Research Article www.acsami.org
Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Area-Controllable Stamping of Semicrystalline Copolymer Ionogels for Solid-State Electrolyte-Gated Transistors and Light-Emitting Devices Hyun Je Kim,† Hae Min Yang,† Jaemok Koo,‡ Moon Sung Kang,‡ Kihyon Hong,*,§ and Keun Hyung Lee*,† †
Department of Chemistry and Chemical Engineering, Inha University, Incheon 22212, Republic of Korea Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea § Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Two types of thin-film electrochemical devices (electrolyte-gated transistors and electrochemical light-emitting cells) are demonstrated using areacontrollable ionogel patches generated by transfer-stamping. For the successful transfer of ionogel patches on various target substrates, thermoreversible gelation by phaseseparated polymer crystals within the ionogel is essential because it allows the gel to form a conformal contact with the acceptor substrate, thereby lowering the overall Gibbs energy of the system upon transfer of the ionogel. This crystallization-mediated stamping provides a much more efficient deposition route for producing thin films of ionically conductive high-capacitance solid ionogel electrolytes. The lateral dimensions of the transferred ionogels range from 1 mm × 1 mm to 40 mm × 40 mm. These ionogel patches are incorporated in organic p-type and inorganic n-type thin-film transistors and electrochemical light-emitting devices. The resulting transistors show sub-1 V device operation with high transconductance currents, and the optoelectronic devices emit orange light through a series of electrochemical redox reactions. These results demonstrate a simple yet versatile route to employ physical ionogels for various solid-state electrochemical device applications. KEYWORDS: transfer-stamping, ionogel, physical gel, electrolyte-gated transistor, electrochemical light-emitting device
1. INTRODUCTION
applications because the reversible formation of physical crosslinks allows the liquid-state processing of ionogels before resolidifying the materials to utilize their functions in thin-film devices.26 To date, several processing methods for ionogels, including spin-coating, aerosol jet printing, and inkjet printing, have been demonstrated.6,26−29 To promote the use of ionogels in a wide variety of thin-film electrochemical devices, however, a facile yet versatile processing strategy that allows the fabrication of ionogels with different feature sizes is desirable. For example, small gel patches are appropriate for electronic devices such as thin-film transistors, whereas large-area gels are suitable for lowcost, large-area solid-state lighting devices. In this regard, an additive transfer-stamping method using a poly(dimethylsiloxane) (PDMS) stamp is attractive because of the low surface energy (19.8 mJ/m2) and the easy processability of PDMS.30−32 Transfer-stamping has been widely applied to deposit various materials including organic small molecules, polymers, nanocrystals, graphene, metals, and inorganic
Ionogels, which are polymer networks swollen by large amounts of ionic liquids, have attracted great attention as promising electrolyte materials in electrochemical devices because they show large specific capacitances, high ionic conductivities, and superior electrochemical stabilities.1−5 The high capacitances of ionogels originate from the formation of thin electric double layers at the ionogel−metal or ionogel− semiconductor interfaces, which effectively counterbalance the external electric potential. High ionic conductivities of ionogels allow fast operation of the corresponding devices because the mobile ions within the gels respond promptly to input signals. Importantly, ionogels are electronically stable within wide potential windows. Therefore, ionogels are promising electrolytes in various electrochemical devices such as thin-film transistors, supercapacitors, sensors, and light-emitting devices.6−16 The structural polymer networks of ionogels can be prepared by chemically forming covalent networks in ionic liquids17−20 or by physically generating temporary networks by noncovalent interactions, including lyophobic phase-separation, hydrogen bonding, metal ligation, and crystallization.21−25 Among these, physical gels provide more effective processing routes in device © XXXX American Chemical Society
Received: August 23, 2017 Accepted: November 16, 2017 Published: November 16, 2017 A
DOI: 10.1021/acsami.7b12712 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces semiconductors on target substrates.33−48 However, direct transfer of ionogel is sparse and we recognize only one paper on this topic using a block polymer, poly(styrene-b-ethylene oxide-b-styrene) (SOS), in an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]).26 Because the surface energy of PDMS is very low, ionogel films prepared on PDMS could in principle be transferred onto any alien substrate (acceptor substrates in this study) with a larger surface energy than that of PDMS. However, to successfully transfer the SOS-based ionogel, the block length of the lyophobic polystyrene chains requires finetuning within a small margin: the chain length should be long enough to form phase-segregated network micelles but short enough to be pulled out from the network cores.26 The previous results showed the feasibility of a transferstamping process using an ionogel; however, the available host polymer was limited to block polymer. The block polymerbased gelation process is typically limited by suitable monomers with appropriate affinities for the ionic liquid of interest, which can be incorporated into the particular block.25 Even when polymerization with specific monomers is possible, it is difficult to synthesize block polymers with target sequences and block lengths because of the involute and variable monomer reactivity under certain reaction conditions.25 To address these issues, we utilize thermoreversible crystallization and fusion of the polymeric crystals of the semicrystalline copolymer-based ionogel for successful transfer of the ionically conductive, high-capacitance ionogel onto various types of acceptor substrates. We expect that crystallization-mediated stamping will provide a universal and versatile ionogel deposition route for a wide range of applications. Note that the phase-separated crystals serve as network crosslinks of the ionogel. In this study, we demonstrate the successful transferstamping of crystallization-induced physical ionogels using flexible PDMS as a donor stamp substrate. The ionogels consist of semicrystalline copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and ionic liquid [EMI][TFSI].49 The weight ratio of P(VDF-HFP) to [EMI][TFSI] was maintained at 1:4 in this study. Thin layers of the physical gels were successfully transferred onto various flat and curved surfaces, with lateral dimensions reaching a few centimeters. This study details a facile processing technique to deposit ionogels with tunable feature sizes, which is potentially advantageous for the applicability of ionogels in various electrochemical devices as test platforms. To demonstrate the versatility of the transfer-stamped ionogels, the gels were employed as gate dielectric materials in electrolyte-gated transistors (EGTs)50 and as luminescent layers in electrochemiluminescent (ECL) devices, when blended with a Rubased luminophore.13 The resulting EGTs and ECLs exhibited desirable device characteristics of high transconductance currents at very low applied voltages and bright orange emission over large areas, respectively.
2.2. Material Characterization. A surface image of the crystallization-based ionogel was obtained using a JEOL JCM-5000 scanning electron microscope (SEM). Differential scanning calorimetry (DSC) measurements were performed on a JADE DSC (PerkinElmer) at temperatures from −60 to 200 °C and at the ramp rate of 10 °C/min. Atomic force microscopy (AFM) images of the acceptor substrates (Al, polyimide (PI), poly(ethylene terephthalate) (PET), and glass) were obtained using an n-Tracer AFM (NanoFocus). Electric impedance spectroscopy was conducted on ionogel capacitors consisting of Au/ionogel/Au using an AUTOLAB spectrometer (Echem-Technology) in the frequency range of 1−106 Hz with the voltage amplitude of 10 mV under ambient conditions. The specific capacitance (C′) was derived using equation C′ = −1/ 2πf Z″A, where f is the excitation frequency, Z″ is the imaginary component of the measured impedance, and A is the area of the Au contact. 2.3. Transfer-Stamping Process. An ionogel layer was spincoated onto a PDMS donor stamp at 1500 rpm for 30 s from a precursor solution consisting of P(VDF-HFP)/[EMI][TFSI]/acetone = 1:4:7 by weight. The average thickness of the ionogel was ∼10 μm. To prepare the PDMS stamp, the PDMS base and the curing agent were mixed at the ratio of 10:1 by weight. After degassing the mixture under vacuum, the mixture was cured at 80 °C for 8 h. The area of the ionogel was controlled by changing the area of the PDMS stamp. For smaller ionogels with areas below 1 cm × 1 cm, ionogel-coated PDMS stamps were cut to desirable sizes with a razor blade and then transferred. For larger ionogels, larger stamps were used. Spin-coated ionogel films are typically thicker at the edges of the stamp, and these thicker edges prevent transfer of the ionogel films. To avoid this issue, we cut the edges of the ionogel-coated stamps using a razor blade and used the flat part. The ionogel-coated PDMS was placed on a target acceptor substrate, and the assembly was heated at 70 °C for 30 s. In this stage, the thermoreversible physical ionogel partially melted to form an intimate conformal contact with the acceptor surface. After cooling the assembly for 30 s at room temperature, the ionogel layer was transferred to the acceptor substrate by gently detaching the PDMS donor stamp. As the ionogel was mechanically supported by the PDMS stamp during transferring, it could be reliably transferred to a target location without physical deformation. We expect the positional accuracy to be further improved by implementing the XYZ translational stage in the transfer-stamping process, but it is beyond the scope of the present study. To stamp the ionogel onto curved surfaces, the substrates were preheated at 70 °C in a convection oven. P3HT and PEDOT:PSS layers were also transfer-stamped onto the desired surfaces following the same procedure. P3HT and PEDOT:PSS were inked onto a PDMS stamp by spin-coating a P3HT precursor solution (3 mg/mL in chloroform) at 2000 rpm for 60 s and solvent-casting a PEDOT:PSS aqueous solution, respectively. The measured thickness of the spin-coated P3HT was ∼40 nm. The PDMS stamp was subjected to ultraviolet/ozone treatment for 30 min prior to the inking of PEDOT:PSS. 2.4. Electrolyte-Gated Transistor Fabrication and Characterization. For p-type P3HT transistors, 3 nm Cr and 37 nm Au layers were sequentially deposited on a substrate by thermal evaporation using a stainless steel stencil mask to form source (S) and drain (D) electrodes. A P3HT semiconductor, an ionogel gate dielectric, and a PEDOT:PSS gate electrode were then transfer-stamped sequentially onto the S/D electrodes. For n-type transistors, an indium gallium zinc oxide (IGZO) semiconductor was generated by a sol−gel process reported previously.51 Al S/D electrodes (50 nm thick) were deposited on IGZO. An ionogel and PEDOT:PSS were sequentially transferred to IGZO. The channel lengths and widths of the p- and n-type transistors were 100 and 1000 μm, respectively. Transistor measurements were conducted on an MS-Tech probe station with a 4200 SCS/F parameter analyzer (Keithley). 2.5. Light-Emitting Device Fabrication and Characterization. An ECL ionogel precursor solution was prepared by blending weighted amounts of P(VDF-HFP), [EMI][TFSI], and Ru(bpy)3Cl2 in acetone. The weight ratio of P(VDF-HFP), [EMI][TFSI], and Ru(bpy)3Cl2 was maintained at 1:4:16. To fabricate the ECL device,
2. MATERIALS AND METHODS 2.1. Materials. P(VDF-HFP) random copolymer with Mn = 130 kg/mol, [EMI][TFSI], and the Ru(bpy)3Cl2 luminophore were purchased from Sigma-Aldrich. A regioregular poly(3-hexylthiophene) (P3HT) semiconductor (4002-EE electronic grade) was purchased from Rieke Metals. Chloroform and acetone solvents were purchased from Sigma-Aldrich. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevios PH 1000) from Heraeus was used as a gate electrode. PDMS (Sylgard 184) from Dow Corning was used as a donor stamp. B
DOI: 10.1021/acsami.7b12712 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces the ECL gel was transfer-stamped from a PDMS stamp to a transparent indium tin oxide (ITO) electrode and then PEDOT:PSS was transferred onto the ECL gel using the stamping method. The luminance of light-emitting devices was characterized by a Konica Minolta CA-210 apparatus. Photoluminescent (PL) and electroluminescent (EL) spectra of the ECL devices were recorded using a RAM Boss PL/EL Measurement System. For PL measurements, a 325 nm He−Cd laser was used as the excitation source.
3. RESULTS AND DISCUSSION 3.1. Fabrication of Area-Controllable and SubstrateVersatile Ionogels. To transfer the crystallization-induced physical ionogel to the acceptor substrates, the gel was first spin-coated onto a PDMS donor stamp. The gel-coated stamp was then placed on a target acceptor substrate with the inked surface facing the acceptor surface. The stamp/substrate assembly was heated at 70 °C for 30 s to improve the physical contact between the ionogel and acceptor surfaces. Subsequently, the assembly was cooled at room temperature for another 30 s. Finally, the PDMS was gently removed to successfully transfer the ionogel to the target substrate. Figure 1 shows the transfer-stamped ionogels on different curved surfaces of metallic Al, organic polymers polyimide (PI)
Figure 2. Transfer-stamped crystallization-induced physical ionogels with feature sizes varying from 9 to 1600 mm2 on a PI substrate. Rhodamine B dye was added to improve the clarity of the optical images.
allows the easy use of ionogels in widespread electrochemical applications. 3.2. Physical Origins of the Ionogel Stamping Process. Figure 3 depicts a schematic of the procedure for transferstamping the crystallization-based physical ionogels onto acceptor surfaces and the corresponding states of the ionogels at each step. To successfully transfer the ionogels, the total Gibbs energy of the system should be lowered through the stamping process. To minimize the total energy upon stamping, the ionogels should be transferred to cover acceptor substrates with higher surface energies, while exposing the lower-surfaceenergy PDMS donor surface.34 The surface energy values of the donor and acceptor materials used in this study are 19.8, 868, 38, 45, and 56 mJ/m2 for PDMS, Al, PI, PET, and soda-lime glass, respectively.52−55 In addition to surface energy, surface roughness is important because the Gibbs energy is proportional to the contact area (Figure 3a). When the ionogel was initially spin-coated onto the PDMS, the gel perfectly wetted the PDMS surface. However, the gel at the air interface was rough, as confirmed by the SEM image in Figure S1, Supporting Information. Simultaneously, the acceptor substrates exhibited finite roughness values, as shown in Figure S2, Supporting Information. The rough natures of both surfaces prevented the formation of intimate conformal contact between the gel layer and the acceptor. Therefore, the gel could not be transferred to the acceptor substrate. To address this issue, the gel was annealed at the elevated temperature of 70 °C, the onset melting temperature of the network crystals. Because the network crystals partially melt at this temperature and the resulting liquidlike solution wets the acceptor surface (Figure 3b), improved physical contact at the ionogel−acceptor interface is achieved. It is noteworthy that the partially melted gel is still solidlike enough to maintain its shape against dewetting. The DSC curve in Figure S3 (Supporting Information) supports the partial melting of the network crystals. Note that dewetting of the ionogel does not occur at elevated processing temperatures because of the decreased surface energy of [EMI][TFSI] at these temperatures; this, in turn, promotes the spreading of the gel electrolyte on the solid surface to minimize the total Gibbs energy of the system.56 Finally, the transfer of the ionogel occurs after the solidlike gel reforms by cooling the assembly at
Figure 1. Transfer-stamped ionogels on various curved surfaces: (a) Al, (b) PI, (c) PET, and (d) glass substrates. The insets show transferstamped ionogels on flat substrates. As the normal ionogel is colorless and transparent, rhodamine B dye was added to improve the clarity of the optical images.
and poly(ethylene terephthalate) (PET), and inorganic sodalime glass. The insets show the transfer-stamped ionogels on flat surfaces of the same materials. The average size of the stamped gels is 1 cm × 1 cm. Because the gels themselves are colorless and transparent, rhodamine B dye is added to improve the clarity of the optical images. Importantly, the dimensions of the stamped gels are easily tunable by varying the sizes of the donor stamps. Figure 2 displays the ionogels transferred to a flexible PI substrate with dimensions ranging from 0.3 cm × 0.3 cm to 4 cm × 4 cm. This simple and facile processing strategy C
DOI: 10.1021/acsami.7b12712 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Schematic of the transfer-stamping process using the crystalline-induced physical ionogel: (a) Ionogel-coated PDMS stamp is placed on a target substrate. The ionogel−acceptor interface shows poor contact because of the roughness of the solid ionogel and substrate surfaces. (b) Physical ionogel achieves conformal contact with the acceptor surface upon heating the assembly at the melting temperature of the network crystals. (c) Upon cooling the assembly, the solid ionogel is reformed and the gel is transferred to the target substrate upon detaching the PDMS.
Figure 4. (a) Schematic of a bottom-contact top-gated P3HT transistor on a SiO2/Si substrate. (b) Representative transfer characteristics (ID vs VG curve) collected in the linear region with VD = −0.1 V. The inset shows an optical image of a transfer-stamped ionogel-gated transistor on SiO2. (c) Representative output curves (ID vs VD curves) with six different gate biases. Statistical summaries of properties from 25 transistors fabricated by transfer-stamping of crystalline-based physical ionogels: (d) hole mobility, (e) on/off ratio, and (f) turn-on voltage (Von).
prepared by sequentially transfer-stamping a polymer semiconductor of regioregular P3HT, an ionogel dielectric, and a PEDOT:PSS gate electrode onto prepatterned S/D electrodes on SiO2/Si substrates.26,52,57 A light-emitting ECL device was also obtained by transferring a luminophore-doped ionogel and a PEDOT:PSS electrode sequentially to a transparent indium tin oxide (ITO) electrode. The average sizes of the stamped gels were 1 and 100 mm2 for the thin-film transistors and lightemitting devices, respectively.
room temperature and then detaching the PDMS from the acceptor substrate (Figure 3c). The thermogravimetric analysis (TGA) curve in Figure S4 (Supporting Information) supports the fact that the ionogel is thermally stable well above the processing temperature used for stamping. 3.3. Thin-Film Electrochemical Devices Using Stamped Ionogels. To investigate the versatility and feasibility of the stamping technique, we fabricated two types of electrochemical devices: electrolyte-gated thin-film transistors and solid-state light-emitting devices. A transistor was D
DOI: 10.1021/acsami.7b12712 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 5. Evolutions in the transfer curves upon cyclic bending of the transfer-stamped ionogel-gated transistors on flexible PI substrates 5000 times to the bending radii of (a) 4 mm and (b) 2 mm. Dependence of (c) normalized maximum current (Imax/Imax,0) and (d) threshold voltage (Vth) vs number of bending cycles, extracted from the transfer curves in (a) and (b).
3.3.1. Transfer-Stamped Ionogels for Electrolyte-Gated Transistors. Figure 4a shows a schematic of a transfer-stamped ionogel-gated transistor. A top-view optical image of the device is displayed in the inset of Figure 4b. The length and width of the transistor channel are 100 and 1000 μm, respectively. Figure 4b shows the representative transfer characteristics via a drain current (ID) versus gate voltage (VG) curve of the transistors on the SiO2/Si substrates. The transfer curve was recorded in the linear regime of a transistor under the applied drain voltage (VD) of −0.1 V. During the measurement, VG was swept from 0.7 to −0.5 V with a step voltage of −0.02 V and the sweep rate of 6 mV/s. Full turn-on and -off in the EGTs were obtained at applied voltages below 1 V and the reasonably high on/off channel current ratio of ∼105. ID in the saturation regime was measured within the same VG range but under a larger VD of −0.5 V. In this condition, the maximum channel current reaching ∼1.3 mA was recorded at very low VG = VD = −0.5 V. In addition, the square root of ID in the saturation region shows a linear correlation with the applied VG, as expected (Figure S5, Supporting Information). Figure 4c shows the output ID versus VD curves at six different fixed VG values. The output curves show clear increases in ID and subsequent pinch-off phenomena with increased VG. Sub-1 V transistor operation with a very high on-current exceeding 1 mA is obtained because of the high capacitance of the ionogel electrolyte (Figure S6, Supporting Information).58 P(VDF-HFP) ionogels with various polymer concentrations (10 and 30 wt %) can be successfully transferred, and the resulting ionogels can also serve well as high-capacitance gate
dielectrics (Figure S7, Supporting Information). All of the transistors exhibit a reliable device performance with lowvoltage operation below 1 V and high on/off current ratio close to 105, which are similar to those in Figure 4b. Statistical studies on the EGTs in Figure 4d−f show that the transfer-stamped ionogel dielectric provides reliable device performance, with the average hole mobility of 1.43 ± 0.75 cm2/V s, on/off current ratio of (7.06 ± 4.25) × 104, and turnon voltage of 0.21 ± 0.09 V. The hole mobility was calculated in the linear regime using the following equations μ=
ep =
L ⎛ ID ⎞ ⎟ ⎜ W ⎝ epVD ⎠
(1)
∫ IG dVG rvA
(2)
where e is the unit charge, p is the hole density of semiconductor P3HT, rv is the gate voltage sweep rate, and A is the area of the PEDOT:PSS contact.59 Figure S8 (Supporting Information) also shows that the transfer-stamped ionogel successfully modulated the electron currents of n-type indium gallium zinc oxide (IGZO) transistors.51 Figure 5 displays the performance variations of EGTs fabricated on flexible PI substrates under 5000 repeated mechanical bending stresses to the different bending radii, r, of 4 and 2 mm. Maximum tensile strains (ε) of 0.31 and 0.63% were applied to the devices for bending radii of 4 and 2 mm, respectively. Tensile strain was calculated by ε = t/2r, where t is E
DOI: 10.1021/acsami.7b12712 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a) Emission spectra with λmax = 610 nm recorded from ECL devices utilizing transfer-stamped ionogels with and without the 5 wt % Ru(bpy)3Cl2 luminophore. The inset shows the chromaticity coordinate of the emitted light from the luminophore-containing device. (b) Luminance vs applied peak-to-peak voltage Vpp curve for ECL devices at an excitation AC bias frequency of 1 Hz. The inset shows an optical image of the ECL device in its on-state. (c) Statistical summary of 25 ECL devices measured at a Vpp of 5.6 V.
the substrate thickness of ∼25 μm.47 The operation of EGTs is well maintained even after the devices are repeatedly bent to r = 4 mm, such that only a small change (
