Toward Near-Foldable Surface Light Sources with Ultimate Efficiency: Ultrathin Substrates Embedded with Micron-Scale Inverted Lens Arrays

Foldable organic light-emitting diodes (OLEDs) are essential building blocks for portable devices with expandable screens as well as more futuristic systems such as wearable or body-attachable electronic devices. Although various approaches have been proposed to realize foldable OLEDs, the efficiency enhancement techniques developed for their rigid counterparts are not always applicable due to the strict thickness limitations, making it challenging to achieve very high efficiency in a foldable OLED. Here, we propose ultrathin substrates embedded with an inverted microlens array (IMLA) as a platform on which to design and realize OLEDs that can be bent at sub-100 μm bending radius while also exhibiting very high light outcoupling efficiency. By noting the periodic arrangement of the patterns in the IMLA, the potential effects of optical diffraction on the overall emission pattern and efficiency enhancement are carefully analyzed by incorporating a bidirectional scattering distribution function via a trans-scale approach. Neutral-plane engineering is also done with finite element method simulations that examine the effect of the IMLA structures on the local modulation of the strain and stress in ultrathin devices, where the feature size of the IMLA is comparable to the overall thickness of the whole device. With the proposed method, highly efficient foldable OLEDs are demonstrated that show the maximum external quantum efficiency to be as high as 58% without optical side effects and that can withstand 10,000 trials of repeated folding cyclic tests at a bending radius of 50 μm.