(A) study on the improvement of efficiency and flexibility of foldable organic light emitting diode

Abstract

With the emergence of foldable smartphones, interest in folding technology has recently become very high. In particular, researches for smart display applications requiring wide screen and portability are being actively carried out. However, the brittle nature of the various materials used in displays remains an obstacle to the implementation of the foldable OLEDs. In this study, we propose a device structure that maintains operability even under repetitive folding with a radius of curvature of less than 1mm using an oxide-based transparent electrode and an encapsulation layer also known to be brittle. For this purpose, we have used an ultra-thin substrate with a thickness of 6 $\mu$ m, and found out that the effect of neutral surface movement caused by the OLEDs elements in the ultra-thin substrate is not negligible. The difference of the neutral plane movement according to the substrate thickness is explained through the finite element analysis simulation. The actual device structure is designed using substrates with different thickness based on the simulation result. Finally, the effect of the neutral plane movement on the thin substrate is verified by evaluating the lifetime of the device according to the presence of bending. In addition, some studies have focused only on the mechanical stability of the device when designing the foldable OLEDs, being limited in terms of efficiency. In particular, foldable OLEDs using very thin substrates have limitations in applying various internal/external structures to increase efficiency. In this study, we explore how to implement an outcoupling structure that is compatible with ultrathin, highly flexible architecture. The local modulation in the neutral plane due to the inclusion of outcoupling structure is carefully investigated through FEM simulation. Furthermore, we analyze the effect of optical diffraction resulting from the outcoupling structure by properly applying geometric and wave optics simulation method in a trans-scale manner. In the case of the final device, the current efficiency turns out to be nearly twice as high as that of the reference device, and the EQE value is more than 50%. By additionally coating the upper optical resin layer, the sandwich structure is created to achieve a highly efficient foldable OLEDs without any change in device performance even after 10,000 repetitions of fully-folding (50 $\mu$ m) bending tests. We believe the results and approach presented in this study will be a good example for designing a high efficiency foldable OLEDs or any emerging thin-film light sources such as perovskite or quantum-dot LEDs.