Tailored ZnO Functional Nanomaterials for Solution‐Processed Quantum‐Dot Light‐Emitting Diodes

Recent improvements in efficiency and luminance of quantum‐dot light‐emitting diodes (QLEDs) promise a versatile technology for next‐generation lighting and display applications. This is accomplished due to the advances in colloidal quantum‐dot (CQD) synthetic methods together with proper engineering of the charge balance in these devices. The exciton quenching mechanisms occurring at the interface between the QD emissive layer and the zinc oxide (ZnO) electron transport layer (ETL) are one of the important parts of the charge transport path, affecting efficiency and long‐term stability. Herein, a comprehensive overview of the advances in the engineering of ZnO‐based ETLs, in terms of device efficiency and operational stability, is attempted. It is specifically highlighted that significant improvements can be achieved using various ZnO ETL defect passivation methods. This review also describes the key requirements for high‐performance QLEDs from the ETL engineering aspect and catalyzes for further interdisciplinary explorations to realize reliable devices for practical applications.

QLED smart displays have also been published. [2b,d,16] However, there has been no comprehensive review on the critical role of the other charge transport layers in these devices. In this review, we focus primarily on ZnO-based functional nanomaterials, exclusively used in solution-processed QLEDs. A wide variety of ZnO-based ETL modification methods have been explored within the research community to date. However, alongside the ongoing improvements on the CQD synthetic methods, there is currently a need for a clear picture of the available knowledge in ZnO ETL engineering to ideally come up with the most effective and scalable of these methods for large-scale manufacturing of next-generation QLED displays and lighting systems. Specifically, we highlight the necessity to focus on long-term operational stability, largely by the passivation of EML/ETL interfacial and ZnO interstitial defects, together with the requirement for improving the overall QLED device efficiency and brightness. Rather than giving a review on the ZnO synthetic methods that have already been reviewed previously, [17] we primarily highlight the advancements achieved on the efficiency and stability parameters from the device engineering point of view with the focus on ZnO-based ETLs. In this review, we first provide a brief introduction to transition metal oxides (TMOs) in hybrid organicinorganic optoelectronic devices, followed by the properties and shortcomings of ZnO ETLs in QLEDs. In the third section, we provide modification methods of ZnO ETLs for improving the QLED device performance and operational stability. Finally, we summarize the evolution of the ZnO ETL modification methods and provide an outlook for the possible directions of the future interdisciplinary research in the field of solutionprocessed QLEDs.

Transition Metal Oxides in Hybrid Optoelectronic Devices
TMOs, thanks to their unique optoelectronic properties, lowtemperature thin-film solution processibility, and excellent charge transport characteristics, are known to be dominant functional materials for numerous applications such as solar cells, light-emitting diodes, field-effect transistors, and photodetectors. [18] Solution-processible p-type TMOs such as nickel oxide (NiO x ) and n-type semiconductors such as molybdenum oxide (MoO 3 ), vanadium oxide (V 2 O 5 ), and tungsten oxide (WO 3 ) with high workfunction as well as n-type TMOs with low workfunction such as ZnO and titanium oxide (TiO x ) have been widely used as charge injection/extraction interlayers in optoelectronic devices. [19] Defects and cation oxidation states in TMOs determine their electronic properties. Workfunction is an important parameter for charge transport and charge generation in devices. Some n-type TMOs (such as V 2 O 5 with a high workfunction of 7 eV) are suitable hole injection materials. [20] MoO 3 with deeplying electronic states is another n-type semiconductor suitable for hole injection that occurs through electron extraction from the highest occupied molecular orbital (HOMO) level of hole transport materials to the CBM of MoO 3 in organic devices. [21] Tungsten oxide (WO 3 ) is a well-known n-type semiconductor with high electrical conductivity used in organic photovoltaics (OPVs), perovskite solar cells, and solar concentrators. [22] The bandgap of WO 3 ranges from 2.4 to 2.8 eV, which makes it a suitable alternative to TiO 2 and ZnO in solar cells. [23] Due to its electronic band structure, WO 3 is also a suitable substitute for PEDOT:PSS in solar cells and OLEDs. [24] These TMOs typically exhibit high carrier mobilities, which largely contributes to device performance. [25] A suitable electronic energy-level alignment between a TMO interlayer and the neighboring organic/ inorganic layers in the device structure enables efficient charge injection and transport.
Crystalline or amorphous thin films of TMOs are typically deposited via a solution-based sol-gel route or from a nanoparticle (NP) suspension, respectively. [17a,26] However, the fabrication of crystalline TMO thin films with high charge carrier mobilities, which requires high temperatures (300-600°C), is not necessary for QLEDs, OLEDs, and OPVs. Therefore, low-temperature (typically <100°C) solution-precipitation synthetic methods for the preparation of amorphous TMO thin films are desired, especially in printed plastic optoelectronic domains. [27] Specifically, in the synthesis processes of ZnO NPs, the widely used precursors are zinc acetate dihydrate (ZnAc 2 ·2H 2 O) [28] and zinc nitrate hydrate (Zn(NO 3 ) 2 ·xH 2 O) in alcoholic or aqueous solvents, to achieve sub-10 nm NPs. [29] The ZnO NPs are eventually dispersed and stored in polar solvents. Depending on the reaction temperature, solvent, and the additives used, the shape and size of the NPs can vary. [30] To prevent aggregation and enhance the colloidal stability of ZnO NPs in the solution, stabilizers such as ethanolamine and polyvinylpyrrolidone (PVP) are usually added. [17a] The final colloidal solution is deposited by printing techniques during the device fabrication process, followed by thermal post-treatment (annealing) at higher temperatures (130-150°C) to evaporate the remaining solvent. [17a,18a,31] Depending on the application, ZnO NPs could be synthesized or grown in various morphologies, sizes, and shapes including nanorods, [32] nanowires, [33] nanorings, [34] nanotubes, and nanobelts. [35] Without the need for any crystalline form of ZnO, solution-processed QLEDs have so far benefited tremendously from adopting amorphous ZnO NPs as suitable ETL nanomaterials.

ZnO Properties as ETLs in QLEDs
Significant progress in the brightness and current efficiency of QLEDs was initially achieved by introducing ZnO NPs as an ETL material in 2008. [36] Colloidal ZnO NPs have since then become the commonly used nanomaterials as the conventional ETLs in QLEDs, [36a,37] due to their desired electronic structures [12,38] and simple and cost-effective synthesis. [39] Furthermore, as a well-known n-type semiconductor, [40] ZnO has a relatively wide bandgap of 3.37 eV, [17a,36b,41] a tunable workfunction of %4.2 eV, [42] and room-temperature excitonic binding energy of 60 meV, [43] making it a suitable candidate for a vast variety of applications. [44] Moreover, using ZnO is advantageous owing to its higher carrier mobility (2 Â 10 À3 cm 2 V À1 s À1 ) [1d] compared with amorphous TiO 2 or common organic ETLs (<1 Â 10 À4 cm 2 V À1 s À1 ), [37a,45] its high excitation binding energy, [46] and excellent chemical and thermal stability. [47] ZnO-based QLEDs can exhibit high transparency, [1d,41a,48] external quantum efficiencies (EQEs) over 20% [49] (single and tandem structures), [41a] and half lifetimes exceeding 10 6 h (for an initial brightness of 100 cd m À2 ) [49a,50] rivaling those of state-of-the-art OLEDs. Moreover, QLEDs exhibit very interesting phenomena such as room-temperature subthreshold turn-on voltage, which is not typically observed in OLEDs. [51] However, to achieve maximum efficiency and brightness and improve the long-term device operational stability, the interstitial and interfacial defects in the ZnO ETL must be passivated. In the following sections, we will review the shortcomings of ZnO ETLs and approaches for minimizing the defects, exclusively in QLED structures reported in the literature.

Shortcomings of ZnO ETLs in QLEDs
The high conductivity of ZnO alongside the facile electron injection from the cathode into the ETL and subsequently into the QDs EML can significantly lower the driving voltage in a QLED device. However, when the QDs EML is in immediate contact with the ZnO ETL, due to the small-energy offset between the CBM of these materials, spontaneous charge transfer occurs at the EML/ETL interface. [52] In this situation, more electrons than holes accumulate in the recombination zone and, due to charge transfer mechanisms, nonradiative exciton dissociation takes place, deteriorating the device performance. [53] Moreover, some of these excess electrons can overcome the EML/HTL energy barrier and cause severe leakage current and consequently lead to heating the device. [54] Hence, due to the high conductivity of ZnO and the small-energy electron injection barrier at the EML/ETL interface, ZnO-based QLEDs are electron-dominant devices. As a result of this charge imbalance, the interfacial electrons accumulated at the EML/HTL interface induce undesired nonradiative Auger recombination and charge the QDs in the EML. [55] The lack of charge neutrality in the QDs EML can eventually result in charged dark states often described as EL blinking. [56] The crystalline structure of ZnO NPs possesses many Zn interstitial defects and oxygen vacancies, which can trap charges and quench radiative excitons, deteriorating the device efficiency and lifetime. [57] 3. Modification Methods for ZnO-Based Electron Transport Layer (ETL) To exploit the desired properties of ZnO-based NPs and minimize their limitations, ZnO ETLs have been modified with various methods in both conventional and inverted QLED architectures. The most researched methods comprise tuning the ZnO NP sizes, doping ZnO with metals, insertion of an ultrathin (insulating) layer at the EML/ETL interface, ZnO ETL post-treatment, etc. One should note that the effectiveness of each of these approaches also depends on the other device parameters and experimental conditions. These methods are described in the following sections and schematically summarized in Figure 1.

Tuning the ZnO Nanoparticle Size
Variation of the quantum confinement effect in ZnO NPs, which originates from tuning the NP size, has a significant influence on the device properties. Exciton dissociation at QDs/ZnO interface, photoluminescence (PL) efficiency, and charge balance in the device are all affected by the NP size. [58] In turn, these properties directly affect the parameters that control the device performance: current density, current efficiency, EQE, turn-on voltage, power efficiency, and brightness (or luminance). [45c,59] Pan et al. reported that the ratio of [Zn 2þ ]:[OH À ] in the ZnO solution determines the size of the NPs, where the [Zn 2þ ]:[OH À ] ¼ 1:0.5 ratio yielded the smallest particle diameter (2.9 nm), while ratios larger than 1:1.5 led to ZnO NP aggregation. [45c] High spatial confinement of charge carriers in smaller QDs increases the bandgap (Figure 2a). The upshifted CBM leads to a stronger driving force for the electron transfer and increased conductivity. On the other hand, the reduced valance band minimum (VBM) level increases the energy offset and blocks the hole transfer from the QDs EML to the ETL, which can improve the charge balance and exciton recombination efficiency in the QDs EML layer. [37a,60] Figure 2a shows the bandgap widening upon NP size reduction and the time-resolved PL measurements and relaxation profiles of radiative excitons for QDs-only and QDs/ZnO/ITO samples (Figure 2b). Pan et al. found that the decay time was longer for the smaller NPs, which was attributed to the reduced exciton quenching and better charge separation. [45c] Interfacial charge separation can account for the changes observed in the relaxation lifetime. This exciton quenching process did not occur in the QDs-only sample, leading to a longer decay time. Figure 2c shows the increased current density in the electrononly device (EOD) using ZnO NPs of various sizes. The smaller NPs exhibited better electron mobility due to the larger grain boundaries, which led to increased conductivity and current density. Figure 2d,e shows the QLED device performance parameters. Thanks to the improved electron injection and efficient prevention of hole leakage provided by the larger bandgap, the device efficiency was substantially improved. As shown in Figure 2d,e, the maximum current and power efficiencies were increased by decreasing the size of ZnO NPs from 5.5 to 2.9 nm. Furthermore, the higher power efficiency was attributed to the lower device turn-on voltage. [45c] In a different study, Moyen et al. reported that ZnO NP size was reduced from 5.43 to 2.8 nm by modifying the reaction temperature from 60 to 0°C. [59] Reducing the temperature subsequently lowered the Zn 2þ ion mobility in solution, which affected the nucleation and growth patterns as well as the NP morphology. Analogous to the previous study, the bandgap was larger for the smaller NP sizes due to the higher spatial confinement of charge carriers. High-resolution transmission electron microscope (HRTEM) images of the smaller NPs (2.8 nm) revealed faceted surfaces instead of an ellipsoidal shape observed for the larger NPs ( Figure 3a). This resulted in a significantly lower surface defect density, despite the much larger surface area, which was attributed to more favorable reaction conditions. [61] In the case of smaller NPs, the increased surface area and grain boundary density should enhance the electron conduction. [61][62][63] Nevertheless, it was observed that electron mobility was reduced in the EODs with the smaller NPs. This was explained in terms of the substantial decrease in the surface defects, which inhibited the electron conduction at a scale that outweighed the effect of increased surface area. This was also supported by the PL emission from the low-defect smaller NPs. Moreover, the electron mobility reduction helped to improve the performance device performance via enabling a better charge balance in the QLED devices. In addition, the passivated defects helped mitigate the exciton dissociation at the QDs/ZnO interface, which elongated the QD radiative decay time. [59] Figure 3 and Table 1 summarize the efficiency parameters for the devices fabricated with ZnO NPs of different sizes. Current and power efficiencies were the highest in the case of devices fabricated with 2.8 nm ZnO NPs, which also exhibited a slightly lower current density due to the reduced electron mobility.

Doping with Lithium (Li)
Doping of pure ZnO with alkaline metals has been demonstrated to improve the efficiency of QLEDs. It has been found that lithium ions (Li þ ) could be utilized to fill the interstitial Zn defects in the ZnO lattice. [64] These interstitial sites act as charge trap states, causing exciton quenching. Therefore, upon doping with Li þ ion, the electron conductivity increases without significantly changing the workfunction and VBM. This improves the device performance by suppressing the interfacial exciton quenching. In addition, it has been observed that, as a result of the occupied interstitial sites, the minimized surface defects prevented the adsorption of moisture and led to better device aerobic stability. [65] 3.2.2. Doping with Gallium (Ga) and Aluminum (Al) (Group III) Doping with group III elements has also been found to be an effective approach to improve the electronic properties of ZnO in QLEDs. Studies have shown that these elements can lower the workfunction of ZnO and that the band alignment favors the suppression of interfacial electron (back-)transfer. [66] In Al-doped ZnO QLED devices with red-emitting QDs, the  Reproduced with permission. [59] Copyright 2020, American Chemical Society.  [69] The authors improved the lifetime and efficiency of the devices via electron-hole balance. In addition, the yttrium-doped ZnO (YZO) ETL inhibited the QD charging and improved the ETL film surface morphology. Among 0, 3, 7, and 9 wt% doping concentrations, the highest EQE and lifetime were achieved with 9 wt% doping level of yttrium. Moreover, the electron mobility data showed that doping with yttrium reduced the electron mobility that consequently controls the conductivity and charge balance in the device.

Doping with Magnesium (Mg)
Magnesium ion (Mg 2þ ) substitution into ZnO nanocrystals is an effective approach for increasing the conduction band of ZnO. For example, 15% Mg doping was reported to exhibit the best charge balance. [70] Upon doping with Mg 2þ ions, the bandgap of ZnO NPs can be widened, [71] the effect of which has been shown to be much stronger than the NP size reduction. [72] Thanks to the similar crystalline structure of MgO and ZnO, as well as similar ionic radii of Mg 2þ and Zn 2þ (0. 57 [54] where the concentration levels were chosen based on the previously published synthesis methods. [65,66c,68,72] According to the dynamic light scattering (DLS) measurements, the average diameter of the NPs was reported to be 5.6 nm. The flat-band energy-level alignments for the undoped ZnO and various metal-doped ZnO ETL materials, alongside those for the other functional layers in the fabricated QLEDs, are schematically displayed in Figure 4a. An upshift in the CBM was observed in the doped ZnO materials. This can therefore slow down the electron injection from the cathode into the ETLs, attenuating the adverse excess electron leakage effect in the devices. [54] A comparison of the device parameters is also given in Figure 4b. QDs with a CdSe (2.3 nm)/ZnS/CdS/ZnS (5 nm) composition were used in yellow-emitting QLEDs (Y-QLEDs). The device performance exhibited a substantial boost when Al-doped ZnO (AZO) ETL was used, despite the minimal CBM shift compared to ZnO. The maximum brightness of the Y-QLEDs with an AZO ETL was higher than the devices fabricated with the other ETLs, the current efficiency improved significantly, and the turn-on voltage was the lowest of the series ( Table 3). This enhancement was attributed to the higher electron conductivity as well as the low surface roughness of the material, which outweighed the benefits of a raised CBM. [54] The maximum current efficiencies for the devices fabricated with Ga-and Mg-doped ETLs were lower compared with the control device. This was in contrast to a 50% improvement in the device performance using Ga-and Mg-doped ZnO materials, demonstrated in a previous study. [72] Alexandrov et al. claimed that this variation originated from the different QD EMLs used in their devices. Specifically, in the study by Kim et al., [72] the spontaneous charge transfer was less severe, thanks to the lower CBM of the CdSe/ZnS red-emitting QDs EML, which increased the electron injection barrier and improved the device charge balance. Finally, QLEDs fabricated with a Li-doped ZnO ETL did not exhibit any significant efficiency improvement. The authors hypothesized that despite the good energy-level alignment between the Ga-and Mg-doped ZnO ETLs and the QDs EML, the raised VBM of these materials reduced their hole blocking ability, leading to hole leakage currents and Joule heating.

Codoping with Lithium and Magnesium
Kim et al. investigated the influence of Li-Mg (10% Li þ and 10% Mg 2þ )-codoped ZnO (sol-gel method) as the ETL in their green-emitting QLEDs (G-QLEDs). [73] As previously shown also in other studies, [74] as a result of 10% codoping with Li þ and Mg 2þ ions, the bandgap of the ETL increased. The charge balance in the device improved as well. In this process, the interstitial Zn sites are filled with Li þ ions, which decrease the number of O─H bonds (i.e., exciton quenching sites) at the Li-Mg-doped ZnO (MLZO) surface. [75] Consequently, the exciton decay time of the QDs on MLZO was measured to be close to that of QDs on bare glass substrates, as shown in Figure 5a.  Mg, 5% [72] 3. 90 5.6 Al, 10% [66c] 3.82 5.6 Li, 5% [65] 3. 93 5.6 Ga, 8% [68] 3.86 6.5 Pure ZnO [17a,36b,41] 3.37 Tunable (3-6 nm) Figure 5b shows the band energy levels of pure ZnO and ZnO doped with Li þ , Mg 2þ , and Li þ -Mg 2þ ions. The CBM level of the MLZO was higher than those of all the other materials studied in this work, and its bandgap (E g ) was found to be the widest. Moreover, Mg doping increased the energy gap between the CBM and Fermi level (E F ) of ZnO, whereas Li doping decreased it. In other words, Li doping increased the electron concentration, while Mg doping had an opposite effect, as also mentioned in the study by Mahmud. [76] Figure 5c shows the current density of the devices with different ETLs, and QLED device metrics with different ETLs are summarized in Table 4. The bandgap widening in the MLZO compared with the LZO, provided a higher energy barrier between the CBM of MLZO and that of the QDs, which could block the electron back-transfer from the QD layer at reverse bias. Despite these results, it is worth mentioning that MZO is still the widely used ETL material in QLEDs to date.

Incorporation of PVP
It has also been found that doping ZnMgO (or MZO) with small concentrations of PVP polymer can significantly suppress the spontaneous interfacial charge transfer at the EML/ETL interface. [63] PVP also acts as a spacer that helps prevent aggregation of colloidal ZnMgO NPs, improves the film quality, provides better air stability, and even offers good protection against plasma damage that can occur during cathode deposition. [48] In addition, doping with electron-blocking PVP can lower the electron leakage current, leading to a better charge balance in the device and to protect it from destructive Joule heating during the device operation. [77] Due to the rise of CBM in PVP-doped ZnMgO (MPZO) compared with pure ZnMgO, the energy barrier for spontaneous electron back-transfer from the EML to the ETL increases. [77b,78] Figure 6 shows the current-voltage curves and efficiency plots for red-emitting QLEDs (R-QLEDs). [63] The efficiency parameters for different doping ratios are also summarized in Table 5. The best maximum EQE, brightness level, and current efficiency were achieved with 10%-Mg and 28%-PVP doping levels. Higher PVP doping levels caused an increased device resistivity due to the insulating nature of the polymer and reduced current density.
In another study by Zhang et al., ZnMgO NPs were mixed with PVP as a hybrid (organic-inorganic) ETL (Figure 7a,b) to improve the charge balance and suppress the interfacial exciton quenching at the EML/ETL interface. [48] The authors showed that addition of PVP can improve the morphology and compactness of the ETL, while also filling the voids in ZnMgO [72,79] and protecting the device from ambient conditions. Moreover, excess electrons and accumulation of electrons at the EML/HTL interface were observed to be reduced, suppressing the leakage current, which indicates the important role of PVP in QLED architectures. Contrary to most of the modification methods proposed previously (i.e., size reduction, doping, etc.), mixing ZnMgO with PVP did not significantly change the CBM of ZnMgO. The insulating nature of PVP, instead, led to reduced conductivity, and the passivation of surface defects (i.e., the exciton quenching sites) in the ETL, which resulted in increased charge balance in the devices. [80] Zhang et al. also reported that increasing the ZnMgO:PVP weight ratio from 20:0 to 20:14.4 wt% significantly reduced the surface roughness and helped suppress  [54] Copyright 2020, Springer Nature. the leakage current. Figure 7c-e shows the G-QLED device performance parameters. The authors also stated that, thanks to the improved compactness, the hybrid ETL can even withstand plasma damage during indium tin oxide (ITO) sputtering, which thus enables fabrication of transparent QLEDs. [48] The best device performance was achieved with 20:9.6 ZnMgO:PVP wt%. The current efficiency values were about 1.33 times higher than those obtained from the control device with pure ZnMgO. A larger amount of PVP resulted in further reductions of the conductivity and current efficiency, together with increased turn-on voltage.

Using Interfacial Modification Layers
To eliminate the unwanted effects of excess electrons and to provide a better charge balance in a QLED, an extremely thin interfacial layer can be inserted at the EML/ETL interface. Most of the interfacial modification layers (IMLs) reported in the literature  are insulating polymers, [74b,81] where their insertion at the EML/ ETL interface helps reduce the electron leakage current. Dai et al. reported for the first time that the use of insulating polymethylmethacrylate (PMMA) polymer as an IML improved the charge balance in their QLEDs.
[41a] A 6 nm PMMA layer prevented excess electron injection from the ZnO ETL to QDs EML, reduced the Auger recombination at the QD/HTL interface, and suppressed the interfacial exciton quenching. Consequently, nonradiative recombination was also suppressed, increasing the exciton decay time, and the EQE of the QLED device increased. The authors further reported a low efficiency roll-off and improved operational stability in their devices. [41a] A similar approach was later adopted by Rahmati et al. to integrate ultrathin QD/PMMA alternating layers for reducing the electron leakage current. [82] Instead of using an insulating IML polymer, Fu et al. employed poly(9-vinlycarbazole) (PVK) semiconducting polymer as an electron blocking layer to improve the charge balance in their QLED devices. Together with polyethyleneimine ethoxylated (PEIE) (to reduce the hole injection barrier), PVK enhanced the maximum EQE, the peak current efficiency, and the brightness of the devices. [83] Other IMLs such as Al 2 O 3 [84] have also been explored. In all these works, the IML thickness optimization has been found to be crucial to control the device performance and operational lifetime.

ZnMgO Interfacial Layer
The use of ZnMgO as an inorganic interfacial layer at the ZnO/ QD interface has been reported to improve QLED device performance. In a work by Nomura et al., Mg doping of ZnO raises the CBM level and decreases the density of oxygen vacancies [85] and the conductivity of ZnO. [72,78] Due to the binding energy of Mg-O being higher than that of Zn-O, it is difficult to generate oxygen vacancies in ZnMgO. [86] Oxygen vacancies generate free electrons affecting the carrier mobility of ZnO materials. [87] Hence, reduction of the number of oxygen vacancies in ZnMgO lowers the conductivity, while upshifting the CBM. [88] Due to the CBM upshift, the conductivity and the excess electron injection can be suppressed by the ZnMgO interfacial layer (ZnO/ZnMgO/QD) in the studied R-QLED devices. [78] A ZnMgO interfacial layer could also help passivate the interfacial exciton quenching sites and suppress nonradiative recombination channels at the QDs/ZnMgO interface. This was demonstrated by Y. Sun et al. using a 13 nm-thick ZnMgO interfacial layer between ZnO and the QDs EML. [78] An inverted redemitting QLED (R-QLED) device with this architecture exhibited a 1.74-fold enhancement in the peak EQE and 1.72-fold in the maximum current efficiency, both compared with the ZnO-ETL control device. Figure 8a,b shows the R-QLED device structure and the band energy levels of the layered structure. The performance parameters for the devices with different thicknesses of the ZnMgO interfacial layer are shown in Figure 8c-e. All devices with a ZnMgO interfacial layer exhibited a higher EQE compared with the ZnO-only control device (Figure 8e). The thinner ZnMgO interlayers (7 and 13 nm) also showed comparable brightness levels at much lower current densities (and a much higher current efficiency). As a result of the upshifted CBM, the energy barrier for electron injection increased, leading to reduced electron injection and leakage currents. While still outperforming the ZnO-only device, a 25 nm ZnMgO layer was deemed too thick, as it resulted in very small current density  and inefficient electron injection, reducing the overall luminance and EQE compared with the thinner interfacial layers.
In metal oxides, oxygen vacancies can introduce intergap states (i.e., charge recombination/quenching sites in the  www.advancedsciencenews.com www.adpr-journal.com bandgap [89] ), but thanks to the reduction of oxygen vacancies upon Mg doping of ZnO, the number of such quenching sites is minimized. This has a positive impact on device performance and is supported by the increased PL lifetime (Figure 8g). Moreover, the elevated CBM level of ZnMgO reduces the probability of interfacial charge back-transfer from the EML to ETL.

Composite ETL (ZnMgO:ZnO)
As mentioned earlier, since Mg 2þ ions are smaller in size compared to Zn 2þ ions, doping ZnO with these ions widens the bandgap (Figure 9a,b). Zhang et al. reported that a ZnMgO: ZnO composite ETL layer is beneficial to concurrently take advantage of the high electron mobility of ZnO and the wider bandgap of ZnMgO to obtain a good charge balance in the device. [90] The wider bandgap of ZnMgO compared with that of ZnO can be further evidenced from the blueshift in the PL peak position of ZnMgO (Figure 9c), and the PL spectra for various ZnMgO:ZnO blend ratios can be found in between. The authors reported that a 3:1 ratio of the ZnMgO:ZnO blend yields the largest bandgap (more blueshift) compared with those with 1:1 and 2:1 ratios. According to Figure 9d, the turn-on voltages for all the devices with composite ETLs were around 2 V, while the control device with ZnO ETL exhibited the highest current density. The reduced current density (at V on ) of the devices with ZnMgO composite ETLs (see the inset in Figure 9d) is another indicator of the wider bandgap of ZnMgO, which could suppress the electron leakage to some extent. Increasing the amount of ZnO in the blend increased the carrier mobility and improved the current density of the composite with respect to pure ZnMgO. The highest current density was achieved with the ZnMgO:ZnO ratio of 3:1 where ZnMgO impeded the charge leakage and helped optimize the excess carrier density from the ZnO ETL.
It was also found that the brightness could be enhanced significantly for the devices using the composites ETLs, attributed to the improved charge balance in the EML due to the wider ETL bandgap. The bandgap widening of ZnO by doping Mg was also explained by Wang et al. and Sun et al. [77b,91] Specifically, the device with a 2:1 ZnMgO:ZnO ratio ETL exhibited the highest luminance, while the device fabricated with the 3:1 ratio showed the lowest brightness, despite the highest current density (Figure 9e). This was attributed to the saturation of luminance intensity because of carrier overflow. The same explanation was provided for the increased EQE of the devices fabricated with the 2:1 (the highest EQE) and 1:1 blend ratios, while the EQE of the composite ETL with the 3:1 ratio was the lowest (Figure 9f ).
The improved charge balance together with the reduced surface roughness and defect passivation prevented exciton quenching and improved the radiative lifetime of the QDs thin films prepared on ITO-coated glass substrates. The PL decay lifetimes of QDs in the absence of an ETL were expectedly the longest (Figure 9g), while among the different compositions of ETL, the 2:1 composite allowed for the longest decay time, which is in agreement with the observed device performance.
www.advancedsciencenews.com www.adpr-journal.com where τ QD=ETL is defined as PL decay lifetime of QDs in the vicinity of the ETL, and τ QD is PL decay time of the QDs on bare glass substrate.

Passivation of ZnO Thin-Film Surface with Ethanedithiol (EDT)
Surface modification of ZnO NPs with ethanedithiol (EDT) was proposed by Lee et al. to improve the efficiency of QLEDs. [93] X-Ray photoelectron spectroscopy (XPS) spectra of the EDTtreated ZnO film showed that Zn─S bond dissociation energy was found to be lower than that of Zn-O, which resulted in reduced oxygen vacancies after the surface was treated with EDT. It is understood that EDT fills the oxygen vacancies in ZnO [94] and increases the surface hydrophobicity as well as the film ambient stability. This is due to the prevention of the NP coarsening, originating from either ambient moisture absorption or oxidation. As a result of surface modification, the CBM energy level of ZnO ETL was upshifted by 0.47 eV (Figure 10), which led to the suppression of interfacial charge transfer and subsequently the nonradiative recombination. The intrinsic dipole moment of EDT and the interfacial dipole moments (due to thiolate ligands, through which the Zn─O bonds are exchanged by Zn-S) induced on the ZnO surface after EDT treatment were considered to be the origin of the increased energy level. [95] The EDT treatment also reduced the carboxylate and hydroxyl ligands (i.e., the exciton quenching sites) on the surface of ZnO NPs; hence, the nonradiative recombination pathways at the QD/ETL interface were suppressed. Performance parameters for the QLEDs fabricated with ZnOtreated ETL and for the control ZnO ETL device are displayed in Figure 10. All loads of EDT led to device efficiency enhancements across the board. In particular, the 0.1 vol.% EDT amount exhibited the highest luminance, EQE, and current efficiency. This was attributed to the passivated surface defects and reduced interfacial charge trapping sites. Overall, the better device operation was linked to more balanced charge distribution in the device. Larger amounts of EDT led to inferior charge balance in the device, accumulation of electrons at the QD/HTL interface, and/or Auger-assisted nonradiative recombination, [83] although still outperforming the performance of the control device with untreated ZnO ETL.
Nguyen et al. investigated the effect of EDT concentration on the performance of QLED devices using 3, 4, and 6 mM of EDT on a CdSe/ZnS QDs EML spin coated on a ZnO ETL. [96] The QLED device performance parameters are shown in Figure 11. The authors claimed that a shift in the vacuum level was induced due to the EDT dipole moments, both at the EML/ETL interface and at the EML surface. The in situ EDT treatment of the QDs Figure 10. a) The band energy levels in G-QLED device layers. The energy level is upshifted after the EDT surface treatment of ZnO ETL. Inverted G-QLED device parameters with various EDT ratios. b) Current density versus voltage, c) luminance versus current density, and d) current efficiency and EQE versus current density. Reproduced with permission. [93] Copyright 2020, Optica. Figure 11. Effect of EDT concentration on G-QLED device performance. a) Current density, b) luminance versus voltage, and c) luminance efficiency versus luminance for the device with and without EDT treatment. Reproduced with permission. [96] Copyright 2019, Royal Society of Chemistry. EML with 3 mM and 4 mM concentration improved the device efficiency, while 6 mM EDT concentration slightly lowered it. Later, in a different study by Chen et al., EDT treatment was employed to alter the conductance in a ZnO film. [97] Similarly, the competing contributions of the dipole moments at the EML/ETL interface and the EML surface were considered to be the reason accounting for the electron and hole current injection or blocking barriers.

Acrylic Resin Encapsulation of QLEDs
Positive aging is an interesting phenomenon observed in QLEDs, which refers to the improvement of efficiency and luminance upon device encapsulation with epoxy and storage, commonly observed a few days after the fabrication day.
[98] Being a mystery for many years, as one explanation, it has just been recently found that this process originates likely from device encapsulation with UV-curable acrylic resins from which outgassed organic acids cause in situ reactions at the ETL/cathode interface. [98b,99] In general, however, this type of positive aging alone would not be useful for commercial applications, as it is normally followed by long-term negative aging (i.e., deterioration of the efficiency and brightness). Chen et al. observed improved performance from their encapsulated devices over a few days of storage, followed by degradation over longer periods ( Figure 12). [99] As an important note, further aging of the devices caused the formation and development of non-emissive dark spots. The authors systematically studied the effect of acids in the resin to investigate the root cause of positive and negative aging. [99] This study will be looked at in more detail later in Section 3.6.2 and 3.6.3.
Similarly, Acharya et al. reported the positive aging effect as a result of QLED device encapsulation with a UV-curable acidic resin stored in a glovebox. [98b] The operational lifetimes and the performances of the R-QLED, G-QLED, and B-QLED were shown to be improved gradually within 1, 3, and 7 days of device storage. The authors proposed that the positive aging mechanism occurs because the water molecules (formed by interaction of acid in UV-curable resin with the device surface) reacted with ambient carbon dioxide (CO 2 ) to form carbonic acid. They found that due to the ZnO surface reaction with carbonic acid, the resulting carbonate groups modified the ZnO/QDs and ZnO/Al interfaces, which led to the reduction of the ZnO surface defect densities and enhancement of the radiative recombination rate. [98b] Su et al. monitored the progression of positive aging over 8 days, which they attributed to the chemical reactions occurring at the ZnMgO/Al interface. [98c] The proposed mechanism, however, was different from Acharya's mentioned above. Specifically, Su et al. observed the formation of aluminium oxide (AlO x ) at the Al/ZnMgO interface that acted as a barrier for electron injection (i.e., Al reacted with oxygen in ZnMgO). This mechanism was accompanied by increased oxygen vacancy sites in ZnMgO, which consequently enhanced the electron concentration and conductivity in the QLEDs. The authors also stated that the oxide barrier reduced the exciton quenching on the metal cathode, which increased the maximum EQE of the R-QLEDs from 15.30% to 18.53%, for G-QLEDs from 11.62% to 15.34%, and for B-QLEDs from 4.93% to 12.97%. In another study, Su et al. also reported the effect of postmetallization on the efficiency enhancement of their QLEDs. Postannealing the device led to metallization of AlZnMgO and formation of the AlO x interlayer. [100] The former reduces the resistance and improves the electron injection, whereas the latter reduces the exciton quenching. Figure 13 shows the device performance parameters for various periods of aging.  Finally, Chen et al. developed a physical and analytical model to investigate the resin encapsulation and positive aging effect on the device performance ( Figure 14). [101] According to them, after encapsulation, the hole leakage current (originating from the defects such as hydroxyl (Zn(OH) 2 ) groups in ZnMgO) was reduced because of defect passivation upon applying the  Reproduced with permission. [101] Copyright 2021, Springer. UV-curable resin. Hence, the device efficiency was improved due to reduced leakage current. They also observed that although it eliminated the unwanted moisture and oxygen, the use of a desiccant hindered the ZnMgO defect passivation and the subsequent efficiency improvement. This was because the desiccant removed the outgassed acidic byproducts in the UV-curable resin. Consequently, the efficiency enhancement expected from positive aging was suppressed, and the lifetime of the device was found to be shorter. On the other hand, when the desiccant was used after the positive aging took place (the devices were reencapsulated after 5 days), the EQE improved up to 1.6-fold, and the device half lifetime was found to be 6-fold longer. The positive aging phenomenon is not uniquely observed in conventional QLEDs. Inverted QLEDs have also been reported to exhibit positive aging behavior. For example, Lee et al. reported that both EQE and current efficiency of their inverted devices with an InP-based EML were increased significantly after 69 days of self-aging due to the reduction of oxygen vacancies in ZnMgO. [102] Consequently, the conductivity of the ETL was reduced, which suppressed the electron injection and improved the charge balance at the ETL/EML interface. A similar mechanism was also studied in another work by Lee et al., where positive aging was found to control the trap states and the exciton quenching at the ETL/EML interface. [103] 3. 6

.2. Mild Acid Treatment of ZnMgO ETL Surface
It was observed that electronic and optoelectronic device encapsulation with UV-curable acrylic resin or controlled treatment with mild organic acids in a glovebox both led to very similar results for shelf aging. [99,104] In the study that was previously mentioned in Section 3.6.1. Chen et al. [99] stored their unencapsulated QLEDs under nitrogen and gradually exposed them to a saturated vapor of acrylic or isobutyric acid over 7 days (Figure 15). Their devices experienced positive aging, analogous to what they observed from the devices encapsulated with UVcurable epoxy. Positive aging was similar both with acrylic and with isobutyric acid treatment. As shown in Figure 15b, the performance parameters of the acid-treated devices improved after 1 day. Extending the storage to 7 days, instead, deteriorated the peak EQE and the current density and increased the turn-on voltage. In contrast, it was observed that the efficiency parameters of the unencapsulated devices without acid treatment remained virtually unchanged after 7 days, which led the authors  to believe both ageing processes were related to the encapsulation with the UV-curable epoxy. The authors concluded that in situ reactions could cause the silver atoms of the cathode to diffuse into the metal-oxide ETL, enhancing the electron conductivity (Figure 15d). Moreover, they found that the acid treatment could modify the ZnMgO surface. During the synthesis of ZnO, hydroxide and acetate groups form on the NP surface. The isobutyric organic acid reacts with the surface hydroxyl groups and increases the carboxylate group density. In turn, they found that the QDs/ZnO interactions are modified in the device structure, which results in the suppression of interfacial exciton quenching processes. Therefore, alongside improved electron conductivity, this led to the observed performance improvement in their study.

Shelf-Life and Negative Aging
In the same study, Chen et al. observed progression of negative aging due to overtreatment with organic acids, which led to acid reaction with the conductive metal-oxide ETL and production of a nonconductive carboxylate salt. [99] This caused the cathode/ETL interface to degrade over time with the consequent formation of pinholes. They found that acid treatment produces water as a byproduct, the accumulation of which triggers some electrochemical and chemical reactions during the device operation and storage. [105] To avoid the unwanted effects of negative aging, Chen et al. employed a bilayer ZnO-ETL in their devices. [99] The bilayer ETL was fabricated with ZnO NPs of two different sizes. Specifically, a layer of so-called "optical" ZnO (O-ZnO) NPs with 2.9 nm diameter was directly deposited on top of the EML, followed by a layer of so-called "conductive" ZnO NPs (C-ZnO) with an optimal diameter of 6.2 nm and a smaller bandgap compared with O-ZnO. The use of magnesium acetate (MgZc 2 ) in the synthesis process of O-ZnO prevented the postsynthesis ripening of the ZnO NPs and greatly improved the colloidal stability. Furthermore, bandgap widening in O-ZnO, as a result of the quantum confinement effect, is stronger in smaller NPs, which acts as an energy barrier to block the hole transfer channel. The higher conductivity of C-ZnO is further advantageous for efficient electron injection from the cathode. Overall, this bilayer ETL arrangement facilitated the electron injection, increased the current density, and improved the charge balance. Figure 15e shows the cross-sectional scanning transmission electron microscopy (STEM) images of the R-QLEDs with the bilayer ETL and their performance characteristics when encapsulated with acid-free epoxy resin. The turn-on voltage for the fresh device was 1.7 V. In these devices, the current density of the freshly made and stored QLEDs at 4.0 V reached higher values compared with that of the control device with the ZnMgO ETL (Figure 15f ). The peak EQE of the device remained relatively unchanged after 180 days (Figure 15g). Moreover, the luminance and operational lifetimes of the freshly made and stored devices were nearly identical and remained high for over 57 h, as shown in Figure 15h. Therefore, alongside an acid-free epoxy resin encapsulation, the bilayer ETL led to air-stable devices with reliable stable operational performances and long shelf-life.

Positive Aging of ZnMgO ETL Using Resistive Switching
Ding et al. reported ZnMgO NPs exhibiting resistive switching behavior. [106] The mechanism behind the phenomenon was explained in terms of a positive aging effect that improves the QLED device performance. According to their work, the positive aging effect could be described based on various mechanisms. First, the chemical reaction between the Al/Ag electrode and ZnMgO improves the electron injection at the interface and increases the conductivity. [98c] Second, diffusion of the cathode atoms into the ETL fills the trap states and enhances the device performance. [107] Oxides like ZnMgO exhibit an intrinsic resistive switching effect, which is responsible for the positive aging of such ETLs in QLEDs. [106] Schematic of resistive switching in ZnMgO in the vicinity of Al and ITO is shown in Figure 16a. According to Ding et al., mobility of oxygen vacancies in such metal oxides is high. [106] Consequently, when an electric bias was applied to ITO/ ZnMgO/Al, randomly distributed off-axis oxygen ions in the ZnMgO crystalline structure were released and migrated toward the anode. As a result, the density of oxygen vacancies increased, which formed a conductive filament through the ETL and enhanced the conductivity (low-resistive state or LRS) (Figure 16b). Under reverse bias, the migrated and accumulated oxygen ions were released and redistributed randomly, resulting in the high-resistive state (HRS) which lowered the conductivity.
A similar effect was observed in ZnMgO when used as the ETL in the vicinity of the QDs EML. The capacity of the electron transport, and consequently, the performance of the QLEDs were increased. Moreover, under an external bias, the off-lattice oxygen ions, which act as trap states, started to move toward the QDs EML (conductive filament mechanism). Active oxygen ions accumulated at the EML/ETL interface oxidized and consequently damaged the QDs EML. Figure 16c shows the schematic of the migration of oxygen ions and conductive filament (oxygen vacancies), which increased the electron transport through ZnMgO. The intrinsic resistive switching that acts on the positive aging of ZnMgO was confirmed by comparison with TPBi (an organic ETL) in a QLED. According to the luminance curves in an accelerated aging regime (Figure 16d), the positive aging was observed only in the device with ZnMgO, while the QLED with TPBi did not show any performance improvement, but rather just a fast negative aging.

Air Exposure of ZnMgO ETL
Chrzanowski et al. reported controlled (humid) air exposure of ZnO-based ETLs as a suitable approach to improve the charge balance in their QLEDs. [108] The authors investigated the effect of water and oxygen on the carrier density in the ETL. It was observed that exposure of the ZnMgO ETL to humid air at room temperature resulted in reduced hole leakage and improved charge balance in the device. Figure 17a illustrates the QLED device structure under investigation. While it had been previously found that oxidation of Al at the ETL/cathode interface can improve the electron injection, [109] Chrzanowski et al. reported that the exposure to ambient air, both before and after Al deposition, improved the charge balance and current density.
www.advancedsciencenews.com www.adpr-journal.com In their study, air exposure of the ETL led to oxygen adsorption as well as passivation of ZnMgO NP surface defects through termination with -OH groups (from chemisorbed water). On the one hand, the passivation of surface defects suppressed the hole leakage current, whereas the adsorbed oxygen acted as an electron scavenger. This indeed led to inhibition of the electron injection and consequently reduction of the electron current, resulting in better charge balance. This was additionally confirmed using unipolar devices. An EOD with ZnMgO ETL exposed to air resulted in lower electron mobility than its unexposed counterpart. When compared with the hole currents from a HOD, it was evident that exposure to air led to more closely matched mobilities in a relevant range of driving voltages. Figure 17c,d shows the device parameter plots for a G-QLED completely fabricated under N 2 compared to the device exposed to humid air (RH of 30% to 60%) before cathode deposition. The improved EQE, power efficiency, and brightness level were indeed attributed to the improvement of charge balance and the leakage current reduction. Z. Chen et al. investigated the influence of the adsorption of oxygen and water molecules on ZnMgO and the performance of the inverted QLED devices. [110] They reported that the ambient H 2 O adsorbed on the ZnMgO surface resulted in high conductivity, while it degraded the QLED efficiency. Annealing in an H 2 O-free glove-box was proposed to preserve the efficiency. Moreover, the authors stated that oxygen adsorption on the ZnMgO surface captures the free electrons. Exposing the ETL film to UV irradiation led to enhancement of the driving current.
In a different work by D. Chen et al., moreover, it was suggested that thiol ligand exchange on the ZnO surface can suppress the negative effect of the O 2 and H 2 O adsorption on the electrical conductivity. [97] According to Chrzanowski et al., [108] lifetime measurements revealed that the air-exposed devices could exhibit muchimproved operational stability (Figure 17e). Two different regimes were identified: Repeating scans under 3 V resulted in reproducible performance, but scanning up to 6 V quickly irreversibly degraded the device performance ( Figure 17f ). The authors further explained that being the major bottleneck to the device stability, this was likely related to the electrochemical degradation of the HTL, which occurred when scanning up to 6 V. They also found signs of electrochemical degradation of the QDs through decreased PL lifetime, possibly related to unstable ligands (Figure 17g). Finally, the authors mention that, while all these results were promising, the use of humid air exposure of ZnMgO might not be a sustainable method to improve performance. This is due to the difficulty in precise controlling of the humidity level and exposure time of the films.

Ethanolamine (EA) Passivation of ZnO Solution
Gradual shelf coarsening of ZnO NPs in colloidal solutions is known to be one of the aging mechanisms that can deteriorate the device performance when used in QLEDs. [111] The coarsening process is controlled by diffusion mechanisms that are accelerated at elevated temperatures. [112] It has been shown that Reproduced with permission. [106] Copyright 2020, American Institute of Physics.
www.advancedsciencenews.com www.adpr-journal.com ethanolamine (EA) stabilization of ZnO NPs can prevent coarsening of ZnO NPs. [113] Noh et al. reported that adding small amounts of ethanolamine to the ZnO solution can improve the colloidal stability of the ZnO NPs. [114] NP coarsening and increased surface roughness result in light scattering and loss of transparency in colloidal solutions. The authors showed that there was an evident loss of transparency in the ZnO NP solutions kept at 4°C and at room temperature after 1-30 days of storage, which appeared to be even more severe at higher temperatures (Figure 18a-c). When EA was instead added to the solution, due to the smaller size of the NPs, a very clear solution was obtained, in contrast to the semiopaque ZnO solution without EA ligands after a month of storage (Figure 18d). Another indication of NP coarsening is the passivation of the green PL emission typically originating from surface defects of ZnO NPs, due to aggregation and formation of bulk-like ZnO. [115] The authors observed that, under UV illumination, the PL intensity was higher in the solution treated with EA, which indicated that the aggregation was effectively suppressed. [114] According to the absorbance spectra and Tauc plots, due to the reduced quantum confinement effect, the stored ZnO NPs showed significantly smaller bandgaps after 21 days (Figure 18f,g), which was attributed to the coarsening. These changes were not observed in the EA-treated solution (Figure 18h,i). NP coarsening and surface roughness progression also result in higher resistivity after aging the ETL, as also reported in other studies. [114,116] As mentioned earlier, it is known that the suppressed electron injection can enhance the Figure 17. a) Device structure and b) the energy band configuration of G-QLEDs. Mechanism of the wet air molecules' adsorption on the ZnMgO ETL surface before aluminum deposition is shown. c) J-V-L and d) EQE and power efficiency versus luminance plots or the devices exposed to wet air in comparison with N 2 . e) G-QLED lifetime measured at different initial luminance for the device with and without exposure of ZnMgO to moist air. f ) Reproducible EQE measurements for 0-3.3 V and 0-6 V (full range) of the G-QLED, and g) the TRPL measurements of the device at 3 V, for 0, 10, and 30 h of aging times. The inset shows the PLQY for different aging times. Reproduced with permission. [108] Copyright 2021, American Chemical Society.
www.advancedsciencenews.com www.adpr-journal.com charge balance in QLEDs, improving the performance. This was consistent with the authors' results, where a QLED device with EA-treated ZnO showcased enhanced device parameters across the board (Figure 18k-n). The EA-treated devices exhibited improved current efficiency, thanks to the lower current density and higher luminance as well as improved operational and shelflife stabilities.

Chlorination of ZnMgO ETL
Chen et al. reported improved QLED device performance by chlorine (Cl) passivation of ZnO-based ETLs. [117] The authors added NaCl precursor to the reaction flask when synthesizing ZnMgO, the result of which was defect passivation of the ETL by Zn─Cl bond formation. The peak EQE after chlorination was improved from 1.54% to 2.99% for a device with a ZnO ETL and from 3% to 4.05% for a device with a ZnMgO ETL. These enhancements were mostly attributed to an increase in the current efficiency. The Cl passivation shifted the ZnMgO CBM level upward, increasing the energy barrier between the Al cathode and the ETL. This inhibited the electron injection from the cathode to the ETL, lowering the electron currents and achieving better charge balance in the device. Moreover, the elevated CBM level also resulted in more facilitated electron injection from the ETL into the QDs EML. [117] Figure 19a shows the QLED device performance with different ETLs. For the chlorinated ZnMgO, the current density and luminescence were lower than those for the other ETLs, but the improved charge balance in the device with the chlorinated ETL led both to the best current efficiency and EQE (Figure 19b,c). Changing the Cl concentration was found to have a minimal effect on the turn-on voltage, while the current density below 3.4 V was slightly higher when using the lower amounts of Cl. A Cl amount of 0.3 mmol increased the peak luminance compared with the higher and lower concentrations used (0.1 and 0.6 m mol, Figure 19d). The peak EQE was also higher for 0.3 m mol Cl (4.05%) and decreased to 3.72% and 3.21% for 0.1 and 0.6 m mol Cl concentrations, respectively. Therefore, the Cl-passivated ETL with an optimal concentration of Cl ions enhanced the QLED device performance, but the excess Cl ions acted as Figure 18. Effect of aging time and temperature on the ZnO solution stability. ZnO kept at 4°C (left) and at room temperature (right) for a) 1 day, b) 7 days, and c) 30 days. d) ZnO solution without (left) and with EA (right) under ambient light, and e) under UV illumination. f ) Absorbance spectra and g) Tauc plots for ZnO NPs without EA stabilization, stored at room temperature over time. h) Absorbance spectra and i) Tauc plots for the EA-treated ZnO solution. j) J-V curves, k) luminance versus voltage, and l) current efficiency versus current density plots for the R-QLEDs. The relative luminance versus voltage plot for the m) QLED device operated over 30 days, and n) the same device under a constant bias, with and without ethanolamine treatment of ZnO. Reproduced with permission. [114] Copyright 2019, Elsevier.

Conclusion and Outlook
In summary, exploiting the full advantage of tailored ZnO-based ETLs has emerged as the best strategy for making efficient and stable conventional and inverted solution-processed QLEDs. Various ZnO NP structural modification methods and ZnO-ETL surface passivation strategies were reviewed. The reported modifications offer efficient charge injection together with minimal EML/ETL interfacial exciton quenching. Since their first introduction to the QLED technology, the evolution of ZnO-ETL modification methods is summarized in Figure 20.
Chronologically, early works showed that the size reduction of ZnO NPs, either by controlling the synthesis temperature or O-H concentration, can lead to bandgap widening and prevention of spontaneous charge back-transfer from the QDs EML to the ZnO ETL. Moreover, the conductivity of the QLED device was controlled by tuning the ETL NP size, which improved the current density and efficiency of the device. As a result, the   Interface-passivating materials were then employed as interlayers to mitigate the unwanted interfacial charge back-transfer at the EML/ETL interface in QLEDs with ZnO ETLs. In the very first investigation of this kind, an ultrathin PMMA was initially used as a single insulating interlayer. Later, other materials such as (semiconducting) PVK polymer and (insulating) Al 2 O 3 were also employed. These materials were studied as ultrathin single or alternating interlayers to help with the suppression of the interfacial exciton quenching without negatively impacting the charge transport in QLEDs. However, provided that this approach has not yet been tried in industry-level manufacturing, it seems that fabrication of such extremely thin interlayers would not be achievable with most large-scale solution-processing techniques (e.g., roll-to-roll printing). On the other hand, large-scale fabrication of inorganic interlayers such as Al 2 O 3 will require thermal evaporation that is not compatible with high-throughput printing. Therefore, the use of ultrathin interface-passivating interlayers might not be easily accessible for future developments of practical QLED-based devices.
In recent years, an effective approach for mitigating the interfacial exciton quenching and improving the efficiency has been to dope ZnO NPs with metals such as Mg, Li, Al, and Ga. Metal doping of ZnO NPs can help to control the electron injection via upshifting the CBM in ZnO-based ETLs. It also decreases the density of oxygen vacancies, reduces the number of O─H bonds (i.e., exciton quenching sites), and fills the interstitial charge trapping sites. More specifically, doping ZnO with Li, Al, Mg, and Ga gives rise to improved charge balance and suppression of the interfacial exciton quenching due to the ETL surface defect passivation. Furthermore, it has been demonstrated that using a composite ETL consisting of ZnO/ZnMgO provides high electron mobility and upshifts the CBM level even more, compared with the individual ZnMgO. Additionally, it has been shown that blending ZnMgO with pure ZnO can facilitate the charge transfer and block the quenching sites. Moreover, incorporation of the insulating PVP polymer into (pure or metal-doped) ZnO ETLs can greatly improve the device efficiency by further passivating the surface defects, controlling the electron leakage, and modification of the film morphology and compactness. Therefore, as also discussed in the following, compared with all the other available methods and due to facile fabrication and better ETL aerobic stability, blending metal-doped ZnO with PVP seems to be the most promising approach for future large-scale manufacturing of QLEDs.
Other methods of defect passivation are adding ethanolamine (EA) to colloidal ZnO solutions after or chlorination of ZnO NPs during the synthesis process. Aside from improving the charge balance by controlling the electron injection, EA prevents ZnO NPs from coarsening and aggregation and elongates the QLED device lifetime. Alternative defect passivation methods such as surface treatment of the ZnO ETL during the fabrication process can also be used to improve the device performance. For instance, ethanedithiol (EDT) increases the hydrophobicity of the surface, reduces the hydroxyl ligands (i.e., exciton quenching sites), and improves the ambient stability of the ZnO NP colloidal solution. However, large-scale printing of QLED-based devices will require use of nontoxic chemicals and solvents, which may cast doubt on the feasibility of some of these in situ (e.g., chlorination) and thin-film post-treatment (e.g., EDT) methods. Aerobic stability of the "post-treated" ETLs is also yet to be explored, which is necessary in most printing technologies. Therefore, future investigations should focus on nontoxic and "green" methods for post-treatment of ZnO ETLs and their ambient stability.
Aside from the aforementioned approaches, it has been found that exposure of ZnO ETL to humid air leads to adsorption of oxygen that can trap the excess electrons injected from the ZnO ETL. Moreover, air exposure of the ZnO ETL passivates the OH groups at the EML/ETL interface and mitigates the interfacial exciton quenching.
Importantly, obtaining high efficiencies from QLEDs without sufficient device operational and shelf-life stability will leave these electroluminescent devices impractical for electrically driven lighting and display applications. As mentioned in Section 3.6, only one previous work has exclusively highlighted the impact of ZnO ETL characteristics on the device shelf-life. Therefore, further experimental explorations are still required to better understand the role of ZnO ETLs on the degradation mechanisms and negative aging over long-term device storage. For instance, studies of shelf-life stability with metal-doped ZnO NPs especially in conjunction with PVP blending are still missing. As mentioned earlier, this is important because charge injection, charge balance, interfacial exciton quenching, and ZnO ETL aerobic stability could be promisingly controlled better with metal-doped ZnO:PVP configurations.
Aside from ZnO NPs, various ETL nanomaterials have been studied recently for their high stability and superior QLED device performances. For example, Z. Chen et al. used SnO 2 NPs as ETL as an alternative to ZnO. [118] According to their study, due to its high conductivity, high transparency, and high stability, SnO 2 could be used as a multifunctional ETL in a variety of architectures such as inverted, noninverted, transparent, and topemitting QLEDs. In addition, M. Chen et al. studied the stabilization of SnO 2 ETL in the QLED with tetramethylammonium hydroxide (TMAH) that prevented the agglomeration of the SnO 2 NPs and increased the operational lifetime and EL efficiency of their QLEDs. [119] Feng et al. reported that the UVozone treatment of the SnO 2 NPs significantly increased the operational lifetime of their devices with preserved efficiency. [120] Apart from the metal oxide materials, organic materials with tunable electronic band structures have also been used as ETLs. Yang et al. introduced triazine-cored organic ETL, 2,4,6-tris(3 0 -(pyridin-3-yl) biphenyl-3-yl)-1,3,5-triazine (TmPPPyTz), to enhance the performance of their R-QLEDs. [121] By tuning the lowest unoccupied molecular orbital (LUMO) level of TmPPPyTz, the authors obtained state-of-art brightness, EQE, diminished PL quenching at the EML/ETL interface, and colorsaturated red EL. Further investigations are still required to explore the advantages and disadvantages of these ETL materials in comparison with ZnO ETLs in QLEDs especially from the device operational stability and shelf-life perspectives.