Department of Materials Science and EngineeringWith increasing consumption of wearable devices, research on flexible displays used as light sources in flexible devices has been conducted in recent years. For realization of flexible, all components of light-emitting diodes (LEDs) must have flexibility. For this reason, studies on organic light-emitting diodes (OLEDs) with high flexibility characteristics are predominant. However, organic semiconductors have disadvantage such as poor color purity with broad full-width half maximum (FWHM) and low-charge carrier mobility. On the other hand, organic-inorganic hybrid perovskites have both characteristics of organic semiconductor materials such as high color purity with narrow full-width half maximum, high charge-carrier mobility and inorganic semiconductor materials such as low processing-cost based on solution process, easy band gap deformation.
Recently, many researches have been conducted to apply perovskite to flexible devices, which have the above advantages, and many flexible devices using perovskite have been developed. However, the study on the deformation characteristics of the developed perovskite flexible LEDs is mainly focused on whether the optical properties are maintained under the deformed state. If all the components deform within the elastic deformation range, they will return to their original state when the external force is removed. When a specific component is subjected to plastic deformation, interface separation phenomenon occurs, which causes a problem in driving the device. For the above reasons, it is important to improve the mechanical characteristics of a weakest material in order to improve the flexibility characteristics of the device. There is a lack of research on flexibility analysis based on mechanical properties.
Although research on flexible devices using perovskite has progressed through the above research. Organic materials react easily with moisture in air and degrade easily, so encapsulation performance of flexible devices must be improved at the same time. Encapsulation performance of flexible devices has been improved through the development of thin film encapsulation (TFE) materials with flexibility characteristic. However, unlike a rigid device using glass as a substrate, a flexible device uses a polymer-based substrate for its deformation characteristics, so that the substrate functions as a moisture infiltration path. Research on substrates for flexible devices with improved encapsulation performance is needed.
In this study, flexibility of perovskite LEDs was analyzed based on the mechanical properties of constituent materials of perovskite LEDs, and substrate for flexible devices with encapsulation properties was developed using silicon dioxide thin films.
The critical bending radius of the perovskite LED was set to be the point where the light intensity starts to decrease when repetitive bending deformation was applied at a specific bending radius. The elastic limit of constituent materials of perovskite LEDs was evaluated using a hole nanoindentation. In-situ micro tensile test was conducted to measure the exact elastic limit of perovskite, which was the weakest material among the constituent materials. The critical bend radius was analyzed using the measured perovskite elastic limit and the distance from the neutral plane of the LED. The critical bending radius based on mechanical properties was compared with the critical bending radius which decreased the actual efficiency.
The substrate for flexible devices with encapsulation performance was developed using silicon dioxide thin films and analyzed for deformation characteristics. Silicon dioxide thin films were fabricated using sol-gel process. The encapsulation performance of the fabricated silicon dioxide thin film was evaluated by commercialized equipment and accelerating exposure test experiment applied to actual devices. The mechanical properties of silicon dioxide were analyzed by in-situ micro tensile test, and repeated bending tests were performed to evaluate critical bending radius applied to a polymer substrate for flexible device.clos
