The schematic energy band diagrams of devices are shown in Figure 4.24. The device with
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doped Rubrene into hole blocking layer for CBP is designed as following:
Device P: ITO/MoO3 (5 nm)/NPB (35 nm)/CBP: Rubrene (6 nm)/DPVBi (20 nm)/CBP (2 nm)/DPVBi
(20 nm)/CBP (2 nm)/BPhen (10 nm)/LiF (0.5nm)/Al (100 nm)
Device Q: ITO/MoO3 (5 nm)/NPB (35 nm)/CBP (6 nm)/DPVBi (20 nm)/CBP: Rubrene (2 nm)/DPVBi
(20 nm)/CBP (2 nm)/BPhen (10 nm)/LiF (0.5nm)/Al (100 nm)
Device R: ITO/MoO3 (5 nm)/NPB (35 nm)/CBP (6 nm)/DPVBi (20 nm)/CBP (2 nm)/DPVBi (20
nm)/CBP: Rubrene (2 nm)/BPhen (10 nm)/LiF (0.5nm)/Al (100 nm)
Device S: ITO/MoO3 (5 nm)/NPB (35 nm)/CBP (6 nm)/DPVBi (10 nm)/CBP: Rubrene (2 nm)/DPVBi
(30 nm)/CBP (2 nm)/BPhen (10 nm)/LiF (0.5nm)/Al (100 nm)
In the device, the NPB is used as hole-injecting layer and hole-transport layer. The CBP layers
are used as the EML and the HBLs. In Fig. 4.24, HBL1, HBL2 and HBL3 represented the HBL
that was close to the anode side, in the middle, and close to cathode, respectively, which forms
so-called the THBL that can control the carrier recombination in the EML to adjust the emission
color of devices. The Rubrene is used as a guest doping material to emit the yellow light for
achieving the white light emission, and the DPVBi is used as blue-emitting layer. The Rubrene is
doped into the CBP layer rather than the DPVBi layer. This is due to the fact that CBP has higher
carrier trapping ability than that of DPVBi. Nevertheless, if the Rubrene is doped into the DPVBi
layer, it will cause the attenuation in luminous efficiency of DVPBi. Figure 4.25 (a) shows the
EL spectra of devices at current density of 100mA/cm2. The peak wavelengths of non-doped
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device are at 416 nm and 468 nm originating from CBP and DPVBi, respectively. It shows the
main peak emission from CBP (HBL1), and minor peak emission from DPVBi (I), which implies
that the recombination zone of excitons is at the range of CBP (HBL1) and DPVBi (I). To
confirm the excitons recombination zone and to achieve white light emission, the Rubrene is
doped into the CBP (HBL1) in device A. The emission peaks of device P are at 460 nm and 564
nm wavelength from DPVBi (I) and Rubrene, respectively. It indicates that most excitons
recombination happens within the DPVBi (I) layer and the CBP (HBL1) layer, resulting in the
yellow light emission of Rubrene. In addition, the emission peak from CBP (HBL1) in device P
is disappeared due to the energy transfer from host to guest, i.e., CBP (HBL1) to Rubrene.
Furthermore, a part of blue light emitted from DPVBi (I) is absorbed by Rubrene molecular,
leading to the reduction in emission peak of DPVBi (I). However, the intensity of yellow light is
higher than that of blue light. Hence, in order to reach the balance between intensity of yellow
light and blue light, the doping position of Rubrene changed to the CBP (HBL2) of device Q,
which can prevent the blue light emitted from the DPVBi (I) layer from absorbing by the
Rubrene. As a result, the relative intensity of yellow to blue emission decreases in device Q. It is
also found that the intensity of blue emission is higher than that of yellow emission, indicating
that most of excitons recombination is at the range of CBP (HBL1)/DPVBi (I) while few
exctions trapping and formation is at CBP (HBL2) layer. In device R, the Rubrene is doped into
the CBP (HBL3), and the intensity of yellow light is almost negligible. It is attributed to the fact
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that the Rubrene doping layer is far away from the exction recombination zone, which is
consistent with previous discussion. The main exctions recombination zone of device is at the
interface of CBP (HBL1)/DPVBi (I), and the intensity of blue emission in device Q is higher
than that of yellow emission, so the thickness of the DPVBi (I) layer nearing the CBP (HBL1)
should be decreased to balance the yellow and blue emission. This is the reason why the device S
based on the structure of device Q is fabricated. In device S, the thickness of DPVBi (I) layer
decreased to 10nm and the thickness of DPVBi (II) layer increased to 30 nm. Compared with
device Q, the yellow intensity in device S is enhanced while the blue intensity is a little decrease.
This is due to the fact that the amount of exciton in CBP (HBL2) increases and the amount of
exciton in DPVBi (I) layer reduces, resulting from a decrease in the thickness of DPVBi (I) layer.
The intensity of yellow emission from Rubrene is almost the same with the intensity of blue
emission from DPVBi in device S by adjusting the thickness of DPVBi layer based on the
structure of device Q. Figure 4.25(b) shows the CIE coordinates of various devices at current
density of 100 mA/cm2. The CIE coordinates of device P-R are (0.431, 0.419), (0.258, 0.331)
and (0.218, 0.292), respectively. It can be seen that the CIE coordinates of device P-R are far
away the standard CIE coordinate of white emission. The CIE coordinate of device S is (0.322,
0.368) and is close to the standard CIE coordinate of white emission.
Figure 4.26 (a) shows the EL spectra of device S at applied voltage of 8-12V. It can be seen
that the EL spectra show 416, 468 and 560 nm wavelengths for CBP, DPVBi and Rubrene,
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respectively. The ratio of blue and yellow emission intensity in EL spectra is almost the same at
different applied voltages, indicating that the excitons recombination zone of device S is steady.
In other words, the ratio is not change as the applied voltage switches. Figure 4.26 (b) shows CIE
coordinates of device S at different applied voltages. The CIE coordinates of device S at 8-12 V
are (0.321, 0.379), (0.324, 0.393), (0.319, 0.389), (0.315, 0.379) and (0.322, 0.368), respectively.
The difference in CIE coordinates of device S is ±△x, y = (0.001, 0.011). The result shows that the
excitons can be confined in the recombination zone by using THBL structure, i.e., the holes are
confined and accumulated in the HOMO level of CBP (HBL1)/ DPVBi (I)/ CBP (HBL2)
potential well in device S due to the existence of potential barrier at the CBP (HBL1)/ DPVBi (I)
and DPVBi (I)/ CBP (HBL2) heterointerfaces. As a result, the balance of electron and hole can
be improved by using the THBL structure, and thus the CIE coordinates stability of device S is
achieved.
The current density-voltage, luminance-voltage and luminous efficiency-current density
characteristics of devices were shown in Fig. 4.27. At a current density of 10mA/cm2, the turn-on
voltages are 6.9V, 9V, 11.2V, and 8.2V for devices P-S, respectively. The turn-on voltage of
device P is lower than that of other devices. It is attributed to the fact that the barrier of energy
level between NPB and CBP is 0.8 eV and is higher than that of between NPB and Rubrene,
resulting that the hole in device P can easily inject into Rubrene of bipolar molecules in CBP
(HBL1) due to a substantially higher carrier affinity [28]. Besides, it can be seen that the turn-on
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voltage of device S is lower than that of device Q. The phenomenon is attributed to the fact that
the DPVBi (I) layer thickness of device S becomes thinner than that of device Q, which enables
Rubrene in CBP (HBL2) to be more close to the recombination zone of exciton. Furthermore, it
is found that the efficiency of device P has a roll-off phenomenon (see Fig. 4.27 (c)). This is due
to the fact that there is a peak wavelength of 460 nm in normalized EL spectra and it enhances as
the increase at applied voltage of 7-10V, as shown in inset of Fig. 4.27 (c). The minority hole
blocked by barrier of NPB/ CBP (HBL1) will cross the barrier of CBP (HBL1) and inject into
DPVBi (I) as the increasing voltage, leading to the decrease of excitons at CBP (HBL1) and the
reduction of energy transfer from CBP host to Rubrene guest, and thereby decrease efficiency of
Rubrene. However, the whole efficiency of device P is reduced with increasing current density.
In contrast, the roll-off phenomenon is barely for device S. The maximum luminance and
maximum luminous efficiency of the device S is 2410 cd/m2, 2.05 cd/A at 12V, respectively. The
result indicates that stable luminous efficiency can be achieved by the improved carrier balance
(as confining of holes).
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