03-10-2012, 05:47 PM
Using Integrated Optical Feedback to Counter Pixel Aging and Stabilize Light Output of Organic LED Display Technology
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INTRODUCTION
ORGANIC light-emitting devices (OLEDs) are brightly
emissive, efficient, color-tunable, planar light sources,
with fast response times, identifying them as a viable contender
for a dominant emissive flat-panel display technology. The past
20 years of OLED technology development has demonstrated
steady improvement of device efficiency and emissive pixel
stability [1], with the external power conversion efficiency of
many OLED structures exceeding the efficiency of the best
liquid crystal display technologies that presently dominate the
flat-panel display market. However, the evolution of OLED
displays as the dominant large-area display technology has been
hampered by the critical challenge of pixel-to-pixel stability.
Individual OLED pixel performance is affected by the quality
of environmental encapsulation, operating temperature change,
device thickness uniformity, and previous operating conditions,
which are all manifested in pixel-to-pixel nonuniformity.
OLED DISPLAY LIFETIME
Stability of pixel brightness in information displays is necessitated
by the high sensitivity of human vision to brightness variation
[2], with an average human eye capable of distinguishing
a 2% difference in relative intensity of neighboring pixels [3].
The conventionally stated metric of OLED half-life, , which
measures 50% change in OLED pixel brightness at constant current
driving conditions, is therefore unsuitable for evaluating viability
of particular OLED technology in information displays
that contain large numbers of neighboring pixels that degrade
different amounts according to usage. Consequently, within the
present work, we define the OLED display lifetime as either
, or , the time it takes an OLED pixel to degrade
to 98%, 97%, or 95% of its initial brightness, respectively, when
operated under constant current conditions. Among these,
is the most conservative estimate on display lifetime, as it considers
the extreme scenario in which one pixel is continually
aged to 98% of initial brightness while its neighbor is not operated.
Nevertheless, the and times are useful in evaluating
viability of individual OLED pixels in displays with simultaneous,
although nonuniform, aging of neighboring pixels.
OPTICAL FEEDBACK SOLUTION
In one proven optical feedback scheme [13], [14], OLED
pixels are driven at a high constant current with a photo-transistor
circuit monitoring and integrating the light output, which
is then compared to the desired output luminance. When the
target output luminance is reached, the OLED is turned off. In
this scheme the OLED is operated at high brightness for some
fraction of the picture frame cycle, which is slowly extended as
the OLED pixel ages and loses efficiency. The useful display
operating lifetime is reached when an OLED pixel needs to stay
turned on for times longer than the entire frame cycle. Implementation
of the active matrix backplane for this optical feedback
scheme is compact and elegant, as each pixel has self-contained
optical monitoring. However, this scheme necessitates
OLED pixel operation at high constant currents, an operating
regime that reduces the quantum efficiency and power efficiency
of OLED pixels, and requires backplanes with transistors capable
of supplying the higher currents. Lower power efficiency
and higher current operation also raises the fraction of power
that contributes towards heating of the display, with higher temperatures
resulting in accelerated device degradation [15]. The
initial OLED driving conditions and display refresh rate ultimately
limit the achievable display lifetime in this feedback
scheme.
CONCLUSION
In this work, we emphasize that the lifetime for an OLED
display requires a different metric than the typically measured
OLED half-life. We estimate that if constant-current-driven
OLEDs are to be used in a display with 2% brightness accuracy
over 10 000 hours of operation, then the OLED half-life has to
be extended beyond 300 000 hours. Additionally, other sources
of operational instability such temperature-dependent current–
voltage characteristics, nonlinear drive-current dependent
device efficiencies, and variation in thicknesses and threshold
voltage can compromise the uniformity of an OLED display.
We suggest that an optical feedback is likely the only solution
to stabilize the light output of an OLED display. By using a
drive-current-correcting optical-feedback scheme, we expect
more than a tenfold display lifetime improvement, while aging
individual OLED pixels to the point of twofold or threefold
decrease in their luminescence efficiency. Using published data
on phosphorescent OLEDs, we conclude that the operating
lifetime of an OLED display can be extended close to the
OLED half-life using a current-correcting optical feedback
with less than a threefold increase in drive current.