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In-Plane Anisotropic Molecular Orientation of Pentafluorene and Its Application to Linearly Polarized Electroluminescence Takeshi Komino,†,‡,§ Hiroyuki Kuwae,§,∥ Akiko Okada,∥ Weixin Fu,∥ Jun Mizuno,⊥ Jean-Charles Ribierre,‡,§ Yuji Oki,§,¶ and Chihaya Adachi*,†,‡,§,# †
Education Center for Global Leaders in Molecular System for Devices, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ‡ Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan § ERATO, Adachi Molecular Exciton Engineering Project, Japan Science and Technology Agency, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ∥ Nano-Science and Nano-Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ⊥ Organization for Nano and Life Innovation, Waseda University, 513 Waseda Tsurumaki-cho, Shinjuku, Tokyo 162-0041, Japan ¶ Department of Electronics, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan # International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: By preparing parallelly aligned 1.9-μm-high SiO2 microfluidic channels on an indium tin oxide substrate surface, the solution flow direction during spin-coating was controlled to be parallel to the grating. Using this technique, a pentafluorene−4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) binary solution in chloroform was spin-coated to embed a 40− 50 nm-thick 10 wt %-pentafluorene:CBP thin film in the channels. In-plane polarized photoluminescence measurements revealed that the pentafluorene molecules tended to orient along the grating, demonstrating that one-dimensional fluid flow can control the in-plane molecular orientation. Furthermore, the dependences of the photoluminescence anisotropy on the spin speed and substrate material suggest that the velocity of the solution flow and/or its gradient in the vertical direction greatly affects the resulting orientation. This indicates that the mechanism behind the molecular orientation is related to stress such as the shear force. The effect of the solution flow on the molecular orientation was demonstrated even in organic light-emitting diodes (OLEDs). Linearly polarized electroluminescence was obtained by applying the in-plane orientation to OLEDs, and it was found that the dichroic ratio of the electroluminescence orthogonal (x) and parallel (y) to the grating is x/ y = 0.75. KEYWORDS: molecular orientation, in-plane orientation, linear polarization, electroluminescence, solution processed thin films
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INTRODUCTION Since the first report of preferential molecular orientation in oligofluorene vapor-deposited thin films by ellipsometry,1 focus on and research of the molecular orientation in thin glassy films composed of small molecules has increased. Although these films were thought to always be isotropic, it is now believed that vapor-deposited glasses can possess anisotropic molecular orientational order. Glassy materials have been widely used in organic thin-film devices since the 1980s because of the advantages provided by their homogeneous pinhole-free morphology over a large area. Interestingly, in addition to oligofluorene glasses, preferential orientations have also been found in many vapor-deposited glasses, making them © XXXX American Chemical Society
potentially applicable to organic optoelectronic devices, especially organic light-emitting diodes (OLEDs).2−5 By extension, the formation mechanisms of oriented films have also attracted increasing attention for possible applications6−12 as well as fundamental research purposes.13−15 Although the mechanism of their orientation processes is still unclear, solution-processed organic films are more advantageous than those in vapor-deposited glasses for printable, low-cost, and high-throughput device fabrication.16−18 Thus, controlling the Received: April 20, 2017 Accepted: July 24, 2017
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DOI: 10.1021/acsami.7b05570 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic diagram of the grated substrate. (b) Chemical structures of the compounds. (c) Schematic diagram of solution flow with a velocity gradient.
coated films on a conductive alignment layer to realize linearly polarized EL with the dichroic ratio of 31.2 (the definition is the ratio of parallel and perpendicular to the nematic director).24 In this study, we attempted to realize in-plane molecular orientation of pentafluorene doped in the CBP host matrix by spin-coating with microfluidic channels fabricated on the substrate surface. We found that fluorene molecules show in-plane linearly polarized photoluminescence (PL) and EL, although the polarization was smaller than those in the literature.24 Because the microfluidic channels provide fluid flow along the channels during spin-coating, these linear polarizations demonstrate that one-dimensional solution flow forms the in-plane anisotropic molecular orientations.
molecular orientation in solution-processed devices is currently an important issue. We recently found that the out-of-plane orientation of oligofluorenes doped in the 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host matrix in spin-coated thin films is controllable.19 Using orientation-controlled heptafluorene:CBP films as a light-emitting layer, we fabricated OLEDs with improved electroluminescence (EL) outcoupling efficiency compared with that expected from randomly orientated oligofluorene molecules.20 Importantly, the above study suggested that glassy films with horizontal orientation are accessible by not only static methods such as vapor deposition but also dynamic methods such as spin-coating. However, the mechanism for the horizontal orientation is still unclear. Therefore, for further applications of such orientations to light outcoupling, it is highly desired to find a factor governing the horizontal orientation. The orientation mechanism should be more complex than those in vapor deposited films. It has been reported that there are some differences in the molecular orientations among vapor-deposited and spin-coated films,21 spin-coated neat and doped films,20 and doped films with and without crystallization.19,22 Among these various conditions, one of the crucial factors governing the molecular orientation is the solution flow during spin-coating, which involves microscopic solution dynamics over the whole area. Recently, one-dimensional orientation has been reported in 2,7-dioctyl[1]benzothieno[2,3,-b][1]benzothiopene-doped polystyrene films fabricated by an off-center spin-coating technique.23 This report led us to believe that in-plane anisotropic fluorene orientation could possibly be achieved even in the conventional spin-coating method by realizing one-dimensional solution flow and that such in-plane orientation could provide direct evidence to clarify the relationship between the solution flow and the molecular orientation. While the aim of this paper is to demonstrate the effect of the solution flow on the molecular orientation in spin-coated films, the attained in-plane orientation can suggest a possible approach for linearly polarized EL. In fact, Chen and co-workers demonstrated that oligofluorenes can be aligned by annealing of the spin
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EXPERIMENTAL SECTION
To realize one-dimensional solution flow, microfluidic channels were fabricated on substrate surfaces as a grating structure. Figure 1a shows a schematic diagram of the substrate. The substrates were fabricated from 130 nm-thick indium tin oxide (ITO)/glass substrates with the surfaces modified by line-patterned SiO2 ridges (for EL measurements). The 130 nm-thick ITO substrates (sheet resistance = 10 Ω square−1) were first covered with SiO2 by chemical vapor deposition at 300 °C. The SiO2 surfaces were then covered by an adhesive layer of hexamethyldisiloxane followed by a 1.1-μm-thick chemical amplified photoresist (TSMR V90, Tokyo Ohka Kogyo Co.). Photolithography was used to pattern the photoresist layer to form a periodic line-andspace structure. After stripping away the photoexposed resist layer, the bare fused silica surfaces were then etched by C3F8/O2 gas using an inductive coupled plasma−reactive ion etching (ICP-RIE) system (RIE-101iPH, Samco Inc.) with ICP power of 300 W. The residual resist was finally removed by O2 plasma treatment. The channel height (h)/width (w)/groove pitch (p) was either h/w/p = 0.5/10/100 or 1.7−1.9/10/50 μm. Hereafter, we differentiate the types of substrate in terms of h (h = 0.5 or 1.9 μm). Figure 2 shows scanning electron microscope images of the substrate, indicating that grating structures were successfully fabricated. Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) or dipyrazino [2,3-f:20,30-h]quinoxaline-2,3,6,7,10,11hexacarbonitrile (HAT-CN), CBP, 9,9,9′,9′,9″,9″,9‴,9‴,9⁗,9⁗decakis(hexyl)-2,7′;2′,7″,2″,7‴;2‴,7⁗-pentafluorene (pentafluorene), 2,8-bis(diphenylphosphonyl)dibenzothiophene (PPT), and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) were used for the holeinjection layer, host matrix, guest emitter, hole-blocking layer, and B
DOI: 10.1021/acsami.7b05570 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
position so that the ITO plane faced the detector side. The current was kept constant at 4 mA cm−2 during the measurements.
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RESULTS AND DISCUSSION Linearly Polarized PL Measurements. To assess the linear polarization of EL, linearly polarized PL measurements were performed using a simple pentafluorene:CBP layer structure. Figure 4a shows the PL spectra obtained at
Figure 2. Scanning electron microscope images of the ITO substrate. electron-transporting layer, respectively. The chemical structures of the molecules are shown in Figure 1b. PEDOT:PSS aqueous solution, HAT-CN, CBP, pentafluorene, and TPBi were purchased from Haraeus Co., LG Chemical Co., Chemipro Kasei Co., American Dye Source Inc., and Jilin OLED Material Tech. Co., respectively, and they were used as received. PPT was synthesized by partly modifying the protocol in ref 25 and sublimated before use. It should be noted that only CBP and pentafluorene were used in the PL measurements. For the PL measurements, a 1 wt % solution of pentafluorene:CBP (1:9 w/w) in chloroform was spun on the ITO substrates to form 40− 50 nm-thick thin films in the channel. The ITO substrates were precleaned by ultrasonication in detergent, deionized water, acetone, and 2-propanol, followed by UV-ozone treatment. The spin speed was varied from 2000 to 8000 rpm. The thickness was estimated by a Dektak surface profiler (Veeco Co.), comparing the channel heights before and after deposition. The top of the ridges was not completely covered by the pentafluorene:CBP layer so that the net thickness could be estimated. Linearly polarized PL spectra of the films were measured, as shown in Figure 3. The sample was subjected to excitation light with wavelength of 375 nm at a right angle from the grating side. The PL was detected by a photomultichannel analyzer (PMA-12, Hamamatsu Photonics, Co.) through a long-pass filter with a cutoff wavelength of 410 nm and a wire grid polarizer. The in-plane rotation angle of the polarizer θ was set so that 0° was parallel to the grooves of the grating substrates, and it was rotated from 0 to 180° with a step of 90° for each spectrum acquisition. OLEDs were fabricated with the structure glass/ITO (130 nm)/ PEDOT:PSS (
