Charge generation layer in stacked organic light-emitting devices

Abstract

Three types of organic-based connection units were examined for use in stacked or tandem organic light-emitting devices, which include (i) Mg-doped tris(8-hydrooxyquinoline) aluminum(III) (Al q3) 4, 4′, 4″ -tris{ N,-(3-methylphenyl)- N -phenylamino}-triphenylamine (m -MTDATA), (ii) Al q3 /tetrafluorotetracyanoquinodimethane (F4-TCNQ)-doped m -MTDATA, and (iii) Mg-doped Al q3 /F4-TCNQ-doped m -MTDATA. Device (iii) shows the highest current efficiency and the differences in device performance can be correlated with the electronic structure of the connection unit and its interface with the neighboring active layers. The working mechanisms of the connection-unit works are discussed in terms of band bending and charge carrier density. The electronic structures of the interface between layers in a connection unit are of particular importance to the device performance. Dopings of Mg in Al q3 and F4-TCNQ in m -MTDATA led to bipolar heterojunction. Removal of either the n -type or the p -type dopants suppresses the band bending and the formation of space charge regions. The charge density accumulated at this interface estimated from Poisson's equation is 1018 cm3, which is respectively 12 and 6 orders of magnitude higher than that in the Mg:Al q3 / m -MTDATA and Al q3 /F4-TCNQ: m -MTDATA connection units. Based on these results, the critical roles of dopants in an efficient connecting unit for stacked organic light-emitting diodes are elucidated. © 2008 American Institute of Physics.