For efficient electron injection,
a method to control the work
functions (WFs) of ZnO electrodes in organic light-emitting devices
(OLEDs) is reported in this study. First, ZnO was modified by doping
of tetraalkylammonium salts (TRAX) into polyethylenimine ethoxylated
(PEIE) for the WF control. Tetrabutylammonium salts (TBAX), where
X = chloride, bromide, iodide, acetate, thiocyanate, and tetrafluoroborate
anions, were doped into PEIE. A WF of nondoped PEIE-modified ZnO was
3.65 eV, whereas TBAX-doped PEIE-modified ZnO exhibited WFs ranging
from 3.52 to 3.00 eV depending on the anion. TBAX salts exhibited
different electron-donating capabilities depending on the anion, and
the doping of TBAX with a large electron-donating capability exhibited
a large WF reduction effect. In addition, tetraethyl- and tetrahexylammonium
chlorides were doped into PEIE. PEIE doped with TRACl containing long
alkyl chains exhibited a large WF reduction effect due to its low
electron-accepting capabilities. In addition, the WF reduction mechanism
was considered by the depth direction analysis of the PEIE:TBAX films.
Finally, the ZnO/PEIE:TRAX bilayers were applied as electron injection
layers in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]
emissive-layer-based OLEDs with an inverted structure. The ZnO/PEIE:TBAX
devices with low WFs exhibited low driving voltages.