Ultrasmooth Quantum Dot Micropatterns by a Facile Controllable

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Ultrasmooth Quantum Dot Micropatterns by a Facile Controllable Liquid-Transfer Approach: Low-Cost Fabrication of HighPerformance QLED Min Zhang,†,‡,# Binbin Hu,‡,# Lili Meng,† Ruixin Bian,† Siyuan Wang,§ Yunjun Wang,§ Huan Liu,*,† and Lei Jiang†

J. Am. Chem. Soc. Downloaded from pubs.acs.org by TUFTS UNIV on 07/03/18. For personal use only.

†

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beihang University, Beijing 100191, P. R. China ‡ Key Lab for Special Functional Materials of Ministry of Education, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475000, P. R. China § Suzhou Xingshuo Nanotech Company, Limited (Mesolight), Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: Fabrication of a high quality quantum dot (QD) film is essentially important for a high-performance QD light emitting diode display (QLED) device. It is normally a high-cost and multiple-step solution-transfer process where large amounts of QDs were needed but with only limited usefulness. Thus, developing a simple, efficient, and low-cost approach to fabricate high-quality micropatterned QD film is urgently needed. Here, we proposed that the Chinese brush enables the controllable transfer of a QD solution directly onto a homogeneous and ultrasmooth micropatterned film in one step. It is proposed that the dynamic balance of QDs was enabled during the entire solution transfer process under the cooperative effect of Marangoni flow aroused by the asymmetric solvent evaporation and the Laplace pressure different by conical fibers. By this approach, QD nanoparticles were homogeneously transferred onto the desired area on the substrate. The as-prepared QLED devices show rather high performances with the current efficiencies of 72.38, 26.03, and 4.26 cd/A and external quantum efficiencies of 17.40, 18.96, and 6.20% for the green, red, and blue QLED devices, respectively. We envision that the result offers a low-cost, facile, and practically applicable solution-processing approach that works even in air for fabricating high-performance QLED devices. processes have been developed for preparing QD films including spin-coating,19−23 transfer printing,10,18,24−27 inkjet printing,28−30 and mist-deposition.31 However, these techniques suffer from limitations such as high cost, complicated multiple steps, demands for template, and shortage of film uniformity. For example, the spin-coating and the mistdeposition normally need a large amount of QD solution but with only a very limited amount actually transferred, which makes it a high-cost process. The transfer printing requires templates, making the process more complicated. Inkjet printing suffers from both the coffee ring effect and easy clog of the jagged nozzle. Thus, developing a simple and low-cost solution-processing approach that can directly fabricate uniform QD films with various micropatterns is important for high-performance QLEDs, but remains a challenge. Here, from inspiration drawn from controllable liquid transfer by the Chinese brush,32−35 we develop a facile

1. INTRODUCTION Quantum dot light emitting diode (QLED) displays, as the most promising next-generation LED display, have attracted a lot of attention due to their stable optical performance, narrow spectral emission bandwidths, and high screen resolution.1−7 As the emitting layer, the quantum dot (QD) film plays an important role in determining the performance of QLED devices.8−11 Generally, the performance of the QD film depends on both the physicochemical property of QDs themselves and the quality of the assembled film, particularly the uniformity and the thickness.12−17 A QD film with suitable thickness is always preferred for efficient charge transfer across the film.18 In particular, the surface roughness of the QD film affects both its charge-transport behavior and the charge balance with the neighboring layer in the multilayered device, which affects the quantum efficiency directly. Meanwhile, considering the high cost of QDs, it is in demand to pattern QD films in the functional area. Therefore, it is important to prepare high-quality micropatterned QD films from the viewpoint of practical applications. So far, various solution © XXXX American Chemical Society

Received: March 16, 2018 Published: June 12, 2018 A

DOI: 10.1021/jacs.8b02948 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

uniformity free from any templates, as shown in Figure 1c. As can be seen from the photoluminescence images under the UV irradiation, all of the green, red, and blue QD films with a width of ca. 500 μm show good homogeneity throughout the entire film (Figure 1d−f). Moreover, the as-prepared QD films show well-defined profiles without any coffee rings or ripples. Nanoparticles in the film are distributed uniformly to form homogeneous color-emitting light. Here, the ligand of octylamine, n-octylmercaptan, and octylamine were particularly used for green, red, and blue QDs, respectively (Figure S1), which show little effect on the film formation in our system. It is rather reasonable that these ligands are all oleophilic, showing similar effects on the interactions among ligands on neighboring QDs and the substrate. When the QD films were integrated into the QLED devices with four functional regions, the uniformly luminous were obtained for all green, red and blue QLEDs without any observable difference in luminous intensity (Figure 1g−i). Note that only a small amount of QD solution was needed for the brush-coating, which suggested a low-cost process. Taken together, the brush-coating enables controllable transfer of QD solution into micropatterns with high uniformity free from any templates, which is an advantage for low-cost fabrication of QLEDs. The uniformity and roughness of QD films were further characterized by a magnified photoluminescence (PL) microscope and an atomic force microscope (AFM), respectively. As shown in Figure 2a−c, the brush-coated QD films exhibit rather good homogeneous distribution within all areas, showing a well-defined profile. For comparison, the spincoated QD films are unable to give any clear profile, despite their certain uniformity (Figure 2d). As has been measured by AFM, the roughness of the brush-coated QD film is as small as ca. 1.10 nm (Figure 2e), which is much smaller than that of the spin-coated one (rms = 2.84 nm, Figure 2f). The film thickness is measured as ca. 8 nm for the brush-coated QD film (Figure S2). Here, the brush-coating show advantages not only in preparing micropatterned QD films but also enables ultrasmooth QD films, which are beneficial for the high performance of QLEDs.36,37 To explore the performance of the brush-coated QD films, the QLED devices with the architecture of glass/ITO/ poly(3,4-ethylenedioxythiophene) are fabricated: poly(styrenesulfonate) (PEDOT:PSS)/poly[9,9-dioctylfluoreneco-N-[4-(3-methylpropyl)]diphenylamine] (TFB)/ CdZnSeS@ZnS core/shell QDs/zinc oxide(ZnO)/Al cathode (Figure 1a). Here, the TFB polymer layer acts as the holetransport layer, and the ZnO layer acts as the electrontransport layer. The CdZnSeS@ZnS QD film, which is fabricated by the brush coating, acts as the emission layer that is inserted between the hole transport layer and the electron transport layer.38−40 As summarized in Figure 3, the as-prepared QLED devices show rather good electroluminescent performances. The symmetric emission peak for the brush-coated QD films at 528, 624, and 468 nm give a narrow full width at half-maximum (fwhm) of 23, 28, and 20 nm for the green, red, and blue QLED devices (Figure 3a1, b1, and c1) respectively. The fwhm of emission spectra and the EL peak position remain unchanged with increasing the driving voltage. This is rather important for practical applications since it suggests the high stability of the EL emission for the QLED devices. For a green QLED, the current density and luminance show clear increase with enlarging the voltage (Figure 3a2), yielding a maximum luminance of over 12000 cd/m2 at 4.4 V.

brush-coating approach to transfer QD solution directly into ultrasmooth micropatterns in a controllable manner, by which high-performance QLED devices were fabricated with rather low cost. It is proposed that the brush-coating enables the dynamic balance of QDs during the whole solution transfer process under the cooperative effect of the Marangoni flow of QDs in the liquid aroused by the asymmetric solvent evaporation and the Laplace pressure different by conical structure. By this approach, QD solution was homogeneously transferred onto the desired area on the substrate without any uncontrollable aggregations, leading to an ultrasmooth micropatterned QD film. Thus, QLED devices with rather high performances were achieved, exhibiting peak current efficiencies of 72.38, 26.03, and 4.26 cd/A and peak external quantum efficiencies (EQE) of 17.40, 18.96, and 6.20% for green, red, and blue QLEDs, respectively. It is worth noting that the QD films were prepared by brushing in air conditions, which is important for large-mass production. We envision that this result offers an alternative simple approach for the low-cost fabrication of high-performance QLED devices.

2. RESULTS AND DISCUSSION For a QLED device with a typical multiple-layered structure, the emitting layer of QD film is crucial (Figure 1a), which

Figure 1. Direct preparation of micropatterned QD film by the Chinese brush. (a) QLED device with a typical multiple-layered stucture. (b) Schematic illustrations of brush-coating process, and the typical CdZnSeS@ZnS core/shell structure of QD nanoparticles. Inset: n-octane and water droplets spread on the TFB layer. (c) Micropatterned green QD film by brush-coating method, where the QDs were only deposited onto specified functional areas. (d−f) Fluorescence microscope images of the as-brushed green, red, and blue QD film under UV irradiation. (g−i) Electroluminescent images of the green, red, and blue QLED devices.

generally derives from a QD solution. By brush-coating, the QD solution is steadily transferred onto the microscale functional area on the substrate in a well-controllable manner, as schematically shown in Figure 1b. The biggest advantage of brush-coating over other solution processes is that it enables direct preparation of micropatterned QD film with rather high B

DOI: 10.1021/jacs.8b02948 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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controllable wetting and dewetting guided by the conical fibers,47 where the dynamic balance of QDs in the solution was maintained without any unexpected aggregation during the entire transfer process. In particular, the solvent of n-octane with rather low surface tension of 21.8 × 10−3 N/m was used in our case, which is liable to spread and difficult to control (Figure 1b and Figure S6). Here, we demonstrated that both polar (toluene) and nonpolar (n-octane) solvents can be used to realize the high-quality QD films (Figure S7), suggesting the small effect of the solvent polarity on the film formation. When the Chinese brush moves at a certain speed V, the large mass of QD solution was steadily held within the brush by the cooperative effect of Laplace pressure difference FL, asymmetrical retention force Fa, and gravity G (Figure 4a, area 1),48−51 leaving the front edge of the solution film under the directional stress of FL via each single fiber and the surface tension (Fγ) at each neighboring fiber (Figure 4a, area 2). Here, the liquid−gas−solid three-phase contact line (TCL) was shaped into multiple small meniscus curves by parallel conical fibers, where multiple parallel Fγ were generated to guide the solution transfer (Figure 4c). To be sure, the biggest advantage of the Chinese brush in transferring QD solution lies in the fact that the FL via each single fiber can help to propel the QDs moving to the base side of fibers, which can be used to counter the Marangoni flow of QDs in the solution aroused by the asymmetric solvent evaporation,52−56 as indicated in Figure 4b. As a cooperative effect of these two driving forces with opposite directions, the dynamic balance of QDs in the solution was enabled during the entire transfer process, leading to a homogeneous QD film. For the QLED display, the film uniformity is necessary not only for large-scale QD films but for microscale patterns of each RGB three-color unit.12 Here, not limited to the microscale, the brush-coating enables a highly uniformed QD film in a large scale, as has been demonstrated by a uniform QD film with a width of ca. 1 cm and a length of ca. 6 cm (Figure 4d). Here, the brushing speed (i.e., the moving speed of brush vs the substrate) of 3 mm/s delivers the high-quality ultrasmooth QD film. In our case, the brushing speed is a crucial parameter to realize the controllable transfer of QD solution, which makes the spreading and dewetting of solution match with each other. That is to say, the brushing speed directly affects the receding pattern of TCL, the duration time for nanoparticles assembly, and the movement and distribution of QD nanoparticles in solution. Here, the QD film prepared with different brushing speeds was characterized by AFM (Figure S8). It is clear that different brushing speeds gave drastically different morphologies of QD films. When the speed is too slow, the dewetting of solution on the substrate is much faster than the spreading of the solution, which makes the movement of QD nanoparticles in solution hard to proceed. As a result, the nanoparticles fail to be uniformly transferred onto the substrate (Figure S8a−c). When the speed is too fast, the dewetting of solution on the substrate is slower than its spreading, which leads to a long duration time for the QD selfassembly. In this situation, the self-assembly of QDs liable to be proceed randomly, leading to a rough QD film (Figure S8d−h). Suitable brushing speed enables the balance of the wetting and dewetting of QD solution on the substrate, where the liquid front edge is the TCL governed by conical fibers. Another parameter affecting the spreading and dewetting of QD solution on the substrate is the temperature, increasing of

Figure 2. PL and AFM characterization of as-prepared QD films. Fluorescence microscope images of green (a1, a2), red (b1, b2), and blue (c1, c2) QD films prepared by brush-coating and by spin-coating (d1, d2, and d3). Among them, a1, b1, and c1 show the inner region of QD films; a2, b2, and c2 show distinct boundaries of the QD films; and d1, d2, and d3 show the QD films by spin-coating where no clear boundary was observed (the scale bar is 10 μm). (e, f) Representative AFM height image and the three-dimensional image show an rms of 1.10 nm for brush-coated QD films and an rms of 2.84 nm for spincoated QD films.

Here, the maximum current efficiency of 72.38 cd/A and the current density of 4.99 mA/cm2 were achieved for the green QLED. This current efficiency value exceeds most reported QLED devices in the literature.1−4,10,22,24,25,28,29,41−43 Moreover, the current efficiency is more than 60 cd/A in a wide current density range (Figure 3a3). The peak EQE of the green QLED is as large as 17.40%, which can be maintained at greater than 15% at current densities in the range of 0.6−20 mA/cm2, corresponding to a luminance in the range of 379− 12000 cd/m2 (Figure 3a2, a3). For red (Figure 3b2, b3) and blue (Figure 3c2, c3) QLED devices, the maximum luminance of 4500 cd/m2 at 4 V and 315 cd/m2 at 5 V, the maximum current efficiency of 26.03 and 4.26 cd/A, and the peak EQE of 18.96% and 6.20% were realized, respectively. Thus, the brushcoated QLED devices give rather good performances, better than those of spin-coated QLED devices (Figures S3 and S4) under the same experimental conditions, which represent a rather high performance for most recently reported QLED devices with similar structures fabricated by other solution processes (Figure S5). It thus offers a facile, low-cost, yet applicable approach for the practical applications of QLEDs. Conical fibers with quasi-parallel configurations are the essence of the controllable liquid transfer of the Chinese brush,44−46 which is responsible for the homogeneous QD film under the operation conditions. As schematically shown in Figure 4, the ultrasmooth QD film is attributable to the C

DOI: 10.1021/jacs.8b02948 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 3. Performance of green, red, and blue QLED devices. (a1, b1, and c1) EL spectral evolutions of three colored QLED devices with increasing driving voltage. (a2, b2, and c2) Current density and luminance characteristics of the driving voltage with the best efficiency in the green, red, and blue QLED devices. (a3, b3, and c3) Current efficiency and EQE characteristics of the current density with the best efficiency in these QLED devices.

3. CONCLUSIONS In this contribution, we present a simple, yet efficient, approach of Chinese brush-coating to prepare ultrasmooth QD micropatterns by using a small amount of QD solution in one step. Under the cooperative effect of the Marangoni flow of QD solution and the Laplace pressure aroused by conical fibers, QD solution was controllably transferred onto the desired area on the substrate homogeneously with rather small roughness. The resulting green, red, and blue QLED devices gave high performances with peak current efficiencies of 72.38, 26.03, and 4.26 cd/A and peak EQEs of 17.40, 18.96, and 6.20%, respectively. Different from other solution-processing approaches, the brush-coating enables fabrication of ultrasmooth QD films with various micropatterns without any template. Therefore, we envision that it will provide a new template-free, low-cost, and facile solution-processing approach to fabricating high-performance QLED devices.

which can largely accelerate the dewetting process. As a result, the balance of the wetting and dewetting of QD solution on the substrate was broken, leading to the aggregation of QDs within the film (Figure S9), which is not favored for highperformance QLED devices. Here, the best condition for yielding a high-qulity QD film in our system is using brushing speed of 3 mm/s at room temperature (ca. 25 °C). Taken together, the self-assmbly of QDs with fine-controlled duration time was enabled under a certain driving force, which makes the liquid transfer quasisteady throughout the whole process. As a result, high-quality QD films are realized. Moreover, the Chinese brush enabled various micropatterned QD films free from any templates. By fixing the brush on a programmable three-axis (x−y−z) motion stage, both the brushing speed and the direction/moving trace can be accurately controlled by software (Figure S10). Thus, various patterns can be directly brushed onto the substrate using a QD solution. As shown in Figure 5, arrays of QD microlines with different widths and spaces were prepared. Parts a−c of Figure 5 show the QD microlines with the widths increasing from 40 and 80 to 180 μm. Parts d−f of Figure 5 show the QD microlines with widths increasing from 60 and 100 to 190 μm. Meanwhile, various curved micropatterns were also enabled by brush-coating as a triangle, a square, a semicircle, and a wave (Figure 5g−j). The well-defined profile and homogeneous uniform fluorescent colors of these patterns were confirmed, which offer a solution for preparing various complex patterns by the Chinese brush.

4. EXPERIMENTAL SECTION Materials. All brushes used in our experiment were purchased from Beijing Jinghua Brush Co., Ltd., and Yuanwang Painting Materials. QD solutions were kindly provided by Suzhou Xingshuo Nanotech Co., Ltd. n-Octane, acetone, and 2-opropanol were purchased from Beijing Chemical Works and used as received. Micropatterned QD Film Preparation by the Chinese Brush. The Chinese brush was cleaned by ultrasonication in deionized water and ethanol for 10 min and then dried using an N2 stream. The substrates were cleaned consecutively in ultrasonic baths of deionized water, acetone, and 2-propanol for 15 min each and then dried using an N2 stream. By the brush-coating, the QD solution was transferred D

DOI: 10.1021/jacs.8b02948 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Devices Preparation. Patterned ITO glass precoated with TFB and PEDOT:PSS was prepared according to the literature under vacuum condition. Red-, green-, and blue-emissive QD films were brush-coated on the top of the TFB layer. Finally, the multilayer samples were loaded into a high-vacuum chamber for spin-coating ZnO as an electron-transport layer, followed by the deposition of an Al cathode. We characterized the voltage−current density/luminance, current density−current efficiency/EQE, and electroluminescence spectral performance of red, green, and blue devices using a Spectrascan PR655. Characterization of the Microstructures. The optical images of the QD patterns were recorded by a digital video camera (Nikon, N1406 D7200). Photoluminescence (PL) images were taken by fluorescence microscope (OLYMPUS, BX53). The microstructures of the QD film were observed by an atomic force microscope (FASTSCANBIO, Bruker, Germany). The performance of QLED devices was characterized with the assist of Suzhou Xingshuo Nanotech Co., Ltd.

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Figure 4. Schematic cartoons of the dynamic balance of QDs during the whole solution transfer process guided by the Chinese brush. (a) Side-view cartoon of the brush-coating process, where the transfer of the QD solution was finely tuned under the direction stress aroused by conical fibers. In area 1, the QD solution was steadily held within the brush as the cooperative effect of FL, Fa, and G. (b) In area 2, the fibers propel the QDs moving to the base side of fibers under the FL and Fγ, which be used to balance the Marangoni flow of the QD solution aroused by the asymmetric solvent evaporation. σ is surface tension. (c) Top-view cartoon of the TCLs guided by the conical fibers, showing multiple meniscus-shaped TCLs within two neighboring fibers, which helps generating the direction stress along the fibers. (d) Optical picture of brushing process where the QD solution was homogeneously transferred onto the substrate with a clear boundary.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b02948. Experimental details, methods, and additional data (Figures S1−S10) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Huan Liu: 0000-0001-9009-7122 Lei Jiang: 0000-0003-4579-728X Author Contributions #

M.Z. and B.H. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21622302, 21574005, 21203055) and the Fundamental Research Funds for the Central Universities. We greatly thank Suzhou Xingshuo Nanotech Co., Ltd., for helpful assistance with the experiments.

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Figure 5. Various QD micropatterns were prepared directly by brushcoating free from any templates. By adjusting the brushing conditions, micropatterned QD films with good uniformity were obtained. Various green QD microlines arrays with stripe widths increasing from (a−c) 40 and 80 to 180 μm and (d−f) 60 and 100 to 190 μm were prepared. (g−j) Some curved QD micropatterns were prepared by brush-coating, such as (g) a triangle, (h) a square, (i) a semicircle, and (j) a wave.

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onto the substrate homogeneously and also in a well-controllable manner. Here, the moving behaviors of the Chinese brush (speed, direction) were accurately controlled by a programmable three-axis (x−y−z) motion stage by using software; while the interaction between the brush and the substrate was fixed as a certain value of H. Substrate temperature is controlled by a hot plate. Spin-coated QD films were fabricated at 2000 rpm for 40 s. Concentration of green, red and blue quantum dots were 13, 17, and 10 mg/mL, respectively. For QD microline arrays, changing the number of fibers led to microlines with different widths. All experiments were performed at room temperature. E

DOI: 10.1021/jacs.8b02948 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/jacs.8b02948 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX