Improving Charge Injection via Blade Coating Molybdenum Oxide

Subscriber access provided by READING UNIV

Article

Improving Charge Injection via Blade Coating Molybdenum Oxide Layer: toward High Performance Large-area Quantum Dot Light-emitting Diodes Qunying Zeng, Zhongwei Xu, Congxiu Zheng, Yang Liu, Wei Chen, Tailiang Guo, Fushan Li, Chaoyu Xiang, Yixing Yang, Weiran Cao, Xiangwei Xie, Xiaolin Yan, Lei Qian, and Paul Holloway ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19333 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 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

ACS Applied Materials & Interfaces

Improving Charge Injection via Blade Coating Molybdenum Oxide Layer: toward High Performance Large-area Quantum Dot Light-emitting Diodes Qunying Zeng, Zhongwei Xu, Congxiu Zheng, Yang Liu, Wei Chen, Tailiang Guo and Fushan Li∗ Fuzhou University, Fuzhou, China Chaoyu Xiang, Yixing Yang, Weiran Cao, Xiangwei Xie, Xiaolin Yan and Lei Qian∗ TCL Corperate Research, Shenzhen, China Paul H. Holloway*

Department of Materials Science and Engineering, University of Florida, Gainesville, United States of America

Abstract:

A solution-processed molybdenum oxide (MoOx) as the hole injection layer by doctor-blade coating was developed to improve the efficiency and lifetime of red emitting quantum-dot light emitting diodes (QD-LEDs). It has been demonstrated that by adding isopropyl alcohol into the MoOx precursor during doctor-blade coating

Authors to whom correspondence should be addressed. Electronic mail: [email protected]; [email protected]; [email protected] 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Page 2 of 31

process, the morphology, composition, and the surface electronic structure of MoOx hole injection layer could be tailored. High quality MoOx film with optimized charge injection was obtained, based on which all-solution-processed highly efficient red emitting QD-LEDs were realized by using low-cost doctor-blade coating technique at ambient conditions. The red QD-LEDs exhibited the maximum current efficiency and external quantum efficiency of 16 cd/A and 15.1 %, respectively. Moreover, the lifetime of red devices initializing at 100 cd/m2 was 3236 hours under ambient conditions,

which

is

about

twice

as

long

as

those

with

conventional

poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) hole injection layer. Large area QD-LEDs with 4 inches emitting areas were fabricated with blade-coating as well, which exhibit high efficiency of 12.1 cd/A for red emissions. Our work paves a new way to the realization of efficient large area QD-LEDs, and the processing and findings from this work can be expanded into next generation lighting and flat-panel displays.

Keywords: Quantum dot light-emitting diodes, Blade coating, Molybdenum oxide, Large-area, Hole injection layer, Lifetime

2


Page 3 of 31 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


Introduction Light emitting quantum dots (QDs) are particles of semiconductor materials with dimensions from 2 to 10 nanometers. These QDs have become excellent candidates for the light emitting applications because of their unique optical properties such as tunable emission wavelength, narrow band emission and high photoluminescence quantum efficiency1-4. Furthermore, such QDs in colloidal phase can be solution processed, such as by spin-coating, inkjet printing, and spray printing. Combined with roll-to-roll manufacturing, quantum-dot light-emitting diodes (QD-LEDs) promise the future of large-area, low-cost, flexible displays5-10. After two decades development, QD-LEDs have achieved remarkable progress in terms of efficiency and stability. Qian et al. reported bright and efficient QD-LEDs using ZnO nanoparticle as electron transport layer (ETL), and improved device lifetime to 270 hours at an initial luminance of 600 cd/m2 without encapsulation8. Peng et al. reported all solution processed QD-LEDs showing the external quantum efficiency (EQE) approaching 20% by using of novel architectures7. Yang et al. demonstrated the blue, green and red 3



emitting QD-LEDs with high EQE over 10% and lifetimes longer than 10,000 hours by optimizing the shell structures of the quantum dots9.

Regarding the hole injection layer (HIL), poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS) has been widely used in solution-processed QD-LEDs, because of high transparency, good conductivity and solution processability. Whereas, long-term stability is still a problem due to its acidity and hygroscopicity, which may corrode the indium-tin-oxide (ITO) substrates and accelerate the degradation of subsequently deposited active layer, leading to reduced device lifetime11-13. Moreover, the work function of PEDOT:PSS (5.0-5.1 eV) would limit the hole injection into hole transporting layer with high work function14. Transition metal oxides such as molybdenum oxide (MoOx) and nickel oxide (NiO) have been extensively studied and regarded as promising alternatives to PEDOT:PSS in organic light emitting diodes (OLEDs) and organic photovoltaic (OPV) devices due to their favorable electronic properties for hole injection, suitable energy levels and stability15-19. Among them, MoOx is one of the most excellent materials with non-toxic and deep lying electronic states13,19-20. Bae et al. have reported the red and white QD-LEDs showing current efficiency of 5.7 cd/A and 3.28 cd/A with inverted structure, in which MoOx was thermally evaporated on top of the spun-cast ZnO/QD layers21-22. However, the vacuum deposition that is commonly employed for MoOx fabrication isn’t suitable for low-cost and large scale production. In addition, the device performance reported so far is not high enough for practical applications. With 4


Page 4 of 31

Page 5 of 31 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


analysis of photoelectron spectroscopy, the previous researches showed that aqueous solution-processed MoOx has almost the same characteristics as the vacuum deposition and the work function is 5.1-6.0 eV13-14,19. Thus, solution-processed MoOx is promising in the fabrication of efficient QD-LEDs.

In previous reports, most of work focused on developing small size QD-LEDs fabricated by spin coating. Practically, spin coating is not suitable for large area devices fabrication and high volume manufacturing. Although inkjet printing and roll-to-roll manufacturing have drawn many attentions from display industry, they still need to overcome a lot of problems to realize large-area devices in terms of high speed and precision formation of signal pixels. Doctor-blade coating is a simple, scalable, cost-efficient and roll to roll compatible process for optoelectronic device fabrication23-25. In comparison with spin coating and ink jet printing, doctor-blade coating shares the advantages of high utilization rate of materials, broad compatibility with different substrates, and continuous large size film. Therefore, it is expected that doctor-blade coating has the advantage in all-solution processed large-area QD-LEDs fabrication. However, to date, it is also less reported QD-LEDs fabricated by the blade coating.

In this work, we report all-solution processed, highly efficient red QD-LEDs with MoOx film as HIL by doctor-blade coating at ambient conditions. It is indicated that the work function and composition of MoOx layer could be adjusted with the introduction of isopropyl alcohol (IPA), which would facilitate the hole injection from 5



anode to hole transport layer (HTL). In comparison with the devices using conventional PEDOT:PSS as a HIL, MoOx significantly improved the lifetime of the devices from 1650 h to 3236 h under ambient conditions with Al2O3 thin film encapsulation. In addition, 4 inches uniform red light emitting devices were fabricated by doctor-blade coating, which exhibited a record-high current efficiency of 12.1 cd/A for the devices of this size. Our work paved a new way for realizing large area uniform luminescence QD-LED devices in ambient conditions.

Results and Discussion As shown in Figure 1a, the QD-LEDs have a device structure of ITO/MoOx/poly(N,N'-bis-(4-butylphenyl)-N,N'-bis(phenyl)benzidine)(poly-TPD)/pol y(N-vinylcarbazole)(PVK)/QDs/zinc oxide (ZnO)/Al (the thickness of each layer is shown in Supporting information). Except for the Al cathode which was thermally evaporated in vacuum chamber, all other layers were sequentially deposited on ITO by doctor-blade coating at ambient conditions. Figure 1b shows a schematic of the flat-band energy level diagram of the layers. All the energy levels were taken from references5,7,13-14. The performances of QD-LEDs significantly depend on the quality of each functional layer, most importantly on the first solution processed layer: MoOx. The strategy for controlling the morphology of MoOx layer is critical to obtain high

6


Page 6 of 31

Page 7 of 31 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


quality HIL, however, there is a challenge for blade coating uniform MoOx layer due to the uncontrollable physicochemical properties of aqueous MoOx precursor. To overcome this difficulty, a small amount of IPA was blended into MoOx (1 wt%) precursor solution. At first, the effects of different contact angles and viscosity on the doctor blade coating process were investigated by the finite element analysis (FEA) using mathematical approximation for simulating the real physical system. By abstracting the blade coating process as a simple 2D/3D model, the transient of blade coating plane was simulated by solving the momentum equations and solving every discrete area of fluid volume fraction. We set the condition with steel blade which has a 25 µm distance to substrate in 22 ℃ surroundings. With the liquid viscosity of 0.96 mPa·s and contact angle of 60.9°, the solution cannot be spread out smoothly on the substrate and small droplets were formed in the drag (Figure 2a). When the liquid viscosity is 1.38 mPa·s and the contact angle is 37.6°, the liquid is spread out very well in a short time, forming a uniform film (Figure 2c). However, with the liquid viscosity of 1.69 mPa·s and contact angle of 28.7°, the liquid does not keep a continuity, so it is difficult to form a successive film (Figure 2d). Figure 2e showed the optical images of the MoOx precursor droplet with different volume ratio of IPA on the UV-ozone treated ITO surface. The contact angles of the droplets were measured by using this method. The MoOx precursor droplet withou the IPA has a larger contact angle of 60.9°. After blending IPA into MoOx precursor, the contact angle was decreased. The contact angle dropped from 60.9° to 47.1°, 37.6° and 28.7°

7



when the volume ratio ascended from 10:0 to 10:0.5, 10:1 and 10:2, respectively (Table S1). With the increasing ratio of IPA from 10:0 to 10:0.5, 10:1 and 10:2, the viscosity of the MoOx:IPA solution arose from 0.96 mPa.s to 1.16 mPa.s, 1.38 mPa.s and 1.69 mPa.s, respectively. The solution with small viscosity leads to discontinuous MoOx film, while the MoOx film is less uniform by using high viscosity MoOx:IPA solution. Figure 3a-d shows scanning electron microscope (SEM) images of the MoOx films coated from MoOx:IPA solutions with various ratios. With the increase of IPA volume ratio in the solution, the MoOx films become continuous and exhibit better surface morphology. The film coated from MoOx:IPA ratio of 10:2 become rougher again and there is an obvious MoOx aggregation in the film due to the increasing viscosity and rapid evaporation rate. The film with MoOx:IPA ratio of 10:1 gives the smoothest morphology. The experimental observations agree well with the simulation results. As shown in Figure 3e-f, the thickness of the MoOx film with MoOx:IPA ratio of 10:1 was about 15 nm. The film was homogeneous and the deviations of the layer thickness was small. The average thickness of the MoOx films with and without IPA did not show obvious difference because it was mainly determined by the precursor concentration. The morphologies of poly-TPD, PVK, QDs and ZnO films fabricated by doctor-blade coating were investigated as well, and the atomic force microscopy (AFM) images are shown in Figure S2–S3 in supporting information. X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the composition of the as-prepared MoOx film. Figure 4a shows the full 8


Page 8 of 31

Page 9 of 31 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


scan of the XPS spectra of the MoOx film coated with the various MoOx:IPA hybrid solution. The Mo 3p3/2 peak at about 398 eV and Mo 3d5/2 peak at about 232.8 eV correspond to a nearly stoichiometric MoO3 film composition26. As shown in Figure 4b, the Mo 3d XPS spectra have doublet peaks corresponding to the Mo 3d3/2 and Mo 3d5/2 peaks for the summation of Mo4+, Mo5+, Mo6+. When blending IPA to MoOx precursor, the core level shifts toward higher binding energies which indicates the increase of Mo6+ proportion26. Correspondingly, with the increase of IPA in the precursor solution, the atomic ratio of O to Mo (x=O: Mo) in the MoOx film increases from 2.80 to 2.91 (Table S3). Blending IPA with low boiling point of 82 ºC, it is proposed that the thermal hydrolysis reaction is promoted during the formation of MoOx film because of high evaporation rate, which leads to the increase of Mo+6 proportion. To further investigate the electronic structure of MoOx layers fabricated with the various MoOx:IPA precursors, the measurements of ultraviolet photoelectron spectroscopy (UPS) were performed. As shown in Figure 4c, the photoemission onset of MoOx is found at 35.8 eV, corresponding to a work function of 5.0 eV. Blending IPA to MoOx precursor solution leads to a shift of the photoemission onset towards lower binding energies, indicating the increase of work function (Table S3). The work function of the MoOx film coated with MoOx:IPA=10:2 is 5.4 eV. Therefore, IPA in MoOx precursor solution can control the rate of thermal hydrolysis reaction, which would affect the growth and composition of the MoOx film and its work function.

9



The efficiency of QD-LEDs devices is greatly influenced by the hole injection and the carrier balance. To explore the impact of MoOx films with the various concentration of IPA, the hole-only devices were fabricated by doctor-blade coating. The structure was ITO/PEDOT:PSS or MoOx:IPA (ratio varied from 10:0 to 10:2)/poly-TPD/PVK/Au (Figure 5a) . As shown on Figure 5b, when the voltage is less than 1.5 V, the leakage current contributes significantly to the current in the devices. With the concentration of IPA of 10:0 and 10:2, the device leakage current is relatively large because the MoOx film is discontinuous and there are a lot of aggregations in the films. With the concentration of IPA of 10:1, the MoOx film is continuous and smooth, thus the leakage current is greatly reduced. When the voltage is more than 1.5 V, the hole current density increases with the concentration of IPA from 10:0 to 10:1 because the blended IPA not only improves the quality of MoOx films, but also modifies the work function of MoOx layer, which facilitates the hole injection and improves the hole current density of devices. While the concentration of IPA further increases to 10:2, the hole current density decreases due to the particle aggregation of MoOx film, which leads to an increased the hole injection barrier. Thus it shows that the MoOx film prepared with MoOx:IPA ratio of 10:1 has smooth morphology, good quality and appropriate work function, which favors the hole injection from MoOx film to poly-TPD and improves the charge balance. The effect of various MoOx roughness on the morphology of poly-TPD was investigated, and the AFM images are shown in Figure 5c. There is little change in the morphology of

10


Page 10 of 31

Page 11 of 31 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


poly-TPD and the roughness is between 0.41-0.5 nm, which further confirms that the transport of the holes is dominated by the MoOx film layer. In order to study the influence of MoOx with different IPA concentrations on the device performance, small area (5×5 mm2) QD-LEDs were fabricated with the HILs of PEDOT:PSS, MoOx, and MoOx:IPA with volume ratio of 10:0.5, 10:1 and 10:2. The normalized electroluminescence (EL) spectrum of a red QD-LED is shown in Figure 6a, with the peak at 632 nm corresponding to Commission Internationale de l’Eclairage (CIE) color coordinates of (0.68, 0.31). Figure 6b shows the current density and luminance versus voltage (J-V-L) curves of the devices. The device performances and the detailed parameters are summarized in Table 1. When the HIL was fabricated from MoOx precursor without IPA, the device efficiency and maximum luminance are lower than those from PEDOT:PSS device. The best device performance came with IPA ratio of 10:1, showing a peak current efficiency of 16 cd/A and EQE of 15.1% with a maximum luminance of 14016 cd/m2. With further increasing the concentration of IPA, the device performance gradually dropped due to the particle aggregation of MoOx film, which leads to an increased hole injection barrier. For the investigation of device reproducibility, 25 red devices using MoOx: IPA (10:1) as HIL were fabricated with doctor-blade coating technique. As shown on figure S4, the statistical data about the EQE of the red devices and the average EQE is estimated to be 13.4 %.

11



The QD-LEDs lifetime test was carried out under ambient conditions (room temperature, relative humidity of 50 %). Device using MoOx:IPA (10:1) HIL was chosen for the lifetime measurement, and PEDOT:PSS-based QD-LED was used as control device. Both devices were encapsulated with Al2O3 film (100 nm) by using atomic layer deposition technique. The devices were tested at a constant driving current density of 13 mA/cm2. As shown in Figure 6e, the initial luminance of the MoOx-based device is L=1980 cd/m2 with encapsulation and L=1920 cd/m2 without encapsulation, and the half lifetime T50 of them are 15 h and 6 h, respectively. The half lifetime T50 of the encapsulated device at an initial brightness of 100 cd/m2 is predicted to be nearly 3236 h by assuming an acceleration factor A=1.8 (see supporting information, “Device lifetime test”), while it is only 1650 h for the PEDOT:PSS-based device27. It is clearly indicated that the introduction of MoOx film not only increases the efficiency of the device, but also improves the stability of the QD-LEDs. To demonstrate the capability of scale-up fabrication with blade coating technique, large size (8×6 cm2, 4 inches) red QD-LEDs were fabricated. As shown in Figure 6f, the devices exhibited uniform red emissions upon external bias stimulus. The maximum average luminance of 5027 cd/m2 and peak current efficiency of 12.1 cd/A were obtained for the red QD-LED. To the best of our knowledge, this is a record-high efficiency for the QD-LEDs of this size. The average luminance was defined and measured with the method in the Supporting Information and Table S4. .

12


Page 12 of 31

Page 13 of 31 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


These results confirmed the feasibility of the fabrication of large area QD-LEDs based on doctor-blade coating technique.

Conclusions We demonstrated the highly efficient red and white QD-LEDs with MoOx:IPA hybrid film as the hole injected layer by using doctor blade coating technique at ambient conditions. It was indicated that the quality of MoOx layer could be significantly enhanced by adding IPA into MoOx precursor. Moreover, blending IPA into MoOx precursor can control the thermal hydrolysis reaction and modify the MoOx film work function. Through the optimization of the MoOx HIL, peak current efficiency as high as 16 cd/A and EQE of 15.1 % were obtained for red QD-LEDs, and the T50 device lifetime of 3236 h under ambient conditions at an initial luminance of 100 cd/m2 could be achieved, which is twice as long as that of PEDOT:PSS-based device. Moreover, 4 inches uniform red light emitting device was fabricated by doctor-blade coating, which exhibit a record-high current efficiency for the devices of this size. Our work paved a new way for realizing large area, highly efficient, and stable luminescence QD-LEDs in ambient conditions.

Methods Materials. All chemicals and reagents were purchased from commercial sources and 13



used as received without further purification. The red emitting CdSe/ZnS QDs were obtained from TCL Co. Ltd. (China) with an emission peak at 632 nm, a full-width at half-maximum (FWHM) of 30 nm and the fluorescence quantum yield of ~85%. MoOx was synthesized by a sol gel method using ammonium heptamolybdate (NH4)6Mo7O24.4H2O as a precursor. (NH4)6Mo7O24.4H2O was dissolved in deionized water with 1 wt% concentration and heated at 85 °C for 1 h in air19. After deposition of the precursor solution on substrate, the film was annealed and decomposed into MoOx, H2O and NH3. The poly-TPD and the PVK were purchased from Sigma Aldrich. The synthesis of ZnO nanoparticles has been slight modified based on previous literatures28-29 (see the Supporting Information).

Fabrication of QD-LEDs by doctor-blade coating. The devices were fabricated on ITO-coated glass substrates (sheet resistance of 20±5 Ω/sq). The substrates were cleaned sequentially in ultrasonic treatment in acetone, ethanol, deionized water for 15 min each. Then the ITO substrates were dried in an oven and illuminated for 16 min with UV-ozone for 15 min to improve the hydrophilicity. The MoOx precursor was dropped onto ITO-coated glass substrate maintained at 80 ºC. A blade was scraped linearly with blade gap of 25µm and the speed of 5 mm/s, then the substrate was annealed at 150 ºC for 20 min. Poly-TPD (10 mg/ml in chlorobenzene), PVK (5 mg/ml in toluene), QDs and ZnO layers was sequentially dropped onto the MoOx film. Then scraped with the speed of 8 mm/s and the blade gap was 50 µm, 45 µm, 45 µm, 60 µm, respectively. The temperature of substrates was kept at 50 ºC during 14


Page 14 of 31

Page 15 of 31 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


blade-coating process. Each active layer was baked after doctor-blade coating using the different temperatures: Poly-TPD (~40 nm) at 120 ºC for 20 min, PVK (~15 nm) at 90 ºC for 20 min, QDs (~35 nm) at 70 ºC for 10 min and ZnO (~40 nm) at 120 ºC for 35 min. In the end, the Al cathode (~100 nm) was deposited by a thermal evaporation system in a vacuum chamber (~ 3×10-3 Pa). The active device area were 5×5 mm2, 8×6 cm2 (4 inches), respectively. The devices were encapsulated with Al2O3 film by atomic layer deposition (ALD). Trimethylaluminum (TMA) was used as the aluminum-based precursor, and used TMA and H2O reaction deposition Al2O3 encapsulation film. The base pressure of the chamber was 40 Pa and the temperature was 90°C during the deposition process, used nitrogen gas as carrier gas and purge gas for 250 ml/min. For each cycle, the TMA pulse and TMA purge sequences were 0.2 s, 6 s respectively, while the H2O pulse and H2O purge sequences were 0.15 s, 10 s, respectively. A thousand cycles were carried out to deposit about 100 nm Al2O3 film.

Characterization and instrumentation. The surface morphologies of the active films were characterized using SEM and AFM (Bruker Multimode 8). XPS was done using a PHI 5000 VersaProbe with the Al Kα (1486.6 eV) photon line. UPS was done with He II light source (40.8 eV) and a VG scienta R4000 analyzer. The electroluminescence spectra were obtained using a Hitachi F-4600 ﬂuorescence spectrophotometer. The J-V-L curves of the devices were recorded by a system 15



incorporating a Topcon SR-3A spectroradiometer and a Keithley 4200 semiconductor characterization system. All tests were carried out under ambient conditions.

Supporting Information The supporting information is available free of charge on the ACS Publications website.

ZnO nanoparticles synthesis; Morphology of the active layers; XPS and UPS spectra of the MoOx film; Device lifetime; Measurement method of average luminance on the 4 inches QD-LEDs.

Acknowledgments This work was supported by the National Natural Science Foundation of China (61377027, 61705040, U1605244), the National Key Research and Development Program of China (2016YFB0401600).

References 1. Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. 16


Page 16 of 31

Page 17 of 31 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


Science 1996, 271 (5251), 933-937.

2. Lim, J.; Bae, W. K.; Kwak, J.; Lee, S.; Lee, C.; Char, K. Perspective on Synthesis, Device Structures, and Printing Processes for Quantum Dot Displays. Opt. Mater. Express 2012, 2 (5), 594-628.

3. Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D.; Zhu, T.; Xu, J.; Yang, C.; Li, Y. Bright, Multicoloured Light-Emitting Diodes Based on Quantum Qots. Nat. Photonics 2007, 1 (12), 717-722.

4. Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulović, V. Emergence of Colloidal Quantum-Dot Light-Emitting Technologies. Nat. Photonics 2013, 7 (1), 13-23.

5. Pan, J.; Chen, J.; Huang, Q.; Khan, Q.; Liu, X.; Tao, Z.; Lei, W.; Xu, F.; Zhang, Z. Flexible Quantum Dot Light Emitting Diodes Based on ZnO Nanoparticles. RSC Adv. 2015, 5 (100), 82192-82198.

6. Leck, K. S.; Divayana, Y.; Zhao, D.; Yang, X.; Abiyasa, A. P.; Mutlugun, E.; Gao, Y.; Liu, S.; Tan, S. T.; Sun, X. W. Quantum Dot Light-Emitting Diode with Quantum Dots inside the Hole Transporting Layers. ACS Appl. Mat. Interfaces 2013, 5 (14), 6535-6540.

7. Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots. Nature 2014, 515 (7525), 96-100.

17



8. Qian, L.; Zheng, Y.; Xue, J.; Holloway, P. H., Stable and Efficient Quantum-Dot Light-Emitting Diodes Based on Solution-Processed Multilayer Structures. Nat. Photonics 2011, 5 (9), 543-548.

9. Yang, Y.; Zheng, Y.; Cao, W.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J.; Holloway, P. H.; Qian, L., High-Efficiency Light-Emitting Devices Based on Quantum Dots with Tailored Nanostructures. Nat. Photonics 2015. 9 (4),259-265.

11. Colvin, V. L.; Schlamp, M. C.; Alivisatos. A. P. Light-Emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer. Nature 1994, 370 (6488), 354-357.

12. So, F.; Kondakov, D. Degradation Mechanisms in Small‐Molecule and Polymer Organic Light‐Emitting Diodes. Adv. Mater. 2010, 22 (34), 3762-3777.

13. Xu, M. F.; Cui, L. S.; Zhu, X. Z.; Gao, C. H.; Shi, X. B.; Jin, Z. M.; Wang, Z. K.; Liao, L. S. Aqueous Solution-processed MoO3 as an Effective Interfacial Layer in Polymer/Fullerene Based Organic Solar Cells. Org. Electron. 2013, 14 (2) 657-664.

14. Liang, J.; Zu, F.-S.; Ding, L.; Xu, M.-F.; Shi, X.-B.; Wang, Z.-K.; Liao, L.-S., Aqueous Solution-Processed MoO3 Thick Films as Hole Injection and Short-Circuit Barrier Layer in Large-Area Organic Light-Emitting Devices. Appl. Phys. Express 2014, 7 (11), 111601. 18


Page 18 of 31

Page 19 of 31 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


15. Meyer, J.; Khalandovsky, R.; Gorrn, P.; Kahn, A. MoO3 Films Spin-Coated from a Nanoparticle Suspension for Efficient Hole-Injection in Organic Electronics. Adv. Mater. 2011, 23 (1), 70-73.

16. Steirer, K. X.; Chesin, J. P.; Widjonarko, N. E.; Berry, J. J.; Miedaner, A.; Ginley, D. S.; Olson, D. C. Solution Deposited NiO Thin-Films as Hole Transport Layers in Organic Photovoltaics. Org. Electron. 2010, 11 (8), 1414-1418.

17. Steirer, K. X.; Ndione, P. F.; Widjonarko, N. E.; Lloyd, M. T.; Meyer, J.; Ratcliff, E. L.; Kahn, A.; Armstrong, N. R.; Curtis, C. J.; Ginley, D. S. Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers. Adv. Energy Mate. 2011, 1 (5), 813-820.

18. Meyer, J.; Hamwi, S.; Kroger, M.; Kowalsky, W.; Riedl, T.; Kahn, A. Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications. Adv. Mater. 2012, 24 (40), 5408-5427.

19. Höfle, S.; Bruns, M.; Strässle, S.; Feldmann, C.; Lemmer, U.; Colsmann, A. Tungsten Oxide Buffer Layers Fabricated in an Inert Sol-Gel Process at Room-Temperature for Blue Organic Light-Emitting Diodes. Adv. Mater. 2013, 25 (30), 4113-4116.

20. Murase, S.; Yang, Y. Solution Processed MoO3 Interfacial Layer for Organic Photovoltaics Prepared by a Facile Synthesis Method. Adv. Mater. 2012, 24 (18), 2459-2462. 19



Page 20 of 31

21. Meyer, J.; Shu, A.; Kröger, M.; Kahn, A. Effect of Contamination on the Electronic

Structure

and

Hole-Injection

Properties

of

MoO3/Organic

Semiconductor Interfaces. Appl. Phys. Lett. 2010, 96 (13), 133308.

22. Bae, W. K.; Lim, J.; Lee, D.; Park, M.; Lee, H.; Kwak, J.; Char, K.; Lee, C.; Lee, S.

R/G/B/Natural

White

Light

Thin

Colloidal

Quantum

Dot-Based

Light-Emitting Devices. Adv. Mater. 2014, 26 (37), 6387-6393.

23. Kwak, J.; Bae, W. K.; Lee, D.; Park, I.; Lim, J.; Park, M.; Cho, H.; Woo, H.; Yoon, D. Y.; Char, K.; Lee, S.; Lee, C. Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure. Nano Lett. 2012, 12 (5), 2362-2366.

23. Liang, J. J; Li, L.; Niu, X. F.; Yu, Z. B.; Pei, Q. B. Fully Solution-Based Fabrication of Flexible Light-Emitting Device at Ambient Conditions. J. Phys. Chem. C 2013, 117 (32), 16632-16639.

24. Kim, J. H.; Williams, S. T.; Cho, N.; Chueh, C. C.; Jen, A. K. Y. Enhanced Environmental Stability of Planar Heterojunction Perovskite Solar Cells Based on Blade‐Coating. Adv. Energy Mate. 2015, 5 (4), 1401229.

25. Deng, Y.; Peng, E.; Shao, Y.; Xiao, Z.; Dong, Q.; Huang, J. Scalable Fabrication of Efficient Organolead Trihalide Perovskite Solar Cells with Doctor-Bladed Active Layers. Energ. Environ. Sci. 2015, 8 (5), 1544-1550.

20


Page 21 of 31 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


26. Sarma, D.D.; Rao, C. N. R. XPES Studies of Oxides of Second- and Third-Row Transition Metals Including Rare Earths. J. Electron. Spectrosc. Relat. Phenom. 1980, 20 (1), 25-45.

27. Li, D. L.; Wu, H.; Zhang, J. P.; Wu, W. L.; Zhu, W. Q. Lifetime Prediction of White OLED Based on MLE under Lognormal Distribution. J. Test. Eval. 2013, 41 (3), 1-5.

28. Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-Temperature Solution Processing Techniques. Nat. Photonics 2013, 8 (2), 133-138.

29. Beek, W. J.; Wienk, M. M.; Kemerink, M.; Yang, X.; Janssen, R. A. Hybrid Zinc Oxide Conjugated Polymer Bulk Heterojunction Solar Cells. J. Phys. Chem. B 2002 109 (19), 9505-9516.

21



Figure Captions

Figure 1. Schematic of device structure, energy levels. (a), As-fabricated devices architecture. (b), Energy level diagram.

Figure 2. Schematic of simulation, contact angle images. (a)-(d), Simulation of the doctor blade coating process with various contact angles and viscosities by using finite element analysis, the inset is 2D images. (e), Contact angle images of various MoOx:IPA droplets.

Figure 3. SEM images. (a)-(d) EM images of the various MoOx:IPA films coated on ITO substrates. (e) The SEM images of MoOx layer with MoOx:IPA=10:0 on the ITO glass. (f) The SEM images of MoOx layer with MoOx:IPA=10:1 on the ITO glass.

Figure 4. XPS and UPS spectra of MoOx films fabricated with various MoOx:IPA hybrid solutions. (a), Full XPS spectra. (b), The XPS spectra of Mo 3d state. (c), Photoemission onsets of UPS spectra.

Figure 5. (a), The structure of the hole-only devices. (b), Current density-voltage (J-V) 22


Page 22 of 31

Page 23 of 31 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


curves for the hole-only devices with PEDOT:PSS and various ratios of MoOx:IPA. (c), AFM images of the poly-TPD fabricated on different MoOx films.

Figure 6. EL spectra, CIE coordinates, device characteristics and lifetime of red QD-LEDs. (a), Normalized EL spectrum of a red QD-LED, the inset is the photograph and CIE coordinates of a turn-on red device. (b), Current density and luminance versus voltage (J-V-L) characteristics of the red QD-LEDs with various HILs. (c), Current efficiency versus current density characteristics of the red QD-LEDs with various HILs. (d), EQE versus current density of the device with various HILs. (e), Lifetime test of the red QD-LEDs. (f), Average luminance-voltage and current efficiency versus current density characteristics of the 4 inches red QD-LED, the inset is the photograph of red QD-LED (at 10V).

Table 1 Optoelectronic characteristics of the QD-LEDs with various HILs. Turn on voltage (VT) was defined as the voltage required to reach the brightness of 1 cd/m2.

23



Fig. 1

24


Page 24 of 31

Page 25 of 31 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


Fig. 2

25



Fig. 3

26


Page 26 of 31

Page 27 of 31 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


Fig. 4

27



Fig. 5

28


Page 28 of 31

Page 29 of 31 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


Fig. 6

29



Hole injected

EQE(%)

ηA (cd/A)

Lmax

Page 30 of 31

ηP (lm/W)

VT (V) layer

(cd/m2)

Peak

at 1000 cd/m2

Peak

at 1000 cd/m2

Peak

at 1000 cd/m2

PEDOT:PSS

2.1

12109

14.1

12.8

13.3

12.1

8.3

8.1@5V

MoOx

1.8

8652

11.2

10.1

10.6

9.5

8.4

[email protected]

MoOx:IPA=10:0.5

1.9

12335

14.1

12.3

13.3

11.6

10.2

9.6@4V

MoOx:IPA=10:1

2

14016

16.0

14.6

15.1

13.8

10.3

[email protected]

MoOx:IPA=10:2

2.3

7621

10.4

8.7

9.8

8.3

5.6

[email protected]

Table 1

30


Page 31 of 31 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


For Table of Contents Only

31


Improving Charge Injection via Blade Coating Molybdenum Oxide

Recommend Documents