.

The storage modulus G′ and the loss modulus G” were mainly manifested in elastic and viscosity characteristics, respectively. Both PDMS and TX-PDMS exhibited quasi-solid behavior in the frequency range from 0.1 to 10 Hz (G’ > G”), with elastic properties dominating, indicating their elastic and reversible crosslinked networks (Fig. 3f)45. As the frequency increased, G’ gradually leveled off and reached a limit value (G’∞). The G’∞ of PDMS (200 kPa) was about 2.86 times that of TX-PDMS (70 kPa), suggesting that the elastic property decreased after the addition of TX. The G” of 0.6% TX-PDMS was greatly reduced at low frequencies due to its large internal viscosol flow and heterogeneous cross-linking network46. In general, the higher loss tangent (tanδ = G”/G’) implied a greater viscoelasticity. The tanδ value of 0.6% TX-PDMS was higher than that of pure PDMS (Fig. 3g), indicating that 0.6% TX-PDMS was more compliant and viscoelastic than pure PDMS. Notably, TX-PDMS exhibited a tanδ value (the average value was 0.42) comparable to that of human skin (the average value was 0.39)47. The highly compliant TX-PDMS used as electrode substrates for flexible EL devices could adapt to the skin tension associated with body movement and improve comfort when worn.

The addition of TX not only endowed PDMS with adhesion, high stretchability and skin-like modulus, but also provided a hydrophilic interface. Pure PDMS exhibited an initial water contact angle of 118° and remained hydrophobic after 10 min (Fig. 3h). Compared with pure PDMS, TX-PDMS presented a lower water contact angle and could further decrease to 81° after 10 min, demonstrating an improved hydrophilicity and the time-dependence of hydrophilic behavior of TX-PDMS. The hydrophilic surface was attributed to TX reducing the surface tension of PDMS, further demonstrating the formation of micelles. The surface of PDMS film was smooth, while 0.6% TX-PDMS film exhibited continuous tiny wrinkles on the surface (Fig. 3i, j), contributing to the improved hydrophilicity of TX-PDMS. Due to the adsorption and diffusion of hydrophilic groups of TX on the liquid-solid interface, the surfactant TX in PDMS changed the initial spherical shape of the water droplets. At the water-PDMS interface, TX was released upon contact with water molecules through hydrogen bonds, thus improving the wettability of TX-PDMS. The TX-PDMS films with hydrophilic surfaces facilitated the subsequent introduction of AgNW-CNC II conductive layers to prepare electrodes with strong interfacial bonding. The high surface compliance and continuous tiny wrinkles allowed TX-PDMS to contact with other materials in a larger area38. The better wettability could improve the contact and interaction between the TX-PDMS surface and other substances46, which could also be used to explain the reason for its better self-adhesive strength.

The AgNW-CNC II/TX-PDMS electrode was prepared by depositing AgNW-CNC II conductive layer on the electrode surface. Small-sized CNC II (average length of 75 ± 20 nm and average diameter of 6 ± 1 nm) with a high transmittance (~96% at 550 nm) was used to disperse AgNWs (Fig. 4a, b). The pure AgNWs was agglomerated in a large area (Supplementary Fig. 6) with an absolute value of the zeta potential of 14.5 mV. Due to the large number of negative charges on the surface, CNC II demonstrated good dispersibility with an absolute value of the zeta potential of 43.6 mV. Therefore, with the help of CNC II, the absolute value of the zeta potential of AgNW-CNC II reached up to 30.1 mV, further suggesting the homogeneous dispersion of AgNW-CNC II nanocomplexes. When the pure AgNWs with a high aspect ratio were exposed to air, many small Ag2O particles grew on the AgNWs surfaces, indicating that the electrode was easily oxidized (Fig. 4c). For AgNW-CNC II, the AgNWs surfaces were smooth due to the close contact between CNC II and AgNWs, suggesting that CNC II could act as a protective layer to prevent the oxidation of AgNWs. Although the AgNWs dispersed well with the help of CNC II, the AgNW-CNC II nanocomplexes was unevenly distributed on the PDMS surface, and the agglomeration of AgNW-CNC II could be clearly observed (Fig. 4d). This was because hydrophobic PDMS caused hydrophilic AgNW-CNC II to accumulate around the droplets during the drying process of the conductive layer, forming a discontinuous and inhomogeneous conductive network under the combined action of subsequent droplet evaporation and surface tension48. In contrast, AgNW-CNC II could be uniformly deposited on the hydrophilic TX-PDMS surface, forming an interconnected network. The raised part in the AFM image was AgNW-CNC II, the exposed part was PDMS substrate. The “coffee ring” effect of AgNW-CNC II/PDMS further suggested the inhomogeneous dispersion of AgNW-CNC II on the pure PDMS surface (Fig. 4e). AgNW-CNC II/PDMS exhibited a rough surface with Ra and Rq of 61.6 nm and 84.6 nm (Supplementary Table 2), respectively. The roughness (Ra = 38.5 nm, Rq = 53.4 nm) of AgNW-CNC II/TX-PDMS surface was significantly reduced, indicating that AgNW-CNC II conductive network were uniformly dispersed on the TX-PDMS substrate with an enhanced interfacial adhesion49.

a TEM image of CNC II. b Transmittance of CNC II suspension (0.5 wt%). Insert: absolute values of the zeta potential of the AgNWs and AgNW-CNC II. c SEM images of AgNWs and AgNW-CNC II. d SEM and e AFM images of the AgNW-CNC II/PDMS and AgNW-CNC II/TX-PDMS electrodes. f Transmittance of AgNW-CNC II/TX-PDMS electrodes with different sheet resistances. g The resistance under different deformation states. h Resistance changes under cyclic tensile strain from 0-400% (Each test was repeated five times and the data were expressed as mean ± SD). i Three cyclic stress-strain curves of AgNW-CNC II/TX-PDMS electrode under 100% strain. j Resistance changes at different strains during 50 stretching-releasing cycles. k Resistance response during 120 stretching-releasing cycles (6000 s) at 100% strain. l Resistance changes during 200 bending cycles.

Electrodes with different sheet resistance were prepared by controlling the loading level of AgNW-CNC II conductive layer. As the amount of AgNW-CNC II increased, the sheet resistance and transmittance of AgNW-CNC II/TX-PDMS electrodes both decreased (Fig. 4f). Notably, the transmittance of electrodes with different sheet resistance were all greater than 70%, which could meet the requirements of optoelectronic device displays50. To balance transmittance and sheet resistance, the electrodes with a sheet resistance of 25 Ω sq-1 and an average transmittance of 79.5% at 550 nm were selected for the assembly of EL device. To visually display the electrode conductivity, the light-emitting diode (LED) and electrode were connected to a simple circuit (Fig. 4g). Obviously, the LED remained bright under bending, folding and twisting, exhibiting the excellent flexibility and conducting stability of the electrodes. However, due to the different deformation degree of bending, folding and twisting, the resistances were slightly different. As the strain increased, the resistance of the electrode increased steadily (Fig. 4h). When the deformation was restored, the resistance almost returned to its original value. The conductive path was not destroyed under large strain, which was attributed to the low modulus of TX-PDMS and the strong interfacial adhesion between the AgNW-CNC II conductive network and TX-PDMS electrode substrate.

Thanks to the high resilience of TX-PDMS matrix, the AgNW-CNC II/TX-PDMS electrode exhibited an excellent fatigue resistance (Fig. 4i), which was an important prerequisite for its application in stretchable EL devices. After 50 stretching-releasing cycles under various strains (20, 50, 100 and 200%) (Fig. 4j), the resistance signals maintained reproducibility. To further investigate the repetitive stretchability of the electrode, a periodic stretching-releasing test was conducted at 100% strain (Fig. 4k and Supplementary Fig. 7). With the increase of the cycle numbers, the resistance gradually increased, which was caused by the loss of the interconnecting parts of the AgNWs and the sliding between the AgNWs51. However, due to the large aspect ratio of AgNWs, the strong interfacial binding between conductive layer and electrode substrate, as well as the dispersion and antioxidant protection of AgNWs by nanocellulose, the electrodes still exhibited excellent conductive stability during cyclic stretching. To investigate the total life cycles of an electrode, 100% strain was chosen as an example. The resistance (about 365 Ω) at 300% strain was used as the final resistance of the electrode at 100% strain during its life cycles. After 1000 cycles, the peak resistance increased from 117.9 Ω to 179.5 Ω. It was expected that after 4020 cycles, the resistance at 100% strain increased to 365 Ω, and the increase in resistance meant that the brightness of the EL device decreased, which did not represent the device failure. In addition, the resistance of the electrodes remained stable during 200 continuous bending cycles (Fig. 4l). These results demonstrated the favorable mechanical flexibility, stretchability, long-term conductive stability and anti-fatigue property of AgNW-CNC II/TX-PDMS electrodes.

The EL device with a three-layer structure was assembled by sandwiching a luminescent layer between two transparent electrodes (Fig. 5a, b). The emission mechanism was as follows: when a critical electric field was applied, electron tunneling occurred at the interface between the dielectric and the luminescent material. These electrons were accelerated under a high electric field to obtain high kinetic energy to form superheated electrons, which excited the luminescent center through impact excitation and ionization mechanisms. Light emission occurred when the excited electron returned to the ground state52. The thickness of the EL device and the luminescent layer were 1.2 mm and 45 µm, respectively. Usually, the interface among the internal layers of the EL device was more easily damaged than other parts. Therefore, a strong and stable interfacial adhesion between the layers was crucial for maintaining a high optoelectronic performance of the devices. Obviously, there was no gap between the electrode and luminescent layer. Phosphor and Cl-BT were evenly dispersed in PDMS and completely wrapped (Fig. 5c), which could effectively avoid dielectric breakdown. The AgNWs were randomly oriented and uniformly distributed in the CNC II film and a stable conductive network was formed on the TX-PDMS surface (Fig. 5d), indicating the excellent dispersion effect of CNC II for AgNWs. The average diameter of phosphor particles was 15 µm (Fig. 5e). After CB treatment, the shape and surface morphology of BT particles did not change, and the dispersion of Cl-BT was better than that of pure BT (Fig. 5f, g). The average diameter of phosphor was 19 times larger than that of BT particles (0.8 µm). This allowed BT to fill the voids between the phosphors, resulting in a denser luminescent layer. Raman, XRD and XPS results showed that Cl atoms were successfully introduced and the prepared Cl-BT was tetragonal phase of barium titanate (T-BT) structure (Supplementary Fig. 8 and Supplementary Note 3). The active substance in chlorinated solvent might be formed by ultrasonic reaction. Ultrasonic treatment could decompose CB into Cl free radicals or chlorine gas53. These Cl radicals or Cl-containing functional groups might react with the surface of BT particles to produce Ba-Cl and Ti-Cl bonds54. Different from cubic phase of barium titanate (C-BT), T-BT presented high crystallinity, uniformity and dielectric constant55.

a Schematic diagram of EL devices. SEM images of b cross sections of EL devices, c luminescent layer, d AgNW-CNC II, e phosphor powders, f BT powders and g Cl-BT particles. h Electroluminescent spectrum in different colors and CIE chromaticity diagram. i XRD patterns of phosphor powders in different colors. j Photographs of EL devices in different colors (scale bar: 1 cm). k Dielectric constant and l dielectric loss of the luminescent layers as a function of frequency. m Relationship between voltage and luminance at 400 Hz. n Photographs of the EL devices driven by a voltage of 100 V, 200 V and 300 V at 400 Hz (scale bar: 1 cm).

The luminescent color of the EL devices could be tuned by controlling the type of metallic elements in the phosphor. By doping Cu2+, Cu2+-Al3+ and Cu2+-Mn2+ in the phosphor, the device could emit green, blue, and orange light, respectively. The typical peaks of blue, orange and green light emission spectrum were centered at 452 nm, 585 nm and 510 nm, respectively (Fig. 5h). The position of Commission Interationale de L’Echairage (CIE) color coordinates were (0.149, 0.123), (0.539, 0.454) and (0.158, 0.245), respectively, proving that the light emitted were in the blue, orange and green regions, respectively. The peaks of phosphors emitting different colors were consistent (Fig. 5i). The diffraction peaks at 2θ = 28.6°, 47.6°, and 56.4° corresponded to the (111), (220) and (311) crystal planes of the ZnS cubic sphalerite structure56. This result was consistent with the Joint Committee on Powder Diffraction Standards (JCPDS) card number 05-056657. At the same voltage and frequency, orange light intensity was lower (Fig. 5j). This was because in orange (ZnS:Cu, Mn) phosphors, Mn2+ captured holes in the AC field and reduced the effective electron-hole recombination, thereby decreasing the emission intensity58. The electroluminescence properties of the different color phosphors were consistent with the results of the photoluminescence (PL) spectra and photoluminescence quantum yield (PLQY) (Supplementary Fig. 9). This color-tunable EL devices offered opportunities for next generation multi-color functional displays.

The luminescent layers with high dielectric constant could enable the device to emit brighter light in similar voltage54. The dielectric constant of the pure PDMS and Phosphor/PDMS were 2.3 and 4.4, respectively (Fig. 5k). The addition of BT particles increased the dielectric constant of BT/Phosphor/PDMS (12.0) by 2.7 times compared to Phosphor/PDMS. Notably, the dielectric constant of the Cl-BT/Phosphor/PDMS reached up to 19. The large increase was ascribed to the accumulation of differential charges at the interface between the Cl-BT particles in the luminescent layer and PDMS due to the strongly electronegative Cl atoms59. Besides, the dielectric constants of luminescent layers were stable from 102 Hz to 105 Hz. Correspondingly, the luminescent layers showed a low dielectric loss in the frequency range (Fig. 5l). This was because Cl-BT particles were small in size and relatively smooth in shape, thus the voids between phosphor and Cl-BT particles were small, which could provide better inter-particle contact to increase the packing density60. The Cl-BT/Phosphor/PDMS luminescent layer with a high dielectric constant and a low dielectric loss in a wide frequency range had great application potential in EL devices with high luminance.

The luminance at different voltages was measured to demonstrate the effect of Cl-BT on the luminescence properties of the devices. At 300 V and 400 Hz, the luminance of the EL device based on Cl-BT (72 cd m-2) increased by about 2.1 times compared with the EL device based on BT (34 cd m-2) (Fig. 5m and Supplementary Fig. 10). Noting that the device could still emit uniform and bright light at 400 V (Supplementary Fig. 11), indicating that the electrical breakdown voltage was above 400 V. The significant improvement of the EL device brightness could be explained by the improvement of the dielectric constant of the luminescent layer. At a fixed frequency, the emission intensity of EL device increased with the increase of applied voltage, and the luminance of EL device based on Cl-BT increased sharply compared with that of EL device based on BT, which was very consistent with Fig. 5n, confirming that luminescent layer with CB-treated BT successfully improved the brightness of EL device.

The stretchability and luminous stability during mechanical cycles was important for the practical application of flexible EL devices. At a small tensile strain (50%), the luminance increased by 1.4% (Fig. 6a), which was mainly attributed to the decrease in the thickness of the luminescent layer increasing the electric field intensity. At higher tensile strains, the reduced luminance was because of the decrease of phosphor density. The luminance remained 88% at 200% strain, manifesting the high stretchability. The small residual strain and almost the same hysteresis loop area in 4 consecutive loading-unloading cycles illustrated that EL devices possessed high fatigue resistance (Supplementary Fig. 12). Importantly, the devices retained the elastic modulus (240 kPa) mimicking human skin, which helped to maintain conformal contact with human skin as a wearable device. After 400 continuous stretching-releasing cycles under 100% and 200% strain, the luminance maintained 85% and 89% of the initial values, respectively (Fig. 6b), indicating the long-term mechanical stability of prepared EL devices. The images showed a slight reduction in brightness when the device was stretched to 200% (Fig. 6c). When the strain restored to 0%, the brightness returned to the original level, further verifying the high stretchability and tensile stability of the EL devices. In addition, the luminous stability during the mechanical tensile cycles demonstrated the excellent interfacial binding of the EL devices.

a Luminance changes under different strains. b Luminance changes under 100% and 200% strain during 400 cycles. c Optical images of the EL device under periodical strain from 0% to 200%. d Photographs under the different states including twisting and cutting. e Luminance changes after 400 min of continuous operation. f Luminance changes after 60d. g Comprehensive performances of as-prepared EL device compared to other similar EL devices reported in the literatures.

During long-term practical application, EL devices inevitably needed to withstand bending, torsion and even local damage. Due to the high aspect ratio of the AgNWs, uniform dispersion of the AgNW-CNC II conductive layer, strong interaction between AgNW-CNC II and TX-PDMS, as well as tight binding between luminescent layer and electrodes, the assembled EL device could emit uniformly bright light when partially damaged (Fig. 6d). In addition, since the top and bottom electrodes were both transparent, a double-sided display could be realized. Such damage-resistant and double-sided luminescent EL devices were expected to open up the possibility of flexible double-sided displays.

The temporal stability was important for EL devices. After 400 min of continuous operation, the luminance reduced by 6% (Fig. 6e and Supplementary Fig. 13). Notably, the luminance barely changed after 60d of storage (Fig. 6f). This remarkable stability depended on the excellent chemical stability of the phosphor, the perfect sealing of the PDMS matrix for phosphor, and the strong interface between the luminescent layer and conductive layer. Compared with other similar EL devices reported in the literatures2,61,62,63,64,65,66,67, our device demonstrated excellent comprehensive performances (Fig. 6g and Supplementary Table 3), especially its stretchability and flexibility at broad temperatures, as well as stable underwater luminescence, which was expected to provide an ideal material platform for the next generation of optoelectronic devices under extreme environments.

Due to the good adhesive ability of TX-PDMS, the EL devices could be well adhered to the surfaces of glass and human skin with a bright display (Fig. 7a). Although the device placed on the human skin was driven by a high voltage, the excellent insulation of PDMS substrate ensured the safety of the process. In general, the safe voltage and current for the human body were 24 V and 20 mA, respectively68,69. Under 300 V and 400 Hz, the current passing through the EL device surface was only about 0.077 µA, indicating the feasibility of the device as a wearable device. The device attached to the round glass bottle exhibited a uniform luminescence, indicating the excellent adhesion to the curved surface. Stable light emission could be achieved at 70 °C and -20 °C, and the device remained flexible (Fig. 7b), demonstrating the excellent temperature tolerance of the device. Because the water repellency of PDMS, complete embedding of phosphor and Cl-BT within the PDMS matrix, as well as tight layer-to-layer bonding of the entire device, the EL device was waterproof with a stable underwater luminous capability (Fig. 7c). More importantly, the luminance of the device remained stable after being immersed into water for 120 min, illustrating the potential of the application in underwater display and light-based information transmission. A stretchable passive EL device array could be obtained by patterning the upper electrode into an array (Fig. 7d). The assembled EL array could be turned on effectively by applying bias voltage. Different array display could be realized by connecting different power supplies. Therefore, more complex patterns of multiple colors in practical applications could be displayed by controlling the electrode shape.

a Photographs of EL device adhered on glass plate, arm and glass bottle (scale bar: 1 cm). b Photographs of the device working at −20 °C and 70 °C (scale bar: 1 cm). c Luminance of the EL device in water. d Schematic diagram and photographs of EL device array (scale bar: 1 cm). e Cell viability assays using L-929 cells by MTT colorimetric assays. f Fluorescent staining images of L-929 cells incubated for 24 h with 100% EL device extract liquid. g SEM of the cell adhered to the EL device after 24 h in the culture.

To evaluate the biocompatibility and implantability of EL devices in human-machine interaction, the assembled EL devices were subjected to a cell viability assay. The cell survival rate of L-929 cells in different concentrations (25%, 50%, 75% and 100%) of the EL device extract after 24 h incubation was higher than 90% (Fig. 7e), indicating the ideal biocompatibility of the EL devices. After incubation with 100% extract for 24 h, the nuclei of living cells were stained with fluorescent dye solution, and an obvious green fluorescent signal was detected in L-929 cell (Fig. 7f). Cells on the device surface exhibited a broader morphological distribution (Fig. 7g). The ability of cells to attach and grow on the device surface was mainly attributed to the addition of surfactants TX to improve the hydrophilicity of the PDMS surface. The results showed that the biocompatible EL devices could be potentially used for biomedical, wearable and implantable electronic devices.

In summary, a sandwich-structured EL device based on two AgNW-CNC II/TX-PDMS electrodes and Cl-BT/Phosphor/PDMS luminescent layer was successfully developed. CNC II served as a dispersing agent, antioxidant and film-former could effectively disperse AgNWs to obtain a uniform conductive network on TX-PDMS electrode substrate. The introduction of surfactants TX not only endowed the EL devices with high stretchability, surface compliance and self-adhesion, but also interacted with CNC II to reduce the surface roughness of electrodes. PDMS served as a dielectric material provided the EL devices with flexibility, stretchability, fatigue resistance, as well as robust interface between luminescent layer and electrode layer. The optimized AgNW-CNC II/TX-PDMS electrode demonstrated a slick surface, favorable conductivity, high transparency, recoverability and durability. The introduction of Cl-BT in luminescent layer improved the dielectric constant, thereby enhancing the brightness of the device. The assembled EL devices with high stretchability, long-term stability, damage resistance, biocompatibility could work normally underwater and over a wide temperature range (-20 ~ 70 °C). This work was projected to provide guidance for the future preparation of wearable displays, especially for applications under extreme environments.

Polydimethylsiloxane (PDMS, Sylgard 184) was obtained from Dow Corning. Triton X-100 (TX, C14H22O(C2H4O)n) and chlorobenzene (CB) were purchased from Aladdin (Shanghai, China). Silver nanowires (AgNWs) were brought from Gu’s New Materials (Suzhou, China). Barium titanate (BaTiO3, 3 ~ 5 μm) and phosphor particles (ZnS: Cu, ZnS: Cu, Al and ZnS: Cu, Mn) were obtained from Keyan Photoelectric Technology (Shanghai, China). Bleached wood pulp (W-50 grade) was bought from Nippon Paper Chemicals (Tokyo, Japan). Sodium hydroxide (NaOH) was obtained from Sinopharm Chemical Reagent (China). Sulfuric acid (H2SO4) was purchased from Nanjing Chemical Reagent (China).

TX-PDMS liquid was prepared by mixing TX (with mass ratios of 0.2, 0.4, 0.6 and 0.8 wt% to PDMS) with PDMS precursor (the ratio of “base” to “curing agent” was 10:1, w/w). Then the TX-PDMS liquid was coated on the glass plate and placed in an oven at 45 °C for 24 h to obtain TX-PDMS films. The TX-PDMS films based on the loading level of TX were labeled as 0.2% TX-PDMS, 0.4% TX-PDMS, 0.6% TX-PDMS and 0.8% TX-PDMS, respectively.

CNC II was prepared according to our previous research70. Specific steps were provided in the Supplementary Note 1. The CNC II aqueous suspension was mixed with the AgNWs suspension (the mass ratio of CNC II:AgNWs was 15%) for sonication to obtain uniform AgNW-CNC II suspension. Then, 1.0 mL of the AgNW-CNC II suspension was uniformly sprayed on the TX-PDMS film using an airbrush. To accelerate the evaporation of the water, the TX-PDMS film was placed on a heating stage at 45 °C. Finally, a uniform AgNW-CNC II/TX-PDMS electrode was obtained. The AgNW-CNC II/TX-PDMS based on the loading level of TX were labeled as AgNW-CNC II/TX0.2%-PDMS, AgNW-CNC II/TX0.4%-PDMS, and AgNW-CNC II/TX0.6%-PDMS, respectively.

0.6 g of BaTiO3 particles were dispersed in 10.0 mL of CB, followed by magnetic stirring and sonication to obtain a uniform BaTiO3-CB (Cl-BT) mixture. 0.5 g of PDMS prepolymer was added into the Cl-BT and stirred vigorously. 1.0 g of phosphor powder was then added and stirred until viscous. Subsequently, 0.05 g of PDMS curing agent was added and uniformly mixed to prepare a high dielectric constant luminescent layer slurry. As a comparison, 0.6 g of BaTiO3 particles without CB treatment, 1 g of phosphor powder and 0.5 g of PDMS liquid (the mass ratio of prepolymer to curing agent was 2:1) were uniformly mixed at room temperature to prepare the luminescent layer slurry.

The luminescent layer paste was spin-coated on the AgNW-CNC II/TX-PDMS electrode and cured at 45 °C for 20 min. Subsequently, the conductive network of the lower electrode was drawn out with copper tape. The upper electrode was placed on the luminescent layer. The conductive network of the upper electrode was drawn out with a copper tape to obtain an EL device, which was then placed in an oven at 45 °C for 30 min to enhance the adhesion between the luminescent layer and the electrodes.

The chemical structures of the samples were analyzed using a fourier transform infrared spectrometer (FTIR, Nicolet iS10, Thermo Scientific Corp, USA). The optical transmittance was measured by UV-Vis spectrophotometer (TU-1810, China). The elements on the surface of the samples were determined by an X-ray photoelectron spectrometer (AXIS UltraDLD, UK). The contact angles were measured using the digital AST contact Angle system (UTM6502, China). The swelling ratio and gel fraction of the samples were tested using toluene as solvent. The initial weight of the sample was recorded as S0. The sample was immersed in a 20 mL glass bottle with toluene for 3 d and the weight of the sample was measured (Ss). Then, the swelling sample was placed in a plastic petri dish for 3 d to dry fully at room temperature, and the weight (Sd) was measured. The swelling ratio (Sw) and the gel fraction (Sg) were calculated according to Sw = Ss/S0, Sg = Sd/S0, respectively. Zeta potential values were measured using the Zetasizer Nano-Zs equipment (Malvern, Worcestershire, UK) with a concentration of 0.05 mg mL-1 and pH value of 6.8. The microstructures and morphologies were observed by field emission scanning electron microscopy (FE-SEM, JSM-7600F, JEOL Ltd, Japan) with an acceleration voltage of 5.0 kV. The film samples were immersed in liquid nitrogen and quenched prior to observation. The adhesion force was measured with a 90-degree peel strength tester (MK-BL-X, China). The rheometer (HAAKE RheoStress 600) was used for frequency scanning within the linear viscoelastic range of the samples. The storage modulus G’ and loss modulus G” were measured in the range of 0.1 ~ 10 Hz. The tensile properties were tested using a universal mechanical testing machine (TY-8000B, China). All the samples were tested five times and then averaged. A four-probe tester (ST-2258C, Suzhou Lattice Electronics Co., LTD., China) was used to test the resistance. The EL devices were driven by AC power (AN97000H, Aino Instruments, China). Photoluminescence spectra and photoluminescence quantum yield of ZnS phosphors were performed by transient steady-state fluorescence spectrometer (FluoroMax-4, China). The luminance was measured using a calibrated wideband optical luminance meter (TES-137, TES Electrical Electronic Corp, Taiwan). Phosphorescence spectra were measured with a spectral radiometer (CS-2000, Konica, Japan). Mouse fibroblast cells (L-929) were chosen as the cell model. The cell viability of the EL device was evaluated by an MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] colorimetric assays. Cell apoptosis assays were characterized by a fluorescence microscope (OLYMPUS BX51). The cellular state and morphology of the EL devices were observed by SEM (Quanta 200).

Data generated in this study are provided in the Main Text and the Supplementary Information. Additional data related to this paper are available from the corresponding author upon reasonable request.

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This study was funded by the National Key Research and Development Program of China (Grant No. 2023YFD2200504), the National Natural Science Foundation of China (Grant Nos. 32371803, 31901274), Independent Innovation Fund for Agricultural Science and Technology of Jiangsu Province (Grant No. CX(23)3059), Youth Top Talent Project of National Forestry and Grassland Administration of China (Grant No. 2023132002), 13th China Special Postdoctoral Science Foundation (Grant No. 2020T130303), China Postdoctoral Science Foundation (Grant No. 2019M661854) and Postdoctoral Science Foundation of Jiangsu Province (Grant No. 2019K142).

Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Joint International Research Lab of Lignocellulosic Functional Materials, College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, 210037, China

Ya Lu, Yuanyuan Chen, Haoyu Sun, Fang Deng, Changtong Mei, Xinwu Xu & Jingquan Han

School of Renewable Natural Resources, Louisiana State University, Baton Rouge, LA, 70803, USA

Qinglin Wu

Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, E3B 5A3, Canada

Huining Xiao

College of Biology and Environment, Nanjing Forestry University, Nanjing, 210037, China

Yiying Yue

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Y.L. and Y.C. conceived and designed the experiments. Y.C., H.S. and F.D. carried out material preparations, device fabrications and characterizations. Q.W. and H.X. assisted with data acquisition. C.M., Y.Y. and X.X. aided with fabrication and test. Y.L. wrote the original manuscript. J.H. conceived the idea, supervised the project and polished the manuscript. All authors discussed the results and commented on the manuscript.

Correspondence to Jingquan Han.

The authors declare no competing interests.

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Lu, Y., Chen, Y., Sun, H. et al. Resilient, environment tolerant and biocompatible electroluminescent devices with enhanced luminance based on compliant and self-adhesive electrodes. npj Flex Electron 8, 37 (2024). https://doi.org/10.1038/s41528-024-00322-2

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Received: 10 February 2024

Accepted: 05 June 2024

Published: 15 June 2024

DOI: https://doi.org/10.1038/s41528-024-00322-2

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