Interfacial charges in organic hetero-layer light emitting diodes probed by capacitance-voltage measurements

The bias dependent capacitance of organic hetero-layer light emitting diodes (LEDs) based on N,N'-diphenyl-N,N'-bis(1-naphtyl)-1,1-biphenyl-4,4 diamine (NPB) and tris(8-hydroxyquinoline) aluminium (Alq(3)) shows below the built-in voltage, a step-like change from a value corresponding to the total organic layer thickness to a higher value given by the Alq(3) layer thickness. The bias and frequency dependent behaviour of the capacitance can be explained by the presence of negative charges with a density of -6 x 10(11) e/cm(2) at the NPB-Alq(3) interface. This leads to an inhomogeneous potential distribution inside the device with a discontinuity of the electric field at the organic-organic interface. (C) 2001 Elsevier Science B.V. All rights reserved.