Utilization of Nanoporous Nickel Oxide as the Hole Injection Layer for Quantum Dot Light-Emitting Diodes.

Nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its excellent hole mobility, high environmental stability, and low-cost fabrication. In this research, a NiOx thin film and nanoporous layers (NPLs) have been utilized as the HIL for the fabrication of quantum dot light-emitting diodes (QLEDs). The obtained NiOx NPLs have spongelike nanostructures that possess a larger surface area to enhance carrier injection and to lower the turn-on voltage as compared with the NiOx thin film. The energy levels of NiOx were slightly downshifted by incorporating the nanoporous structure. The amount of Ni2O3 species is higher than that of NiO in the NiOx NPL, confirming its good hole transport ability. The best QLED was achieved with a 30 nm thick NiOx NPL, exhibiting a maximum brightness of 68 646 cd m-2, a current efficiency of 7.60 cd A-1, and a low turn-on voltage of 3.4 V. More balanced carrier transport from the NiOx NPL and ZnO NPs/polyethylenimine ethoxylated (PEIE) is responsible for the improved device performance.

Introduction

Since the first quantum dot (QD)-based light-emitting diodes (QLEDs) reported by Alivisatos and co-workers,[1] many scientists have launched a great deal of research to pursue high brightness and efficiency of QLEDs. Colloidal cadmium selenide (CdSe) QDs have attracted enormous research interest because of their excellent properties, such as high photoluminescence (PL) quantum efficiency, narrow spectral bandwidth, high color purity, ease of color tuning without changing the composition, and low-cost solution processability.[2−8]

The existing QLED structure has the configuration of anode/hole injection layer (HIL)/hole transporting layer (HTL)/CdSe QDs/electron transporting layer (ETL)/electron injection layer (EIL)/cathode, which is similar to that of general organic light-emitting diodes. In common QLED devices, zinc oxide (ZnO) is the most promising ETL for QLEDs as it has high electron mobility and high transmittance in the visible range.[9,10] ZnO thin films are commonly prepared by the sol–gel method,[11,12] followed by high-temperature calcination over 400 °C to promote crystallinity and high carrier mobility.[13] It is generally considered that a high calcination temperature is unfavorable for low-cost manufacturing and is incompatible with flexible substrates. To avoid this problem, Meulenkamp proposed a low-temperature synthesis of ZnO nanoparticles (NPs) with a diameter ranging from 2 to 7 nm,[14] which were homogeneously dispersed in alcoholic solvents and required only 150 °C to remove the solvent to form smooth ZnO thin films after spin coating. For the HIL, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is extensively used in QLEDs owing to its high conductivity,[4] suitable energy level for hole injection,[6] and electron-blocking ability.[15] To enhance hole transport from the HIL to the CdSe emissive layer, an HTL is usually inserted. Three semiconducting polymers have been reported as the HTL in QLED fabrication, namely, poly(N-vinylcarbazole) (PVK), poly[N-(4-butylphenyl)-N′,N″-diphenylamine] (poly-TPD), and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB).[4,16,17]

Although QD materials and related QLEDs have been developed with high brightness and efficiency,[18,19] there are still two major problems to overcome. One is imbalanced electron–hole transport and the other is insufficient device lifetime. As mentioned above, ZnO is the most commonly used ETL in QLEDs. Compared with ZnO, the hole transport materials utilized in QLEDs, including PEDOT:PSS, poly-TPD, and TFB, have much lower hole mobility that restricts device performance.[10] For obtaining high brightness and efficiency of QLEDs, several approaches have been proposed to improve charge balance. Zinc magnesium oxide (ZnMgO) NPs have been prepared by doping Mg2+ in ZnO NPs to decelerate the electron mobility, which effectively solves the drawback of charge carrier imbalance.[10,20] The optimized QLED based on the ZnMgO ETL showed a 3.67-fold enhancement of current efficiency from 3.74 to 13.73 cd A–1 and a 3.67-fold increase in external quantum efficiency (EQE) from 2.58 to 9.46%. The second approach is to insert an electron-blocking layer (EBL) between the CdSe emissive layer and the ZnO ETL. The EBL can not only effectively prohibit excessive electron injection into the CdSe QDs but also prevent the reverse transfer of electrons from CdSe QDs to the ETL, thereby preserving superior emissive efficiency. Dai et al. demonstrated high-performance QLEDs by inserting an insulating poly(methyl methacrylate) layer between the QD layer and the ZnO ETL to optimize charge balance in the device with the highest EQE of 20.5% and an average EQE of 18.7% from 27 devices.[21] Jin et al. introduced a poly(p-phenylene benzobisoxazole) layer between the CdSe QDs and ZnO layer to block the excess electron injection, achieving a maximum EQE of 16.7% and an average EQE > 14%.[22] Another approach is to increase the hole transport ability of the HTL to achieve electron–hole charge balance in QLEDs. Our group blended a wetting and dispersing additive BYK-P105 into PEDOT:PSS to improve the hole mobility. The best QLED achieved a very high brightness of 139 909 cd m–2 and a current efficiency of 27.2 cd A–1, with a low turn-on voltage of 3.8 V.[23]

As mentioned in the previous paragraph, the device lifetime is not sufficient for commercialization because of the usage of PEDOT:PSS, which is often used as the hole transport layer in typical QLEDs. PEDOT:PSS is widely known for its acidic and hydrophilic nature that could accelerate the deterioration of device performance and reduce the device lifetime.[4,24] To solve this issue, a series of solution-processed p-type metal oxides with high carrier mobility and superior stability, such as nickel oxide (NiOx), vanadium oxide (VOx), tungsten oxide (WOx), and molybdenum oxide (MoOx), have been investigated as alternatives to PEDOT:PSS and have been introduced as the HIL for QLEDs.[25−28] Among the above metal oxide materials, NiOx is most favored because of its superior electron-blocking ability compared with the others.[25,29] Zhang et al. utilized NiO nanocrystals (NCs) as the HIL for fabricating QLEDs with the configuration of indium tin oxide (ITO)/NiO/TFB/CdSe QDs/ZnO NCs/Al.[24] The NiO NCs were prepared from their precursor nickel acetylacetone in tert-butanol in an autoclave at 200 °C for 24 h, with their valence band (VB) and conduction band (CB) at −6.17 and −2.45 eV, respectively. The best QLED showed a peak luminance of up to 25 580 cd m–2 and a current efficiency of 5.38 cd A–1. Sun et al. utilized a UV–ozone-enhanced NiOx HIL for QLEDs with the structure of ITO/NiOx/PVK/CdSe QDs/ZnMgO NPs/Al.[4] The low-temperature-annealed NiOx film was subjected to UV–ozone treatment for 20 min to enhance its conductivity. The optimized device showed a maximum current efficiency and a peak EQE of 45.8 cd A–1 and 10.9%, respectively. Moreover, a 3.2-fold enhancement of the device lifetime was attained by replacing the organic PEDOT:PSS with inorganic NiOx. Cao et al. demonstrated high-efficiency and stable QLEDs consisting of a solution-processed Cu-doped NiO HIL.[25] A maximum luminance of 61 030 cd m–2 and a current efficiency of 45.7 cd A–1 were obtained from the optimized device, with an almost 4-fold operation lifetime enhancement compared with the PEDOT:PSS-based QLED.

In this research, green QLEDs with the configuration of ITO/NiOx/CdSe QDs/ZnO NPs/polyethylenimine ethoxylated (PEIE)/LiF/Al were fabricated and evaluated. As described above, the utilization of PEDOT:PSS may lead to corrosion of the ITO electrode and reduced device stability. We selected NiOx as the HIL to avoid the above drawbacks. Moreover, different types of nanostructured NiOx layer, including nanoporous layers (NPLs) and a thin film, were prepared for comparison. From literature survey, we realized that NiOx NPLs have not been utilized as the HIL in QLEDs. Nanoporous materials can facilitate greater charge transfer due to the increased surface area. By combining two inorganic metal oxides NiOx and ZnO with high hole/electron mobility, balanced electron–hole transport and augmented device performance of QLEDs are easier to achieve. The best QLED based on a NiOx NPL as the HIL and PEIE as an interfacial layer achieved a high brightness of 68 646 cd m–2 and a current efficiency of 7.6 cd A–1, with a low turn-on voltage of 3.4 V. Our results open up new opportunities for the selection of carrier transport layers and fabrication of QLEDs for future commercialization.

Results and Discussion

The top-view and cross-sectional scanning election microscopy (SEM) images of NiOx NPLs from different concentrations (0.075, 0.0875, and 0.1 M) of NiSO4 aqueous solutions are shown in Figure . It is seen that these NiOx NPLs have spongelike nanostructures and staggered networks, indicative of a large surface area. Moreover, the NiOx NPL from the 0.075 M NiSO4 solution possesses the smallest pore size, while the one from the 0.1 M precursor solution has the largest pore size. In other words, the pore size of NiOx becomes larger gradually as the concentration of NiSO4 precursor solution increases. The thicknesses of different NiOx NPLs are determined to be 30, 70, and 100 nm, respectively, grown from 0.075, 0.0875, and 0.1 M NiSO4 solutions. The morphology and roughness of the NiOx thin film and NPLs with different thicknesses were investigated by atomic force microscopy (AFM). The corresponding topographic images and average roughness (Ra) of samples are displayed in Figure S1 in the Supporting Information. The NiOx thin film showed a quite smooth surface with a relatively low Ra value of 2.0 nm. On the other hand, the porous topologies of the NiOx NPLs from AFM experiments are in accordance with SEM observation. The Ra values of 30, 70, and 100 nm thick NiOx NPLs are measured to be 7.59, 9.38, and 17.9 nm, respectively. It is clearly seen that the surface roughness of the NiOx NPLs becomes higher with increasing NPL thickness.

Figure 1

Top-view and cross-sectional SEM images of NiOx NPLs with different thicknesses of (a, b) 30, (c, d) 70, and (e, f) 100 nm.

The X-ray diffraction (XRD) patterns of the resulting NiOx thin film and 30 nm thick NPL are shown in Figure . The XRD peaks are enlarged to clearly observe diffraction signals of NiOx. Both samples exhibit intense peaks at 2θ = 37.2, 43.3, and 62.9°, corresponding to the (111), (200), and (220) planes,[31] indicative of a cubic crystal structure.[32] These diffraction peaks are well matched with our previous report, which demonstrated the utilization NiOx HTL for fabricating inverted perovskite solar cells with high efficiency and stability. Besides, we notice that the peak intensity of the NiOx NPL is similar with that of the NiOx thin film. To date, the prepared NiOx NPL and thin film have shown similar crystalline structure and crystallinity.

Figure 2

XRD patterns of the NiOx thin film and 30 nm thick NPL on the ITO substrate.

The transmission spectra of the NiOx thin film and NPLs with different thicknesses of 30, 70, and 100 nm are displayed in Figure a. It is seen that all samples possess moderate transmittance higher than 65% from 350 to 490 nm, and they show even higher transmittance of up to 90% from 490 to 780 nm, which is beneficial for light-emitting application. The absorption spectra of the resulting NiOx thin film and NPLs are shown in Figure b. All samples preserve similar absorption behavior and show the absorption edge at ca. 350 nm, corresponding to an optical band gap of 3.51 eV. Besides, a small absorption band is located at 420 nm, which could be explained by crystalline defects that lead to the electronic transition from the oxygen vacancies to the VB of the metal oxide.[33] The different nanostructures of NiOx do not affect its optical properties.

Figure 3

(a) Transmission and (b) absorption spectra of the NiOx thin film and NPLs with different thicknesses of 30, 70, and 100 nm.

To investigate the energy levels of the NiOx thin film and 30 nm thick NiOx NPL, ultraviolet photoelectron spectroscopy (UPS) measurements were carried out, and the corresponding UPS spectra are depicted in Figure . The work function (φ) was evaluated by subtracting the secondary electron cutoff position from the incident He I photon energy (hν = 21.22 eV).[34] Hence, the Fermi levels (EF) of the NiOx thin film and 30 nm thick NiOx NPL are determined to be −4.83 and −4.69 eV, respectively. The valence band level (EVB) can be deduced from the equation EVB = EF – EFermi edge, where EFermi edge represents the Fermi edge level.[35] The conduction band level (ECB) can be calculated with the equation ECB = EVB + Eg.[36] The EVB values of the NiOx thin film and 30 nm thick NiOx NPL are −5.13 and −5.21 eV, respectively. In addition, the ECB values of the NiOx thin film and 30 nm thick NiOx NPL are −1.62 and −1.70 eV, respectively. The above results indicate that the change in nanostructure of NiOx does not significantly alter its energy levels.

Figure 4

UPS spectra of the NiOx thin film and 30 nm thick NPL in the (a) secondary electron cutoff and (b) Fermi edge region.

The hole transport ability of nonstoichiometric NiOx arises from interstitial oxygen and Ni vacancies in the crystalline structure. X-ray photoelectron spectroscopy (XPS) measurements were carried out for identification of the Ni3+/Ni2+ ratio. Figure a,b reveals the Ni 2p3/2 band of the NiOx thin film and NiOx NPL, respectively. The multicomponent band can be deconvoluted into four different states at 853.9 (Ni2+), 855.8 (Ni3+), 861.1 (Ni2+ satellite), and 864.2 eV (Ni3+ satellite), which are consistent with a previous report.[31] The Ni3+/Ni2+ ratios are calculated to be 1.14 and 1.27 for the NiOx thin film and NiOx NPL, respectively. It can be seen that the amount of Ni2O3 species is higher than stoichiometric NiO in the NiOx NPL, confirming that the NiOx NPL possesses higher hole mobility. The increased Ni3+ concentration indicates enhanced hole transport ability. Figure c,d shows the O 1s spectra of the NiOx thin film and NiOx NPL and are fitted with two states at around 529.2 (O2– from NiO) and 531.1 eV (O2– from Ni2O3).[30]

Figure 5

XPS spectra of Ni 2p3/2 signals of (a) the thin film and (b) 30 nm thick NPL and O 1s signals of (c) the thin film and (d) 30 nm thick NPL.

The graphical illustration of the QLED based on the NiOx NPL is shown in Figure a, which is constructed with the configuration of ITO/NiOx NPL/PVK/CdSe QDs/ZnO NPs/PEIE/LiF/Al. A QLED based on a 40 nm thick NiOx thin film was also fabricated and evaluated for comparison. The energy-level diagram of the whole device is illustrated in Figure b. In our devices, we chose NiOx as the hole injection material since its VB is close to the work function of ITO. The VB and CB of the NiOx thin film and NiOx NPL were deduced from the above-mentioned UPS experiments. Next, PVK was adopted as the HTL between NiOx and CdSe QDs for stepwise hole transportation from NiOx to the emissive layer. The highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of PVK were reported to be −5.8 and −2.2 eV, respectively.[21,37] In addition to the hole transport ability, PVK also shows good electron-blocking ability because of the high-lying LUMO level. As for the ETL, the CB of PEIE-modified ZnO is −2.9 eV,[38] while LiF/Al is evaporated as the electrode with a work function of −2.8 eV.[39] Hence, a small energy barrier between ZnO/PEIE and LiF/Al electrode was achieved for electron injection and transportation. The cross-sectional SEM image of the whole device is shown in Figure c. In this device, the 30 nm thick NiOx NPL was used as the HIL. The thicknesses of the NiOx NPL/PVK, CdSe QDs, ZnO/PEIE, and LiF/Al are estimated to be 40, 35, 25, and 100 nm, respectively. The top-view SEM image of NiOx/PVK is provided as Figure S1 in the Supporting Information. The porous structure of NiOx can be clearly seen since PVK penetrated into pores.

Figure 6

Illustrations of (a) the device architecture, (b) energy-level diagram, and (c) cross-sectional SEM image of the QLED.

The brightness–voltage and current efficiency–current density characteristics of all QLEDs are shown in Figure a,b, respectively. All device performances from the four QLEDs based on the NiOx thin film and NiOx NPLs with different thicknesses of 30, 70, and 100 nm are summarized in Table . The maximum brightness (Lmax) and current efficiency (CEmax) of the QLED based on the NiOx thin film were measured to be 22 614 cd m–2 and 13.1 cd A–1, respectively, with a high turn-on voltage (Von) of 8.0 V. Using the 30 nm thick NiOx NPL as the HIL, the Lmax was significantly augmented to 68 646 cd m–2 and Von was reduced to 3.4 V, with a CEmax of 7.6 cd A–1. The Lmax and CEmax of the QLEDs based on 100 and 70 nm thick NPLs were substantially reduced to 615–10,928 cd m–2 and 0.07–1.32 cd A–1, respectively. It is seen that the device performance became lower when the thickness of the NiOx NPL was increased. The reason for this phenomenon is explained as follows. The thicknesses of the upper layers PVK and CdSe QDs are relatively thinner than that of the NiOx NPL. With the thinnest NiOx NPL of ∼30 nm utilized as the HIL, a flat and sandwiched device structure was built; the QLED with the lowest Von was observed, as revealed in Figure a. When the thickness of the NiOx NPL became thicker up to 70 nm or even 100 nm, the deposited PVK and CdSe QDs could not cover the whole surface of the NiOx NPL and flakes of NiOx penetrated through the PVK and CdSe layers to the top surface. Therefore, the device efficiency based on the 70 or 100 nm thick NiOx NPL was reduced. Besides, we observed that QLEDs using NiOx NPLs as the HIL have a smaller Von than the one based on the NiOx thin film. This is understandable since nanoporous structures possess more surface area to enhance carrier injection and generation of light.[40,41]Figure c reveals the snapshot of the QLED based on the 30 nm thick NiOx NPL driven at 10 V and its electroluminescence spectrum centered at 532 nm, exhibiting a bright green emission. To the best of our knowledge, NiOx NPLs were utilized for the first time as the HIL to construct QLEDs.

Figure 7

(a) Brightness–voltage and (b) current efficiency–current density characteristics of all QLEDs based on the NiOx thin film and NiOx NPLs with different thicknesses of 30, 70, and 100 nm; (c) EL spectrum and snapshot of the QLED based on the 30 nm thick NiOx NPL at 10 V; and (d) current–voltage characteristics of the hole-only and electron-only devices.

Table 1

Device Performance of All QLEDs Using NiOx as the HTL

NiOx type	Von (V)a	Lmax (cd m–2@V)	Jmax (mA cm–2@V)	CEmax (cd A–1@V)
thin film	8.0	22 614@19.0	772@20.0	13.1@13.6
30 nm NPL	3.4	68 646@18.5	1490@13.2	7.6@11.3
70 nm NPL	4.1	10 928@13.4	1230@13.4	1.32@10.1
100 nm NPL	4.0	615@8.6	941@8.6	0.07@8.2

Defined as the operating voltage when the brightness reached 1 cd m–2.

To compare carrier transport ability between the NiOx thin film and NPL, two hole-only devices made of the ITO/NiOx film or 30 nm thick NPL/Al and one electron-only device of the structure ITO/ZnO NPs/PEIE/LiF/Al were fabricated to measure their current–voltage characteristics, as shown in Figure d. The device based on the NiOx NPL showed a higher current value, indicative of better hole transport ability than the NiOx thin film. The hole mobility of the 30 nm thick NiOx NPL was calculated from the space-charge limited current model J = (9/8)εrε0μeV2/d3, assuming εr = 11.9,[42] ε0 = 8.854 × 10–12 F m–1, and thickness d = 40 nm for the NiOx thin film and 30 nm for the NiOx NPL. Hence, the hole mobility of the NiOx thin film was estimated to be 2.70 × 10–5 cm2 V–1 s–1, which is close to the value in a previous report.[43] The hole mobility of the NiOx NPL was also calculated to be 3.25 × 10–5 cm2 V–1 s–1, revealing a higher value than that of the NiOx thin film. Moreover, the current of the NiOx NPL device was closer to that of the ZnO NPs/PEIE device, indicating more balanced carrier transport that is responsible for the improved device performance.

The device based on the NiOx thin film has more balanced carrier recombination under a low current density below 180 mA cm–2, and it exhibits a peak efficiency of 13.1 cd A–1 at 18 mA cm–2. On the contrary, the NiOx NPL device shows a lower current efficiency under a low current density, possibly due to the exposed NiOx nanostructure after PVK deposition, as shown in Figure S2 in the Supporting Information. This would lead to direct contact of NiOx and the CdSe active layer and a leakage current that decreases current efficiency. On the other hand, the leakage current for the device based on the NiOx thin film was small at a low driving voltage due to its smooth surface. The leakage current became more significant with increasing driving voltage, which resulted in a sharp decrease in current efficiency. Besides, the device based on the 30 nm thick NiOx NPL always showed a higher luminance than the one based on the NiOx thin film. Therefore, the device based on the NiOx NPL exhibited a higher performance when the current density was increased over 180 mA cm–2 or higher.

Conclusions

Nanoporous NiOx was successfully prepared using a coprecipitation method to serve as the HIL in QLEDs. The experimental results showed that the NiOx NPL and thin film have similar crystalline structures and high transmittance of up to 90%. The XPS results indicated that the NiOx NPLs have more Ni2O3 species than NiO, revealing a better hole transport property. The UPS results proved that the nanostructural change of NiOx slightly alters its energy levels. The best device exhibited a maximum brightness of 68 646 cd m–2, a current efficiency of 7.6 cd A–1, and a turn-on voltage of 3.4 V. More balanced carrier transport from NiOx NPLs and ZnO NPs/PEIE was achieved in our QLEDs.

Experimental Section

Materials

ITO glass substrates (7 Ω sq–1) were purchased from Merck. Nickel(II) sulfate heptahydrate (purity 98%) was purchased from Acros. PEIE (37 wt % in water) and PVK were purchased from Alfa Aesar and TCI, respectively. CdSe QDs were provided by Opulence Optronics Co., Ltd. from Taiwan. Colloidal ZnO NPs in n-butanol were synthesized according to our previous report.[23] Other reagents and solvents were bought from Alfa Aesar, Acros, or ECHO Chemical Co., Ltd. and used without further purification.

Preparation of the NiOx Thin Film

To prepare NiOx precursor solution, 0.141 g of nickel(II) sulfate heptahydrate (NiSO4·7H2O), 30 μL of ethanolamine, and 5 mL of isopropanol (IPA) were mixed and heated at 80 °C with stirring in a sealed glass vial overnight. The color of the precursor solution became translucent green after dissolution. The precursor solution was then spin-coated at 2000 rpm for 30 s into a thin film on the ITO substrate, followed by heating at 80 °C for 20 min. The dried film was calcinated at 300 °C for 1 h to obtain the final NiOx thin film.

Preparation of NiOx NPLs

The preparation of NiOx NPLs was carried out using a modified coprecipitation method.[30] First, three different weights (1.05, 1.123, and 1.4 g) of NiSO4·7H2O were dissolved in 50 mL of deionized (DI) water to form three NiSO4 aqueous solutions with different concentrations (0.075, 0.0875, and 0.1 M). Meanwhile, three different weights (0.253, 0.296, and 0.338 g) of potassium persulfate (K2S2O8) were dissolved in 50 mL of DI water to give three K2S2O8 aqueous solutions with different concentrations (0.019, 0.022, and 0.025 M). Second, 16 mL of 0.075 M (or 0.085 or 0.1 M) NiSO4 solution, 12 mL of 0.019 M (or 0.022 or 0.025 M) K2S2O8 solution, and 4 mL of aqueous ammonia (25–28%) were mixed and shaken for 5 min to give three solution mixtures with different concentrations. Third, the above solution mixtures were poured into individual Petri dishes and precleaned ITO substrates were immersed horizontally for 4 min with the ITO side upward. The deposited films were rinsed with DI water to remove loosely bound particles and further dried at 80 °C for 20 min in ambient air, followed by calcination at 300 °C for 1 h to obtain NiOx NPLs.

Device Fabrication

Regular QLEDs with the configuration of ITO/NiOx/PVK/CdSe QDs/ZnO NPs/PEIE/ LiF/Al were fabricated. The ITO substrates were cleaned sequentially with a detergent, DI water, acetone, and IPA under ultrasonication for 30 min each, followed by nitrogen purge and ultraviolet–ozone exposure for 20 min. The NiOx thin film or NPLs were deposited on the cleaned ITO according to Sections and 4.3. The substrates were then transferred into a nitrogen-filled glovebox. PVK (in chlorobenzene, 8 mg mL–1) was spin-coated on top of the NiOx film or NPLs at 3000 rpm for 30 s, followed by drying at 150 °C for 20 min. CdSe QDs (in n-octane, 12 mg mL–1) were spin-coated into a thin film on the PVK layer at 2000 rpm for 30 s and heated at 150 °C for 30 min. ZnO NPs were deposited on top of the CdSe layer by spin coating at 2000 rpm for 30 s, followed by baking at 150 °C for 20 min. A thin layer of PEIE was deposited from its 0.4 wt % solution in 2-ethoxyethanol on ZnO NPs by spin coating at 5000 rpm for 30 s and heated at 110 °C for 20 min. Finally, 0.5 nm of LiF and 100 nm of aluminum electrodes were deposited by thermal evaporation under a base pressure of ∼10–6 Torr. The active area of each device was 1 mm2 for performance evaluation.

Characterization Methods

The top-view and cross-sectional micrographs of the NiOx surface and fabricated devices were investigated with an ultra-high-resolution ZEISS Crossbeam scanning electron microscope. The surface morphology and roughness of the NiOx thin film and NPLs were studied using a Bruker Innova atomic force microscope. The absorption and PL spectra of CdSe QDs were recorded with a Princeton Instruments Acton 2150 spectrophotometer equipped with a Xe lamp as the light source. The UPS measurements for the NiOx films and NPLs were performed on a Thermo VG-Scientific/Sigma Probe spectrometer. A He I (hν = 21.22 eV) discharge lamp was used as the excitation source. The XPS measurements were conducted on a Thermo K-Alpha X-ray photoelectron spectrometer for elemental composition analysis of NiOx. The XRD patterns and crystallinity of NiOx were measured using a Rigaku D/MAX2500 X-ray diffractometer. The current density–voltage characteristics of hole- and electron-only devices were measured using an Agilent 4155C semiconductor parameter analyzer. The performance and electroluminescence spectra of QLEDs were recorded using an Agilent 4155C semiconductor parameter analyzer and an Ocean Optics USB2000+ spectrometer.