Using focused ion beam and scanning electron microscopy, we investigated the effect of particulate contaminants on current leakage defects in organic light-emitting diode devices ( 200 nm thick organic layer ). Flaked particles from sputtering targets and resist-related particles were investigated as examples of particulate contaminants. Three types of defects were identified corresponding to the dimensions and locations of the particulate contaminants. Large particles ( > 3 mu m in height) on the substrate formed short-circuit-type defects. They prevented an organic layer from being deposited on the anode, resulting in localized exposure of the anode surface. The cathode contacted the anode at these locations. Small particles (0.5- 1.0 mu m in height and 1.0- 3.0 mu m in length ) on the substrate caused localized thinning at the places where the organic layer was deposited on the particles. This localized thinning of the organic layer apparently increased the Fowler - Nordheim tunneling current and the resultant avalanche multiplication, leading to leakage current. Very minute particles of lithium-oxide (Li2O) (0.03- 0.05 mu m in diameter ) in the cathode buffer layer caused current leakage. The local electric field concentration due to these minute particles and the resultant carrier injection is the possible cause of current leakage. (c) 2007 The Electrochemical Society.