Electroluminescence “phenomenon was first discovered in 1923 when Lossew was studying SiC geophone. In 1936, physicist Destriau discovered that the suspended medium containing trace copper impurities can emit visible light under alternating current field, so this luminescence phenomenon is also called Desriau effect. Because the transparent and conductive indium tin oxide technology was not mature at that time, the research progress in the next twenty years was very slow [1]. It was not until 1952 that Sylvania Electric Company showed the first electroluminescent phosphor screen at the MIT Solid State Conference [2]. In China, Beijing Institute of Physics and Changchun Institute of Physics first studied this field, and made a series of progress around luminous brightness, stability and multi-color [3].
For human wearable devices, it is often necessary to bear large-scale complex three-dimensional deformation [4]; In order to realize the stretchability of zinc sulfide electroluminescent devices, it is necessary to ensure that each layer of multilayer materials has stretchability and the stretching ratio needs to match each other. Among them, the transparent stretchable electrode as the light-transmitting layer is the research difficulty and hot spot at present.
The traditional electronic system will be fatigued, corroded or damaged in the process of frequent operation, and will deteriorate with the passage of time, which will lead to the failure of electronic equipment. Zinc sulfide electroluminescent devices have been widely integrated into flexible multi-functional electronic systems, playing a vital role as light-emitting components. The life and mechanical stability of this light-emitting device are often severely limited with local damage. In addition, due to the characteristics of high-frequency and high-voltage driving power supply of electroluminescent devices, when the strain on the light-emitting devices exceeds the mechanical endurance limit, it is easy to cause the local electric field to be too large and the light-emitting devices will be broken down and damaged [5].
The preparation of zinc sulfide electroluminescent devices matches the common preparation methods of flexible electronics devices, such as screen printing and inkjet printing, and has become a research hotspot in the field of electronic skin [6-8]. As a flexible display device, pixel resolution, waterproof and microcircuit manufacturing technology are the current difficulties.
Working principle of zinc sulfide electroluminescent device
Taking Cu2+-doped ZnS electroluminescent powder as an example, its luminescence principle is that by doping zinc sulfide crystal with metal ions, defects can be introduced into ZnS material, and then a shallow defect band can be formed, as shown in Figure 1. When AC voltage is applied to zinc sulfide electroluminescent powder, electrons are accelerated to become overheated electrons under the action of external electric field; Then impact the luminescent center of ZnS∶Cu2+ to excite or ionize it; Then the electrons in the luminescent center will produce light during the process of de-excitation or recombination. The structure of the luminescent center determines the formation of the emission spectrum. The electroluminescent powder of ZnS∶Cu2+ has two luminescent centers, namely, the green luminescent center (in the t2 state of Cu2+) and the blue luminescent center (in the E state of Cu2+), so its luminescent color is the result of the joint action of the two luminescent centers. The blue light-emitting center is closer to the valence band than the green light-emitting center, so the holes in the blue light-emitting center are easier to ionize under the action of electric field, that is, the hole life of the blue light-emitting center is shorter than that of the green light-emitting center. Therefore, under the action of low frequency electric field, ZnS∶Cu2+ electroluminescent powder is mainly green; However, under the action of high frequency electric field, it is mainly blue.
Generally speaking, zinc sulfide electroluminescent devices are divided into five layers, as shown in Figure 2, which are the back electrode, dielectric layer, luminescent layer, transparent electrode and transparent substrate from top to bottom. Among them, AC voltage is applied between the back electrode and the transparent electrode, and the dielectric layer is made of high dielectric constant material, such as barium titanate, which is mainly used to adjust the alternating electric field distribution between the light-emitting layer and the dielectric layer, enhance the electroluminescent brightness, prevent local avalanche breakdown and increase the luminous uniformity. Transparent electrodes and transparent plates are usually indium tin oxide-polyethylene terephthalate (ITO-PET) films, so that the luminescent layer can be radiated through the transparent layer [13-16]. In the practical application field, in order to realize the new application of zinc sulfide electroluminescent devices, the shape of the devices is often redesigned, such as electroluminescent fibers.
Research hotspot of EL devices in wearable field
In recent years, with the rapid development of flexible electronics industry, light-emitting display has become an indispensable part of wearable integrated devices. Zinc sulfide electroluminescent material has shown its unique application in the field of flexible wearable due to its advantages of simple structure, stable light emission and adjustable color, and has become a research hotspot in recent years, as shown in Figure 3. The research directions of flexible wearable devices based on zinc sulfide electroluminescent materials mainly include improving the flexibility and stretchability of photoelectric devices [19-20], endowing electrode materials and dielectric materials with self-healing [21-22], expanding the color diversity of electroluminescent devices and Multi-color)[23] switching [23]. At present, this zinc sulfide-based electroluminescent material is mainly used in new luminescent electronic skin.
As a traditional photoelectric material, zinc sulfide electroluminescent material has triggered a new round of research hotspot in the new wave of flexible electronics, which has attracted extensive attention from academia and industry. With the deepening of scientific research, stretchable electroluminescent devices have shown great application potential in stretchable display, lighting and biomedical fields, so they have received extensive attention and made rapid development. Compared with traditional electronic devices, stretchable electronic devices break through the limitation of rigid silicon substrate, and can keep normal function in the states of stretching, compression and bending. However, at present, the dielectric constant of the light-emitting layer of the stretchable device based on alternating current electroluminescence is low, and a high driving voltage is needed to achieve sufficient brightness. The luminous efficiency of zinc sulfide alternating current electroluminescent devices is not high, and there is still a gap compared with the mainstream organic electroluminescent devices such as OLED. In addition, due to the limitation of device manufacturing process, the current stretchable luminescent display can only realize simple pattern display. For AC electroluminescent devices, high dielectric materials can effectively enhance the excitation electric field at the electroluminescent particles and improve the luminous brightness of the devices. Based on the above reasons, the industrialization direction of zinc sulfide-based electroluminescent materials in flexible electronics still needs to be explored continuously.