Inkjet-Printed Oxide Thin-Film Transistors Based on Nanopore-Free

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Functional Inorganic Materials and Devices

Inkjet-Printed Oxide Thin-Film Transistors Based on Nanopore-Free Aqueous-Processed Dielectric for ActiveMatrix Quantum-Dot Light-Emitting Diodes Displays Yuzhi Li, Penghui He, Siting Chen, Linfeng Lan, Xingqiang Dai, and Junbiao Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08258 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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ACS Applied Materials & Interfaces

Inkjet-Printed Oxide Thin-Film Transistors Based on NanoporeFree Aqueous-Processed Dielectric for Active-Matrix QuantumDot Light-Emitting Diodes Displays Yuzhi Li,# Penghui He,# Siting Chen,# Linfeng Lan,* Xingqiang Dai, and Junbiao Peng State Key Laboratory of Luminescent Materials and Devices, South China University of Technology Guangzhou 510640, China

ABSTRACT: Inkjet-printed thin-film transistor (TFT) backplane for active-matrix light-emitting diode display is drawing much attention for the advantages of low material waste and simple fabrication processes without vacuum deposition and photolithography steps. Herein, for the first time, solution-processed quantum-dots light-emitting diode (QLED) array displays driven with inkjet-printed oxide TFT backplane were realized and demonstrated using a general “solvent printing” method. To suppress nanopore formation in the thick oxide films, carbon-free aqueous inks were employed for gate dielectrics. No nanopores was found in the whole 120 nm-thick gate dielectrics. However, compared to the organic inks, the aqueous inks have very low viscosity, resulting in uncontrollable ink spreading especially in trans-line printing. The ink easily shrinks on the low-surface-energy area and spreads on the high-surface-energy area, leading to serious uniformity problems (the upper lines even break at the top of underlying lines). To solve the problem, a “solvent printing” method was employed to form coffee-line surface-energy patterns, which were uniform without shape distortion. The surface-energy patterns can restrain the ink spreading and tune the morphology of the printed films. As a result, multilayer TFT arrays with ideal shapes were achieved. The mobilities of the printed top-gate TFTs in the backplane array were 3.13 ± 0.87 cm2 V−1 s−1 for switching TFTs and 2.22 ± 0.38 cm2 V−1 s−1 for driving TFTs. Finally, an active-matrix red QLED character display based on the printed oxide TFT backplane and solution-processed QLEDs was demonstrated. The “solvent printing” method opens a general route for inkjet-printed multilayer electronic devices. KEYWORDS: inkjet printing, oxide, thin-film transistors, AMQLED, solvent printing

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1. INTRODUCTION Oxide semiconductors integrated in thin-film transistors (TFTs) generally exhibit significantly higher field-effect mobility (> 10 cm2 V−1 s−1) and better stability compared to amorphous silicon and organic semiconductors.1-3 Oxide high-k dielectrics offer higher unit-area capacitance, which can lower the operating voltage and subthreshold swing of TFTs.4-6 TFTs based on oxide semiconductors and oxide high-k dielectrics are attractive for their good electrical properties. Vacuum-processed oxide TFTs have been applied in active matrix (AM) display productions, but their cost is relatively high due to the employed expensive vacuum and photolithography systems. In contrary, oxide TFTs fabricated by solution-based techniques are more attractive for its low fabrication costs and high throughput.7-9 Among all of the solution-based techniques, drop-on-demand inkjet printing offers a digital way to directly deposit films with desired patterns, leading to low material waste.10,11 Lee et al.12 reported TFTs based on inkjet-printed oxide semiconductor in 2007. Since then, interests on printed oxide TFTs have grown continuously. However, most of researches on inkjet-printed oxide TFTs focused on the optimization of oxide compositions, ink formulas, printing parameters, and post-annealing treatments.13-17 Fundamental problems of printing high-quality and good-reproducibility multilayer oxide TFT backplanes for active-matrix displays are still unresolved. Generally, most of the oxide precursor inks are based on organic solvents,15,18-21 but oxide films fabricated with organic solvents have lots of carbon impurities and nanopores, especially for those with large thickness (e.g. oxide gate dielectric films). Instead, oxide precursor ink based on aqueous solution can produce carbonimpurities- and nanopores-free oxide films due to their carbon-free nature. However, the viscosity of the aqueous ink is relatively low, resulting in uncontrollable ink spreadingespecially in trans-line printing. The landed aqueous ink easily shrinks on the low-surface-energy area and spreads on the high-surface-energy area, leading to serious uniformity problems (the upper lines even break at the top of underlying lines). In this work, an active-matrix quantum-dots light-emitting diodes (AMQLEDs) character display driven by inkjet-printed oxide TFTs was developed and demonstrated for the first time. Aqueous precursor ink was employed to produce high-quality and nanopore-free thick oxide dielectric layers, 2


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which exhibited high breakdown electric field and low leakage current. The influence of aqueous and organic inks on the structure of printed oxide films was investigated. A “solvent printing” method was employed to form coffee-line surface-energy patterns, which can restrain the ink spreading and tune the morphology of the printed films. As a result, oxide TFT arrays with ideal shapes (good step coverage without any coffee rings and trans-line distortions) were achieved. The mechanism for the formation of high-fidelity oxide film patterns and coffee-ring-free films based on the surface-energy pattern assisted inkjet printing method was also discussed.

2. EXPERIMENTAL SECTION 2.1. Precursor Solutions. Cytop solution was prepared by mixing Cytop solute (CTL-107MK, Asahi Glass) and the solvent (CT-SOLV180, Asahi Glass) with a volume ratio of 1:5. Aqueous AlOx precursor ink was prepared by dissolving 0.4-M Al(NO3)3·xH2O into deionized water. The preparing routes of the InGaO precursor ink and the InSnO (ITO) precursor ink were described in our previous work.19 All of the aforementioned oxide precursor materials, including the solutes and solvents, were purchased from Sigma-Aldrich. Positive-tone photosensitive polyimide (DL1000) for passivation layer was purchased from Toray Industries, Inc. PEDOT:PSS (Clevios P CH 8000) purchased from Heraeus Electronic Materials Division was diluted by ethanol with a volume ratio of 1:5. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(p-butylphenyl))-diphenylamine)] (TFB, American Dye Source, Inc.) was dissolved into chlorobenzene (Sigma-Aldrich) with a concentration of 10 mg mL−1.The red (CdSe/CdS/ZnS) QDs dispersed in octane with a concentration of 20 mg mL−1 was purchased from Suzhou Xingshuo Nanotech Co., Ltd. The ZnO nanoparticles dispersed in ethanol (30 mg mL−1) was purchased from Guangdong Poly OptoElectronics Co. 2.2. Device Fabrication. 2T1C pixel design (Figure S1) was employed to drive solutionprocessed QLED. To avoid the influence of passivation layer on the electrical properties of devices, top gate structure was employed for printed oxide TFTs. The key fabrication steps of printed oxide TFT backplane are shown in Figure 1b-e. Briefly, a 60-nm ITO film was deposited on glass substrate by sputtering, and then was patterned by photolithography to form pixel electrodes (anode for QLED), source/drain (S/D) electrodes, data lines and Vdd lines (Figure 1b). Next, InGaO channel layers of driving and switching TFTs were prepared by inkjet printing (Figure 1c). Then, AlOx films 3



as dielectric layers were deposited via inkjet printing to cover channel and S/D regions (Figure 1d). After that, ITO films were inkjet-printed to form gate electrodes of switching and driving TFTs (Figure 1e), the printed gate electrodes of driving TFTs electrically connect with the source electrodes of switching TFTs. The gate electrodes of switching TFTs were printed twice to ensure conductive enough. The aforementioned inkjet printing processes were conducted using a Dimatix (DMP-2800) printer with a 10 pL cartridge, and the surface-energy pattern assisted inkjet printing method was employed to prepare the oxide films, which the details of processing parameters were summarized in Table S1. Briefly, Cytop solution was spin-coated onto a substrate at 3000 rpm for 40 s to produce a ~6 nm Cytop layer. Then, the Cytop layer was etched by printing pure Cytop solvent (CT-SOLV180) to form desired surface-energy patterns. The patterned Cytop layer was treated by oxygen plasma and ultraviolet in sequence to remove undesired residue and improve the wettability of the substrate. Next, the oxide precursor ink was printed into the surface-energy patterns. After that, the deposited oxide precursor film was soft-baked at 100 °C for 5 min and hardbaked at 350 °C for 1 h in sequence. The prepared oxide TFT backplane was passivated by patterned DL1000 with a thickness of ~1 μm. The structure of QLED is shown in Figure 1a, all of the functional layers except anode (ITO) and cathode (Al) were prepared by spin-coating, which the processing parameters were shown in Table S2. A 150-nm Al film as cathode was deposited through thermal evaporation. Finally, the AMQLED was encapsulated using epoxy glue and glass in a N2filled glove box. 2.3. Characterizations of Oxide Films and Devices. The structure of the printed oxide films were characterized by transmission electron microscopy (TEM, FEI Titan Themis 200) equipped with an energy dispersive X-ray spectrometer (EDS). Surface morphology of oxide films were characterized by atomic force microscope (AFM, Bruker Multimode 8). 3D morphology and surface profile of oxide films were measured by Veeco NT 9300 and Dektak 150 surface profiler, respectively. The surface tension of oxide precursor inks was measured using Attension Theta Lite (TL100). The TFT electrical measurements were conducted using a semiconductor parameter analyzer (Agilent 4155C) under ambient conditions.

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Figure 1. (a) Schematic cross-sectional diagram of the pixel in AMQLED character display. The fabrication flow for inkjet-printed oxide TFT backplane: (b) preparing ITO electrodes by sputtering and photolithography; (c) inkjetprinting InGaO channel layers of switching and driving TFTs; (d) inkjet-printing AlOx dielectric films; (e) inkjetprinting ITO gate electrodes of switching and driving TFTs.

3. RESULTS AND DISCUSSION The precursor inks are important for their printability and film quality. Usually, the organic solvents with additives to increase viscosity are preferred to the oxide precursor inks, because of the better printability compared to aqueous solvent. However, nanopore or pinhole related defects are easily formed due to the carbon related impurities and larger organic molecules compared to water molecules. For the oxide films with small thickness in TFTs, such as the oxide semiconductor layer, nanopore or pinhole related defects can be neglected, because the solvent molecules can easily percolate through the oxide film.22 For the gate dielectric layer, however, the thickness are too large (>100 nm for good step coverage to avoid breakdown) for the organic solvent molecules to percolate through it from the bottom. During post annealing, the residual solvents will be evaporated or decomposed, leading to formation of nanopores, pinholes and carbon residue impurities (see Figure 2a).23 In the contrary, the oxide films prepared with carbon-free aqueous inks have fewer nanopores and no carbon impurities, because the water molecules can easily percolate through the oxide precursor film (see Figure 2b). However, the viscosity of the aqueous solution is relatively low for 5



inkjet printing. The flowability of aqueous inks is generally high for its low viscosity, which make the spreading and merging of the individual droplets hard to control and thus forming poor-defined patterns.24 The drying microenvironment also has an inevitable influence on pattern fidelity (related to the pining and depinning of droplets) as well as the morphology of the printed films.25,26 Therefore, the printability of oxide precursor ink is generally poor than polymer-based inks which have relatively high viscosity and relatively strong interaction. Adding polymers to aqueous inks can improve the printability, but it induces new impurities and nanopores to oxide films after annealing, and thus causes the degeneration of electrical performance of the oxide films.

Figure 2. Schematic diagrams of oxide films prepared based on (a) organic precursor and (b) aqueous precursor.

Another key issue for printing TFT backplanes is to print films across different surfaces (transline printing), especially across the films with surface energy different from the substrate, where the ink easily shrinks on the film surface (with lower surface energy) and accumulates on the substrate surface (with higher surface energy), causing serious uniformity or even disconnecting problems (see Figure 3c). Moreover, the coffee ring phenomenon existed in printed films also causes nonuniform surface morphology.

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Figure 3. Surface-energy pattern assisted inkjet printing: (a1) Spin-coating Cytop layer on substrate; (a2) Etching Cytop layer selectively by printing pure solvent; (a3) Formation of Cytop coffee line after plasma treatment; (a4) Inkjet-printing oxide precursor ink onto surface-energy pattern; (a5) Formation of oxide film after annealing. (b) Surface profile of Cytop coffee lines. 3D morphology images of (c) direct trans-line printed oxide film and (d) surface-energy pattern assisted trans-line printed oxide film.

To resolve the aforementioned issues, surface-energy patterns (Cytop coffee line patterns) prepared by a “solvent printing” method were employed to define the patterns of printed oxide films, as shown in Figure 3a. The hydrophobic Cytop layer was etched selectively by inkjet-printing pure solvent. When the solvent droplets landing on the substrate, the underlying Cytop layer was dissolved, and the dissolved solute was migrated to the three-phase contact line along with the outward capillary flow during the evaporation of the solvent, forming coffee line at the three-phase contact line (see Figure 3a2). After that, plasma treatment was performed to remove the residues, thereby leaving Cytop coffee line (see Figure 3a3,b). And the height of the Cytop coffee line reduced from ~60 nm to ~25 nm after plasma treatment. The great difference on surface energy of Cytop 7



(water contact angle large than 110°) and plasma-treated surface (water contact angle less than 5°) would confine the inks to spread within the coffee lines (see Figure 3a4). Therefore, the surfaceenergy pattern defines the pattern of printed oxide film regardless of flowability of ink. It is worth noting that there was almost no pattern distortion (see Figure 3d, S2b) for the transline printing oxide films with the surface-energy pattern assisted inkjet printing method, while poor pattern fidelity with serious distortion and coffee rings was found for those with direct printing method (see Figure 3c, S2a). Cytop coffee lines across different surfaces were uniform, which was ascribed to the uniform solvent etching. In other words, when the solvent droplet landed on the Cytop layer, the spreading and pinning of the droplet was determined by the Cytop layer regardless of the underlying substrates. Therefore, Cytop coffee lines across the different surfaces showed high pattern fidelity. In addition, the ultraviolet treatment made the surface energy of substrates high enough to realize fully spreading for ink within the surface-energy patterns. As a result, the transline printed oxide films exhibited good pattern fidelity without distortion. A qualified printed film should not only require undistorted film shape, but also require uniform surface without coffee rings. As known, the coffee-ring effect is caused by uneven evaporation rate. The faster evaporation rate of solvent near the periphery of droplet results in the outward capillary flow, which brings matter to the edge and builds up a ring-shaped ridge (named coffee ring) at the edge.27 The coffee rings of underlying layer critically influence the properties of the upper layer and the subsequent device performance.28 Generally, reducing intensity of the outward capillary flow or triggering the inward Marangoni flow can weaken or eliminate the coffee ring effect. The inward Marangoni flow (Ma), which can replenish the solute loss caused by the outward capillary flow, is expressed as a function of surface tension gradient (Δγ) and viscosity (η) of the ink droplet:29 ∆γ Ma ∝ η

( 1)

Figure 4a shows the relationship of the surface tension versus concentration for the aqueous AlOx precursor inks. It could be seen clearly that the surface tension increases with the increase of solute concentration, which is contrary to the results reported elsewhere.30 The increase in surface tension with increasing solute concentration is attributed to higher density of Al complex (Al3+―nH2O, inset I of Figure 4a), and the binding strength of Al3+―H2O is higher than that between neighboring water molecules. Since the intensity of solvent evaporation at the droplet edge is higher than that in 8


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the middle, the concentration at the edge will become higher than that in the middle with the evaporation of droplet. As a result, the surface tension of the ink near the droplet edge is larger than that near the middle of the droplet (Δγ < 0). It means that Ma < 0, which causes an outward Marangoni flow (inset II of Figure 4a). Therefore, in the case of the aqueous AlOx precursor inks, it is impossible to suppress the coffee-ring effect by the Marangoni flow. The surface-energy pattern played a key role to eliminate the coffee-ring effect for the printed oxide films based on aqueous AlOx precursor inks. It was found that the surface morphology can be tuned (and the coffee-ring effect can be eliminated) by simply adjust the amounts of the ink droplets with the help of the surface-energy pattern. The outward capillary flow velocity (v(r)) can be expressed as:27 v(r) ∝ (R - r ) -λ λ =

π - 2θ 2π - 2θ

(2) ( 3)

where R is the radius of the droplet, r is the distance to the center of the droplet, and θ is the contact angle. It can be deduced from Equation (2) and (3) that the capillary flow velocity increases with the decreasing θ, which means that the outward matter migration rate increases with the evaporation of solvent in droplet. In addition, the relationship between v(r) and η has the form:31 v(r) ∝

1 η

(4)

which means that viscous resistance can reduce the intensity of matter migration. Because the solution concentration increases with the evaporation of droplet, thereby increasing the viscosity of ink, and the effect of viscosity on matter migration rate would be more and more significant. It is reasonable to infer that the higher initial contact angle of droplet is in favor of elimination of coffee ring phenomenon, because the lower outward matter migration at initial evaporation stage and viscous resistance starts to exhibit significant inhibition for outward flow at high contact angle stage. Because aqueous AlOx precursor inks are confined in the region enclosed by the hydrophobic surface-energy pattern, the initial contact angle can be increased by simply filling more inks into the region enclosed by the surface-energy pattern. As a result, the film morphology can be tuned by the filled amount of ink droplets. By using this method, the film morphology is regardless of surface energy of the substrates, so the morphology of the printed film on different surface is the same (see Figure 3d). Figure 4b shows the surface profile and 3D morphology of printed aqueous-based AlOx 9



films after optimizing the amounts of ink droplets. The AlOx film exhibited a convex surface topology without coffee ring, which is beneficial to construct reliable TFTs.

Figure 4. (a) Surface tension versus concentration for AlOx precursor inks. Inset I: Schematic diagram of Al complex in aqueous solution. Inset II: Fluid flow in printed AlOx precursor droplet on substrate. (b) Surface profile and 3D morphology image of printed AlOx film.

Figure 5a shows the polarizing microscope image of a TFT in the printed oxide TFT backplane. It can be clearly seen that all the layers of the printed oxide TFT is uniform with good surface coverage, suggesting that the surface-energy patterns effectively restrain the spreading of liquid phase inks. Good uniformity of the printed oxide patterns also indicates that the influence of drying microenvironment on printed patterns is negligible due to the restraint of surface-energy patterns. TEM images were taken to have an insight into the functional layers of the printed TFT. As shown in Figure 5b, the printed AlOx film with a thickness of ~120 nm shows a homogeneous structure free of nanopores, indicating the gases from the decomposition of the aqueous precursor can easily diffuse out. In contrast, nanopores can be clearly seen in the ITO layer which was printed with organic solvent based ink (aqueous inks for ITO cannot be prepared, because SnCl2 is unstable in water). The gases produced by the decomposition of organic solvents cannot easily diffuse out from thick ITO film, forming nanopores at the lower layer of the film.32 The high-resolution TEM and corresponding fast Fourier transform (FFT) images (Figure 5c,d) reveal that the ITO layer is constituted with nanocrystalline and amorphous phase, while the InGaO film is crystalline.

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Figure 5. (a) Polarizing microscope image of a printed oxide TFT. (b) Cross-sectional TEM image of channel region of printed oxide TFT. High-resolution cross-sectional TEM images and corresponding FFT patterns of (c) ITO and (d) InGaO layer.

It is known that the surface roughness is a parameter that evaluates the quality of the prepared film. The smooth surface is critical for reliable and high-performance TFTs. The AFM images (Figure 6) collected from the printed oxide films showed that the root mean square (RMS) values were 0.39, 0.19, and 0.31 nm for InGaO, AlOx, and ITO films, respectively. The surfaces of all printed oxide films are quite smooth with RMS values less than 1 nm, which is comparable with the spin-coated films.33,34 The low surface roughness of the AlOx film is ascribed to its amorphous nature and the use of an aqueous-based ink. The gas-induced nanopores and crystalline nature of ITO film increase its surface roughness.

Figure 6. Surface morphology of inkjet-printed (a) InGaO, (b) AlOx and (c) ITO films.

To investigate the dielectric properties of the printed AlOx dielectric films, capacitors with a structure of ITO/AlOx/ITO (the thickness of AlOx layer is ~120 nm) were fabricated and characterized. Figure 7 shows the areal capacitance of printed AlOx film as a function of frequency. 11



The areal capacitance at 1 kHz was ~54.1 nF cm−2 and the dielectric constant was ~7.3, close to the value of solution-processed AlOx reported elsewhere,32,35,36 but smaller than that of alumina ceramic (> 9), which is mainly attributed to the existence of metal hydroxide in printed AlOx films.37 The frequency dependence of areal capacitance is also attributed to the hydroxyl groups in the films.3840

The inset of Figure 7 shows leakage current density versus electric field to evaluate the electrical

properties of printed AlOx films. It was found that the printed AlOx exhibited good insulating properties with a low leakage current density of 6.01×10-7 A cm−2 at 2 MV cm−1 and a high breakdown electric field of larger than 6 MV cm−1.41 For comparison, the AlOx film with organic solvent was prepared by dissolving 0.4-M Al(NO3)3·xH2O into a blended solvent containing 2methoxyethanol and ethylene glycol (1/1). Compared to the AlOx film with aqueous solvent, the film with organic solvent exhibited higher leakage current and lower breakdown filed, as shown in Figure S3.

Figure 7. Areal capacitance and leakage current density of printed AlOx dielectric film.

Figure 8a-d show typical output and transfer characteristics of printed oxide TFTs. It can be clearly seen that both switching and driving TFTs showed saturated behavior and clear linear regions without significant current crowding at low VDS, indicating good contacts between the S/D electrodes and the oxide channel layer.42 The saturation mobility (μsat) was extracted by fitting a straight line to the plot of the square root of the IDS versus VGS and using the following equation: IDS =

WμsatCi 2L

(VGS - Vth)2

(5)

where Ci is the areal capacitance of the dielectric, Vth is the threshold voltage, and W and L are the 12


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channel width and length, respectively. The calculated μsat of ten switching devices at different region of the TFT backplane is in the range of 3.13 ± 0.87 cm2 V−1 s−1 (see Figure 8g), while that of ten driving devices is in the range of 2.22 ± 0.38 cm2 V−1 s−1 (see Figure 8h). Generally, the mobility of the printed TFT is lower than that of spin-coated TFT, because the printed films have lower film density and poorer interface contacts than the spin-coated ones. Both of switching and driving TFTs show good transfer characteristics with a current on/off ratio larger than 106 and little hysteresis. The average gate leakage current (|IG|) for the printed devices was less than 0.1 nA, indicating relatively good insulating property of the printed AlOx dielectric films. To investigate the driving capability of the printed oxide TFTs for QLEDs, one pixel containing two TFTs (the switching TFT and the driving TFT), one capacitor and one QLED was fabricated (see Figure 1e). Figure S4 shows the electrical characteristics of solution-processed red QLED. The device shows a turn-on voltage (1 cd m−2) of ~1.9 V, a maximum brightness of ~95500 cd m−2, and a peak current efficiency of 14.5 cd A−1. Figure 8e shows the driving current output characteristic of the pixel. During testing, the gate of switching TFT was set to 10 V (Vselect = 10 V), and the cathode of QLED was grounded; when the drain voltage in switching TFT (Vdata) was set to 0 V, 5 V and 10 V, respectively, the drain current of the driving TFT (Idd) was collected while the drain voltage of the device (Vdd) swept from 0 V to 10 V. It could be clearly seen from the output curves and insets that Vdata could effectively tune the current and brightness of the QLED when the switching TFT is turned on. Figure 8f shows the transfer characteristic of the pixel. During testing, the gate of switching TFT and the drain of driving TFT were both set to 10 V (Vselect = 10 V, Vdd = 10 V), and the cathode of QLED was grounded; Idd was collected while Vdata swept from -5 V to 10 V. It could be seen clearly from the transfer curve and insets that the QLED could be well controlled from off-state to on-state. To further demonstrate the feasibility of printed oxide TFTs applied in display, an AMQLED character display demo was fabricated by integrating inkjet-printed oxide TFT backplane and solution-processed red QLEDs. Figure 9a,b show the polarizing microscope image of printed oxide TFT backplane pixels and the operating image of AMQLED character display demo, respectively. To the best of our knowledge, this is the first active-matrix display demo based on inkjet-printed oxide TFT backplane. It indicates that the printed oxide TFT technique could be a promising way for low cost display manufacturing.

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Figure 8. Typical (a) output and (b) transfer characteristics of printed switching TFT in backplane. Typical (c) output and (d) transfer characteristics of printed driving TFT in backplane. (e) Output and (f) transfer curves collected from one pixel containing 2T1C and one QLED, the inset is the photograph of QLED driven by printed TFTs. Statistical distributions of mobility for (g) switching TFTs and (h) driving TFTs.

Figure 9. (a) Polarizing microscope image of printed oxide TFT backplane pixels. (b) Operating image of AMQLED character display demo based on printed oxide TFT backplane.

4. CONCLUSIONS In summary, an AMQLED character display driven by inkjet-printed oxide TFTs was developed and demonstrated. Aqueous precursor ink was employed to produce high-quality nanopore-free thick oxide dielectric layers, which exhibited high breakdown field and low leakage current. A “solvent printing” method was employed to form coffee-line surface-energy patterns, which can restrain the ink spreading and tune the morphology of the printed films. As a result, oxide TFT 14


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backplane with ideal shapes (good step coverage without any coffee rings and trans-line distortion) were achieved. The mobilities of the printed top-gate TFTs in the backplane were 3.13 ± 0.87 cm2 V−1 s−1 for switching TFTs and 2.22 ± 0.38 cm2 V−1 s−1 for driving TFTs.

ASSOCIATED CONTENT Supporting Information (1) Pixel circuit diagram of AMQLED display (Figure S1); (2) Top-view 3D morphology images of trans-line printed oxide films (Figure S2); (3) Leakage current density versus electric field curve for AlOx film with organic solvent (Figure S3); (4) Electrical characterizations of solution-processed red QLEDs (Figure S4); (5) Processing parameters of TFT fabrication (Table S1); (6) Processing parameters of functional layers of QLED (Table S2).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Linfeng Lan: 0000-0002-6477-2830 Author Contributions #Yuzhi

Li, Penghui He, and Siting Chen contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Program (Grant no. 2016YFB0401105), the National Natural Science Foundation of China (Grant nos. 51673068, and 61204087), the Guangdong

Province

Science

and

Technology

Plan

(Grant

nos.

2017A050503002,

2014B010105008, and 2016B090906002), the Guangdong Natural Science Foundation (Grant no. 15



2017A030306007), and the Fundamental Research Funds for the Central Universities.

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