Development of Phosphorescent White OLED withHigh Power Efficiency and Long Lifetime
Tomoyuki Nakayama＊, Kunimasa Hiyama＊, Keiichi Furukawa＊, and Hirofumi Ohtani＊

Abstract
White OLED device with extremely high power effi-１Introduction

ciency and long lifetime was developed in which blue,

Because of recent environmental protection and en-

yellow-green and red phosphorescent emitters wereergy supply situations, eco-friendly lighting sources ofused. The performances achieved were 64 lm/W, andlow energy consumption are in demand. An enormous

10000 hours of lifetime at initial luminance of 1000cd/m2

amount of research effort goes into the field, and, overwith a light out-coupling technique. The device also ex-

the past several years, particular attention has been

hibited the good durability important in practical use.

paid to inorganic and organic light-emitting diodes as

New technologies, such as blue phosphorescent mate-

new lighting source candidates.

rials and a sophisticated organic layer structure wereOrganic light-emitting diodes (OLEDs) are regarded

applied to the device. Our hope is that these technolo-

as a powerful candidate because they are an areagies will open the door to the practical use of OLEDs as

lighting source and can be driven at voltages as low aslighting sources.

just several volts, and, further, they contain no materials (e.g. mercury) that are harmful to the human bodyor the environment. As a lighting source for illumination or display backlight, a white light is usually required. It is well known that the use of plural lightemissive materials such as blue, green, and red is useful in obtaining white emission.
This raises several questions: When a white OLED
is used as lighting sources, how much efficiency is required? How much external emission quantum efficiency (EQE) and drive voltage are needed? It seems
that a power efficiency nearly matching that of thecurrent lighting sources is necessary to replace themwith OLED lighting, depending on the application of
the light source. The power efficiency of current lighting sources in general is about 10 to 20lm/W for electric bulbs and 60 to 90lm/W for fluorescent lamps.
Fig.1 shows the relationship between power efficiency and drive voltage at given EQE values for an
OLED which emits warm white color. From this figure,as an example, it is understood that a drive voltage of4V and an EQE of 35% are necessary to obtain thepower efficiency of 60lm/W. Because the lowest drive
voltage of a white OLED is limited to about 3V bytriplet excitation energy of the blue phosphorescentmaterial, an EQE of over 25% is needed to obtain at＊Advanced Materials Technology R&D Laboratories
Konica Minolta Technology Center, Inc.,

least 60lm/W.
OLEDs fall roughly into two classifications defined

KONICA MINOLTA TECHNOLOGY REPORT VOL.5（2008）115

by their emission mechanism: fluorescent OLEDs andphosphorescent OLEDs. It is well known that thephosphorescent OLED is advantageous from the aspect of emission quantum efficiency １）. The emission
quantum efficiency of fluorescent OLED is only one
quarter that of a phosphorescent OLED, and, theoretically, a phosphorescent OLED can reach 100% .

Fig. 2

２

Photograph of the white OLED device developed

Device structure and evaluation

The structure of our phosphorescent white OLED
device is shown schematically in Fig.3. Organic layers
were evaporated on an indium tin oxide (ITO) anode
whose thickness was 110nm under pressure of less
Fig.1

Power efficiency vs. driving voltage

Therefore, developing a phosphorescent OLED
seemed the better course in obtaining higher power
efficiency. Indeed, Y. Tung et al. reported a phosphorescent white OLED with high efficiency ２）. Its EQEreached 16% , and the power efficiency was 18.4lm/W.
However, there was a severe problem in obtaining aphosphorescent white OLED: the lifetimes of blue
phosphorescent materials have been too short forpractical application in a phosphorescent OLED.
Recently, however, an improvement of the lifetimeof blue phosphorescent materials was reported. M. S.
Weaver et al. reported blue phosphorescent OLEDs
with improved operating stability ３）. B. W. D’Andrade

than 4 × 10 − 5 Pa. A hole injection layer (HIL), holetransport layer (HTL), emissive layers, inter layer, andelectron transport layer (ETL) were evaporated on theanode in that order. An aluminum (Al) cathode with athickness of 100nm was evaporated on the electrontransport layer.
Emissive layers have blue, yellow-green and red
phosphorescent dopants and host material. The blue
dopant and the host material were developed and synthesized by our laboratory. The yellow-green (YD-85)
and red (RD-61) phosphorescent dopants were obtained
from Universal Display Corporation. The electrontransport material used in the ETL was also developedand synthesized by our laboratory.

et al. reported a phosphorescent white OLED with
14% EQE and a long lifetime ４）. However there is still
estrangement with theoretical maximum EQE of aphosphorescent OLED.
We engaged in the development of blue phosphorescent materials with a long lifetime, and, at the sametime, we pushed forward with development of an organic layer structure to obtain higher power efficiency.
As a result, we were able to develop a phosphorescent
white OLED of high performance. Fig.2 shows an example of the white OLED device we developed. In thispaper, we discuss the major performances of and an
outline of the technology incorporated in our white
OLED device.

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Fig. 3

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Device structure of the white OLED

Glass cap encapsulation was accomplished using UV

３．２

Efficiencies and lifetime

hardening epoxy glue in a pure, dry nitrogen atmo-Fig.4 shows the dependence of EQE on luminance. Itsphere. Oxygen concentration was less than 1 ppm,

shows that an EQE of 34% was obtained at 1000cd/and vapor concentration was less than 0.1ppm in them 2.

encapsulation environment. An absorbent was included

nance side of the peak at approximately 100cd/m2, thein the device.

dependence of the EQE on the luminance is small.

Though the EQE falls slightly on the high lumi-To improve the light out-coupling efficiency of theTherefore, we can assume that the organic layer

device, a diffusive light out-coupling film was affixed

structure maintains good carrier balance over a wide

on surface of the glass substrate on the side of the de-range of luminance.
The data shown in Fig.4 relates to the device withvice opposite the organic layers.
Emission areas of our devices were 4mm2 or 15cm2.

light out-coupling film shown in Fig.3. By affixing theThe efficiencies were nearly the same in both devices.

light out-coupling film, the amounts of light emitted

Lifetime was measured for 4mm2 device.

outside of the device rose about 1.6 times. The EQE ofThe current-voltage-luminance characteristics andemission spectra of the devices were measured by

the device without the light out-coupling film was 20%
at 1000cd/m2.

spectrophotometer (CS1000: Konica Minolta Sensing,Fig.5 shows the dependence of luminance on driveInc.). Light distribution was measured to calculatevoltage. As seen in the figure, a drive voltage of about

power efficiency. Device lifetime was defined as the3.6V at 1000cd/m2 was obtained.

time taken to reduce luminance to half of the initial luminance under continuous drive conditions. Storage
stability was evaluated via an accelerated test condition of 85℃.

３The white OLED’s performance

３．1

Major characteristics

Major characteristics and performance of our white
OLED are shown in Table 1. Our white color device
had a color temperature of 4500K, CIE1931 color coordinates of (0.37, 0.42), and an EQE of 34%, A drive voltage of 3.6V and a power efficiency of 64lm/W were obtained at a luminance of 1000cd/m 2 . The light
distribution was almost Lambertian. The lifetime of thedevice was estimated to be 10000 hours at an initial lu-Fig. 4 D ependence of EQE on luminance of the device shownin Fig. 3.

minance of 1000cd/m2. These performances were virtually unchanged after over 500 hours of storage at
85℃.
Table 1

P erformance of our phosphorescent white OLED at
1,000cd/m2

Fig. 5 D ependence of luminance on voltage of the device
shown in Fig. 3.

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117

Fig.6 shows the dependence of power efficiency on

３．３

Storage stabilityluminance. By achieving the high EQE and the low

Storage stability is also important in practical use.

drive voltage shown in Fig.4 and Fig.5, a power effi-Fig.8 shows the storage stability of luminance and col-ciency of 64lm/W was obtained at 1000cd/m2. Poweror measured under an accelerated test condition ofefficiency reached about 90lm/W at 100cd/m2.

85℃. Luminance and color were measured at a constant current of 2.5mA/cm2 and at room temperature.
In this figure, the dots indicate luminance and the triangles indicate color shift ⊿E, where ⊿E as definedby equation (2) indicates the degree of color change.
⊿E ＝ √(x(t)-x(0))2+(y(t)-y(0))2⑵Here, x(t) and y(t) indicate color coordinates x and yin a CIE1931 color diagram at storage time t, while x(0)and y(0) indicate color coordinates x and y before storage.
The color shift ⊿E was less than 0.005 after storage
for 520 hours at 85℃, and the change in luminanceFig. 6 D ependence of power efficiency on luminance of thedevice shown in Fig. 3.

was less than 1% .
In addition, no outbreaks of dark spot or decreases
in the emission area were observed, as shown in theFig.7 shows the operating stability of the device

photographs in Fig.9. In Fig.9, on the left our device isshown in Fig.3 under continuous driving conditions.

in a driving state before storage, while in the right theThe initial luminance was 4800cd/m . It is known thatdevice in a driving state after storage of 520 hours at

the relationship between lifetime and initial luminance85℃.

2is expressed by the following equation (1).
t(L1) = t(L2) x (L2 ／ L1)X⑴Here, t(L1) and t(L2) are lifetimes measured at initialluminance L1 and L2 respectively. Here x is an acceleration coefficient, and its value is generally between 1.5
and 2. Assuming that x is 1.5, which is a conservative
estimate, a lifetime of 1000 hours measured with an
initial luminance of 4800cd/m2 is equivalent to the lifetime of 10000 hours with an initial luminance of1000cd/m2.
Fig. 8 S torage stability of luminance and color at 85°C. Dots (●)
indicate luminance and triangles (▲) indicate color shift ⊿E.

Fig. 7 O
 perational stability under continuous driving conditions. Initialluminance was 4,800cd/m2 of the device shown in Fig.3.

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Fig. 9 P hotographs of OLED in driving state before and afterdurability test

KONICA MINOLTA TECHNOLOGY REPORT VOL.5（2008）

４

EQE, the EQE of the white light is calculated to beKey technologies

４．１

only about 15 % , well below 20 % , which is generally

Material

said to be theoretical maximum EQE of phosphores-The most important materials technology involved

cent OLED.

with our device are blue phosphorescent materials.

However, we found that we could raise EQE by op-Table 2 shows the performances of FIr(pic), which is atimizing the carrier trapping property and using an

typical blue phosphorescent dopant, and KMBD-342, aenergy transfer mechanism in the emissive layer, in-blue phosphorescent dopant developed by our labora-

cluding the red dopant, which had the lowest EQE

tory. The data shown in Table 2 were obtained for aamong the dopants that we used. By using such a te-blue OLED device whose layer structure was ITO

chique, as a result, the EQE of our white OLED

(110nm) / HIL / α-NPD / blue emissive layer / BAlq

reached 20% without light out-coupling film even

/ LiF / Al (100nm).

though EQE of each individual dopant was less than
20%. Further, the EQE of our white OLED reached

Table 2

Performance of blue phosphorescent materials

34% when we utilized a light out-coupling film asshown in Fig.4.
４．２．２

Carrier transport layers

Doping technology was introduced to the electrontransport layer to achieve low drive voltage, i.e. ourelectron transport layer was so-called n-type electron
Even though the emission efficiency of FIr(pic) istransport layer. The drive voltage fell to about 3V athigh, its lifetime is well known to be too short for that1000cd/m2 as shown in Fig.10. This contributed great-material to be applied to practical devices.

ly to the improvement of power efficiency.

We found a dopant, KMBD-342, and a host material

In Fig.10, the dots indicate the luminance-voltage

which was suitable for KMBD-342. As shown in(L-V) characteristics of the device, which uses the de-Table 2, KMBD-342’
s EQE was 17%. Althoughveloped carrier transport materials, and the squares

KMBD-342’s EQE was not much higher than that ofindicate L-V characteristics of a device using conven-FIr(pic), the lifetime of KMBD-342 was dramaticallytional carrier transport materials, such as CuPc as holehigher than that of FIr(pic), reaching a 16000-hour life-injection material, α-NPD as the hole transport mate-time with an initial luminance of 300cd/m2. These bluerial, Alq3 as the electron transport material and LiF as

dopant and host materials contributed greatly to ob-the electron injection material. Both devices were

taining the lifetime of 10000-hour lifetime of our whitegreen phosphorescent OLEDs using GD48 as a green

OLED.

phosphorescent emitter and CBP as the host material

４．２

Organic layer design

To obtain high power efficiency, it is necessary to simultaneously raise EQE and lower drive voltage. As
well as material technology described above, the layer
design of the white emissive layers contributed to raising EQE, and the layer design of the ETL contributed
greatly to lowering the drive voltage.
４．２．１

White emissive layers

We designed the emissive layers to achieve 20%
EQE without a light out-coupling technique.

The indi-vidual EQEs of the blue, yellow-green, and red dopantsin a layer structure which was similar to the white
OLED were 17 % , 17 % and 12 % respectively. If each
dopant in the white OLED emits light with the above

Fig. 10KONICA MINOLTA TECHNOLOGY REPORT VOL.5（2008）

Dependence of luminance on voltage

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in the emissive layer. GD48 was obtained from Universal Display Corporation.

References１）M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and
S. R. Forrest,“Very high-efficiency green light-emitting devices based on electrophosphorescence”, Appl. Phys. Lett. 75,

５

p. 4, 1999.

Future works

２）Y. Tung, T. Ngo, M. Hack, J. Brown, N. Koide, Y. Nagara, Y.

As shown in Fig.11, the performance of our OLED
device surpassed that of electric bulbs and reached thedomain of fluorescent lamps. Although further study

Kato, H. Ito,“A high efficiency phosphorescent white OLED
for LCD backlight and display applications”, SID Symposium
Digest, 35, p. 48-51, 2004.
３）M. S. Weaver, Y. J. Tung, B. D’Andrade, J. Esler, J. J. Brown, C.
Lin, P. B. Mackenzie, R. W. Walters, J. Y. Tsai, C. S. Brown, S.
R. Forrest, M. E. Thompson,“Advances in Blue Phosphorescent Organic Light-Emitting Devices”, SID Symposium Digest, 37, p. 127-130, 2006４）B. W. D’
Andrade, J-Y. Tsai, C. Lin, M. S. Weaver, P. B. Mackenzie, J. J. Brown,“Phosphorescent white organic light-emitting diodes for displays and lighting”, Proceedings of The
13th International Display Workshops, p. 469-472, 2006.
(An additional note ; The contents described in this paper waspresented at Society for Information Display 2007 International
Symposium (Digest of technical papers Vol.38, p.1018) )Fig. 11 L
 ight source trends: combined power efficiency andlifetime

will be needed in the future, encouraging characteristics of durability, such as the storage stability essential
to practical use, were obtained.
In the future, further improvement of power efficiency and lifetime will be necessary if higher luminance is to be achieved. But gaining higher luminancewill establish a foothold in the proliferation of thistechnology, and we hope that the technologies discussed in this paper lead to the practical use of white
OLEDs as lighting sources for illumination, backlights,and more.

６Conclusion

We developed a phosphorescent white OLED device
with a power efficiency of 64lm/W and a lifetime of
10000 hours at an initial luminance of 1000cd/m2. An
EQE of 34 % and a low drive voltage of near 3.6V at
1000cd/m2 were realized. This performance was due tothe development of organic materials, such as blue
phosphorescent and electron transport materials, andto progress in organic layer structure design.

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