OSRAM ORBEOS Components - Manual (Page 14)

14
RELIABILITY
4.2. Degradation
4.3. Lifetime
1.00
0.75
0.50
0.25
0
0 2,500 5,000 7,500 10,000 12,500 15,000 17,500
Operating time [h]
Relative luminance [L/L0]
Initial luminance = 50 % of standard
Standard initial luminance
Fig. 10. Luminance degradation characteristics of an OLED with two different initial
luminance values (normalized luminance values). The lower curve shows a drop to
50 % of the initial value after approximately 5,000 hours. By reducing the initial lu-
minance by a factor of 2, the time to reach the half value nearly triples to 14,000
hours (upper curve).
In parallel to the luminance decay, the driving voltage will in-
crease during operation in constant current mode. ORBEOS ®
OLEDs currently show a voltage increase of less than 1.0 V
after reaching 50 % of the initial luminance.
4.2.2. Ambient temperature
The luminance degradation is a process which requires acti-
vation energy. This means that for higher ambient temperature,
the decay rate is also higher. As another rule of thumb, it can
be estimated that a temperature increase of about 20 K cur-
rently leads to reduction of lifetime by approximately 50 %.
Since the OLED is sensitive to higher temperatures, a heat man-
agement system is recommended in order to keep operating
temperature low and the lifetime of the OLED high. Depending
on the individual integration level, e.g. in a well-insulated envi-
ronment, accumulation of heat needs to be avoided.
The major type of degradation during operation which can be
observed quite easily is the decay of luminance. Driven under
constant current conditions, the luminance of an OLED typi-
cally degrades according to a stretched exponential behavior,
meaning that the initial degradation rate is higher than the
degradation rate in the mid or later state of operation. The
luminance degradation effect is dependent on several side
parameters:
4.2.1. Absolute value of initial luminance
The higher the luminance of the OLED, the more charge car-
riers are transported through the device which can initiate
degradation mechanisms. Additionally, the internal OLED
temperature increases due to higher energy input resulting in
an accelerated degradation. Depending on the stack archi-
tecture, the effect can be more or less pronounced. As a rule
of thumb, it can be estimated that the lifetime can be tripled
when the initial luminance is decreased by 50 %. This is indi-
cated in fi gure 10 (above).
The luminance degradation is the most important parameter
for the defi nition of lifetime and reliability. For a single OLED
device, the lifetime is usually defi ned as the time interval in
which the luminance has dropped to a certain percentage of
its initial value. The operating conditions (constant current
driving, ambient temperature etc.) remain unchanged during
this period of time.
Most common in use are L50 and L70 values, representing
the time intervals after which 50 % and 70 % of the initial
luminance values have been reached. In contrast to other
light sources the luminance is taken as reference quantity
and not luminous intensity or luminous fl ux, because for a
large area emitter the luminance can be determined easier
and quicker.