Understanding Dark Spot Formation and Growth in Organic Light-Emitting Devices by Controlling Pinhole Size and Shape

Abstract

We report on our in-situ experimental observations of dark spots in organic light-emitting diodes using optical microscopy. Uniformly sized silica microparticles are used to intentionally create size- and shape-controllable pinholes on the cathode protective layer. Subsequently, the pinholes trigger the initial formation of dark spots, which we then monitor. Due to the use of particles of various diameters, we are able to linearly associate the growth rate with pinhole size. This allows us to estimate the original pinhole sizes that give rise to the dark spots and to study their distribution. Our studies verify that pinholes on the protective layer create pathways for water or oxygen diffusion, which controls the dark spot growth rate. The pinhole size dependence illustrates that the pinhole perimeter (not the area) determines the amount of water or oxygen diffusing into the diodes at a certain time.