Folding-stability criteria of thin-film encapsulation layers for foldable organic light-emitting diodes

We have investigated thin-film encapsulation layers that can be stable without the occurrence of cracks even when the layer is repeatedly folded with a small radius down to 1 mm. The folding stability criteria were also identified for the cases where the encapsulation layer deviates by a certain distance from the stress neutral plane inside a foldable device. An organic-inorganic multilayer structure having the inorganic layer thickness of 5 nm has always been shown to be stable against repeated folding with a radius of 1 mm even though the multilayer is located far from the stress neutral plane. In addition, the folding stability of the organic-inorganic multilayer encapsulation layer was evaluated when contaminant particles were present on the surface of foldable devices. To this end, the step coverage over the contaminant particles and the folding-stability criteria of the encapsulation structure were investigated. It was confirmed that the organic-inorganic multilayer structure was stable to repeated folding in a radius of 1 mm when the coating angle covering the particles was loosened to 140° or more through planarization and thin-film encapsulation.