可撓性顯示器受彎曲及扭轉的應力分析

An investigation of bending and torsional mechanism for FOLED

李凱迪(Kai-Di Li)

指導教授 ： 張正憲

國立臺灣大學/工學院/應用力學研究所/碩士(2011年)

https://doi.org/10.6342/NTU.2011.10271

摘要

本文主要為探討五層結構的可撓性顯示器(FOLED)在受彎曲及受扭轉兩種負載下的力學行為分析。由於歷年來研究顯示FOLED的破壞主要是由硬膜ITO所控制，因此在理論分析上針對調整水氧阻隔層的材料參數、尺寸厚度以降低ITO上的應力負載並減少各個層間剪應力的差異性。然而一昧的降低ITO上的應力值反而會造成塑膠基板上的應力值超過降伏應力因而導致基板的永久變形，所以再納入塑膠基板的降伏應力做為考量因素，希望ITO與PET能同時達到破壞及降伏並做出最佳化設計，最後再加入基板的黏彈特性以有限元素模擬分析之。

由分析結果發現，FOLED在以降低ITO應力值的原則設計下，彎曲最佳化的水氧阻隔層厚度為18µm、扭轉最佳化的為17µm且其ITO上的應力值皆大概為無水氧阻隔層時的0.5倍。以彎曲至最小曲率半徑、扭轉至最大扭轉角所需最佳化水氧阻隔層厚度分別為2.13µm及2.14µm。以上兩種設計目標彎曲與扭轉最佳化所需的水氧阻隔層厚度均相當接近，結果顯示FOLED經由最佳化後能兼顧彎曲與扭轉兩種能力。考慮基板的黏彈特性，得知在長時間固定負載下由於材料鬆弛PET的應力值會隨著時間減小而ITO上的應力值會則愈來愈大。

關鍵字

FOLED ； 水氧阻隔層 ； ITO ； 有限元素法 ； 黏彈性 ； 彎曲應力 ； 剪應力

並列摘要

This paper focuses on investigating the mechanical mechanism of Flexible Organic Light-Emitting Diode (FOLED) under bending and twisting. In general, FOLED is composed of five layers, which involve cathode, OLED, ITO (anode), buffer, and PET layer. Based on previous researches, they have demonstrated that the failure of FOLED is primarily caused by the interfacial delamination in ITO. However, for making the FOLED, reducing the stress in ITO would cause the stress in PET higher than PET’s yielding stress, thus leading to a permanent deformation of PET. Therefore, the Yielding of PET should be taken into consideration for optimization design. Finally, we implement the finite element analysis based on the property of viscoelasticity for PET.

From our analysis, based on the concept of reducing the stress in ITO, the optimized material properties of buffer layer are summarized as follows: reduce bending stress in ITO 18µm ; reduce shear stress in ITO 17µm. On the other hand, according to the concept of bending to the smallest curvature of radius and twisting to the biggest twist angle of FOLED, the optimized thicknesses of buffer layer are 2.13µm and 2.14µm respectively. For the thickness of buffer layer, the above results in the bending cases are similar to the twisting cases. That means the optimization design of FOLED could have not only great bending but also twisting ability. Additionally, considering the property of viscoelasticity for PET, it is found that the stress in PET seems gradually decay due to long-term loading and in ITO would increase along with the time.

並列關鍵字

FOLED ； Buffer Layer ； ITO ； finite element ； viscoelasticity ； bending stress ； shear stress

參考文獻

[1] S. Timoshenko , "Theory of Plates & Shells, " 1940.

[2] P. H. Townsend, D. M. Barnett, and T. A. Brunner, "Elastic relationships in layered compositemedia with approximation for the case of thin-films on a thick substrate , "Journal of Applied Physics Vol. 62, Issue 11, pp.4438-4444, 1987.

[3] Z. Suo, E. Y. Ma, H. Gleskova, and S. Wagner, "mechanics of rollable and foldable film-on-foil electronics," Journal of Applied Physics, Vol. 74, pp.1177–1179, 1999.

[4] Chien-Jung Chiang, Chris Winscom, Steve Bull, Andy Monkman, "Mechanical modeling of flexible OLED devices, "Organic Electronics 10 (2009) 1268–1274.

[5] Y. Leterrier, L. Medico, F. Demarco, J.-AE Manson, P. Bouten, J. DeGoede, and J.A. Nairn, "Mechanical properties of transparent functional thin films for flexible display," 46th Annual Technical Conference Proceedings, 2003.