High-Performance Quantum Dot Light-Emitting Diodes Based on Al

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Functional Inorganic Materials and Devices

High-Performance Quantum Dot Light-Emitting Diodes Based on Al-Doped ZnO Nanoparticles Electron Transport Layer Heng Zhang, Jiangliu Wei, Pai Liu, Shuming Chen, and Shengdong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04754 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 11, 2018

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ACS Applied Materials & Interfaces

High-Performance Quantum Dot Light-Emitting Diodes Based on Al-Doped ZnO Nanoparticles Electron Transport Layer Yizhe Sun1,2, Weigao Wang2, Heng Zhang2, Qiang Su2, Jiangliu Wei4, Pai Liu3,4, Shuming Chen2*, Shengdong Zhang1* 1

Institute of Microelectronics, Peking University, Beijing, P. R. China, 100871 [email protected]

2

Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, P. R. China, 518055 [email protected]

3

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, P. R. China, 510640 4

Guangdong Poly Optoelectronics Co., Ltd., Jiangmen, P. R. China, 529040

Abstract ZnO nanoparticles (NPs) are widely used as the electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) owing to their suitable electrical properties. However, due to the well-aligned conduction band levels, electrons in QDs can be spontaneously transferred to adjacent ZnO NPs, leading to severe exciton dissociation, which reduces the proportion of radiative recombination and deteriorates the device efficiency. In this work, Al-doped ZnO NPs are thoroughly investigated as replacement of ZnO for QLEDs. The energy band structures of Al-doped ZnO are modified by adjusting the concentration of Al dopants. With increasing Al content, the work function and the conduction band edge of ZnO are gradually raised, and thus the charge transfer at the interface of QDs/ETL is effectively suppressed. Consequently, the green QLEDs with 10% Al-doped ZnO NPs exhibit maximum current efficiency and external quantum efficiency of 59.7 cd/A and 14.1%, which are about 1.8-fold higher than 33.3 cd/A and 7.9% of the devices with un-doped ZnO NPs. Our work suggests that Al-doped ZnO NPs can serve as a good electron transport/injection 1 / 21

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

material in QLEDs and other optoelectronic devices.

Keywords: QLEDs; electron transport layer; metal oxide; Al-doped ZnO nanoparticles; charge transfer 1. Introduction In recent years, colloidal quantum dots (QDs) have gained growing attention owing to their unique material properties, such as size-controllable emission wavelength, high color purity, good intrinsic stability and efficient band edge emission, which make them useful for applications in biological probe, signal detection and information display.1-5 Particularly, quantum-dot light emitting diodes (QLEDs) have been intensively studied over the past few years, and the performance of the devices has been substantially improved because of the rapid development of materials and device structures.6-10 For example, the external quantum efficiency (EQE) of state-of-the-art QLEDs is higher than 20%, which is comparable to that of organic light emitting diodes (OLEDs).11,12 Therefore, together with their own unique advantages, QLEDs have become one of the promising candidates for next generation flat-panel display and solid-state lighting. In typical QLEDs, zinc oxide (ZnO) nanoparticles is widely used as the electron transport layer (ETL) due to its advantages of high mobility, suitable energy level and simple solution processibility.13-15 Because of the improvement of electron injection, QLEDs with ZnO ETL exhibit significantly enhanced EQEs and high brightness, which surpass those of the devices without ZnO.16-19 Nevertheless, when the QDs film is in contact with metal oxides with high work function, interfacial charge transport will occur spontaneously, which is known as a quenching mechanism for QDs fluorescence. In QLEDs, due to the aligned conduction band levels, the electrons in QDs can transfer to ZnO readily, leading to severe exciton dissociation and significant efficiency reduction, consequently.20-22 Up to now, a variety of solutions had been proposed to alleviate the charge transfer process and most of these methods could be divided into two categories: (1) inserting 2 / 21


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a wide bandgap modification layer to block this quenching pathway. For instance, by inserting inorganic Al2O3 layer at QDs/ZnO interface, the spontaneous electron transport has been effectively restricted and thus the QLEDs exhibited higher efficiency and longer life time over 110 000 hours, as reported by Ji et al.23 In 2014, Peng et al introduced an insulating poly(methylmethacrylate) (PMMA) layer between QDs and ZnO NPs, and realized an extremely high EQE of 20% for the red QLEDs. The PMMA layer is confirmed to maintain charge neutrality of QDs and prolong the average lifetime of excitons.11 (2) doping ZnO to modify its electronic structure. For example, according to Wu’s report, by doping Ga, the Fermi level can be effectively raised while the band gap can be broaden, and as a result, the interfacial charge transfer between QDs and ZnO NPs was profoundly weakened.24 Generally speaking, both methods are certainly helpful to prevent exciton dissociation and QD charging induced by metal oxides. However, the introduction of modification layers adds the complexity in device fabrication. In addition, some of the interlayers are composed by insulating organic materials that may increase the driving voltage and deteriorate the device stability. Moreover, most of the modification layers used in QLEDs are so thin that it is extremely difficult to precisely control their thicknesses. In these respects, adjusting the electronic properties of ZnO NPs via doping seems to be a more reliable scheme. In this contribution, we report on the efficient QLEDs by using aluminum doped zinc oxide (AZO) NPs as the ETL without any modification. Substitutional doping of ZnO NPs with Al is successfully realized and the electronic structure of ZnO can be controlled in different degree by modulating the concentration of Al dopants. Meanwhile, the spontaneous charge transfer process induced by metal oxides is effectively suppressed by the application of AZO NPs, which is verified by photoluminance (PL) tests. As a consequence, the green QLEDs with 10% Al doped ZnO ETL exhibit the maximum current efficiency (CE) of 59.7 cd/A and the maximum EQE of 14.1%, which are about 1.8-fold higher than those of the control devices. In addition, the AZO ETLs are also compatible with the red and the blue QLEDs, indicating the capacity of AZO NPs in fabricating full color QLEDs with high efficiency. 3 / 21



2. Results and discussion The synthesis of AZO NPs was guided by a modified method according to the previous reports.25 To fully investigate the influence of Al dopant on the electronic characteristics ZnO NPs and the performance of QLEDs, the doping concentration of Al was varied from 0, 5, 10 to 15 at%. Correspondingly, these samples are referred to as ZnO, AZO5, AZO10 and AZO15, respectively. Figure 1(a) shows the X-ray diffraction (XRD) patterns of a series of AZO NPs with different Al contents. In spite of the different doping concentration, all AZO samples exhibit peak characteristics of ZnO with hexagonal wurtzite structure (compared with the standard card PDF #36-1451), which suggests that the Al3+ ions are incorporated into ZnO lattice by occupying the Zn2+ sites and excludes the formation of Al2O3 phase.26, 27 Meanwhile, as shown in the magnifying curves in Figure S1, the positions of the diffraction peaks at 31.7° (1 0 0) and 36.2° (1 0 1) are shifted slightly towards higher angle with the increase in doping level, which may signify that the doping process is effective and the strain brought into the ZnO matrix is not intense. Besides, it is evident that the diffraction peaks at 31.7° and 36.2° become incrementally broad with increasing Al content, especially for AZO15. The broader peaks suggest that smaller NPs are formed because of the smaller radius of Al3+ (0.53 Å) than that of Zn2+ (0.74 Å).

To further confirm the crystal phase, the samples ZnO and AZO10 were characterized by transmission electron microscope (TEM). As shown in Figure 1(b) and 1(c), both samples are well dispersed with nearly spherical shapes. The average diameters of ZnO and AZO10 NPs are about 3.69 and 3.20 nm, respectively. The size difference of these NPs is observed more clearly from the high-resolution TEM images (lower right of Figure 1(b) and 1(c)) and the decreased particle size via Al doping is in accordance with the XRD patterns. What’s more, the diffraction patterns labelled as (1 0 0), (1 0 1) and (1 1 0) lattice planes of wurtzite ZnO could be found from the selected area electron diffraction (SAED) patterns (upper left of Figure 1(b) and 1(c)), implying the samples have the same lattice structure.28 4 / 21


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Intensity (a.u.)

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70

80

2-Theta (deg)

Figure 1. (a) XRD patterns of ZnO and AZO NPs with different Al content. TEM images of (b) ZnO and (c) AZO10 NPs. Lower-right inserts show the respective high-resolution TEM images and upper-left inserts show the respective SAED patterns.

The chemical composition and the bonding states of ZnO and AZO10 NPs were characterized by X-ray photoelectron spectroscopy (XPS) and the C1s peak at 284.6 eV was used to calibrate the energy scale. As shown in Figure 2(a), the XPS wide scans of the samples exhibit the main element peaks of Zn 2p and O1s visibly, which indicates the presence of these elements. Figure 2(b) shows the amplifying Zn 2p core line spectra of ZnO and AZO10 where the peaks at ~1021.8 eV and ~1044.9 eV correspond to Zn 2p3/2 and Zn 2p1/2 respectively, confirming the covalent ZnO structure.29 Because the electronegativity of Al (1.61) is similar with that of Zn (1.65), the binding energy positions of Zn 2p3/2 and Zn 2p1/2 are not shifted obviously in the AZO10 film. As shown in the inset of Figure 2(b), no resolvable Al 2p peak is obtained in the un-doped ZnO sample. In contrast, the Al 2p core level spectrum at ~74.2 eV can be detected in the AZO10 film although the intensity of this peak is not strong due to the low content of Al in the sample. The binding energy position of Al 2p peak at ~74.2 eV is similar with that of Al2O3, implying the formation of Al-O bonds and the existence of Al in the ZnO lattice.30

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Intensity (a.u.)

(a) Intensity (a.u.)

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Binding Energy (eV)

Figure 2. (a) XPS wide scans of ZnO and AZO10 films. (b) Zn 2p core line spectra of ZnO and AZO10 films. The insert shows the Al 2p core line spectra.

The electronic structures of AZO NPs with different doping levels were characterized by ultraviolet photoelectron spectroscopic (UPS) measurement. Figure 3(a) and 3(b) display the secondary-electron cutoff and the valence band onset regions of the samples. Generally, the work function (WF) value is estimated from the difference between the incident light energy (21.2 eV) and the energy cutoff (Ecutoff). Besides, the valence band maximum (VBM) can be calculated according to the equation: VBM = 21.2 - (Ecutoff - Eonset )

(1)

where Eonset is the valence band onset energy. As a result, the WFs of ZnO, AZO5, AZO10 and AZO15 are measured to be 3.47, 3.40, 3.31 and 3.20 eV below the vacuum level, respectively. Compared to the un-doped ZnO NPs, the WFs of the AZO NPs are gradually decreased with increasing Al content, which represents an upward shift in Fermi level induced by the donor doping. Whereas, the VBM levels of are almost unchanged, which are calculated to be 7.43, 7.39, 7.40 and 7.43 eV for samples of ZnO, AZO5, AZO10 and AZO15, respectively. As shown in Figure 3(c), the absorption spectra (Figure S2) of all samples are illustrated as the plots of (αhν)2 versus hv, where α is the absorbance coefficient and hv is the photo energy. Hence, the bandgaps (Eg) are able to be extracted from the absorption onset of the linear 6 / 21


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region, and finally, the conduction band minimum (CBM) values are deduced to be 4.03, 3.93, 3.88, and 3.86 eV for ZnO, AZO5, AZO10 and AZO15 NPs films, respectively. More details of the energy bands are summarized in Table S1. Apparently, the increment in Al concentration leads to the wider bandgaps of AZO NPs, as confirmed by the blue shifts of the absorption edges in Figure 3(c). These results are in agreement with the earlier investigations and could be explained by Burstein-Moss theory.31 That is, extra free electrons supplied by Al dopants can occupy the lowest states of the conduction band and make it harder for valence electrons to be excited into higher energy levels. Consequently, owing to the broadening band gaps, the CBM levels of AZO NPs are elevated effectively, which could serve as an energy barrier to prohibit the spontaneous electron transfer between QDs and ETL, giving rise to the improved device efficiency.

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Intensity (a.u.)

(a) ZnO

AZO5

17.73

18.3

18.0

AZO10

17.80 17.7

17.4 18.3

AZO15

18.00

17.89

18.0

17.7

17.4 18.3

18.0

17.7

17.4 18.3

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17.7

17.4

Binding energy (eV)

(b) Intensity (a.u.)

ZnO

3.96 7

6

5

4

3

2

3.99 17

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AZO15

AZO10

AZO5

4.23

4.09 17

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Binding energy (eV)

(c) AZO5

ZnO

AZO10

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(α hν ) (a.u.)

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3.40 3.2

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3.6

3.2

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3.57

3.4

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3.6

Binding energy (eV)

Figure 3. UPS spectra of (a) secondary-electron cutoff and (b) valence-band edge regions of ZnO, AZO5, AZO10 and AZO15 films. (c) (αhν)2-hν plots converted from the absorption spectra of the samples.

To evaluate the influence of Al concentration on the performance of devices, typical

green

QLEDs

with

the

structure

of

ITO/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/poly(9-vinlycarbazole) (PVK)/QDs/AZO NPs/Al were fabricated. The structure and the energy band alignment of the devices are schematically shown in 8 / 21


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Figure 4. With the widened bandgaps and lifted CBMs, AZO ETL may help to suppress exciton dissociation at the interface of QDs/metal oxides.

Figure 4. (a) Schematic structure and (b) energy level alignment of the green QLED with different ETLs.

To verify our hypothesis, the electroluminescence (EL) results of QLEDs with different ETLs are shown in Figure 5. The optimum ETL thickness is found to be ~40 nm. QLEDs with AZO ETLs exhibit the similar current density–voltage (J-V) characteristics with the reference device, as shown in Figure 5 (a). However, at large driving voltages, the luminance is enhanced greatly. Especially, for QLED with AZO10 NPs, the maximum luminance reaches to 577 200 cd/m2 at 12 V, which is significantly higher than 328 400 cd/m2 for the reference device, suggesting the adoption of Al dopants can effectively enhance the device efficiency. The variations of CE and EQE with current density are shown in Figure 5(b) and 5(c), respectively, and the detail values of maximum CE and EQE are summarized in Figure 5(d). Initially, as we expect from the variation of luminance, CE and EQE are trended to increase by using ETL with higher Al concentration, and the best performance is achieved in AZO10-based QLED. To be precise, with the application of AZO10 NPs, the maximum CE and EQE are enhanced from 33.3 cd/A and 7.9% to 59.7 cd/A and 14.1%, respectively, revealing an enhancement factor of ~1.8 compared with the ZnO-based device. However, when the Al content is further increased to 15%, the efficiencies are decreased to 41.2 cd/A and 9.9%, respectively. According to our 9 / 21



previous report, the intragap states originated from oxygen vacancies may act as trap sites, quenching excitons and deteriorating the efficiency of QLEDs.32 To investigate whether the Al dopants can affect the oxygen vacancies, XPS was performed to characterize the concentration of oxygen vacancies of AZO samples. As shown in Figure S3, O1s spectra of all AZO samples are detected and de-convoluted into two Gaussian distributions, which are centered at 529.9±0.1 eV and 531.3±0.1 eV, corresponding to the oxygen in oxide lattices and the oxygen vacancies, respectively.33 The concentration of oxygen vacancies is clearly higher in AZO15 film than that in other samples, which works in concert with the defect emission spectra (Figure S4) of all samples and can be responsible for the decreased device efficiency.

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Al doping concentration (%)

Figure 5. (a) Current density and luminance versus voltage characteristics of QLEDs based on ZnO, AZO5, AZO10 and AZO15 NPs, respectively. (b) CE and (c) EQE versus current density characteristics of different ETLs. (d) Maximum CE and EQE as a function of Al doping concentration.

To further investigate the influence of Al concentration on the performance of 10 / 21


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QLEDs, steady PL and time-resolved PL measurement were performed to five samples with structure of QDs, QDs/ZnO, QDs/AZO5, QDs/AZO10 and QDs/AZO15. As shown in Figure 6(a), the QDs alone shows the highest PL intensity. However, when the QDs are in direct contact with ZnO NPs, the PL intensity is decreased, indicating excitons in the emitting layer are greatly quenched by the metal oxide. The quenching of excitons is caused by interfacial charge transfer, which charges QD and promotes the non-radiative Auger recombination, as reported by many groups. Fortunately, by replacing the problematic ZnO with AZO10, the interfacial charge transfer can be blocked, which helps to hold the neutrality of QDs, and thus enhances the PL intensity greatly. Figure 6 (b) shows the time-resolved PL decay data, which are fitted by a three-exponential model (summarized in Table S2). The PL decay is significantly accelerated when the QDs are in close vicinity to ZnO NPs, indicating that a new channel has been opened up for the relaxation of the excitons in addition to the intrinsic radiative and non-radiative decay channels. Nevertheless, the exciton lifetime of QDs is recovered from 3.27 ns to 3.45 ns by replacing ZnO with AZO15, signifying that the exciton quenching is alleviated. Here, the average exciton lifetime is estimated by the following equation τav = (A1 τ21 +A1 τ21 +A1 τ21 )/(A1 τ1 +A2 τ2 )

(2)

where τi and Ai are the time components and the corresponding ratios, respectively.34 Besides, electron transfer rate kET is calculated by kET =1/τav(QDs/ETLs) -1/τav(QDs)

(3)

Figure 6(c) plots the detail values of τav and kET of QDs adjacent to different ETLs. It is evident that the exciton lifetime is prolonged for QDs interfaced to AZO15 and the kET value is decreased accordingly, further verifying the enhanced device performance is derived from the weakened charge transfer process. Up to now, the interfacial property between QDs and metal oxides has been extensively studied, and the electron transfer from the excited QDs to the metal oxides has been deemed to stem from the small energy offset of their conduction band potentials.35,36 For this reason, the uplifted WF and CBM induced by Al doping can effectively block the 11 / 21



spontaneous electron transfer at the interface of QDs/ETL, leading to more efficient radiative recombination in the emitting layer. This process is briefly illustrated in Figure 6(d).

PL Intensity (a. u.)

QDs QDs/ZnO QDs/AZO5 QDs/AZO10 QDs/AZO15

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w/o

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ZnO

AZO5

3.45

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AZO10

AZO15

7 6

ETL type

Figure 6. (a) PL spectra and (b) time-resolved PL decay curves of different samples, QDs, QDs/AZO5, QDs/AZO10, and QDs/AZO15. (c) Exicton lifetime and electron transfer rate as functions of different ETLs. (d) Illustrated scheme for electron transfer and exciton dissociation induced by metal oxides. The electron transfer rate kET can be effectively decreased by the application of Al-doped ZnO NPs.

Red and blue QLEDs were also fabricated to verify whether AZO10 can be used to improve the performance of full-color devices. The device characteristics of the red and the blue QLEDs are shown in Figure S5. Figure 7(a) displays the normalized EL spectra of the R-, G- and B-QLEDs, showing the central emission peaks at 624, 529 and 467 nm, with the full-width at half-maximum (FWHM) of 28, 33 and 24 nm, respectively. Owing to the narrow FWHMs, the CIE coordinates are nearly located at 12 / 21


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the spectral locus, as shown in Figure 7(b), signifying a wide color gamut could be achieved for the displays used our R-, G- and B-QLEDs. Figure 7(c) displays the photographs of the operating R-, G- and B-QLEDs with bright and uniform emission. Besides, as listed in Table 1 which summarizes the key parameters of all devices, the optimized R-, G- and B-QLEDs exhibit significantly improved device efficiency with maximum EQEs of 11.1, 14.1 and 4.0%, respectively, which are improved 1.5, 1.8 and 1.2-times, indicating the huge application potential of AZO NPs as the ETL for full-color QLEDs. To examine the repeatability of the high performance, the histogram of maximum EQE of 24 green QLEDs based on AZO10 from 6 batches is shown in Figure S6. These devices exhibit an average EQE of 13.5%, which proves the feasibility of AZO10 as the ETL in QLEDs. Moreover, owing to the application of AZO as the ETL, the spontaneous electron transfer at the interface of QDs/ETL is highly alleviated which in turn reduces the charging effect inside QDs and improves the stability of the devices. As shown in Figure S7, when the devices are driven at a constant current of 0.5 mA under ambient condition, QLEDs with AZO10 exhibit a longer half-lifetime T50 of 80.3 h at an initial luminance of 5409 cd/m2, which is more than 2 times longer than that of the control device which exhibits a lifetime of 35.8 h at a lower initial luminance of 3521 cd/m2.

(a)

Normalized EL intensity (a.u.)

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0.8 0.6 0.4 0.2 0.0

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Figure 7. (a) EL spectra of the R-, G- and B-QLEDs. (b) CIE coordinates of the R-, G- and B-QLEDs. (c) Photographs of the R-, G- and B-QLEDs under operation.

Table 1. Summarized EL performances of the R-, G-, and B-QLEDs with ZnO or AZO10 NPs as 13 / 21



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the ETL

Color

Red Green Blue

ETL ZnO AZO10 ZnO AZO10 ZnO AZO10

λmax (nm)

FWHM (nm)

624

28

529

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467

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CE (cd/A) VT (V)

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at 1000 mA/cm2

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9.9 15.3 33.3 59.7 2.2

9.2 13.2 29.3 52.3 2.1

7.2 11.1 7.9 14.1 3.3

6.7 9.6 6.9 12.4 3.2

3.8

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2.6

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3.9

3. Conclusions In conclusion, Al-doped ZnO NPs have been synthesized with the standard crystal properties and have been utilized as the ETL in the solution-processed QLEDs, leading to the enhanced device performance compared to ZnO-based QLEDs. We found that the energy band structure of ZnO can be tailored by doping Al. Furthermore, confirmed by our PL measurements, the spontaneous electron transfer at the interface of QDs/ETL could be effectively suppressed, which is attributed to the decreased WFs and CBMs induced by the Al dopants. As a result, QLED based on AZO10 NPs exhibited the best device performance with maximum CE and EQE increased from 33.3 cd/A and 7.9% to 59.7 cd/A and 14.1%, respectively, representing a 1.8-fold enhancement compared with the reference device. Besides, the efficiencies of red and blue QLEDs with AZO10 have been greatly improved as well, suggesting the AZO NPs can be universally applied to improve the performance of full-color QLEDs for the next generation display application.

4. Experimental Section Synthesis of Al-doped ZnO nanoparticles. The synthesis of aluminum doped zinc oxide nanoparticles were modified from the reported method25. Zn(OAc)2·2H2O (54.7 mmol), dimethyl sulfoxide (480 ml) and absolute ethanol (80 ml) were loaded in a three neck round bottom flask and vigorously stirred. A stock solution made by dissolving aluminum nitrite in ethanol (0.2 mol/L) was swiftly injected. After 14 / 21


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injection, another stock solution of potassium hydroxide dissolved in ethanol (0.4 mol/L) was added to the mixture dropwise, and the reaction was allowed to process for three hours. The as-synthesized nanoparticles were washed by ethanol and ethyl acetate, followed by introduction of a certain amount of ethanolamine to stabilize the nanoparticles. The sample was finally redispersed in absolute ethanol for further use.

QLED

fabrication.

All-solution

processed

QLEDs

with

structure

of

glass/ITO/PEDOT:PSS (~40 nm)/PVK (~20 nm)/QDs/Al-doped ZnO (~40 nm)/Al (~100 nm) were fabricated. For comparison, device with un-doped ZnO was also prepared. Prior to the spin-coating process, the patterned ITO substrates were cleaned in detergent diluted with deionized water for 30 min, followed by soaking in ultrasonic deionized water for 15 min and baking in an oven at 60 °C for 30 min. Then, the substrates were cleaned with a UV-ozone cleaner for 25 min to provide a well surface for PEDOT:PSS deposition. After that, PEDOT:PSS was spin-coated at 3000 rpm for 45 s, followed by baking at 120 °C for 20 min in air. Then the PVK dissolved in chlorobenzene with a concentration of 10 mg/mL was spin-coated at 3500 rpm and annealed at 120 °C for 15 min. For different color QLEDs, the QDs (commercially obtained from Mesolight Inc.) emitting layers were spin-coated from n-octane solution and baked at 100 °C for 5~6 min. The green QDs (CdZnSeS/ZnS/oleic acid, quantum yield ~85%) with a concentration of 10 mg/mL was spin-coated at 3000 rpm for 45s. The red QDs (CdZnSe/ZnS/OT, quantum yield ~85%) with a concentration of 15 mg/mL was spin-coated at 2500 rpm for 45s. The blue QDs (CdZnSeS/ZnS/OT, quantum yield ~85%) with a concentration of 10 mg/mL was spin-coated at 3000 rpm for 45s. Subsequently, different ETL samples (ZnO, AZO5, AZO10 and AZO15 NPs) were deposited at a speed of 2500 rpm for 45s from a 25 mg/mL ethanol solution. Finally, the Al cathode with a thickness of 100 nm was thermally evaporated in a vacuum evaporator with a base pressure of 5×10-4 Pa.

Characterization. The thicknesses of the solution processed films were measured using a Bruker DektakXT Stylus Profiler. The phase analysis of different metal oxide 15 / 21



Page 16 of 21

samples was performed by a powder X-ray diffractometer (D8 Advance, Bruker) and the TEM images were detected via high-resolution TEM (JEM-3200FS, JEOL). XPS and UPS results were obtained from an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher). Besides, measurements of the absorbance, steady PL and time-resolved PL of different samples were carried out using Edinburgh FS5 spectrofluorometer. The current density-luminance-voltage (J-V-L) characteristics were measured by a dual-channel Keithley 2614B programmable source meter and a PIN-25D calibrated silicon photodiode. The electroluminescence (EL) spectra of QLEDs were measured by fiber optic spectrometer (USB 2000, Ocean Optics) in the normal direction. All measurements were performed in air at room temperature.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (61775090 and 61574003), the Guangdong Natural Science Funds for Distinguished Young Scholars (2016A030306017), the Guangdong Special Funds for Science and Technology Development (2017A050506001), the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20170307105259290),

the

National

Key

R&D

Program

of

China

(2016YFB0401702), the Shenzhen Peacock Plan (KQTD2015071710313656), and the Guangdong Provincial Science and Technology Project (2016B090906001).

Supporting Information Supporting Information. Enlarged XRD view of ZnO, AZO5, AZO10 and AZO15 NPs; Absorption spectra of AZO NPs with different doping levels; Detailed energy characteristics; O1s XPS spectra of AZO NPs with different doping levels; Defect emission spectra of different AZO samples; Summarized components of the three-exponential fitting curves; Current density and luminance versus voltage, CE versus current density, and EQE versus current density characteristics of R- and B-QLEDs; Histogram of maximum CEs of G-QLEDs; Lifetime curves of the green QLEDs 16 / 21


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ToC Figure and caption

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High-Performance Quantum Dot Light-Emitting Diodes Based on Al

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