The mechanical reliability design of thin-film encapsulation for laminated-based flexible organic light emitting diodes using semi-analytical method

The multi-layered flexible organic light emitting diodes (FOLEDs) are composed of a flexible substrate, organic/inorganic layers, and thin-film encapsulations (TFE), where the multi-barriers are useful to eliminate vapor and oxygen permeation and guarantee the long-term reliability of electronic devices. However, there will be defects (e.g., particle, pinhole, and delamination) in the manufacturing process, and the longitudinal tensile stress in the inner materials will result in cracks and crack propagations when the flexible electronic devices subject to transverse loads and bending moments. Therefore, it is worthy to evaluate the influence of internal stresses and different defects on reliability designs of FOLEDs. In this paper, the semi-analytical models are established to evaluate the mechanical behavior of the laminated-based FOLED devices. In material failure model, the displacement values of each layer combined with global interpolation functions are used to describe displacement filed of the entire structure, and the constitutive model of the FOLEDs is the superposition of each lamina’s constitutive equation. The governing equations of FOLEDs are derived through the principle of virtual displacements, and then the stress and strain distributions of FOLEDs along the thickness direction are obtained from displacement fields. This paper further investigates the fracture toughness and interfacial reliability of FOLEDs model with different defects using the strain energy release rate (SERR). The results are validated against those of the finite element analysis. Moreover, the computational efficiency and capability of development method are illustrated by comparing computing time of the finite element analysis. With fast semi-analytical method, the failure and fracture behaviors of thin-film encapsulation for laminated-based FOLEDs are estimated and discussed.

I. Introduction

The characteristics of fast response time, high resolution, high efficiency, and flexibility show that the flexible organic light emitting diode (FOLED) is a promising flat panel display technology [1], [2]. The FOLED consists of stacked lamina with different material properties, where the organic layers are sensitive to oxygen and water vapor, and the multi-barrier encapsulation are applied to protect electronic devices against environmental degradation. Thus many researchers have concentrated on the water vapor transmission rate (WVTR) of thin-film barriers [3], [4]. Nevertheless, It’s should be addressed that the mechanical characteristics of FOLEDs are also need be paid important attention for the reliability design [5], such as the material ultimate strength and fracture toughness. There are some cracks and defects that generate in thin-film encapsulation (TFE) during fabrication process, and those tiny cracks can propagate when the FOLEDs structure is under the bending-induced tensile stresses or strains. The bending behavior of laminated FOLEDs are often presented when the structures subjected to the transverse loads and bending moments.