The Role of Defects in Single- and Multi-Layer Barriers for Flexible Electronics
Thin transparent layers on polymer films are used to drastically enhance the permeation barrier properties of polymer webs while at the same time maintaining the flexibility and optical transparency. Applications range from food packaging films to encapsulation films for solar cells or flexible electronics. Very low water vapor and oxygen transmission rates are required for the encapsulation of flexible organic electronic devices such as organic solar cells and organic light emitting diodes(OLED). The density and distribution of defects and particles on a barrier film surface is a critical aspect for using barrier films as substrate for or encapsulation of organic light emitting diodes. Not only the barrier layers need to have a low defect density, but also the substrate particles and defects need to be covered and planarized. One approach to low-defect permeation barrier films is the deposition of multi-layer stacks combining oxide permeation barrier layers with polymer planarization layers. This paper gives an overview about multi-layer permeation barrier technologies in comparison to common single layer techniques. The distribution and origin of defects in sputtered single layer barriers on different polymer substrates as well as the evolution of these defects in multi-layer barrier systems are investigated in detail. The focus thereby lies on a combination of sputtered barrier layers with a hybrid-polymer (ORMOCER®) interlayer applied from liquid phase. The influence of substrate polymer surface quality and processing steps on defects in single- and multilayers is characterized by using scanning electron microscopy, optical defect inspection and an optical calcium test. The relationship between the defect density and distribution and the water vapor permeability on different polymer substrate is investigated. Finally, multilayer barrier films are used as a transparent substrate for small molecule OLED devices to evaluate the effect of barrier layer defects in an application relevant environment also giving an outlook on current challenges on the topic of device encapsulation.