Chapter 1. Introduction
1.4 Stretchable and transparent AgNW electrodes and device applications
1.4.1 Electroluminescence
Stretchable alternating-current electroluminescent (ACEL) devices are a new form of deformable electronics that are believed to be one of the most promising technologies for next- generation display and lighting applications. With the development of highly transparent and stretchable electrodes, the stretchable ACEL devices have been reported that can withstand large mechanical deformations such as bending, twisting, stretching, and folding. The stretchable ACEL devices also can be successfully applied to the soft and wearable electronics with excellent conformability on the arbitrary curvilinear surfaces. In accordance with recent progresses in the stretchable ACEL devices, the wide applications including bio-medical devices, photosensitive vision sensors, interactive displays, electronic skins, and robotics have been intensively researched. However, the degree of stretchability is still not enough to satisfy the high demand on mechanical requirements of stretchable ACEL devices.
Thus, the development of transparent and stretchable electrodes having high stretchability and stability is still required.
Compared to the light-emitting diodes consist of p-type and n-type layer with the complex structure of p-n junction at the interface, the ACEL devices have simple structure with a single emissive layer composed of dielectric and phosphors sandwiched between top and bottom electrodes without the specific requirements for energy band matching that facilitates their easy application in cost effective, large-scale, flexible, and stretchable displays. Besides, capability of frequent overturning of the applied electric field results in an exclusion of the charge accumulation. The operation mechanism of ACEL devices are based on the field-induced impact excitation of electrons for light emission. The injected charge in the phosphor layers are accelerated under high applied electric field, resulting in the impact excitation of the luminescent centers by high-energy electrons (hot electrons) and radiative relaxation of the luminescent centers in the phosphors.
The most widely used phosphor materials for the ACEL device are ZnS doped with different impurities such as Cu, Al, and Mn for different color emissions. The integration of phosphors with elastomeric dielectric materials and stretchable AgNW electrodes facilitates the fabrication of deformable EL devices that can bring a plethora of new technologies of smart displays in a near future.
However, the high operating voltage and low efficiency of the ACEL devices still limit the use of the ACEL device as the wearable and soft electronic devices. Recently, Wang and co-workers developed stretchable and self-deformable ACEL devices fabricated with simple all-solution processable method (Figure 1.14a).85 The spray-coated AgNW/PDMS stretchable electrodes and ZnS:Cu microparticles are used for the fabrication of stretchable ACEL devices. The stretchable ACEL device shows good mechanical compliance with a sustained performance under tensile strain up to 100% due to the well-
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maintained electrical conductivity from AgNW networks. In addition, fabricated ACEL device is used as self-deformable dielectric elastomer actuators (DEAs) showing the dynamic shape variable display.
The soft ACEL device achieved area strain of 55% at the actuation voltage of 5 kV. After that, the stretchable ACEL devices with waterproof function was demonstrated (Figure 1.14b).86 The ACEL devices consist of stretchable conductor that comprises AgNW on the PDMS substrate covered by polyurethane urea (PUU) film. Here, the strong hydrogen bonds between hydroxylated PDMS and PUU enable the water-resistance, mechanically stable, and stretchable device applications. The fabricated device can be elongated up to tensile strain of 150% and can work in the water over 30 min. Nowadays, high-k materials are introduced to improve the device efficiency by decreasing the working voltage of ACEL devices. The stretchable ACEL devices composed of top and bottom AgNW electrodes and the active layer with ZnS:Cu EL microparticles and barium titanate (BaTiO3) high-k material are reported (Figure 1.14c).87 The emissive matrix with high dielectric constant will focus the electric field to the phosphor microparticles, resulting in the enhanced light emission upon same applied voltages. The fabricated EL-BaTiO3-PDMS device shows much higher luminance of ~121 cd m-2 compared to the EL-PDMS device which shows luminance of ~16 cd m-2 at the same driven voltage and frequency of 250 V and 2.2 kHz. Further, the stretchable and foldable seven-segment display was demonstrated that can be used under indoor lighting conditions. More recently, bright stretchable EL devices based on AgNW electrode and high-k thermoplastic elastomer were fabricated for enhancing working efficiency (Figure 1.14d).88 Here, the EL device comprises the AgNW network embedded in high-k thermoplastic polyurethane (TPU) elastomer and phosphor EL layer with high-k dielectric matrix of e-poly- ((vinylidene fluoride)-hexafluoropropylene) (e-PVDF-HFP). The dielectric constant of e-PVDF-HFP is above 10 over the wide range of frequencies. The ACEL device with e-PVDF-HFP shows 192 cd m-2 with a drive voltage of 180 V and 2 kHz square wave. The development of AC-driven EL devices have provided the new perspectives for the realization of smart lighting and display technologies due to the simple structure and direct integration with AC power sources. Despite the remarkable progress in lowering the driving voltage and enhancing luminance, the stable and durable stretchable ACEL devices with extremely low power consumption should be researched for the future wearable and portable soft optoelectronic and electronic display applications.
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Figure 1.14. Stretchable ACEL devices based on AgNW electrodes. (a) Stretchable and self- deformable ACEL devices (J. Wang et al. Adv. Mater. 2015, 27, 2876). (b) Stretchable and waterproof EL devices (B. You et al. ACS Appl. Mater. Interfaces 2017, 9, 5486). (c) Bright stretchable ACEL devices based on high permittivity composites (F. Stauffer and K. Tybrandt Adv. Mater. 2016, 28, 7200).
(d) Bright stretchable EL devices based on AgNW electrodes and high-k thermoplastic elastomers (Y.
Zhou et al. ACS Appl. Mater. Interfaces 2018, 10, 44760).
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