The use of charge transfer interlayers to control hole injection in molecular organic light emitting diodes

In order to improve the performance of the indium-tin oxide (ITO) electrode frequently used as the anode in electroluminescent devices, we report its modification using ultrathin films of C-60 and 11, 11, 12,1 2-tetracyanonaphtho-2,6-quinodimethane (TNAP). In both cases the interaction between the film and the ITO substrate is found to shift the work function of the electrode, thereby modifying the barrier to hole injection in the model system ITO\TPD\Alq(3)\Al (where TPD is N,N',-bis(3-methylphenyl)-N,Aldiphenyl-1,1'-biphenyl-4,4'-diamine and Alq(3) is tris(quinolin-S-olato) aluminium). Scanning Kelvin probe measurements show kthat the ITO work function is increased by as much as 0.25 eV with a TNAP overlayer, whilst C-60 overlayers are found to reduce the work function by a comparable amount. The former has been attributed to a charge transfer effect, however, for C-60 overlayers the variation in the electrostatic potential across the inter-face cannot be attributed to charge transfer alone. The performance of devices incorporating these modified ITO electrodes are rationalised in terms of the work function modification, film thicknesses and the hole transport properties of the two films. (C) 2002 Elsevier Science B.V. All rights reserved.