6.1 Auger-assisted charge neutralization model
6.1.2 Charge neutralization model accompanying Auger-assisted electron
CBM of ZnO to QDs (luminance from all devices is observed after the threshold point) [107].
the energetically aligned mid-gap states of adjacent ZnO nanoparticle. The only difference of our results compared to the reference is that the brightening of luminance is weaker and the increased luminance begins to decrease again in short timescale. In our case, the energy offset between the CBM of ZnO nanoparticles and that of InP/ZnSe/ZnS QDs would bring the lower level of positively charged QDs in the equilibrium state.
Moreover, the lower VBM of InP/ZnSe/ZnS QDs than that of CdSe/CdS facilitates the hole injection into QDs, and brings more hole accumulation at the interface of ZnO/QD, which could lessen the effects of neutralization of QD emitter during the operation; the charging behavior is stronger than neutralization process in our devices. Therefore, the brightening of luminance is less than the results from the literature.
A modified model from the reference could explain why the efficiency and luminance of the device with thinner QD layer overtake those from the thicker QD layer at a higher driving current density (Figure 6.1.4).
Figure 6.1.4 Illustrated process for neutralization of charged QD and Auger-assisted electron injection through interactions between InP/ZnSe/ZnS QDs and ZnO nanoparticles.
Electrons are accumulated at the CBM of ZnO nanoparticles near the interface with QDs because of the 0.7 eV of the energy barrier to the QDs. Holes are also accumulated more at the VBM of InP/ZnSe/ZnS QDs near the interface of ZnO nanoparticles.
However, the accumulated holes could be efficiently extracted to the adjacent mid-gap states of ZnO layer which is well aligned with the VBM of QD layer, and then the extracted hole could recombine with the electron accumulated at the ZnO/QD interface.
The energy generated from the recombination transfers to the other electrons at the ZnO/QD interface followed by the electron injection into the CBM of QD, which is the
extraction could be facilitated by the even strong electric field during the operation in the same direction, which would reduce the field-induced quenching problem. Finally, the neutralization of charged QD and Auger-assisted electron injection would remain the QDs in an emissive, less charged state under the high current density operation. Again, the thinner QD layer close to the ZnO would have more stable uncharged state through the explained mechanism, but a thicker QD layer, which has more distance from the ZnO/QD interface, would experience stronger luminance quenching and remain unstable during the operation. Thicker QD layer also needs a high electric field to operate the device, which would cause a more severe field-induced quenching [80]. Even though the hole extraction from QD to ZnO layer is possible in this mechanism, the electron block property against exciton dissociation still works because very low current density (c.a.
1.4 ×10-4 mA/cm2) is observed when the device is operated in the reverse bias, as shown in Figure 6.1.5.
-3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
10-4 1.2x10-4 1.4x10-4 1.6x10-4 1.8x10-4 2x10-4 2.2x10-4 2.4x10-4 2.6x10-4 2.8x10-4 3x10-4
Reverse driving -1 Reverse driving -2
Current density [mA/cm2 ]
Voltage [V]
Figure 6.1.5 Current density-voltage characteristic with first reverse bias sweep. (YH202) Interestingly, the conventional QLEDs with different QD thickness show slightly different trends on the device performance as shown in Figure 6.1.6. A trade-off between luminance and efficiency is not observed in Figure 6.1.6b. Moreover, the current efficiency drops immediately as luminance increase in the most of the conventional QLEDs, which is not observed from the inverted QLEDs in our experiments. In the conventional structure, we only used the organic charge transport layer, and then the electron is very easily accumulated at the interface of QD/poly-TPD because of the facilitated electron injection through an only small contact barrier between QD/TPBi interfaces. The accumulated charges at the interface of QD/poly-TPD have no possibility to be neutralized with adjacent organic layers; rather, the charged QD leads to
unstable current efficiency along the driving voltage because the generated excitons in the RZ undergo Auger recombination quenching with the excess holes (i.e. positive charges). This quenching process is severe because the RZ is close to the interface of QD/TPD (Figure 6.1.6c), which is indicated by the parasitic emission of poly-TPD at the peak of 425 nm in the EL spectrum of all devices with different QD concentration. Consequently, the current efficiency of all devices begins to decrease seriously by the luminance quenching process from the beginning of the EL of the device.
0 100 200 300 400 500 600
0 1 2 3 4 5
0 1 2 3 4 5 6 7 8 9 10
10-6 10-5 10-4 10-3 10-2 10-1 100 101 102
3 mg/ml 7 mg/ml 9 mg/ml Current density [mA/cm2]
Voltage [V]
3 mg/ml 7 mg/ml 9 mg/ml
Current efficiency [cd/A]
Luminance [cd/m²]
(a) (b)
400 500 600 700 800
10-2 10-1 100
3 mg/ml 7 mg/ml 9 mg/ml
Normalized EL intensity [a.u.]
Wavelength [nm]
0 50 100 150 200 250 300
0 20 40 60 80
100 3 mg/ml @ 5 V, 700 cd/m2
Luminance [cd/m2 ]
Time [sec]
(c) (d)
Figure 6.1.6 (a) A band diagram, (b) current efficiency-luminance curves and (c) photoluminescence spectra for the conventional QLEDs with different QD layer conditions. (d) Luminance changes as a function of time, which is measured in the more optimized structure that has maximum current efficiency and luminance of 8.8 cd/A and 3200 cd/m2, respectively. (YH37, YH38, YH39, (d) YH81)
Moreover, the luminance drops from the beginning in the lifetime measurement shown in Figure 6.1.6d also have the good agreement of no neutralization process in a conventional device with 3 mg/mL QD solution. On the other hand, the thinner QD layer (i.e. lower concentration) increases the performance of the conventional device regarding both luminance and current efficiency by the modulated charge balance. In contrast, the
unbalance at the RZ near the QD/poly-TPD interface due to the high resistance of the thick emitter layer, which results in the low device performance.
Additionally, the superior efficiency with thicker QD layer at a low luminance region is not observed. It can be addressed by the following hypothesis. The QD layer part which is close to the interface with TPBI has a low probability of generating the excitons because the fast electron injection from the cathode and the low hole mobility of the QD layer still confine the RZ near the QD/poly-TPD interface. Therefore, even thicker QD layer has low exciton recombination efficiency even in the low luminance region. The influence of carrier mobility and thickness of QDs on the RZ will be discussed further in the following chapters.
6.2 Optimization trends according to the thickness of QD layer