Improved band alignment for hole injection by an interfacial layer in organic light emitting devices

We demonstrate that a thin organic interfacial layer of 3,4,9,10 perylenete tracarboxylic dianhydride (PTCDA) can be utilized to improve the band align ment of N,N'-di-(3-methylphenyl)N,N'diphenyl-4,4'diaminobiphenyl (TPD) film s on [indium-tin-oxide (ITO)] (InSnO) substrates in, e.g., organic electrol uminescent devices. A photoemission study of the highest occupied molecular orbital (HOMO) and vacuum level position as a function of the organic over layer thickness reveals that due to chemisorptive bonding a thin PTCDA inte rlayer results in a reduced barrier between the Fermi level of ITO and the HOMO of TPD. Furthermore we detect a new molecular state 0.6 eV below the F ermi level at the PTCDA/ITO interface. Both effects are expected to improve the hole injection from the ITO anode into the TPD hole transport layer, e .g., in organic light emitting devices. (C) 2000 American Institute of Phys ics. [S0003-6951(00)05134-2].