Performance Improvement of Quantum Dot Light-Emitting Diodes Using a ZnMgO Electron Transport Layer with a Core/Shell Structure

Highly efficient and all-solution processed quantum dot light-emitting diodes (QLEDs) with high performance are demonstrated by employing ZnMgO nanoparticles (NPs) with core/shell structure used as an electron transport layer (ETL). Mg-doping in ZnO NPs exhibits a different electronic structure and degree of electron mobility. A key processing step for synthesizing ZnMgO NPs with core/shell structure is adding Mg in the solution in addition to the remaining Mg and Zn ions after the core formation process. This enhanced Mg content in the shell layer compared with that of the core X-ray photoelectron spectroscopy showed a higher number of oxygen vacancies for the ZnMgO core/shell structure, thereby enhancing the charge balance in the emitting layer and improving device efficiency. The QLED incorporating the as synthesized ZnMgO NP core/shell A exhibited a maximum luminance of 55,137.3 cd/m2, maximum current efficiency of 58.0 cd/A and power efficiency of 23.3 lm/W. The maximum current efficiency and power efficiency of the QLED with ZnMgO NP core/shell A improved by as much as 156.3% and 113.8%, respectively, compared to the QLED with a Zn0.9Mg0.1O NP ETL, thus demonstrating the benefits of ZnMgO NPs with the specified core/shell structure.

In typical QLEDs, the energy barrier for electron injection from the cathode to the emitting layer (EML) is smaller than that of the holes from the anode to the EML. ZnO nanoparticles (NPs) are widely employed in the electron transport layer (ETL) of QLEDs, given that the mobility of electrons in ZnO NPs is much larger than that of holes in the hole transport layer (HTL) [20]. These mismatches result in an electron-hole charge imbalance in the QD EML, thus increasing the possibility of non-radiative exciton recombination and deterioration of device performance [21][22][23]. In addition, a high number of oxygen-deficient surface states in ZnO NPs adsorb molecular O 2 and H 2 O from the air [20]. The adsorbed into the synthesized solution to precipitate the ZnMgO NPs. The volume ratio of the ethyl acetate and synthesized solution was 3:1. After 3 h, a white powder had formed and precipitated. ZnO NPs were obtained by centrifuging the solution.
To synthesize Zn 0.9 Mg 0.1 O NPs, 0.2962 g of zinc acetate dihydrate (Zn(CH 3 COO)·2H 2 O) powder (Sigma-Aldrich) and 0.03292 g of magnesium acetate tetrahydrate (Mg(CH 3 COO)·4H 2 O) powder (Sigma-Aldrich), as precursor materials, were dissolved in 15 mL of DMSO. TMAH (0.421 g) was dissolved in 5 mL of EtOH. The mixture of the TMAH solution and mixed solution of Zn(CH 3 COO)·2H 2 O and 2Mg(CH 3 COO)·4H 2 O) was stirred at room temperature for 24 h to ensure that they were fully dissolved. Ethyl acetate (Sigma-Aldrich) was poured into the synthesized solution to precipitate the Zn 0.9 Mg 0.1 O NPs. The volume ratio of the ethyl acetate and synthesized solution was 3:1. After 3 h, a white powder had formed and precipitated. Zn 0.9 Mg 0.1 O NPs were obtained by centrifuging the solution.
To synthesize the ZnMgO NP solution used as the core of the ZnMgO NP core/shell structure, 0.263 g of zinc acetate dihydrate (Zn(CH 3 COO)·2H 2 O) powder (Sigma-Aldrich) and 0.033 g of magnesium acetate tetrahydrate (Mg(CH 3 COO)·4H 2 O) powder (Sigma-Aldrich), as precursor materials, were dissolved in 12 mL of DMSO. TMAH (0.421 g) was dissolved in 5 mL of EtOH. The mixture of the TMAH solution and mixed solution of Zn(CH 3 COO)·2H 2 O and 2Mg(CH 3 COO)·4H 2 O) was stirred at room temperature for 5 min to complete the synthesis of the ZnMgO NP core.
To synthesize the shell layers of ZnMgO with a core/shell structure, Mg(CH 3 COO)·4H 2 O powder (Sigma-Aldrich) in amounts of 0.033 and 0.049 g, as precursor materials, was dissolved in 3 mL of DMSO to create two Mg(CH 3 COO)·4H 2 O solutions (A and B). Each solution was poured into the as synthesized core solution, followed by stirring at room temperature for 24 h to ensure full dissolution. After 3 h, a white powder had formed and precipitated. After centrifuging the solution, ZnMgO NPs with a core/shell structure were obtained. There was Zn in the solution prepared for synthesizing the shell layer. The ZnMgO NP shell layer was grown onto the core using the remaining Zn and Mg ions formed during core synthesis. As such, we believe that the shell layer had more Mg than the core layer. Hereinafter, the ZnMgO NPs-based ETLs synthesized using 0.033 g of Mg(CH 3 COO)·4H 2 O powder in the shell is denoted as ZnMgO NP core/shell A and that using 0.049 g of Mg(CH 3 COO)·4H 2 O as ZnMgO NP core/shell B.

Device Fabrication
QLEDs were fabricated using the following process. An indium tin oxide (ITO) thin film with a sheet resistance of ≤10 Ω/ on glass substrate was patterned by a photolithography method. The patterned ITO was used as an anode. The ITO-patterned glass substrate was ultrasonically cleaned with acetone, isopropyl alcohol, methanol and deionized water. Then, the ITO-patterned glass substrate was spin-coated with three layers, as described in the following.
Poly(N-vinyl-carbazole) (PVK) (Sigma-Aldrich) was dissolved to 1.2 wt% in toluene; the dissolved PVK solution was deposited onto as-patterned ITO glass substrate by spincoating at a speed of 600 rpm for 5 s at room temperature, followed by spin-coating at a speed of 1500 rpm for 15 s at room temperature. Then, CdSe/ZnS QDs (Zeus) were dissolved at 5 mg/mL in heptane to create the EML; this solution was then spin-coated onto the ITO/PVK substrate at a speed of 3000 rpm for 5 s at room temperature. Solutions of ZnMgO-based ETLs comprising ZnO NPs, ZnMgO NPs and ZnMgO NPs with a core/shell structure (A/B) were spin-coated onto the ITO/PVK/QD substrate at a speed of 2000 rpm for 20 s at room temperature. Then, the multilayered substrates were loaded into a high-vacuum deposition chamber (Cetus OL 100; Celcose, Hwasung, Korea) (background pressure, 6 × 10 −7 Torr) to deposit the Al cathode layer (150 nm thick) at an evaporation rate of 1.2 Å/s, which was patterned by an in-situ shadow mask to form an active emitting area of 4 mm 2 .

Characterizations
X-ray diffraction (XRD; D/Max-2200pc; Rigaku, Tokyo, Japan) was used to confirm the formation of the ZnMgO NP-based ETLs (comprising ZnO NPs, Zn .9 Mg 0.1 O NPs, or ZnMgO NPs with an A/B core/shell structure), with Cu-Kα radiation applied to the centrifuged ZnMgO NPs. Field-emission transmission electron microscopy (FE-TEM) (Tecnai F30 S-Twin; JEOL Ltd., Tokyo, Japan) was used to determine the actual particle size of the ZnMgO-based ETLs. XPS measurements were conducted using a Nexsa XPS system (ThermoFisher Scientific, Waltham, MA, USA) and UPS analysis with a He (I) 21.22-eV gas discharge lamp, to characterize the O 1s level and valence band maximum (VBM) of the ZnMgO-based ETL thin films, respectively. The transmittance and reflectance were measured using a spectrophotometer (UV-1650PC; Shimadzu Corp., Kyoto, Japan), with normally incident monochromatic light at the sample surface side. Current density-voltageluminance (J-V-L) was evaluated using a computer-controlled source meter (2400; Keithley Instruments, Cleveland, OH, USA) and luminance meter (LS100; Konica Minolta, Tokyo, Japan). Electroluminescence (EL) spectra were recorded using a spectroradiometer (CS1000; Konica Minolta).  103) and (112) planes, which was attributed to the ZnO phase. Notably, the peaks broadened as the Mg content in the ZnO NPs increased (Figure 1). This is attributable to the Zn-MgO NPs with the core/shell structure having a greater amount of Mg than Zn 0.9 Mg 0.1 O NPs. Notably, the Mg content in ZnMgO core/shell B was higher than that in ZnMgO core/shell A.

Results and Discussion
(background pressure, 6 × 10 −7 Torr) to deposit the Al cathode layer (150 nm thick) at an evaporation rate of 1.2 Å/s, which was patterned by an in-situ shadow mask to form an active emitting area of 4 mm 2 .

Characterizations
X-ray diffraction (XRD; D/Max-2200pc; Rigaku, Tokyo, Japan) was used to confirm the formation of the ZnMgO NP-based ETLs (comprising ZnO NPs, Zn.9Mg0.1O NPs, or ZnMgO NPs with an A/B core/shell structure), with Cu-Kα radiation applied to the centrifuged ZnMgO NPs. Field-emission transmission electron microscopy (FE-TEM) (Tecnai F30 S-Twin; JEOL Ltd., Tokyo, Japan) was used to determine the actual particle size of the ZnMgO-based ETLs. XPS measurements were conducted using a Nexsa XPS system (ThermoFisher Scientific, Waltham, MA, USA) and UPS analysis with a He (I) 21.22-eV gas discharge lamp, to characterize the O 1s level and valence band maximum (VBM) of the ZnMgO-based ETL thin films, respectively. The transmittance and reflectance were measured using a spectrophotometer (UV-1650PC; Shimadzu Corp., Kyoto, Japan), with normally incident monochromatic light at the sample surface side. Current density-voltage-luminance (J-V-L) was evaluated using a computer-controlled source meter (2400; Keithley Instruments, Cleveland, OH, USA) and luminance meter (LS100; Konica Minolta, Tokyo, Japan). Electroluminescence (EL) spectra were recorded using a spectroradiometer (CS1000; Konica Minolta). Figure 1 shows the typical XRD patterns of the ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO core/shell A and ZnMgO core/shell B. All samples exhibited the hexagonal wurtzite structure of ZnO. The XRD patterns of ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B showed reflection in the (100), (002), (101), (102), (110), (103) and (112) planes, which was attributed to the ZnO phase. Notably, the peaks broadened as the Mg content in the ZnO NPs increased ( Figure 1). This is attributable to the ZnMgO NPs with the core/shell structure having a greater amount of Mg than Zn0.9Mg0.1O NPs. Notably, the Mg content in ZnMgO core/shell B was higher than that in ZnMgO core/shell A.      Figure 2c,e show the average diameter of the core layers pre-synthesized for ZnMgO NP core/shells A and B (3.64 and 3.95 nm, respectively). After completing the shell formation process on the pre-synthesized core layer, the diameter of ZnMgO NP core/shells A and B was measured as 4.54 and 4.25 nm, respectively, as shown in Figure 2d,f. The average diameters of the completed ZnMgO NPs with the core/shell structure increased after the shell formation process. This implies that ZnMgO shell layers with higher Mg content were grown on the ZnMgO cores.

Results and Discussion
Zn0.9Mg0.1O NPs was estimated to be 4.69 and 4.51 nm, respectively. Figure 2c,e show the average diameter of the core layers pre-synthesized for ZnMgO NP core/shells A and B (3.64 and 3.95 nm, respectively). After completing the shell formation process on the presynthesized core layer, the diameter of ZnMgO NP core/shells A and B was measured as 4.54 and 4.25 nm, respectively, as shown in Figure 2d,f. The average diameters of the completed ZnMgO NPs with the core/shell structure increased after the shell formation process. This implies that ZnMgO shell layers with higher Mg content were grown on the ZnMgO cores. The thin films fabricated using the ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were further investigated by XPS. Figure 3 shows the O 1s peaks in the XPS spectra of the thin films, which carries information about the quality of The thin films fabricated using the ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were further investigated by XPS. Figure 3 shows the O 1s peaks in the XPS spectra of the thin films, which carries information about the quality of the NPs. The thin films exhibited an asymmetric O 1s spectra. The spectra were deconvoluted into three binding states, attributed to oxygen in the metal oxide lattice  [35,36]. The VBM levels of the thin films of ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were estimated to be 6.94, 7.21, 7.66 and 7.87 eV below the vacuum level, respectively. Figure  4c plots (⍺hν) 2 against the photon energy (hν), where ⍺, h and ν are the absorption coefficient, Planck's constant and radiation frequency, respectively. The absorption coefficient was calculated based on the UV/Vis data. The optical band gaps of the various films were evaluated using a Tauc plot of the UV/Vis absorption spectra. The optical band gaps of the films with the ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were estimated to be 3.39, 3.61, 3.76 and 3.70 eV below the vacuum level, respectively. Using the optical bandgap and VBM values, the CBM level of the ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were calculated to be 3.60, 3.62, 3.79 and 3.75 eV below the vacuum level, respectively. Figure 4d shows a schematic diagram of the flat-band energy levels of the QLEDs with ZnO NPs, Zn0.9Mg0.1O The electronic energy level configuration of the ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B was investigated using UPS. The UPS spectra coinciding with the secondary-electron cutoff and VBM regions are shown in Figure 4a,b. The VBM level was calculated from the incident photon energy of 21.22 eV, onset energy in the valence band region (E onset ) and high-binding energy cutoff region (E cutoff ) using the following equation: VBM = 21.22 − (E cutoff − E onset ) [35,36]. The VBM levels of the thin films of ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were estimated to be 6.94, 7.21, 7.66 and 7.87 eV below the vacuum level, respectively. Figure 4c plots (αhν) 2 against the photon energy (hν), where α, h and ν are the absorption coefficient, Planck's constant and radiation frequency, respectively. The absorption coefficient was calculated based on the UV/Vis data. The optical band gaps of the various films were evaluated using a Tauc plot of the UV/Vis absorption spectra. The optical band gaps of the films with the ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were estimated to be 3.39, 3.61, 3.76 and 3.70 eV below the vacuum level, respectively. Using the optical bandgap and VBM values, the CBM level of the ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were calculated to be 3.60, 3.62, 3.79 and 3.75 eV below the vacuum level, respectively. Figure 4d shows a schematic diagram of the flat-band energy levels of the QLEDs with ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B. Figure 4e shows the schematic of the fabricated QLEDs with ZnMgO-based ETLs. Except for the vacuum evaporation of Al cathode, all layers were spin coated to prepare the QLEDs. Figure 4f shows a photographic image of a device. It is shown in Figure 4f that the device consisted of 4 active-light-emitting QLEDs. The device area was 25 × 25 mm 2 and activelight-emitting area was 2 × 2 mm 2 . Al cathode, all layers were spin coated to prepare the QLEDs. Figure 4f shows a photographic image of a device. It is shown in Figure 4f that the device consisted of 4 activelight-emitting QLEDs. The device area was 25 × 25 mm 2 and active-light-emitting area was 2 × 2 mm 2 .     Figure 5 shows the electroluminescent (EL) characteristics of the QLEDs with various ZnMgO NP-based ETLs. Figure 5a shows the current density (J) and luminance (L) curves as a function of the applied voltage (V). It is well known that the current density can be attributed not only to the electron/hole injection barrier, but also to the mobility of the ETL and HTL. Although the energy barrier between the Al cathode and ETL of ZnO NPs was larger compared to those between the Al cathode and ETLs of Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B, the current density of the QLED with the ZnO NP ETL exhibited the highest value. This indicates that the mobility of the ZnO NP ETL should be the highest. Figure 5a also shows that the current density of QLED with Zn 0.9 Mg 0.1 O NPs was higher than those of QLEDs with a core/shell structure. This suggests that, for the ZnMgO NP shell layer with greater Mg content compared to the ZnMgO NP core layer, there was a reduction in the mobility of ZnMgO NPs due to an increase in oxygen vacancy content. From Figure 5a, the turn-on voltage of the QLEDs with ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B was estimated to be 4.2, 6.1, 4.1 and 5.6 V, respectively; the turn-on voltage was defined as the voltage at which the luminance reached 1 cd/m 2 . The QLED with ZnMgO NP core/shell A exhibited the lowest turn-on voltage. Figure 5a also shows that the maximum luminance of the QLEDs fabricated with ETLs of ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B (50,784.7, 88,235.0, 55,137.3 and 20,716.9 cd/m 2 , respectively). Figure 5b shows that the maximum current efficiency of the QLEDs with ETLs of ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B was 21.2, 37.1, 58.0 and 45.4 cd/A, respectively. The maximum current efficiency of the QLEDs with Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B was enhanced by 75%, 173.6% and 114.2%, respectively, compared to the QLED with a ZnO NP ETL. The maximum current efficiency of the QLEDs with core/shell ZnMgO NP A and core/shell ZnMgO NPs B was enhanced by 156.3% and 122.4%, respectively, compared to the QLED with a Zn 0.9 Mg 0.1 O NPs ETL. Because the ZnMgO NP shell layers, which have higher Mg content than the ZnMgO NP core layer, have more oxygen vacancies than Zn 0.9 Mg 0.1 O NPs without the shell, the mobility of electrons in ZnMgO NP ETLs with a core/shell structure should be lower than that of a ZnMgO NP ETL without a core/shell structure. Thus, the optimal charge balance was achieved in the QLED with a ZnMgO NP core/shell A ETL, likely due to appropriate reduction of the electron mobility and energy barrier between the cathode and ETL with ZnMgO core/shell layers, which enhanced the charge balance. Figure 5c shows that the maximum power efficiency of the QLEDs with ZnO NP ETL, Zn 0.9 Mg 0.1 O NP ETL, ZnMgO NP core/shell A ETL and ZnMgO NP core/shell B ETL was 10.9, 10.9, 23.3 and 9.6 lm/W, respectively. The maximum power efficiency of the QLED with ZnMgO NP core/shell A ETL was enhanced by 113.8%, compared to the QLED with ZnO NP ETL and Zn 0.9 Mg 0.1 O NP ETL. It is obvious that the QLED with ZnMgO NP core/shell A ETL considerably enhances the power efficiency in comparison with the QLEDs with ZnO NP and Zn 0.9 Mg 0.1 O NP ETLs without core/shell structure. Figure 5d shows the PL spectrum from the CdSe/ZnS QD and the EL spectra from the QLEDs with ZnO NP ETL, Zn 0.9 Mg 0.1 O NP ETL, ZnMgO NP core/shell A ETL and ZnMgO NP core/shell B ETL. The characteristic parameters of the spectra are summarized in Table 1. There is no parasite blue emission from PVK so that the color purity of the device is ensured. This also indicates that charge recombination occurred only in the EML. We note that there are blue-shifted EL peaks for QLEDs with ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B compared to the PL peak of the QD. We attribute this blue-shift to the electric-field induced Stark effect [37,38] under high volage and current.

Conclusions
ZnO-based metal oxide NPs of ZnO, Zn0.9Mg0.1O, ZnMgO NP core/shell A a ZnMgO NP core/shell B were synthesized as an ETL for QLEDs and highly efficie QLEDs were developed with the NPs. XRD and TEM analyses revealed the single-cr talline nature of the NPs. The electronic energy level structure and oxygen vacancy co tent of the ZnO NPs, Zn0.9Mg0.1O NPs, ZnMgO NP core/shell A and ZnMgO NP core/sh B were investigated using UPS and XPS, respectively. The maximum current efficien and maximum power efficiency of the QLEDs with a ZnMgO NP core/shell A ETL w enhanced by 156.3% and 113.8%, respectively, compared to the QLED with a Zn0.9Mg0 NP ETL. As a result, an appropriate balance of the mobility of the ETLs and energy barr between the cathode and ETL was achieved in the QLED with ZnMgO NP core/shell the improvement in current efficiency and power efficiency is likely due to appropri reduction of the electron mobility and energy barrier between the Al cathode and ZnM NP ETL with a core/shell structure.

Data Availability Statement:
The data that support the findings of this study are available from corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in

Conclusions
ZnO-based metal oxide NPs of ZnO, Zn 0.9 Mg 0.1 O, ZnMgO NP core/shell A and ZnMgO NP core/shell B were synthesized as an ETL for QLEDs and highly efficient QLEDs were developed with the NPs. XRD and TEM analyses revealed the single-crystalline nature of the NPs. The electronic energy level structure and oxygen vacancy content of the ZnO NPs, Zn 0.9 Mg 0.1 O NPs, ZnMgO NP core/shell A and ZnMgO NP core/shell B were investigated using UPS and XPS, respectively. The maximum current efficiency and maximum power efficiency of the QLEDs with a ZnMgO NP core/shell A ETL were enhanced by 156.3% and 113.8%, respectively, compared to the QLED with a Zn 0.9 Mg 0.1 O NP ETL. As a result, an appropriate balance of the mobility of the ETLs and energy barrier between the cathode and ETL was achieved in the QLED with ZnMgO NP core/shell A; the improvement in current efficiency and power efficiency is likely due to appropriate reduction of the electron mobility and energy barrier between the Al cathode and ZnMgO NP ETL with a core/shell structure.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.