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Efficient All-Solution Processed Quantum Dot Light Emitting Diodes Based on Inkjet Printing Technique Yang Liu,† Fushan Li,*,† Zhongwei Xu,† Congxiu Zheng,† Tailiang Guo,† Xiangwei Xie,‡ Lei Qian,‡ Dong Fu,‡ and Xiaolin Yan‡ †
Institute of Optoelectronic Technology, Fuzhou University, Fuzhou 350002, People’s Republic of China TCL Corperate Research, Shenzhen 518057, People’s Republic of China
‡
S Supporting Information *
ABSTRACT: Quantum dot light emitting diodes (QLEDs) are increasingly attractive owing to their compatibility with the inkjet printing process and potential application in low-cost large-area full-color pixelated display. The strategy for controlling the morphology of the 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 the quantum dot film formed by inkjet printing. We achieved coffee ring-free and low-roughness quantum dot film, and all-solution processed QLEDs with normal structure were fabricated for the first time. The devices have a low turnon 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 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
1. INTRODUCTION Quantum dot light-emitting diodes (QLEDs) are attractive for the promising application in the next-generation lighting and display technologies.1−5 The interest in QLEDs, aroused in 1995 for the first time,6 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 composition.4,7−9 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%, low efficiency rolloff, and a long operational lifetime of more than 100 000 h at 100 cd m−2, making the performance of this red device comparable to that of the vacuum-deposited organic lightemitting diodes (OLEDs).10 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 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 the display field. © 2017 American Chemical Society
Inkjet printing has attracted increasing attention in pixelated display as a mask-free, material-effective, and patterneddeposition method to form functional patterns in optoelectronic and electronic devices,12−16 which is clearly superior to other solution-processing methods such as spin coating, blade coating, and so on. Several studies about inkjet printing QLEDs have been reported. Haverinen et al. reported inkjet-printed QLEDs with quantum dots (QDs) dissolving in chlorobenzene.15 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 multilayers. To solve this problem, Lee’s group fabricated red QLEDs with an inverted device structure.17 Recently, 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) layer.18 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 Received: April 18, 2017 Accepted: July 11, 2017 Published: July 11, 2017 25506
DOI: 10.1021/acsami.7b05381 ACS Appl. Mater. Interfaces 2017, 9, 25506−25512
Research Article
ACS Applied Materials & Interfaces
Prefiltered ZnO nanoparticles were then spin coated at the rotation speed of 1500 rpm and baked at 70 °C for 30 min. All of 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 F4600 fluorescence spectrophotometer. The EL and photoluminescence (PL) microscopic images of QD layer morphology were characterized using a fluorescent microscope (Olympus BX51M). The surface tension and viscosity of QD 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 an ambient environment.
solution processes, the strategy for controlling the morphology of the quantum dot layer is definitely critical for realizing allsolution 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 be carefully addressed. First, preparing QD ink with orthogonal solvents not only does not dissolve underneath films but also makes it suitable for inkjet printing due to the lower boiling point and viscosity of regular solvents such as octane and toluene. Second, the coffee ring is removed and the smoothness of inkjet-printed QD layer is enhanced to stop the upper layer penetrating into the QD layer, which may lead to inefficient exciton recombination. In this paper, we succeed in depositing coffee ring-free lowroughness QD film on 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 QD film, solution processed QLEDs with normal structure were fabricated for the first time. The luminance of the devices increases to about 2000 cd/m2 at 6 V and reaches 12 100 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 QLED displays.
3. RESULTS AND DISCUSSION The schematic of the normal-structure bottom-emitting device is shown in Figure 1a, consisting of an ITO/PEDOT:PSS/poly-
2. EXPERIMENTAL SECTION Materials. All 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):polystyrenesulfonate (PEDOT:PSS) and poly-TPD were purchased from Xi’an Polymer Light Technology Corp. Poly(9-vinlycarbazole) (PVK) was supplied by Sigma Co. Ltd. ZnO nanoparticles were synthesized using an adapted procedure according to previous reports with slight modification.19 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, 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 was 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 the light-emission layer were fabricated in the following procedure. Prepatterned 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 of oxygen plasma treatment to increase the work function and enhance the hydrophilicity of the substrate surface. Prefiltered 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) was sequentially deposited on the PEDOT:PSS layer by spin coating and then was heated at 150 °C 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 μm. For the spin-coated QD film, QD solution (20 mg/mL in decane) was deposited on the substrate at the rotation speed of 2000 rpm and heated in the dark at 70 °C for 30 min.
Figure 1. (a) Schematic of the inkjet-printed layers in QLEDs. (b) Energy level diagram for the QLEDs. (c) Optical image of the asprepared QD inks 1−5 (from left to right) containing 100, 90, 80, 70, and 0% cyclohexylbenzene under illumination. (d) Flying process of droplets in various times (from left to right, 30, 75, 255, and 455 μs).
TPD/QD/ZnO NPs/Ag. As shown in the energy level diagram in Figure 1b, the band alignment between ZnO NPs, Al, and QDs allows efficient electron injection from cathode Ag into the QD layer, while the valence band offset at the QD/ZnO NPs helps to confine the holes injected from poly-TPD to the QD layer. Compared with other common hole transport materials in QLEDs such as PVK or poly(9,9-dioctylfluoreneco-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 deposit QD inks. Traditional solvents of QDs such as toluene and octane are not suitable for inkjet printing due to low boiling point and low viscosity, tending to block the 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 as25507
DOI: 10.1021/acsami.7b05381 ACS Appl. Mater. Interfaces 2017, 9, 25506−25512
Research Article
ACS Applied Materials & Interfaces
Considering the TCL pinned during the whole evaporation process, a large part of QDs immigrate into a typical ring-like pattern along the periphery of the droplet, the so-called coffee ring phenomenon. High viscosity leads to the suppression of the coffee ring by slowing the outward capillary flow.18,20 Table 1 summarizes the
prepared QD inks under a UV light is shown in Figure 1c. The flying process of droplets in varied time illustrates stable nonsatellite-point droplets were jetted out and dropped on substrates (Figure 1d). Figure 2a shows the schematic of the inkjet printing process on substrates with separator, where the thickness of the QD
Table 1. Composition, Boiling Point, Viscosity, Surface Tension, and Contact Angle on Poly-TPD Layer of Various Quantum Dots Inksa ink
CHB/ decane (V/V)
1 2 3 4 5
10/0 9/1 8/2 7/3 0/10
boiling point (°C)
viscosity (cP)
surface tension (mN/m)
contact angle (degree)
237
2.81 2.35 2.09 1.81 0.98
34.6 32.8 31.3 29.7 23.9
8.78 7.49
