Research Of Stretchable Organic Light-Emitting Devices Based On Wrinkled Structure

Display technology is an important way for humans to acquire and disseminate information,and the pursuit of high-quality displays has promoted the research and industrialization of display technology.With decades of development,displays have evolved from the earliest larger cathode ray tubes(CRTs)to flat-panel liquid crystal displays(LCDs)and organic light-emitting devices(OLEDs),which have gradually decreased in size and weight.Benefiting from the flexible nature of organic optoelectronic materials,bendable and rollable displays have been commercialized.While with the development of stretchable electronics,stretchable displays,as a promising future display technology,have shown broad application prospects in wearable devices,electronic skin and three-dimensional display and have become a hotspot in scientific research and industry.Light-emitting devices are the central part of displays and are the focus of research on stretchable displays.Various schemes have been developed to realize stretchable luminescent devices,including intrinsically stretchable devices based on elastic materials,stretchable arrays based on island-bridge structures and wrinkled devices based on ultra-thin flexible devices.Among them,the wrinkled stretchable OLEDs have compatible materials,device structures and preparation processes with traditional flat OLEDs.Thus,they have better practicability and stability and exhibit great potential for development.However,some problems still need to be solved in the present research on wrinkled stretchable OLEDs.In this thesis,we focus on the following three points:First,the negative effect of large-sized periodic wrinkled structures(with a period of several hundred microns)on display quality;Second,the negative effect of optical mode losses in OLEDs on the luminance efficiency;Third,when facing new applications such as transparency display,the preparation of highly transparent stretchable OLEDs and the analysis of the effects of tensile deformation on device transparency.In view of the above problems,this thesis focuses on the research of stretchable OLEDs based on wrinkled structures to minimize the size of the wrinkles,reduce the optical mode loss in the device and improve the transparency of the device by optimizing the preparation process and device structure design.The specific research work is summarized in the following three aspects:1.Stretchable OLEDs with a wrinkled period of 70μm were prepared to reduce the impact of wrinkles on display quality.Preparation of wrinkled stretchable devices requires complicated steps such as stripping,transfer and pasting,resulting in incomplete contact between ultrathin devices and elastic substrate.It will lead to irregular wrinkles with large size due to inhomogeneous stress.Aiming at this problem,we use a transfer-free process of depositing ultra-thin parylene substrate and OLED directly on the surface of pre-stretched PDMS elastic films.After releasing the tensile force,the compressive stress of PDMS led to the spontaneous formation of periodic wrinkled structures in the ultrathin OLED.Parylene films are tightly bonded to PDMS,thus making the interaction force between them uniform and void-free,which effectively inhibits the formation of defects and disordered wrinkles and improves the orderliness of wrinkles and device yields.According to the theoretical formula,the wrinkled size was tuned by changing parameters such as the thickness of the parylene film.The wrinkled structure with a period of 70μm,invisible to the human eyes,was finally obtained,which is the smallest period of ordered wrinkles reported to date for stretchable OLEDs.The stretchable OLED exhibited high luminance of 98700 cd/m~2and excellent current efficiency of 70.3 cd/A,comparable to that of the reference flat device.Meanwhile,the stretchable OLED maintained 90%of its initial luminance after2000 cycles of stretching at 20%strain,demonstrating its excellent cyclic stretching stability.After in-situ flexible encapsulation,the stretchable OLED can be exposed to air for 600 minutes without a significant decrease in luminance.2.The luminescent efficiency enhancement and stretchability of OLEDs are simultaneously achieved by utilizing random wrinkled structures.Due to the difference in thermal expansion coefficient between parylene and PDMS,the parylene film spontaneously form a randomly wrinkled structure during film deposition.The substrate with disordered wrinkles changes the propagation path of the light emitted from the OLED,causing broadband light scattering,which in turn can reduce the optical mode loss and improve the luminance efficiency of the OLED.After optimizing the wrinkled size,the luminance and current efficiency of the OLED based on disordered wrinkles were improved by 37%and 18%,respectively,compared to planar devices.At the same time,the micro-cavity effect and angular dependence of the device are suppressed.After 200 cycles of the stretch-release process at 0%to 5%tensile strain,the stretchable OLED can still maintain 40%of the initial luminance.3.Two dimensional(2D)stretchable transparent OLEDs based on bi-metallic transparent electrodes.By selecting suitable seed layer materials to modify the deposited surface and suppress island growth during metal deposition,ultrathin metal films(thickness<10 nm)with both high transmittance and conductivity were obtained as cathode and anode electrodes.The flexible transparent OLED showed 73%transmittance at 550 nm wavelength,and the two-side electrodes exhibited almost symmetrical luminescence properties with a sum of the current efficiency of 30.1 cd/A.The 2D stretchable transparent OLEDs were obtained by combining the ultrathin devices with transparent elastic tape.The device reached a maximum tensile degree of50%and still maintained 80%of its initial efficiency after 200 cycles of stretching.In summary,through systematic research and optimization of the preparation process and device structure of wrinkle-assisted stretchable OLEDs,we have achieved stretchable OLEDs with invisible wrinkles,high luminance efficiency and high transparency,making a beneficial exploration to promote the application and development of stretchable displays.