Solution-Processed Double-Junction Quantum-Dot Light-Emitting

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Solution-Processed Double-Junction Quantum-Dot Light-Emitting Diodes with an EQE of Over 40% Piaoyang Shen,† Fan Cao,† Haoran Wang,† Bin Wei,† Feijiu Wang,‡ Xiao Wei Sun,*,§ and Xuyong Yang*,†

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/02/19. For personal use only.

†

Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai 200072, China ‡ Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan § Department of Electrical and Electronic Engineering, Southern University of Science and Technology, 1088 Xue-Yuan Road, Shenzhen 518055, China S Supporting Information *

ABSTRACT: Despite the rapid development in quantum-dot light-emitting diodes (QD-LEDs) with a single junction, it remains a big challenge to make tandem QD-LEDs with high performance. Here, we report solution-processed doublejunction tandem QD-LEDs with a high external quantum efficiency of 42.2% and a high current efficiency of 183.3 cd A−1, which are comparable to those of the best vacuumdeposited tandem organic LEDs. Such high-efficiency devices are achieved by interface engineering of fully optimized single light-emitting units, which improves carriers’ transport/injection balance and suppresses exciton quenching induced by ZnO, and design of an effective interconnecting layer consisting of poly(4-butylphenyl-diphenylamine) (poly-TPD)-mixed poly(9-vinylcarbazole) (PVK)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/polyethylenimine ethoxylated-modified ZnO. KEYWORDS: quantum dots, tandem structure, light-emitting diodes, electroluminescence, interfacial engineering

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INTRODUCTION Light-emitting devices employing quantum dots (QD-LEDs) as emitters have recently attracted considerable attention for lighting and display applications thanks to their many advantageous properties including tunable emission spectra, saturated color, wide color gamut, and low fabrication cost.1−9 With the rapid development of high-quality QD materials and LED architectures, the performance has been significantly improved since the first QD-LED was demonstrated.10−15 For example, the highly efficient red-emitting QD-LEDs with an over 20% external quantum efficiency (EQE) were realized by improving the carrier balance in devices and suppressing the photoluminescence (PL) quenching of QDs caused by the contacted zinc oxide (ZnO) electron-transporting layer (ETL).16 Nevertheless, the overall performance of the current QD-LEDs is still lower than that of commercially available organic LEDs (OLEDs), especially for tandem ones.17−19 The tandem device structure by stacking two or more lightemitting units is an effective approach to realize highperformance LEDs, which has widely been utilized in commercial OLEDs for lighting and display applications.20−22 However, significant challenges need to be overcome for tandem QD-LEDs: (i) a solvent strategy to protect the underlying functional layers from damage during multiple solution coatings, (ii) perfect interface contact of the interconnecting layer (ICL) with emitting QDs, and (iii) © XXXX American Chemical Society

efficient generation of electrons and holes at the ICL to better balance the carriers injected into the sub-LED unit. Recently, Zhang et al. reported a highly efficient (an EQE of 27.6%) tandem QD-LED with an ICL structure of ZnMgO/Al/ 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile/MoO3 fabricated by a combined vacuum thermal deposition and solutionprocess method.23 Nevertheless, the progress of tandem QDLEDs still lags behind compared with those with mature technologies for tandem OLED devices, primarily attributed to the easy damage of functional films in the fabrication of tandem QD-LED devices caused by multiple solvent treatments. In this work, we demonstrate a 42.2% EQE tandem QDLED consisting of two green light-emitting units fabricated by utilizing an all-solution process. The key elements to obtain such high-performance double-junction tandem QD-LEDs are as follows: (i) the high-quality solution-processed fabrication of multilayer functional films; (ii) interface engineering for achieving efficient light-emitting units by introducing a polyethylenimine ethoxylated (PEIE) ultrathin layer between the QDs emissive layer (EML) and the top ZnO ETL to better balance carrier injection/transport and reduce the metal oxideReceived: October 29, 2018 Accepted: December 19, 2018 Published: December 19, 2018 A

DOI: 10.1021/acsami.8b18940 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of the layers in the device structure (ITO/PEIE-modified ZnO/QDs/PVK/PEDOT:PSS/Al) and the corresponding cross-sectional TEM images. The thicknesses of solution-processed functional layers correspond to those in our optimized QD-LEDs. (b) Energy level diagram of the device. (c) Valence band edge regions of ZnO NPs and ZnO/PEIE films. (d) Ecut‑off region of the pure ZnO NP film and PEIEmodified ZnO NP films, respectively. (e) J−V curves of the resulting single-carrier devices.

larger than the hole mobility (2.5 × 10−6 cm2 V−1 s−1) of PVK, which leads to an unbalanced carrier transport/injection within the LED device. These cause a carrier injection imbalance and thus limit the device’s performance. The PEIE interlayer deposited on the ZnO ETL has an insulating nature due to its wide bandgap (6.2 eV)26 and can upshift the conduction band minimum (CBM) of ZnO, which causes a large energy barrier for electron injection. Therefore, the excess electrons into EML are effectively blocked and more balanced carrier injection is obtained. The upshifted energy level can be attributed to the PEIE with neutral amine groups having a high dipole moment, resulting in the decreased work function (WF) for ZnO ETL because of the formation of interface dipoles.27 We used ultraviolet photoelectron spectroscopy to analyze the effects of PEIE on the energy levels of ZnO (Figure 1c,d). The secondary electron cut-off (Ecut‑off) of ZnO can be increased from 17.69 to 18.03 eV by increasing the PEIE concentration (0−0.8 wt %), and the WFs are reduced from 3.53 to 3.19 eV (WF = 21.22 eV − Ecut‑off). Consequently, the CBM of ZnO modified with PEIE is upshifted from 3.79 to 2.86 eV, as calculated by VBMs and the bandgap of materials obtained from their absorption spectra (Supporting Information, Figure S1). The improved charge balance in QD-LED devices was further confirmed by the fabrication of single-carrier devices. The current density for the single-electron device (ITO/ZnO/PEIE/QDs/Al) is obviously decreased and very close to the density values of hole injection after PEIE (0.4 wt %) is deposited on the ZnO ETL compared with that of the electron-only device without PEIE (Figure 1e). In addition to the significantly improved electrical properties of ZnO ETL, the deposition of a PEIE ultrathin layer on QDs also plays another role in avoiding severe PL quenching of QD EML caused by the conductive ZnO NPs. Currently, the

induced exciton quenching; (iii) the effective ICL of poly(4butylphenyl-diphenylamine) (poly-TPD)-doped poly(9-vinylcarbazole) (PVK)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/PEIE-modified ZnO for better connecting with QD-LED units, which allows more efficient charge generation. The outstanding electroluminescence (EL) performance and low fabrication cost potential of the resulting tandem QD-LEDs are critical steps toward the practical application of the electrically driven quantum dot display technology.

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RESULTS AND DISCUSSION Figure 1a shows the schematic device architecture and its corresponding cross-sectional transmission electron microscopic (TEM) image of our solution-processed single-junction QD-LEDs with the inverted structure consisting of the indium tin oxide (ITO) cathode, PEIE-modified ZnO nanoparticle (NP) ETL, CdSe/ZnS QDs EML, PVK hole-transporting layer (HTL), PEDOT:PSS with the addition of the isopropanol (IPA) hole-injection layer, and the top Al anode. The inverted structure is advantageous for the OLEDs/QD-LEDs in display application, because the bottom ITO transparent cathode can be conveniently integrated onto n-type thin-film transistor (TFT) backplanes.24,25 PVK is employed as the HTL because of its deep highest occupied molecular orbital level (∼5.7 eV), which facilitates holes’ injection into the QDs EML. PEIE is used as a surface modifier layer to adjust the electron transport/injection properties. Note that injecting electrons into the QDs layer is easier owing to the smaller energy barrier in the conduction bands of ZnO ETL and QDs EML compared with hole injection from the corresponding device energy level diagram depicted in Figure 1b. In addition, the electron mobility (∼1.8 × 10−3 cm2 V−1 s−1) of ZnO is much B


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decrease in current injection owing to the blocking of excess electrons. The resulting QD-LED with the 0.4 wt % PEIE concentration exhibits the best performance (Supporting Information, Figure S2), with a high luminance of up to 30 730 cd m−2, more than 2-fold brightness enhancement compared with the reference QD-LED (13 660 cd m−2). Besides, the turn-on voltage (VT) of 4 V for the QD-LED device with PEIE is also close to that of the reference device. The current efficiency−EQE−L (CE−EQE−L) curves of the device are depicted in Figure 3d,e. With the deposition of the PEIE thin layer, the efficiency of the QD-LED is drastically improved, with a peak CE of 94.3 cd A−1 corresponding to a 21.9% EQE, 6-fold higher than that of the device without PEIE (3.8% and 16.5 cd A−1). This is the highest efficiency value reported for the single-junction QD-LEDs (Supporting Information, Table S1). In addition, the stability of the QDLED with PEIE is also improved drastically, which benefits from the better electron−hole balance. As shown in Figure 3f, after a continuous current driving test, the resulting QD-LED with PEIE displays about 3-fold lifetime enhancement under the same initial luminance (100 cd m−2). Different types of HTLs were also investigated to optimize hole injection for enhancing the device performance. Poly(9,9dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine) (TFB), poly-TPD, and PVK are the widely used HTLs for fabricating solution-processed QD-LED devices,22,34−36 and a series of QD-LEDs with these three kinds of HTLs are fabricated in the present work. The PVK-based QD-LED shows a higher luminance and efficiency than poly-TPD-based QD-LEDs (max. L: 17 450, peak EQE: 3.2%) and TFB-based QD-LEDs (max. L: 9427, peak EQE: 2.7%) (Figure 4a,b). This is mainly attributed to the better wetting ability of PEDOT:PSS with the addition of IPA on the underlying PVK film (Supporting Information, Figure S3). However, the QD-LED with poly-TPD HTL exhibits a lower VT (∼2 V) and more efficient current injection (Figure 4c) because of the higher hole mobility (10−4 cm2 V−1 s−1) of poly-TPD compared with that (2.5 × 10−6 cm2 V−1 s−1) of PVK. It is worth noting that a low turn-on voltage is an important prerequisite for effective integration with TFT to obtain highperformance active-matrix QD-LEDs. Owing to the strong dependence of hole-injection on the HTLs used, we further improved the LED performance by mixing a small amount of poly-TPD with a higher hole mobility in PVK HTL. The maximum EQE for the resulting device reaches 22.2% when 5 wt % poly-TPD is added into PVK HTL, and the VT is simultaneously decreased to 3.5 V. Meanwhile, it was found that the turn-voltage can be continuously decreased to 2.5 V with an increasing ratio of poly-TPD in PVK HTL to 40 wt %, which is accompanied by a decreased EQE. The tandem QD-LEDs with two identical subunits are fabricated by an all-solution process after the single-junction QD-LEDs are fully optimized, in which a poly-TPD (5 wt %)-mixed PVK/PEDOT:PSS/PEIE-modified ZnO heterojunction is designed as the ICL. One of the main challenges for making solution-processed tandem QD-LED devices with a high performance is the easy damage of the underlying layers when depositing overlayers for the multilayer thin-film device fabrication. In our case, all the functional layers in the resulting tandem device are clearly observed in the cross-sectional TEM and element mapping images (Figures 5a and S4), and the total number of solution-processed layers reaches 10. The

highest efficiencies reported for all the red (R)-, green (G)-, and blue (B)-colored QD-LEDs adopted ZnO NPs as the ETL.16,28,29 However, the PL emission of QDs can be quenched by metal-oxides, which is mainly attributed to the ultrafast nonradiative process originating from inefficient trion emission.30,31 In our case, this is confirmed by the PL emission and PL lifetime increase of the QD films after the PEIE interlayer is inserted between the QDs and ZnO layer (Figure 2). The PL emission intensity as well as PL average lifetime are

Figure 2. PL emission intensity (a) and PL lifetime (b) of the QDs, QDs deposited on ZnO NPs without PEIE, and QDs deposited on ZnO NPs with different concentrations of PEIE.

drastically decreased when the QD films are directly in contact with ZnO NPs. With the insertion of a PEIE interlayer, the PL emission is obviously improved, and the corresponding average PL lifetime of QDs is also increased from 4.05 to 5.9 ns as the deposition concentration of PEIE reaches 0.8 wt %, suggesting that the PEIE layer can lead to the charge neutrality of QDs and thus preserve the high luminescent efficiency of QDs. Figure 3a presents the electroluminescence (EL) spectrum of the single-junction QD-LEDs with 0.4 wt % PEIE. The pure green emission of QDs (peaking at ∼534 nm) with a relatively narrow full width at half maximum of ∼28 nm is observed, illustrating an efficient exciton recombination within the QD EML. The EL spectrum is broadened and red-shifted about 6 nm compared with the PL emission peak of the QD solution, which is originated from a combined effect of the Förster resonant energy transfer within QDs and the Stark effect dependent on the external electric field.32,33 The inset displays a color-saturated, uniform and very bright green emission recorded at 300 cd m−2 from the operating device with an area of 2 mm × 2 mm. Figure 3b,c shows the current density (J)− voltage (V) and luminance (L)−voltage (V) curves of the green QD-LEDs with and without PEIE. We can see that the insertion of the PEIE thin interlayer results in a drastic C


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Figure 3. (a) PL and EL spectra of QDs in devices. (b) J−V, (c) L−V, (d) current efficiency (CE)−L, and (e) EQE−L curves of the resulting QDLED. (f) Operating lifetime characteristics of devices. The devices encapsulated with ultraviolet curing epoxy are measured in air.

Figure 4. (a) L−V, (b) EQE−L, and (c) J−V curves of the resulting QD-LEDs with different HTLs. (d) Turn-on voltage and EQE dependence on the ratio of poly-TPD in PVK.

is no obvious difference in the EL emission spectra of singlejunction and double-junction QD-LEDs, the important parameters of the double-junction tandem device are almost 2-fold as compared with the single-junction device. The J−L− V characteristics of the resulting tandem devices show a high brightness of 40 180 cd m−2 and a relatively low VT of ∼8 V (Figure 5d). Furthermore, the resulting tandem device yields a record efficiency in QD-LEDs, with an ultrahigh EQE of up to 42.2% corresponding to a CE of 183.3 cd A−1 (Figure 5e). After the maximum EQE value, the device efficiency is gradually decreased with further increasing applied voltage (the

device energy band diagram of the double-junction tandem devices is depicted in Figure 5b. Our ICL can well connect with the two sub-LED units and efficiently generate carriers that are subsequently injected into the subunits under voltage driving. The symmetric J−V curve for the charge generation and injection of the ICL-only device indicates the efficient carrier generation in our solution-processed ICL (Supporting Information, Figure S5).19 Besides, in the visible spectrum range, the ICL also has a high transmittance of >90%, which is very beneficial for enhancing light outcoupling (Supporting Information, Figure S6). As shown in Figure 5c, although there D


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Figure 5. (a) Device architecture of the resulting double-junction tandem device and its corresponding TEM image. (b) Device energy level diagram. (c) EL spectra of single- and double-junction QD-LEDs. J−L−V (d) and CE−EQE−L (e) curves of the tandem QD-LED.

current density is about 100 mA cm −2 ) (Supporting Information, Figure S7). Meanwhile, the operational stability of the tandem QD-LED is also enhanced more than 2 times compared with that of the single QD-LED (Supporting Information, Figure S8).

Xuyong Yang: 0000-0003-3597-1491 Author Contributions

P.S., F.C., and H.W. fabricated and characterized the doublejunction QD-LEDs. P.S., F.C., H.W., B.W., F.W., X.W.S., and X.Y. analyzed and discussed the experimental results. All authors contributed to the manuscript.

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CONCLUSIONS In summary, we have demonstrated the best-performing tandem QD-LEDs by an all-solution process, with a high EQE of 42.2% and a luminance of over 40 000 cd m−2. The excellent EL performance of the tandem QD-LEDs is attributed to the more balanced carrier injection/transport, suppressed emission quenching of QDs, and effective ICL with an efficient charge generation capability and controlled electrical properties. The efficiency of our devices presented here is the highest reported in QD-LEDs. We believe that these results would offer an important step for the realization of high-efficiency solution-based QD-LEDs.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from the National Natural Science Foundation of China (Nos. 61605109, 51675322, and 61735004).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18940. Experiment details, TEM, X-ray diffraction, adsorption spectra of QDs and ZnO nanoparticles, EL characteristics of tandem QD-LEDs with different PEIE concentrations, contact angle measurement of films, histogram of peak EQEs of devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.S.). *E-mail: [email protected] (X.Y.). ORCID

Bin Wei: 0000-0003-4157-2737 Feijiu Wang: 0000-0003-3139-1086 E


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Solution-Processed Double-Junction Quantum-Dot Light-Emitting

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