Hole injection from an ITO|PEDT anode into the hole transporting layer of an OLED probed by bias induced absorption

Poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonic acid) (PEDT:PSS) is commonly used as buffer layer between indium tin oxide anode and the emitting layer in organic light emitting diodes. To understand the beneficial effect of PEDT:PSS to the device performance, the interface between the buffer layer and a hole transport layer (HTL), i.e. 1,3,5-tris-(N,N-bis(4,5-methoxy-phenyl)aminophenyl)benzene (TDAPB), has been investigated by spectroscopical means. The number of radical cations in the HTL has been monitored quantitatively by bias induced absorption measurements as a function of voltage. The results are discussed in terms of (a) chemical interaction between TDAPB and PEDT:PSS and (b) charge accumulation at the interface of TDAPB/tris(8-hydroxyquinolinato)aluminium (Alq(3)). The in situ formation of TDAPB(+) after deposition is believed to be the reason for ohmic contacts at the interface, improving hole injection into the HTL. (C) 2003 Elsevier B.V. All rights reserved.