Ultrastable Quantum-Dot Light-Emitting Diodes by Suppression of

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Ultra-Stable Quantum-Dot Light Emitting Diodes by Suppression of Leakage Current and Exciton Quenching Processes Han Zhang, Ning Sui, Xiaochun Chi, Yinghui Wang, Qinghui Liu, Hanzhuang Zhang, and Wenyu Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09246 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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ACS Applied Materials & Interfaces

Ultra-Stable

Quantum-Dot

Light

Emitting

Diodes

by

Suppression of Leakage Current and Exciton Quenching Processes Han Zhang, Ning Sui, Xiaochun Chi, Yinghui Wang, Qinghui Liu, Hanzhuang Zhang,* and Wenyu Ji* College of Physics, Jilin University, Changchun 130012, China

Abstract A study of hybrid inverted quantum-dot light-emitting diodes (QD-LEDs) constructed with and without Al2O3 interlayers is presented. The Al2O3 interlayers are deposited at ZnO/QDs or/and QDs/4,4'-Bis(carbazol-9-yl)biphenyl (CBP) interfaces, resulting in large improvement of device performance, including luminance, current efficiency, and device lifetime. Especially, the devices with QD emitters sandwiched by two Al2O3 layers exhibits outstanding performance, the longest operation lifetime and mediate efficiency. The maximum current efficiency of 15.3 cd/A is obtained, an enhancement factor of 35% in comparison to that (11.3 cd/A) of conventional device without Al2O3 layer. Moreover, device lifetime is also largely enhanced, over 110,000 hours for the device containing two Al2O3 interlayers, nearly 40% enhancement relative to that of conventional device that shows a lifetime of only 80,000 hours. Based on electrical property and photoluminescence spectroscopy studies, we

*

[email protected]; [email protected]

ACS Paragon Plus Environment


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demonstrate that the Al2O3 interlayers play crucial roles in suppressing the leakage current across the device and reducing exciton quenching induced by ZnO. Key words: quantum dot; QLED; exciton quenching; leakage current; insulating layer;


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1. Introduction: Colloidal quantum dots (QDs) have been the subject of intense research over the past three decades due to their unique size-dependent optical properties， high photoluminescence (PL) quantum efficiency, low-cost solution processability and potential applications for optoelectronic devices.1-5 Especially, many researchers and industries have taken enormous efforts aimed at applying QDs in the next generation lighting and display technologies.1-14 Since the first QDs based light-emitting diodes (QD-LEDs) reported by A.P. Alivisatos,2 steady progress has been achieved by optimizing the material synthesis and device design. As a result, the device performance has been comparable to that of fluorescent organic LEDs that have been extensively investigated for over two decades.15,16 In common QD-LEDs, the emitting layer consists of a thin or monolayer QD layer sandwiched between two charge transport layers. As we know, due to interaction with adjacent charge transport layers, QDs may be charged and interfacial charge transfer processes are active, which will quench the QD fluorescence. To date, it remains unclear how the fluorescence property of single QDs is affected under device environment, but it is confirmed that a thin emission layer is highly susceptible to the initial surface and interface charge traps, resulting in exciton quenching through various non-radiation pathways, such as Auger recombination, QD charging and electron or/and energy transfer from QDs to the charge transport materials, etc. All the non-radiative processes must lead to a deteriorated photoelectric performance of QD-LEDs. In order to alleviate the effect of charge transport layer on the QD emitting processes, much effort was devoted to



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material synthesis and device structure design. In the aspect of material synthesis, “Giant” quantum dots were synthesized and used as the emissive layer in QD-LEDs, in which the thick shell plays a key role in suppressing the Auger recombination in the QDs and improves the device performance.17-19 For the device design, a great deal effort has been made to reduce the leakage current and suppress the exciton quenching induced by the charge transport layer. One feasible approach is to introduce a buffer layer

into the

QD-LEDs

at

QD/charge-transport-layer

interface,

such

as

polyethylenimine ethoxylated (PEIE),20 poly(9-vinlycarbazole (PVK),21 Cs2CO3,22 polyvinylpyrrolidone (PVP),23 and poly(methylmethacrylate) (PMMA).12 In particular, the QD-LED with an external quantum efficiency of 20.5% and device lifetime over 100,000 hours was reported by Peng group in 2014,12 which is hailed as a milestone in the QD-LED technology. In their device, a PMMA insulator layer was inserted between QDs and ZnO nanoparticle layer, which optimized the charge balance in the device and suppressed the exciton quenching induced by ZnO nanoparticles. Moreover, the PMMA layer is a benefit to the reduction of hole leakage current across ZnO layer. All in all, the buffer layers mentioned above, especially the insulating buffer layers, have a similar effect on QD-LEDs and play a crucial role in determining the device photoelectric properties and help to improve QD-LED performance. However, the QD-LED possessing the highest external quantum efficiency was built by inserting an organic insulating buffer layer of PMMA.12 As we know, almost all of the organic materials are sensitive to moisture and oxygen, which limits the stability of QD-LEDs. As concerned above, it should be desired to construct a QD-LED with


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an inorganic buffer layer instead of the organic one due to better photoelectric properties and stability of inorganic materials. In this study, the effect of an inorganic Al2O3 interlayer on device performance is systematically investigated by employing a standard inverted device architecture consisting of ITO/ZnO/QDs/ 4,4'-Bis(carbazol-9-yl)biphenyl (CBP)/MoO3/Al. We demonstrate that the inorganic Al2O3 interlayer is benefit to improve the QD-LED performance, including photoelectric performance and operating stability. More significant, the device with the QD emissive layer sandwiched between two Al2O3 inorganic insulating layers exhibits the longest operation lifetime and mediate efficiency. Our steady-state and time-resolved PL (TRPL) studies are well consistent with the photoelectric results obtained for QD-LEDs, which further elucidates the influence of interlayers on device performance. The first Al2O3 layer at ZnO/QDs interface suppresses the exciton quenching induced by ZnO nanoparticles and reduces the hole leakage current across the ZnO layer. The second Al2O3 layer at CBP/QDs interface alleviates the leakage of electrons from QDs to CBP. Consequently, QD-LED with higher current efficiency of 14.5 cd/A and longer operating lifetime over 110,000 hours is obtained. Our work offers a simple, reliable and cost-effective approach to enhance the QD-LED performance. 2. Experimental details The QDs and ZnO nanoparticles were prepared according to the methods depicted in our previous paper.24 The QDs possess the structrue of CdSe/CdS/ZnS. Before fabricating the QD-LEDs, patterned indium tin oxide (ITO)-glass substrates were



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cleaned sequentially using detergent, di-ionized water, acetone, ethanol, and di-ionized water, and were then treated for 7 min under UV-ozone. The standard device (Device A) with a structure of ITO/ZnO (45 nm)/QDs (30 nm) /CBP (55 nm)/MoO3 (7 nm)/Al (100 nm) were fabricated with solution processes combining with thermal evaporation technology. Firstly, the ZnO film was deposited onto the UV-ozone treated ITO substrates at 2000 rpm from a 25 mg/ml ZnO nanoparticle ethonal solution and then annealed at 150°C for 30 min. The QDs were dispersed in toluene (5 mg/ml) and spin-coated onto the ZnO layer at 2500 rpm and then annealed at 150 °C for 30 min. In order to investigate the effect of Al2O3 layer on the device performance, we built another three QD-LEDs with similar architecture to the standard device (Device A) but inserting Al2O3 layers at different interfaces: Device B: ITO/ZnO/Al2O3/QDs/CBP/MoO3/Al Device C: ITO/ZnO/QDs/Al2O3/CBP/MoO3/Al Device D: ITO/ZnO/Al2O3/QDs/Al2O3/CBP/MoO3/Al The Al2O3 layer was deposited by spin-coating the Al2O3 precursor solution and then annealed at 150°C for 40 min. The Al2O3 precursor solution was prepared according to the method reported in Ref. [25]. The freshly prepared aluminum acetate colloid (2 mg) was dissolved in 1 ml 2-methoxy ethanol and 4 µl ethanolamine. The thickness of Al2O3 interlayer is estimated by measuring the thickness of ITO/ZnO and ITO/ZnO/Al2O3 samples through AFM measurements. The statistical results show that the two types of samples possess almost the same thicknesses (~45 nm) within experimental error. So we estimate that the thickness of Al2O3 layer is less than 2 nm.


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All of the spin-coating procedures were carried out in a nitrogen-filled BRAUN glove box. Finally, CBP, MoO3 and aluminum (Al) layers, respectively serving as hole transport layer, hole injection layer, and cathode, were successively deposited with the thermally evaporated technology at a pressure of less than 4.5×10-4 Pa. The layer thicknesses were 55, 7, and 100 nm for CBP, MoO3, and Al, respectively. Characteristics: The characteristics of current density and luminance versus voltage were measured by a programmable Keithley model 2400 power supply combined with a Minolta Luminance Meter LS-110. The electroluminescence (EL) spectra of the devices were obtained through Ocean Optics Maya 2000-PRO spectrometer. The room temperature absorption spectrum was measured with an ultraviolet/visible spectrometer (UV 1700, Shimadzu) and the PL spectrum of the QDs in toluene and QD films were recorded by a Hitachi F-4500 spectrophotometer and Ocean Optics spectrophotometer (MAYA 2000pro),respectively, under an excitation wavelength of 400 nm. TRPL measurements were carried out with Edinburgh Instruments FL920 spectrometer, utilizing a 450 nm excitation light source. The transmission electron microscopy (TEM) images were recorded on a Philips TECNAI G2. The atomic force microscopy (AFM) images were recorded by a Nanosurf EasyScan2 FlexAFM. The energy-dispersive X-ray spectroscopy (EDS) spectra were obtained from by X-ray energy dispersive spectrometer (Hitachi S4800). 3. Results and discussion In order to investigate the effect of Al2O3 interlayer on the device performance, a set of inverted QD-LEDs with and without interlayer were fabricated as shown in Figure



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1(a). The behind mechanism of device performance improvement is explored by inserting the inorganic Al2O3 interlayer at different interfaces. The energy level alignment of the device fabricated in our work is shown in Figure 1(b). The PL and absorption spectra of the QDs in toluene are shown in Figure 1(c). The PL peak of QDs in toluene is 600 nm with a full width at half maximum (FWHM) of 42 nm. The exciton absorption peak is not obvious due to the thick alloying shells outside the QDs. Inset is the TEM image of the QDs used in our work. In addition, the size distribution and elemental analysis based on EDS spectra were shown in Figure S1 and S2 in the Supporting Information. The QDs possess an average diameter of ~7.7 nm according to the size distribution analysis. As we know, leakage current will produce much Joule heat in the devices, which must degrade the device lifetime and luminous efficiency. Thus, higher operation stability of QD-LEDs is achievable by reducing the leakage current. Figure 2 shows the optical-electrical characteristics of QD-LEDs with and without Al2O3 interlayer. We can see that the current density of the QD-LEDs is decreased with the insertion of Al2O3 interlayer due to the increased injection or/and transport barrier to the carriers (electrons and holes), which is attributed to the insulator nature of inorganic Al2O3. Additionally, it is worth to noting that the current density of QD-LEDs containing Al2O3 interlayer is much lower than that of device without Al2O3 layer at low driving voltage range (< 2.5 V). In other words, the introduction of Al2O3 interlayer reduces the leakage current across QD-LEDs. Another crucial phenomenon is that the current density of device C is much lower than that of device B at the low operating voltage


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range, although both of devices all contain only one Al2O3 interlayer. It means that the Al2O3 interlayer inserted at QDs/CBP has a larger effect on the device leakage current than that at ZnO/QDs interface, which indicates that the leakage current primarily originates from electron leakage from QDs to CBP layer in this kind of QD-LEDs. In addition, we can observe that the current density of device B is similar to (slight lower than) that of device A at the voltage range from 2.5 to 8V, which reveals that the Al2O3 interlayer at ZnO/QDs interface has little effect on the conduction properties of device B. In other words, this Al2O3 interlayer may primarily play a role in passivating the trap states on ZnO nanoparticle surface. We will explain this in the following text by TRPL measurement. Figure 2 (b) shows the voltage versus luminance characteristics of the QD-LEDs. As can be seen, the turn-on voltage (defined as the driving voltage at luminance of 1 cd/m2) is similar for all the QD-LEDs, which indicates that the introduction of Al2O3 interlayer could not highly increase the barrier to both electrons and holes injected form charge transport layers to QDs. But the luminance of device B is higher than that of device A at the driving voltage range from 2.5 to 3.5 V, which should originate from the passivation of ZnO nanoparticle surface by Al2O3, hence reducing the exciton quenching induce by ZnO. We also performed the AFM measurements for samples of ITO/ZnO and ITO/ZnO/Al2O3 as shown in Figure S3. The AFM results show that the introduction of Al2O3

layer

have

no

obvious

influence

on

the

ZnO

morphology,

the

root-mean-squares (RMS) are 5.35 and 5.40 nm for ZnO film without and with Al2O3 layers,

respectively.

AFM

images

of

ITO/ZnO/Al2O3/QDs


and


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ITO/ZnO/Al2O3/QDs//Al2O3 are shown in Figure S4. The RMS are 1.33 and 1.44 nm for QD layers without and with Al2O3, respectively. These results indicate that the Al2O3 layer has no effect on the QD surface morphology. All the results discussed above present an oriented conclusion to us that the Al2O3 interlayers not only reduce the leakage current, but also suppress the exciton quenching in the QD-LEDs. As shown in Figure 3, the luminous efficiencies of QD-LEDs are enhanced with the insertion of Al2O3 interlayers, especially at low driving current (less than 10 mA/cm2) or voltages. These results are in well accordance with the optical-electrical characteristics of devices exhibited in Figure 2. Moreover, the efficiencies are further enhanced when an Al2O3 interlayer was inserted at QDs/CBP interface in devices C and D, which reveals that the leakage current primarily occurs at QDs/CBP interface and this Al2O3 interlayer blocks the electron leaked from QDs to CBP. As a result, devices C and D possess higher efficiencies at low driving current. To demonstrate the hypothesis proposed above, we studied the effect of Al2O3 interlayers on the fluorescence properties of QDs through both fluorescence lifetime and steady-state PL spectroscopy measurements. Five samples were prepared, S1: ITO/ZnO/QDs/CBP, S2: ITO/ZnO/Al2O3/QDs/CBP, S3: ITO/ZnO/QDs/Al2O3/CBP, S4: ITO/ZnO/Al2O3/QDs/Al2O3/CBP, and S5: Glass/QDs/CBP. The fluorescence decay curves are shown in Figure 4, which are well fitted by a two-exponential equation and shown with white lines. In addition, the fluorescence decay data for the QDs in toluene are also recorded as shown in the Figure S5 in the Supporting Information. The exciton lifetimes for different film samples are summarized in Table


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1. We can see that the exciton lifetimes are shortened in the samples with QDs sandwiched between different active layers compared with that in sample with QDs directly deposited on glass substrate. However, the exciton lifetime is increased compared to S1, when the Al2O3 layer is inserted at ZnO/CBP interface. As we know, when the quantum dots are directly contacted with the ZnO layer, a spontaneous charge transfer process will occur, resulting in QDs being positively charged as reported previously,12.14 which leads to a shortened exciton lifetime and emission quenching of QDs. The insertion of a thin Al2O3 layer can suppress the QDs/ZnO interfacial interaction, increasing the exciton lifetime of QDs from 24.9 to 28.5 ns. In addition, it is worthy to note that inserting Al2O3 interlayer at the QDs/CBP interface also leads to a longer exciton lifetime relative to S1. We have demonstrated that the energy/electron transfer from QDs to CBP can be ignored.26 Herein, we also carried out the fluorescence decay measurements for the samples with structures of ITO/ZnO/QDs and ITO/ZnO/QDs/CBP, and the results demonstrate that the exciton lifetimes are almost the same in these two samples (25.1 and 24.9 ns for samples of ITO/ZnO/QDs and ITO/ZnO/QDs/CBP, respectively), which indicates that the CBP layer has little effect on the QD dynamics processes and the increase of exciton lifetime in S3 should originate from the interaction between Al2O3 and QDs. There are many reports on the fluorescence enhancement with QDs under gaseous environments,27,28 or processed with oxygen and water.29,30 In most cases, the behind mechanism for the fluorescence enhancement is attributed to passivation of surface trap states. Considering the closed correlation between fluorescence and exciton



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lifetime, we think the solution processed Al2O3 also plays a similar role in passivating the QDs. But the physics mechanism is not clear and more investigations are needed. In order to further demonstrate the effect of Al2O3 interlayer on the QD film fluorescence emission, we also carried out the steady-state PL measurement and the results are shown in Figure 5. The sample of S6 with the same structure to S3 but post-processed by ethanol after QD layer deposition31 is also shown to distinguish the effect of Al2O3 and ethanol on the fluorescence properties of QD layer. Inset is the enlarged PL spectra to exhibit the PL intensity clearly. Note that the PL intensity is enhanced when an Al2O3 buffer layer is inserted at ZnO/QDs or QDs/CBP interface due to the suppression of the interaction between QDs and ZnO or passivation of the QD surface trap states. The steady-state PL results are in accordance with those of TRPL ones. All the results reveal that perhaps, the Al2O3 layer should help to maintain QD emitter in charge neutrality state and preserve their superior fluorescence properties. However, it is worth to noting that device C with a single layer Al2O3 layer exhibits the best efficiency but sample S4 with two Al2O3 layers shows the highest PL intensity (or efficiency). This disparity should be due to the different excitation source for PL and EL. For the PL measurements, the excitation conditions for every sample are identical, but it is not the case for EL processes. Relative to device C, the introduction of the second Al2O3 layer at QDs/CBP interface in the device D should result in a different carrier injection processes. As can be seen from Figure 3, the roll-off of device D is larger than that of device C, which indicates that the carrier injection into the device D is more imbalanced than that in device C. So the efficiency


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of device D is a little lower than that of device C. Figure 5(b) shows the normalized PL spectra of different samples, S1, S2, S3, S4, and S6. As can be seen, the PL spectra possess the same profile for every sample, which indicates that the introduction of Al2O3 only affect the surface of the QDs instead of changing the QD structure. It is well known that both the formation of charged QDs and the presence of interfacial electron/energy transfer pathways will decrease the PL quantum yield (PLQY) of the QDs. According to the discussion above, the insertion of Al2O3 interlayers must suppress the occurrence of QD charging or/and electron/energy transfer processes between charge transport layers and QDs, consequently, the fluorescence performance is mostly preserved like the case of QDs on glass substrate. The PLQY of QD films and exciton lifetimes in different samples are summarized in Table 1. We can find that the PLQYs of the QD films are substantially improved with the introduction of Al2O3 interlayers. The lifetimes of excitons in QDs are also increased due to the suppression of nonradiative pathways. In particular, the sample of S4 with QDs sandwiched two Al2O3 interlayers shows the highest PL intensity and longest exciton lifetime among the samples containing ZnO nanoparticles. These results suggest that both of Al2O3 interlayers play a role in modifying the interfaces and improving the QD fluorescence emission. Most notably, the operation stability of QD-LEDs, including stable EL emission at different bias and long-term operation stability, is significant to the industrial production and commercialization applications. We also assess this parameter through EL and device lifetime measurements. Figure 6 shows the normalized EL spectra of



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QD-LEDs with and without Al2O3 interlayers. Note that not any obvious change is occurred for the EL spectra of devices with and without Al2O3 interlayers, so we can make sure that the exciton recombination zone is not changed in the QD-LEDs, i.e., the excitons are still formed in QDs after the interlayer introduction into the devices. For the device lifetime measurements, different driving voltages were applied across device A and D in order to achieve similar device luminance and ensure the fair comparison between two devices. Each device tested at a constant driving current density, 50 mA/cm2 for all of the devices. Our QD-LEDs were simply sealed by ultraviolet-curable resin with a glass cover for the lifetime test. The device lifetime is estimated by the relation of L0nT50 = const.32 L0 is the initial luminance, 4930 and 5792 cd/m2 for devices A and D, respectively. The n is an acceleration exponent, referred to as acceleration factor. According to the literature,32 the values of acceleration factor n were in the range of 1.5 to 2 for the phosphorescent emitters based organic LEDs. Due to the similar characteristics of QDs to phosphorescent emitters, such as nearly 100% quantum yield, the low limited n value of 1.5 is chosen in the device lifetime measurements of QD-LEDs in order to ensure the data reliability.12 The device lifetime under luminance of L0 is the value of half lifetime (T50) that is defined as the time for the luminance to decrease to half of the initial luminance. As shown in Figure 7, the device lifetime Tlifetime for device at luminance of 100 cd/m2, calculated by L0nT50 = 100n Tlifetime, is over 79,000 and 119,000 hours for devices A and D, respectively. An enhancement factor of 47% is obtained for the device lifetime when two Al2O3 interlayers are introduced. In addition, we also


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estimated the stability of the devices B and C with an original luminance of 4956 and 6849 cd/m2, respectively, at operating current density of 50 mA/cm2. The operation lifetimes are 82,000 and 95,000 hours at 100 cd/m2 for devices B and C, respectively, falling in between that of device A and D. This is an inspiring result because the device lifetime of over 110,000 hours is enough to the commercialization for QD-LEDs. Conclusions In conclusion, the inorganic Al2O3 interlayer plays two important roles in improving the device performance. Firstly, it reduces the leakage current across the device and results in better confinement of both electrons and holes in the QDs where they can form excitons and then realize radiative emission. Secondly, it suppresses the exciton quenching induced by ZnO nanoparticles, preserving the high QY nature of QDs. Moreover, the Al2O3 interlayer is achieved by a solution process and annealing at low temperature (only 150 oC), which ensures this process is comparable to the manufacturing techniques of QD-LEDs. Our results lay the foundation for rational design of QD-LED structure and offer a simple and feasible approach to fabricating efficient QD-LEDs. It is obvious that these techniques can be extended to other colored inverted QD-LEDs. We believe that these techniques would lead to efficient and stable electroluminescent devices based on QDs for display and lighting applications.



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Supporting Information The Supporting Information is available in the online version of the paper (http://pubs.acs.org). TEM images of CdSe/CdS/ZnS core/shell QDs and corresponding size distribution histograms; Energy-dispersive X-ray spectroscopy (EDS) spectra of QDs and fluorescence decay curve of QDs in toluene; AFM images of ITO/ZnO, ITO/ZnO/Al2O3,

ITO/ZnO/Al2O3/QDs,

and

ITO/ZnO/Al2O3/QDs//Al2O3

samples.


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Acknowledgements This work was supported by the program of the National Natural Science Foundation of China (Nos. 1167040699, 11474131, 21573094, and 11274142).



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Figure captions Figure 1. (a) The schematic structure of the QD-LED with QD emissive layer sandwiched between two inorganic Al2O3 layers; (b) Flat-band energy level diagram of different materials used in this work; (c) The PL and absorption spectra of QDs in toluene. Inset is the TEM image of QDs. Figure 2. (a) Current density-voltage and (b) luminance-voltage properties of QD-LEDs. Figure 3. Current density versus efficiency curves for the devices. Figure 4. Time-resolved PL spectra of samples with and without Al2O3 interlayers. S1:ITO/ZnO/QDs/CBP,

S2:ITO/ZnO/Al2O3/QDs/CBP,

S3:

ITO/ZnO/QDs/

Al2O3/CBP, S4: ITO/ZnO/Al2O3/QDs/Al2O3/CBP, and S5:Glass/QDs/CBP. Figure 5. (a) PL spectra of different samples, S1, S2, S3, S4. The sample of S6 with the same structure to S1 but post-processed by ethanol after QD layer deposition is also shown to further demonstrate that the effect on fluorescence properties of QD layer is originated form Al2O3 but not ethanol. Inset is the enlarged PL spectra to show the PL intensity clearly. (b) Normalized PL spectra of different samples, S1, S2, S3, S4 and S6. Figure 6. Normalized EL spectra of QD-LEDs. Figure 7. Stability data for QD-LEDs with and without Al2O3 interlayers.



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Figure 1


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Figure 2



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Figure 3


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Figure 4



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Figure 5


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Figure 6



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Figure 7


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Table 1 Summary of the PL quantum yield (ηPL) and decay lifetime for different samples.

ηPL

Lifetime (ns)

S1: ZnO/QDs/CBP

45

24.9

S2: ZnO/Al2O3/QDs/CBP

49

28.5

S3: ZnO/QDs/Al2O3/CBP

47

26.4

S4: ZnO/Al2O3/QDs/Al2O3/CBP

54

29.0

S5: Glass/QDs

68

37.2

Sample



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Ultrastable Quantum-Dot Light-Emitting Diodes by Suppression of

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