Splitting of the mechanical neutral plane is a promising concept for foldable displays because it reduces the folding stress in each layer of the display. We verified the splitting concept experimentally and revealed a linear relation between the relative position of the neutral plane and the logarithm of the adhesive's elastic modulus. As the modulus decreased, the position of the neutral plane approached that of perfect splitting. On the basis of the neutral‐plane splitting concept, we developed 5.5‐inch full high‐definition foldable active matrix organic light‐emitting diode (AMOLED) displays, which endured 150 k inward folding cycles and 150 k outward folding cycles with folding radii of 3 and 5 mm, respectively. This study is expected to improve the flexibility of designing foldable AMOLED displays, enabling better balance of the portability versus practicality trade‐off in mobile displays.
Neutral-plane splitting is essential for the durability and the usability of foldable displays. In this study, we use folding stiffness measurements and finite element method simulations to establish the relationship between split neutral planes' positions and folding stiffness. Based on the relationship, the folding stiffness is predicted from the split neutral-planes position.
Splitting of neutral plane is a promising concept for foldable displays because of reducing folding stress in each layer composing them. We verified the concept experimentally and developed a 5.5‐inch full HD foldable AMOLED display, which endured 150k inward folding cycles with a radius of 3 mm.
SUMMARYNowadays, semiconductor quantum dots have attracted intense attention as emissive materials for light-emitting diodes, due to their high photoluminescence quantum yield and the controllability of their photoluminescence spectrum by changing the core diameter. In general, semiconductor quantum dots contain large amounts of organic ligands around the core/shell structure to obtain dispersibility in solution, which leads to solution processability of the semiconductor quantum dot. Furthermore, organic ligands, such as straight alkyl chains, are generally insulating materials, which affects the carrier transport in thin-film light-emitting diodes. However, a detailed investigation has not been performed yet. In this paper, we investigated the luminance characteristics of quantum-dot lightemitting diodes containing ZnCuInS 2 quantum dots with different carbon chain lengths of alkyl thiol ligands as emitting layers. By evaluating the CH 2 /CH 3 ratio from Fourier-transform infrared spectra and thermal analysis, it was found that approximately half of the oleylamine ligands were converted to alkyl thiol ligands, and the evaporation temperature increased with increasing carbon chain length of the alkyl thiol ligands based on thermogravimetric analysis. However, the photoluminescence quantum yield and the spectral shape were almost the same, even after the ligand-exchange process from the oleylamine ligand to the alkyl thiol ligand. The peak wavelength of the photoluminescence spectra and the photoluminescence quantum yield were approximately 610 nm and 10%, respectively, for all samples. In addition, the surface morphology of spin coated ZnCuInS 2 quantum-dot layers did not change after the ligand-exchange process, and the root-mean-square roughness was around 1 nm. Finally, the luminance efficiency of an inverted device structure increased with decreasing carbon chain length of the alkyl thiol ligands, which were connected around the ZnCuInS 2 quantum dots. The maximum luminance and current efficiency were 86 cd/m 2 and 0.083 cd/A, respectively. key words: semiconductor quantum dot, quantum dot light-emitting diode, ligand exchange, alkyl thiol
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