Research progress on stretchable organic light-emitting devices based on buckled structures

In recent years, with the development of flexible and wearable electronic devices, the demand for stretchable devices has been rising, which has promoted the rapid development of stretchable electronic devices. Stretchable display is an important part in stretchable electronic systems and plays the role of information transmission and human-machine interaction. The stretchable light-emitting device is one of the core components of stretchable displays. With the continuous development of materials, processes and device structure design, the research of stretchable light-emitting devices has been developed rapidly and the device performance has been significantly improved. A variety of strategies have been proposed to make light-emitting devices stretchable. Among them, stretchable organic light-emitting devices (SOLED) based on buckled structures have shown greater application potential in wearable electronic devices, bioelectronic skin, smart clothing and other fields due to their excellent optoelectronic performance and mechanical stretchability and have become one of the most promising candidates for stretchable displays. This paper reviews the research progress of SOLEDs based on buckled structures. Firstly, the formation mechanism of buckled structures and related theories are introduced. Common strategies for the preparation of buckled structures include prestretch-release of elastomer substrates, mechanical compression, solvent or heat-induced deformation and substrate molding, etc. Among them, prestretch-release process can obtain devices with large stretchability and is widely used for the preparation of SOLED with buckled structures. The theoretical model based on the bilayer structure can predict the wavelength and amplitude of the wrinkles. However, in practical applications, the buckled process and the final morphology of the wrinkles are more diverse and complex, which requires different analysis. In order to decrease the bending strain of the OLEDs with buckled structures, ultrathin and flexible films are the preferred substrates. Then the SOLEDs are classified into four types according to the stretch dimension and the morphology of the wrinkles. They are one-dimensional (1D) SOLEDs with random and ordered buckled structures and two-dimensional (2D) SOLEDs with random and ordered buckled structures. Random wrinkles can be easily formed by attaching the flexible OLEDs on a pre-stretched elastic substrate. The morphology of the wrinkles is not controllable. On the contrary, ordered wrinkles with controllable morphology need fine preparation process and structure design. Through the performance comparison, it can be seen that SOLEDs based on ordered wrinkles have better cyclic stretching stability, and 2D stretchable devices can better adapt to 3D surface than 1D stretchable devices. Therefore, 2D SOLEDs with ordered wrinkles have greater application potential. However, the stretchability, device performance and fabrication technology of 2D SOLEDs with ordered wrinkles cannot meet application requirements and need to be improved. Finally, some challenges and future prospects of SOLEDs with buckled structures are briefly discussed. SOLEDs based on buckled structures face some common problems. For example, the circular curved buckled structure makes the whole device appear uneven light emission, the process of pre-stretching of the elastic substrate restricts the preparation of large size devices, there is serious image distortion under tensile states, and the elastic substrate is slow to recover to the initial state after several stretching cycles. Therefore, it is important to develop the preparation technology of ordered wrinkles without pre-stretching, image distortion suppression strategy and pixel compensation strategy, and optimize the properties of elastic substrates.