AbstractThe thin electron injection layers between the cathode and the light
emitting polymer layer in polymer light emitting diodes (PLEDs) have been
shown to have a big impact on the final device performance. Usually, in
PLEDs low work function metals like Ba, Mg or Ca are used to reduce the
energy barrier between the cathode and the polymer thus providing a better
electron injection from the cathode. Also salts like LiF, NaF,
Cs2CO3 and CsF have recently been shown to function
as electron injection layers in light emitting devices. From these,
especially caesium carbonate (Cs2CO3) results into
high efficiency diodes both as a solution processed electron injection layer
in PLEDs, as well as an n-dopant in the electron transport layer in vacuum
deposited small molecule based OLEDs. The functional mechanism of Cs2CO3 as
a pure interlayer is not yet fully understood. The proposed mechanisms
include the n-doping of the organic layer with Cs2CO3,
the thermal decomposition of Cs2CO3 and following
formation of caesium metal or the formation of an n-doped CsO2
layer. In this study the phenomena resulting from the combination of a
hole-dominant alkoxy-phenyl-substituted poly(phenylene vinylene) (PPV) based
light emitting polymer with a highly efficient electron injection layer of
Cs2CO3 in light emitting diodes has been
investigated. As a result, diodes with about 35 % higher efficiency were
achieved with PPV-Cs2CO3 structure in comparison to
the traditional PPV-Ba structure. Additionally to the increased efficiency,
also the lifetime of the Cs2CO3-diodes is comparable
to the Ba-diodes implying that the long-term stability of the diodes is not
affected by the optimized Cs2CO3-cathode. The strong
increase in the electron injection of the Cs2CO3
diodes is apparently caused by a highly conductive, n-doped layer resulting
from the charge transfer reaction between Cs2CO3 and
PPV, where the magnitude of the reaction and resulting effects strongly
depend on the amount of the applied Cs2CO3. The
conclusion of the n-doped layer can be drawn from the LIV, impedance and
photoluminescence measurements of the diodes with Ba and
Cs2CO3 cathodes before, during and after electrical
stressing.
Mechanism study on highly efficient polymer light-emitting diodes utilizing double-layered alkali halide electron injection layer
