Efficient All-Solution Processed Quantum Dot Light Emitting Diodes

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Efficient All-solution Processed Quantum Dot Light Emitting Diodes Based on Ink-jet Printing Technique Yang Liu, Fushan Li, Zhongwei Xu, Congxiu Zheng, Tailiang Guo, Xiangwei Xie, Lei Qian, Dong Fu, and Xiaolin Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05381 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Applied Materials & Interfaces

Efficient All-solution Processed Quantum Dot Light Emitting Diodes Based on Ink-jet Printing Technique Yang Liu, Fushan Li*, Zhongwei Xu, Congxiu Zheng, Tailiang Guo Institute of Optoelectronic Technology, Fuzhou University, Fuzhou 350002, People’s Republic of China Xiangwei Xie, Lei Qian, Dong Fu, Xiaolin Yan TCL Corperate Research, Shenzhen 518057, People’s Republic of China

Abstract Quantum dot light emitting diodes (QLEDs) are increasingly attractive owing to their compatibility with ink-jet printing process and the potential application in low-cost large-area full-color pixelated display. The strategy for controlling the morphology of quantum dot layer is definitely critical for realizing all-solution processed QLEDs with high performance, which certainly requires in-depth thinking regarding the design of ink composition and their optimization in the printing process. Herein, by carefully controlling the quantum dot ink composition and physicochemical properties, we demonstrate that the viscosity, contact angle and the three-phase contact line moving would affect the final morphology of quantum dots film formed by inkjet printing. We achieved coffee-ring-free and low-roughness quantum dots film and all solution-processed QLEDs with normal structure were fabricated for the first time. The devices have a low turn-on voltage of 2.0 V, a luminance of 12100 cd/m2 at the voltage of 12 V and a maximum current efficiency of 4.44 cd/A at the luminance of 1974 cd/m2, which is the best result to date for inkjet-printed red QLEDs. The results 1

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will pave the way for future application of inkjet printing in solution-processed pixelated QLED display.

Keywords: Quantum dots; QLEDs; Inkjet printing; Coffee ring; All-solution process.

*Corresponding author: [email protected] (F. Li)

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1.

Introduction

Quantum dots light-emitting diodes (QLEDs) are attractive for the promising application in the next-generation lighting and display technologies 1-5. The interest on QLEDs, aroused in 1995 for the first time6, stems from their outstanding characteristics, such as saturated colors, narrow emission peak, high luminescent efficiency, and adjustable emission wavelength via varying the quantum dot size and composition4, 7-9. Recently, impressive progress in the performance of QLEDs has been reported. Peng et al. reported QLEDs with high external quantum efficiencies of up to 20.5 per cent, low efficiency roll-off, and a long operational lifetime of more than 100,000 hours at 100 cd m-2, making the performance of this red device comparable to the vacuum-deposited organic light-emitting diodes (OLEDs)10. Yixing Yang et al. fabricated a full series of blue, green and red QLEDs, all with high external quantum efficiencies over 10%11. All-solution-processed high-performance flexible QLEDs have also been reported by Dae Kyoung Kim et al.2 Compared with OLEDs, their work indicates that QLEDs can exhibit comparable efficiency and operational lifetime via solution processes. However, the devices mentioned above, rather than pixelated red-green-blue (RGB) pattern, are just single lighting units fabricated by spin coating, limiting its application in display field. Inkjet printing has attracted increasing attention in pixelated display as a mask-free, material-effective, and patterned-deposition method to form functional patterns in optoelectronic and electronic devices12-16, which is clearly superior to other solution-processing methods, such as spin coating, blade coating, and so on. Several 3



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studies about inkjet printing QLEDs have been reported. Hanna M. Haverinen et al. reported

inkjet-printed

QLEDs

with

quantum

dots

(QDs)

dissolving

in

chlorobenzene15. However the performance of the devices is inferior, which can be attributed to the coffee ring effects, the pinhole and solvent perturbation to the underlying polymer film during the inkjet process, considering the essential factor of the interface between multi-layers. To solve this problem, Changhee Lee’s group fabricated red QLEDs with an inverted device structure.

17

Recently, Junbiao Peng et

al. promoted the current efficiency up to 4.5 A/cd by introducing PEI film into inverted-structured devices as the modification of zinc oxide nanoparticle (ZnO NPs) layer18. However, the inserting of PEI film increases the turn-on voltage of the devices to 5.1 V at the same time. To fabricate efficient pixelated QLEDs with full solution processes, the strategy for controlling the morphology of quantum dot layer is definitely critical for realizing all-solution processed QLEDs with high performance, which certainly requires in-depth thinking regarding the design of ink composition and their optimization in the printing process. It should be noted that, two issues at least should to be carefully addressed. First, preparing QDs ink with orthogonal solvents not only indissolvable to underneath films but also suitable for inkjet printing, due to the lower boiling point and viscosity of regular solvents, such as octane and toluene. Second, removing coffee ring and enhancing the smoothness of inkjet-printed QDs layer to stop the upper layer penetrating into QDs layer, which may lead to inefficient exciton recombination. In this paper, we succeed to deposit coffee-ring-free low-roughness QDs film on 4


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poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine

(poly-TPD)

layer

by

introducing mixed solvents of decane and cyclohexylbenzene. On the basis of the high-quality QDs film, solution-processed QLEDs with normal structure were fabricated for the first time. The luminance of the devices increases up to about 2000 cd/m2 at 6 V, and reaches 12100 cd/m2 at the voltage of 12 V. The maximum current efficiency is 4.44 cd/A at the luminance of 1974 cd/m2, which is the best result for inkjet-printed red QLEDs. The results will pave the way for future application of inkjet printing in solution-processed pixelated QLEDs display.

2.

Experimental Section

Materials: All the materials mentioned in the following parts were obtained from commercial suppliers and used without further purification. The red emitting CdSe/ZnS QDs were obtained from Guangdong Poly Opto Electronics Co. Ltd. with an emission peak at 630 nm, and a full-width at half-maximum (FWHM) < 35 nm. Poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and poly-TPD were purchased from Xi'an Polymer Light Technology Corp. Poly(9-vinlycarbazole) (PVK) were supplied by Sigma Co. Ltd. ZnO nanoparticles were synthesized using an adapted procedure according to a previous reports with slight modification19. Zinc acetate (2.95 g, 13.4 mmol) was dissolved in methanol (125 ml) with vigorous stirring at 65 °C. Subsequently, a solution of KOH (1.48 g, 23 mmol) in methanol (65 ml) was added dropwise at 60-65 °C over a period of 15 min. After 1.5 h, the nanoparticles started to precipitate 5



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and the solution became turbid. After 2.5 h, the heater and stirrer were removed and the nanoparticles were allowed to precipitate for an additional 2 h. Precipitate and supernatant were separated and the precipitate was washed twice with methanol (20 ml). After the washing steps, n-Butanol were added to disperse the precipitated ZnO nanoparticle solution with a concentration of ~12 mg/ml. Before use, the ZnO nanoparticle solution was filtered through a 0.22 µm polyvinyl difluoride syringe filter.

Fabrication of QLEDs: QLEDs with inkjet-printed QDs as light-emission layer was fabricated in the following procedure. Pre-patterned ITO-coated glass substrates with low surface energy separators were cleaned by ultrasonication in deionized (DI) water for 10 min. The substrates were dried by nitrogen stream and heated at 150 °C for 5 min, then followed by 2 min oxygen plasma treatment to increase the work function and enhance the hydrophilicity of the substrates surface. Pre-filtered PEDOT:PSS was spin coated onto the substrate and baked at 150 °C for 15 min. Hole transport material poly-TPD (10 mg/ml in chlorobenzene) were sequentially deposited on the PEDOT:PSS layer by spin coating, then was heated at 150 ºC for 30 min. The inkjet printing of QDs on substrates was accomplished by using a Microfab JETLAB 2 equipped with a 30 µm diameter piezoelectric-driven inkjet nozzle and a motorized stage with the accuracy of 5 um. For the spin-coated QDs film, QDs solution (20 mg/ml in decane) was deposited on substrate at the rotate speed of 2000 r.p.m and was heated in the darkness at 70 °C for 30 min. Pre-filtered ZnO nanoparticle was then spin coated at the rotate speed of 1500 r.p.m and baked at 6


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70 °C for 30 min. All the solution processes were carried out in ambient environment. Finally, the substrates were transferred to a vacuum chamber below 3×10-3 Pa to deposit silver cathode (100 nm). The as-fabricated devices were encapsulated by the cover glasses using ultraviolet-curable resin.

Characterization: The surface morphologies of the films were characterized by atomic force microscopy (AFM, Bruker Multimode 8). Electroluminescence (EL) spectra was recorded with a Hitachi F-4600 fluorescence spectrophotometer. The electroluminescence (EL) and photoluminescence (PL) microscopic images of QDs layer morphology were characterized by fluorescent microscope（Olympus BX51M）. The surface tension and viscosity of QDs inks were obtained from surface tension meter (CNSHP BZY-1) and Brookfield Rotational Viscometer (DV2T) at room temperature, respectively. The contact angles were determined using an instrument (SL200 KS, Kono, USA), and luminance-voltage-current density curves were measured using a system incorporating a Topcon SR-3A spectroradiometer and a Keithley 4200 semiconductor characterization system. All tests were carried out in ambient environment.

3.

Results and Discussion.

The schematic of the normal-structure bottom-emitting device is shown in Figure 1 a, consisting of an ITO/PEDOT:PSS/poly-TPD/QDs/ZnO NPs/Ag. As shown in the energy level diagram in Fig 1b, the band alignment between ZnO NPs, Al and QDs allows efficient electron injection from cathode Ag into the QDs layer, while the 7



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valence band offset at the QD/ZnO NPs helps to confine the holes injected from poly-TPD to the QDs layer. Compared with other common hole transport materials in QLEDs,

such

as

PVK

or

poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB), the poly-TPD is resistant to nonpolar solvent after annealing 30 min at 150 °C to some degree, making it easier to deposited QDs inks. Traditional solvents of QDs, like toluene and octane, are not suitable for inkjet printing due to low boiling point and low viscosity, tending to block nozzle and form satellite points. To solve this problem, a series of inks with higher boiling point and viscosity were prepared. The optical image of the as-prepared QDs inks under a UV light is shown in Fig 1c. The flying process of droplets in varied time illustrates stable non-satellite-point droplets were jetted out and dropped on substrates (Fig 1d). Figure 2a shows the schematic of inkjet printing process on substrates with separator, where the thickness of QDs layer can be tuned by the concentration of QDs in inks and the drops in a single pixel. Figure 2b and its higher resolution counterparts (Figure 2c) exhibit the morphology characteristics of printed dot films onto plain substrates coated with poly-TPD. The dots array in Figure 2 b1-b5 (PL microscopic images) correspond to QDs inks 1-5 containing 100, 90, 80, 70 and 0% (volume ratios) cyclohexylbenzene, respectively. When it comes to inks of pure cyclohexylbenzene (Figure 2 b1, c1), the coffee ring does not appear in the dots. Many bright areas, however, scatter on the even film, which can be attributed to the undesirable dissolvability and aggregation during of solvent evaporation. As shown in Figure 2 b2, 8


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c2, when 10% decane with volume ratios was added, the dots show better coverage and uniformity in distribution. With the increasing volume of decane, dots on poly-TPD layer exhibit a black hole in the center and their uniformities get worse, corresponding to a coffee ring effect. For coffee ring effect, the three-phase contact line (TCL) is self-pinned immediately around the droplet periphery when the inks use decane as single solvent drop on poly-TPD film. Owing to the much lower contact angle (

Efficient All-Solution Processed Quantum Dot Light Emitting Diodes

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