JangGyeong-Pil1
YangJi-Hun1
KimSu-Young1
ChaeYoung-Bin†
ChoiHyuk-Doo1
MoonDae-Gyu1
LeeKyoung-Ho1
KimChang-Kyo†
-
(Department of Electronic Materials, Devices, and Equipment Engineering, Soonchunhyang
University, Asan 31536, Korea)
Copyright © The Institute of Electronics and Information Engineers(IEIE)
Index Terms
Quantum dot light-emitting diode, Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O nanoparticles, electron transport layer, charge balance, exciton quenching
I. INTRODUCTION
Quantum dot light-emitting diodes (QLEDs) have emerged as prominent candidates for
next-generation displays and lighting due to their remarkable attributes including
exceptional color purity, tunable emission wavelengths, high efficiency, and solution
processability [1-6]. The continuous advancements in fabrication technology have led QLEDs to steadily
approach the performance levels of organic light-emitting diodes (OLEDs) [1-11]. The efficiency and functionality of QLEDs are significantly influenced by the engineering
and selection of functional layers. While most QLEDs adopt a bottom-emitting structure,
top-emission (TE) structures have attracted increasing attention due to their adaptability.
TE-QLEDs minimize the effects of the many thin-film transistors integrated onto the
substrate, thus expanding the range of compatible substrates to include even opaque
materials. [12,13]. These TE structures can be effectively fabricated onto silicon or metal foils, thereby
enhancing their potential applications in displays. Several metal oxides such as ZnO
nanoparticles (NPs) were solution-processable for QLED and OLED fabrications. ZnO
nanoparticles (NPs) are widely used as electron transport layers (ETLs) in QLEDs due
to their high mobility, transparency, and conduction band level compatibility with
quantum dots (QDs) [9,11, 14-19]. However, QLEDs employing ZnO NP ETLs often experience imbalanced charge injection,
wherein electron injection and transport surpass hole injection and transport [19-21]. This imbalance can lead to the accumulation of excessive electrons at the interface
between the QD emission layer (EML) and ZnO NP ETLs, potentially charging the QDs
and increasing the risk of non-radiative Auger recombination [14,20,21]. Additionally, ZnO NPs often contain many defects arising from oxygen vacancies [21] and surfaces of ZnO NPs synthesized in air are known to be rich of hydroxide oxygen
which act as exciton quenching sites affecting performance [22]. Direct contact between a QD emission layer (EML) and ZnO NP ETLs rich in defects
and hydroxide oxygen results in spontaneous electron transfer at the interface, causing
exciton quenching [23-27]. These phenomena are facilitated by interfacial charge transfer and/or intragap-assisted
non-radiative recombination centers [20], significantly influencing device performance. Moreover, certain studies suggest
that holes might also transfer from QDs to the intragap state of ZnO NPs [6], inducing exciton dissociation within the QD EML, negatively influencing device performance.
A straightforward way to suppress exciton quenching is to passivate on the surface
of the ZnO using an insulating layer of poly(methyl methacrylate) (PMMA), as demonstrated
by Dai et al. [15]. Another approach involves inserting a Mg-doped ZnO (ZnMgO) NP interlayer between
the ZnO NP ETL and QD EML to suppress exciton quenching and reduce the electron current,
thereby enhancing the charge balance in QLEDs [28,29]. Heo et al. fabricated QLEDs with ZnMgO interfacial layers between the QD EML and
ZnO NP ETLs, resulting in an 86.9% increase in external quantum efficiency (EQE).
The stepped interfacial electronic structure is preventing direct contact between
the QD EML and ZnO NP ETLs [28]. The use of ZnMgO as ETLs in QLEDs allows tuning of mobility, conductivity, surface
defects and conduction band levels, contributing to improved charge balance and reduced
exciton quenching [30,31]. Some groups have developed highly efficient QLEDs using a ZnMgO core-shell structure
as an ETL, passivating defects in ZnO NPs, attributed to Mg in the shell layer [32,33]. Currently, many researchers are focusing on direct contact between QDs and ZnMgO
rather than between QDs and ZnO NPs.
In this study, we aimed to optimize the charge balance and reduce interfacial quenching
in TE-QLEDs by solely adjusting the thickness of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs. The goal was to effectively mitigate the concentration of hydroxide oxygen
in Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NPs varying Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL, lowering exciton quenching and ultimately enhancing the radiative recombination
efficiency within the QD layer. Through X-ray photoelectron spectroscopy (XPS) measurements,
we investigated hydroxide oxygen states to explore interfacial electronic properties.
Additionally, we examined oxygen vacancy states to analyze electron injection and
electron transport in Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs. According to
the results, QLEDs with a 30-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O ETL
demonstrated a remarkable performance with a high level of efficiency due to an extremely
low concentration of hydroxyl group, resulting in a low exciton quenching. We found
that a TE-QLED with a 30-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETL demonstrated
outstanding performance, with a maximum current efficiency of 91.92 cd/A and a maximum
EQE of 21.66%. We believe that the use of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs of appropriate thickness will allow the fabrication of high-performance TE-QLEDs
for next-generation display and lighting applications.
II. MATERIALS AND METHODS
2.1 Material Syntheses
To synthesize Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NPs [33], we dissolved 0.2962 g zinc acetate dihydrate [Zn(CH3COO)${\cdot}$2H2O] powder (Sigma-Aldrich,
St. Louis, MO, USA) and 0.03292 g of magnesium acetate tetrahydrate [Mg(CH3COO)${\cdot}$4H2O]
powder (Sigma-Aldrich) in 15 mL dimethyl sulfoxide (DMSO) and 0.421 g of TMAH (Sigma
Aldrich) in 5 mL of EtOH. The two solutions were mixed and stirred at room temperature
for 24 h. Ethyl acetate (Kanto Chemical Co., Tokyo, Japan) was added to 33% of the
volume of the mixed solutions precipitating Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs as a white powder over 3 h; the NPs were retrieved via centrifugation.
2.2 Device Fabrication
TE-QLED fabrication employed a solution-process approach. The glass substrate is underwent
meticulous cleaning in acetone, isopropyl, alcohol, methanol, and deionized water
and then subjected to oxygen plasma treatment. Poly (3,4-ethylenedioxythiophene):poly
(styrene sulfonate) (PEDOT:PSS) (Al4083; Heraeus, Hanau, Germany) in isopropanol (Daejung
Chemicals and Metals, Gyeonggi, Republic of Korea) was spin-coated onto the glass
substrate, followed by baking at 60$^{\circ}$C for 10 m in air to eliminate residual
water. The resulting 25-nm-thick PEDOT:PSS film served as the buffer layer between
the glass substrate and Ag anode, enhancing adhesion during Ag evaporation onto the
glass substrate and improving the electrical conductivity of the Ag anode. Subsequently,
glass substrates with PEDOT:PSS buffer layers were placed in a high-vacuum deposition
chamber (Cetus OL 100; Celcose, Gyeonggi, Republic of Korea) under a pressure of 6
${\times}$ 10$^{-}$$^{7}$ Torr.
In the chamber, a 150-nm-thick Ag anode layer was deposited at an evaporation rate
of 1.2 Å/s. The anode layer was patterned as desired using an in-situ shadow mask.
A bank layer was photolithographically fabricated on the Ag anode to eliminate leakage
current between the anode and cathode. SU-8 2002 (Kayaku Advanced Materials, MA, USA)
was deposited onto the glass/PEDOT:PSS/Ag substrate via spin-coating at 500 rpm for
5 s at room temperature, followed by spin-coating at 3,000 rpm for 30 s at room temperature.
The SU-8 thin film thus formed was soft-baked at 90℃ on a hot plate for 2 min to evaporate
the solvent and create the thin film. The substrate with the SU-8 thin film was aligned
with the mask of the bank layer pattern on a mask aligner (ABM/6/350/NUV/DDD/M; ABM,
CA, USA) and exposed to UV light of 365 nm for 15.6 s. The SU-8 layer was thus polymerized
and crosslinked, becoming insoluble. After UV exposure, baking proceeded at 95℃ on
a hot plate for 2 min. The UV-exposed SU-8 was immersed in SU-8 developer for 1 min
to remove unreacted materials, followed by a 10-s rinse in isopropyl alcohol (IPA)
(Daejung Chemicals and Metals, Gyeonggi, Republic of Korea). A nitrogen jet removed
residual moisture. Thus, all developer and cleaning solutions were eliminated. To
enhance adhesion, a hard bake ran on a 150℃ hot plate for 30 min, yielding the bank
layer. To create the hole injection layer (HIL), an additional PEDOT: PSS layer was
spin-coated onto the glass/PEDOT: PSS/Ag substrate within the bank as described above.
To create the hole transport layer (HTL), poly(9 vinylcarbazole) (PVK) dissolved in
toluene was coated onto the pre-fabricated glass/PEDOT: PSS/Ag/PEDOT: PSS substrate
within the bank, and the PVK film was annealed in air at 60℃ for 10 min. The PVK film
thickness was 40 nm. To fabricate the QD EML, CdSe/ZnS QDs (Zeus, Gyeonggi, Republic
of Korea) were dissolved to 5 mg/mL in heptane, and the solution was spin-coated onto
the glass/PEDOT: PSS/Ag/PEDOT: PSS/PVK substrate within the bank; the QD film thickness
was 10 nm. The Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP solution was then spin-coated
onto the glass/PEDOT:PSS/Ag/PEDOT:PSS/PVK/QD substrate within the bank. Finally, a
15-nm-thick Ag cathode was deposited onto the glass/PEDOT:PSS/Ag/PEDOT:PSS/ PVK/QD/Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP substrate within the bank using the in-situ method employed for Ag anode deposition;
the TE-QLEDs were now complete (Fig. 1).
Fig. 1. Schematic of TE-QLED fabrication.
2.3 Characterization
The X-ray diffraction (XRD; D/Max 2200pc; Rigaku, Tokyo, Japan) patterns obtained
using Cu-K${\alpha}$ radiation revealed the crystal structures of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs. Field-emission transmission electron microscopy (FE-TEM) (Tecnai F30 S-Twin;
JEOL Ltd., Tokyo, Japan) determined the particle sizes. An XPS system (PHI Quantera-II;
Ulvac-PHI, Kanangawa, Japan) was used to investigate the composition of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP films. Transmittance and reflectance measurements were derived with a spectrophotometer
(UV-1650PC; Shimadzu Corp., Kyoto, Japan) that delivered monochromatic light normally
incident to the sample surface. The current density-voltage-luminance (J-V${-}$L)
characteristics were derived using a computer-controlled source meter (2400; Keithley
Instruments, Cleveland, OH, USA) and a luminance meter (LS100; Konica Minolta, Osaka,
Japan). Electroluminescence (EL) spectra were recorded using a dedicated spectroradiometer
(CS1000; Konica Minolta).
III. RESULTS AND DISCUSSION
Fig. 2 shows the characteristic 2${\theta}$ XRD patterns of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs, elucidating their crystal structure. The XRD analysis of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs revealed a hexagonal wurtzite lattice, with reflections across the (100), (002),
(101), (102), (110), (103), and (112) planes. These reflections closed matched those
of ZnO, confirmed by comparison with the Joint Committee on Powder Diffraction Standards
(JCPDS) (Card Number 1-1136). Minor deviations in peak positions and alterations in
interatomic distances were observed. Notably, when the XRD patterns of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs were compared with the those of MgO on the JCPDS (Card Number 1-1235), entirely
different patterns emerged. This disparity is attributed to the rocksalt structure
of MgO. Thus, the presence of the identical hexagonal wurtzite structure in the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs as in ZnO, distinct from the rocksalt structure of MgO, signifies successful incorporation
of Mg into ZnO.
Fig. 2. X-ray diffraction patterns XRD of Zn0.9Mg0.1O nanoparticles (NPs) over a 2θ range of 20-80°.
Fig. 3. Field-emission transmission electron micrographs of Zn0.9Mg0.1O NPs.
Fig. 4. X-ray photoelectron O 1s spectra of Zn0.9Mg0.1O NP thin films of varying thickness deposited onto a PEDOT:PSS/Ag/PEDOT:PSS/PVK/QD substrate: (a) 10-nm-thick films; (b) 30-nm-thick films; (c) 50-nm-thick films; (d) 70-nm-thick films; (e) Area ratios of the three peaks changes according to thickness
Fig. 5. Current density versus applied voltage of electron-only devices (EODs) with Zn0.9Mg0.1O NPs of various thicknesses (10, 30, 50, and 70 nm) and the hole-only device (HOD).
Fig. 6. Electroluminescence (EL) characteristics of TE-QLEDs with Zn0.9Mg0.1O NP ETLs of different thicknesses: 10, 30, 50, and 70 nm: (a) Current density-voltage curves as a function of voltage; (b) luminance curves as a function of voltage; (c) current efficiency and external quantum efficiency curves as a function of current density.
Fig. 7. Normalized photoluminescence spectrum of the CdSe/ZnS quantum dot (QD) and the EL spectra of TE-QLEDs with 10-, 30-, 50-, and 70-nm-thick Zn0.9Mg0.1O NP ETLs.
Fig. 3 shows a typical FE-TEM image of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NPs, the
average diameter of which was 4.61 nm.
XPS measurements were conducted to examine the oxygen configuration of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film as a function of varying thickness, providing information about the quality
of the NPs, as shown in Fig. 4. This figure illustrates the normalized O 1s spectra of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs deposited onto the PEDOT: PSS (25 nm)/Ag (150 nm)/PEDOT:PSS (25 nm)/QD (10 nm)
substrate. By employing deconvolution, three peaks were extracted, each corresponding
to specific components, including the metal oxide lattice bond (OM), oxygen vacancies
O2- ions (OV), and hydroxide oxide (-OH) [21,34]. The area of each peak exhibited variations depending on the thickness of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film. OT represents the overall O 1s peak. Fig. 4(e) includes a distribution graph illustrating the ratio of the areas of the three peaks
areas concerning the thickness of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP thin
film. The ratios of the peak area (OV/OT) of 10-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film, 30-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP thin film, 50-nm-thick
Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP thin film, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film were calculated to be 35.46%, 27.20%, 25.26%, and 21.70%, respectively.
On the other hand, the ratios of the peak area (-OH/OT) of 10-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film, 30-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP thin film, 50-nm-thick
Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP thin film, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film were estimated to be 9.36%, 0.57%, 3.67%, and 5.10%, respectively. As
the thickness increased, there was a clear tendency toward a greater concentration
of metal oxide lattice bond, while the concentration of oxygen vacancies decreased.
These vacancies act as trapping sites, and their increase leads to higher carrier
concentration, improved conductivity, and reduced resistance of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP thin film [23,28,35]. Conversely, hydroxyl oxygen species can act as an exciton quenching sites, affecting
the device efficiency.
Electron-only devices (EODs) and hole-only devices (HOD) were fabricated to investigate
hole injection, electron injection, and electron transport. Four EODs that differed
in Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP thickness (10, 30, 50, and 70 nm) were
developed (refer to Fig. 6), along with an HOD. The EOD structures comprised Ag (150 nm)/Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs (10, 30, 50, 70 nm)/QD (10 nm)/Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NPs (10,
30, 50, 70 nm)/Ag (150 nm). The HOD structure involved Ag (150 nm)/PEDOT: PSS (25
nm)/PVK (40 nm)/QD (10 nm)/Ag (150 nm). The thickness of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP layers was identical on both sides of the EOD QDs. The additional layer of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs on the anode blocked the arrival of holes from the anode and prevented excessive
current flow. Electrons were injected from the cathode into the conduction band minimum
(CBM) level of the QDs via the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NPs. The EOD
measurements revealed that as the thickness of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs increased, the current density decreased, consistent with the XPS results, which
revealed fewer oxygen vacancies as the thickness of Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NPs increased. This reduced conductivity and increased the resistance of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL. The HOD current density fell to within the range of the current densities
associated with EODs having thicknesses 30 and 50 nm. This implies that the optimal
charge balance of the QD EML can be achieved by a TE-QLED with a Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
ETL thickness of 30 or 50 nm.
Fig. 6 illustrates the EL characteristics of the TE-QLEDs with Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs of varying thickness (10, 30, 50, and 70 nm). In Fig. 6(a), the curves for current density are presented as a function of applied voltage. Note
that as the thickness of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs increases,
the current density decreases markedly, attributable to a reduction in conductivity
that reflects the fewer oxygen vacancies within the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL, as supported by the XPS results. This decrease is also associated with a reduced
electric field attributable to the increased ETL thickness. Fig. 6(b) shows luminance versus voltage curves. The turn-on voltages at 1 cd/m2 for TE-QLEDs
with 10-, 30-, 50-, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs
were extrapolated to be 4.15 V, 4.06 V, 4.57 V, and 4.52 V, respectively. Thus, the
TE-QLED with a 30-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETL exhibited
the lowest turn-on voltage; exciton quenching was minimized at the interface between
the QD and Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETL, attributable to a decrease
in hydroxide oxygen species (as confirmed by XPS analysis) and attainment of an optimal
charge balance in the QD EML. Fig. 6(b) shows the maximum luminance values of TE-QLEDs with 10-, 30-, 50-, and 70-nm-thick
Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs; these were estimated to be 30,560.1
cd/m$^{2}$, 257,307.4 cd/m$^{2}$, 157,337.4 cd/m$^{2}$, and 59,782.06 cd/m$^{2}$,
respectively. The TE-QLED with a 30-nm-thick Zn $_{\mathrm{0.9}}$ Mg$_{\mathrm{0.1}}$O
NP ETL exhibited the highest luminance. This correlated with the better-balance charge
carriers of electrons and holes within the QD EML in TE-QLEDs with 30- or 50-nm-thick
Zn $_{\mathrm{0.9}}$ Mg$_{\mathrm{0.1}}$O NP ETLs, as confirmed by analysis of the
EODs and HOD (refer to Fig. 3). Additionally, the reduced concentration of hydroxide oxygen species in the Zn $_{\mathrm{0.9}}$
Mg$_{\mathrm{0.1}}$O NPs (as shown by the XPS data in Fig. 4) minimized exciton quenching at the interface between the QD EML and Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL, thus enhancing luminance. Fig. 6(c) shows the current efficiencies and EQEs of TE-QLEDs with Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs with thicknesses of 10, 30, 50, and 70 nm; these were 9.68 cd/A, 90.92 cd/A,
55.33 cd/A, and 20.98 cd/A, respectively. The maximum EQEs for TE-QLEDs with 10-,
30-, 50-, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs were 3.28%,
21.66%, 13.22%, and 6.08%, respectively. Remarkably, the TE-QLED with a 30-nm-thick
Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETL exhibited an outstanding maximum current
efficiency and a maximum EQE of 90.92 cd/A and 21.66%, respectively, in line with
the above discussion. It is essential to choose an appropriate Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL thickness to balance the charge carriers of electrons and holes and minimize
the concentration of hydroxide oxygen species that engage in quenching at the interface
between the QD and Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs. Table 1 summarizes the key parameters of TE-QLEDs with 10-, 30-, 50-, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs.
Fig. 7 shows the normalized photoluminescence (PL) spectrum of the CdSe/ZnS QD and the EL
spectra of TE-QLEDs with 10-nm-thick, 30-nm-thick, 50-nm-thick, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs. A summary of the characteristic parameters is given in Table 2. The EL peak of the TE-QLED with a 10-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL is centered at 524 nm, matching the PL peak of the CdSe/ZnS QD. In contrast,
the EL peaks of TE-QLEDs with 30-nm-thick, 50-nm-thick, and 70-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs are centered at 532 nm, i.e., are red-shifted compared to the PL peak of the
CdSe/ZnS QD. Such red shifts can be attributed to Föster energy transfer, microcavity
effect and the Stark effect, which develops at high voltage and current [36,37].
Table 2 shows that the spectral FWHM increases with the thickness of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETLs. However, a thickness of 10 nm is too low to exert a significant impact. The
TE-QLED with a 10-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETL exhibited
the narrowest FWHM of 32.8 nm. The absence of parasitic PVK emission confirms that
device emission was primarily attributable to electron-hole recombination within the
CdSe/ZnS QD EML, in turn attributable to the good charge balance between holes and
electrons within that layer.
Table 1. The performance parameters of TE-QLEDs with various thickness of Zn0.9Mg0.1O NPs
Thickness of
Zn0.9Mg0.1O NP ETL
(nm)
|
Turn-on voltage at
1 cd/m2
(V)
|
Current Density at 9 V
(mA/cm2)
|
Maximum
Luminance
(cd/m2)
|
Maximum
Current Efficiency
(cd/A)
|
Maximum EQE
(%)
|
10
|
4.15
|
315.75
|
30560.06
|
9.68
|
3.28
|
30
|
4.06
|
42.75
|
257307.40
|
90.92
|
21.66
|
50
|
4.57
|
20.75
|
157337.40
|
55.33
|
13.22
|
70
|
4.65
|
25.75
|
59782.06
|
20.98
|
6.08
|
Table 2. Normalized photoluminescence (PL) spectrum of the CdSe/ZnS QD and the electroluminescence (EL) spectra of TE-QLEDs with Zn0.9Mg0.1O NPs of varying thickness
Sample
|
Peak (nm)
|
FWHM (nm)
|
PL of QD
|
524
|
33.8
|
EL of TE-QLED with
10 nm Zn0.9Mg0.1O NP
|
524
|
32.8
|
EL of TE-QLED with
30 nm Zn0.9Mg0.1O NP
|
532
|
35.5
|
EL of TE-QLED with
50 nm Zn0.9Mg0.1O NP
|
532
|
40.4
|
EL of TE-QLED with
70 nm Zn0.9Mg0.1O NP
|
532
|
41.9
|
IV. CONCLUSIONS
TE-QLEDs were fabricated inside an SU-8 bank formed on a PEDOT:PSS/Ag anode. XPS measurements
revealed that the concentrations of oxygen vacancies and hydroxyl species changed
as the thickness of the Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O NP ETLs varied. As
the thickness increased, the number of oxygen vacancies decreased; this reduced conductivity,
as confirmed by EOD. Notably, HOD current density remained within the range of that
of EODs with 30- and 50-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O layers. A
good QD EML charge balance is ensured by using such TE-QLEDs. When a 30-nm-thick Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL was employed, the hydroxide oxygen concentration was lowest, minimizing the
exciton quenching at the interface of the QD and Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL and thus improving the charge balance within the QD EML, significantly enhancing
QLED efficiency. The maximum TE-QLED current efficiency and EQE were 90.92 cd/A and
21.66%, respectively. Our results suggest that Zn$_{\mathrm{0.9}}$Mg$_{\mathrm{0.1}}$O
NP ETL layers of appropriate thickness will enhance the performance of the TE-QLEDs
required for next-generation display and lighting applications.
ACKNOWLEDGMENTS
This research was partially supported by the Soonchunhyang University Research
Fund and also supported by Korea Institute for Advancement of Technology (KIAT) grant
funded by the Korea government (MOTIE) (P0012453, The Competency Development Program
for Industry Specialist).
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Gyeongpil Jang received the B.S. and M.S. degrees from Soonchunhyang University,
Korea, in 2021 and 2023, respectively. He is now with Samsung Display. His research
interests include QLEDs, OLEDs, and synthesis of Nanomaterials.
Ji-Hun Yang received the B.S. and M.S. degrees from Soonchunhyang University,
Korea, in 2021 and 2023, respectively. His research interests include QLEDs, OLEDs,
and synthesis of Nanomaterials.
Su-Young Kim received the B.S. and M.S. degrees from Soonchunhyang University,
Asan, Korea, in 2022 and 2024, respectively. His current interest is synthesis of
nanomaterials and quantum dot light-emitting diodes.
Young-Bin Chae received the B.S. and M.S. degrees from Soonchunhyang University,
Asan, Korea, in 2022 and 2024, respectively. His current interest is synthesis of
nanomaterials and quantum dot light-emitting diodes.
Hyuk-Doo Choi received the B.S. and Ph.D. degrees in electrical and electronics
engineering from Yonsei University, Seoul, Republic of Korea, in 2009 and 2014, respectively.
From 2014 to 2017, he worked for LG Electronics as a senior research engineer. Since
2018, he has been an assistant professor with the Department of Electronic Materials,
Devices, and Equipment Engineering in Soonchunhyang university. His research interests
include deep learning, and unsupervised learning.
Dae-Gyu Moon received his Ph.D. degree from KAIST in 1994. He was employed at
LG Display from 1993 to 1998, and subsequently served as a postdoctoral fellow in
Oxford University from 1999 to 2000. Following his time at Oxford, he joined at KETI
and worked there from 2001 to 2005. Since then, he has held his position of a professor
at Soonchunhyang University. His research interests are in the field of display such
as QLEDs, OLEDs, and microLEDs.
Kyoung-Ho Lee received his Ph.D. degree from Virginia Polytechnic Institute and
State University, VA, USA in 1993. Presently, he serves as a Professor in the Department
of Electronic Materials, Devices, and Equipment Engineering, Soonchunhyang University,
Asan, Korea, where he is also holds the position of Dean in the Office of Academic
Affairs. His current interests include material synthesis through the sol-gel method,
ceramic glass, and MLCC.
Chang-Kyo Kim received his Ph.D. degree from Vanderbilt University, TN, USA in
1992. Presently, he holds the position of Professor in the Department of Electronic
Materials, Devices, and Equipment Engineering at Soonchunhyang University, Korea.
In addition to his role as Head of the department, he serves as the Director of Display
New Technology Institute at Soonchunhyang University. His current research interests
focus on OLEDs, QLEDs, and microfabrication.