# White Organic Light-Emitting Diodes Using Two Phosphorescence

Authors: Tomoya Inden; Tomoya Hishida; Masayuki Takahashi; Masato Ohsumi; Naoki Ohtani
Journal: International Journal of Photoenergy
 (2012)

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## Abstract

We fabricated two kinds of white organic light-emitting diodes (WOLEDs); one
consisted of two emissive materials of red and blue, and the other of three
emissive materials of red, green, and blue. The red and blue emissive materials
were phosphorescent. We evaluated the thickness dependence of the CIE
coordinate, the external quantum efficiency (EQE), and the luminance by changing
the thicknesses of the Ir(btp)2acac and FIrpic layers. Samples consisting of
three emissive materials revealed the best CIE coordinate and the best EQE in
the same sample structure. On the other hand, the samples consisting of two
emissive materials revealed the best CIE coordinate and the best EQE in
different structures. The best CIE coordinate of (0.33, 0.36) was observed by
changing the thicknesses of the stacked active layers. The best EQE was
9.73%, which was observed in the sample consisting of different thickness
of stacked active layers.


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## Body

## 1. Introduction

Research on organic light-emitting diodes (OLEDs) has been attracting much interest
for applications to full-color flat-panel displays and illumination light sources.
In particular, white OLEDs (WOLEDs) are the candidate for novel illumination light
sources. However, WOLEDs should be improved in their efficiency and operating
lifetime for practical use. To solve these problems, most of the recent research on
WOLEDs has used phosphorescent emissive materials instead of fluorescent ones. These
days the highest external quantum efficiency (EQE) of green emission using
phosphorescent material exceeds 20% [1].WOLEDs can be fabricated by two methods. The most common method is using three
primary color emissive materials in an emitting layer (EML). Sun et al. [2] reported that a hybrid WOLED with a
fluorescent blue emitter and phosphorescent red and green emitters showed the
highest EQE of 18.7%. Another method to make white emission only uses two
complimentary colors.In this paper, we fabricated two kinds of WOLEDs. One consists of two emissive
materials of red and green, and the other consists of three emissive materials of
red, green, and blue. The former is called an RGB-type sample, and the latter one is
an RB-type sample. The red and blue emissive materials are phosphorescent materials.
[Bis(2-(2′-benzothienyl)pyridinato-N,C3′)iridium(acetylacetonate)]
(Ir(btp)2-acac) is used as a red emissive phosphorescent material, and [Bis
(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium] (FIrpic) is
used as a blue emissive phosphorescent material. In addition, [Tris
(8-hydroxyquinolinato)-aluminum] (Alq3) is used as a green emissive and
electron-transporting fluorescent material. We evaluated the thickness dependence on
the CIE coordinate, the external quantum efficiency, and the luminance by changing
the thicknesses of the Ir(btp)2acac and FIrpic layers.


## 2. Experimental Procedure and Sample Structures

### 2.1. Organic Materials

Figures1 and 2 show the energy band diagram of the RGB- and RB-type
samples. The HOMO-LUMO values are estimated from [3].Figure 1
Energy diagram in all RGB-type samples.Figure 2
Energy diagram in all RB-type samples.[Dicarbazolyl-3,5-benzene] (MCP) can be used as a host material in the emitting
layers because it has a higher triplet energy than FIrpic. In previous studies,
[4,4′-Bis(N-carbazolyl)-1,1′-biphenyl] (CBP) has usually been used
as the host material of green or red phosphorescent materials because CBP has
higher triplet energy than they do [4].
Its triplet energy, however, is lower than FIrpic, indicating that EQE cannot be
improved by combining CBP and FIrpic. Thus, we use MCP as the host material in
both the Ir(btp)2acac and FIrpic layers.In RGB-type samples, the Alq3 layer is an electron-transporting layer (ETL), and
[2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline] (BCP) is used as an ETL in the
RB-type samples. In addition,
[4,4′,4″-tris-(3-methylphenylphenylamino)
triphenylamine] (m-MTDATA) is used as a hole-transporting layer on the ITO
electrode in both types of samples. m-MTDATA also works as an electron-blocking
layer. In addition, the discontinuity of the HOMO levels of m-MTDATA and FIrpic
play a role in hole blocking, improving the radiative efficiency.In the RB-type samples, we caused a radiative recombination in the FIrpic layer
because the discontinuity of the HOMO levels between BCP and FIrpic works as a
hole-blocking interface.


### 2.2. Sample Fabrication

The organic materials were deposited on sufficiently cleaned ITO-coated glass
substrate by using a vacuum evaporator. The pressure was under 3 ×
10−4 Pa, and the deposition rates of the HTL and
the ETL were between 0.1 and 0.2 nm/s and were monitored with a quartz
oscillator. After deposition of all the organic materials, LiF and Al were
deposited as a cathode. The mesa area was 4 mm2.The following are the sample structures of the RGB type and RB type:RGB type:(A)
ITO/m-MTDATA/FIrpic + MCP(25 nm)/Ir(btp)2acac +
MCP(5 nm)/Alq3/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%.
(B)
ITO/m-MTDATA/FIrpic + MCP(25 nm)/Ir(btp)2acac +
MCP(5 nm)/Alq3/LiF/Al:
deposition rate of FIrpic was between 0.3 and
0.5 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 20 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(C)
ITO/m-MTDATA/FIrpic + MCP(20 nm)/Ir(btp)2acac +
MCP(5 nm) /FIrpic + MCP(5 nm)/Alq3/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;RB type:(D)
ITO/m-MTDATA/Ir(btp)2acac+MCP(5 nm)/FIrpic +
MCP(25 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(E)
ITO/m-MTDATA/Ir(btp)2acac + MCP(5 nm)/FIrpic +
MCP(25 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.3 and
0.5 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 20 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(F)
ITO/m-MTDATA/Ir(btp)2acac + MCP(1 nm)/FIrpic +
MCP(29 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(G)
ITO/m-MTDATA/Ir(btp)2acac + MCP(1 nm)/FIrpic +
MCP(29 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.3 and
0.5 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 20 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%,
deposition rate of MCP was fixed between 2.0 and
2.3 nm/s in all samples.We evaluated the current-voltage (I-V) characteristics, the EQEs, the current
density-luminance (J-L) characteristics, the CIE coordinates, and the EL
spectra. The I-V characteristics were measured with a semiconductor parameter
analyzer (4155C, Agilent Technologies). The luminance and the EL spectra of all
the samples and the photons per unit of time were measured with an integrating
sphere connected CCD spectrophotometer (USB 2000, Ocean Optics). The CIE
coordinates were measured with a color-luminance meter (CS-100A, KONIKA
MINOLTA). All measurements were performed in air.


## 2.1. Organic Materials

Figures1 and 2 show the energy band diagram of the RGB- and RB-type
samples. The HOMO-LUMO values are estimated from [3].Figure 1
Energy diagram in all RGB-type samples.Figure 2
Energy diagram in all RB-type samples.[Dicarbazolyl-3,5-benzene] (MCP) can be used as a host material in the emitting
layers because it has a higher triplet energy than FIrpic. In previous studies,
[4,4′-Bis(N-carbazolyl)-1,1′-biphenyl] (CBP) has usually been used
as the host material of green or red phosphorescent materials because CBP has
higher triplet energy than they do [4].
Its triplet energy, however, is lower than FIrpic, indicating that EQE cannot be
improved by combining CBP and FIrpic. Thus, we use MCP as the host material in
both the Ir(btp)2acac and FIrpic layers.In RGB-type samples, the Alq3 layer is an electron-transporting layer (ETL), and
[2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline] (BCP) is used as an ETL in the
RB-type samples. In addition,
[4,4′,4″-tris-(3-methylphenylphenylamino)
triphenylamine] (m-MTDATA) is used as a hole-transporting layer on the ITO
electrode in both types of samples. m-MTDATA also works as an electron-blocking
layer. In addition, the discontinuity of the HOMO levels of m-MTDATA and FIrpic
play a role in hole blocking, improving the radiative efficiency.In the RB-type samples, we caused a radiative recombination in the FIrpic layer
because the discontinuity of the HOMO levels between BCP and FIrpic works as a
hole-blocking interface.


## 2.2. Sample Fabrication

The organic materials were deposited on sufficiently cleaned ITO-coated glass
substrate by using a vacuum evaporator. The pressure was under 3 ×
10−4 Pa, and the deposition rates of the HTL and
the ETL were between 0.1 and 0.2 nm/s and were monitored with a quartz
oscillator. After deposition of all the organic materials, LiF and Al were
deposited as a cathode. The mesa area was 4 mm2.The following are the sample structures of the RGB type and RB type:RGB type:(A)
ITO/m-MTDATA/FIrpic + MCP(25 nm)/Ir(btp)2acac +
MCP(5 nm)/Alq3/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%.
(B)
ITO/m-MTDATA/FIrpic + MCP(25 nm)/Ir(btp)2acac +
MCP(5 nm)/Alq3/LiF/Al:
deposition rate of FIrpic was between 0.3 and
0.5 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 20 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(C)
ITO/m-MTDATA/FIrpic + MCP(20 nm)/Ir(btp)2acac +
MCP(5 nm) /FIrpic + MCP(5 nm)/Alq3/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;RB type:(D)
ITO/m-MTDATA/Ir(btp)2acac+MCP(5 nm)/FIrpic +
MCP(25 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(E)
ITO/m-MTDATA/Ir(btp)2acac + MCP(5 nm)/FIrpic +
MCP(25 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.3 and
0.5 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 20 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(F)
ITO/m-MTDATA/Ir(btp)2acac + MCP(1 nm)/FIrpic +
MCP(29 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.1 and
0.3 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 10 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%;
(G)
ITO/m-MTDATA/Ir(btp)2acac + MCP(1 nm)/FIrpic +
MCP(29 nm)/BCP/LiF/Al:
deposition rate of FIrpic was between 0.3 and
0.5 nm/s,
deposition rate of Ir(btp)2acac was between 0.1 and
0.2 nm/s,
average doping density of FIrpic was 20 mol%,
and
average doping density of Ir(btp)2acac was
10 mol%,
deposition rate of MCP was fixed between 2.0 and
2.3 nm/s in all samples.We evaluated the current-voltage (I-V) characteristics, the EQEs, the current
density-luminance (J-L) characteristics, the CIE coordinates, and the EL
spectra. The I-V characteristics were measured with a semiconductor parameter
analyzer (4155C, Agilent Technologies). The luminance and the EL spectra of all
the samples and the photons per unit of time were measured with an integrating
sphere connected CCD spectrophotometer (USB 2000, Ocean Optics). The CIE
coordinates were measured with a color-luminance meter (CS-100A, KONIKA
MINOLTA). All measurements were performed in air.


## 3. Results and Discussion

### 3.1. RGB-Type Samples

Figure3 shows the J-L characteristics of
the RGB-type samples. This result clearly shows that the luminance becomes lower
with an increase of doping density. This is most likely caused by concentration
quenching. In addition, the gradients of the J-L characteristics in the three
samples is almost equal, because the relation between the current density and
the disappearance of the photons are almost equal. When the value of the applied
voltage is between 0 and 18 V, sample A shows a maximum luminance of
39500 cd/m2. On the other hand, sample B shows a maximum
luminance of 20300 cd/m2 with the same applied voltage.Figure 3
J-L characteristics in all RGB-type samples.Figure4 shows the J-EQE characteristics
of the RGB-type samples. The measurement range of the current density is between
25 and 1250 mA/cm2. When the current density is less than
25 mA/cm2, the photon counting does not work due to a lack
of photons. The EQE in sample A is larger than the others due to the large
concentration quenching in samples B and C. With increasing current density, the
EQE in the three samples sharply decreases. This is attributed to the effect of
the triplet-triplet annihilation (T-T annihilation) in the large current density
region. T-T annihilation means the disappearance of the triplet excitons, which
occurs when their density in the emitting layers becomes very large [5].Figure 4
J-EQE characteristics in all RGB-type samples.Table1 shows the CIE coordinate in the
RGB-type samples. The best white emission is observed in sample A. The CIE
coordinate is (0.33, 0.41). The nearest white coordinate in sample B is (0.35,
0.44). Compared with sample A, sample B’s CIE coordinate is slightly red
shifted at the same current. In addition, the CIE coordinates of both samples
are blue shifted with increasing current density, indicating that the radiative
recombination region is changed to wider-bandgap material.Table 1
CIE coordinate in all RGB-type samples.
Current (mA)
Sample
1
5
10
20
A
(0.37, 0.48)
(0.35, 0.45)
(0.33, 0.41)
(0.33, 0.42)
B
(0.40, 0.46)
(0.36, 0.46)
(0.35, 0.45)
(0.35, 0.44)
C
(0.45, 0.49)
(0.45, 0.48)
(0.43, 0.45)
(0.38, 0.46)The operating property in sample C is the worst of the RGB-type samples. The
best white coordinate in sample C is (0.38, 0.46). Compared with the others, the
CIE coordinate shows a red shift. This means that the radiative recombination
occurs mainly in the Ir(btp)2acac layer due to the narrow bandgap. Thus, many
triplet excitons gather in the Ir(btp)2acac layer, leading to T-T annihilation
as well as the decrease of luminance and EQE.Finally, Figures5, 6, and 7 show the
EL spectra of all the RGB-type samples. The EL intensities are normalized at the
intensity of Ir(btp)2acac. The EL intensities of both FIrpic and Alq3 in sample
A are stronger than those of both FIrpic and Alq3 in sample B. This means that
the EL spectrum in sample A is wider than that in sample B. Therefore, the CIE
coordinate in sample A is better for generating white color emission. In
addition, the EL intensities of both FIrpic and Alq3 in sample C are small and
result in the red shift of the CIE coordinate.Figure 5
EL spectra in sample A. Injected currents are 1, 5, and 10 mA.Figure 6
EL spectra in sample B. Injected currents are 1, 5, and 10 mA.Figure 7
EL spectra in sample C. Injected currents are 1, 5, and 10 mA.We evaluated the EL spectra of the three samples at the same currents of 1, 5,
and 10 mA. The EL spectrum of every RGB-type sample depends on the
applied voltage. In particular, with an increase of the applied voltage, the EL
intensity from the Ir(btp)2acac layer becomes weaker, while the EL intensity
from Alq3 becomes stronger. However, the EL emissions from the three primary
colors can always be observed within the range of every applied voltage. This
means that the energy transfer is not caused by the applied voltage; the
radiative recombination occurs in the same emitting layers.These results concerning to the RGB-type samples clearly demonstrate that the
best CIE coordinate and the best EQE are observed in sample A.


### 3.2. RB-Type Samples

Figure8 shows the J-L characteristics of
the RB-type samples. Samples E and G containing larger doping densities reveal
smaller luminance than samples D and F. This is also caused by concentration
quenching. In addition, the gradients of the J-L characteristics in the same
sample structures are almost equal. This result is similarly observed in the
RGB-type samples, meaning that the relation between the current density and the
disappearance of the photons is almost equal when the sample structure is
identical. When the range of the applied voltage is between 0 and 18 V,
the maximum luminance shows 71135 cd/m2 in sample D. The
second highest maximum luminance shows 48108 cd/m2 in sample F
with the same applied voltage.Figure 8
J-L characteristics in all RB-type samples.Figure9 shows the J-EQE characteristics
in all the RB-type samples. The injected current densities are the same as the
RGB-type. The EQEs of all four samples increase with decreasing current density,
because EQE is proportional to the amount of photons and inversely proportional
to the current density.Figure 9
J-EQE characteristics in all RB-type samples.Sample D shows the maximum EQE of the RB-type samples, which is 9.73%. This
sample is the most efficient among the RB type.When the doping density of FIrpic is changed between 10 and 20 mol%,
the range of the decrease of EQE is almost equal among the same sample
structures. This indicates that the decrease of luminance due to concentration
quenching depends on the doping density. In addition, the decrease of EQE among
the large current density region is caused by T-T annihilation. Thus, the
concentration quenching and T-T annihilation enter at the same time from a
certain current density region.To consider the influence of the Ir(btp)2acac layer thickness, we compared the
luminance and EQE properties between samples D and F and samples E and G. The
samples consisting of a 5 nm Ir(btp)2acac layer reveal better luminance and EQE
properties than those consisting of a 1-nm Ir(btp)2acac layer. Since the
discontinuity of the HOMO levels of Ir(btp)2acac layer and FIrpic layer is
0.6 eV as shown in Figure2, many
holes are trapped at the interface. Thus, higher luminance and the EQE in the
samples D and F were actively caused by the recombination of the carriers
between the Ir(btp)2acac layer and FIrpic layer. The ultrathin Ir(btp)2acac
layer entered the separation of the careers between the Ir(btp)2acac and FIrpic
layer (Figure 2). The small luminance and
the EQE observed in samples E and G can be attributed to the increase of the
nonradiative indirect transition due to the ultrathin Ir(btp)2acac layer. Table
2 shows the CIE coordinate in all
the RB-type samples. The best white color emission is observed in sample F. The
CIE coordinate was (0.33, 0.36). The second best white coordinate is observed in
sample D, which reveals the highest luminance. All the RB-type samples similarly
show a blue shift, indicating that the recombination region partially moved to
the FIrpic layer.Table 2
CIE coordinate in all RB-type samples.
Current (mA)
Sample
1
5
10
20
D
(0.45, 0.40)
(0.37, 0.38)
(0.34, 0.49)
(0.31, 0.38)
E
(0.48, 0.48)
(0.45, 0.46)
(0.44, 0.46)
(0.45, 0.43)
F
(0.38, 0.36)
(0.33, 0.36)
(0.31, 0.39)
(0.30, 0.38)
G
(0.38, 0.40)
(0.36, 0.40)
(0.34, 0.40)
(0.34, 0.39)Figures10, 11, 12, and 13 show the EL spectra in all the RB-type
samples. The EL intensity of FIrpic is stronger than Ir(btp)2acac in samples D
and F, reflecting weak concentration quenching. Thus, the emitting color in all
the RB-type samples depends on the thickness of Ir(btp)2acac and the doping
density in the EML materials.Figure 10
EL spectra in sample D. Injected currents are 1, 5, and 10 mA.Figure 11
EL spectra in sample E. Injected currents are 1, 5, and 10 mA.Figure 12
EL spectra in sample F. Injected currents are 1, 5, and 10 mA.Figure 13
EL spectra in sample G. Injected currents are 1, 5, and 10 mA.The profile of the EL spectra depends on the applied voltage. With an increase of
the applied voltage, the EL intensity from Ir(btp)2acac layer becomes weaker,
and the EL intensity from FIrpic becomes relatively strong. This means that the
carriers in the Ir(btp)2acac layer move to the adjacent wide-bandgap material
FIrpic due to the change of the carrier mobility by the large applied
voltage.


## 3.1. RGB-Type Samples

Figure3 shows the J-L characteristics of
the RGB-type samples. This result clearly shows that the luminance becomes lower
with an increase of doping density. This is most likely caused by concentration
quenching. In addition, the gradients of the J-L characteristics in the three
samples is almost equal, because the relation between the current density and
the disappearance of the photons are almost equal. When the value of the applied
voltage is between 0 and 18 V, sample A shows a maximum luminance of
39500 cd/m2. On the other hand, sample B shows a maximum
luminance of 20300 cd/m2 with the same applied voltage.Figure 3
J-L characteristics in all RGB-type samples.Figure4 shows the J-EQE characteristics
of the RGB-type samples. The measurement range of the current density is between
25 and 1250 mA/cm2. When the current density is less than
25 mA/cm2, the photon counting does not work due to a lack
of photons. The EQE in sample A is larger than the others due to the large
concentration quenching in samples B and C. With increasing current density, the
EQE in the three samples sharply decreases. This is attributed to the effect of
the triplet-triplet annihilation (T-T annihilation) in the large current density
region. T-T annihilation means the disappearance of the triplet excitons, which
occurs when their density in the emitting layers becomes very large [5].Figure 4
J-EQE characteristics in all RGB-type samples.Table1 shows the CIE coordinate in the
RGB-type samples. The best white emission is observed in sample A. The CIE
coordinate is (0.33, 0.41). The nearest white coordinate in sample B is (0.35,
0.44). Compared with sample A, sample B’s CIE coordinate is slightly red
shifted at the same current. In addition, the CIE coordinates of both samples
are blue shifted with increasing current density, indicating that the radiative
recombination region is changed to wider-bandgap material.Table 1
CIE coordinate in all RGB-type samples.
Current (mA)
Sample
1
5
10
20
A
(0.37, 0.48)
(0.35, 0.45)
(0.33, 0.41)
(0.33, 0.42)
B
(0.40, 0.46)
(0.36, 0.46)
(0.35, 0.45)
(0.35, 0.44)
C
(0.45, 0.49)
(0.45, 0.48)
(0.43, 0.45)
(0.38, 0.46)The operating property in sample C is the worst of the RGB-type samples. The
best white coordinate in sample C is (0.38, 0.46). Compared with the others, the
CIE coordinate shows a red shift. This means that the radiative recombination
occurs mainly in the Ir(btp)2acac layer due to the narrow bandgap. Thus, many
triplet excitons gather in the Ir(btp)2acac layer, leading to T-T annihilation
as well as the decrease of luminance and EQE.Finally, Figures5, 6, and 7 show the
EL spectra of all the RGB-type samples. The EL intensities are normalized at the
intensity of Ir(btp)2acac. The EL intensities of both FIrpic and Alq3 in sample
A are stronger than those of both FIrpic and Alq3 in sample B. This means that
the EL spectrum in sample A is wider than that in sample B. Therefore, the CIE
coordinate in sample A is better for generating white color emission. In
addition, the EL intensities of both FIrpic and Alq3 in sample C are small and
result in the red shift of the CIE coordinate.Figure 5
EL spectra in sample A. Injected currents are 1, 5, and 10 mA.Figure 6
EL spectra in sample B. Injected currents are 1, 5, and 10 mA.Figure 7
EL spectra in sample C. Injected currents are 1, 5, and 10 mA.We evaluated the EL spectra of the three samples at the same currents of 1, 5,
and 10 mA. The EL spectrum of every RGB-type sample depends on the
applied voltage. In particular, with an increase of the applied voltage, the EL
intensity from the Ir(btp)2acac layer becomes weaker, while the EL intensity
from Alq3 becomes stronger. However, the EL emissions from the three primary
colors can always be observed within the range of every applied voltage. This
means that the energy transfer is not caused by the applied voltage; the
radiative recombination occurs in the same emitting layers.These results concerning to the RGB-type samples clearly demonstrate that the
best CIE coordinate and the best EQE are observed in sample A.


## 3.2. RB-Type Samples

Figure8 shows the J-L characteristics of
the RB-type samples. Samples E and G containing larger doping densities reveal
smaller luminance than samples D and F. This is also caused by concentration
quenching. In addition, the gradients of the J-L characteristics in the same
sample structures are almost equal. This result is similarly observed in the
RGB-type samples, meaning that the relation between the current density and the
disappearance of the photons is almost equal when the sample structure is
identical. When the range of the applied voltage is between 0 and 18 V,
the maximum luminance shows 71135 cd/m2 in sample D. The
second highest maximum luminance shows 48108 cd/m2 in sample F
with the same applied voltage.Figure 8
J-L characteristics in all RB-type samples.Figure9 shows the J-EQE characteristics
in all the RB-type samples. The injected current densities are the same as the
RGB-type. The EQEs of all four samples increase with decreasing current density,
because EQE is proportional to the amount of photons and inversely proportional
to the current density.Figure 9
J-EQE characteristics in all RB-type samples.Sample D shows the maximum EQE of the RB-type samples, which is 9.73%. This
sample is the most efficient among the RB type.When the doping density of FIrpic is changed between 10 and 20 mol%,
the range of the decrease of EQE is almost equal among the same sample
structures. This indicates that the decrease of luminance due to concentration
quenching depends on the doping density. In addition, the decrease of EQE among
the large current density region is caused by T-T annihilation. Thus, the
concentration quenching and T-T annihilation enter at the same time from a
certain current density region.To consider the influence of the Ir(btp)2acac layer thickness, we compared the
luminance and EQE properties between samples D and F and samples E and G. The
samples consisting of a 5 nm Ir(btp)2acac layer reveal better luminance and EQE
properties than those consisting of a 1-nm Ir(btp)2acac layer. Since the
discontinuity of the HOMO levels of Ir(btp)2acac layer and FIrpic layer is
0.6 eV as shown in Figure2, many
holes are trapped at the interface. Thus, higher luminance and the EQE in the
samples D and F were actively caused by the recombination of the carriers
between the Ir(btp)2acac layer and FIrpic layer. The ultrathin Ir(btp)2acac
layer entered the separation of the careers between the Ir(btp)2acac and FIrpic
layer (Figure 2). The small luminance and
the EQE observed in samples E and G can be attributed to the increase of the
nonradiative indirect transition due to the ultrathin Ir(btp)2acac layer. Table
2 shows the CIE coordinate in all
the RB-type samples. The best white color emission is observed in sample F. The
CIE coordinate was (0.33, 0.36). The second best white coordinate is observed in
sample D, which reveals the highest luminance. All the RB-type samples similarly
show a blue shift, indicating that the recombination region partially moved to
the FIrpic layer.Table 2
CIE coordinate in all RB-type samples.
Current (mA)
Sample
1
5
10
20
D
(0.45, 0.40)
(0.37, 0.38)
(0.34, 0.49)
(0.31, 0.38)
E
(0.48, 0.48)
(0.45, 0.46)
(0.44, 0.46)
(0.45, 0.43)
F
(0.38, 0.36)
(0.33, 0.36)
(0.31, 0.39)
(0.30, 0.38)
G
(0.38, 0.40)
(0.36, 0.40)
(0.34, 0.40)
(0.34, 0.39)Figures10, 11, 12, and 13 show the EL spectra in all the RB-type
samples. The EL intensity of FIrpic is stronger than Ir(btp)2acac in samples D
and F, reflecting weak concentration quenching. Thus, the emitting color in all
the RB-type samples depends on the thickness of Ir(btp)2acac and the doping
density in the EML materials.Figure 10
EL spectra in sample D. Injected currents are 1, 5, and 10 mA.Figure 11
EL spectra in sample E. Injected currents are 1, 5, and 10 mA.Figure 12
EL spectra in sample F. Injected currents are 1, 5, and 10 mA.Figure 13
EL spectra in sample G. Injected currents are 1, 5, and 10 mA.The profile of the EL spectra depends on the applied voltage. With an increase of
the applied voltage, the EL intensity from Ir(btp)2acac layer becomes weaker,
and the EL intensity from FIrpic becomes relatively strong. This means that the
carriers in the Ir(btp)2acac layer move to the adjacent wide-bandgap material
FIrpic due to the change of the carrier mobility by the large applied
voltage.


## 4. Conclusions

We fabricated and evaluated two kinds of WOLEDs. The luminance and EQE of both RGB-
and RB-type samples were found to depend on the doping density and the T-T
annihilation. For the RB-type samples, the thickness of the Ir(btp)2acac layer
strongly affected the luminance and the EQE properties. The RGB-type samples exhibit
the best CIE coordinate and the best EQE in the same sample structure; the best CIE
coordinate was (0.34, 0.39), the maximum EQE was 9.73%, and the maximum
luminance was 71135 cd/m2. The RB-type samples exhibit the best
CIE coordinate and the best EQE in different structures; the best CIE coordinate was
(0.33, 0.36), the maximum EQE was 7.46%, and the maximum luminance was
48108 cd/m2. The EQE can exceed 10% by a current
injection less than 25 mA/cm2 using five organic materials. This
means that high performance white OLEDs can be fabricated using a few organic
materials and simple sample structures.



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*Source: 703496-2012-02-27.xml*