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Fabrication and performance evaluation of microfluidic organic light emitting diode
Accepted Manuscript Title: Fabrication and performance evaluation of microfluidic organic light emitting diode Authors: Takashi Kasahara, Shigeyuki Matsunami, Tomohiko Edura, Juro Oshima, Chihaya Adachi, Shuichi Shoji, Jun Mizuno PII: DOI: Reference:
S0924-4247(12)00777-7 doi:10.1016/j.sna.2012.12.031 SNA 8147
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-4-2012 24-12-2012 27-12-2012
Please cite this article as: T. Kasahara, S. Matsunami, T. Edura, J. Oshima, C. Adachi, S. Shoji, J. Mizuno, Fabrication and performance evaluation of microfluidic organic light emitting diode, Sensors and Actuators: A Physical (2010), doi:10.1016/j.sna.2012.12.031 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.
Fabrication and performance evaluation of microfluidic organic
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light emitting diode
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Takashi Kasaharaa, Shigeyuki Matsunamib, Tomohiko Edurab, Juro Oshimab,c, Chihaya Adachib,
Nano-Science and Nano-Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-
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a
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Shuichi Shojia, Jun Mizunod
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8555, Japan. b
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744
Nissan Chemical Industries, Ltd., 2-10-1 Tsuboi-nishi, Funabashi, Chiba 274-8507, Japan
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c
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Motooka, Nishi, Fukuoka 819-0395, Japan
d
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Institute for Nanoscience and Nanotechnology, Waseda University, 513 Wasedatsurumakicho,
Shinjuku, Tokyo 162-0041, Japan.
*Corresponding author. Takashi Kasahara
Email: [email protected] Phone: +81-3-5286-3384
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Fax: +81-3-3204-5765 Address: 3-4-1 Okubo
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City: Shinjuku, Tokyo
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ZIP Code: 169-8555
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an
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Country: JAPAN
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Abstract
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In this study, we fabricated a microfluidic organic light emitting diode (OLED) and evaluated its
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performance. The microchip consisted of a 3 × 3 matrix array of OLED pixels in SU-8
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microchannels sandwiched by indium tin oxide (ITO) anode and cathode pairs. Liquid organic semiconductors introduced into the microchannels were employed as the light emitters.
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Electroluminescence was successfully observed at the emitting area of the
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microchannels. A current density of 0.298 mA/cm2 was measured at 70 V. In our prototype microfluidic OLED, the patterning of the liquid emitters was confirmed in the
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microchannels on a single chip. This result shows that the proposed structure can be
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applicable for liquid-based display.
Keywords
Microfluidic OLED; Liquid OLED; liquid organic semiconductor; SU-8 microchannel
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1. Introduction Organic light emitting diode (OLED) displays consisting of solid-state organic semiconductors
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have attracted considerable attention as potential candidates for next generation flat panel
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displays because of the many advantages they offer over conventional display systems; these
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advantages include self-emission, wide view-angle, reduced weight, and reduced panel
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thickness [1,2]. In addition, OLEDs can be fabricated on plastic substrates for flexible display applications [3,4]. Further, optoelectronic devices based on liquid emitting materials such as
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electrochemiluminescence (ECL) cells [5-8] and liquid OLEDs [9,10] have been recently
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reported. The use of liquid emitters is expected to provide flexibility and robustness of the displays; such features are difficult to achieve using solid-state emitters. Many of these devices
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were fabricated as simple liquid emitter structures positioned between a pair of electrodes on
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substrates. However, multicolor emissions from a single device require precise volume control and transfer of small amounts of liquid emitters. In the past decade, microfluidic devices have been developed for a wide range of applications such as chemical and biological analyses; these devices offer several advantages in terms of the delivery and mixing of small amounts of reagents [11]. The microchannel structures of such devices are usually fabricated via microelectro mechanical systems (MEMS) technologies using various materials including silicon, quartz, glass, and polymers. The use of optically transparent
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substrates in these devices enables easy observation and detection. The epoxy-based negative photoresist SU-8 is considered a suitable material for microchannel fabrication due to its
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excellent lithography properties, thermal stability (glass transition temperature above 200 °C),
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and good solvent resistance as long with high optical transparency. Our research group has
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developed methods of fabricating high-aspect-ratio 3D SU-8 microfluidic structures [12]. Moreover, the next generation microfluidic devices are expected to require the incorporation of
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electrodes into microchannels [13-15]. Transparent electrodes such as indium tin oxide (ITO)
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facilitate the optical microscopic observation of fluidic behavior in the microchannels. ITO has
[16,17].
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been widely used as an anode in OLEDs due to its high transparency and low sheet resistance
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In this study, we proposed and fabricated a prototype microfluidic OLED, which is a novel
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liquid OLED realized using microfluidic technology [18]. The SU-8-based microfluidic chip, which was integrated with ITO anode and cathode pairs, was fabricated using photolithography, vacuum ultraviolet treatment in the presence of oxygen gas (VUV/O3) [19-21], and lowtemperature bonding techniques through the use of epoxy- and amine-terminated self-assembled monolayers (SAMs) [22,23]. A liquid organic semiconductor was employed as the emitting material and injected into the emitting areas of the microfluidic OLED.
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2. Concept and principle of microfluidic OLED The concept of the microfluidic OLED is illustrated in Fig. 1. Various fresh liquid organic
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semiconductors are continuously injected from the inlets into the light emitting areas through
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the microchannels using syringe pumps. Electroluminescence was generated by the
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radiative recombination of electron-hole pairs in the liquid organic semiconductors under an appropriate DC voltage. After passing through the microchannels, the liquid
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emitters are collected at the outlets. The continuous injection of the emitters is expected to
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prevent performance degradation of the microfluidic OELD. The emission wavelengths of the
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organic semiconductors.
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microfluidic OLED can be controlled by varying the composition and mixing of the liquid
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3. Experimental procedure
3.1. Microfluidic OLED fabrication Fig. 2 shows the design of our microfluidic OLED [18]. The microchip has a 3 × 3 matrix OLED array embedded in the SU-8 microchannels. The microchannels are sandwiched between the ITO anodes on a glass substrate and the polyethylene naphthalate (PEN) film with the ITO cathodes. The depth of the microchannels is chosen to be approximately 6 µm because this depth leads to enhancement of the OLED performances, while the channel widths are 1000,
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1250, and 1500 µm in order to ensure smooth flow of the liquid emitters. The fabrication process of the microfluidic OLED is shown in Fig. 3. The anode and cathode
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substrates were separately fabricated and subsequently bonded to form enclosed ITO electrode-
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embedded microchannels using two kinds of self-assembled monolayers (SAMs). An epoxy-
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terminated SAM of 3-glycidyloxypropyltrimethoxysilane (GOPTS) was utilized for the anode substrate, while an amine-terminated SAM of 3-aminopropyltriethoxysilane (APTES) was
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formed on the cathode substrate.
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For the anode substrate, an ITO-coated glass substrate with a 135-nm-thick ITO layer and a sheet resistance of 10 Ω/sq was used. The substrate was ultrasonic cleaned with acetone and
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isopropyl alcohol (IPA) for 10 and 5 min, respectively, and dried on a hot plate at 120 °C for 20
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min. The ITO anodes were patterned by conventional photolithography and wet-etching with a
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dilute aqua regia solution of HCl:HNO3:H2O in the ratio of 5:1:6 (Fig. 3 (a)). The SU-8 3005 (Microchem Co.) photoresist was spun on the patterned ITO-coated glass substrate at 4000 rpm for 20 s to obtain a 6-µm-thick resist layer and soft-baked at 100 °C for 10 min. For the formation of the open microchannels on the ITO anodes, UV exposure was performed using a radiation dose of 250 mJ/cm2 at a wavelength of 365 nm. The post-exposure bakes were carried out at 65 °C for 2 min and at 95 °C for 5 min. Finally, the anode substrate was developed in the SU-8 developer (Microchem Co.) at room temperature, followed by hard baking at 180 °C for
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30 min (Fig. 3 (b)). For the selective GOPTS-SAM formation on only the SU-8 layer of the anode substrate, the ITO anodes in the microchannels were covered by a sacrificial layer of
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positive resist (Tokyo Ohka Kogyo Co., TSMR-V90) (Fig. 3 (c)). On the cathode substrate part,
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ITO-coated PEN film substrate was used. The thickness of the PEN was 150 µm, while that of
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the ITO layer was 350 nm with a sheet resistance of 25 Ω/sq. The ITO cathodes were fabricated using the same process as the ITO anodes (Fig. 3 (d)). The inlet and outlet of the microchannels
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were mechanically punched out using a sharpened needle tip (Fig. 3 (e)). Before the SAM
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formation processes, the anode and cathode substrates were pre-treated by VUV/O3 using a Xe2* excimer lamp source (Ushio Inc., UER20-170) for the formation of hydroxyl groups on
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the SU-8, PEN, and ITO cathode surfaces (Fig. 3 (f)) [19-21]. The anode and cathode
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substrates were immersed in 1% (v/v) GOPTS and 5% (v/v) APTES solutions prepared in water,
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respectively, for 20 min (Fig. 3 (g)). The anode substrate was subsequently rinsed with acetone and IPA to remove both the sacrificial resist and any unbound GOPTS-SAM, while the cathode substrate was rinsed with ethanol. Finally, the surfaces were bonded under contact pressure of 1.5 MPa at 140 °C for 5 min to form amine-epoxy bonds using a bonding machine (EV Group Co., EVG520HE) (Fig. 3 (h)).
3.2. Evaluation of the fabricated microfluidic OLED
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The microfluidic OLEDs that were fabricated without and with the selective GOPTS-SAM formation process of the anode substrate were investigated using a scanning acoustic
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microscope (PVA TePla Analytical Systems GmbH, SAM 300) at an acoustic frequency of 175
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MHz.
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In order to evaluate the characteristics of the fabricated microfluidic OLED, the following two types of assessments were performed: the observation of the photoluminescence and
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electroluminescence of the liquid emitters in the microchannels, and the current density-voltage
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(J-V) measurements [18]. Liquid organic semiconductor (PLQ) (Nissan Chemical Industries, Ltd.) was used as liquid emitter. Tributylmethylphosphonium
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bis(trifluoromethanesulfonyl)imide (TMP-TFSI) (Tokyo Chemical Industry Co., Ltd.) was
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employed as an electrolyte and introduced into the PLQ at its concentration of 0.25 wt% to
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obtain efficient carrier injection electroluminescence. Fig. 4 illustrates the experimental setup of the fabricated microfluidic OLED. For each experiment, the microfluidic OLED was clamped between top and bottom acrylic resin plates within glass windows for optical observations. The spring-loaded probes were used for the electrical connection of the anode and cathode contact pads to the source meter, as shown in Fig. 4 (a). The inlet and outlet nozzles were connected to the top plate. The liquid emitters were introduced manually into the microchannels with the syringes through the inlet nozzles, while
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the used liquid emitters were collected from the outlet nozzles. The flow of the liquid emitter was detected by photoluminescence under 365-nm UV-light irradiation. The appropriate DC
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voltages were applied to the device using a source meter, and subsequently, the resulting
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electroluminescence was recorded with a digital camera. The J-V characteristics were measured
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using a source meter (Keithley Instruments, Inc., Model 2400 SourceMeter). The flow of the
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PLQ was stopped when operating voltages were applied to the microfluidic OLED.
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4. Results and discussion
Fig. 5 shows the image of the fabricated microfluidic OLED. Figs. 6 (a) and (b) show the
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scanning acoustic microscope images of the fabricated microfluidic OLED without and with the
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selective GOPTS-SAM formation on the anode substrate, respectively. From Fig. 6 (a), it can be
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observed that without the selective GOPTS-SAM formation, several defects are formed in the microchannels; these defects not only form obstacles to the smooth flow of the liquid emitters but also lead to electrical shorts between the anode and cathode. We speculate that these defects were probably formed by the chemical reaction between the GOPTS- and APTES-SAMs in the microchannels during the bonding process. When pressure was applied between the anode and cathode substrates, the PEN film with the ITO cathodes deformed towards the GOPTS-SAM formed-ITO anodes, thereby leading to bonding of the film and the anodes at their contact
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points. In contrast, no obvious defects were observed in the microchannels for the microfluidic OLED fabricated via selective GOPTS-SAM formation on the anode substrate, as shown in Fig.
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6 (b); this lack of defects indicates that the air-gap structures between each anode and cathode
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pair were successfully fabricated. In addition, despite the formation of a 350-nm-thick ITO layer
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on the PEN film, no significant voids were observed at the bonded interfaces between the ITO
the use of a flexible PEN film as the lid substrate.
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cathodes and the SU-8 photoresist as well as between the PET and SU-8; this is probably due to
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Fig. 7 shows the images that demonstrate the working of the fabricated microfluidic OLED. The liquid emitters were continuously passed through the microchannels consisting of the ITO anode
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and cathode pairs. Consequently, the photoluminescence of the liquid emitters was observed
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under UV radiation, as shown in Fig. 7 (a). The flow behavior of the PLQ was recorded by
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digital camera. Its flow rate was approximately 9 nl/min. Further, the working of the
OLED confirmed that there was no leakage of the liquid emitters at the bonded interface. The OLED array exhibited electroluminescence under static condition of the liquid emitter when 70 V was applied without photo-excitation, as shown in Figs. 7 (b)-(e). This result indicated that the gap structures between the anode and cathode pairs were preserved when a DC voltage was applied to the microfluidic OLED. Fig. 8 shows the J-V characteristics of the fabricated microfluidic OLED. In the J-V
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measurements, the liquid emitter was injected into the 1500-µm-wide microchannel. A varying DC voltage (0 to 100 V) was applied in 1 V steps to the microfluidic OLED. The current density
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was proportional to the applied voltage up to 40 V, which is Ohmic behavior (J ! V1). As it is
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clearly seen, the current density increased steeply, and its value was 0.298 mA/cm2 at 70 V. The
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current density was proportional to the square of the voltage at higher than 40 V (J ! V2), which may indicate the space charge limited current (SCLC). SCLC behavior was also observed in the
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liquid OLED with other liquid organic semiconductors reported by Hirata et al. [10]. On the
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other hand, upon comparison with solid-state OLEDs, the fabricated microfluidic OLED required a higher driving voltage because of the presence of the double-layer structures with a
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liquid emitting layer and APTES-SAM layer sandwiched between ITO anode and cathode pairs.
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In addition, the emitting layer used was thicker than that usually employed in solid-state
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OLEDs. Furthermore, the liquid OLEDs have a fundamental problem of short lifetime [10], whereas solid-state OLEDs of more than 100,000 h half-decay lifetime have been reported [24]. Our future studies will focus on improving the electroluminescence characteristics by optimizing the device structure as well as investigating device lifetime under the flow and static condition of the liquid emitters. The disadvantage of liquid OLEDs could be overcome by the proposed microfluidic OLED. Moreover, flexible plastic-based anode substrate is another choice to fabricate flexible microfluidic OLED because the liquid emitters can change their
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shape easily in the microchannels.
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Conclusions
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We proposed and fabricated a prototype microfluidic OLED that combines the working
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principle of microfluidic and liquid-emitter-based OLEDs. The microfluidic OLED is composed of SU-8 microchannels integrated with ITO anode and cathode pairs. In order to ensure the
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heterogeneous assembly, the pre-treatment processes of the anode and cathode substrates
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included an optimized selective GOPTS-SAM formation on the channel substrate, and the uniform formation was confirmed via a scanning acoustic microscope. The liquid emitters were
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made to flow through the microchannels, and electroluminescence was obtained under the
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appropriate voltage. A current density of 0.298 mA/cm2 was obtained when a voltage of 70 V
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was applied. In the light of our results, we believe that microfluidic OLEDs have considerable potential for use in variable multi-colored light emitting devices.
Acknowledgements
This work was partly supported by Japan Ministry of Education, Culture, Sports Science & Technology Grant-in-Aid for Scientific Basic Research (S) No. 23226010 and by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading
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Innovative R&D on Science and Technology (FIRST Program),” initiated by the Council for Science and Technology Policy (CSTP). The authors thank for Nanotechnology Support Project
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of Waseda University for their technical advices.
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Fig. 1. Concept of microfluidic OLED. Fig. 2. Design of prototype microfluidic OLED. (a) Whole and (b) cross-sectional illustrations. Fig. 3. Fabrication process of microfluidic OLED.
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Fig. 4. Experimental setup of fabricated microfluidic OLED. (a) Device assembly and (b)
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evaluation of the device characteristics.
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Fig. 5. Image of fabricated microfluidic OLED.
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Fig. 6. Scanning acoustic microscope images of fabricated microfluidic OLED. (a) Without and (b) with selective GOPTS-SAM formation on anode substrate.
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Fig. 7. Demonstration of fabricated microfluidic OLED. (a) Photoluminescence and
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electroluminescence of (b) 1000-µm-width, (c) 1250-µm-width, (d) 1500-µm-width microchannels, and (e) all microchannels.
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Fig. 8. Current density-voltage (J-V) characteristics of fabricated microfluidic OLED.
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Fig. 1
Anode substrate
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Liquid organic semiconductor
Syringe
Inlet
Microchannel Outlet
DC voltage
Collection
Cathode substrate
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Fig. 2
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(a)
PEN
d
Outlet
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ITO cathode
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Inlet
SU-8 ITO anode Glass
Cathode pad
Anode pad
(b)
PEN ITO SU-8 ITO Glass
6
1000 1250 1500
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Unit: µm
Fig. 3
Anode substrate ITO (a)
Cathode substrate (d)
ITO
Glass
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(b)
PEN
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Outlet
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TSMR
Inlet
d
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(c)
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SU-8 (e)
(f)
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O Si O O O
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GOPTS
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VUV/O3 treatment
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APTES
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Press and heat Page 21 of 26
Fig. 4
(a)
Spring-loaded probe
Bottom plate
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Microfluidic OLED
Inlet nozzle
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Spring loaded probe
d
Glass window
(b)
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Outlet nozzle Top plate Digital camera
UV lamp Source meter ------. ----uA ------. ----V
Syringe Page 22 of 26
ITO cathodes
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Fig. 5
PEN
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Inlet
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ITO anodes
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Fig. 6
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(c)
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10 mm
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d te
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Current density (mA/cm2)
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Voltage (V)
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S0924-4247(12)00777-7 doi:10.1016/j.sna.2012.12.031 SNA 8147
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-4-2012 24-12-2012 27-12-2012
Please cite this article as: T. Kasahara, S. Matsunami, T. Edura, J. Oshima, C. Adachi, S. Shoji, J. Mizuno, Fabrication and performance evaluation of microfluidic organic light emitting diode, Sensors and Actuators: A Physical (2010), doi:10.1016/j.sna.2012.12.031 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.
Fabrication and performance evaluation of microfluidic organic
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light emitting diode
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Takashi Kasaharaa, Shigeyuki Matsunamib, Tomohiko Edurab, Juro Oshimab,c, Chihaya Adachib,
Nano-Science and Nano-Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-
an
a
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Shuichi Shojia, Jun Mizunod
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8555, Japan. b
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744
Nissan Chemical Industries, Ltd., 2-10-1 Tsuboi-nishi, Funabashi, Chiba 274-8507, Japan
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c
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Motooka, Nishi, Fukuoka 819-0395, Japan
d
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Institute for Nanoscience and Nanotechnology, Waseda University, 513 Wasedatsurumakicho,
Shinjuku, Tokyo 162-0041, Japan.
*Corresponding author. Takashi Kasahara
Email: [email protected] Phone: +81-3-5286-3384
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Fax: +81-3-3204-5765 Address: 3-4-1 Okubo
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City: Shinjuku, Tokyo
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ZIP Code: 169-8555
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ed
M
an
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Country: JAPAN
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Abstract
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In this study, we fabricated a microfluidic organic light emitting diode (OLED) and evaluated its
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performance. The microchip consisted of a 3 × 3 matrix array of OLED pixels in SU-8
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microchannels sandwiched by indium tin oxide (ITO) anode and cathode pairs. Liquid organic semiconductors introduced into the microchannels were employed as the light emitters.
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Electroluminescence was successfully observed at the emitting area of the
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microchannels. A current density of 0.298 mA/cm2 was measured at 70 V. In our prototype microfluidic OLED, the patterning of the liquid emitters was confirmed in the
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microchannels on a single chip. This result shows that the proposed structure can be
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applicable for liquid-based display.
Keywords
Microfluidic OLED; Liquid OLED; liquid organic semiconductor; SU-8 microchannel
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1. Introduction Organic light emitting diode (OLED) displays consisting of solid-state organic semiconductors
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have attracted considerable attention as potential candidates for next generation flat panel
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displays because of the many advantages they offer over conventional display systems; these
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advantages include self-emission, wide view-angle, reduced weight, and reduced panel
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thickness [1,2]. In addition, OLEDs can be fabricated on plastic substrates for flexible display applications [3,4]. Further, optoelectronic devices based on liquid emitting materials such as
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electrochemiluminescence (ECL) cells [5-8] and liquid OLEDs [9,10] have been recently
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reported. The use of liquid emitters is expected to provide flexibility and robustness of the displays; such features are difficult to achieve using solid-state emitters. Many of these devices
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were fabricated as simple liquid emitter structures positioned between a pair of electrodes on
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substrates. However, multicolor emissions from a single device require precise volume control and transfer of small amounts of liquid emitters. In the past decade, microfluidic devices have been developed for a wide range of applications such as chemical and biological analyses; these devices offer several advantages in terms of the delivery and mixing of small amounts of reagents [11]. The microchannel structures of such devices are usually fabricated via microelectro mechanical systems (MEMS) technologies using various materials including silicon, quartz, glass, and polymers. The use of optically transparent
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substrates in these devices enables easy observation and detection. The epoxy-based negative photoresist SU-8 is considered a suitable material for microchannel fabrication due to its
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excellent lithography properties, thermal stability (glass transition temperature above 200 °C),
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and good solvent resistance as long with high optical transparency. Our research group has
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developed methods of fabricating high-aspect-ratio 3D SU-8 microfluidic structures [12]. Moreover, the next generation microfluidic devices are expected to require the incorporation of
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electrodes into microchannels [13-15]. Transparent electrodes such as indium tin oxide (ITO)
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facilitate the optical microscopic observation of fluidic behavior in the microchannels. ITO has
[16,17].
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been widely used as an anode in OLEDs due to its high transparency and low sheet resistance
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In this study, we proposed and fabricated a prototype microfluidic OLED, which is a novel
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liquid OLED realized using microfluidic technology [18]. The SU-8-based microfluidic chip, which was integrated with ITO anode and cathode pairs, was fabricated using photolithography, vacuum ultraviolet treatment in the presence of oxygen gas (VUV/O3) [19-21], and lowtemperature bonding techniques through the use of epoxy- and amine-terminated self-assembled monolayers (SAMs) [22,23]. A liquid organic semiconductor was employed as the emitting material and injected into the emitting areas of the microfluidic OLED.
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2. Concept and principle of microfluidic OLED The concept of the microfluidic OLED is illustrated in Fig. 1. Various fresh liquid organic
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semiconductors are continuously injected from the inlets into the light emitting areas through
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the microchannels using syringe pumps. Electroluminescence was generated by the
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radiative recombination of electron-hole pairs in the liquid organic semiconductors under an appropriate DC voltage. After passing through the microchannels, the liquid
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emitters are collected at the outlets. The continuous injection of the emitters is expected to
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prevent performance degradation of the microfluidic OELD. The emission wavelengths of the
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organic semiconductors.
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microfluidic OLED can be controlled by varying the composition and mixing of the liquid
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3. Experimental procedure
3.1. Microfluidic OLED fabrication Fig. 2 shows the design of our microfluidic OLED [18]. The microchip has a 3 × 3 matrix OLED array embedded in the SU-8 microchannels. The microchannels are sandwiched between the ITO anodes on a glass substrate and the polyethylene naphthalate (PEN) film with the ITO cathodes. The depth of the microchannels is chosen to be approximately 6 µm because this depth leads to enhancement of the OLED performances, while the channel widths are 1000,
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1250, and 1500 µm in order to ensure smooth flow of the liquid emitters. The fabrication process of the microfluidic OLED is shown in Fig. 3. The anode and cathode
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substrates were separately fabricated and subsequently bonded to form enclosed ITO electrode-
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embedded microchannels using two kinds of self-assembled monolayers (SAMs). An epoxy-
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terminated SAM of 3-glycidyloxypropyltrimethoxysilane (GOPTS) was utilized for the anode substrate, while an amine-terminated SAM of 3-aminopropyltriethoxysilane (APTES) was
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formed on the cathode substrate.
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For the anode substrate, an ITO-coated glass substrate with a 135-nm-thick ITO layer and a sheet resistance of 10 Ω/sq was used. The substrate was ultrasonic cleaned with acetone and
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isopropyl alcohol (IPA) for 10 and 5 min, respectively, and dried on a hot plate at 120 °C for 20
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min. The ITO anodes were patterned by conventional photolithography and wet-etching with a
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dilute aqua regia solution of HCl:HNO3:H2O in the ratio of 5:1:6 (Fig. 3 (a)). The SU-8 3005 (Microchem Co.) photoresist was spun on the patterned ITO-coated glass substrate at 4000 rpm for 20 s to obtain a 6-µm-thick resist layer and soft-baked at 100 °C for 10 min. For the formation of the open microchannels on the ITO anodes, UV exposure was performed using a radiation dose of 250 mJ/cm2 at a wavelength of 365 nm. The post-exposure bakes were carried out at 65 °C for 2 min and at 95 °C for 5 min. Finally, the anode substrate was developed in the SU-8 developer (Microchem Co.) at room temperature, followed by hard baking at 180 °C for
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30 min (Fig. 3 (b)). For the selective GOPTS-SAM formation on only the SU-8 layer of the anode substrate, the ITO anodes in the microchannels were covered by a sacrificial layer of
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positive resist (Tokyo Ohka Kogyo Co., TSMR-V90) (Fig. 3 (c)). On the cathode substrate part,
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ITO-coated PEN film substrate was used. The thickness of the PEN was 150 µm, while that of
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the ITO layer was 350 nm with a sheet resistance of 25 Ω/sq. The ITO cathodes were fabricated using the same process as the ITO anodes (Fig. 3 (d)). The inlet and outlet of the microchannels
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were mechanically punched out using a sharpened needle tip (Fig. 3 (e)). Before the SAM
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formation processes, the anode and cathode substrates were pre-treated by VUV/O3 using a Xe2* excimer lamp source (Ushio Inc., UER20-170) for the formation of hydroxyl groups on
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the SU-8, PEN, and ITO cathode surfaces (Fig. 3 (f)) [19-21]. The anode and cathode
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substrates were immersed in 1% (v/v) GOPTS and 5% (v/v) APTES solutions prepared in water,
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respectively, for 20 min (Fig. 3 (g)). The anode substrate was subsequently rinsed with acetone and IPA to remove both the sacrificial resist and any unbound GOPTS-SAM, while the cathode substrate was rinsed with ethanol. Finally, the surfaces were bonded under contact pressure of 1.5 MPa at 140 °C for 5 min to form amine-epoxy bonds using a bonding machine (EV Group Co., EVG520HE) (Fig. 3 (h)).
3.2. Evaluation of the fabricated microfluidic OLED
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The microfluidic OLEDs that were fabricated without and with the selective GOPTS-SAM formation process of the anode substrate were investigated using a scanning acoustic
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microscope (PVA TePla Analytical Systems GmbH, SAM 300) at an acoustic frequency of 175
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MHz.
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In order to evaluate the characteristics of the fabricated microfluidic OLED, the following two types of assessments were performed: the observation of the photoluminescence and
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electroluminescence of the liquid emitters in the microchannels, and the current density-voltage
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(J-V) measurements [18]. Liquid organic semiconductor (PLQ) (Nissan Chemical Industries, Ltd.) was used as liquid emitter. Tributylmethylphosphonium
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bis(trifluoromethanesulfonyl)imide (TMP-TFSI) (Tokyo Chemical Industry Co., Ltd.) was
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employed as an electrolyte and introduced into the PLQ at its concentration of 0.25 wt% to
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obtain efficient carrier injection electroluminescence. Fig. 4 illustrates the experimental setup of the fabricated microfluidic OLED. For each experiment, the microfluidic OLED was clamped between top and bottom acrylic resin plates within glass windows for optical observations. The spring-loaded probes were used for the electrical connection of the anode and cathode contact pads to the source meter, as shown in Fig. 4 (a). The inlet and outlet nozzles were connected to the top plate. The liquid emitters were introduced manually into the microchannels with the syringes through the inlet nozzles, while
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the used liquid emitters were collected from the outlet nozzles. The flow of the liquid emitter was detected by photoluminescence under 365-nm UV-light irradiation. The appropriate DC
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voltages were applied to the device using a source meter, and subsequently, the resulting
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electroluminescence was recorded with a digital camera. The J-V characteristics were measured
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using a source meter (Keithley Instruments, Inc., Model 2400 SourceMeter). The flow of the
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PLQ was stopped when operating voltages were applied to the microfluidic OLED.
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4. Results and discussion
Fig. 5 shows the image of the fabricated microfluidic OLED. Figs. 6 (a) and (b) show the
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scanning acoustic microscope images of the fabricated microfluidic OLED without and with the
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selective GOPTS-SAM formation on the anode substrate, respectively. From Fig. 6 (a), it can be
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observed that without the selective GOPTS-SAM formation, several defects are formed in the microchannels; these defects not only form obstacles to the smooth flow of the liquid emitters but also lead to electrical shorts between the anode and cathode. We speculate that these defects were probably formed by the chemical reaction between the GOPTS- and APTES-SAMs in the microchannels during the bonding process. When pressure was applied between the anode and cathode substrates, the PEN film with the ITO cathodes deformed towards the GOPTS-SAM formed-ITO anodes, thereby leading to bonding of the film and the anodes at their contact
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points. In contrast, no obvious defects were observed in the microchannels for the microfluidic OLED fabricated via selective GOPTS-SAM formation on the anode substrate, as shown in Fig.
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6 (b); this lack of defects indicates that the air-gap structures between each anode and cathode
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pair were successfully fabricated. In addition, despite the formation of a 350-nm-thick ITO layer
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on the PEN film, no significant voids were observed at the bonded interfaces between the ITO
the use of a flexible PEN film as the lid substrate.
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cathodes and the SU-8 photoresist as well as between the PET and SU-8; this is probably due to
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Fig. 7 shows the images that demonstrate the working of the fabricated microfluidic OLED. The liquid emitters were continuously passed through the microchannels consisting of the ITO anode
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and cathode pairs. Consequently, the photoluminescence of the liquid emitters was observed
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under UV radiation, as shown in Fig. 7 (a). The flow behavior of the PLQ was recorded by
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digital camera. Its flow rate was approximately 9 nl/min. Further, the working of the
OLED confirmed that there was no leakage of the liquid emitters at the bonded interface. The OLED array exhibited electroluminescence under static condition of the liquid emitter when 70 V was applied without photo-excitation, as shown in Figs. 7 (b)-(e). This result indicated that the gap structures between the anode and cathode pairs were preserved when a DC voltage was applied to the microfluidic OLED. Fig. 8 shows the J-V characteristics of the fabricated microfluidic OLED. In the J-V
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measurements, the liquid emitter was injected into the 1500-µm-wide microchannel. A varying DC voltage (0 to 100 V) was applied in 1 V steps to the microfluidic OLED. The current density
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was proportional to the applied voltage up to 40 V, which is Ohmic behavior (J ! V1). As it is
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clearly seen, the current density increased steeply, and its value was 0.298 mA/cm2 at 70 V. The
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current density was proportional to the square of the voltage at higher than 40 V (J ! V2), which may indicate the space charge limited current (SCLC). SCLC behavior was also observed in the
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liquid OLED with other liquid organic semiconductors reported by Hirata et al. [10]. On the
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other hand, upon comparison with solid-state OLEDs, the fabricated microfluidic OLED required a higher driving voltage because of the presence of the double-layer structures with a
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liquid emitting layer and APTES-SAM layer sandwiched between ITO anode and cathode pairs.
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In addition, the emitting layer used was thicker than that usually employed in solid-state
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OLEDs. Furthermore, the liquid OLEDs have a fundamental problem of short lifetime [10], whereas solid-state OLEDs of more than 100,000 h half-decay lifetime have been reported [24]. Our future studies will focus on improving the electroluminescence characteristics by optimizing the device structure as well as investigating device lifetime under the flow and static condition of the liquid emitters. The disadvantage of liquid OLEDs could be overcome by the proposed microfluidic OLED. Moreover, flexible plastic-based anode substrate is another choice to fabricate flexible microfluidic OLED because the liquid emitters can change their
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shape easily in the microchannels.
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Conclusions
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We proposed and fabricated a prototype microfluidic OLED that combines the working
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principle of microfluidic and liquid-emitter-based OLEDs. The microfluidic OLED is composed of SU-8 microchannels integrated with ITO anode and cathode pairs. In order to ensure the
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heterogeneous assembly, the pre-treatment processes of the anode and cathode substrates
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included an optimized selective GOPTS-SAM formation on the channel substrate, and the uniform formation was confirmed via a scanning acoustic microscope. The liquid emitters were
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made to flow through the microchannels, and electroluminescence was obtained under the
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appropriate voltage. A current density of 0.298 mA/cm2 was obtained when a voltage of 70 V
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was applied. In the light of our results, we believe that microfluidic OLEDs have considerable potential for use in variable multi-colored light emitting devices.
Acknowledgements
This work was partly supported by Japan Ministry of Education, Culture, Sports Science & Technology Grant-in-Aid for Scientific Basic Research (S) No. 23226010 and by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading
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Innovative R&D on Science and Technology (FIRST Program),” initiated by the Council for Science and Technology Policy (CSTP). The authors thank for Nanotechnology Support Project
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of Waseda University for their technical advices.
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Fig. 1. Concept of microfluidic OLED. Fig. 2. Design of prototype microfluidic OLED. (a) Whole and (b) cross-sectional illustrations. Fig. 3. Fabrication process of microfluidic OLED.
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Fig. 4. Experimental setup of fabricated microfluidic OLED. (a) Device assembly and (b)
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evaluation of the device characteristics.
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Fig. 5. Image of fabricated microfluidic OLED.
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Fig. 6. Scanning acoustic microscope images of fabricated microfluidic OLED. (a) Without and (b) with selective GOPTS-SAM formation on anode substrate.
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Fig. 7. Demonstration of fabricated microfluidic OLED. (a) Photoluminescence and
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electroluminescence of (b) 1000-µm-width, (c) 1250-µm-width, (d) 1500-µm-width microchannels, and (e) all microchannels.
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Fig. 8. Current density-voltage (J-V) characteristics of fabricated microfluidic OLED.
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Fig. 1
Anode substrate
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Liquid organic semiconductor
Syringe
Inlet
Microchannel Outlet
DC voltage
Collection
Cathode substrate
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an
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Fig. 2
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(a)
PEN
d
Outlet
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ITO cathode
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Inlet
SU-8 ITO anode Glass
Cathode pad
Anode pad
(b)
PEN ITO SU-8 ITO Glass
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1000 1250 1500
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Unit: µm
Fig. 3
Anode substrate ITO (a)
Cathode substrate (d)
ITO
Glass
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(b)
PEN
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Outlet
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TSMR
Inlet
d
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(c)
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SU-8 (e)
(f)
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(h)
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VUV/O3 treatment
Si
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Press and heat Page 21 of 26
Fig. 4
(a)
Spring-loaded probe
Bottom plate
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Microfluidic OLED
Inlet nozzle
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Spring loaded probe
d
Glass window
(b)
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Outlet nozzle Top plate Digital camera
UV lamp Source meter ------. ----uA ------. ----V
Syringe Page 22 of 26
ITO cathodes
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Fig. 5
PEN
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Outlet
Inlet
10 mm
Glass
ITO anodes
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Fig. 6
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5 mm
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(b)
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(c)
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10 mm
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d te
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Current density (mA/cm2)
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Fig. 8
100 10-1
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