5.3 ITO/QDs interfaces design to control electron injection
5.3.1 Electron injection behavior through the ITO/QD interface condition
Role of aluminum oxide layer inserted at ITO/QD interface
In order to develop efficient inverted QLEDs, the design of the interface between the cathode (ITO) and QD is quite important because of the 1.6 eV of higher electron injection barriers compared to the holes. Therefore, it is important to understand the electron injection behavior through different ITO/QD interface condition. The insertion of a thin buffer or injection layer at the ITO/QD interface is the simple and effective strategy to control the charge injection. There are variety of interfacial materials to modify the energy level of the ITO, including organic buffer layers, such as copper phthalocyanine (CuPc), polyethyleneimine (PEI), polyethylene oxide (PEO) and poly-allylamine (PAA), and inorganic insulator buffer layer, such as LiF, Al2O3, SiO2, HfOx
and MoO3 [122–124]. Among the different strategies, an ultra-thin Al2O3 insulating material has an effect of improving the current efficiency and power efficiency of OLEDs [123,125,126]. The several barrier reduction models were suggested to explain the enhanced device performance through the insulating buffer layer [123,125,127].
Here, Al2O3 layer fabricated by atomic layer deposition (ALD) at the interface between ITO and QD is applied to improve device efficiency. For the explanation of our results, the barrier reduction model from L. Zhou et al. and S.T. Zhang et al. is adopted [123,125]. They found that the accumulation of hole or electron at the interface of buffer/organic layer is necessary for the electron or hole injection through the buffer layer, which could apply to our QLED device with an Al2O3 insulating buffer layer (i.e.
hole accumulation at the Al2O3/QD interface).
Figure 5.3.1a shows a band diagram of inverted QLEDs with inserted Al2O3 layer.
In order to have efficient hole injection from the anode, thermally evaporated TCTA HTL, MoO3 HIL, and Ag anode were chosen for the inverted device structure. Figure 5.3.1b shows the J-V curves of different devices. With increasing the thickness of Al2O3, the driving voltage requested to achieve a certain current density is significantly increased. The threshold voltage also increases as the thickness of Al2O3 increases, 2.3 V, 2.6 V, 3.2 V, and 3.5 V for 0.0 nm, 0.4 nm, 0.8 nm and 1.6 nm of Al2O3 thickness, respectively. Even though the high driving voltages, the maximum device efficiency
performance decreased again with 1.6 nm of the Al2O3 layer as shown in Figure 5.3.1c.
Moreover, Figure 5.3.1e also shows the device with 0.8 nm of Al2O3 has highest power efficiency. The parasitic emission at the blue region in EL spectra of the device with an Al2O3 buffer layer in Figure 5.3.1f indicates that the RZ shifted from QD layer to near the QD/TCTA interface, while the device without Al2O3 shows clear spectrum without any parasitic emission. The highest intensity of parasitic emission with 0.8 nm of Al2O3
measured at 4 V means the largest RZ shift. According to the L. Zhou et al., the electron injection in the device with Al2O3 buffer layer can be explained not only the thermionic injection (n1 in Figure 5.3.2a) but also the quantum tunneling injection (n2L and noxTL in Figure 5.3.2a) [125]. Electron tunneling probability noxTL strongly depends on the thickness of oxide layer, while thermionic injection n1L depends on the energy offset between ITO and semiconductor layer. In our device, the n1L could not be enhanced efficiently even though the reduced work function of ITO by the increased Al2O3
thickness (Table 5.1) because the effects of reduced noxTL through the thicker Al2O3
layer is more dominant.
Table 5.1 Different film and device characteristics depending on the thickness of Al2O3. (The values of surface potential are based on the reference [123])
Al2O3 thickness
(nm)
Surface potential
[123]
(eV)
Vth
(V)
Voltage
@ 1 cd/m2
Max.
efficiency cd/A
cd/m2
@ 80 mA/cm2
0.0 4.7~4.9 2.3 4.3 0.13 100
0.4 4.7 2.6 4.5 0.25 46
0.8 4.5 3.2 5.0 0.47 28
1.6 4.4 3.5 7.3 0.07 30
However, the thermionic injection could increase more efficiently by the stronger band bending of Al2O3 buffer layer (i.e. vacuum level shift of oxide layer, eVoxH) and QD layer (esH) after the driving voltage increase as shown in the scheme of Figure 5.3.2b. The vacuum level shift of oxide layer is facilitated by the increase of the voltage drop across the buffer layer by the efficient hole accumulation at the Al2O3/QD interface, which is attributed to the sufficient hole injection into the valence band of QD by our efficient hole transport system in the device (i.e. TCTA/MoO3/Ag). Therefore, electron injection could be increased by the facilitated both n1H and noxTH. On the other hand, the current density requested to achieve a certain voltage is still getting lower as the thickness of Al2O3 layer increase (Figure 5.3.1b), which indicates that the decreasing
rate of noxT is larger than the increasing rate of thermionic injection n1. Once again, the increased electron injection has more strong effects on increasing the current efficiency and power efficiency even though the reduced total current density by a thicker insulation layer. Moreover, the shifted RZ recognized by the TCTA emission from the device with Al2O3 is another reasonable agreement of the enhanced electron injection by the Al2O3
buffer layer.
(a) (b)
1 10 100 1000
0.01 0.1
0.0 nm 0.4 nm 0.8 nm 1.6nm
Current efficiency [cd/A]
Current density [mA/cm2]
(c) (d)
4 5 6 7 8 9 10 11 12
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.35 0.0 nm
0.4 nm 0.8 nm 1.6 nm
Power efficiency [lm/W]
Voltage [V]
400 420 440 460 480 500
0.00 0.01 0.02 0.03 0.04 0.05 0.06
0.07 0.0 nm
0.4 nm 0.8 nm 1.6 nm
400 450 500 550 600 650 700 750 800 0.0
0.2 0.4 0.6 0.8 1.0
Normalized EL intensity [a.u.]
Wavelength [nm]
Normalized EL intensity [a.u.]
Wavelength [nm]
(e) (f)
Figure 5.3.1 (a) A band diagram of QLEDs with inserted Al2O3 buffer layer, (b) Current density-voltage, (c) current efficiency-current density, (d) luminance-voltage, (e) power
(a) (b)
Figure 5.3.2 The schematic of band bending diagram under the relatively (a) low and (b) high electric field.
However, the enhancements of current efficiency are more distinct under the low current density (Figure 5.3.1c, < 10 mA/cm2). These changes can be addressed by the increased parasitic emission in EL spectra through the modification of RZ with the insertion of the Al2O3 buffer layer. The increased electron injection into QD by the reduced oxide tunneling barrier under higher current density could induce a severe Auger luminance quenching by the excess of electrons at the RZ [22,53]. Moreover, the luminance quenching becomes more serious problems when the exciton recombines near the QD/TCTA interface or at the TCTA layer because of the non-radiative exciton recombination by the defect states at the interface or the energy loss through the inefficient organic EL. Therefore, the device with 0.8 nm of the Al2O3 layer, which has largest parasitic emission, has higher current efficiency drop as the current density increase. The high power efficiency only under low voltage (i.e. below 7.5-9 V) from the devices also corresponds with above explanation.
There is another interesting double diode behavior on J-V-L characteristics in Figure 5.3.1b,d. The luminance and current density of the devices with 0.4 nm and 0.8 nm of the Al2O3 buffer layer are increased exponentially again at the 9.1 V and 8.3 V, respectively. As the hole injection increases at the high electric field, more holes can be accumulated at the Al2O3/QD interface as shown in Figure 5.3.2b, which will increase the vacuum level shift of oxide layer (eVoxH) by the increasing voltage drop across the
buffer layer. In addition, the strong electric field will enhance the band bending of QD layer (esH). Both band-bending under the high electric field now enhance the quantum tunneling injection n2H, which increases the current density again by more electron injection into QDs. However, the 0.8 nm of Al2O3 could not have efficient quantum tunneling injection n2H because the electron tunneling probability noxT is still relatively too low. For the device without Al2O3,the hole can be easily dissociated to the counter electrode in the absence of buffer layer. Therefore, the double diode behavior cannot be observed from both conditions. Additionally, the reduced plasmon-exciton interaction could also contribute the enhanced current efficiency. The separation of plasmon in the electrode and exciton in the QD by insertion of the buffer layer may suppress the luminance quenching [126,128].
Since the ultra-thin PEI interfacial dipole layer could reduce the work function of ITO just as thin Al2O3 layer [122,124], the different concentration of PEI solution for the various thickness was applied in our inverted QLEDs. Figure 5.3.3 shows the current efficiency as a function of luminance characteristics comparing the results from the Al2O3 layer. The current efficiencies from the device with PEI increased via the similar mechanism of the Al2O3 layer. However, the enhanced value is higher, and the efficiencies are relatively stable as luminance increase than the devices with the Al2O3
layer. Since solution processed PEI interfacial layer have lower dielectric constant (i.e.
3.8 [129]) compared to the ALD Al2O3 (i.e. 8~9 [130]), the electron tunneling probability (noxT) in the PEI layer is higher than that of the Al2O3 layer. Therefore, the electron injection through PEI is more efficient than Al2O3 buffer layer combining the effects of improved thermionic injection with thicker PEI layer (n1) [131].
0 20 40 60 80 100 120 140 160
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
0.0 nm 0.4 nm 0.8 nm 1.6 nm
Current efficiency [cd/A]
Luminance [cd/m²]
0 20 40 60 80 100 120 140 160 0.0
0.2 0.4
0.6 0.0 wt%, PEI
0.025 wt%
0.05 wt%
0.1 wt%
0.2 wt%
Current efficiency [cd/A]
Luminance [cd/m²]
(a) (b)
Figure 5.3.3 Current efficiency-voltage characteristics of the devices with (a) Al2O3 and
Electron injection from ITO to QD without any buffer layer
(a) (b)
1 10
10-5 10-4 10-3 10-2 10-1 100 101 102 103 104
ITO/QDs/TCTA/MoO3/Ag ITO/ZnO/QDs/TCTA/MoO3/Ag
Current density [mA/cm2 ]
Voltage [V]
2 3 4 5 6 7 8 9 10 11
10-1 100 101 102 103
ITO/QDs/TCTA/MoO3/Ag ITO/ZnO/QDs/TCTA/MoO3/Ag Luminance [cd/m2 ]
Voltage [V]
(c) (d)
Figure 5.3.4 The band diagram of (a) quasi HOD and (b) bipolar inverted device and their (c) current density-voltage and (d) luminance-voltage characteristics. (YH180, YH203)
There is an interesting behavior from the device fabricated without any buffer layer (ITO/QD/TCTA/MoO3/Ag). This device is basically designed as the hole only device (HOD, Figure 5.3.4a) because 1.6 eV of energy offset between ITO to CBM of QDs would not allow the electron injection. However, Figure 5.3.4d shows that the EL is observed from 4 V (i.e. turn-on voltage) while the threshold voltage from quasi-HOD is around 2 V. The higher turn-on voltage compared to the threshold voltage indicates the electron injection for the exciton generation and recombination only begins over the 4 V, while the hole injection occurs over the 2 V. On the other hand, the threshold voltage of the bipolar device (Figure 5.4.3b) is also around 2 V. According to the observation of the similar threshold voltage from both devices, it can be expected that the threshold voltage is confined by the hole injection barrier into QD layer, not by that of electron injection. In addition, the current density of quasi-HOD is similar to the bipolar inverted device, which also indicates that hole is majority carrier in our inverted device structure.
Therefore, the reducing the hole injection barrier would bring the lower threshold voltage
from the device (i.e. low driving voltage) and higher device performance (see the Chapter 6.2.2 ).
(a) (b)
Figure 5.3.5 (a) Schematic energy level diagram of ITO/QD/TCTA/MoO3/Ag HOD, and (b) an illustration of the Auger assisted electron injection process at the ITO/QD interface:
recombination of interfacial exciton, resonant energy transfer between from the interfacial exciton to the accumulated electrons, injection of the high energy electron into CBM of QD, and radiative recombination in QD emitter.
Besides the efficient hole injection at low voltage, the possible electron injection over around 4 V is more interesting. The electron injection and the recombination process can be explained based on the Auger-like energy up-conversion process [108]. According to the L. Qian et al., energy up-conversion process can be facilitated in the condition of efficient carrier injection from the electrodes, an accumulation of charge carriers at the interface, and nanoparticle materials for efficient surface interaction [107]. Our system is suitable for the following conditions (Figure 5.3.5a). The hole injection into QD is sufficient according to the J-V curves in Figure 5.3.4a, and the injected holes can be accumulated at the edge of the quantum well close to the interface of ITO/QD due to a wide band gap ZnSe/ZnS shell and a localized hole wave function in the InP core. For the electron will be accumulated at the ITO/QD interface by the high offset from ITO to CBM of QD. The Auger assisted energy up-conversion process can also be boosted up by the nanocrystal properties of InP-based QD. Therefore, the possible Auger assisted electron injection process is depicted in the Figure 5.3.5b. The accumulated electrons
transferred to the accumulated electrons at the ITO/QD followed by the Auger assisted electron injection into the CBM of QD. Finally, the generated excitons in QD have radiative recombination. However, the injection barrier from ITO to CBM of QD should be reduced by inserting HTL or HTL to develop more efficient inverted QLEDs.