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emaining following assembly.
Light extraction from the OLED device
is a primary concern for increasing OLED
panel efficiency and luminaire efficacy. In
conventional OLEDs, it is estimated that
only 20% of the generated light exits the
OLED due to refractive index mismatch 12 .
Films that redirect light between the
substrate and the electrode can be used to
enhance light output. Photonic crystals can
provide improved coupling to enable light
to exit the OLED.
OLED printing processes
SMT assembly makes use of various
printing and deposition processes in the
production of electronic products, e.g.,
solder paste screen printing, conformal
coating, conductive adhesive dispensing,
etc. Printing technologies for OLED
technology embrace a number of printing
technologies that move beyond the typical
SMT space. The ability to print many of
the materials that comprise an OLED SSL
device makes such fabrication a compelling
value proposition.
The OLED substrate will determine
whether R2R or sheet fed processes will
be feasible. Rigid glass substrate systems
require sheet fed processes, while flexible
substrates such as polymer and metal
foils can be used with either process.
While R2R printing produces long sheets
of OLED material the OLED devices
still need to be excised from the web.
Depending upon the manufacturing
flow and subsequent processing steps,
sheet fed printing that produces near-net
final shapes may be as cost effective as
R2R printing with its subsequent slitting
and excising steps. Engineering analysis
is required to determine the optimum
system.
OLED printing techniques can be
differentiated by the method they use to
feed the substrate to the printing process.
R2R processes print on long continuous
material films, while sheet fed printing
uses individual sheets in discrete sizes.
Relative to sheet fed processes, R2R
processes minimize the load/un-load time
between printing stations. However, sheet
fed processes can be made to mimic R2R
printing with inline techniques.
A simplified R2R process flow for
producing an OLED light source is
depicted in Figure 7 13 . In this schematic
diagram, the transparent substrate is
printed with the anode layer and then
successive layers of functional inks are
in-line printed to produce the OLED
structure. For white light production,
red, green and blue emissive layers can
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Figure 7. Schematic of a R2R OLED printing
process.
Figure 9. Left—Osram ORBEOS product. Right—Philips Lumiblade product.
be printed, but if a single light color is
required then only printing that ink would
be necessary. White light can also be
produced by printing a blue light emissive
layer and adding a phosphor layer that
converts the blue light to white light,
which is similar to crystalline LED SSL
practices. Within this basic process flow
numerous permutations are possible to
address cost effective manufacturing. The
encapsulation and completed module steps
will be discussed in later sections.
Gravure, flexographic and slot-die
coating processes can be used to print or
coat the inks to produce an OLED device.
An alternative to these printing processes
are inkjet printing processes. While the
first three processes require contract
between dispensing equipment and the
substrate, inkjet printing is a non-contact
process. Inkjet printing processes have
been developed for fabricating OLED
displays 14 , Figure 8. An OLED suitable
for SSL applications could be simpler to
produce since it would not require the
polysilicon TFT and the pixel sizes could
be larger.
Screen-printing technology (which
is an SMT staple) uses a masked screen
to determine the location where the ink
will be deposited. With both flat bed
and rotary machines available, it has
wide applicability because it can print
on many types of surfaces and substrates
Printed electronics for flexible solid-state lighting
Figure 8. Schematic of an inkjet OLED printing
process.
and is compatible with a wide range of
ink viscosities. It is suitable for printing
relatively thick layers to produce electrical
conductors and dielectric layers. These
are required: a) to produce interconnects
between the OLED light engine and power
sources, b) to generate printed bus bars to
bring electrical power to the OLED pixels,
c) to provide conductors for control lines
to offer unique features and d) to provide
insulating layers where appropriate. Bus
bars and interconnect conductors can be
formed from silver or carbon inks. Silver
inks have higher conductivity while carbon
is lower cost. Bus bars and interconnect
lines must be positioned so that they offer
the lowest shadowing effects.
OLED encapsulation processes
In an OLED SSL device, the OLED light
source generates the light (Assembly Level
1 and 2) while the light engine (Assembly
Level 2) enables it to couple to the
luminaire. Assembly Level 2 must provide
the light engine environmental protection
(e.g., humidity resistance, oxygen
resistance, etc.) and physical protection
(e.g., abrasion resistance, etc.). Assembly
Level 2 also provides the opportunity for
improving light extraction.
Printed OLEDs are thin structures
with electrodes on the top and bottom
surfaces. Since the separation distance
between the electrodes is small and the
Global SMT & Packaging Southeast Asia – Winter 2010 – 17