Home Effect of magnesium doping on NiO hole injection layer in quantum dot light-emitting diodes
Article Open Access

Effect of magnesium doping on NiO hole injection layer in quantum dot light-emitting diodes

  • Nayoon Lee ORCID logo , Van Khoe Vo ORCID logo , Hyo-Jun Lim ORCID logo , Sunwoo Jin , Thi Huong Thao Dang ORCID logo , Heewon Jang , Dayoung Choi , Joon-Hyung Lee , Byoung-Seong Jeong EMAIL logo and Young-Woo Heo ORCID logo EMAIL logo
Published/Copyright: September 11, 2024
Download Article (PDF)
Become an author with De Gruyter Brill
Submit Manuscript Author Information
Nanophotonics
From the journal Nanophotonics Volume 13 Issue 25

Abstract

This study reports on the fabrication of quantum dot light-emitting diodes (QLEDs) with an ITO/Ni1−x Mg x O/SAM/TFB/QDs/ZnMgO/Al structure and investigates the effects of various Mg doping concentrations in NiO on device performance. By doping Mg into the inorganic hole-injection layer NiO (Ni1−x Mg x O), we improved the band alignment with the hole-injection layer through band tuning, which enhanced charge balance. Optimal Mg doping ratios, particularly a Ni0.9Mg0.1O composition, have demonstrated superior device functionality, underscoring the need for fine-tuned doping levels. Further enhancements were achieved through surface treatments of Ni0.9Mg0.1O with UV-Ozone (UVO) and thermal annealing (TA) of the ZnMgO electron transport layer. Consequently, by optimizing Mg-doped NiO in QLED devices, we achieved a maximum external quantum efficiency of 8.38 %, a brightness of 66,677 cd/m2, and a current efficiency of 35.31 cd/A, indicating improved performance. The integration of Mg-doped NiO into the QLED structure resulted in a device with superior charge balance and overall performance, which is a promising direction for future QLED display technologies.

Keywords: QLED; NiO; Ni1−x Mg x O; ZnMgO; hole injection layer

1 Introduction

Quantum dot light-emitting diodes (QLEDs) are emerging as next-generation displays to replace existing OLEDs due to advantages such as high color purity, easy solution processing, high quantum yield, and chemical stability of quantum dots [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Charge transport layer (CTL) of a QLED, which comprises hole injection layers (HIL), hole transport layers (HTL), and electron transport layers (ETL), facilitates the injection of holes from the anode and electrons from the cathode. The charges that reach the emissive layer through this process form excitons, which then recombine, resulting in the emission of light [7], [12], [13]. Maintaining an appropriate balance between holes and electrons is crucial for enhancing the luminous efficiency of QLEDs [14], [15]. However, organic materials used in HILs and HTLs are sensitive to moisture, high current density, light, and heat, which undermine the long-term stability of QLEDs [11], [16], [17], [18], [19], [20].

To address these issues, researchers are actively exploring QLED devices based on stable inorganic materials such as NiO, Cu2O, PMA (phosphomolybdic acid), Cl@TPA (Cl-passivated tungsten phosphate), and MoO3 [21], [22], [23], [24]. Among these materials, NiO, which is an intrinsic p-type oxide semiconductor, exhibits high stability, high carrier mobility, and high optical transmittance in the visible spectrum and is easily processed in solutions and doped, making it a promising candidate for use in QLED HILs [8], [11], [25], [26], [27], [28], [29]. However, numerous surface defects on NiO can induce quenching, which hinders smooth hole injection from NiO to the hole transport layer, thereby impeding device performance [30], [31]. To address this problem, researchers have explored various approaches, including surface passivation, improved material synthesis, post-treatment procedures, interface modifiers, and the development of alternative materials [11], [32], [33], [34], [35]. Despite advances in device performance improvement, challenges remain regarding the chemical and physical compatibility with NiO, the complexity of the process, and the difficulty in controlling the quality and scalability of advanced techniques [13].

Therefore, this study addresses these challenges by optimizing band alignment through band tuning and by improving charge balance through doping. Among the dopants, MgO was selected because its crystal structure is identical to that of NiO, and its minimal lattice mismatch is approximately 0.8 % [36], [37]. The similar ionic radius of MgO and NiO indicate that smooth intermixing across the entire compositional range is expected [38]. Furthermore, MgO has a bandgap of approximately 7.8 eV, which is expected to lower the energy barrier with the HTL when doped into NiO, promoting balanced carrier injection for efficient radiative recombination [33], [39].

2 Experimental

2.1 Materials selection

Nickel nitrate hexahydrate (98 %) and hexane (95 %) were purchased from Samchun. Patterned 50 nm ITO-coated glass was purchased from AMG. Tetramethylammonium hydroxide pentahydrate (98 %, TMAH) was purchased from Alfa-Aesar. Magnesium nitrate hexahydrate (99 %), zinc acetate dihydrate (99 %), magnesium acetate tetrahydrate (98 %), dimethyl sulfoxide (anhydrous, 99.9 %, DMSO), 4-(Trifluoromethyl) benzoic acid, and molybdenum oxide (99.5 %) were purchased from Sigma Aldrich. TFB (ADS259BE) was purchased from American Dye Source, CdSe/ZnS (Core/Shell) (CZO-530H) dispersed in Hexane Quantum Dots was purchased from Global Zeus, aluminum (99.999 %, metal pellets) was purchased from ITASCO. Ethanol (anhydrous, 99.9 %) was purchased from Daejung. All chemicals were used without further purification.

2.2 Synthesis of doped Ni1−x Mg x O nanoparticles (NPs)

The synthesis method for Ni1−x Mg x O nanoparticles is as follows. (1−x), (x = 0, 0.05, 0.1, 0.15, 0.2) moles of nickel nitrate hexahydrate and x mole of magnesium nitrate hexahydrate were dissolved in DI water. A 50 wt% TMAH (tetramethylammonium hydroxide pentahydrate) aqueous solution was then added dropwise until the pH reached 10. After an additional 10 min of stirring, the resulting nickel magnesium hydroxide was washed by centrifugation. To remove residual TMAH and other organic materials, DI water was added, and centrifugation was repeated several times. The material was then dried by freeze-drying. After drying, nickel magnesium hydroxide was annealed at 270 °C for 2 h to obtain black Ni1−x Mg x O NPs.

We attached a carboxyl-functional group organic molecule to the Ni1−x Mg x O NPs and dispersed them in anhydrous ethanol at a concentration of 30 g/L. The completed solution was filtered through a 0.45 μL PVDF filter before use.

2.3 Synthesis of ZnMgO

The synthesis method of ZnMgO NPs in this study was slightly modified. 9.5 mmol of zinc acetate dihydrate and 0.5 mmol of magnesium acetate tetrahydrate were dissolved in 40 mL of anhydrous dimethyl sulfoxide (DMSO), while 10 mmol of tetramethylammonium hydroxide pentahydrate (TMAH) was dissolved in 10 mL of anhydrous ethanol. The TMAH solution was then added dropwise to the DMSO mixture while stirring at room temperature for 1 h. The resulting solution was combined with acetone, which served as an antisolvent, to precipitate ZnMgO, which was subsequently washed by centrifugation. Finally, the ZnMgO nanoparticles were dispersed in anhydrous ethanol and filtered through a 0.45 μL PTFE filter before use.

2.4 Fabrication of QLED devices

The patterned ITO-coated glass substrates were ultrasonically cleaned with detergent, acetone, ethanol, and distilled water for 10 min. After cleaning the substrates, they were dried using a nitrogen gun. Subsequently, the substrates were subjected to a 15-min UV-Ozone (UVO) treatment to eliminate organic residues and enhance the hydrophilicity of the surface. A Ni1−x Mg x O solution was spin-coated onto the ITO substrates at 3,000 rpm for 60 s, followed by annealing at 100 °C for 15 min. The Ni1−x Mg x O-coated ITO substrates were transferred to a glove box under a nitrogen atmosphere. Next, 4-(Trifluoromethyl) benzoic acid dissolved in anhydrous ethanol (2.5 mg/mL) was spin-coated for 20 s at 5,000 rpm, followed by annealing at 100 °C for 5 min. TFB was dissolved in chlorobenzene (8 mg/mL) and then spin-coated onto the Ni1−x Mg x O layer at 4,000 rpm for 30 s, followed by annealing at 150 °C for 15 min.

CdSe/ZnS green quantum dots (QD) at a concentration of 5 mg/mL were coated on top of TFB by spinning at 2,000 rpm for 20 s, followed by annealing at 80 °C for 15 min. ZnMgO, dispersed in anhydrous ethanol, was spin-coated on top of the QDs at 3,000 rpm for 30 s, followed by annealing at 80 °C for 10 min. Subsequently, aluminum was deposited via thermal evaporation at a growth rate of 2 Å/s to 2.3 Å/s under a pressure of less than 2 × 10−6 torr. The QLED device structure of ITO/Ni1−x Mg x O/SAM/TFB/QDs/ZnMgO/Al is shown in Figure 1.

Figure 1: Schematic of the QLED device structure evaluated in this study.
Figure 1:

Schematic of the QLED device structure evaluated in this study.

2.5 Characterization

X-ray Diffraction (XRD) was used to investigate the crystallinity and presence of secondary phases in Ni1−x Mg x O NPs using a Malvern Panalytical X’pert PRO equipped with a CuKα X-ray source. Transmission electron microscopy (TEM) was used to determine the average particle size of Ni1−x Mg x O NPs and obtain cross-sectional images of Ni0.9Mg0.1O-based QLED by Titan G2 ChemiSTEM Cs Probe. Atomic force microscopy (AFM) was adopted to investigate the film surface roughness Park Systems (XE-100). The transmittance and absorbance data of the Ni1−x Mg x O thin film were collected using an Agilent Cary 5,000 UV–Vis–NIR Spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) was conducted on a Ni1−x Mg x O thin film to assess the effects of varying Mg doping levels on the valence band maximum (VBM) and work function (WF) using an Axis ultra-delay line detector with a He I source (hv = 21.21 eV) at KBSI, Daejeon. J–V Characteristics of HOD were measured using a Keithley 2,400 source meter. The room-temperature photoluminescence (PL) spectra of thin films under excited wavelength at 325 nm were acquired using a Ramboss-Star Microscope Raman measurement system (DongWoo Optron Co., Ltd). A confocal microscope with a 40×(air) objective (MicroTime-200, Picoquant, Germany) was used for the TRPL investigation excited by a 470 nm laser. The J–V properties, functionality, and brightness of the devices were evaluated using a Konica Minolta CS-2,000 spectroradiometer coupled with a Keithley 2,400 Sourcemeter.

3 Results and discussion

Figure 2 shows the X-ray diffraction (XRD) patterns for undoped nickel oxide (NiO) and magnesium-doped nickel oxide (Ni0.9Mg0.1O). The patterns display several peaks corresponding to the crystallographic planes of NiO materials, such as (111), (200), (220), (222), and (311) planes, which are indicative of the rock salt structure listed on the JCPDS card #47-1049 [40], [41], [42]. The asterisks mark the peaks, which were attributed to silicon (Si) for the XRD peak shift correction. As shown in Figure 2, there was no difference in the XRD main peaks for Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2), indicating that no secondary phase formed due to Mg doping and that good crystallinity was maintained.

Figure 2: Comparison of XRD patterns of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) NPs.
Figure 2:

Comparison of XRD patterns of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) NPs.

To investigate the changes in the actual particle size before and after Mg doping, we conducted TEM analysis on undoped NiO and Ni1−x Mg x O NPs. Figure 3a shows the TEM image of undoped NiO NPs, where individual particles can be observed. Figure 3b displays a histogram of the particle size distribution for NiO, with a superimposed black bell curve indicating a mean particle size of approximately 4.38 nm. Figure 3c presents the TEM image of Ni0.9Mg0.1O NPs, where the particles appear uniformly distributed and Figure 3d features a histogram of the particle size distribution for Ni0.9Mg0.1O, with a superimposed red bell curve showing the mean particle size of about 4.56 nm.

Figure 3: The particle size before and after Mg doping. (a) TEM image and (b) the average particle size of NiO NPs; (c) TEM image and (d) average particle size of Ni0.9Mg0.1O NPs.
Figure 3:

The particle size before and after Mg doping. (a) TEM image and (b) the average particle size of NiO NPs; (c) TEM image and (d) average particle size of Ni0.9Mg0.1O NPs.

As shown in Figures 3a–d and S1, the average particle size of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) is slightly larger than that of undoped NiO NPs. Despite Mg doping, the particle size distribution remained relatively narrow, indicating that the doping process did not significantly affect the uniformity of the NPs. In addition, both sets of NPs exhibit a symmetric distribution around the mean, as indicated by the shape of the bell curves. Overall, Mg doping resulted in a relatively small change in the average particle size and a slight broadening of the size distribution of the NiO NPs.

To investigate surface roughness of Ni1−x Mg x O thin film, we conducted AFM measurement on Ni1−x Mg x O thin film. Without Mg doping, the Root mean square (RMS) roughness is 3.2 nm. The RMS roughness of Ni0.95Mg0.05O, Ni0.9Mg0.1O, Ni0.85Mg0.15O, and Ni0.8Mg0.2O are 3.9, 4.0, 3.1, and 4.8 nm, respectively.

To analyze the effect of Mg doping on optical properties, Ultraviolet–Visible (UV–Vis) light transmittance spectra were examined for Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) at various doping concentrations. The samples were spin-coated onto glass and thermally treated at 100 °C. As shown in Figure 4a, the lines represent the percentage of light transmittance over a range of wavelengths. The spectra indicate that as the Mg doping concentration increased, the transmittance increases, above 80 %, across the visible spectrum (400–700 nm). This is attributed to the increase in bandgap due to Mg doping [38].

Figure 4: The effect of Mg doping on optical properties. (a) Transmittance of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) films (inset graph shows the Tauc plot); (b) bandgap energy versus the Mg doping concentration.
Figure 4:

The effect of Mg doping on optical properties. (a) Transmittance of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) films (inset graph shows the Tauc plot); (b) bandgap energy versus the Mg doping concentration.

Figure 4b shows the relationship between the Mg dopant at various doping concentrations (x = 0, 0.05, 0.1, 0.15, 0.2) and bandgap energy. The plot reveals a distinct trend indicating that the bandgap energy increases with increasing Mg doping concentration. The bandgap of MgO (approximately 7.8 eV) is larger than that of NiO (approximately 3.8 eV). When MgO is doped into NiO, the Ni 3d states become localized. As a result, the VBM shifts to a deeper energy level, and the CBM shifts to a lower energy level, leading to an increase in the bandgap [43].

We performed ultraviolet photoelectron spectroscopy (UPS) for Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) to investigate the effects of Mg concentration on the valence band maximum (VBM) and work function (WF). Figure 5a shows two sets of UPS spectra for undoped NiO and different Mg-doped NiO samples. The left set focuses on the binding energy range typically associated with deeper core levels, while the right set examines energies near the Fermi level, providing insights into the valence band structure. The spectra show changes in peak positions due to Mg doping in the NiO samples, revealing electronic-state alterations induced by Mg doping.

Figure 5: The impact of Mg concentration on the valence band maximum (VBM) and work function (WF). (a) UPS spectra of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) films deposited on ITO substrates (secondary electron cut-off on left-side and valence band edge on right-side); (b) band diagram illustrating the changes in band structure with Mg doping.
Figure 5:

The impact of Mg concentration on the valence band maximum (VBM) and work function (WF). (a) UPS spectra of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) films deposited on ITO substrates (secondary electron cut-off on left-side and valence band edge on right-side); (b) band diagram illustrating the changes in band structure with Mg doping.

Based on the UPS result, the band diagram in Figure 5b illustrates the changes in VBM with increasing Mg doping concentration. The VBM is the highest energy state that electrons can occupy at absolute zero temperature [44]. As the Mg doping concentration increased, the VBM shifted toward higher binding energies, indicating systematic changes in the electronic properties of NiO upon Mg doping. Specifically, with increased Mg doping, the VBM of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) films shifted to deeper levels at −5.35, −5.37, −5.41, −5.44, and −5.48 eV respectively. This shift indicates a reduction in the hole-injection barrier between the hole transport layer (HTL) and the hole injection layer (HIL) at the interface, facilitating smoother hole injection in doped NiO than in undoped NiO. However, as the doping level increases, the energy gap between the Fermi energy level and the VBM also increases, indicating a decrease in the p-type characteristics.

Figure 6a illustrates the current density–voltage (J–V) characteristics of Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2) (Structure: ITO/Ni1−x Mg x O/Au). When the Mg doping concentration was 5 % or 10 %, there was an increase in current density. However, as the Mg doping concentration increased to 15 % and 20 %, the current density decreased, which was attributed to we determined the energy gap of E F – E v through UPS analysis, and according to the calculation formula p = N v ⁡ exp − E F − E v k T , we confirmed that the hole concentration decreases with Mg doping. Therefore, despite the decrease in hole concentration with Mg doping, we found that the hole injection barrier decreased, increasing the number of injected holes. As a result, the current density increased when 10 % Mg was doped in the J–V characteristic evaluation. As shown in Figure 6b, the highest current density in the HOD was achieved using Ni0.9Mg0.1O, which was attributed to the reduced hole-injection energy barrier increasing the number of injected holes. In contrast, Ni0.8Mg0.2O exhibited a lower current density; despite its reduced hole-injection energy barrier, E F – E v increases, leading to a decrease in hole concentration and thus reducing hole injection characteristics, resulting in lower device performance.

Figure 6: Electrical properties. (a) Current density–voltage (J–V) curves of ITO/Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2.)/Au for conductivity evaluation; (b) current density–voltage (J–V) curves of ITO/Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2.)/SAM/TFB/QD/Au for the hole-only device (HOD).
Figure 6:

Electrical properties. (a) Current density–voltage (J–V) curves of ITO/Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2.)/Au for conductivity evaluation; (b) current density–voltage (J–V) curves of ITO/Ni1−x Mg x O (x = 0, 0.05, 0.1, 0.15, 0.2.)/SAM/TFB/QD/Au for the hole-only device (HOD).

To evaluate the performance of N1−x Mg x O as a hole-injection layer in QLED devices, we fabricated QLED devices with the following structure: ITO/Ni1−x Mg x O/self-assembled monolayer (SAM)/TFB/QDs/ZnMgO/Al. Figure 7a shows the energy level diagram of the QLED device. The figure illustrates the alignment of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of different layers within the device structure. It also provides valuable information for the structural and energetic design of devices by identifying the layers and their respective energy levels that facilitate charge injection and transport [33].

Figure 7: QLED devices performance with various concentrations of Mg dopant. (a) Schematic of a QLED structure; (b) cross-section of a QLED device obtained by TEM; (c) electroluminescence (EL) spectra of a Ni0.9Mg0.1O QLED device; (d) J–V–L curve of a QLED; (e) EQE and current efficiency curve versus current density; (f) EQE and current efficiency curves according to luminance.
Figure 7:

QLED devices performance with various concentrations of Mg dopant. (a) Schematic of a QLED structure; (b) cross-section of a QLED device obtained by TEM; (c) electroluminescence (EL) spectra of a Ni0.9Mg0.1O QLED device; (d) J–V–L curve of a QLED; (e) EQE and current efficiency curve versus current density; (f) EQE and current efficiency curves according to luminance.

Figure 7b shows a cross-sectional transmission electron microscopy (TEM) image of a QLED device, revealing the different layers used in its construction. The thicknesses of each layer in the ITO/Ni1−x Mg x O/TFB/QDs/ZnMgO/Al structure are approximately 54, 40, 13, 11, 48, and 109 nm, respectively. The image shows that each multilayer was well deposited by the colloidal solution process. Figure 7c shows the emission spectra at different voltages for the QLED device. The graph illustrates the variations in intensity and peak wavelength of the emitted light as a function of applied voltage, indicating that as the voltage increases, the devices emit stronger light, which peaks within the green region of the visible spectrum (as shown in the inset photograph).

The performance of the QLED devices with various Mg doping levels is shown in Figure 7d–f, while Table 1 summarizes these results. Devices employing Ni0.9Mg0.1O as the HIL achieved superior performance, reaching an external quantum efficiency (EQE) of 5.11 %, luminance of 39,280 cd/m2 and a current efficiency of 21.28 cd/A.

Table 1:

Summary of QLED device performance at various Mg dopant concentrations.

Mg Concentration (%) EQEmax (%) L max (cd/m2) CE (cd/A)
0 2.76 29,003 11.33
5 3.66 38,396 15.02
10 5.11 39,280 21.28
15 4.19 27,065 17.43
20 4.76 7,425 19.85

Figure 6b shows an unexpected outcome regarding the hole injection barrier. Although theory indicates that increasing Mg doping would lower the valence band maximum (VBM), leading to a reduced hole-injection barrier, a device employing a Ni0.8Mg0.2O film as the HIL did not improve, possibly due to the decrease in carrier concentration. Therefore, Ni0.9Mg0.1O, doped with a moderate amount of Mg proved to be the most effective hole-injection layer. Further optimization of the Ni0.9Mg0.1O films led to improved QLED devices. This result demonstrates that a balanced approach to doping rather than maximizing the Mg concentration is crucial for achieving optimal device performance.

To enhance the performance of this QLED device, we optimized the post-process by depositing Ni0.9Mg0.1O via spin coating followed by heat treatment and then applying UV-ozone (UVO) treatment for 5, 10, and 15 min. Furthermore, following the deposition of the ZnMgO electron transport layer (ETL), the material was subjected to thermal annealing (TA) under several conditions: 10 min, 1 h, and 2 h.

Figure 8a shows the current density (mA/cm2) as a function of voltage (V) for devices subjected to different durations of UVO treatment and thermal annealing (TA). Treatments are indicated by the first and second numbers in each legend entry, respectively (e.g., “15 min/1 h” indicates 15 min of UVO treatment followed by 1 h of thermal annealing). All devices exhibited typical diode behavior, with the current density increasing exponentially with voltage. Notably, longer treatment times resulted in higher current densities at a given voltage, indicating improved charge transport or reduced resistance within the device.

Figure 8: QLED devices performance with additional treatment. (a) J–V–L curve of QLED devices; (b) EQE and current efficiency (CE) curve versus current density; (c) EQE and current efficiency curve according to luminance.
Figure 8:

QLED devices performance with additional treatment. (a) J–V–L curve of QLED devices; (b) EQE and current efficiency (CE) curve versus current density; (c) EQE and current efficiency curve according to luminance.

Figure 8b and c shows the external quantum efficiency (EQE) and current efficiency as functions of current density and luminance. Notably, the EQE decreased with increasing current density, which is a trend observed in optoelectronic devices [45]. The optimized combination of a 15-min UVO treatment on the Ni0.9Mg0.1O layer and subsequent thermal annealing (TA) for 2 h on the ZnMgO significantly enhanced the performance. The resulting device achieved an external quantum efficiency (EQE) of 8.38 %, with a luminance of 66,677 cd/m2 and a current efficiency of 35.31 cd/A, indicating superior performance. The UVO treatment applied to the Ni0.9Mg0.1O film effectively reduced the hole injection barrier and improved hole transport to quantum dots (QDs), thus minimizing electron accumulation and enhancing the overall device efficiency (Table 2).

Table 2:

Summary of QLED device performance at various UVO/TA times.

UVOa/TAa (time) EQEmax (%) L max (cd/m2) CE (cd/A)
0 min/10 min 5.11 39,280 21.28
15 min/10 min 5.23 35,413 21.85
0 min/1 h 5.47 35,011 22.93
15 min/1 h 5.61 35,958 23.46
0 min/2 h 4.66 33,268 19.51
5 min/2 h 6.15 38,871 25.85
10 min/2 h 6.35 40,180 26.73
15 min/2 h 8.38 66,677 35.31

  1. aUVO: UV-Ozone treatment, TA: thermal annealing.

To investigate the effect of Mg doping on the quantum dot (QD) layer characteristics, photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were conducted. Figure 9a shows the PL intensities of various samples as a function of wavelength in the range of 490–570 nm. The curves represent PL from different layers in the ITO/NiO/QD and ITO/Ni0.9Mg0.1O/QD structures, including the self-assembled monolayer (SAM) and TFB, which are likely hole transport layers. The PL intensity was significantly higher for samples with the Ni0.9Mg0.1O layer, both with and without SAM/TFB treatment, than for NiO. This increase was attributed to the deeper valence band maximum (VBM) resulting from increased Mg doping, as shown in Figure 5b, which improves the band alignment between NiO and the other layers. Consequently, hole accumulation and QD charging was reduced, leading to suppressed non-radiative recombinations such as Auger recombination [11], [33].

Figure 9: Optical characterizations. (a) Photoluminescence (PL) and (b) time-resolved photoluminescence (TRPL) of a Ni1−x Mg x O (x = 0, 0.1) deposited on ITO.
Figure 9:

Optical characterizations. (a) Photoluminescence (PL) and (b) time-resolved photoluminescence (TRPL) of a Ni1−x Mg x O (x = 0, 0.1) deposited on ITO.

The time-resolved photoluminescence (TRPL) spectra depicted in Figure 9b show the normalized decay of PL intensity over time (in nanoseconds) after excitation. TRPL data provide insights into the charge recombination kinetics, which are crucial for optimizing the performance of optoelectronic devices [6]. The presence of Ni0.9Mg0.1O, with and without SAM/TFB layers, influenced the PL decay profile, thereby influencing the charge dynamics and recombination processes in the QDs. The lower decay rates in Ni1−x Mg x O indicate improved charge separation or transfer to the QDs, leading to a longer radiative recombination lifetime. Consequently, compared with pure NiO, Ni0.9Mg0.1O exhibited a suppression of the quenching effect. The QD layer of Ni0.9Mg0.1O exhibited an extended PL lifetime and higher PL intensity, which were attributed to the reduced trap density [11], [33] (Table 3).

Table 3:

Comparison of photoluminescence (PL) and time-resolved photoluminescence (TRPL) of a Ni1−x Mg x O (x = 0, 0.1) deposited on ITO.

Sample A 1 (%) τ 1 (ns) A 2 (%) τ 2 (ns) A 3 (%) τ 3 (ns) τ ave (ns)
Ni0.9Mg0.1O/SAM/TFB/QD 2.5 6.8 6.3 18 0.3609 70 25.3
NiO/SAM/TFB/QD 2.4 7.6 5.6 19 0.3092 72 25
Ni0.9Mg0.1O/QD 3.6 4.6 3.29 16 0.1864 67 22
NiO/QD 3.4 4.1 2.9 15 0.1448 67 20.3

4 Conclusions

This study investigates quantum dot light-emitting diodes (QLEDs) using an ITO/Ni1−x Mg x O/SAM/TFB/QDs/ZnMgO/Al configuration to investigate the impact of varying Mg dopant concentrations on device performance. The integration of MgO into the NiO HIL enhanced band alignment via band tuning, thereby enhancing charge balance. Notably, the Ni0.9Mg0.1O composition was the most effective doping concentration, highlighting the critical role of the precise doping concentration. Further improvements were achieved through UV-Ozone surface treatments of Ni0.9Mg0.1O and thermal annealing of the ZnMgO electron transport layer, which mitigated surface defects and reduced quenching effects. These optimizations resulted in a QLED with a maximum external quantum efficiency (EQE) of 8.38 %, luminance of 66,677 cd/m2, and current efficiency of 35.31 cd/A. The successful incorporation of Mg-doped NiO into the QLED structure is expected to offer enhanced charge balance and superior overall performance, indicating a significant advancement in QLED display technology.

Corresponding authors: Byoung-Seoug Jeong, Department of Hydrogen and Renewable Energy, Kyungpook National University, Daegu 41566, Republic of Korea; and KNU Advanced Material Research Institute, 34986 Kyungpook National University , Daegu 41566, Republic of Korea, E-mail: ; and Young-Woo Heo, School of Materials Science and Engineering, 34986 Kyungpook National University , Daegu 41566, Republic of Korea; and KNU Advanced Material Research Institute, 34986 Kyungpook National University , Daegu 41566, Republic of Korea, E-mail:

Nayoon Lee and Van Khoe Vo contributed equally to this work.

  1. Research funding: This work was funded by Ministry of Trade, Industry & Energy (MOTIE, Korea) supporting the Materials and Components Technology Development Program [No. 20016195] and supporting the Industrial Technology Innovation Program [No. 20010427]. This study was supported by the BK21 Four project funded by the Ministry of Education, Korea (2120231314753).

  2. Author contributions: NL: Collecting data, synthesizing solutions, writing, and preparing original drafts. VKV: Collecting data, investigating, making devices, and preparing original drafts. H-JL: Investigating, and analyzing data. SJ: Investigating powder, and solution. THTD: Investigating powder, and solutions. HJ: Synthesizing solutions. DC: Measuring devices. J-HL: Reviewing, editing. B-SJ: writing, Reviewing, editing, validating. Y-WH: Reviewing, editing, validating, supervising. All authors read and approved the final manuscript.

  3. Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  4. Data availability: The data presented in this study are available upon request from the corresponding authors.

References

[1] Z. Liao, et al.., “Ultralow roll-off quantum dot light-emitting diodes using engineered carrier injection layer,” Adv. Mater., vol. 35, no. 47, p. 2303950, 2023. https://doi.org/10.1002/adma.202303950.Search in Google Scholar PubMed

[2] E. Jang and H. Jang, “Review: quantum dot light-emitting diodes,” Chem. Rev., vol. 123, no. 8, pp. 4663–4692, 2023. https://doi.org/10.1021/acs.chemrev.2c00695.Search in Google Scholar PubMed

[3] Q. Li, et al.., “Boosting electroluminescence performance of all solution processed InP based quantum dot light emitting diodes using bilayered inorganic hole injection layers,” Photonics Res., vol. 10, no. 9, pp. 2133–2139, 2022. https://doi.org/10.1364/prj.467604.Search in Google Scholar

[4] M. F. Prodanov, V. V. Vashchenko, and A. K. Srivastava, “Progress toward blue-emitting (460–475 nm) nanomaterials in display applications,” Nanophotonics, vol. 10, no. 7, pp. 1801–1836, 2021. https://doi.org/10.1515/nanoph-2021-0053.Search in Google Scholar

[5] J. Kim, D. Hahm, W. K. Bae, H. Lee, and J. Kwak, “Transient dynamics of charges and excitons in quantum dot light-emitting diodes,” Small, vol. 18, no. 29, p. 2202290, 2022. https://doi.org/10.1002/smll.202202290.Search in Google Scholar PubMed

[6] H. Wan, et al.., “Nickel oxide hole injection layers for balanced charge injection in quantum dot light-emitting diodes,” Small, vol. 20, no. 34, p. 2402371, 2024. https://doi.org/10.1002/smll.202402371.Search in Google Scholar PubMed

[7] J. H. Chang, et al.., “Unraveling the origin of operational instability of quantum dot based light-emitting diodes,” ACS Nano, vol. 12, no. 10, pp. 10231–10239, 2018. https://doi.org/10.1021/acsnano.8b03386.Search in Google Scholar PubMed

[8] Y. Deng, et al.., “Solution-processed green and blue quantum-dot light-emitting diodes with eliminated charge leakage,” Nat. Photonics, vol. 16, no. 7, pp. 505–511, 2022. https://doi.org/10.1038/s41566-022-00999-9.Search in Google Scholar

[9] H. Jung, et al.., “Two-band optical gain and ultrabright electroluminescence from colloidal quantum dots at 1000 A cm−2,” Nat. Commun., vol. 13, no. 1, p. 3734, 2022. https://doi.org/10.1038/s41467-022-31189-4.Search in Google Scholar PubMed PubMed Central

[10] Z. Zhang, et al.., “High-brightness all-polymer stretchable LED with charge-trapping dilution,” Nature, vol. 603, no. 7902, pp. 624–630, 2022. https://doi.org/10.1038/s41586-022-04400-1.Search in Google Scholar PubMed

[11] F. Wang, et al.., “Highly efficient and super stable full-color quantum dots light-emitting diodes with solution-processed all-inorganic charge transport layers,” Small, vol. 17, no. 12, p. 2007363, 2021. https://doi.org/10.1002/smll.202007363.Search in Google Scholar PubMed

[12] E. Yoon, K. Y. Jang, J. Park, and T. Lee, “Understanding the synergistic effect of device architecture design toward efficient perovskite light-emitting diodes using interfacial layer engineering,” Adv. Mater. Interfaces, vol. 8, no. 3, p. 2001712, 2021. https://doi.org/10.1002/admi.202001712.Search in Google Scholar

[13] S. Rhee, et al.., “Steering interface dipoles for bright and efficient all-inorganic quantum dot based light-emitting diodes,” ACS Nano, vol. 15, no. 12, pp. 20332–20340, 2021. https://doi.org/10.1021/acsnano.1c08631.Search in Google Scholar PubMed

[14] S. Rhee, et al.., “Versatile use of 1,12-diaminododecane as an efficient charge balancer for high-performance quantum-dot light-emitting diodes,” ACS Photonics, vol. 10, no. 2, pp. 500–507, 2023. https://doi.org/10.1021/acsphotonics.2c01638.Search in Google Scholar

[15] L. Kong, et al.., “Stability of perovskite light-emitting diodes: existing issues and mitigation strategies related to both material and device aspects,” Adv. Mater., vol. 34, no. 43, p. 2205217, 2022. https://doi.org/10.1002/adma.202205217.Search in Google Scholar PubMed

[16] M. Azadinia, P. Chun, Q. Lyu, G. Cotella, and H. Aziz, “Differences in electron and hole injection and auger recombination between red, green, and blue CdSe-based quantum dot light emitting devices,” ACS Nano, vol. 18, no. 2, pp. 1485–1495, 2024. https://doi.org/10.1021/acsnano.3c07999.Search in Google Scholar PubMed

[17] P. H. Lee, et al.., “Highly crystalline colloidal nickel oxide hole transport layer for low-temperature processable perovskite solar cell,” Chem. Eng. J., vol. 412, p. 128746, 2021. https://doi.org/10.1016/j.cej.2021.128746.Search in Google Scholar

[18] F. Cheng, et al.., “85 °C/85%-Stable n-i-p perovskite photovoltaics with NiOx hole transport layers promoted by perovskite quantum dots,” Adv. Sci., vol. 9, no. 26, p. 2201573, 2022. https://doi.org/10.1002/advs.202201573.Search in Google Scholar PubMed PubMed Central

[19] W. Chen, et al.., “Engineering of dendritic dopant-free hole transport molecules: enabling ultrahigh fill factor in perovskite solar cells with optimized dendron construction,” Sci. China Chem., vol. 64, pp. 41–51, 2020. https://doi.org/10.1007/s11426-020-9857-1.Search in Google Scholar

[20] X. Sun, et al.., “Hole-injection-barrier effect on the degradation of blue quantum-dot light-emitting diodes,” ACS Nano, vol. 18, no. 7, pp. 5898–5906, 2023. https://doi.org/10.1021/acsnano.3c12840.Search in Google Scholar PubMed

[21] H. S. Kim and C. K. Kim, “All-solution-processed quantum-dot light emitting diodes with nickel oxide nanoparticles as a hole injection layer,” Mol. Cryst. Liq. Cryst., vol. 678, no. 1, pp. 33–42, 2019. https://doi.org/10.1080/15421406.2019.1597526.Search in Google Scholar

[22] Q. Wu, et al.., “Efficient tandem quantum-dot LEDs enabled by an inorganic semiconductor-metal-dielectric interconnecting layer stack,” Adv. Mater., vol. 34, no. 4, p. 2108150, 2022. https://doi.org/10.1002/adma.202108150.Search in Google Scholar PubMed

[23] F. Cao, et al.., “All-inorganic quantum dot light-emitting diodes with suppressed luminance quenching enabled by chloride passivated tungsten phosphate hole transport layers,” Small, vol. 17, no. 19, p. 2100030, 2021. https://doi.org/10.1002/smll.202100030.Search in Google Scholar PubMed

[24] F. Cao, et al.., “High-efficiency and stable quantum dot light-emitting diodes enabled by a solution-processed metal-doped nickel oxide hole injection interfacial layer,” Adv. Funct. Mater., vol. 27, no. 42, p. 1704278, 2017. https://doi.org/10.1002/adfm.201704278.Search in Google Scholar

[25] P. Nandi, et al.., “NiO as hole transporting layer for inverted perovskite solar cells: a study of X-ray Photoelectron spectroscopy,” Adv. Mater. Interfaces, vol. 11, no. 8, p. 2300751, 2024. https://doi.org/10.1002/admi.202300751.Search in Google Scholar

[26] M. H. Li, J. H. Yum, S. J. Moon, and P. Chen, “Inorganic p-type semiconductors: their applications and progress in dye-sensitized solar cells and perovskite solar cells,” Energies, vol. 9, no. 5, p. 331, 2016. https://doi.org/10.3390/en9050331.Search in Google Scholar

[27] M. Taeño, D. Maestre, and A. Cremades, “An approach to emerging optical and optoelectronic applications based on NiO micro- and nanostructures,” Nanophotonics, vol. 10, no. 7, pp. 1785–1799, 2021. https://doi.org/10.1515/nanoph-2021-0041.Search in Google Scholar

[28] M. Bonomo, “Synthesis and characterization of NiO nanostructures: a review,” J. Nanopart. Res., vol. 20, no. 8, p. 222, 2018. https://doi.org/10.1007/s11051-018-4327-y.Search in Google Scholar

[29] F. Jiang, W. C. H. Choy, X. Li, D. Zhang, and J. Cheng, “Post-treatment-free solution-processed non-stoichiometric NiOx nanoparticles for efficient hole-transport layers of organic optoelectronic devices,” Adv. Mater., vol. 27, no. 18, pp. 2930–2937, 2015. https://doi.org/10.1002/adma.201405391.Search in Google Scholar PubMed

[30] Y. Zhang, L. Zhao, H. Jia, and P. Li, “Study of the electroluminescence performance of NiO-based quantum dot light-emitting diodes: the effect of annealing atmosphere,” Appl. Surf. Sci., vol. 526, p. 146732, 2020. https://doi.org/10.1016/j.apsusc.2020.146732.Search in Google Scholar

[31] J. M. Caruge, J. E. Halpert, V. Bulović, and M. G. Bawendi, “NiO as an inorganic hole-transporting layer in quantum-dot light-emitting devices,” Nano Lett., vol. 6, no. 12, pp. 2991–2994, 2006. https://doi.org/10.1021/nl0623208.Search in Google Scholar PubMed

[32] Y. Yu, et al.., “Low-temperature solution-processed transparent QLED using inorganic metal oxide carrier transport layers,” Adv. Funct. Mater., vol. 32, no. 3, p. 2106387, 2022. https://doi.org/10.1002/adfm.202106387.Search in Google Scholar

[33] Y. Jiang, et al.., “All-inorganic quantum-dot light-emitting diodes with reduced exciton quenching by a MgO decorated inorganic hole transport layer,” ACS Appl. Mater. Interfaces, vol. 11, no. 12, pp. 11119–11124, 2019. https://doi.org/10.1021/acsami.9b01742.Search in Google Scholar PubMed

[34] Q. Chen, et al.., “Gate-tunable all-inorganic QLED with enhanced charge injection balance,” J. Mater. Chem. C Mater., vol. 8, no. 4, pp. 1280–1285, 2020. https://doi.org/10.1039/c9tc06088j.Search in Google Scholar

[35] X. Yang, et al.., “High-efficiency all-inorganic full-colour quantum dot light-emitting diodes,” Nano Energy, vol. 46, pp. 229–233, 2018, https://doi.org/10.1016/j.nanoen.2018.02.002.Search in Google Scholar

[36] J. Deng, M. Mortazavi, N. V. Medhekar, and J. Zhe Liu, “Band engineering of Ni1-xMgxO alloys for photocathodes of high efficiency dye-sensitized solar cells,” J. Appl. Phys., vol. 112, no. 12, p. 123703, 2012. https://doi.org/10.1063/1.4769210.Search in Google Scholar

[37] Y. Chen, et al.., “Lattice distortion and electronic structure of magnesium-doped nickel oxide epitaxial thin films,” Phys. Rev. B, vol. 95, no. 24, p. 245301, 2017. https://doi.org/10.1103/PhysRevB.95.245301.Search in Google Scholar

[38] R. C. Boutwell, M. Wei, A. Scheurer, J. Mares, and W. Schoenfeld, “Optical and structural properties of NiMgO thin films formed by sol-gel spin coating,” Thin Solid Films, vol. 520, no. 13, pp. 4302–4304, 2012. https://doi.org/10.1016/j.tsf.2012.02.065.Search in Google Scholar

[39] G. Li, et al.., “Overcoming the limitations of sputtered nickel oxide for high-efficiency and large-area perovskite solar cells,” Adv. Sci., vol. 4, no. 12, p. 1700463, 2017. https://doi.org/10.1002/advs.201700463.Search in Google Scholar PubMed PubMed Central

[40] M. Bao, et al.., “Plate-like p-n heterogeneous NiO/WO3 nanocomposites for high performance room temperature NO2 sensors,” Nanoscale, vol. 6, no. 8, pp. 4063–4066, 2014. https://doi.org/10.1039/c3nr05268k.Search in Google Scholar PubMed

[41] Y. Zeng, et al.., “An ultrastable and high-performance flexible fiber-shaped Ni–Zn battery based on a Ni–NiO heterostructured nanosheet cathode,” Adv. Mater., vol. 29, no. 44, p. 1702698, 2017. https://doi.org/10.1002/adma.201702698.Search in Google Scholar

[42] X. Sun, et al.., “Three-dimensionally ‘curved’ NiO nanomembranes as ultrahigh rate capability anodes for Li-ion batteries with long cycle lifetimes,” Adv. Energy Mater., vol. 4, no. 4, p. 1300912, 2014. https://doi.org/10.1002/aenm.201300912.Search in Google Scholar

[43] W. Xie, et al.., “The effect of sputtering gas pressure on the structure and optical properties of MgNiO films grown by radio frequency magnetron sputtering,” Appl. Surf. Sci., vol. 405, pp. 152–156, 2017, https://doi.org/10.1016/j.apsusc.2017.02.028.Search in Google Scholar

[44] H. Ishii, K. Sugiyama, E. Ito, and K. Seki, “Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces,” Adv. Mater., vol. 11, no. 8, pp. 605–625, 1999. https://doi.org/10.1002/(SICI)1521-4095(199906)11:8<605::AID-ADMA605>3.0.CO;2-Q.10.1002/(SICI)1521-4095(199906)11:8<605::AID-ADMA605>3.3.CO;2-HSearch in Google Scholar

[45] D. Tian, et al.., “A review on quantum dot light-emitting diodes: from materials to applications,” Adv. Opt. Mater., vol. 11, no. 2, p. 2201965, 2023. https://doi.org/10.1002/adom.202201965.Search in Google Scholar

Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/nanoph-2024-0239).

Received: 2024-04-30
Accepted: 2024-08-22
Published Online: 2024-09-11

© 2024 the author(s), published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

  1. Frontmatter
  2. Editorial
  3. Current trends in nanophotonics
  4. Review
  5. Applications of surface enhanced Raman scattering (SERS) spectroscopy for detection of nucleic acids
  6. Research Articles
  7. Design of optical Kerr effect in multilayer hyperbolic metamaterials
  8. A tiny Drude scatterer can accurately model a coherent emitter in nanophotonics
  9. Strong coupling spontaneous emission interference near a graphene nanodisk
  10. Long-range molecular energy transfer mediated by strong coupling to plasmonic topological edge states
  11. Thermal radiation forces on planar structures with asymmetric optical response
  12. Molecular surface coverage standards by reference-free GIXRF supporting SERS and SEIRA substrate benchmarking
  13. Effect of magnesium doping on NiO hole injection layer in quantum dot light-emitting diodes
  14. Anomalous reflection for highly efficient subwavelength light concentration and extraction with photonic funnels
  15. Nanometric probing with a femtosecond, intra-cavity standing wave
  16. Considerations for electromagnetic simulations for a quantitative correlation of optical spectroscopy and electron tomography of plasmonic nanoparticles
  17. Free-electron coupling to surface polaritons mediated by small scatterers
https://doi.org/10.1515/nanoph-2024-0239
Keywords for this article
QLED; NiO; Ni1−x Mg x O; ZnMgO; hole injection layer
Creative Commons
BY 4.0

Articles in the same Issue

  1. Frontmatter
  2. Editorial
  3. Current trends in nanophotonics
  4. Review
  5. Applications of surface enhanced Raman scattering (SERS) spectroscopy for detection of nucleic acids
  6. Research Articles
  7. Design of optical Kerr effect in multilayer hyperbolic metamaterials
  8. A tiny Drude scatterer can accurately model a coherent emitter in nanophotonics
  9. Strong coupling spontaneous emission interference near a graphene nanodisk
  10. Long-range molecular energy transfer mediated by strong coupling to plasmonic topological edge states
  11. Thermal radiation forces on planar structures with asymmetric optical response
  12. Molecular surface coverage standards by reference-free GIXRF supporting SERS and SEIRA substrate benchmarking
  13. Effect of magnesium doping on NiO hole injection layer in quantum dot light-emitting diodes
  14. Anomalous reflection for highly efficient subwavelength light concentration and extraction with photonic funnels
  15. Nanometric probing with a femtosecond, intra-cavity standing wave
  16. Considerations for electromagnetic simulations for a quantitative correlation of optical spectroscopy and electron tomography of plasmonic nanoparticles
  17. Free-electron coupling to surface polaritons mediated by small scatterers
Sign up now to receive a 20% welcome discount
Institutional Access
How does access work?
Have an idea on how to improve our website?
Please write us.
© 2025 De Gruyter Brill
Downloaded on 11.9.2025 from https://www.degruyterbrill.com/document/doi/10.1515/nanoph-2024-0239/html
Scroll to top button