colour AMOLED Pixel Matrix

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TFT-directed Electroplating of RGB Luminescent Films without a Vacuum or Mask towards a Full-colour AMOLED Pixel Matrix Rong Wang, Donglian Zhang, You Xiong, Xuehong Zhou, Cao Liu, Weifeng Chen, Weijing Wu, Lei Zhou, Miao Xu, Lei Wang, Linlin Liu, Junbiao Peng, Yuguang Ma, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04487 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Materials & Interfaces

TFT-directed Electroplating of RGB Luminescent Films without a Vacuum or Mask towards a Fullcolour AMOLED Pixel Matrix Rong Wang,ab Donglian Zhang,a You Xiong,a Xuehong Zhou,a Cao Liu,a Weifeng Chen,a Weijing Wu,a Lei Zhou,c Miao Xu,c Lei Wang,ac Linlin Liu,a* Junbiao Peng,a Yuguang Ma,ab* Yong Caoa a

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. b

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun

130012, P. R. China. c

Guangzhou New Vision Opto-electronic Technology Co., Ltd. Guangzhou 510530, P. R.

China. KEYWORDS: TFT, electroplating, AMOLED, RGB full-colour films, Raman mapping

ABSTRACT: The thin-film transistor (TFT) driving circuit is a separate electronic component embedded within the panel itself to switch the current for each pixel in active-matrix organic light-emitting diode displays. We reported a TFT-directed dye electroplating method to fabricate pixels; this would be a new method to deposit films on pre-patterned electrode for organic full-

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color display, where TFT driving circuit provide a switching current signal to drive and direct dye depositing on selected RGB pixels. A prototype patterned colour pixel matrix was achieved, as high-quality light-emitting films with uniform morphology, pure RGB chromaticity and stable output. With the co-efforts of scientists and industrialists, organic light-emitting diodes (OLEDs) and the corresponding display products have entered the market in recent years. The size of pixels is increasingly decreasing (to typically dozens of microns) to increase the resolution, which inevitably increases the manufacturing challenges and cost. Currently, the most mature technology to realize full-colour OLED displays with large area and high resolution is fine metal mask evaporation.1-3 Mobile phones with OLED displays based on this technology have high contrast ratios relative to liquid crystal displays (LCDs), making them suitable for usage outdoors and under sunlight. However, their complex processing, which require fine masks and a vacuum, incurs high cost and limits the panel size. To exploit the advantages of organic materials, maskless colour pattern approaches emphasizing the low-cost potential have been explored, typically as ink-jet printing (IJP).4-7 IJP is an efficient, highly cost-effective, and feasible mass manufacturing technology that shows great promise for use in polymer lightemitting flat panel displays. Other technologies, such as laser-induced transfer printing8-10 and photolithography11-13 are also being developed for fabricating higher-resolution panel displays. However, for high-quality OLED displays, films with uniform morphology, high colour purity, no colour interference and precisely deposited on pixel matrix are required. Currently, electrochemical synthesis has become an important method for material synthesis; this approach can simplify reactions, avoid the use of dangerous reagents, and reduce contaminants and cost.14,15 Among electrochemical synthesis methods, electroplating (EP) has


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been proven to be an efficient method for the preparation of electro-active and conducting polymer films and for cost-efficient fabrication in terms of both materials and equipments.16-18 In the EP method, the precursors are electrochemically oxidized or reduced, and the coupling reaction between the monomers occurs at the electrode interface with the deposition of the polymer film on electrode surface. In the past few years, our group has developed EP method to allow precursors containing luminescent dyes to simultaneously polymerize and deposit on a conductive substrate. After optimizing the precursors structure and EP parameters, the EP films are highly luminescent, possess an insoluble cross-linking structure, and exhibit relatively smooth morphology, and their thickness are controllable.19-22 These properties have been well demonstrated in prototype OLEDs and passive matrix OLED (PMOLED).21 However, in PMOLED displays, a large voltage drop exists across one-pixel lines from one side to another, which limits their application in large-size displays. Since both the deposition of EP films and the operation of display require an electrical process, electroplating films on PMOLED backplane have also met the same problem of film inhomogeneity, and this voltage drop effect is more severe in EP technology than the display technologies. More importantly, all red pixel lines, green pixel lines and blue pixel lines on PMOLED backplane have to be connected into three electrodes, respectively, for the electroplating of red, green and blue (RGB) films, which would complicate the fabrication process of PMOLED backplanes and increase the difficulty. Thus, the introduction of an active-matrix technology with switching thin-film transistor (TFT) driving circuit would be very important for high-quality EP films and display applications. Active-matrix technology is one of the most important concepts in the field of large-area displays. In this method, each pixel is controlled by a separate TFT driving circuit, which can effectively decrease the chromatic aberration and the voltage drop in the displays. This approach


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is widely applied in commercial flat panel displays, such as LCDs and especially the burgeoning OLED displays market.23-25 Here, a novel method of TFT-directed dye electroplating (TDDE) is introduced for the direct and efficient deposition of organic luminescent dyes onto the pixel without a vacuum or mask. The deposition process on the selected pixels can be addressed and driven by the switching current signal of TFT driving circuits, which makes TDDE a maskless colour pattern method. In this work, we successfully used TFT driving circuit as a switch to control the “on” or “off” state of EP process, where the TFT driving circuits can direct the dyes depositing on the selected pixels and stop dyes depositing on the unselected pixels. After optimized parameters involving dye precursors, electrode modification (1~2 nm gold layer) and TFT parameters (gate voltage), high-quality light-emitting films with uniform morphology and high fluorescence yield are obtained. Furthermore, patterned coloured films are achieved by controlling the work states of the TFT driving circuits and changing the RGB precursors in an orderly manner. A prototype OLED device matrix showing pure RGB chromaticity and stable output is also achieved. The molecular structures of the RGB dye precursors are shown in Chart 1, which are fluorene-based compounds with an emission-adjustive unit and peripheral carbazole groups, and namely, OCNzC (red), OCBzC (yellow green) and OCPC (blue), respectively. Carbazole is a highly electro-active group exhibiting a relatively low oxidation potential, relatively efficient coupling between the carbazyl radical cation and structurally well-defined cross-link products (predominantly dimeric carbazyl).19-22 In addition, the eight peripheral carbazoles provide higher coupling and luminesce efficiencies.22 Alkyl chains separate the fluorescent units (with a high oxidation potential) from the electro-active carbazole group (with a low oxidation potential), ensuring that the electroplating process has no impact on the fluorescence.21 According to our


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previous work,26 herein, we used an ultra-thin gold nanoparticle (AuNP) layer to modify ITO (ITO/AuNP) pixels to improve the EP behaviour of dye precursors and achieve higher-quality luminescent films. The chosen TFT was a metal oxide TFT (MOTFT) with an n-type Ln-IZO active layer, and its device structure was shown in Figure S1. As an alternative to low-temperature poly-silicon (LPTS), MOTFT exhibits high mobility, excellent uniformity and low cost and is considered to be the ideal TFTs for active matrix OLED (AMOLED) displays.27,28 As shown in Figure 1.b, the TFT driving circuit is consisted of two TFTs and one capacitor (2T1C), which is the most basic driving structure to realize an OLED display.29,30 The switching TFT (T1) was used for addressing, the driving TFT (T2) was used to control the luminance and grey level by adjusting the current through the OLED, and the capacitor (C) was used to store charge. The T2 we used had a threshold voltage of -1 V and a linear mobility of about 17 cm2V-1s-1. The output curve shown in Figure S2 indicated that the Ln-IZO TFT operated in the “on” state when exposed to a positive gate voltage (Vg) and operated in the “off” state when exposed to a negative Vg, which acted as a switch. As with OLEDs, EP process is also driven by the current through the electrolytic cell, therefore, in principle, the “on” or “off” state of the EP process can be controlled by the “on” or “off” state of TFT driving circuits. Here the “on” state of the EP process indicates that the EP signal can be input into the pixel electrode, where the electrode reaction of dye precursors can occur and luminescent films can be deposited on the pixels; while the “off” state means that the EP signal can’t be input into the pixel electrode, where the electrode reaction of dye precursors can’t occur and no films deposited on the pixels. To investigate the feasibility of incorporating the switch character of TFT driving circuits and EP technology, the 2T1C driving circuit was connected in series into the electrolytic circuit, as


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shown in Figure 1.a: the gate electrode of T1 was connected to scan line (Vsel) and used to controll the “on” or “off” state of T1; the drain electrode of T1 (which can also be regarded as the gate electrode of T2 through the “on”-state T1) was connected to the data line (Vdata), which was used to controll the current through the pixel; the drain electrode of T2 was connected to the source line (Vdd), and the EP signal was input into the pixel electrode through Vdd. In brief, T1 was used to control the “on” or “off” state of T2, while T2 was used to control the “on” or “off” state of the EP process. Because each TFT driving circuit can be used for its corresponding pixel, various films can be deposited on different pixels by controlling the TFT driving circuit of each pixel, as shown in Scheme 1. Therefore, by systematically controlling the TFT driving circuit states and changing the RGB precursors, we can deposit RGB full-colour films on an AMOLED backplane. Taking OCBzC as an example, when we deposited yellow green films, we conducted the EP process in the OCBzC electrolyte, set all T1 to the “on” state, and EP signals were given to all Vdd. However, only the T2 of the yellow green pixels operated in the “on” state, while other T2 operated in the “off” state; thus, OCBzC films only deposited on the yellow green pixel electrodes. With this method, RGB films can be systematically deposited on the corresponding pixel electrodes. Furthermore, our experimental results suggested that the deposited order of the RGB dyes had no influence on the quality of the RGB films, which demonstrated that there was no interference between each operation during film deposition. This was attributed to the insoluble cross-linked structure of the RGB EP films, while RGB precursors were small molecules and could be dissolved in the organic solvent. According to the process in Scheme 1, we deposited RGB films on a 3×3 pixel AMOLED backplane. Figure S3 presented the cyclic voltammetry (CV) curves of RGB precursors


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deposited onto ITO/AuNP pixel electrodes, which exhibited similar EP behaviour due to their similar chemical structures. Fluorescence microscope measurements revealed high-quality films with uniform fluorescence (shown in Figure 2.a). Films with smooth and compact morphology (Rms of 1.01 nm, 1.21 nm and 0.98 nm for OCNzC, OCBzC and OCPC, respectively) were also characterized by atomic force microscope (AFM, as shown in Figure S4). Therefore, we have obtained EP RGB films on small size ITO/AuNP pixels, whose morphology, fluorescence and uniformity have reached up to the requirements for OLEDs. Finally, we fabricated prototype devices with the following structure: ITO/Au(1 nm)/OCNzC(60 nm) or OCBzC(65 nm) or OCPC(60 nm)/TPBi(35 nm)/CsF(1 nm)/Al(100 nm). These devices exhibited red, yellow green and blue light-emitting peaks of 638 nm, 562 nm, and 421/445 nm, respectively, as shown in Figure 2.b. Additionally, other arrangement mode of RGB films on a 6×5 pixel AMOLED backplane was also fabricated by controlling the TFT driving circuit of each pixel, and the devices exhibited RGB emission as shown in Figure S5. These results demonstrated the successful production of high-quality RGB films with satisfactory emission on an AMOLED backplane by combining the switch function of TFT driving circuits with the EP technique. The EP behaviour of these three precursors was similar because of the same EP functional groups, where in most cases the result of OCBzC is given as an example. Figure 3.a (black line) showed the first 14 cycles CV curve of OCBzC on an ITO/AuNP pixel electrode (200 µm×200 µm) without using TFT driving circuit, which was similar to that on the normal ITO electrode.26 In this case, the real potential of the pixel electrode surface (Vreal-w/o TFT) was equal to the input potential (Vinput) given by the electrochemical workstation (Vreal-w/o

TFT=Vinput)

(here, we

neglected the voltage drop of electrolyte and electrode). However, when we used TFT as a switch to drive the EP process with the parameters of Vg=10 V, Vdd=EP signal=0~0.835 V,


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TFT works at the linear region, where TFT was equivalent to a pure resistance (RTFT) and it would cause a voltage drop (VTFT). Therefore, in this case, the real potential of the pixel electrode surface (Vreal-with

TFT)

was Vreal-with

TFT=Vinput-VTFT,

which is lower than Vinput.

According to the Ohm law, VTFT=i×RTFT, where i is the current through the electrolytic cell and can be given by the electrochemical workstation. The RTFT of TFT at linear region can be calculated from the slope of the linear region of output curve (Figure S1), which gave a RTFT of 53 kΩ, 28 kΩ, 18 kΩ and 7.8×106 kΩ when given a Vg of 10 V, 15 V, 20 V and -5 V, respectively. Therefore, Vreal-with TFT can be calculated from the equation of Vreal-with TFT=VinputVTFT=Vinput- i×RTFT. Herein, we calculated the Vreal-with TFT in the case of Vg=10 V, RTFT=53 kΩ, i= the last cycle of the CV curves in Figure 3.a (back line) that gave the maximal current of electrolytic cell. And Figure 3.b showed the variation of Vinput and Vreal-with TFT, where there was only a slight decrease of Vreal-with TFT at 0.5~0.7 V and 0.835 V. Additionally, with higher Vg (lower RTFT) and lower i, the VTFT would be decreased further, which would provide a better Vreal-with

TFT

thus a better EP behavior and better EP films. The EP behaviour of 2,3-

dihydrothieno-1,4-dioxin (EDOT) and its EP films (PEDOT) with different Vg were shown in Figure S6 and Figure S7, which also demonstrated that the higher Vg, the better Vreal-with TFT thus a better EP behaviour and better EP films. However, when Vg=-5 V, RTFT was sufficiently large (>7.8×106 kΩ) to shut off the electrolytic circuit, and the EP process was inhibited. All these results suggested that TFT can be used as an ideal switch to control the “on” or “off” state of the EP process by adjusting Vg of TFT. According to the EP behaviour of OCBzC and the analysis above, we finally chose a Vg of 10 V to driven the EP process of OCBzC. The CV curves of OCBzC using TFT driving circuit as a switch to control the EP process were shown in Figure 3.a, where the CV curve with Vsel=10 V,


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Vdata=10 V (red line) was similar to that without using the TFT (black line). There was only a slight current decrease caused by the small voltage drop of the TFT, which indicated that the EP process occurred regularly when the TFT operated in the “on” state. While the current was close to 0 (10-11 A, blue line) when Vsel=10 V, Vdata=-5 V, which indicated that the EP process was inhibited when the TFT operated in the “off” state. The yellow green polymer films were observed clearly on the ITO/AuNP pixel electrode without using TFT driving circuit or when Vsel=10 V, Vdata=10 V, while no polymers (no fluorescence) were observed when Vsel=10 V, Vdata=-5 V (Figure 4.a and d). The compact and smooth surface morphology (Rms of 1.20 nm and 1.21 nm of OCBzC films deposited without TFT driving circuit and Vsel=10 V, Vdata=10 V, respectively) were also determined by AFM, as shown in Figure 4.b and e. The Raman spectrum resulting from the vibration and rotation of a molecule can be used to qualitatively and quantitatively analyse samples through the characteristic peaks and peak intensity. The Raman mapping of a certain characteristic peak can reveal the distribution and quantity of a sample. The Raman mapping images (Figure 4.c and f) of the peak intensity at 1607 cm-1 (attributed to the vibration of the fluorene group) of the OCBzC EP films revealed that both films have similar intensities and distributions, thus leading to a similar thickness. The boundary of EP films is very important for fineness of this method. In our previous work, we characterized the boundary of EP films deposited on patterned ITO by atomic force microscope (AFM), which exhibited a very ‘‘sharp” edge with the edge roughness

colour AMOLED Pixel Matrix

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