DETERMINATION OF FRONTIER ORBITAL ALIGNMENT AND BAND BENDING AT AN ORGANIC SEMICONDUCTOR HETEROINTERFACE BY COMBINED X-RAY AND ULTRAVIOLET PHOTOEMISSION MEASUREMENTS

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq(3))/N,N'-di-(3-methylphenyl) -N,N' diphenyl-4,4' -diaminobiphenyl (TPD) heterojunction, used in org anic light-emitting diodes (OLED), was determined by growing a TPD lay er in several steps on a thick Alq(3) substrate layer. After each grow th step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy, The offset of the HOMO maxima at the inter face was determined to be -0.13 eV from Alq(3) to TPD. By including th e known HOMO-lowest occupied molecular orbital (LUMO) gaps for both mo lecules into the evaluation, the offset of the LUMO minima was determi ned to be -0.33 eV from Alq(3) to TPD. These values are consistent wit h previous assumptions that this interface represents a higher barrier for electron injection from Alq(3) to TPD than for hole injection fro m TPD to Alq(3). (C) 1998 American Institute of Physics, [S0003-6951 ( 98)03434-2].