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Inkjet-printing line film with varied droplet-spacing
Accepted Manuscript Inkjet-printing line film with varied droplet-spacing Lan Mu, Zhanhao Hu, Zhiming Zhong, Congbiao Jiang, Jian Wang, Junbiao Peng, Yong Cao PII:
S1566-1199(17)30405-6
DOI:
10.1016/j.orgel.2017.08.012
Reference:
ORGELE 4259
To appear in:
Organic Electronics
Received Date: 30 May 2017 Revised Date:
10 August 2017
Accepted Date: 11 August 2017
Please cite this article as: L. Mu, Z. Hu, Z. Zhong, C. Jiang, J. Wang, J. Peng, Y. Cao, Inkjet-printing line film with varied droplet-spacing, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.08.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Inkjet-printing line film with varied droplet-spacing Lan Mu, Zhanhao Hu, Zhiming Zhong, Congbiao Jiang, 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
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510640, P. R. China.
*
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Corresponding authors. E-mail addresses: [email protected] (J.Peng), [email protected] (J. Wang)
Abstract: Compared to the conventional cell-shaped pixel structure, linear bank structure for solution-processed organic light-emitting diode (OLED) displays has the advantage of
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simple process, and large aperture-ratio. However, the prolonged drying time of the inkjet-printed ink in the linear bank forms a convex surface profile in the longitude To
achieve
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direction.
uniform
film
surface
of
the
inkjet-printed
poly
(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), a novel printing method by varying the droplet-spacing is introduced. It is found that a larger droplet density at the line ends than that at the line center can compensate the inward flow of the ink, enabling systematic control of the film thickness along the line direction. As a result, uniform PEDOT:PSS line pattern is achieved. OLED display panels with line pixels are 1
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fabricated with varied droplet-spacing demonstrating better brightness uniformity than
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those inkjet-printed with constant droplet-spacing.
Keywords: Inkjet printing, Organic light-emitting diode, Surface morphology, Ink
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formulation, OLED display
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1. Introduction
Organic light-emitting diodes (OLEDs) technology has been commercialized in recent years. Up to date, all the commercial OLED displays are fabricated by thermally evaporating multilayer functional materials in vacuum.[1] In 1991, Heeger et al.
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introduced the solution process to OLED manufacturing.[2] In 2013, our group demonstrated the world’s first all-solution processed polymer OLED display.[3] The solution process is typically carried out at room temperature, which significantly reduces
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the production cost. Among all the solution processes, inkjet-printing is the most versatile
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deposition tool as a noncontact, mask-free, material-saving, and on-demand patterning technique.[4] Recently, through the development of materials and processes, the printed film morphology and the device efficiency of inkjet-printed OLED displays have been greatly improved.[5–10] The narrowed performance gap between printed and evaporated devices shows the promising future of the inkjet-printed organic electronics.[5,11,12]
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To pattern primary color pixels in a solution-processed OLED display via inkjet-printing, banks are usually constructed on the substrate to confine the spreading of
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inkjet-printed droplets and guide the ink flow.[13,14] The conventional pixel structure in displays consists of cells surrounded by bank walls, in the shape of, for example, dots, squares or rectangles. For printed OLED, the oval shape (shown in Figure 1a) is commonly
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employed due to better ink wetting at the pixel corners. However, the major drawback of
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cell pixels is their low aperture ratio. To overcome such problem, a linear bank structure (Figure 1b) inside which ink is printed as a continuous line pattern across the display.[15,16] Such bank structure greatly simplifies the printing process and increases the aperture ratio by eliminating half of the bank walls.
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Conventionally, each pixel is filled with inks by printing evenly spaced droplets, which subsequently coalesce into a single continuous film.[17,18] Though the approach is simple and effective on cell-shaped pixels, on much elongated linear banks, the impact of fluid
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flow along the line direction is magnified and can sometimes cause non-uniformity of the
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film thickness. To prevent coffee ring effect, double-solvent ink formulation is often adopted. The surface tension difference of the solvents forms an inward flow (Marangoni flow) to compensate the outward flow (Capillary flow).[19,20] In a linear bank structure, drying time of the ink is prolonged due to the increased ink amount in a single bank structure. As a result, the Marangoni flow along the line direction transports excessive solutes from the line ends to the line center. Thus, the linear film forms a convex surface 3
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profile. The uneven film thickness can lead to brightness non-uniformity across the display.
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Previous studies on modifying line-patterned film morphology focused on suppressing coffee rings formed perpendicular to the lines.[15,17,18] In our contribution, we target to
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achieve uniform film thickness along the longitudinal direction. The hole injection material, poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) is
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selected as the ink in the investigation. By printing PEDOT:PSS droplets with constant droplet-spacing, the ink contracts during drying and forms a line with gradually increased thickness from line ends to the center. A novel printing scheme is developed to solve the problem. By printing PEDOT:PSS droplets with varied spacing, i.e. the spacing at the line
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ends is the smallest, and gradually increases toward the center, the accumulation of ink at the center is suppressed, and the film-thickness variation is systematically modulated. A uniform line film morphology is achieved by fine adjusting the spacing parameters and
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droplet number. The fabricated OLED display panel shows a much superior brightness
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uniformity compared to conventional constant-spacing printing. The simplicity and effectiveness of the inkjet-printing process with varied droplet-spacing could be applied in inkjet-printing any inks to obtain flat film.
2. Experiments
2.1. Inks Preparation:
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PEDOT:PSS (Clevios™ P CH 8000) is purchased from Heraeus Electronic Materials Division,
and
used
as
the
hole
injection
layer
(HIL).
Poly
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(dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) is synthesized in our lab, and used as the light emission layer. The HIL ink is formulated by diluting PEDOT:PSS with 60 vol% deionized water (DI water), and 10 vol% ethylene glycol (EG).
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After all the components are mixed, the ink is ultrasonically stirred for 30 seconds at room
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temperature. The viscosity of the inks is obtained by a Brookfield Rotational Viscometer (LVDV-I+) at room temperature. A OneAttension Theta Lite (TL100) is used to measure the surface tension of the inks.
2.2. Display Panel Fabrication:
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The display panel is made by etching polyimide (PI) coated ITO glass into linear banks. The panel size is 15 mm × 15 mm. The linear bank is 8 mm long, 50.5 µm wide, and 1.5 µm
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high. Each panel has 113 linear banks. Before inkjet-printing PEDOT:PSS ink, the patterned substrate is cleaned in ultrasonic bath of DI water, and isopropyl alcohol in
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sequence, followed by UV-O3 cleaning for 10 min. The substrate is maintained at room temperature during printing process. PEDOT:PSS ink is inkjet-printed into the banks at room temperature, by a JetLab II printer manufactured by MicroFab Technologies, Inc., from a 30 µm nozzle. The volume of each PEDOT:PSS droplet is about 16 pL. After the ink is printed, the substrate is annealed at 180 ºC on a hot plate in a nitrogen filled glove
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box for 10 min. The thickness and the surface morphology of the PEDOT:PSS layer are characterized by a Dektak 150 surface profiler (Bruker Corp.) and Veeco NT 9300.
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To fabricate the light-emission device, a 70 nm thick PF-FSO layer is spin-casted from p-xylene solution onto the printed PEDOT:PSS layer, followed by 20 min baking at 140 ºC on a hot plate in a nitrogen filled glove box. The device is completed by thermally
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3. Results and Discussion
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evaporating the metal cathode of Ba/Al in the vacuum under a pressure of 1 × 10-4 Pa.
In the experiment, PEDOT:PSS ink is diluted by 60 vol% DI water, and 10 vol% EG (ink properties are provided in Supporting Information Table S1). The addition of high
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boiling-point solvent EG greatly improves jetting stability of the ink, and suppresses coffee ring effect (Figure S1).[21,22] The linear bank has a dimension of 8 mm × 50.5 µm as schematically shown in Figure 2a. The substrate has a total of 113 banks. PEDOT:PSS
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droplets were inkjet-printed into the linear banks with constant droplet-spacing of 35 µm.
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The ink was printed in every two lines for better viewing. During the ink drying process, the local vapor pressure is higher at the line center than at the line ends (Figure 2b). The fast solvent evaporation rate at the line ends leaves more EG content at the line ends than at the line center. The contrast of solvent surface tension generates a Marangoni flow towards the center. Since the ink amount in a line bank is much larger than that of a single droplet (Figure S1), the ink drying time is prolonged, and the inward Marangoni flow transports
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excessive PEDOT:PSS to the center. The PEDOT:PSS drying dynamic is shown in Figure S2. Four minutes after printing, the PEDOT:PSS lines are still not completely dried.
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During the drying process, the liquid boundary moved from the line ends slowly toward the center, bringing solutes along the way. After total solvent evaporation (about 6 min later or more), the film surface along the longitude direction shows a convex profile as shown in
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Figure 2c (15 locations on the printed line are sampled for measuring the film thickness).
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In order to achieve uniform film thickness along the line direction, a novel inkjet-printing scheme with varied droplet-spacing is developed. By setting small droplet-spacing at the edge and large droplet-spacing at the center, the solute amount is larger at the edge than that at the center. The inward Marangoni flow will transport solute
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from the edge to the center to form a uniform distribution of PEDOT:PSS. The inkjet-printing process with varied droplet-spacing is compared against the process with constant droplet-spacing in Figure 3. The simplest droplet-spacing variation is to change
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the spacing value in an arithmetic progression.
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As shown in Figure 3, we define each spacing as di, the common difference as ∆d, and
the droplet number of half the line as n. The droplet-spacing setting is symmetric to the center point. Since the half length of the line pixel is 4 mm, the droplet-spacing values can be determined using the following formulas: +
∆ = 4 mm
= + − 1∆ 7
(1) (2)
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For the constant droplet-spacing (∆d = 0) in Figure 3a, di and n is set to be 35 µm and 114, respectively. The film thickness at the center region is about 49 nm (Figure 2c). Using
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the result as a starting point, two approaches are designed to vary the droplet-spacing. In the first approach, the largest spacing of the center droplets (dn) is fixed to 35 µm, and the spacing towards the line ends is reduced gradually. The second approach is fixing the
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smallest spacing at the line ends (d1) at 35 µm, and gradually increasing the spacing toward
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the center. Various spacing difference ∆d is applied in both approaches as shown in Table 1. Since the diameter of a single droplet on the substrate is about 60 µm (Figure. S1), the largest spacing (dn) is kept under 60 µm in order to form a continuous thin film. When the droplet spacing is less than 27.5 µm, the printed ink will overflow the bank and cause
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cross-contamination between adjacent lines. Therefore, the smallest droplet spacing (d1) is set at 27.5 µm. The distribution of droplet density per millimeter across half the line pixel is plotted in Figure S3. Both approaches have the largest droplet density at the line ends, and
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monotonically decreased density towards the center. The density is kept around 28 drops
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per millimeter at the center in the first approach, while the density is kept around 28 drops per millimeter at the line end in the second approach. The printed PEDOT:PSS line films show gradually changed surface profiles with
varied droplet-spacing. Figure 4a and d are the film thickness in the longitude direction. The line film inkjet-printed with constant droplet-spacing at n = 114 serves as a reference. The average thickness of the line (representing the relative content of ink in each bank) printed through the two approaches are plotted in Figure 4b and e. It’s expected that the 8
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average film’s thickness decreases as the total number of droplets n decreases.
To evaluate the film uniformity, the ratio of the largest thickness at the center to the
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smallest at the ends (Hmax/Hmin) is compared in Figure 4c and f. As shown in Figure 4a and b, the line inkjet-printed with constant droplet-spacing (d1 = dn = 35 µm
n = 114)
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produces the most significant convex surface profile. Hmax/Hmin reaches 1.75 with the film thickness of 28 nm at the ends, and 49 nm at the center. For n = 119, 123, and 128
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with fixed dn = 35 µm, the smallest spacing d1 are 32.5 µm, 30 µm, and 27.5 µm, respectively. As depicted in Figure 2b, the excessive solutes at the line ends will be transported inward by the Marangoni flow during solvent evaporation. As a result, Hmax/Hmin decreases as n increases. At n = 128, Hmax/Hmin is reduced to about 1.13, giving
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an almost flat film surface. For n = 110, 106, 103, 99, and 94 with fixed d1 = 35 µm, the largest spacing dn are 37.5 µm, 40 µm, 42.5 µm, 45 µm, and 50 µm, respectively. Increasing the droplet-spacing towards the line center suppresses the convex surface
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profile. As a result, Hmax/Hmin decreases as n decreases. At n = 94, Hmax/Hmin decreases to
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1.03. The line film surface becomes flat. The film thickness difference Hmax-Hmin is plotted as a function of the droplet density difference in Figure 5. It is revealed that the film surface profile is governed by the droplet density contrast. Meanwhile, the average film thickness can be modulated by adjusting the total printed droplet number. For the films with best uniformity, n = 128 yields an average film thickness of 40 nm and n = 94 yields about 27 nm. Therefore, high quality thin film with desired thickness is achievable 9
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via systematic tuning of the spacing variation parameters. To demonstrate the impact of the film uniformity on light emission, complete display
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panel with linear banks is fabricated. The OLED device structure is shown in Figure 6a. A blue polymer, poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) is spin-coated on top of PEDOT:PSS as the light-emitting layer.[23] For better
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comparison, the substrate is divided into two parts. As shown in Figure 6b, on the upper
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half of the panel, PEDOT:PSS is inkjet-printed with varied droplet-spacing, i.e. n = 128 with fixed dn = 35 µm, to achieve the best film uniformity. On the lower half of the panel, PEDOT:PSS is inkjet-printed with the constant droplet-spacing of n = 114 with d1 = dn = 35 µm. For the pixel lines printed by constant droplet-spacing, since PEDOT:PSS layer’s
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thickness diminishes towards the line ends, charge injection at the line edge region is not as efficient as that at the line center region. As a result, the line edge region of the panel barely emits light as shown in Figure 6b. By contrast, for the pixel lines printed by varied
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thickness.
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droplet-spacing, the whole lines give a homogenous brightness due to the uniform film
The pixels’ brightness is read from the display image in Photoshop as gray scale, and
illustrated in Figure 7. As expected, the brightness of the pixel lines printed by constant droplet-spacing, sharply drops at the line edge region, while the brightness of the pixel lines printed by varied droplet-spacing remains constant across the whole line. Of note, though the PEDOT:PSS film thickness of the pixel lines printed by constant 10
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droplet-spacing varies from about 30 nm to 50 nm (Fig. 2c, Fig. 5), the brightness variation across the non-shrunk region is almost identical to that of the pixel lines printed with varied
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droplet-spacing. The reason is that PEDOT:PSS is conducting polymer with relative high electric conductivity compared to the light-emitting material. Therefore, the light-emitting device’s brightness is not sensitive to PEDOT:PSS thickness in the range of about 30 to 50
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nm. The brightness variation mainly comes from the non-uniformity of the spin-coated
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PF-FSO layer. The brightness uniformity, 1 - (Lmax - Lmin)/Lmax,[24] is about 75%, showing that spin-coating film on substrate with pixel structures can’t obtain high uniform surface profile.[25]
Besides the brightness non-uniformity, there are other defects of the display, such as
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bright spots, dark spots, etc. Dark spots are caused by either the dusts, or the residue photoresist. Bright spots are caused by the over-etched bank walls. The bank walls are patterned by photolithography. Some parts of the walls are over-etched. As a result, the
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spin-coated PF-PFO film is thinner across the over-etched walls than the film in other area,
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since the over-etched walls can’t block the spin-coated ink as well as the regular walls. Under the same operation voltage, the thin light-emission layer will emit more lights due to the large electric field across the film.
4. Conclusions
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To inkjet-print linear films with high uniformity along the longitude direction, a printing scheme with varied droplet-spacing is successfully developed to turn the surface
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profile from convex to flat. A simple arithmetic progression of the droplet-spacing setting is introduced to suppress the solute accumulation at the line center. It is found that the surface profile is mainly governed by the droplet density difference along the line, while
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the average thickness is determined by the total droplet number in each linear bank. By fine
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tuning the droplet-spacing parameters, printing at n = 128, ∆d = 0.059 µm, and n = 94, ∆d = 0.161 µm, result in flat film surface. The OLED display panel ink-jet printed with varied droplet-spacing confirms the uniformity of the film. The technique reduces the task to further adjust the solvents of the ink, and could be applied in inkjet-printing any inks to
Acknowledgements
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obtain uniform films along the longitude direction.
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The authors are deeply grateful to the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655004) founded by MOST, National
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Natural Science Foundation of China (51521002， U1601651， 51573056, 51373057), and National Key Research and Development Program of China (2016YFB0401400) for their financial supports.
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[24]
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d (µm)
dn (µm)
∆d (µm)
128
27.5
35
0.059
123
30
35
0.041
119
32.5
35
0.021
114
35
35
0
110
35
37.5
0.023
106
35
40
0.048
103
35
42.5
0.074
99
35
45
0.102
94
35
50
0.161
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n
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Table 1. Printing set-up.
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Figure Captions: Figure 1. (a) Conventional pixel bank structure. (b) Linear bank structure.
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Figure 2. (a) Schematic illustration of the patterned ITO substrate with linear banks. (b) Schematic illustration of the film drying process. (c) Surface morphology of the inkjet-printed film with constant droplet-spacing d = 35 µm.
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Figure 3. Schematic illustration of inkjet-printing with (a) constant droplet-spacing, and (b)
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varied droplet-spacing.
Figure 4. (a, d) The surface morphology of the printed line films. (b, e) The dependence of the average film thickness on the total number of the droplets. (c, f) The dependence of Hmax/Hmin ratio on the total number of the droplets.
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Figure 5. The dependence of Hmax-Hmin on the printing droplet density difference. Figure 6. (a) The OLED device structure. (b) Light-emission images of the OLED display
(lower half).
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panel inkjet-printed with varied droplet-spacing (upper half) and constant droplet-spacing
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Figure 7. The pixel brightness in the units of gray scale along the line of the OLED display panel operated at 5V.
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
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Appendix A. Supplementary data Supporting Information
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Inkjet-printing line film with varied droplet-spacing
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Lan Mu, Zhanhao Hu, Zhiming Zhong, Congbiao Jiang, Jian Wang*, Junbiao Peng*, and Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou
*
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510640, P. R. China.
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Corresponding authors. E-mail addresses: [email protected] (J.Peng), [email protected] (J. Wang)
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1. Rheological properties of the PEDOT:PSS ink and the solvents Table S1. Rheological properties of the PEDOT:PSS ink and the solvents. Boiling point (ºC)
Viscosity (cP)
EG Water 3:6:1 (CH 8000:DI:EG)
197.3 99.9 -
25.66 1.009 6.95
Surface Tension (mN/m)
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Composition
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46.49 72.7 67.33
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Table S1 lists the rheological properties of the PEDOT:PSS ink (diluted by 60 vol% DI water, and 10 vol% EG) and its solvents. Adding high boiling point EG prevents clogging of the nozzle, thereby improves jetting stability of the ink. The mixed solvents help suppress coffee-ring effect (Figure S1).
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2. Surface morphology of the printed droplet
Figure S1. Surface morphology of an inkjet-printed PEDOT:PSS droplet on ITO. (a) The three-dimensional picture was taken by the white light interferometer. (b) The surface profile shows no coffee-ring formation.
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3. The drying dynamic of the printed ink.
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Figure S2. The ink’s contact line moves from the line end to the line center. Microscopic images are taken at t = 30 s, 60 s, 2 min, 3 min, and 4 min. 4. The droplet density along the linear bank
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n = 128 n = 123 n = 119
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n = 114 n = 110 n = 106 n = 103
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Droplet Density (/mm)
40
20
n = 99 n = 94
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0
1
2
3
Location (mm)
Figure S3. The droplet density along the linear bank.
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4
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Highlights: 1. Ink-jet printed linear film with constant droplet-spacing shows non-uniform surface. 2. Printing with varied droplet-spacing is developed to turn the surface from convex to flat.
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3. The surface profile is mainly governed by the droplet density difference along the line.
4. Larger droplet density at the line ends than that at the center compensates the inward flow.
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5. The technique eliminates the need for multiple solvents in ink formulation.
S1566-1199(17)30405-6
DOI:
10.1016/j.orgel.2017.08.012
Reference:
ORGELE 4259
To appear in:
Organic Electronics
Received Date: 30 May 2017 Revised Date:
10 August 2017
Accepted Date: 11 August 2017
Please cite this article as: L. Mu, Z. Hu, Z. Zhong, C. Jiang, J. Wang, J. Peng, Y. Cao, Inkjet-printing line film with varied droplet-spacing, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.08.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Inkjet-printing line film with varied droplet-spacing Lan Mu, Zhanhao Hu, Zhiming Zhong, Congbiao Jiang, 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
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510640, P. R. China.
*
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Corresponding authors. E-mail addresses: [email protected] (J.Peng), [email protected] (J. Wang)
Abstract: Compared to the conventional cell-shaped pixel structure, linear bank structure for solution-processed organic light-emitting diode (OLED) displays has the advantage of
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simple process, and large aperture-ratio. However, the prolonged drying time of the inkjet-printed ink in the linear bank forms a convex surface profile in the longitude To
achieve
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direction.
uniform
film
surface
of
the
inkjet-printed
poly
(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), a novel printing method by varying the droplet-spacing is introduced. It is found that a larger droplet density at the line ends than that at the line center can compensate the inward flow of the ink, enabling systematic control of the film thickness along the line direction. As a result, uniform PEDOT:PSS line pattern is achieved. OLED display panels with line pixels are 1
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fabricated with varied droplet-spacing demonstrating better brightness uniformity than
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those inkjet-printed with constant droplet-spacing.
Keywords: Inkjet printing, Organic light-emitting diode, Surface morphology, Ink
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formulation, OLED display
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1. Introduction
Organic light-emitting diodes (OLEDs) technology has been commercialized in recent years. Up to date, all the commercial OLED displays are fabricated by thermally evaporating multilayer functional materials in vacuum.[1] In 1991, Heeger et al.
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introduced the solution process to OLED manufacturing.[2] In 2013, our group demonstrated the world’s first all-solution processed polymer OLED display.[3] The solution process is typically carried out at room temperature, which significantly reduces
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the production cost. Among all the solution processes, inkjet-printing is the most versatile
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deposition tool as a noncontact, mask-free, material-saving, and on-demand patterning technique.[4] Recently, through the development of materials and processes, the printed film morphology and the device efficiency of inkjet-printed OLED displays have been greatly improved.[5–10] The narrowed performance gap between printed and evaporated devices shows the promising future of the inkjet-printed organic electronics.[5,11,12]
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To pattern primary color pixels in a solution-processed OLED display via inkjet-printing, banks are usually constructed on the substrate to confine the spreading of
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inkjet-printed droplets and guide the ink flow.[13,14] The conventional pixel structure in displays consists of cells surrounded by bank walls, in the shape of, for example, dots, squares or rectangles. For printed OLED, the oval shape (shown in Figure 1a) is commonly
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employed due to better ink wetting at the pixel corners. However, the major drawback of
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cell pixels is their low aperture ratio. To overcome such problem, a linear bank structure (Figure 1b) inside which ink is printed as a continuous line pattern across the display.[15,16] Such bank structure greatly simplifies the printing process and increases the aperture ratio by eliminating half of the bank walls.
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Conventionally, each pixel is filled with inks by printing evenly spaced droplets, which subsequently coalesce into a single continuous film.[17,18] Though the approach is simple and effective on cell-shaped pixels, on much elongated linear banks, the impact of fluid
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flow along the line direction is magnified and can sometimes cause non-uniformity of the
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film thickness. To prevent coffee ring effect, double-solvent ink formulation is often adopted. The surface tension difference of the solvents forms an inward flow (Marangoni flow) to compensate the outward flow (Capillary flow).[19,20] In a linear bank structure, drying time of the ink is prolonged due to the increased ink amount in a single bank structure. As a result, the Marangoni flow along the line direction transports excessive solutes from the line ends to the line center. Thus, the linear film forms a convex surface 3
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profile. The uneven film thickness can lead to brightness non-uniformity across the display.
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Previous studies on modifying line-patterned film morphology focused on suppressing coffee rings formed perpendicular to the lines.[15,17,18] In our contribution, we target to
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achieve uniform film thickness along the longitudinal direction. The hole injection material, poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) is
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selected as the ink in the investigation. By printing PEDOT:PSS droplets with constant droplet-spacing, the ink contracts during drying and forms a line with gradually increased thickness from line ends to the center. A novel printing scheme is developed to solve the problem. By printing PEDOT:PSS droplets with varied spacing, i.e. the spacing at the line
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ends is the smallest, and gradually increases toward the center, the accumulation of ink at the center is suppressed, and the film-thickness variation is systematically modulated. A uniform line film morphology is achieved by fine adjusting the spacing parameters and
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droplet number. The fabricated OLED display panel shows a much superior brightness
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uniformity compared to conventional constant-spacing printing. The simplicity and effectiveness of the inkjet-printing process with varied droplet-spacing could be applied in inkjet-printing any inks to obtain flat film.
2. Experiments
2.1. Inks Preparation:
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PEDOT:PSS (Clevios™ P CH 8000) is purchased from Heraeus Electronic Materials Division,
and
used
as
the
hole
injection
layer
(HIL).
Poly
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(dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) is synthesized in our lab, and used as the light emission layer. The HIL ink is formulated by diluting PEDOT:PSS with 60 vol% deionized water (DI water), and 10 vol% ethylene glycol (EG).
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After all the components are mixed, the ink is ultrasonically stirred for 30 seconds at room
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temperature. The viscosity of the inks is obtained by a Brookfield Rotational Viscometer (LVDV-I+) at room temperature. A OneAttension Theta Lite (TL100) is used to measure the surface tension of the inks.
2.2. Display Panel Fabrication:
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The display panel is made by etching polyimide (PI) coated ITO glass into linear banks. The panel size is 15 mm × 15 mm. The linear bank is 8 mm long, 50.5 µm wide, and 1.5 µm
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high. Each panel has 113 linear banks. Before inkjet-printing PEDOT:PSS ink, the patterned substrate is cleaned in ultrasonic bath of DI water, and isopropyl alcohol in
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sequence, followed by UV-O3 cleaning for 10 min. The substrate is maintained at room temperature during printing process. PEDOT:PSS ink is inkjet-printed into the banks at room temperature, by a JetLab II printer manufactured by MicroFab Technologies, Inc., from a 30 µm nozzle. The volume of each PEDOT:PSS droplet is about 16 pL. After the ink is printed, the substrate is annealed at 180 ºC on a hot plate in a nitrogen filled glove
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box for 10 min. The thickness and the surface morphology of the PEDOT:PSS layer are characterized by a Dektak 150 surface profiler (Bruker Corp.) and Veeco NT 9300.
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To fabricate the light-emission device, a 70 nm thick PF-FSO layer is spin-casted from p-xylene solution onto the printed PEDOT:PSS layer, followed by 20 min baking at 140 ºC on a hot plate in a nitrogen filled glove box. The device is completed by thermally
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3. Results and Discussion
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evaporating the metal cathode of Ba/Al in the vacuum under a pressure of 1 × 10-4 Pa.
In the experiment, PEDOT:PSS ink is diluted by 60 vol% DI water, and 10 vol% EG (ink properties are provided in Supporting Information Table S1). The addition of high
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boiling-point solvent EG greatly improves jetting stability of the ink, and suppresses coffee ring effect (Figure S1).[21,22] The linear bank has a dimension of 8 mm × 50.5 µm as schematically shown in Figure 2a. The substrate has a total of 113 banks. PEDOT:PSS
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droplets were inkjet-printed into the linear banks with constant droplet-spacing of 35 µm.
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The ink was printed in every two lines for better viewing. During the ink drying process, the local vapor pressure is higher at the line center than at the line ends (Figure 2b). The fast solvent evaporation rate at the line ends leaves more EG content at the line ends than at the line center. The contrast of solvent surface tension generates a Marangoni flow towards the center. Since the ink amount in a line bank is much larger than that of a single droplet (Figure S1), the ink drying time is prolonged, and the inward Marangoni flow transports
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excessive PEDOT:PSS to the center. The PEDOT:PSS drying dynamic is shown in Figure S2. Four minutes after printing, the PEDOT:PSS lines are still not completely dried.
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During the drying process, the liquid boundary moved from the line ends slowly toward the center, bringing solutes along the way. After total solvent evaporation (about 6 min later or more), the film surface along the longitude direction shows a convex profile as shown in
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Figure 2c (15 locations on the printed line are sampled for measuring the film thickness).
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In order to achieve uniform film thickness along the line direction, a novel inkjet-printing scheme with varied droplet-spacing is developed. By setting small droplet-spacing at the edge and large droplet-spacing at the center, the solute amount is larger at the edge than that at the center. The inward Marangoni flow will transport solute
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from the edge to the center to form a uniform distribution of PEDOT:PSS. The inkjet-printing process with varied droplet-spacing is compared against the process with constant droplet-spacing in Figure 3. The simplest droplet-spacing variation is to change
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the spacing value in an arithmetic progression.
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As shown in Figure 3, we define each spacing as di, the common difference as ∆d, and
the droplet number of half the line as n. The droplet-spacing setting is symmetric to the center point. Since the half length of the line pixel is 4 mm, the droplet-spacing values can be determined using the following formulas: +
∆ = 4 mm
= + − 1∆ 7
(1) (2)
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For the constant droplet-spacing (∆d = 0) in Figure 3a, di and n is set to be 35 µm and 114, respectively. The film thickness at the center region is about 49 nm (Figure 2c). Using
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the result as a starting point, two approaches are designed to vary the droplet-spacing. In the first approach, the largest spacing of the center droplets (dn) is fixed to 35 µm, and the spacing towards the line ends is reduced gradually. The second approach is fixing the
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smallest spacing at the line ends (d1) at 35 µm, and gradually increasing the spacing toward
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the center. Various spacing difference ∆d is applied in both approaches as shown in Table 1. Since the diameter of a single droplet on the substrate is about 60 µm (Figure. S1), the largest spacing (dn) is kept under 60 µm in order to form a continuous thin film. When the droplet spacing is less than 27.5 µm, the printed ink will overflow the bank and cause
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cross-contamination between adjacent lines. Therefore, the smallest droplet spacing (d1) is set at 27.5 µm. The distribution of droplet density per millimeter across half the line pixel is plotted in Figure S3. Both approaches have the largest droplet density at the line ends, and
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monotonically decreased density towards the center. The density is kept around 28 drops
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per millimeter at the center in the first approach, while the density is kept around 28 drops per millimeter at the line end in the second approach. The printed PEDOT:PSS line films show gradually changed surface profiles with
varied droplet-spacing. Figure 4a and d are the film thickness in the longitude direction. The line film inkjet-printed with constant droplet-spacing at n = 114 serves as a reference. The average thickness of the line (representing the relative content of ink in each bank) printed through the two approaches are plotted in Figure 4b and e. It’s expected that the 8
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average film’s thickness decreases as the total number of droplets n decreases.
To evaluate the film uniformity, the ratio of the largest thickness at the center to the
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smallest at the ends (Hmax/Hmin) is compared in Figure 4c and f. As shown in Figure 4a and b, the line inkjet-printed with constant droplet-spacing (d1 = dn = 35 µm
n = 114)
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produces the most significant convex surface profile. Hmax/Hmin reaches 1.75 with the film thickness of 28 nm at the ends, and 49 nm at the center. For n = 119, 123, and 128
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with fixed dn = 35 µm, the smallest spacing d1 are 32.5 µm, 30 µm, and 27.5 µm, respectively. As depicted in Figure 2b, the excessive solutes at the line ends will be transported inward by the Marangoni flow during solvent evaporation. As a result, Hmax/Hmin decreases as n increases. At n = 128, Hmax/Hmin is reduced to about 1.13, giving
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an almost flat film surface. For n = 110, 106, 103, 99, and 94 with fixed d1 = 35 µm, the largest spacing dn are 37.5 µm, 40 µm, 42.5 µm, 45 µm, and 50 µm, respectively. Increasing the droplet-spacing towards the line center suppresses the convex surface
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profile. As a result, Hmax/Hmin decreases as n decreases. At n = 94, Hmax/Hmin decreases to
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1.03. The line film surface becomes flat. The film thickness difference Hmax-Hmin is plotted as a function of the droplet density difference in Figure 5. It is revealed that the film surface profile is governed by the droplet density contrast. Meanwhile, the average film thickness can be modulated by adjusting the total printed droplet number. For the films with best uniformity, n = 128 yields an average film thickness of 40 nm and n = 94 yields about 27 nm. Therefore, high quality thin film with desired thickness is achievable 9
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via systematic tuning of the spacing variation parameters. To demonstrate the impact of the film uniformity on light emission, complete display
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panel with linear banks is fabricated. The OLED device structure is shown in Figure 6a. A blue polymer, poly (dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) is spin-coated on top of PEDOT:PSS as the light-emitting layer.[23] For better
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comparison, the substrate is divided into two parts. As shown in Figure 6b, on the upper
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half of the panel, PEDOT:PSS is inkjet-printed with varied droplet-spacing, i.e. n = 128 with fixed dn = 35 µm, to achieve the best film uniformity. On the lower half of the panel, PEDOT:PSS is inkjet-printed with the constant droplet-spacing of n = 114 with d1 = dn = 35 µm. For the pixel lines printed by constant droplet-spacing, since PEDOT:PSS layer’s
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thickness diminishes towards the line ends, charge injection at the line edge region is not as efficient as that at the line center region. As a result, the line edge region of the panel barely emits light as shown in Figure 6b. By contrast, for the pixel lines printed by varied
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thickness.
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droplet-spacing, the whole lines give a homogenous brightness due to the uniform film
The pixels’ brightness is read from the display image in Photoshop as gray scale, and
illustrated in Figure 7. As expected, the brightness of the pixel lines printed by constant droplet-spacing, sharply drops at the line edge region, while the brightness of the pixel lines printed by varied droplet-spacing remains constant across the whole line. Of note, though the PEDOT:PSS film thickness of the pixel lines printed by constant 10
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droplet-spacing varies from about 30 nm to 50 nm (Fig. 2c, Fig. 5), the brightness variation across the non-shrunk region is almost identical to that of the pixel lines printed with varied
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droplet-spacing. The reason is that PEDOT:PSS is conducting polymer with relative high electric conductivity compared to the light-emitting material. Therefore, the light-emitting device’s brightness is not sensitive to PEDOT:PSS thickness in the range of about 30 to 50
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nm. The brightness variation mainly comes from the non-uniformity of the spin-coated
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PF-FSO layer. The brightness uniformity, 1 - (Lmax - Lmin)/Lmax,[24] is about 75%, showing that spin-coating film on substrate with pixel structures can’t obtain high uniform surface profile.[25]
Besides the brightness non-uniformity, there are other defects of the display, such as
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bright spots, dark spots, etc. Dark spots are caused by either the dusts, or the residue photoresist. Bright spots are caused by the over-etched bank walls. The bank walls are patterned by photolithography. Some parts of the walls are over-etched. As a result, the
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spin-coated PF-PFO film is thinner across the over-etched walls than the film in other area,
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since the over-etched walls can’t block the spin-coated ink as well as the regular walls. Under the same operation voltage, the thin light-emission layer will emit more lights due to the large electric field across the film.
4. Conclusions
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To inkjet-print linear films with high uniformity along the longitude direction, a printing scheme with varied droplet-spacing is successfully developed to turn the surface
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profile from convex to flat. A simple arithmetic progression of the droplet-spacing setting is introduced to suppress the solute accumulation at the line center. It is found that the surface profile is mainly governed by the droplet density difference along the line, while
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the average thickness is determined by the total droplet number in each linear bank. By fine
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tuning the droplet-spacing parameters, printing at n = 128, ∆d = 0.059 µm, and n = 94, ∆d = 0.161 µm, result in flat film surface. The OLED display panel ink-jet printed with varied droplet-spacing confirms the uniformity of the film. The technique reduces the task to further adjust the solvents of the ink, and could be applied in inkjet-printing any inks to
Acknowledgements
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obtain uniform films along the longitude direction.
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The authors are deeply grateful to the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655004) founded by MOST, National
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Natural Science Foundation of China (51521002， U1601651， 51573056, 51373057), and National Key Research and Development Program of China (2016YFB0401400) for their financial supports.
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[24]
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d (µm)
dn (µm)
∆d (µm)
128
27.5
35
0.059
123
30
35
0.041
119
32.5
35
0.021
114
35
35
0
110
35
37.5
0.023
106
35
40
0.048
103
35
42.5
0.074
99
35
45
0.102
94
35
50
0.161
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n
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Table 1. Printing set-up.
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Figure Captions: Figure 1. (a) Conventional pixel bank structure. (b) Linear bank structure.
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Figure 2. (a) Schematic illustration of the patterned ITO substrate with linear banks. (b) Schematic illustration of the film drying process. (c) Surface morphology of the inkjet-printed film with constant droplet-spacing d = 35 µm.
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Figure 3. Schematic illustration of inkjet-printing with (a) constant droplet-spacing, and (b)
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varied droplet-spacing.
Figure 4. (a, d) The surface morphology of the printed line films. (b, e) The dependence of the average film thickness on the total number of the droplets. (c, f) The dependence of Hmax/Hmin ratio on the total number of the droplets.
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Figure 5. The dependence of Hmax-Hmin on the printing droplet density difference. Figure 6. (a) The OLED device structure. (b) Light-emission images of the OLED display
(lower half).
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panel inkjet-printed with varied droplet-spacing (upper half) and constant droplet-spacing
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Figure 7. The pixel brightness in the units of gray scale along the line of the OLED display panel operated at 5V.
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Figure 1.
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Figure 2.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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Figure 7.
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Appendix A. Supplementary data Supporting Information
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Inkjet-printing line film with varied droplet-spacing
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Lan Mu, Zhanhao Hu, Zhiming Zhong, Congbiao Jiang, Jian Wang*, Junbiao Peng*, and Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou
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510640, P. R. China.
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Corresponding authors. E-mail addresses: [email protected] (J.Peng), [email protected] (J. Wang)
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1. Rheological properties of the PEDOT:PSS ink and the solvents Table S1. Rheological properties of the PEDOT:PSS ink and the solvents. Boiling point (ºC)
Viscosity (cP)
EG Water 3:6:1 (CH 8000:DI:EG)
197.3 99.9 -
25.66 1.009 6.95
Surface Tension (mN/m)
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Composition
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46.49 72.7 67.33
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Table S1 lists the rheological properties of the PEDOT:PSS ink (diluted by 60 vol% DI water, and 10 vol% EG) and its solvents. Adding high boiling point EG prevents clogging of the nozzle, thereby improves jetting stability of the ink. The mixed solvents help suppress coffee-ring effect (Figure S1).
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2. Surface morphology of the printed droplet
Figure S1. Surface morphology of an inkjet-printed PEDOT:PSS droplet on ITO. (a) The three-dimensional picture was taken by the white light interferometer. (b) The surface profile shows no coffee-ring formation.
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3. The drying dynamic of the printed ink.
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Figure S2. The ink’s contact line moves from the line end to the line center. Microscopic images are taken at t = 30 s, 60 s, 2 min, 3 min, and 4 min. 4. The droplet density along the linear bank
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n = 128 n = 123 n = 119
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n = 114 n = 110 n = 106 n = 103
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Droplet Density (/mm)
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Location (mm)
Figure S3. The droplet density along the linear bank.
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Highlights: 1. Ink-jet printed linear film with constant droplet-spacing shows non-uniform surface. 2. Printing with varied droplet-spacing is developed to turn the surface from convex to flat.
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3. The surface profile is mainly governed by the droplet density difference along the line.
4. Larger droplet density at the line ends than that at the center compensates the inward flow.
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5. The technique eliminates the need for multiple solvents in ink formulation.