  The performance of solution-processed organic light emitting diodes (OLEDs)
is often limited by non-uniform contacts. In this work, we introduce
Ni-containing solution-processed metal oxide (MO) interfacial layers inserted
between indium tin oxide (ITO) and
poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) to improve
the bottom electrode contact for OLEDs using the poly(p-phenylene vinylene)
(PPV) derivative Super-Yellow (SY) as an emission layer. For ITO/Ni-containing
MO/PEDOT:PSS bottom electrode structures we show enhanced wetting properties
that result in an improved OLED device efficiency. Best performance is achieved
using a Cu-Li co-doped spinel nickel cobaltite [(Cu-Li):NiCo2O4], for which the
current efficiency and luminous efficacy of SY OLEDs increased, respectively,
by 12% and 11% from the values obtained for standard devices without a
Ni-containing MO interface modification between ITO and PEDOT:PSS. The enhanced
performance was attributed to the improved morphology of PEDOT:PSS, which
consequently increased the hole injection capability of the optimized
ITO/(Cu-Li):NiCo2O4/PEDOT:PSS electrode.
