Chloride-Passivated Mg Doped ZnO Nanoparticles For Improving

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Chloride-Passivated Mg Doped ZnO Nanoparticles For Improving Performance of Cadimum-Free Quantum-Dot Light-Emitting Diodes Fei Chen, Zhenyang Liu, Zhongyuan Guan, Zheming Liu, Xu Li, Zhenbo Deng, Feng Teng, and Aiwei Tang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00722 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Chloride-Passivated Mg Doped ZnO Nanoparticles For Improving Performance of Cadmium-Free Quantum-Dot Light-Emitting Diodes Fei Chen,† Zhenyang Liu,† Zhongyuan Guan, † Zheming Liu, † Xu Li,‡ Zhenbo Deng, *,† Feng Teng, †, ‡ Aiwei Tang *,† †

School of Science, Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing JiaoTong University, Beijing 100044, China ‡

Hebei Key Laboratory of Optic-electronic Information and Materials College of

Physics Science and Technology，Hebei University，Baoding 071002, Heibei, China

Corresponding authors. [email protected]; [email protected]

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ABSTRACT Colloidal ZnO nanoparticles (NPs) are widely used as an electron-transporting layer (ETL) in the solution-processed quantum-dot light-emitting diodes (QD-LEDs). However, the inherent drawbacks including surface defect sites and unbalanced charge injection prevent the device from realizing their further performance enhancement. In this work, a series of Mg doped ZnO (ZnO:Mg) and chloride-passivated

ZnO

(Cl@ZnO)

NPs

were

synthesized

by

using

a

solution-precipitation strategy, and they exhibited tunable optical bandgaps and upward-shift of conduction-band maximum (CBM). Solution-processed QD-LEDs based on cadmium-free Cu-In-Zn-S/ZnS (CIZS/ZnS) nanocrystals (NCs) were fabricated by using ZnO:Mg and Cl@ZnO NPs as the ETLs, whose maximum peak external quantum efficiency (EQE) was nearly twice as high as that of QD-LEDs using ZnO NPs as the ETL (EQE = 1.54%). To take advantage of the benefits of ZnO:Mg and Cl@ZnO NPs, Cl@ZnO:Mg NPs were developed through the integration of Mg doping and Cl-passivation. Surprisingly, the cadmium-free QD-LEDs with the Cl@ZnO:Mg NPs as the ETL exhibited a maximum peak EQE of 3.72% and current efficiency of 11.1 cd A-1, which could be enhanced to be 4.05% and 12.17 cd A-1 by optimizing the Cl amount, respectively. The positive effects of the Mg doping and Cl-passivation on the cadmium-free QD-LEDs are primarily ascribed to the reduced electron injection barrier of ETL/the emitting layer interface and slower electron mobility, which can be verified by the spectroscopy

(UPS)

measurements

and

ultraviolet photoelectron

current-voltage

characteristics

electron-only devices. KEYWORDS: QD-LEDs; Cadmium-free; Interfacial engineering, Cl@ZnO:Mg

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INTRODUCTION Since the pioneering work on the QD-LEDs with organic/quantum dots (QDs) composites as an emitting layer was reported by Alivisatos et al. more than two decades ago, this new type of display and lighting technology have attracted considerable attention due to their size-tunable emission color and narrow emission linewidth.1-5 In particular, great progress has been made in the device performance improvement of QD-LEDs by optimizing the nanostructures and device architectures in the past few years.6-13 In these QD-LEDs with superior performance, however, cadmium-based QDs are one of the most commonly used emitters. To meet increasingly stringent environmental demands, governments and organizations have introduced some regulations to avoid the release of cadmium elements. As a result, different types of cadmium-free QD-LEDs have become actively pursued, in which the QD-LEDs based on multinary copper-based semiconductor NCs have aroused great research interests due to their low toxicity, earth abundance and high luminescence efficiency.14-22 To date, much effort has been made to improve the device performance of the QD-LEDs based on copper-based semiconductor NCs in terms of surface engineering of QDs and device structure optimization. For example, Zhong’s group recently developed an effective in-situ ligand exchange strategy for preparing hydroxyl-terminated CuInS2-based NCs capped with 6-mercaptohexanol, which were used as an emitting layer to fabricate an inverted QD-LED. Due to an improved electron mobility and injection efficiency, the QD-LEDs exhibited a maximum luminance of 8735 cd m-2 and a peak EQE of 3.22%.20 Additionally, in our previous work, a solution-processed approach was adopted to fabricate red, yellow and green-colored QD-LEDs using Cu-In-Zn-S/ZnS NCs as emitters, in which colloidal ZnO NPs were incorporated into the devices as the ETL. By optimizing the ZnO thickness and device aging time, the peak EQE of yellow QD-LEDs could reach up to 2.4%.21 As a matter of fact, such a QD-LED hybrid architecture has recently gained attention due to their outstanding performance, which often comprises of an 3


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organic semiconductor as the hole-transporting layer (HTL) and colloidal metal oxide NPs as the ETLs.23 Unlike the QD-LEDs using small organic molecules as the ETLs, the working mechanism of the QD-LEDs using metal oxide NPs as the ETL is more likely dominated by charge injection than by energy transfer.23 Among different types of metal oxide NPs, ZnO NPs are regarded as one of the most popular ETLs for fabrication of highly efficient QD-LEDs due to their high electron mobility and hole-blocking properties.5, 24 However, the unbalance of charge injection induced by large energy barrier between ZnO and different functional layers often brings about poor device performance and stability. Moreover, ZnO NPs often possess numerous surface defects induced by chemical bonding between surface atoms and coordinating ligands due to large surface-to-volume ratios, which may quench the luminescence of QDs.10,

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Hence, much attention has been paid to

exploring different strategies to overcome these intrinsic shortcomings.25-34 Among different approaches to resolve the charge unbalance and reduce the surface defects of ZnO NPs, substitutional doping and incorporation of organic polymers are often used to tailor electronic structure and to passivate the surface of ZnO NPs, respectively.30-34 For example, Choi et al. fabricated an inverted QD-LED based on CdSe/ZnS QDs by using a polyethylenimine ethoxylated (PEIE) modified ZnO NPs as the ETL, and the combination of PEIE and ZnO NPs can facilitate the electron injection and charge balance. Thus, the QD-LED exhibited a low turn-on voltage of 2-2.5 V and a maximum luminance of 8600 cd m-2 as well as a maximum current efficiency of 1.53 cd A-1.30 In addition, doping is an effective and low-cost method to tailor the electronic structure of ZnO NPs, which enables the fabrication of highly bright QD-LEDs. For instance, Yang’s group reported a solution-processed Cu-In-S-based QD-LED using alloyed ZnMgO NPs with different electronic energy levels as the ETLs, and the device performance of the QD-LEDs exhibited a strong dependent on the type of ZnMgO ETLs. As a result, the maximum current efficiency of 5.75 cd A-1 and EQE of 2.19% could be realized in the Cu-In-S/ZnS-based QD-LEDs using Zn0.95Mg0.05O as the ETL.34 4


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Despite impressive progress in the QD-LEDs using different types of ZnO NPs as the ETL, there is no end to improve the device performance of cadmium-free QD-LEDs through the optimization of nanostructures and surface of ZnO ETL. Very recently, Sargent et al. developed chloride-passivated ZnO (Cl@ZnO) NPs from a solution of presynthesized ZnO NPs after Cl-treating, which exhibited decreased surface defect densities and a favorable electronic band alignment. Therefore, the charge extraction from the active layer could be improved, which brought about the best power conversion efficiency of 11.6% in the colloidal QDs photovoltaic cells.35 Inspired by this study, we developed colloidal Cl@ZnO:Mg NPs using a solution-precipitation approach at relatively low temperature (20-50 oC), which combined the tailoring electronic structure of ZnO:Mg NPs and reduced surface defect sites of Cl@ZnO NPs. The solution-processed Cl@ZnO:Mg NP solid films exhibited a larger optical bandgap, a decreased luminescent intensity and an upshift of CBM compared with the ZnO:Mg, Cl@ZnO and ZnO films. All-solution-processed QD-LEDs were fabricated with a device architecture consisting of the cadmium-free CIZS/ZnS NC layer sandwiched between an organic HTL and an ETL. The device with Cl@ZnO:Mg NPs exhibited a peak EQE of 3.72% and a peak current efficiency of 11.1 cd A-1, which had a substantial improvement compared with the devices using Cl@ZnO, ZnO:Mg and ZnO NPs as the ETLs. By optimizing the Cl amount, the peak EQE of the device could be enhanced to 4.05%, which was 2.63-fold as high as that of the device using pure ZnO NPs as the ETL. Such a EQE value is comparable to that of the best QD-LEDs based on CIZS/ZnS NCs reported previously. The significant improvement of the device performance is mainly derived from a more favorable energy level alignment and charge injection balance. The proposed interfacial engineering approach provides a promising method to boost the device performance of QD-LEDs.

EXPERIMENTAL SECTION Materials. Zinc chloride (ZnCl2, 99.9%), copper(I) chloride (CuCl, 99.9%), zinc(II) acetate (Zn(OAc)2, 99.9%), 1-dodecanethiol (DDT, 98%), oleic acid (OA, AR), 5


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oleylamine (OM, 80%), zinc acetylacetonate (Zn(acac)2, 97%), indium(III) chloride (InCl3, 99.9%), magnesium acetate tetrahydrate (Mg(OAc)2.4H2O, 99.98%), zinc(II) acetate dihydrate (Zn(OAc)2.2H2O, 99.99%), and sodium chloride (NaCl, 99.99%) were purchased from Shanghai Aladdin Reagent Company. 1-Octadecene (ODE, 90%) was purchased from Alfa Aesar. Ethyl alcohol (HPLC) and dimethyl sulfoxide (DMSO, 99.9%) were purchased from Acros. Tetramethylammonium hydroxide pentahydrate (TMAH, 98%) was purchased from Sigma-Aldrich. Sulfur powder (AR), ethanol, toluene and acetic ether were purchased from Beijing Chemical Reagent, China. All of the chemicals were used without further purification. Synthesis of CIZS/ZnS NCs. The synthesis of CIZS/ZnS NCs was according to our previous work.36 Typically, CuCl (0.075 mmol), InCl3 (0.5 mmol), Zn(OAc)2 (1 mmol) and S powder (3 mmol) were loaded into a 25 mL four-necked flask, and then ODE (10 mL), DDT (3 mL), OA (1 mL) and OM (1 mL) were added in sequence. Subsequently, the mixture was heated up to 100 ºC under nitrogen flow till the solution became clear, following by elevating the reaction temperature to 230 ºC slowly. The reaction was kept at this temperature for 10 min, and then the Zn stock solution (a mixture consisting of 1 mmol of Zn(OAc)2, 1.5 mL of ODE, 0.4 mL of OM, 0.2 mL of OA and 0.1 mL of DDT was heated to 170 ºC under nitrogen and kept for 30 min.) was injected into the mixture at 230 oC and then the reaction system was heated to 250 ºC. After reaction for 30 min, the reaction was cooled down to room temperature. Finally, the CIZS/ZnS NCs were obtained by washing and precipitated using toluene and ethanol for three times, and then the sample was dispersed in toluene at a concentration of 10 mg mL-1. Synthesis of Colloidal Cl@ZnO:Mg NPs. The colloidal Cl@ZnO:Mg NPs were synthesized by using a solution-precipitation process according to the synthetic recipe of colloidal ZnO NPs reported by Qian et al.7 Typically, 10 mL of Mg(OAc)2, Zn(OAc)2 and NaCl solution in DMSO (0.5 mol L-1) and 4 mL of TMAH solution in ethanol (0.55 mol L-1) were mixed and stirred for 1 h under the reaction temperature of 40 ºC in ambient air, in which the mass fraction of Mg(OAc)2 was 5%. Subsequently, the as-obtained products were washed and precipitated by adding 6


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ethanol and acetic ether for twice. Finally, the resulting Cl@ZnO:Mg NPs were dispersed in ethanol at a concentration of 30 mg mL-1. As a comparison, ZnO:Mg, Cl@ZnO and ZnO NPs were also synthesized by using the aforementioned procedures except in the absence of NaCl or Mg sources or the both precursors. Fabrication of solution-processed cadmium-free QD-LEDs. All the devices were fabricated on glass substrates pre-patterned with ITO film and the sheet resistance is ～20 Ω sq-1. All the ITO-coated glass-substrates were sonicated and cleaned sequentially by detergent, deionized water, acetone and isopropanol, and each procedure was kept for 30 min. Subsequently, the as-cleaned substrates were treated by ultraviolet ozone in air for 25 min. Afterwards, the poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate (PEDOT:PSS) (AI 4083) was spin-coated from aqueous solution onto the ITO-coated substrates which were then transferred to a N2-filled glove box right after an annealing process at 140 ºC for 15 min in air. Next, poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB) (ADS 259, 8 mg mL-1 in chlorobenzene) was spin-coated onto the PEDOT:PSS layer and annealed at 150 °C for 30 min. In turn, CIZS/ZnS NCs (10 mg mL-1 in toluene) and different types of ETLs (30 mg mL-1 in ethanol) were spin-coated onto the TFB layer, respectively, following by baking at 60 °C for 30 min. Finally, the Al cathode layer was deposited via a thermal evaporation process and the effective area is 4 mm2. Characterization. Absorption spectra were measured on an Ocean Optics USB 2000 spectrophotometer and the photoluminescence (PL) spectra were performed on a FLUORAT-02-PANORAMA spectrophotometer. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffraction meter using a Cu Ka radiation source (λ=1.54056 Å). X-ray photoelectron spectroscopy (XPS) was performed using a VG ESCALAB 220i-XL spectrometer with a 300 W Al Kα radiation source and all binding energies for different elements were calibrated with respect to the C1s line at 284.8 eV from the contaminant carbon. Transmission electron microscopy (TEM) images were taken using a JEM-1400 transmission electron microscope with an acceleration voltage of 100 kV. A Hitachi S-4800 field emission scanning electron microscope (SEM) was used to characterize the cross-sectional morphology of the 7


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device. UV photoelectron spectroscopy (UPS) was carried out on a Kratos AXIS Ultra DLD spectrometer by utilizing a He I photon source (21.22 eV). The time-resolved PL spectra were recorded on an Edinburgh F900 steady/transient state fluorescence spectrometer. The current density-luminance-voltage (J-L-V) characteristics of QD-LEDs were recorded on a Keithley 2400 source meter and Photo Research-735, and the electroluminescence (EL) spectra were taken using an OceanOptics USB 2000 spectrometer with the devices driven at a constant current using a Keithley 2400 source meter. All the measurements were performed at room temperature.

RESULTS AND DISCUSSION Different types of ETLs including ZnO, ZnO:Mg, Cl@ZnO and Cl@ZnO:Mg NPs were synthesized and their XRD patterns are depicted in Figure 1a. All the samples possess an archetypal wurtzite-type ZnO structure, and no other diffraction peaks from MgO or NaCl are observed, which indicates that the appropriate Mg doping content and Cl-passivation could not bring about any lattice strain and thus their crystal structures are kept unchanged. The representative TEM images of ZnO and Cl@ZnO:Mg NPs shown in Figure 1b and c indicate that the ZnO and Cl@ZnO:Mg NPs have a similar morphology and size distribution, and their average size is estimated to be less than 5 nm. The optical absorption spectra of the ZnO, ZnO:Mg, Cl@ZnO and Cl@ZnO:Mg NP films are presented in Figure 1d, and the optical bandgaps can be achieved by Tauc plots between (αhν)2 and photon energy (hν), which are given in Figure 1e. The as-obtained ZnO and Cl@ZnO films have identical optical bandgap with 3.52 and 3.53 eV, which are consistent with the value of Cl@ZnO reported previously.35 As compared to ZnO NPs, an obvious blue-shift in wavelength is observed in the absorption spectra of ZnO:Mg and Cl@ZnO:Mg NPs (Figure 1d). Accordingly, the optical bandgaps are estimated to be 3.58 and 3.62 eV for ZnO:Mg and Cl@ZnO:Mg NPs (Figure 1e), which indicates the incorporation of Mg2+ ions into ZnO changes the electronic structure, which is consistent with the previous results from ZnMgO and ZnO:Ga NPs.32, 34 The larger optical bandgap of doped ZnO NPs can be attributed to the so-called Burstein-Moss effect.32 The PL 8


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spectra of the four samples shown in Figure S1 of the Supporting Information (SI) indicate that the different types of ZnO NPs exhibit a broad luminescence in the visible range from 500 to 550 nm due to the presence of structural or surface defects in the samples.25 As compared to ZnO and ZnO:Mg NPs, a decrease of PL intensity is observed in Cl@ZnO and Cl@ZnO:Mg NPs shown in Figure S1a and b, respectively. This suggests that the surface defect traps are restrained due to Cl-passivation to some extent.35 To further consolidate the deduction, the time-resolved PL measurement was performed, and the lifetime of the Cl@ZnO and Cl@ZnO:Mg NPs is reduced as compared to ZnO and ZnO:Mg NPs (Figure.S1c and d), respectively. The PL decay results further confirm that the surface defect traps are suppressed.

Figure 1. (a) XRD patterns of ZnO, Cl@ZnO, ZnO:Mg and Cl@ZnO:Mg NPs, and the bottom lines represent the standard diffraction lines of wurtzite-type ZnO structure (JCPDS card No. 36-1451); Typical TEM images of (b) ZnO NPs and (c) Cl@ZnO:Mg NPs; (d) The corresponding absorption spectra of the solid films; (e) the Tauc plots between (αhν)2 and photon energy.

The XPS technique was used to analyze the chemical composition and the binding affinity of Cl to ZnO NPs. Figure 2a-c depicts the high-resolution Zn 2p, Mg 1s and 9


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Cl 2p signals of ZnO:Mg, Cl@ZnO and Cl@ZnO:Mg NPs. The survey spectra of Cl@ZnO:Mg NPs shown in Figure S2 of SI confirm the presence of Zn, O, Mg and Cl in the respective products. As shown in Figure 2a, two peaks of Zn 2p are located at around 1044.2 and 1021.1 eV for ZnO:Mg, which correspond to Zn 2p1/2 and Zn 2p3/2, respectively. For Cl@ZnO:Mg NPs, a slight peak shift from 1021.1 to 1021.6 eV of Zn 2p3/2 signal as compared to ZnO:Mg NPs, which indicates that the Cl is bound directly to ZnO due to the different binding energies of Zn-O and Zn-Cl bonds.35 Figure 2b shows the XPS signal of Mg 1s for the three products, and no any peak is observed in Cl@ZnO NPs, suggesting the absence of Mg elements in the Cl@ZnO NPs. By comparison the Mg 1s signal of ZnO:Mg and Cl@ZnO:Mg NPs, a slight peak shift toward higher binding energy is observed, which indicates the formation of Mg-Cl bond, because the binding energy of Mg-Cl bond is larger than Mg-O bond. The Cl 2p spectra of the three samples shown in Figure 2c demonstrate the incorporation of Cl into Cl@ZnO and Cl@ZnO:Mg NPs. The Cl 2p signal of Cl@ZnO NPs can be fitted into a single peak at 198.7 eV, which can be attributed to Zn-Cl bond. In contrast, the Cl 2p signal of Cl@ZnO:Mg NPs is fitted into double peaks at around 198.5 eV and 200 eV, and the former one arises from Zn-Cl bond and the latter one is attributed to the Mg-Cl bond. Based on the aforementioned XPS analysis, it can be concluded that the doping of Mg ions and the Cl-passivation are realized in Cl@ZnO:Mg NPs.

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Figure 2. High-resolution XPS signals of (a) Zn 2p, (b) Mg 1s, (c) Cl 2p for ZnO:Mg, Cl@ZnO and Cl@ZnO:Mg NPs. Table 1 Summary of energy information of ZnO, ZnO:Mg and Cl@ZnO:Mg films obtained from UPS and absorption results

Figure 3. UPS spectra of (a) the high-binding energy secondary electron cut-off regions and (b) the valence-band edge regions of ZnO, ZnO:Mg, and Cl@ZnO:Mg NP films deposited on ITO; (c) Band alignments of ZnO, ZnO:Mg, and Cl@ZnO:Mg NPs.

The electronic structure of different types of ZnO NP films is an important parameter to evaluate their potentials as the ETLs in the solution-processed QD-LEDs, and thus UPS measurement was utilized to study the electronic structures of ZnO, ZnO:Mg and Cl@ZnO:Mg NP films. Figure 3a and b presents the UPS results of secondary-electron cutoff and valence-band regions of the three samples. The valence-band maximum (VBM) level can be estimated from the incident photon energy (21.22 eV), the high-binding energy cut-off (Ecut-off) (Figure 3a), and the onset energy in valence-band region (Eonset) (Figure 3b) based on the following equation of VBM=21.22-(Ecut-off-Eonset).32 Thus, the VBM positions of ZnO, ZnO:Mg and Cl@ZnO:Mg NP films are calculated to be 7.03, 6.95 and 6.94 eV below the vacuum level, respectively. In combination with the optical bandgaps determined by the 11


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UV-Vis spectra shown in Figure 1d, the CBM levels are estimated to be 3.51, 3.37, and 3.32 eV below the vacuum level for ZnO, ZnO:Mg and Cl@ZnO:Mg NP films, respectively. The band alignments of three samples are given in Figure 3c. The detailed UPS parameters including the Ecut-off, Eonset, VBM, bandgap and CBM are summarized in Table 1. It can be seen in Figure 3c and Table 1 that the bandgap becomes widened and the conduction-band edge up-shifts towards vacuum energy level with the doping of Mg ions into ZnO NPs and Cl-passivation on the surface of ZnO NPs. The upward shift of CBM level in Cl@ZnO:Mg is beneficial for the electron injection from ETL to the emitting layer in QD-LEDs. To demonstrate the performance of different types of ZnO NPs as the ETLs in QD-LEDs, all-solution-processed QD-LEDs are fabricated with a typical multilayered structure of ITO/PEDOT:PSS/TFB/NCs/ETL/Al, in which the PEDOT:PSS and TFB are used as the hole-injection layer (HIL) and HTL, and different types of ZnO NPs are used as the ETL. Moreover, cadmium-free CIZS/ZnS NCs are selected as an emitting layer, which has an emission peak of 557 nm and a relative photoluminescence quantum yield of around 80% (Figure S3a). The XRD pattern and TEM image shown in Figure S3b and c indicate that the CIZS/ZnS NCs exhibits a cubic zinc-blende structure and a mean size of less than 5 nm. Figure 4a depicts the schematic device structure of the QD-LEDs, whose feature is all-solution-processing with orthogonal solvents except the Al cathode. The cross-sectional SEM image of a typical QD-LED is given in Figure S4, and different layers with clear boundaries can be observed. The schematic energy level diagram of the QD-LEDs is presented in Figure 4b, and the work function of ITO and Al as well as the energy values of PEDOT:PSS, TFB and CIZS/ZnS NCs are taken from previous reports.11,

21, 34

Considering that different types of ZnO NPs have an electron affinity of ～3.32-3.51 eV and an ionization potential of ～6.94-7.01 eV below vacuum energy level, the conduction band offsets at the ETL/NCs and ETL/Al interfaces are small enough to provide efficient electron injection from the Al cathode into the CIZS/ZnS layer. In contrast, the valence band offset at ETL/NCs interface is big enough to confine the 12


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holes within the CIZS/ZnS layer and thus the charge recombination efficiency can be improved to a great extent. Figure 4c gives the comparison between the PL spectrum of CIZS/ZnS NCs in toluene and electroluminescence (EL) spectrum of QD-LEDs using Cl@ZnO:Mg as the ETL under the bias of 5 V. It can be seen that there is little shift between PL and EL maximum, which suggests that the EL of the QD-LEDs mainly stems from the CIZS/ZnS NCs. The insets of Figure 4c present the digital photos of the CIZS/ZnS NCs dispersed in toluene before and after illumination under UV light (top panel) as well as the EL emission under the driving voltage of 5 V, and an identical bright emission is observed from the CIZS/ZnS NC solution and the corresponding QD-LEDs.

Figure 4. (a) Schematic device structure and (b) energy level diagram of all-solution-processed QD-LEDs with a multilayered structure of ITO/PEDOT:PSS/TFB/NCs/ETL/Al; (c) Top panel: PL spectrum of CIZS/ZnS NCs dispersed in toluene and the digital photos of the products before and after illumination under UV light; Bottom panel: EL spectrum of the QD-LEDs using Cl@ZnO:Mg NPs as the ETL, and the digital photos of the emission from the device under the driving voltage of 5 V.

According to previous reports, doping of metal ions into ZnO NPs is an effective method to tailor the electronic structures, which is often determined by the synthetic parameters, such as doping concentration and reaction temperatures.30-34 To compare the device performance of QD-LEDs using different types of ZnO NPs as the ETL, a systematic study was carried out to optimize the device performance of the QD-LEDs using ZnO:Mg NPs as the ETLs. Firstly, different ZnO:Mg NPs were synthesized by 13


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varying the reaction temperature and Mg doping contents, and the reduction of reaction temperature and increase of doping Mg concentration bring about a continuous blue-shift of absorption peak, while the hexagonal wurtzite structure is still retained (Figure S5). Subsequently, solution-processed QD-LEDs were fabricated by using different ZnO:Mg NPs as the ETLs, and thus the device performance can be optimized. Figure S6 depicts the J-L-V characteristics and device efficiencies versus current density for the devices using ZnO:Mg as the ETLs synthesized at different temperature with a Mg doping concentration of 5 wt%, and a maximum brightness of 3644 cd m-2 can be obtained in the QD-LED using ZnO:Mg synthesized at 40 °C. In this case, the EQE and current efficiency can reach up to 3.00 % and 8.89 cd A-1, and the corresponding device parameters are summarized in Table S1. When a series of ZnO:Mg NPs with different doping concentrations synthesized under 40 oC were used as the ETL to fabricate QD-LEDs, the maximum brightness of 3644 cd m-2 and the peak EQE of 3.00% can be obtained in the device using ZnO:Mg as the ETL with a doping concentration of 5 wt% (Figure S7). All the device parameters are summarized in Table S2. As a result, the optimized device efficiency of the QD-LEDs with ZnO:Mg NPs is obtained when the reaction temperature is 40 oC and the Mg content is 5 wt%. Moreover, the ZnO NPs synthesized at different reaction temperature were also used as the ETL to fabricate QD-LEDs, and the optimized device performance were achieved when the ZnO NPs are synthesized at 40 oC (Figure.S8). On the basis of these results, Cl@ZnO and Cl@ZnO:Mg NPs are synthesized with the Mg doping content of 5 wt% under 40 oC, and they are used as an ETL in cadmium-free QD-LEDs to study their superiorities over conventional ZnO, Cl@ZnO and ZnO:Mg NPs, which will be discussed in the following section. Figure 5 displays the device performance of the QD-LEDs using ZnO, Cl@ZnO, ZnO:Mg, Cl@ZnO:Mg NPs as the ETLs, and the Mg doping content is 5 wt% and the Cl dosage is 0.1 mmol. The variation of current density and luminescence with the applied voltage is shown in Figure 5a, and a lower current density is observed in the devices using Cl@ZnO, ZnO:Mg and Cl@ZnO:Mg NPs as ETLs under the driving voltage above 3V, which suggests that the electron injection through the ETLs is 14


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restrained to some extent.31 Such a probable suppression of the electron injection is in favor of the charge injection balance and thus promotes the device improvement. Actually, the maximum luminance of the device using ZnO NPs as the ETL is 2894 cd m-2, and an enhanced luminance of 3570, 3644 and 3185 cd m-2 can be achieved at 5.4 V in the devices using Cl@ZnO, ZnO:Mg and Cl@ZnO:Mg NPs as ETLs. The current efficiency and EQE as a function of current density for different QD-LEDs are depicted in Figure 5b, and the device using pure ZnO NPs as the ETL displays a peak EQE value of 1.54%, corresponding to the current efficiency of 4.56 cd A-1, which is equivalent to the mean peak efficiencies of our previous work. When the Cl@ZnO and ZnO:Mg NPs are used as the ETLs in the cadmium-free QD-LEDs, the maximum EQE and current efficiency are substantially increased to about 3% and 8.89 cd A-1, respectively. Such a value is higher than that of the Cu-In-S/ZnS-based QD-LEDs using ZnMgO NPs as the ETL reported previously.34 It is encouraging that the device using Cl@ZnO:Mg NPs as the ETL exhibits a peak EQE of 3.72% and a peak current efficiency of 11.08 cd A-1, which is enhanced by 2.4 times as compared with the undoped ZnO NPs-based devices. It should be noted that such a high EQE and current efficiency are achieved at the luminance over 500 cd m-2, which is critical to evaluate the practical display and lighting applications. The exciting enhancement of the device efficiency confirms that the combination of Mg-doping and Cl-passivation strategies can really boost the device performance of cadmium-free QD-LEDs. Moreover, the Cl@ZnO:Mg-based QD-LEDs not only exhibit an improved device performance but also have a good reproducibility, and the statistical histograms of the peak EQE and current efficiencies for 50 devices from different batches are shown in Figure 5c and d. The mean peak EQE and current efficiency can be estimated to be 2.84±0.82 % and 8.45±2.49 cd A-1, indicating a good reproducibility of the device performance. Figure S9 shows the typical EL spectra of the CIZS/ZnS-based QD-LEDs using different ETLs under the different driving voltage of 4-7 V, and a steady increase of the EL intensity is observed without any obvious shift of peak positions. Moreover, no parasitic EL emission from TFB layer is observed in the EL spectra, implying the exciton recombination mainly occurs in the CIZS/ZnS layer. 15


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The corresponding EL images of the four devices operating at 5 V are depicted in the insets of Figure S9, and bright yellow-green emission is observed in the four devices.

Figure 5. (a) The J-L-V characteristics and (b) current efficiency and EQE as a function of current density for the QD-LEDs using ZnO, Cl@ZnO, ZnO:Mg and Cl@ZnO:Mg NPs as the ETLs; Statistical histograms of (c) EQE and (d) current efficiency for 50 devices based on Cl@ZnO:Mg NPs as ETL.

The advantages of Cl@ZnO:Mg NPs over the other three counterparts have been demonstrated in the device performance comparison of the cadmium-free QD-LEDs. Based on the aforementioned UPS results shown in Figure 3 that the upward shift of CBM in Cl@ZnO:Mg NPs toward vacuum level has been established, and the energy 16


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barrier between Al cathode and ETL is increased as compared with ZnO:Mg and ZnO NPs, and thus the rate of electron injection into ETL becomes slow. The reduced electron injection and transport of Cl@ZnO:Mg NPs is also verified by the electron-only devices with a structure of ITO/ETL/NCs/ETL/Al (Figure S10), and the current density is decreased with an incorporation of

Mg ions and Cl-passivation .

To further obtain the information on the charge injection and transport, the hole-only device with a structure of ITO/TFB/NCs/MoO3/Al was also fabricated, and the current density is much lower than that of the electron-only device with pure ZnO NPs (Figure S10), which leads to the imbalance of the charge injection. With the incorporation of Mg ions and the passivation of Cl ions, the difference of the current density between the electron-only and hole-only devices is reduced, which is beneficial to realize the charge injection balance, and has a positive effect on the device performance. In addition, the electron transfer between the CIZS/ZnS NCs and Cl@ZnO:Mg NPs is prohibited, which is inferred from the PL decay curves of various ETL/NCs films (Figure S11). To sum up, due to the reduced energy barrier between the emitting layer and the ETL, the electron injection into the emitting layer through the ETL becomes more efficient, which reduces the number of the electrons accumulated at the interface and the possibility of the interfacial recombination. Since deep valence-band energy level is obtained in the different types of ZnO NPs, the energy barrier between the emitting layer and the ETL is high enough to block the holes injection into ETL, and the recombination of holes and electrons is confined in the emitting layer, leading to enhanced charge recombination efficiency. Therefore, it is because Cl@ZnO:Mg NPs offer reduced electron mobility and a more favorable electrical band alignment that the device efficiency of the QD-LEDs is improved greatly.

17


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Figure 6. (a) The J-L-V characteristics and (b) EQE as a function of current density for the

QD-LEDs using Cl@ZnO:Mg NPs as the ETL with different Cl amounts.

Apart from Mg doping, Cl-passivation also plays a significant role in the device performance enhancement of cadmium-free QD-LEDs using Cl@ZnO:Mg NPs as the ETL. To optimize the device performance, different Cl@ZnO:Mg NPs were synthesized by using variable Cl amount from 0.1 to 0.6 mmol, which were used as the ETLs to fabricate solution-processed QD-LEDs. The UV-Vis absorption spectra shown in Figure S12 indicates that no obvious shift of absorption wavelength of Cl@ZnO:Mg NP films is observed by varying the Cl amount, and the corresponding optical bandgap is changed from 3.66 to 3.70 eV with an increase of the Cl amount from 0.1 to 0.6 mmol. Figure 6a and b depicts the J-V-L characteristics and the variation of EQE with the current density for the QD-LEDs using Cl@ZnO:Mg with different Cl amount as the ETLs. As shown in Figure 6a, the three devices exhibit an identical turn-on voltage of 3.4 V, and little change is observed in the current density above 3.4 V, but higher current density at below 3.4 V is obtained in the device using Cl@ZnO:Mg NPs with less Cl amount. Furthermore, the maximum luminance of the device is increased from 3185 to 3955 cd m-2 with an increase of Cl amount from 0.1 18


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to 0.3 mmol, but further increasing Cl amount to 0.6 mmol leads to a decrease of the maximum luminance to 3367 cd m-2. The EQE as a function of current density for the devices using different Cl@ZnO:Mg NPs as the ETLs is depicted in Figure 6b, and the peak EQE is increased from 3.72 % to 4.05% and then decreased to 3.21% with an increase of Cl amount from 0.1 to 0.6 mmol. Therefore, it is reasonable to believe that Cl-passivated ZnO NPs with appropriate amount of Cl ions are beneficial for the device performance enhancement of QD-LEDs, but the presence of excess Cl ions in the ZnO NPs may act as charge traps to reduce the exciton recombination in the emitting layer. The best-performing device with Cl@ZnO:Mg NPs as the ETL exhibits a superior peak EQE of 4.05% and peak current efficiency of 12.17 cd A-1, which stands for the best value of the CIZS/ZnS-based QD-LEDs by just optimizing the device architecture and interfacial engineering. We believe there is enough room to further enhance the device performance of the CIZS/ZnS-based QD-LEDs through the combination of such an interfacial engineering strategy and nanostructure optimization, and the further work is going on in our group.

CONCLUSIONS In summary, a significant improvement of the device performance has been obtained in the cadmium-free CIZS/ZnS-based QD-LEDs by optimizing the electronic structure and surface passivation of ZnO NPs as the ETLs. As compared to the EQE of 1.54% for the device using ZnO as the ETL, a nearly 2-fold enhancement is realized in the devices using Cl@ZnO and ZnO:Mg NPs as the ETLs, respectively. The Mg doping and Cl-passivation offer an upward-shift of CBM, reduced electron mobility and decreased surface defect traps, which facilitate the electron injection balance and exciton recombination in the emitting layer. With these benefits, the device with Cl@ZnO:Mg as the ETL exhibits an optimized peak EQE of 4.05% and current efficiency of 12.17 cd A-1, respectively. These device efficiencies are comparable to the highest value ever reported for the cadmium-free QD-LEDs based on CIZS/ZnS NCs. Such an interfacial engineering method may hold promising potentials in the future optoelectronic devices. 19


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXX.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (A. W. Tang) *E-mail: [email protected] (Z. B. Deng) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is partly supported by the National Natural Science Foundation of China (Nos. 61735004 and 61674011), and Beijing Natural Science Foundation (No. 4172050) and National Key Research and Development Program of China (No. 2016YFB0700703). The author (A. T.) appreciates the support from the “Excellent One Hundred” project of Beijing JiaoTong University.

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A significant improvement is obtained in the cadmium-free quantum-dot light-emitting diodes using Cl@ZnO:Mg nanoparticles as the electron-transporting layer. 95x65mm (300 x 300 DPI)


Chloride-Passivated Mg Doped ZnO Nanoparticles For Improving

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