Highly Efficient and Fully Solution-Processed Inverted Light-Emitting

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Highly Efficient and Fully Solution-Processed Inverted Light Emitting Diodes with Charge Control Interlayers Yan Fu, Wei Jiang, Daekyoung Kim, Woosuk Lee, and Heeyeop Chae ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05092 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Materials & Interfaces

Highly Efficient and Fully Solution-Processed Inverted Light Emitting Diodes with Charge Control Interlayers

Yan Fu,

†

†, §

Wei Jiang,§ Daekyoung Kim,# Woosuk Lee,§ and Heeyeop Chae§, #, *

College of Materials Science and Engineering, Jilin Institute of Chemical Technology, Jilin, 132022, P. R. China

§

School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea

#

Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea

E-mail: Prof. H. Chae [email protected]

Keywords: quantum-dot light-emitting diode, inverted device, sandwich QD structure, all solution processing, electron blocking layer, charge balance

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ABSTRACT In this work, we developed charge control sandwich structure around QD layers for the inverted QLEDs, the performance of which is shown to exceed that of the conventional QLEDs in terms of the external quantum efficiency (EQE) and the current efficiency (CE). The QD light emitting layer(EML) is sandwiched with two ultrathin interfacial layers: one is is a poly(9-vinlycarbazole) (PVK) layer to prevent excess electrons and the other is a polyethylenimine ethoxylated (PEIE) layer to reduce hole injection barrier. The sandwich structure resolves the imbalance between injected holes and electrons, and brings the level of balanced charge carriers to a maximum. We demonstrated the highly improved performance of 89.8 cd/A of current efficiency, 22.4 % of external quantum efficiency and 72,814 cd m-2 of maximum brightness with the solution-processed inverted QLED. This sandwich structure (PVK/QD/PEIE), as a framework, can be applied to various QLED devices for enhancing performance.

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INTRODUCTION Quantum-dot light-emitting diodes (QLEDs) are a competitive alternative to organic light-emitting diodes (OLEDs) and a potential candidate for next generation display and solidstate lighting owing to their unique features, such as good color purity, widely tunable emission, and simple solution-based processing.1-7 The rapid advance and growth in QLED research have led to extraordinary progresses in the quantum efficiency of the device since the introduction of the first QLEDs8. The evolvement of QLEDs involved optimizing the chemical synthesis of QDs,9-14 enhancing the photoluminescence quantum yield (PL QY), and designing proper device architectures15-18 to bring the external quantum efficiency (EQE) to its maximum. Inverted device structure rather than the conventional structure is preferred for future QLEDs because the transparent indium tin oxide (ITO) as the bottom cathode can be directly connected to an n-type thin film transistor (TFT)19 that is in general faster than a p-type TFT. Furthermore, unlike the conventional structure, the first spin-cast layer of ZnO nanoparticles (NPs) on ITO can go through a high temperature annealing without damaging other layers. With regard to fabrication, solution processing is preferred because of its cost-efficiency and easy processability.20-21 Therefore, inverted QLEDs that are fabricated solely by solution processing are considered here. Considering that ZnO NPs made significant contribution in QLED development as an effective electron transport layer22 (ETL), it is not surprising that extensive efforts have been made for the modification of ZnO and extra interlayers to control electron transport effectively.

They

include

inserting

a

thin

insulating

polymer,

poly[(9,9-bis(30-

(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene(PFN)23, and utilizing Cesium Carbonate (CsCO3) doped aluminum-zinc-oxide (AZO)24-25, polyethylenimine (PEI)

3



or polethylenimine ethoxylated (PEIE)26-27 modified surface of ZnO, Al doped ZnO (AZO)/Li doped ZnO (LZO)28, Mg doped ZnO (ZnMgO)29-30, and Ga doped ZnO31. Much more important than the modification of ZnO for the device efficiency, it has been found, is the balancing between injected holes and electrons that form excitons at the interfaces of ETL/emitting layer (EML) and of hole transport layer (HTL)/EML. Peng et al., for example, reported achieving an EQE of 20.5% by inserting an insulating layer of poly (methyl methacrylate) (PMMA) between ETL and EML to block excess electron overflow, and a thin layer of poly(9-vinlycarbazole) (PVK) between HTL and EML to make up for insufficient hole injection.32 This approach, however, is not suitable for solution-processed inverted QLEDs, because the QD emission layer (EML) is easily damaged during subsequent organic hole transport layer (HTL) deposition.33 It inevitably causes a serious rough and nonuniform morphology of the surface of QD EML, resulting in a substantial leakage current in the device. Zhang et al.34, for another, used double Al2O3 interlayers as interfacial modifiers on both sides of EML ( ETL/Al2O3/EML/Al2O3/HTL), resulting in an increase of the current efficiency from 11.3 cd/A to 15.3 cd/A. The alumina layer on the ETL side suppressed the exciton quenching induced by ZnO and reduced leakage current. We formally introduce here a sandwiched QD layer as an interfaced QD EML for fully solution-processed inverted QLEDs, in light of the substantial benefits that can be accrued from the double interfacial layers around the EML. On the strength of significant efficiency improvement that was attained with the introduction of a thin polymeric surface modifier of polyethylenimine ethoxylated (PEIE) between QD EML and HTL in our previous study,35 PEIE was adopted as a material for the interface modification layer (IML) on the HTL side of the QD EML. The introduction of the sandwiched QD layer is shown to remarkably enhance the performance of QLEDs. A material of PVK for the electron side interfacial layer needs to be chosen for the proposed sandwich structure of QD emission layer (EML) now that the 4


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material of PEIE has already been chosen on the basis of our prior work, for the interfacial modification layer (IML) on the hole side.

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RESULTS AND DISCUSSION To block excess electron overflow from ETL to EML and to reach a balance between injected electrons and holes, an insulator could be used as was attempted32 for the conventional device structure. However, a good physical interface between ZnO nanoparticles and QDs is required for good devices. Poly(9-vinlycarbazole) (PVK) is one of the most frequently used hole transport materials (HTM) in QLEDs due to strong hole transport properties and weak electron transport properties (see Figure S1 for the absorption and PL spectra of PVK solution), which allows better control of charge transfer compared to other insulating materials. The material has also been used as a matrix for blending with QDs. Hole injection efficiency36-37 has been found enhanced when QDs were mixed with PVK in a conventional device structure. Improved charge transport balance and suppressed photoluminescence quenching were observed for an inverted device structure when PVK was mixed with QDs physically or bonded with QDs through a ligand.38 These results indicate that PVK is quite compatible with QDs. Furthermore, it has quite high an energy barrier of around 1.8 eV against electron flow (Figure 3a). Therefore, PVK was used as the electron side IML for the sandwiched QD layer in this work. To assess the interface quality with PVK inserted between ETL and QD EML, the surface roughness of the PVK/ZnO layer was measured by atomic force microscopy (AFM) for various PVK thicknesses coated on the ZnO films. High surface roughness is known to lead to trapping of excitons.28,30,39 With the inserted PVK, the root mean square roughness (RMS) of ZnO film was reduced from 0.77 nm to 0.40 nm (Figure 1). This result indicates that an ultrathin interlayer of PVK makes the surface of ZnO film smooth and provides a flat film morphology that is suitable for subsequent QD EML coating in this inverted device structure. 6


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To examine the role that the interfacial layers of PVK and PEIE play in this sandwiched QD layer, photoluminescence (PL) was measured individually for both sides of the interfaced QD EML. As shown in Figure S2a, the maximum PL intensity of QD/P-TPD layers increased by more than a factor of 2 after inserting PEIE IML because the hydrophilic surface preserves the integrity of the EML. Moreover, the contact angle of QD layer and QD/PEIE layer was measured (Figure S3) and the result showed that the contact angle decreased from 111º to 51º after depositing PEIE layer on QD layer. Meanwhile, on the other side of the interfaced QD EML, the PL intensity of ZnO/QD layers increased by a factor of 1.3 upon inserting the interfacial layer of PVK (Figure S2b). However, the PL intensity of ZnO/QD layers is lower than that of QD layer only, indicating that there are strong exciton quenching sites at the interface between the metal oxide and the EML due to OH-bond or surface defect of ZnO film.28, 30, 39 Therefore, we believe that inserting PVK IML has the effect of reducing the quenching sites. To further examine the effects of the inserted PVK interlayer on emission of QD, we investigated steady-state and time-resolved PL (TRPL) of the multilayer with and without the PVK interfacial layer (Glass/ ZnO/ w or w/o PVK/ QD/ PEIE/ P-TPD) (Figure 2). The steadystate PL (Figure 2a) shows that a 1.2 times increase in PL results when the PVK layer is inserted (red vs. black line), which is similar to the results in Figure S2b. The fluorescence decay curves (Figure 2b) were fitted by a bi-exponential equation. According to the fitting results, the average exciton decay time becomes longer with the insertion of the interfacial layer (7.2 ns vs. 6.4 ns). The longer exciton decay time indicates that the PVK layer effectively suppresses exciton quenching induced by metal oxide nanoparticles (NPs). With these background investigations in hand, we fabricated an inverted device with the sandwiched QD layer (see Supporting Information for all solution-processed fabrication 7



procedure). The inverted device structure comprises indium tin oxide (ITO) cathode / ZnO NPs / poly(9-vinlycarbazole) (PVK) / QDs / PEIE / poly (N,N9-bis(4-butylphenyl)-N,N9bis(phenyl)-benzidine) (poly-TPD) / MoOx / Al anode, where ZnO NPs constitute the ETL, CdSe@ZnS/ZnS QDs the EML, poly-TPD the HTL, and MoOx the hole injection layer (HIL). The flat-band energy level diagram of an inverted QLED with sandwich structure of quantum dot layer (PVK/QD/PEIE) is shown on Figure 3a. The absorption and PL spectra of the green CdSe@ZnS/ZnS gradient-composition QDs are shown in Figure S4a. According to the transmission electron microscopy (TEM) and HRTEM (inset) images in Figure S4b, the average diameter of the QDs is 13 ± 0.55 nm. Absolute PL quantum yield of the QDs solution is approximately 90%. Energy-dispersive X-ray spectroscopy (EDS) spectra of the QDs are shown in Figure S4c. The interfacial layer of PEIE is not readily distinguishable in the crosssectional TEM image of the device in Figure 3b, but the integrity of the QD layer indicates the existence of PEIE layer; The ultrathin interfacial layer of PVK can be found as a straight line filled in the gap the interface layer of ZnO/QD. The sandwiched QD EML has an ultrathin PVK layer on the electron transport side of the QD layer which blocks excess electron injection from ETL to the EML, and an ultrathin PEIE layer on the hole side which facilitates injection of holes from HTL to the EML. For optimal device performance, the thicknesses of these two layers have to be selected so as to balance the injected charge carriers at the highest level possible for radiative recombination. The thicknesses of the interfacial layers were varied for an optimal set and they were found to be 7.5 nm for the hole side IML of PEIE and 6 nm for the electron side IML of PVK. Figure 4a shows the optical-electrical characteristics of the optimal device along with the same device without the PVK layer for comparison. A significant reduction in leakage current density is observed in the figure when PVK is introduced. Although PVK is a conductive 8


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polymer, the turn-on voltage (luminescence ≥ 1 cd/m2) is increased from 4.0 V to 5.75 V with the presence of PVK because it hampers the flow of electrons with its electron injection barrier. More specifically, electron injection should overcome a large potential energy barrier (1.8 eV) due to PVK having the lowest unoccupied molecular orbital (LUMO) about -2.2 eV and ZnO NPs possessing conduction band (CB) about −4.0 eV (Figure 3a). This relatively high turn-on voltage could be lowered in due time with a more suitable sandwich structure. Figure 4b shows the external quantum efficiency and the current efficiency (CE) as a function of the luminance of the devices. The maximum CE and EQE of the device with the optimal PVK thickness of 6 nm reach 89.8 cd/A and 22.4%, respectively, which are more than two times the efficiencies of the device without PVK layer. These efficiencies are the highest ever reported in the literature.5,

32, 35, 40-42

The optimal PVK thickness is a compromise

between a thin layer causing insufficient electron blocking and a thick layer leading to insufficient election injection. The results obtained experimentally with electron-only and hole-only devices43-45 (Figure S5) support that the positive and negative charge carriers are well balanced at the optimal PVK thickness. It is noted that the lowest surface roughness on ZnO ETL was achieved when the PVK layer was 6 nm. Extremely good external quantum efficiency up to 20.5% was achieved32 as early as 2014 and the EQE reached 21% recently40 with a maximum CE of 80 cd/A for the QLEDs with the conventional device structure. In contrast, the performance of QLEDs with the inverted structure has been far behind compared to the QLEDs with the conventional structure. Only recently, did the EQE reach 15% level for the inverted device.35 The EQE of 22.4% achieved here, up to 24% as shown shortly, is a significant jump from the 15% level for the inverted device, outpacing the EQE of the conventional device for the first time.

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The exceptional EQE of 22.4% at 0.41 mA/cm2 and brightness of 370 cd/m2 with a CE of almost 90 cd/A does not roll-off46 much over a wide range of luminance. In fact, as apparent from Figure 4b, the quantum efficiency is maintained above 19% for the luminance range between 60 cd/m2 and 5,600 cd/m2. The roll-off in efficiency with increasing luminance is more relevant to practical applications than the roll-off with increasing current density, and the luminance when the efficiency is reduced to 90% of its maximum ranges from 1,000 to 10,000 cd/m2 for the state of the art OLEDs.46 The fact that this value is 5,600 cd/m2, closer to the high end of 10,000 cd/m2, for the inverted QLED with the sandwiched quantum EML bodes well for high power applications. The slightly higher CE and EQE are observed without PVK when the luminance over 10,000 cd/m2. This is attributed to the increased carrier accumulation at higher voltage, which probably induces narrow exciton recombination region at interface layer of PVK/QD and leads to Joule heating and exciton-exciton annihilation. The electroluminescence spectrum of the inverted QLED at 9V is given in Figure 4c. The green at 525 nm has a full width at half maximum of 20 nm. The circle shown in the inset corresponds to Commission Internationale de l’Eclairage (CIE) (1931) color coordinates of (0.13, 0.79), which represents a saturated green emission ideal for display applications. The fully solution processed inverted QLED can be fabricated with good reproducibility. Shown in Figure 4d is a histogram for the maximum EQEs obtained for 30 devices tested. The average is 21% ± 3%, indicating good reproducibility, particularly considering that the devices were taken out of glove box for aluminum deposition and then again for device measurements.

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CONCLUSIONS In summary, we have put forward the sandwich structure as a framework for the quantum dot layer of QLEDs. Introduction of this structure to an all-solution processed inverted QLED has been shown to lead to the best ever performance in terms of EQE and CE. The performance of QLEDs of inverted device structure has been far behind that of the conventional QLEDs. Better performance of the inverted device obtained here than that of the conventional device bodes well for its use in conjunction with n-type transistors that are faster than the p-type. The sandwich structure is a framework. As such, it could be framed in various forms for different purposes with selected materials and the functionalities that can be provided by the functional groups attached for enhancing the performance of QLEDs in general. ASSOCIATED CONTENT Supporting Information Methods and all experimental details are available in Supporting Information. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H.C) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS 11



This work was financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2015R1A2A2A01003520). This work was also supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2012M3A6A7054855).

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Polyethylenimine

Ethoxylated-Mediated

All-Solution-Processed

High-Performance

Flexible Inverted Quantum Dot-Light-Emitting Device. ACS Nano 2017, 11 (2), 1982-1990. (36) Wang, L.; Chen, T.; Lin, Q.; Shen, H.; Wang, A.; Wang, H.; Li, C.; Li, L. S. Highperformance Azure Blue Quantum Dot Light-Emitting Diodes via Doping PVK in Emitting Layer. Org. Electron. 2016, 37, 280-286. (37) Dong, D.; Wu, W. J.; Lian, L.; Feng, D. X.; Su, Y. Z.; Li, W. W.; He, G. F. Enhanced Performances of Quantum Dot Light-Emitting Diodes with Doped Emitting Layers by Manipulating the Charge Carrier Balance. J. Mater. Chem. C 2017, 5 (20), 5018-5023. (38) Fokina, A.; Lee, Y.; Chang, J. H.; Park, M.; Sung, Y.; Bae, W. K.; Char, K.; Lee, C.; Zentel, R. The Role of Emission Layer Morphology on the Enhanced Performance of LightEmitting Diodes Based on Quantum Dot-Semiconducting Polymer Hybrids. Adv. Mater. Interfaces 2016, 3 (18), 1600279. (39) Liu, S.; Ho, S.; Chen, Y.; So, F. Passivation of Metal Oxide Surfaces for Highperformance Organic and Hybrid Optoelectronic Devices. Chem. Mater. 2015, 27 (7), 25322539.

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(40) Titov, A.; Acharya, K.; Wang, C.; Hyvonen, J.; Tokarz, J.; Holloway, P. H. 6- 3: Quantum Dot LEDs: Problems & Prospects, SID Symposium Digest of Technical Papers, 2017, 48 (1), 58-60. (41) Mashford, B. S.; Stevenson, M.; Popovic, Z.; Hamilton, C.; Zhou, Z.; Breen, C.; Steckel, J.; Bulovic, V.; Bawendi, M.; Coe-Sullivan, S. High-Efficiency Quantum-Dot Light-Emitting Devices with Enhanced Charge Injection. Nat. Photonics 2013, 7 (5), 407-412. (42) Li, Z.; Hu, Y.; Shen, H.; Lin, Q.; Wang, L.; Wang, H.; Zhao, W.; Li, L. S. Efficient and Long- life Green Light- emitting Diodes Comprising Tridentate Thiol Capped Quantum Dots. Laser & Photonics Reviews 2017, 11 (1), 1600227. (43) Li, J.; Xu, L.; Wang, T.; Song, J.; Chen, J.; Xue, J.; Dong, Y.; Cai, B.; Shan, Q.; Han, B. 50-Fold EQE Improvement up to 6.27% of Solution-Processed All-Inorganic Perovskite CsPbBr3 QLEDs via Surface Ligand Density Control. Adv. Mater. 2017, 29 (5), 1603885. (44) Li, J.; Jin, H.; Wang, K.; Xie, D.; Xu, D.; Xu, X.; Xu, G. High Luminance of CuInS2Based Yellow Quantum Dot Light Emitting Diodes Fabricated by All-solution Processing. Rsc. Adv. 2016, 6 (76), 72462-72470. (45) Shen, H. B.; Cao, W. R.; Shewmon, N. T.; Yang, C. C.; Li, L. S.; Xue, J. G. HighEfficiency, Low Turn-on Voltage Blue-Violet Quantum-Dot-Based Light-Emitting Diodes. Nano Lett. 2015, 15 (2), 1211-1216. (46) Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll- off in Organic Light- Emitting Diodes. Adv. Mater. 2013, 25 (47), 6801-6827.

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Figure 1. AFM surface images of

ZnO/PVK film: (a) without PVK, (b) ZnO with 3 nm

PVK, (c) ZnO with 6 nm PVK, (d) ZnO with 11 nm PVK, and (e) ZnO with 13 nm PVK (RMS: root mean square (height)). 18


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(a) PL intensity (a.u.)

Glass/ZnO/QD/PEIE/P-TPD Glass/ZnO/PVK/QD/PEIE/P-TPD

450

475

500

525

550

575

600

Wavelength (nm) 0

10

(b) Normalized PL Intensity

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


Glass/ZnO/QD/PEIE/P-TPD Glass/ZnO/PVK/QD/PEIE/P-TPD

-1

10

-2

10

-3

10

0

400

800

1200

1600

2000

Time (ns)

Figure

2.

(a)

Steady-state

PL

spectra

of

Glass/ZnO/QD/PEIE/P-TPD

and

Glass/ZnO/PVK/QD/PEIE/P-TPD, (b) Time-resolved PL of Glass/ZnO/QD/PEIE/P-TPD and Glass/ZnO/PVK/QD/PEIE/P-TPD.

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Figure 3. (a) Flat-band energy level diagram of an inverted QLED with sandwich structure of quantum dot layer (PVK/QD/PEIE). (b) Cross-sectional transmission electron microscopy image of an inverted QLED.

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8

1

10

7

10

6

10

-1

2

2

Current Density (mA/cm )

10

w/o PVK 3 nm PVK 6 nm PVK 11 nm PVK 13 nm PVK

5

10

10

4

10

-3

10

3

10 -5

10

Luminance (cd/m )

3

(a) 10

2

10

1

-7

10

10

0

10

-9

10

0

1

2

3

4

5

6

7

8

9

10

11

Voltage (V)

3

10

2

10

1

10

0

10

-1

10

4

10

3

10

2

10

1

10

w/o PVK 3 nm PVK 6 nm PVK 11 nm PVK 13 nm PVK

-2

10

-3

10

-4

10

0

10

1

10

2

10

3

4

10

10

0

10

-1

10

-2

5

10

External Quantum Efficiency (%)

5

(b)10 Current Efficiency (cd/A)

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


10

2

Luminance (cd/m )

Figure 4. Device performance of an inverted QLED with different interfacial layer thickness for the sandwich structure of quantum dot layer: (a) current density–voltage–luminance (J–V– L) characteristics (b) Current efficiency and external quantum efficiency characteristics. (c) EL spectrum of inverted QLED collected at 9V. (Insert figure: corresponding CIE (1931) color coordinates (0.13, 0.79) marked with a circle. (d) Distribution of EQE for 30 QLEDs fabricated.

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The Table of Contents Entry

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Highly Efficient and Fully Solution-Processed Inverted Light-Emitting

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