Polymer as an Additive in the Emitting Layer for High-Performance

Article

Polymer as an Additive in the Emitting Layer for HighPerformance Quantum Dot Light-Emitting Diodes Feng Liang, Yuan Liu, Yun Hu, YingLi Shi, Yuqiang Liu, ZhaoKui Wang, Xuedong Wang, Baoquan Sun, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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ACS Applied Materials & Interfaces

Polymer as an Additive in the Emitting Layer for High-Performance Quantum Dot Light-Emitting Diodes

Feng Liang, Yuan Liu, Yun Hu, Ying-Li Shi, Yu-Qiang Liu, Zhao-Kui Wang, Xue-Dong Wang, Bao-Quan Sun and Liang-Sheng Liao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China

*

Address correspondence to [email protected] (L. S. Liao)

KEYWORDS: quantum dots; light-emitting diodes; additive; charge balance; morphology

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ABSTRACT: A facile but effective method is proposed to improve the performance of quantum dot light-emitting diodes (QLEDs) by incorporating a polymer, poly (9-vinlycarbazole) (PVK), as an additive into the CdSe/CdS/ZnS quantum dots (QDs) emitting layer (EML). It is found that the charge balance of the device with the PVK-added EML was greatly improved. In addition, the film morphology of the hole-transporting layer (HTL) which is adjacent to the EML is substantially improved. The surface roughness of the HTL is reduced from 5.87 nm to 1.38 nm, which promises a good contact between the HTL and the EML resulting in low leakage current. With the improved charge balance and morphology, a maximum external quantum efficiency (EQE) of 16.8% corresponding to the current efficiency of 19.0 cd/A is achievable in the red QLEDs. The EQE is 1.6 times as high as that (10.5%) of the reference QLED comprising a pure QDs EML. This work demonstrates that incorporating some polymer molecules into the QD EML as additives could be a facile route toward high-performance QLEDs.


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INTRODUCTION

Colloidal quantum dot light-emitting diodes (QLEDs) have attracted great attention owing to their outstanding properties, such as tunable colors, saturated emission, narrow emission linewidth, inherent photophysical stability, and facile fabrication process, which highlight their potential in the next generation flat-panel display and the solid-state lighting technologies.1-7 Since the first QLEDs were demonstrated,1 there have been intensive studies on the improvement of the device performance by investigation of both quantum dots (QDs) materials8-15 and device structures16-20 as well as by fundamental understanding on the device physics,21-26 such as how to balance charge injection in the devices.

If a QLED has balanced charge injection in the emission layer (EML), high luminous efficiency can be achieveable in the device. However, it is a great challenge to fabricate QLEDs with balanced charge injection to realize balanced electron-hole recombination. For examples, in terms of the QDs materials in the EML, II-VI semiconductors QDs (such as CdSe) are widely utilized due to their high photoluminescence quantum yield (PLQY). In order to maintain the high PLQY, the QDs usually contain an inorganic semiconductor core/shell (CdSe/CdS/ZnS) and an organic ligand (oleic acid). The ligands are able to assist the growth and stabilization of QDs, to passivate surface atoms of the QDs, and to disperse QDs in organic solvents. However, fatty acids of the ligand with long chains would affect the electrical properties of the QD layer, leading to unbalanced hole and electron conduction. On the other hand, in terms of device structure, although some materials, such as ZnO nanoparticles, with high electron mobility and suitable energy levels, are usually 3 ACS Paragon Plus Environment


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adopted as an electron-transporting layer (ETL), hole-transporting layer (HTL) in the devices still suffers from a series of problems. These problems include large injection barrier, high density surface-trap states, and poor contact at the interface between the HTL and the QD EML. This will also result in unbalanced charge injection and low quantum efficiency.

Great efforts have been put to improve charge injection balance either by QD material modification27 or by device architecture innovation.28-30 For instances, Jin and Peng et al. demonstrated polystyrene

a

forward

device

structure,

ITO/poly(3,4-ethylenedioxythiophene)

sulfonate

(PEDOT:PSS)/poly

(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)-benzidine (Poly-TPD)/PVK/QDs/PMMA/ZnO/Ag,

by

inserting

an

insulating

poly(methylmethacrylate) (PMMA) layer between the ZnO ETL and QDs EML to slow down electron transport aiming to achieve improved charge balance. As a result, a prominent current efficiency of 17.5 cd/A for a red QLED has been achieved.19 Ji, Zhang,

and Zhao et al. introduced organic material N,N’-dicarbazolyl-3,5-benzene (mCP) with a deep highest-occupied-molecular-orbital (HOMO) energy level (-6.1 eV) as a stepwise HTL in their inverted device structure, ITO/ZnO/QDs/mCP/CBP/MoO3/Al, to enhance the hole injection and confine charge carrier in QDs, and achieved high current efficiency of 16 cd/A for a red QLEDs.30 The aforementioned examples demonstrated that either slowing down electron transport in the ETL or enhancing hole transport in the HTL could effectively improve the electron-hole balance in the devices. Rationally, there would be an alternative way to achieve improved electron-hole recombination balance within the EML 4 ACS Paragon Plus Environment

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by mixing a hole-transporting material in the layer. Ma and the co-workers doped CdSe/CdS QDs into PVK layer and achieved light emissions from both CdSe/CdS QDs and the PVK in their devices with a forward structure of ITO/(PEDOT:PSS)/PVK:QDs/Al.31 Although the purpose of the work was to achieve white light emission (with the current efficiency of 0.21 cd/A) from the QLEDs by doping a small amount of QDs into the matrix of PVK, the combination of QDs and PVK indicated that it may be possible to control the charge injection balance by adding a small amount of PVK into the QDs EML. Li and Shen et al. doped PVK into the QDs EML in their forward structure, ITO/PEDOT:PSS/ poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine](TFB)/QDs:PVK/ZnO/ Al, and indeed achieved improved charge injection balance in the EML resulting in blue emission with high current efficiency of 6.41 cd/A corresponding to the external quantum efficiency (EQE) of 8.76%.32 Therefore, it clearly indicates that balancing the charge injection in the QDs EML could play a critical role in fabricating high-performance QLED devices.

In order to achieve higher efficiency from QLEDs, we have been fabricating our devices with

an

inverted

structure

of

ITO/ZnO/QDs/4,4-bis(carbazole-9-yl)biphenyl

(CBP)/MoO3/Al. However, unlike the forward structure, in the inverted structure, not only unbalanced charge injection in the EML but also QDs induced rough surface morphology of the HTL are the major problems we have to deal with. Therefore, searching for a practical way to improve both charge injection balance in the EML and surface morphology of the HTL in the inverted structure is critically important to realize high-performance QLEDs. Here, we report that incorporating a small amount of polymer such as PVK as an 5 ACS Paragon Plus Environment


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additive, instead of a large portion as previously reported, into the QDs EML, both charge injection in the EML and morphology of the HTL adjacent to the EML can be simultaneously improved. As confirmed by the hole/electron-only devices, adding a small amount of PVK (5 wt%) into the QDs EMLs can facilitate the hole injection/transport and suppress the electron transport resulting in improved charge injection balance as well as enhanced electron-hole recombination. Moreover, adding PVK in the EML, the surface roughness of the HTL adjacent to the EML can be dramatically reduced from 5.87 nm to 1.38 nm, which would also be beneficial to the reduction of the trap states at the EML/HTL interface resulting in improved efficiency. With balanced charge injection and smooth surface, a maximum current efficiency of 19 cd/A corresponding to the EQE of 16.8% has been achieved successfully. The EQE is 1.6-fold as high as that of the control device. In addition, blue-, green-, and red-emitting devices are all demonstrated, which indicate the potentials of this method in fabricating high-performance QLEDs.

RESULTS AND DISCUSSION

A schematic diagram and the corresponding cross-sectional scanning electron microscopy (SEM) image of the inverted multilayer red-emitting QLED device are shown in Figures 1a and 1b, respectively. The multilayer structure consists of the patterned ITO as the cathode, zinc oxide (ZnO) nanocrystals film (80 nm) as the electron-injecting layer (EIL)/electron-transporting layer (ETL), cadmium selenide (CdSe)/cadmium sulfide (CdS)/zinc sulfide-alloyed (ZnS)/oleic acid (core/shell/ligand) QDs as the EML (~25 nm), 4,4-bis(carbazole-9-yl)biphenyl (CBP) layer as the hole-transporting layer (HTL, 60 nm), 6 ACS Paragon Plus Environment

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molybdenum trioxide (MoO3) as the hole-injecting layer (HIL, 5 nm), and Al (120 nm) as the anode. The PLQY of QDs is 85%. Meanwhile, the PL average lifetime of these red-emitting QDs is about 22.9 ns (as shown in Figure S1a). These outstanding optical characteristics indicate that QDs are capable of forming an efficient EML in the QLEDs. Figure 1c shows the absorption and the photoluminescence (PL) spectra of the red-emitting CdSe/CdS/ZnS core shell QDs dispersed in toluene at room temperature. The PL band has its maximum peak at 624 nm with narrow full width at half-maximum (FWHM) of 32 nm. From the transmission electron microscope (TEM) of QDs shown in Figure 1d, the QDs have a uniform size distribution, with an average diameter of 7.6 nm (Figure S1b). The quantum dots with the uniform size will lead to the symmetric PL band in our study. Furthermore, the high-resolution transmission electron microscopy (HRTEM) image (the inset of Figure 1d) exhibits the lattice fringes (interplanar distances of 1.34 and 1.51 Å) throughout the whole QD, which proves considerably high crystallinity of these CdSe/CdS/ZnS core shell QDs.

Figures 2a and 2b show the schematic of the energy level diagrams of the multilayers in the QLED devices based on a pristine QDs EML and a PVK-added QDs EML. The energy levels for the red-emitting QDs and ZnO were obtained by the ultraviolet photoelectron spectroscopy (UPS) and the optical measurements (Figure S2, Figure S3 and Figure S4, Supporting Information). The bandgap of the red-emitting QD in Figure 2 was estimated from the UV-Vis absorption cutoff of the red-emitting QD. Other energy level values were taken from the literatures.33-35 At the ITO (cathode) side, the ZnO nanocrystals layer act as the ETL. The suitable conduction/valence bands can simultaneously lead to 7 ACS Paragon Plus Environment


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both efficient electron-injection and efficient hole-blocking in the QLEDs. At the aluminum electrode (anode) side, the holes can be efficiently injected into the HTL through the MoO3/CBP interface, and then availably transported to the CBP/QDs interfaces due to the high hole mobility of the CBP.36 Since the energy barrier at the CBP/QDs interface (0.4 eV) is not small enough, holes intended to accumulate at the interface with difficulty to inject into the EML. On the other hand, the electron mobility of the QDs in the EML is greater than their hole-mobility. This is because that the electron wave function has the overlap with the shell region whereas the hole wave function are only confined to the CdSe core, which is consistent with the previously reported charge mobility of the core/shell QDs.19 As a result, electrons could inject or transport in the EML more easily than holes causing the unbalanced charge recombination within the layer. To confirm this hypothesis, a hole-only device based on pristine QDs EML with structure of ITO/PEDOT: PSS (~20 nm)/QDs (~ 25 nm)/CBP (60 nm)/MoO3 (5 nm)/Al and an electron-only device with structure of ITO/ZnO (80 nm)/QDs (25 nm)/Al were fabricated. Each of the layers in the hole/electron-only devices has an identical thickness to its corresponding layer in our fabricated QLEDs. As shown in Figure 2c, the current density of the electron-only device based on pristine QDs EML is much larger than that of the hole-only device based on pristine QDs EML. Therefore, it would be understandable that the unbalanced electrons and holes in the device will cause unbalanced electron-hole recombination as well as the other negative effects such as polaron-exciton quenching. On the other hand, since the current density of the hole-only device based on the pristine QDs EML is almost two orders of magnitude lower than that of hole-only device based on PVK layer (Figure S6), by adding 8 ACS Paragon Plus Environment

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PVK into the QDs EML, the bulk hole conduction of the QDs EML will certainly be enhanced. It is indeed clearly shown in Figure 2c that PVK as an additive in the QDs EML simultaneously facilitates the hole injection/transport and suppresses the electron transport in the layer. Therefore, the charge balance based on PVK-added QDs EML is greatly improved. These results reveal that the charge balance and the location of the recombination zone in the device can be modulated by incorporating PVK into the QDs EML.

Furthermore, it is found that the QDs EML on the substrate has nonnegligible influence on the film morphology of the vacuum deposited HTL adjacent to the EML. Figure 3 displays the contact angle and the atomic force microscopy (AFM) measurements of these EMLs and the CBP layers deposited on the corresponding EMLs. Impressively, Figures 3a and 3b show the typical shapes of the water droplets on the pristine QDs EML and the PVK-added EML, respectively. The contact angle for water on neat QDs films is 92.2°, which is much larger than that of the 5 wt% PVK-added EML (45.7°). Besides, the contact angles for 1 wt% PVK-added EML, 9 wt% PVK-added EML, 13 wt% PVK-added EML, and a pure PVK layer are 71.2°, 46.7°, 74.0°, and 86.3°, respectively, as shown in Figure S8 (the lowest contact angle corresponds to the 5 wt% PVK-added EML). The Cassie's model demonstrates that the liquid drop does not fill the crevices of the rough surface but sits on a composite surface consisting of the solid material and air.37 The linear and flexible organic polymer PVK can interact with the QDs film and change the surface structure of the QDs EML. As a result, the aggregation behavior of the CBP molecules deposited on the top of the QDs EML will be affected accordingly. As compared with the 9 ACS Paragon Plus Environment


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pristine QDs EML, the 5 wt% PVK-added EML has the highest surface energy among the PVK-added EML. Thus, it would form the best film morphology of the HTL in our fabricated QLEDs.

Figures 3c-f show the AFM images of QDs film, PVK-added QDs film, QDs/CBP film and PVK-added QDs/CBP film, respectively. Both pristine QDs and PVK-added QDs EML films show similar roughness. The root-mean-square (RMS) surface roughnesses of pristine and PVK-added QDs films are 1.17 nm and 0.95 nm, respectively. However, the morphology of the CBP layer changed dramatically. Without PVK in the QDs EML, the RMS surface roughness of the CBP film formed on the pristine QDs EML is as high as 5.87 nm. By adding PVK into the QDs EML, the RMS surface roughness is reduced to 1.38 nm, which promises a good contact between the EML and the HTL. This good contact formed on the smooth EML surface could facilitate hole-injection at this interface and prevent leakage current in the QLEDs. The CBP layer with large roughness (5.87 nm) formed on the pristine QDs can be attributed to the aggregation of the CBP molecules during the thermal evaporation, which is also not beneficial for device lifetime as we reported before.38 In a short conclusion, adding a small amount of PVK into the QDs EML can simultaneously improve both the charge balance in the EML and the morphology of the HTL in the QLEDs.

We fabricated the red-emitting QLEDs based on both the PVK-added EML and the pristine QDs EML for comparison. As shown in Figure 4a, firstly, when the drive voltage is lower than the turn-on voltage (~2 V), for instance, at 1 V, the current density look like 10 ACS Paragon Plus Environment

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inconsistently fluctuated with increasing its doping concentration. Actually, the current density of the devices would reflect the micro electrical shorts (a kind of leakage current) in the devices in some extent, which is closely related to surface roughness in the devices. Shown in Figure S9, since the CBP layers in the devices with pristine QD EML and with 1.0 wt% PVK-added EML have the highest RMS values (5.87 nm and 4.55 nm, respectively), the leakage currents are the highest among the four devices. The CBP layer in the device with 13 wt% PVK-added EML has higher RMS value (2.59 nm) than that of the device with 5 wt% PVK-added EML (1.38 nm), the leakage current is higher than that of the device with 5 wt% PVK-added EML accordingly. Therefore, the QLED based on 5 wt% PVK-added EML possesses the lowest leakage current due to its lowest RMS. Secondly, with increased PVK concentration in the QDs EML (the drive voltage of 2-7 V), the current density decreases (at the same drive voltage) correspondingly, which could be ascribed to the reduced electron injection in the EML. Meanwhile, as seen from current densities-voltage curves, it is found that the turn-on voltage is almost negligibly affected for devices with and without PVK, since these devices based on the pristine QDs, and PVK-added EMLs show the almost same value of about 2.0 V (Table S1). Figures 4b and 4c

demonstrate

the

current

density-luminance

(J-L)

characteristics,

current

efficiency-luminance-EQE properties of these devices. As summarized in Table 1, by adding PVK into the QDs EML, the performance of the device is improved. It reaches the highest efficiency in the 5 wt% PVK-added EML based QLEDs, with a maximum current efficiency of 19.0 cd/A and EQE of 16.8%. The EQE is 1.6 fold as high as that of the reference device (pristine QDs) with a maximum current efficiency of 12.5 cd/A and EQE 11 ACS Paragon Plus Environment


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of 10.5%. Further increasing the ratio (13 wt%) of the PVK, the QLED device exhibits increased drive voltage and the deceased brightness. Figure 4d presents the normalized EQE of the QLEDs based on the pristine QDs EML and the 5 wt% PVK-added QDs EML. It can be seen that the 5 wt% QDs EML based QLEDs show low efficiency roll-off than that of the pristine QDs based QLEDs, which can be ascribed to improved charge balance in the EML. The lifetimes of the QLED devices based on the pristine QDs EML and the 5 wt% PVK-added QDs EML were tested during the continual operation at a constant current density corresponding to an initial luminance (L0) of 2000 cd/m2. The control QLEDs were fabricated with the same QDs and characterized under the same conditions (i.e., encapsulation, initial brightness, humidity and temperature). As shown in Figure S10, the lifetimes for the devices with and without PVK at luminance of 100 cd/m2 is estimated to be 119 h and 1004 h, respectively. The EL spectra obtained at 5 mA/cm2 of these QLED devices are shown in Figure 4e. All of them have their maxima around 632 nm with a FWHM of 38 nm. Impressively, the photograph of our fabricated QLED based on the 5 wt% PVK-added EML driven at the current density of 5 mA/cm2 is demonstrated as the bottom inset of Figure 4e. What’s more, the EL spectra of QLEDs based on pristine QDs and the 5 wt% PVK-added EMLs are shown at half-exponential coordinates in Figure 4f. There are no EL signals from 380 nm to 550 nm at a high current density of 20 mA/cm2, indicating that the PVK itself hasn’t emitted photons. Photoluminescence spectrum of PVK dispersed in toluene is shown in Figure S5. Thus, the EL behaviors of QLED devices based on PVK-added are not affected by incorporating PVK into the EMLs. Therefore, it can be concluded that the performance of the QLEDs can be substantially enhanced by improving 12 ACS Paragon Plus Environment

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the charge balance and the film morphology of the HTL in the QLEDs based on the PVK-added EMLs.

Using the optimized device structure, RGB-emitting QLEDs based on red, green and blue QDs were also fabricated. As shown in Figure S11, these RGB-emitting devices based on the 5 wt% PVK-added EMLs show the maximum EQE of 16.8% for red QLED devices, 10.0% for green QLED devices, and 4.0% for blue QLED devices, respectively. For comparison, these reference devices (pristine RGB-emitting QDs) show the maximum EQE of 10.5% for red QLEDs, 7.5% for green QLEDs, and 2.4% for blue QLEDs. It is noted that these RGB-emitting QLED devices based on 5 wt% PVK-added EMLs show enhanced efficiency performances. Therefore, this facile method by adding PVK into the EML contributes to the enhanced performance in these red-emitting, green-emitting, and blue-emitting QLEDs. The efficiency performances of the reference devices with and without PVK additive were summarized in Table S1. In our case, green and blue QLED devices were fabricated using the same device structure as the red QLED device, without further optimizing the green and blue device structure with matched CTL. The green and blue QDs inherently possess a larger bandgap than the red QDs according to the normalized UV-Vis spectra of these RGB QDs (Figure S3). Thus a larger potential energy barrier at green/blue QDs and CTL interfaces increase the difficulty of charge injection, leading to a low EQE. Taking into account of the effects of device structure and surface ligands on efficiency performances, it can be optimized by developing new efficient QDs and new device structure in the near future.



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Impressively, Figure 5 exhibits the operated large-area flexible RGB-emitting devices with the active area of 2.0×2.0 cm2 driven at the applied voltage of 5.0 V and the EL spectra of these devices. Blue, green and red QLEDs show peak emission at 460, 528, and 632 nm with the FMHW of 38, 33, and 22 nm, respectively. The Commission Internationale de l’Eclairage (CIE) color coordinates of each color are (0.69, 0.31), (0.21, 0.74), and (0.14, 0.05), respectively. With such saturated emission, the color gamut is larger than that of National Television Systems Committee (NTSC) standard (~115% of NTSC 1931).

CONCLUSION

We have demonstrated that adding a small amount of polymer PVK into the QDs emission layer can simultaneously improve the charge balance in the EML and enhance the morphology of the HTL, which can contribute to the high-performance QLED. As is confirmed by the hole/electron-only devices, the PVK in the EMLs facilitates the hole injection/transport and suppresses the electron transport, leading to the optimized charge balance and the enhanced electron-hole recombination. Also, with the additive PVK, the roughness (RMS = 1.38 nm) of the obtained HTL is much lower than that (RMS = 5.87 nm) of the reference HTLs. Based on the PVK-added EMLs, we can achieve highly efficient, saturated RGB QLED devices comprising of multilayer structures. Impressively, the maximum current efficiency of 19 cd/A corresponding to the EQE of 16.8% for the red-emitting device was achieved in our fabricated devices based on the PVK-added EMLs. The EQE 16.8% is enhanced by 1.6-fold as compared with that (10.5%) of the reference 14 ACS Paragon Plus Environment

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QLED devices. Our results show a simple but efficient strategy toward high-performance QLEDs for the next generation display and solid-state lighting.



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EXPERIMENTAL DETAILS

Materials. The red, green, and blue CdSe/CdS/ZnS core–shell QDs were provided by Poly OptoElectronics Co., Ltd. PVK (average molecular weight, 25000-50000 g/mol) and zinc acetate hydrate (99.9%, powder) were purchased from Sigma-Aldrich. CBP and MoO3 were purchased from LumTec. Tetramethylammonium hydroxide (TMAH) (99.99%, powder), toluene, dichloromethane, n-butylalcohol, ethyl acetate, and dimethylsulphoxide (DMSO) were purchased from Alfa-Aesar. Ethanol (reagent grade, 99%) and acetone (reagent grade, 99%) were purchased from Sinopharm Chemical Reagents. All reagents were used as received without any further purification.

Synthesis of ZnO Nanoparticles. The ZnO nanoparticles (NPs) used in this study were synthesized through a sol-gel method. A solution of 0.1 M zinc acetate in dimethyl sulfoxide (DMSO) and 10 mL of 0.5 M tetramethylammonium hydroxide (TMAH) in ethanol were mixed and stirred for 1.0 h in ambient atmosphere. The prepared product was collected by centrifugation and then washed twice with methanol. The transparent precipitate was dispersed in n-butyl alcohol to form a ZnO NPs solution with a concentration of ~30 mg/ml. TEM image of ZnO NPs and AFM image of ZnO NPs film were shown in Figure S7.

Fabrication of QLED Devices. The conventional QLED devices and large-area flexible devices were fabricated on the indium tin oxide (ITO) coated glass and polyethylene terephthalate (PET) substrates, respectively, with a square resistance of ~15 Ω/square. All the substrates were ultrasonically cleaned in acetone, ethanol, deionized water, and 16 ACS Paragon Plus Environment

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isopropanol in sequence and followed by an ex situ UV ozone treatment in air for 5 min. The conventional QLED devices and large-area flexible devices were fabricated based on the same device structure. For conventional QLED devices, the ZnO nanoparticles were deposited onto the cleaned and UV ozone treated ITO substrates by spin-coating process from a 30 mg/ml ZnO butanol solution and then annealed at 120°C for 15 min in a glove box. Afterwards, the QDs were spin-coated onto ZnO layer QD toluene solution (10 mg/ml) at 2000 rpm and then annealed at 110 °C for 10 min in the same glove box. The thickness of ZnO and QD is around 80 nm and 25 nm, respectively, determined by SEM measurements. Finally, the HTL of CBP (60 nm), hole injection layer of MoO3 (5 nm), and anode of Al (120 nm) were thermally evaporated in high vacuum at pressure below 4 × 10-6 Torr. Finally, the devices were encapsulated using a cover glass with epoxy in a glove box. The active areas of QLEDs device were 3×3 and 20×20 mm2，respectively.

Characterizations. The cross-sectional SEM image of the multilayer of the inverted QLED device was characterized by a high-resolution scanning electron microscope (SEM) (Carl Zeiss, Supra 55). The absorption spectra were measured using the UV-Vis spectrophotometer (Lambda 750, PerkinElmer) and the PL spectra were measured using the Hitachi F-4600 fluorescence spectrophotometer. TEM image of core-shell QDs was recorded using Tecnai f20 electron microscope (U.S.A. FEI company). HRTEM image were recorded using Philips CM 200 electron microscope. Atomic force microscopy (AFM) images were obtained using a Veeco Multimode V instrument. Contact angle measurements were performed using a Dataphysics OCA 20 (U.S.A.) video-based optical goniometer with the sessile drop method. Energy levels were investigated by ultraviolet 17 ACS Paragon Plus Environment


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photoemission spectroscopy (UPS) in a Kratos AXIS Ultra-DLD ultrahigh vacuum photoemission spectroscopy system with HeI excitation (hν = 21.22 eV). The spectra of the devices were measured with Electroluminescence (EL), and current-voltage (I-V) characteristics were measured by a constant current source (Keithley 2400s Source Meter) combined with a photometer (Photo Research PR 655 spectrophotometer). The half-lifetime T50 was measured under the same conditions using an aging system made by Shanghai University. The EQE values were calculated by the ratio of the number of photons emitted by the device to the number of electrons injected.39 All device measurements were performed under air ambient conditions.

ASSOCIATED CONTENT

Supporting Information

Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

Time-resolved PL decay of R-QDs, Size distribution of R-QDs, UPS results and optical characteristics of R-, G-, B-QDs, and ZnO, PL spectrum of PVK film, Electrical measurements on the hole-only devices based on QDs and PVK, TEM image of ZnO, AFM image of ZnO NPs film, water droplet on EML with pristine QDs or various PVK added EML or PVK film substrate, AFM images of the evaporated CBP HTLs on various QDs EMLs, operational lifetime characteristics of R-QLEDs, device performance of the RGB QLEDs based on pristine QDs EML and 5.0 wt% PVK added EML.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes: The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS The authors acknowledge financial support from the National Key R&D Program of China (Grant No. 2016YFB0400700) and from the Natural Science Foundation of China (Grant No. 61575136). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology (Nano-CIC) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We thank Poly OptoElectronics Co., Ltd for supplying the QDs for use in this work. We thank Shui-Kai Lu for assistance with transmission electron microscopy experiments. We also thank Peng Wen for assistance with cross-sectional scanning electron microscopy experiments.



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Figures and Tables

Figure 1. (a) Schematic diagram of the multilayer QLED device. (b) Cross-sectional SEM image of the multilayer of the inverted QLED device. (c) Absorption (black circles line) and PL (red triangles line) spectra of CdSe/CdS/ZnS core-shell QDs. (d) TEM image of core-shell QDs with a scale bar of 20 nm. The inset shows the HRTEM image of one single QD with the scale bar of 2 nm.



a

b

-2

-2 -2.3

-2.6

-6.7 -7.4

-8 -9

PVK

-6 -7

-4.3

CBP

R-QDs

-5 -4.7

ZnO

-4.2-5.9

-6.3

Energy (eV)

-4.3

ITO -4.1

-5.9

-4.3

-5.8 -6.3 -6.7

-7.4

-8 -9

-9.7

-10

Al

MoO3

-7

-4

Al

MoO3

-6

R-QDs

ITO -4.1 -4.3 -5 -4.7 ZnO

Energy (eV)

-4

-2.6

-3

CBP

-3

-9.7

-10

c 10

1

-2

Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1

10

-3

10

-5

10

ITO/PEDOT:PSS/QDs/CBP/MoO3/Al -7

10

-9

10

0.1

ITO/PEDOT:PSS/QDs:PVK/CBP/MoO3/Al ITO/ZnO/QDs/Al ITO/ZnO/QDs: PVK/Al

Voltage (V)

1

2

3

Figure 2. Diagrams of energy level and exciton recombination zone in the devices. (a) Device based on pristine red-emitting QDs as active layer. (b) Optimized device based on PVK-added QDs as active layer. (c) Electrical measurements on the hole-only devices and the electron-only devices.


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Figure 3. (a) Photograph of the water droplet on the pristine QDs EL, and the corresponding contact angles (CA) is 92.2°. (b) Photograph of the water droplet on 5 wt% PVK-added EML, and the corresponding contact angles (CA) is 45.7°. (c, d) AFM images of pristine QDs, and QDs: 5 wt% PVK EMLs. (e, f) AFM images of the formed CBP HTL on the above-depicted QDs EML (in Figure 3c), and QDs: 5 wt% PVK (in Figure 3d) EML.



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Figure

4.

(a)

density-luminance

Current

density-voltage

(J-V)

(J-L)

characteristics,

(c)

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characteristics current

and

(b)

current

efficiency-luminance-EQE

characteristics (d) EQE/EQEmax-current density characteristics for QLEDs based on pristine QD and 5 wt% PVK-added QDs, (e) Normalized EL spectra of QLEDs based on different emitting layers. The under inset is the photograph of device at 5 mA/cm2. (f) EL spectra after the logarithm based on pristine QD and 5 wt% PVK-added QDs at a high current density (at 20 mA/cm2). The EL spectra are shown at half-exponential coordinates to clearly exhibit no EL emissions from PVK. 28 ACS Paragon Plus Environment

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Figure 5. (a-c) Photographic images of RGB large-area (2.0×2.0 cm2) flexible QLED devices. (d) Normalized EL spectra of RGB QLED devices. The inset shows the CIE color coordinates corresponding to our fabricated RGB QLED devices (solid lines) and National Television Systems Committee (NTSC) standard (dashed line).



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Table 1. Summaries of the device performance of QLEDs based on the pristine QDs and the PVK-added QDs EMLs

a)

Devices

λmax (nm)

Vturn-on (V)

Pristine QDs

628

1 wt%

ηC(max)/ ηC (1000)a) ηEQE(max)/ηEQE(1000)b) (cd/A)

(%)

1.9

12.5/7.2

10.5/6.0

632

2.0

13.0/10.2

11.5/9.0

5 wt%

632

2.0

19.0/13.2

16.8/11.6

13 wt%

632

2.1

12.5/8.7

11.0/7.7

Maximum current efficiency (ηc.max), and current efficiency (ηc.1000) measured at a

brightness of 1000 cd/m2; b)Maximum external quantum efficiency (ηEQE.max.), and EQE (ηEQE.1000) measured at 1000 cd/m2.


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ToC graph


Polymer as an Additive in the Emitting Layer for High-Performance

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