Management of singlet and triplet excitons for
efficient white organic light emitting devices
Yiru Sun*, Noel C. Giebink*, Hiroshi Kanno*‡, Biwu Ma†, Mark E. Thompson†,
Stephen R. Forrest*+
Supplementary Information
Table:
Selected
electrophosphorescent
WOLED
architectures
with
their
corresponding performance characteristics.
ηp [lm/W]
CRI
Refs
11.0, 11.0†, 7.8‡
22.1, 16.1†, 5.8 ‡
85
This
work
Phosphorescent triple-doped
12*, 8.0†, 3.2‡
26*, 13.3†, 1.8 ‡
80
1
Two phosphor doped layers
12*, 9.3†, 4.1‡
10*, 4.6†, 1.3 ‡
<60
2
Architecture
Exciton managed
phosphorescent
fluorescent/
ηext [%]
Single dopant phosphorescent
6.4
12
67
3
Multiple phosphor doped layers
5.2
6.4
83
4
Phosphorescent triplet excimers
4
4.4
78
5
Blue-fluorescent/
Red-phosphorescent
1.4
0.92
<60
6
Phosphorescent sensitizer
1.8
4.1
<60
7
Here, ηext and ηp are the forward viewing external quantum and power efficiencies,
respectively. Where ηext or CRI is not reported, a value is calculated from the
luminous efficiency and spectral data provided in the reference. *Values at 10-3
mA/cm2; †Values at 1mA/cm2; ‡Values at 102 mA/cm2; Values without specific
notation are the maximum reported.
Luminance characteristics of the fluorescent/phosphorescent white organic light
2
Luminance (cd/m )
emitting device.
10
5
10
4
10
3
10
2
10
1
10
0
10
-1
10
-2
10
Figure.
1.
-3
-2
-1
0
1
10
10
10
10
10
2
Current Density (mA/cm )
Forward
viewing
2
10
3
luminance–current
density
of
the
fluorescent-phosphorescent WOLED
Non-Radiative exciton transfer in the white fluorescent/phosphorescent organic
light emitting device
To determine if triplets are lost due to formation directly on BCzVBi molecules or
otherwise wasted through non-radiative channels, we compare two structures: Device
III, which is similar to the same device in the text, has structure: [NPD(30nm)/5wt.-%
BCzVBi:CBP(5nm)/CBP(4nm)/5wt.-%Ir(ppy)3:CBP(20nm)/CBP(4nm)/5wt.-%BCz
VBi:CBP(5nm)/BCP(40nm)], and Device IV with structure: [NPD(30nm)/CBP
(9nm)/5wt.-%Ir(ppy)3:CBP (20nm)/CBP (9nm)/BCP (40nm)] fabricated at the same
time. Here, Device IV omits the BCzVBi doping at the edges of the emission layer,
but is otherwise identical to Device III.
If charge trapping and significant triplet
losses on the blue dopant do in fact occur, then the electrophosphorescent emission
should be stronger in Device IV than in Device III. However, the unnormalized EL
spectra of these two devices show that Ir(ppy)3 emission intensity decreases, instead
of increases, for Device IV when the EML is not doped with BCzVBi, as shown in
Fig. 2. The external quantum efficiency (EQE) is 5.8% for Device III, and 2.8% for
Device IV. The lower EQE of Device IV is arises since excitons form in the NPD
layer (broad NPD emission is observed at a wavelength of 430nm in Device IV), and
hence this portion of triplets cannot diffuse into the CBP layer due to the intervening
energy barrier. The doping of BCzVBi at the emission layer edges results in increased
the Ir(ppy)3
emission while eliminating that from NPD, strongly suggesting that the
presence of the blue dopant does not result in a loss in triplets.
Non-normalized EL (a.u)
5
Device III (BCzVBi /Irppy /BCzVBi)
Device IV (CBP /Irppy /CBP)
J=100mA/cm
4
2
3
2
1
0
400
500
600
700
Wavelength (nm)
800
Figure 2. Unnormalized electroluminescence (EL) spectra of Device III and IV.
One further consideration is that triplet excitons might be formed by direct trapping
on the BCzVBi since it has a lower triplet energy than CBP. To conclusively
demonstrate this mechanism does not occur with BCzVBi as the dopant, we
fabricated a further device (Device V structure: NPD (30nm)/ 3wt.-% DCM2: CBP
(10nm)/ CBP (4nm)/3wt.-% Ir(ppy)3: CBP (20nm)/ CBP (4nm)/ 3wt.-% DCM2:
CBP (10nm)/ BCP (40nm)) using the same device structure as in Device III, except
that BCzVBi is replaced with the red dopant DCM2, which has a HOMO energy of
5.26eV.8
DCM2 is excited primarily by direct recombination at the dopant
molecules resulting from both electron and hole trapping,9 instead of energy transfer
from the host. In Device V, the Ir(ppy)3 emission at =511nm is negligible (see Fig. 3,
below) since excitons are directly formed on, and triplets are then trapped on the
DCM2, and cannot subsequently diffuse to the Ir(ppy)3-doped region. However, the
unnormalized EL spectra in Fig. 3 show that Ir(ppy)3 emission is >50 times that in
Device III. That is, we can conclude that BCzVBi does not trap, and hence waste
triplets as does DCM2. Given the large difference in Ir(ppy)3 emission intensity from
Device III and Device V, the origin of highly efficient phosphorescent emission in
Device III and the WOLED can only be attributed to CBP triplet diffusion from the
BCzVBi:CBP exciton formation region.
Device III
Device VI
Unnormalized EL (a.u.)
0.20
0.15
0.10
0.05
0.00
300
400
500
600
700
800
Wavelength (nm)
Fig. 3: Comparsion of emission from Devices III and V.
Ir(ppy)3 emission in Device V.
Note the lack of
No direct transfer can occur from DCM to
Ir(ppy)3 in Device V due to their considerable spatial separation.
* Department of Electrical Engineering, Princeton Institute for the Science and
Technology of Materials (PRISM), Princeton University, Princeton, New Jersey
08544, USA
† Department of Chemistry, University of Southern California, Los Angeles,
California 90089, USA
‡ Currently on leave from Sanyo Electric Co., Ltd., Osaka Japan 573-8534
+
Current address: Departments of Electrical Engineering and Computer Science,
Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor,
MI 48109. email: stevefor@umich.edu
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