Coffee-Ring-Free Quantum Dot Thin Film Using ... - ACS Publications

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Coffee-Ring-Free Quantum Dot Thin Film Using Inkjet Printing from a Mixed-Solvent System on Modified ZnO Transport Layer for LightEmitting Devices Congbiao Jiang, Zhiming Zhong, Baiquan Liu, Zhiwei He, Jianhua Zou,* Lei Wang, Jian Wang, JunBiao Peng,* and Yong Cao

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Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Inkjet printing has been considered an available way to achieve large size full-color RGB quantum dots LED display, and the key point is to obtain printed film with uniform and flat surface profile. In this work, mixed solvent of 20 vol % 1,2-dichlorobenzene (oDCB) with cyclohexylbenzene (CHB) was used to dissolve green quantum dots (QDs) with CdSe@ ZnS/ZnS core/shell structure. Then, by inkjet printing, a flat dotlike QDs film without the coffee ring was successfully obtained on polyetherimide (PEI)-modified ZnO layer, and the printed dots array exhibited great stability and repeatability. Here, adding oDCB into CHB solutions was used to reduce surface tension, and employing ZnO nanoparticle layer with PEI-modified was used to increase the surface free energy. As a result, a small contact angle is formed, which leads to the enhancement of evaporation rate, and then the coffee ring effect was suppressed. The printed dots with flat surface profile were eventually realized. Moreover, inverted green QD-LEDs with PEImodified ZnO film as electron transport layer (ETL) and printed green QDs film as emission layer were successfully fabricated. The QD-LEDs exhibited the maximum luminance of 12 000 cd/m2 and the peak current efficiency of 4.5 cd/A at luminance of 1500 cd/m2. KEYWORDS: coffee ring, flat dotlike film, inkjet printing, contact angle, surface free energy, quantum dots light-emitting device

1. INTRODUCTION Colloidal quantum dots light-emitting diodes (QD-LEDs) have attracted great interest due to theirs fascinating properties, like high saturated color emission, great photostability, and tunable emission owing to size dependence.1 Such attractive properties make QD an excellent candidate for next generation full-color display.2−4 The first QD-LEDs were reported about 20 years ago, and great progress has been made in enhancement of device performances, which is close to that of organic LEDs.5,6 Recently, full color QD-LEDs with high performance have been reported by Qian’s group,6 which indicates the commercialization of QD-LEDs in the future. However, it is difficult to pattern an individual RGB QDs layer onto a pixelated display panel for the integration of full-color QD-LEDs with solution process. More recently, transfer printing,7−9 electrohydrodynamic jet printing (EHD-jet),10 and mist deposition11 processes have been proposed as alternative methods for QD patterning. Nevertheless, the QDs were first spin-coated on a transfer stamp, which causes great loss of solution for transfer printing. EHD-jet needs a high electric field between substrate and nozzle to eject ink from nozzle causing bad stability and repeatability. For mist deposition, net volume solution © 2016 American Chemical Society

deposited on all pixels was far less than the overall volume sprayed out from nozzle, and serious mass lost was occurred. Considering these facts and the expensive price of pure QDs, inkjet printing is the best candidate for RGB pattern at low materials consumption. For inkjet printing, ejected materials can be deposited on required points accurately with little materials loss, and inkjet printing has been utilized in many areas of materials and device fabrication, such as polymer LEDs,12−15 small molecular LEDs,16−18 organic TFT.19,20 In recent years, there are some works about inkjet printing of QDs.10,21,22 However, the performance of QD-LEDs with printed QDs emission layer are very poor, for example, Jabbour’s group has reported inkjet printing of red quantum dots LEDs21 with peak luminance about 400 cd/m2 at a high bias of 17 V and a strong exciplex emission between hole transport layer (HTL) and electron transport layer (ETL). Meanwhile, these works did not discuss more about the morphology of inkjet-printed film, which has great influence on Received: July 15, 2016 Accepted: September 9, 2016 Published: September 9, 2016 26162

DOI: 10.1021/acsami.6b08679 ACS Appl. Mater. Interfaces 2016, 8, 26162−26168

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ACS Applied Materials & Interfaces

Figure 1. Dots printing onto substrate S1. (a) PL microscopic images of dots array (scale bar: 200 μm); (b) 3D morphology image; and (c) film thickness profile of each single dot. (a1−a4) Quantum dots inks with volume ratios of 0, 10, 20, and 30% oDBC.

performance of QD-LEDs. As we all know, the printed film without the coffee ring pattern or pinhole was desired for highperformance printed QD-LED or OLED. In this work, we used mixed solvent of CHB and oDCB with volume ratio of 8:2 to dissolve green quantum dots with CdSe@ZnS/ZnS core/shell structure, and the surface tension and viscosity of QDs solution (30 mg/mL) were as low as 31.77 mN/m and 2.45 cP, respectively. Moreover, PEImodified ZnO film with high surface free energy of 55.79 mN/m was selected as substrate, and printed QDs film without the coffee ring was achieved. The relationship of the properties of QD inks, substrates, and their interaction between droplet and substrates were studied. In time, inverted green QD-LEDs with PEI-modified ZnO film as ETL and printed green QDs film as emission layer were fabricated. The green QD-LEDs exhibit a maximum luminance of 12 000 cd/m2 and a peak current efficiency of 4.5 cd/A at luminance of 1500 cd/m2. The results will be beneficial for future fabrication of full-color active matrix QD-LED display based on inkjet printing.

these two substrates via inkjet printing technology. For quantum dots inks, green quantum dots CdSe@ZnS/ZnS powder with average diameter around 12 nm (TEM image of green quantum dots is showed in Figure S1) was purchased from China Beijing BEIDA JUBANG Science & Technology Co., Ltd., and dissolved in mixed solvent of CHB (cyclohexylbenzene)/oDCB (1,2-dichlorobenzene) at 30 mg/mL to make QDs inks, and the quantum yields of QD solution with mixed solvent and QD film are about 81 and 38%, respectively. Then, each quantum dots ink was printed onto substrates S1 and S2 at room temperature by JetLab II printer (Micro Fab Technologies Inc.) with the diameter of 20 μm nozzle, and the printed QDs films were transferred into vacuum chamber to remove residual solvents for 2 h. For the inkjet-printed QD-LED, the structure of device and fabrication are similar to the work we had reported.24 The architecture of device is ITO/ZnO-PEI (40 nm)/inkjet-printed QDs (25 nm)/ TCTA(20 nm)/NPB(40 nm)/MoO3 (8 nm)/Al (200 nm). PEImodified ZnO film worked as electron transport layer (ETL) and was spin-coated on ITO cathode with thermal annealing at 120 °C for 10 min. The QDs emission layer was inkjet-printed on ETL with a thermal treatment at 160 °C for 10 min. Subsequently, double HTLs to enhance device performance were deposited on QDs layer by vacuum evaporation.24 Finally, MoO3 as hole injection layer and Al as anode were deposited in sequence, and the thicknesses were 8 and 200 nm, respectively. The electrical characteristics measurement was the same as that we have reported.24 The properties of QDs inks, such as surface tension and viscosity, were obtained from OneAttension Theta Lite (TL100) and Brookfield Rotational Viscometer (LVDV-I+) at room temperature. The contact angle of inks on two substrates were acquired from OneAttension Theta Lite (TL100) as well. The thickness of each layer was measured by a Dektak 150 surface profiler. 3D morphology images and AFM images were obtained by Veeco NT 9300 and Bruker Multimode 8, respectively. Electroluminescence (EL) and photoluminescence (PL) spectra were measured by USB 2000+ (Ocean Optics) and Lambda950 respectively.

2. EXPERIMENTAL SECTION For the QD-LED, poly(9-vinylcarbazole) (PVK) has been used as HTL widely in conventional QD-LEDs, and ZnO as ETL in inverted QD-LEDs has been reported extensively. In order to research characteristics of printed QDs film on PVK and ZnO films, two kinds of substrates were prepared: glass/ITO/PVK (substrate S1) and glass/ITO/ZnO/PEI (substrate S2). Here PVK (Sigma-Aldrich) with concentration of 8 mg/mL was dissolved in chlorobenzene. The ZnO nanoparticle was synthesized per procedures in previously reported literature23 and dissolved in ethanol with the concentration of 30 mg/ mL. Substrate S1 was fabricated by spin-coating PVK solution on the ITO substrate with thickness of 40 nm, followed by being baked at 140 °C for 30 min. For substrate S2, 40 nm thick ZnO nanoparticle film was deposited on the ITO substrate and annealed at 120 °C for 10 min. Subsequently, polyetherimide (PEI) was dissolved in ethyl alcohol with a concentration of 0.5 mg/mL, then spin-coated on ZnO film with spin speed of 2500 rpm for 35 s to obtain a high surface free energy substrate. Then, the quantum dots film was deposited onto

3. RESULTS AND DISCUSSION 3.1. Dots Printing onto Different Substrate. Figure 1 shows the morphology characteristics of printed dot films onto 26163


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Figure 2. Dots printing onto substrate S2. (A) PL microscopic images of dots array (Scale bar: 200 μm); (B) 3D morphology image; and (C) film thickness profile of each single dot. (A1−A4) Quantum dots inks with volume ratios of 0, 10, 20, and 30% oDBC.

a higher value of WR/R ratio indicates weaker coffee ring effect and better surface morphology. The WR/R ratio against the volume ratio of oDCB is displayed in Figure 3; with increasing

substrate S1 with different oDCB volume ratios (oDCB/CHB) in quantum dots inks. As Figure 1a shows, the radiuses of printed dot films on substrate S1 are almost the same as those when the volume ratios of oDCB increase from 0 to 30% in the ink, but these films illustrate serious coffee ring effect after solvent evaporation, which means most of QDs gathers at the edge of printed film. 3D morphology image in Figure 1b and film thickness profile in Figure 1c confirm that inkjet-printed films on substrate S1 demonstrate different coffee ring structures. From Figure 1b,c, it is easy to find that the printed dots radius for each inks are 63.1, 74.5, 68.2, and 67.3 μm, respectively. There are some changes for the radius of the printed dots with adding oDCB into inks; this may be attributed to dissolution of PVK and cause the movement of contact line. The morphology characteristics of printed QDs film on substrate S2 are shown in Figure 2. It can be easily found that the spread radius of all printed dots are around 60 μm due to good solvent corrosion resistance of PEI, while printed dots on substrate S1 show a large radius when adding oDCB into inks. Besides, those films on substrate S2 exhibit much slighter coffee ring effect, compared with films printed on substrate S1. Moreover, the printed film without the coffee ring pattern is eventually obtained with 20 vol % oDCB blended into ink, but the coffee ring effect occurs when the ratio of oDCB increased to 30 vol %. In general, the films printed on substrate S2 exhibit much better surface profile than those on substrate S1. To quantize the degree of the coffee ring effect formed by printed film, we introduced the ratio of the ring wall width WR to the radius of printed dot R. The width of the coffee ring wall is defined as WR which is the width at the thickness of mean of valley and peak in the coffee ring structure, as Figures 1c and 2C illustrate. The WR/R ratio has a maximum value of 1; the ring wall width WR equals the dot radius at that time, which means printed film without the coffee ring structure. Moreover,

Figure 3. Curve of WR/R ratio versus the oDCB percentage in the mixed solvent ink.

volume ratio of oDCB from 0 to 30%, the value of WR/R increased from 0.27 to 0.58 for films on substrate S1. Obviously, if the percentage of oDCB continued to increase, then the WR/R value may reach up to 1, and a flat film without the coffee ring effect will be obtained. However, when the percentage of oDCB higher than 30%, the interface between PVK and QDs film will become unacceptable because an excess of oDCB in QDs solution can dissolve the subjacent PVK layer; because of the bad solubility of green QDs in oDCB, QDs will precipitate out. Though the concave ring becomes smaller with the increase of WR/R ratio, printed films still show severe classic coffee ring pattern. Compared to the surface profile on substrate S1, the films printed on substrate S2 show a slight coffee ring effect. The WR/R ratio of films printed on substrate 26164


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the contact angle reaches the minimum value of 8.19°, a flat printed film without the coffee ring is obtained. Actually, the formation of the coffee ring results from solute gathering at pin line due to supplement of solution. Thus, restraining the solute movement can suppress the coffee ring effect, the evaporation rate of solvent, and velocity of solute are two important factors. The evaporation time relates to the solvent evaporation rate, which is the solute move time as well. Meanwhile, low translational speed of solute and short movement time make solute difficult to gather at pin line to form the coffee ring structure. Therefore, enhancing the evaporation rate to shorten the evaporation time, which means there is not enough time for solute moving from center to edge to form the coffee ring pattern, and increasing viscosity to reduce solute movement rate, which means it is more difficult for solute to move from center to edge with lower velocity, these two methods to suppress the coffee ring effect have been studied for many years.26,27 For short evaporation time, a small contact angle is in favor of enhancing evaporation rate.26,27 Thus, the WR/R values of printed films on these two substrates increase with reduction of contact angle, and a flat film without the coffee ring structure can be realized with smallest contact angle. However, a larger WR/R value of printed film on substrate S1 is obtained when the percentage of oDCB is 30 vol %, and this may be caused by the advance of contact line, which reduces the contact angle.28,29 Moreover, the evaporation rate of droplets on substrate S2 is higher than that on substrate S1 due to the small contact angle; this is why films printed on substrate S1 show much better surface profile and higher WR/R values of on substrate S2. Higher viscosity causes stronger viscous resistance, making it harder to meet each other for adjacent solute around contact line. In order to restrain the coffee ring effect, high viscosity and fast evaporation rate are desired at the same time. However, it is easy to find from Table 1 that the ink with the lowest surface tension also shows the smallest viscosity, which will lead to a more serious coffee ring effect. Why the coffee ring effect still weakens while solute movement strengthened as well is due to low viscosity. To further investigate the roles of contact angle and viscosity play in formation of the coffee ring, a dimensionless time factor f describes the ratio of particle movement time (τparticle) and solvent evaporation time (τevap), which relate to the viscosity (η) and contact angle (θ), respectively, as eq 1 shows.27

S2 are much closer to maximum value of 1. When the volume ratio of oDCB is 20% in the ink, the printed film with flat surface profile is eventually obtained. In contrast with films on substrate S1, WR/R values of films on substrate S2 are much higher, and these films show a more homogeneous surface. The reason why the surface profiles of films on these two substrates exhibit a huge difference will be given in the following sections. 3.2. Mechanism of Restraining Coffee Ring Effect. As observed in the previous section, the forming behaviors of different inkjet-printed films should result from different properties of solutions, substrates, and interactions between droplet and substrates. The Table 1 summarized the basic Table 1. Viscosity, Surface Tension, and Contact Angle on Substrates S1 and S2 of Different Quantum Dots Inksa contact angle (θ, deg) inks

CHB/oDCB (V/V)

viscosity (η, cP)

surface tension (γ, mN/m)

S1*

S2*

Ink1 Ink2 Ink3 Ink4

10/0 9/1 8/2 7/3

3.14 3.03 2.45 2.5

41.31 40.63 31.77 36.39

13.46 11.57 9.19 10.77

11.47 10.50 8.19 9.99

a

S1: substrate of glass/ITO/PVK; S2: substrate of glass/ITO/ZnO/ PEI.

properties of solution, surface free energy of substrates, and contact angle of different solutions on substrates. From Table 1, we can see that with an increase of oDCB volume ratio from 0 to 20% in mixed solvents, the viscosity and surface tension decrease from 3.14 cP and 41.31 mN/m to 2.45 cP and 31.77 mN/m. When the volume ratio of oDCB reached 30%, however, the viscosity will increase to 2.5 cP, and surface tension increases to 36.39 mN/m. We know that the viscosity and surface tension of pure solvent CHB are 3.68 cP and 34.5 mN/m, after QDs dissolved in CHB solvent, the viscosity reduced to 3.14 cP while surface tension increased to 41.31 mN/m, the variation is due to interaction between solvent molecular and surface ligands of quantum dots.25 With oDCB added into solution, both viscosity and surface tension decrease because of small viscosity and surface tension of oDCB. (The viscosity and surface tension of solvent oDCB are 1.33 cP and 36.6 mN/m.) However, adding more oDCB leads to an increase in the surface tension of ink that we attribute to the bad dissolvability of green QDs in oDCB. Hence, when 30 vol % oDCB is mixed into solution, more QDs dissolve in CHB, causing the increase of surface tension. Therefore, the ink exhibits the lowest viscosity and surface tension when adding 20 vol % into CHB solution. The contact angle of droplets on substrate S1 decreases from 13.46 to 9.19° with the volume ratio of oDCB increasing from zero to 20%, however, when 30 vol % oDCB added into CHB solution, contact angle increases to 10.77° (shown in Table 1). Besides, the contact angle of four inks on substrate S2 shows the same variation trend with those on substrate S1: Lower surface tension of solution leads to smaller contact angle. Hence, with the decrease of surface tension, the contact angle decreased for these two substrates. Moreover, the contact angle on substrate S1 is larger than those on substrate S2; this is because the surface free energy of substrate S2 is 55.79 mN/m, much higher than that of substrate S1 (44.68 mN/m). When

f=

τparticle τevap

∝

η θ

(1)

Figure 4 plots the η value versus volume ratio of oDCB with θ different substrates and shows that for these two substrates the η value increases with the increase volume ratio of oDCB and θ reaches the highest value at oDCB volume ratio of 20 vol %. Comparing the η values of these two substrates, η values of θ θ substrate S2 are larger than those of substrate S1 for ink with oDCB ratios of 0, 10, 20, and 30 vol %. According to the previous discussion, we find that the coffee ring effect recedes gradually with increase of the η value for these two substrates. θ Especially, the printed dots exhibits prefect homogeneous surface with the highest η value of substrate S2. When adding θ 20 vol % oDCB into CHB solution, both contact angle and viscosity decrease to the lowest value, but the η value still θ

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Figure 4. Curve of substrates.

η θ

large films, and we successfully printed quantum dots lines with good surface morphology by inkjet printing multiple droplets of ink (adding 20 vol % oDCB) with droplet spacing of 50 μm and line spacing of 200 μm on PEI-modified ZnO substrate (Figure 5). As the PL image (Figure 5A) shows, inkjet-printed solid lines with width of 125 μm exhibit great uniformity without any bulging or scalloped. AFM image (Figure 5B) and 3D morphology image (Figure 5C) show that the printed line has a smooth and flat surface, and the surface root mean squared values (RMS) is about 3.87 nm, showing good film morphology which is necessary for better performance of QDLED. For a large-sized display, emission uniformity is very important which relates to many factors, such as thickness uniformity. Thus, we have scanned six points of one printed line with length of 15 mm and find that these six points all have the same thickness, as the Figure 5D shows. The printed lines film with good surface profile is in favor of the future fabrication of AM-QLED based on inkjet printing. 3.4. Inkjet-Printed QD-LED. QD-LED with inkjet-printed QDs light-emission layer was fabricated. The ITO was patterned with low surface energy separators, polyimide (PI), as shown in Figure 6A. The cross section of PI separators is trapezoid with an upper base of 10 μm and a bottom base of 20 μm; the patterned ITO has an effective area of 233.8 μm × 74.6 μm for each subpixel. Figure 6B is the light-on image of subpixel, and every subpixel shows great luminance uniformity. Moreover light-emitting zone equals the effective area of ITO, which suggests the printed film overspreads the whole subpixel and has good thickness uniformity without the coffee ring structure. Figure 6C is optical image of inkjet-printed green QD-LED with a bias of 6 V. The inkjet-printed device expresses great emission homogeneity without any dead pixels, and this is very important for the large size display. Figure 7A,B shows the electrical characteristics of the inkjet printing device: The turn-on voltage (1 cd/m2) is about 5.1 V,

value versus volume ratio of oDCB with different

reaches its peak, which means the evaporation rate is accelerated greatly due to smallest contact angle though the solute movement becomes easiest with lowest viscosity. Comparing the time of close QDs meeting others, the solvent evaporation time is much shorter, causing the suppression of the coffee ring effect. The coffee ring effect disappears when evaporation is enhanced with small contact angle, though solute movement was strengthened as well due to low viscosity. Therefore, a higher η value indicates that it is more difficult to θ form the coffee ring pattern because the time of adjacent QDs meeting others is shortened severely via reducing the contact angle. In other words, a higher η value suggests better surface θ morphology of printed film. Here η is the viscosity of ink and θ is the contact angle of droplets spreading on substrates. 3.3. Lines Printing onto PEI-Modified Substrate. Line printing offers a feasible approach to remove the pixel well structure.18 Furthermore, line printing is the basis of printing

Figure 5. (A) PL image, scale bar: 200 μm; (B) AFM image of the center area (part of the selected area in A); (C) 3D morphology image of selected area in A; and (D) film thickness profile of 6 points in one printed lines. 26166


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the Figure S2. The current efficiency of QD-LED with inkjetprinted emission layer is just half that of the control device; one possible reason for the efficiency reduction is that the inkjetprinted film has a larger roughness compared with that of spincoated film. (The morphology of spin-coated film is depicted in the Figure S3.) In addition, at the luminance of 100 cd/m2, the driving voltage is about 6.8 V; the current density and current efficiency are 2.99 mA/cm2 and 2.95 cd/A, respectively. At a high luminance of 1000 cd/m2, the driving bias just increases to 7.6 V, while the current efficiency has increased by 42%, reaching up to 4.25 cd/A. The inset of Figure 7B shows the PL spectrum of QDs film and EL spectrum of inkjet-printed device at 6 V. As can be seen, both EL and PL spectrum demonstrate a pure green emission and the full-width at half-maximum (fwhm) is 29 nm and great saturated emission from QDs with the Commission International de l’Eclairage (CIE) coordinate of (0.20, 0.74). The inkjet-printed devices show great color purity; the printed QDs ink spreads the whole subpixel continuously causing pure green light emission without any exciplex emission.

Figure 6. (A) Micro image of subpixel with inkjet-printed QDs film from CHB/oDCB (80/20, V/V) solution; (B) light-on image of subpixel; and (C) light-on image of inkjet-printed green QD-LED with a bias of 6 V.

4. CONCLUSIONS By adding 1,2-dichlorobenzene (oDCB) into the green quantum dots solution to reduce the surface tension and selecting a PEI-modified ZnO film with high surface free energy as substrate, the solvent evaporation rate was enhanced greatly, and the coffee ring effect was restrained. A flat surface profile CdSe@ZnS/ZnS core/shell structure quantum dots film has been deposited by inkjet printing. To explain this phenomenon, the time scale of solvent evaporation and QDs movement were considered, which were related to the contact angle and viscosity, respectively. The η value was evaluated, and the best θ

flat printed film was achieved with the highest η θ

η θ

value when

20% oDCB was added into CHB. A high value meant that the solvent evaporated more quickly than adjacent QDs meeting each other, so the coffee ring effect was suppressed. Last, goodperformance green QD-LED with inverted structure was successfully fabricated by inkjet printing QDs layer, having a peak current efficiency of 4.5 cd/A and a maximum luminance of 12 000 cd/m2. In addition, green QD-LED showed a current efficiency of 4.25 cd/A at luminance of 1000 cd/m2 and 4.29 cd/A at luminance of 5000 cd/m2, relatively high performance without roll-off. Beyond a doubt, this work is beneficial to the future fabrication of a large full-color QD-LED display based on inkjet printing.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. Characteristics of QD-LED with inkjet-printed emission layer. (A) Current density−voltage−luminance and (B) current efficiency−current density curves of QD-LEDs. Inset: PL spectrum of spin coated green QDs film and EL spectrum of QD-LED based on inkjet-printed emission layer under a voltage of 6 V.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08679. TEM image of green quantum dots, characteristic of QDLEDs with inkjet-printed and spin-coated emission layers, and morphology of spin-coated QDs film (PDF)

the maximum luminance is 12 000 cd/m2, and the peak current efficiency is 4.5 cd/A at the luminance of 1500 cd/m2. A control device with spin-coated emission layer has been fabricated as well. The control device shows a turn-on voltage of 5.2 V, maximum luminance of 15 000 cd/m2, and peak current efficiency of 10.7 cd/A. The corresponding current density−voltage−luminance (J−V−L), current efficiency− current density (CE−J) characteristics of QD-LEDs with inkjet-printed and spin-coated emission layer can be found in

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 26167


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(18) Liu, H.; Xu, W.; Tan, W.; Zhu, X.; Wang, J.; Peng, J.; Cao, Y. Line Printing Solution-Processable Small Molecules with Uniform Surface Profile via Ink-Jet Printer. J. Colloid Interface Sci. 2016, 465, 106−111. (19) Kang, B. J.; Oh, J. H. Control of the Crystalline Structure of Inkjet-Printed Semiconductor Layers Using Overlap Condition and Surface Wettability. J. Micromech. Microeng. 2015, 25, 055011. (20) Grimaldi, I. A.; Barra, M.; De Girolamo Del Mauro, A.; Loffredo, F.; Cassinese, A.; Villani, F.; Minarini, C. Inkjet Printed Perylene Diimide Based OTFTs: Effect of the Solvent Mixture and the Printing Parameters on Film Morphology. Synth. Met. 2012, 161, 2618−2622. (21) Haverinen, H. M.; Myllylä, R. a.; Jabbour, G. E. Inkjet Printing of Light Emitting Quantum Dots. Appl. Phys. Lett. 2009, 94, 073108. (22) Haverinen, H. M.; Myllylä, R. a.; Jabbour, G. E. Inkjet Printed RGB Quantum Dot-Hybrid LED. J. Disp. Technol. 2010, 6, 87−89. (23) Qian, L.; Zheng, Y.; Choudhury, K. R.; Bera, D.; So, F.; Xue, J.; Holloway, P. H. Electroluminescence from Light-Emitting polymer/ ZnO Nanoparticle Heterojunctions at Sub-Bandgap Voltages. Nano Today 2010, 5, 384−389. (24) Jiang, C.; Liu, H.; Liu, B.; Zhong, Z.; Zou, J.; Wang, J.; Wang, L.; Peng, J.; Cao, Y. Improved Performance of Inverted Quantum Dots Light Emitting Devices by Introducing Double Hole Transport Layers. Org. Electron. 2016, 31, 82−89. (25) Craik, A. D. D. Thomas Young on Fluid Mechanics. J. Eng. Math. 2010, 67, 95−113. (26) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. a. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827−829. (27) Shen, X.; Ho, C.-M.; Wong, T.-S. Minimal Size of Coffee Ring Structure. J. Phys. Chem. B 2010, 114, 5269−5274. (28) Li, R.; Shan, Y. Contact Angle and Local Wetting at Contact Line. Langmuir 2012, 28, 15624−15628. (29) Ralston, J.; Popescu, M.; Sedev, R. Dynamics of Wetting from an Experimental Point of View. Annu. Rev. Mater. Res. 2008, 38, 23− 43.

ACKNOWLEDGMENTS This work was supported by the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655004), National Natural Science Foundation of China (No. 61574061), and Science and Technology Project of Guangdong Province (2015B090914003).

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