Stacked metal cathode for high-contrast-ratio polymeric
light-emitting devices
S. H. Li, H. Liem, C. W. Chen, E. H. Wu, Z. Xu, and Y. Yang
a͒
Department of Materials Science and Engineering, University of California at Los Angeles,
Los Angeles, California 90095
͑Received 13 September 2004; accepted 16 February 2005; published online 1 April 2005͒
An extremely high optical absorbing film made of alternating aluminum–silver layers was used as
cathode in polymeric light-emitting devices ͑PLEDs͒. Physical properties of the cathodes were
characterized by I–V measurement, atomic force microscopy, and x-ray photoemission
spectroscopy. As a result of the slow evaporation rate, each pair of the aluminum–silver layer was
shown to be in the form of aluminum–aluminum oxide nanoclusters embedded in an amorphous
charge conducting network of silver. The nanoclusters helped to absorb and scatter the ambient light
effectively. The use of four alternating layers structure in conventional PLEDs demonstrated 126%
enhancement of contrast under 1000 lx ambient illumination. The I–V characteristics of the black
cathode PLEDs remained intact when compared with reference PLEDs. This technology offers
precise control of the cathode quality in terms of its reflectivity and conductivity. © 2005 American
Institute of Physics. ͓DOI: 10.1063/1.1897440͔
The fabrication of polymeric light-emitting diodes
͑PLEDs͒ with high brightness and efficiency has been shown
feasible during the last decade and some device prototypes
are being considered for market entry. An additional and pos-
sibly more important advantage is the availability of a broad
range of substrates and device structures which can be used
to further enhance the device performance. The cathode
evaporated onto the device is usually a low work function
metal or metal alloy, such as aluminum ͑Al͒, magnesium–
silver ͑Mg–Ag͒, lithium–aluminum ͑Li–Al͒, and lithium
fluoride–aluminum ͑LiF–Al͒ to reduce operating voltage and
enhance device performance.
1–3
The ambient light, however,
after transmitting through the polymer layer, will be reflected
by the metal cathode and coupled out of the device through
the ITO coated substrate. As a result, the contrast ratio of
PLEDs will be significantly reduced by reflected ambient
light, and thus prohibiting the PLEDs for outdoor applica-
tions. There are many attempts to enhance the contrast ratio
by inserting an absorption and conductive inorganic layer
between the polymer layer and the cathode.
4,5
However, the
need of sputtering process for deposition brings about dam-
age on the polymer layer. Recently, a black cathode consist-
ing of a silicon monoxide doped Al film and an overlaid Al
layer has been developed for organic light-emitting diodes,
and exhibited superior device performances.
6
However, in
practice, the proposed technique causes the black conductive
films porous even with precise control of the Al–silicon mon-
oxide ratio during coevaporation. In this letter we demon-
strate an evaporation technique and cathode structure, with
primary focus being on the physical properties of Al–Al ox-
ide nanoclusters which forms the essential element for the
unique light absorbing ability. Since only one more type of
metal is introduced in the material system, this technology
requires no modification of the existing evaporation system.
The inset of Fig. 1͑a͒ illustrates the black cathode
device with a structure of glass/ITO/poly͑3,4-ethylene-
dioxythiophene͒͑500 Å͒/green fluorine-containing copoly-
mer ͑5BTF8͒͑800 Å͒/calcium͑2͒ acetylacetonate
͓͑Ca͑acac͒2͔͒/ ϫ4Al͑40 Å͒–Ag ͑40 Å͒/Ag ͑280 Å͒. The
readers are referred to Ref. 7 for fabrication details. It will be
shown later that each pair of the Al–Ag layer is used to
provide sufficient absorption of the ambient illumination,
while maintaining good electron injection to the polymeric
layer. The merit of using black cathode for PLEDs was com-
a͒
Electronic mail: yangy@ucla.edu
FIG. 1. ͑a͒ Electroluminescence of the black cathode PLEDs and the control
device. Inset shows the device configuration of the black cathode PLEDs ͑M
is either Al or Ag͒ and ͑b͒ contrast ratio of the black cathode PLEDs and the
control device under different ambient illumination. The inset shows the
current–voltage characteristics of the PLEDs.
APPLIED PHYSICS LETTERS 86, 143514 ͑2005͒
0003-6951/2005/86͑14͒/143514/3/$22.50 © 2005 American Institute of Physics86, 143514-1