Mechanically enhanced hybrid nano-stratified barrier with defect suppression mechanism for highly reliable flexible OLEDs
Understanding the mechanical behaviors of encapsulation barriers under bending stress is important when fabricating flexible organic light-emitting diodes (FOLEDs). The enhanced mechanical characteristics of a nano-stratified barrier were analyzed based on the defect suppression mechanism, and then experimentally demonstrated. Following the Griffith model, naturally-occurring cracks, which were caused by Zn etching at the interface of nano-stratified structure, can curb the propagation of defects. Cross section images after bending tests provided remarkable evidence to support the existence of a defect suppression mechanism. Many visible cracks were found in a single Al2O3 layer, but not in the nano-stratified structure, due to the mechanism. The nano-stratified structure also enhanced the barrier’s physical properties by changing the crystalline phase of ZnO. In addition, experimental results demonstrated the effect of the mechanism in various ways. The nano-stratified barrier maintained a low water vapor transmission rate after 1,000 iterations of a 1 cm bending radius test. Using this mechanically enhanced hybrid nano-stratified barrier, FOLEDs were successfully encapsulated without losing mechanical or electrical performance. Finally, comparative lifetime measurements were conducted to determine reliability. After 2000 hours of constant current driving and 1,000 iterations with 1 cm bending radius, the FOLEDs retained 52.37% of their initial luminance, which is comparable to glass-lid encapsulation, with 55.96% retention. Herein, we report a mechanically enhanced encapsulation technology for FOLEDs using nano-stratified structure with a defect suppression mechanism.