applied sciences

Article Graphene Oxide/Polystyrene Bilayer Gate Dielectrics for Low-Voltage Organic Field-Effect Transistors

Sooji Nam 1,* , Yong Jin Jeong 2, Joo Yeon Kim 1, Hansol Yang 2 and Jaeyoung Jang 2,*

1 Realistic Display Research Group, Electronics and Telecommunications Research Institute, Daejeon 34129, Korea; [email protected] 2 Department of Energy Engineering, Hanyang University, Seoul 04763, Korea; [email protected] (Y.J.J.); [email protected] (H.Y.) * Correspondence: [email protected] (S.N.); [email protected] (J.J.); Tel.: +82-2-2220-2334 (S.N.); +82-42-860-1479 (J.J.)


Received: 3 December 2018; Accepted: 17 December 2018; Published: 20 December 2018


Abstract: Here, we report on the use of a graphene oxide (GO)/polystyrene (PS) bilayer as a gate dielectric for low-voltage organic field-effect transistors (OFETs). The hydrophilic functional groups of GO cause surface trapping and high gate leakage, which can be overcome by introducing a layer of PS—a hydrophobic polymer—onto the top surface of GO. The GO/PS gate dielectric shows reduced surface roughness and gate leakage while maintaining a high capacitance of 37.8 nF cm−2. The resulting OFETs show high-performance operation with a high mobility of 1.05 cm2 V−1 s−1 within a low operating voltage of −5 V.

Keywords: graphene oxide; polystyrene; gate dielectric; low voltage; organic field-effect transistor

1. Introduction Organic field-effect transistors (OFETs) have attracted great attention because of their potential applications in flexible and bendable displays with high field-effect mobility, low-temperature processability, and flexibility [1,2]. Although high-performance OFETs have been consistently reported in the past decade, most of the studies have focused on devices employing rigid inorganic dielectrics, such as silicon dioxide (SiO2) and aluminum oxide [3–5]. Therefore, polymer dielectrics, such as poly(methyl methacrylate), poly(vinyl alcohol), polyimide, and poly(4-vinylphenol), have been extensively studied as alternatives to inorganic dielectric layers in OFETs because the former offer the advantages of solution processes and the need for flexible device applications [6–9]. However, single layers of such polymer dielectrics have some limitations with regard to working in low-voltage driving OFETs. Realization of low-voltage OFETs necessitates the use of high-capacitance (Ci) gate dielectrics. −1 In general, Ci can be expressed as Ci = k·ε0·d , where k is the dielectric constant, ε0 is the permittivity of the dielectric material, and d is the dielectric thickness. From this equation, it is clear that the use of a high-k gate dielectric film or a reduction of the thickness of the dielectric layer is necessary for increasing Ci. With only a few exceptions, polymer dielectrics generally have low k; thus, the thickness of the polymer layer must be reduced to a scale of at least a few nanometers to increase Ci. However, the insulating properties of polymer dielectrics degrade severely with a reduction in film thickness [10]. Therefore, it is highly challenging to develop novel gate dielectric materials that simultaneously are thin and exhibit good dielectric properties with high k for application to low-voltage OFETs. On account of its outstanding electrical, mechanical, and chemical properties, graphene has been intensively studied for application as a semiconductor, electrode, and encapsulation layer in OFETs [4,11–14]. In particular, graphene derived from graphene oxide (GO), which is referred to as reduced GO (rGO), has attracted much attention for application to flexible devices because of its

Appl. Sci. 2019, 9, 2; doi:10.3390/app9010002 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2 2 of 8 thinness, excellent electrical properties, and mechanical flexibility [15–17]. Most researchers have focused on achieving good electrical properties of rGO; however, it is notable that GO has potential to be used as a gate dielectric layer of OFETs since it possesses insulating properties and a very high k [18–20]. Nevertheless, neither the dielectric properties of GO-based materials nor their application as dielectric layers of OFETs has been studied satisfactorily. To enable the use of GO as a dielectric layer of OFETs, its surface properties need to be modified because the hydrophilic hydroxyl and epoxide functional groups on the GO surface can function as trap sites and reduce the device characteristics. Several studies have attempted to passivate the hydrophilic surface of gate dielectrics with hydrophobic polymers to realize high-performance and reliable OFETs [21,22]. Here, we report on the feasibility of using GO as the gate dielectric layer of low-voltage OFETs. We attempted to modify the surface of GO by overlaying it with the hydrophobic polymer polystyrene (PS). Through atomic force microscopy (AFM), X-ray diffraction (XRD), and two-dimensional grazing incidence X-ray diffraction (2D-GIXD) analysis, we confirmed that the GO/PS bilayer gate dielectric reduced the surface roughness and increased the crystallinity of the organic semiconductor. We also found that the GO/PS bilayer gate dielectric resulted in a reduced gate leakage current, which, in turn, drastically enhanced the electrical properties of OFETs within a low operating voltage of −5 V.

2. Materials and Methods

2.1. Film Characterization GO was synthesized using the modified Hummers’ method, as shown in Figure1; further details of this synthesis have been reported in a previous paper [16]. Final dried GO flakes were dispersed in a deionized water/methanol (2:1) mixed solvent (1.5 wt %) by using a sonicator. For film characterization, GO solutions were spin-coated onto Si substrates at 1200 rpm for 60 s and cured at 120 ◦C for 30 min. To form the GO/PS bilayer, 0.7 wt % PS solution in toluene was spin-coated onto the GO films at 4000 rpm for 45 s, and then cured at 90 ◦C for 20 min. The GO and PS solutions were spin-coated in air. The film thickness was measured by ellipsometry (J.A. Woollam). The surface morphologies and Ci were characterized using an atomic force microscope (MultiMode SPM, Digital Instruments Inc., Santa Barbara, CA, USA) and the HP 4284A precision LCR meter (Agilent, Santa Clara, CA, USA), respectively. To investigate the crystallinity of the organic semiconductor films, a 50 nm-thick pentacene film was deposited onto various dielectric layers (GO, GO/PS, and PS). The pentacene films were investigated via XRD and 2D-GIXD experiments, which were performed at the 10C1 and 4C2 beamlines (wavelength = 1.38 Å), respectively, at the Pohang Accelerator Laboratory in Korea. The measurements of Ci and leakage current were performed using a metal–insulator–metal structure.

2.2. Device Fabrication For the fabrication of OFETs, highly p-doped Si wafers were used as gate electrodes and substrates. After piranha cleaning and consecutive DI water washing of the substrates, the GO solutions were spin-coated onto them at 1200 rpm for 60 s and cured at 120 ◦C for 30 min. Then, 0.7 wt % PS solution in toluene was spin-coated onto the GO films at 4000 rpm for 45 s, and cured at 90 ◦C for 20 min. A 50 nm-thick pentacene film, serving as the active layer, was deposited onto the GO/PS dielectric at a rate of 0.2 Å s−1 by means of an organic molecular beam deposition system. The film deposition conditions were kept identical for the film analysis and the device. Finally, Au source/drain electrodes with a thickness of 100 nm each were deposited using a thermal evaporator (Figure1b). Electrical measurements were conducted using Keithley 2400 and 236 source/measure units. The electrical properties were evaluated in the dark under ambient conditions. The device parameters were calculated on the basis of the transfer curves in the saturation regime by using the 2 equation Id = (W/2L)·µ·Ci·(Vg − Vth) ; here, Id, µ, Vg, and Vth are the drain current, field-effect mobility, gate voltage, and threshold voltage, respectively; W and L are the channel width and length, respectively; and Ci is the capacitance per unit area of the GO/PS gate insulator. Appl. Sci. 2019, 9, 2 3 of 8 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 8

FigureFigure 1. ( 1.a)( Schematica) Schematic illustration illustration of procedure of procedure for synthesi for synthesizingzing graphene graphene oxide oxide (GO) (GO) by modified by modified Hummers’ method.Hummers’ (b) Schematic method. (deviceb) Schematic structure device of organic structure field-effect of organic transistor field-effect (OFET) transistor used (OFET)in this study. used in (c this) Height- c modestudy. atomic ( ) Height-mode force microscopy atomic (AFM) force microscopy topographs (AFM) of GO topographs pieces obtained of GO from pieces high obtainedly diluted from solution highly (the diluted solution (the inset shows the height profile of a GO slice). inset shows the height profile of a GO slice). 3. Results 2.2. Device Fabrication 3.1. Film Characterization For the fabrication of OFETs, highly p-doped Si wafers were used as gate electrodes and substrates.The surface After morphologies piranha cleaning of the and synthesized consecutiv GO,e GO/PSDI water bilayer, washing and pentaceneof the substrates, deposited the on GO the GO/PSsolutions bilayer were spin-coated were studied onto by them AFM. at Figure 1200 1rpmc shows for 60 the s and synthesized cured at GO120 pieces°C for 30 obtained min. Then, from 0.7 a highlywt % PS diluted solution solution; in toluene the pieceswas spin-coated are in the formonto ofthe flat GO sheet-type films at 4000 structures rpm for with 45 s, an and average cured sheetat 90 size°C for of 20 about min. 600 A nm50 nm-thick (with some pentacene size deviation film, serving from sheet as the to active sheet). layer, The insetwas ofdeposited Figure1 conto shows the theGO/PS height dielectric profile at of a arate 1.5–2.0 of 0.2 nm Å GOs–1 by slice, means which of an reveals organic that molecular the GO flakes beam aredeposition well separated system. byThe a singlefilm deposition layer. Figure conditions2a,b, respectively, were kept show identical the surfacefor the morphologiesfilm analysis ofand the the GO device. film (thickness Finally, Au of 70source/drain nm) and PS-coated electrodes GO with (GO/PS a thickness bilayer, of total 100 thicknessnm each ofwere 75 nm)deposited film used using in thisa thermal system. evaporator As can be seen(Figure in the1b). AFM Electrical images, measurements coating the PSwere layer conduc on theted GO using surface Keithley can significantly 2400 and 236 reduce source/measure the surface root-mean-squareunits. The electrical (R qproperties) of the GO were film, fromevaluated 4.91 nm in tothe 2.17 dark nm, under and improve ambient the conditions. uniformity. The Figure device2c showsparameters the morphology were calculated of the on pentacene the basis filmof the deposited transfer curves on the GO/PSin the satura film;tion the averageregime by grain using size the is 2 Appl.approximatelyequation Sci. 2018 Id , =8 ,( xW 250FOR/2L nm,PEER)·μ·C and iREVIEW·(VgR -q Vroughness th ) ; here, I isd, μ 7.8, V nm.g, and Vth are the drain current, field-effect mobility,4 of 8 gate voltage, and threshold voltage, respectively; W and L are the channel width and length, respectively; and Ci is the capacitance per unit area of the GO/PS gate insulator.

3. Results

3.1. Film Characterization The surface morphologies of the synthesized GO, GO/PS bilayer, and pentacene deposited on the GO/PS bilayer were studied by AFM. Figure 1c shows the synthesized GO pieces obtained from a highly diluted solution; the pieces are in the form of flat sheet-type structures with an average sheet size of about 600 nm (with some size deviation from sheet to sheet). The inset of Figure 1c shows the height profile of a 1.5–2.0 nm GO slice, which reveals that the GO flakes are well separated by a single layer.Figure Figures 2. Height-modeHeight-mode 2a,b, respectively, AFM topographs show the of surface ( a) GO film, film,morphologies ( b) polystyrene of the (PS)-coated GO film GOGO(thickness (GO/PS)(GO/PS) offilm, film, 70 nm) and PS-coatedand (c) pentacene GO (GO/PS filmfilm deposited bilayer, onontotal GO/PSGO/PS thickness bilayer bilayer of dielectric. dielectric. 75 nm) film used in this system. As can be seen in the AFM images, coating the PS layer on the GO surface can significantly reduce the surface root- To measure the gate dielectric properties of the GO and GO/PS bilayer films, metal–insulator– mean-square (Rq) of the GO film, from 4.91 nm to 2.17 nm, and improve the uniformity. Figure 2c metal capacitors (herein, Au–GO–heavily p-doped Si wafer or Au–GO/PS–heavily p-doped Si wafer) shows the morphology of the pentacene film deposited on the GO/PS film; the average grain size is were fabricated. Figure 3 shows the measured Ci and electric-field-dependent gate leakage current approximately 250 nm, and Rq roughness is 7.8 nm. density. The Ci value of GO decreases from 85 to 62.4 nF cm–2 (average: 73.3 nF cm–2) and that of GO/PS decreases from 40.3 to 34.8 nF cm–2 (average: 37.8 nF cm–2), as the frequency increases from 10 kHz to 1 MHz, which agrees well with the reported Ci value of a GO film with a similar thickness [13]. Although the Ci value of the GO/PS bilayer is smaller than that of the GO film because of the low dielectric constant of PS, the former Ci is high enough for GO/PS to serve as a dielectric layer for low-voltage operation of OFETs. As can be observed from the data in Figure 3a, the Ci values of the GO film and the GO/PS bilayer decrease by about 27% and 14%, respectively, with an increase in the frequency to up to 1 MHz; this indicates that the GO/PS bilayer has fewer mobile impurities on the dielectric surface than does the GO single layer. Figure 3b shows the electric-field-dependent gate leakage current. The GO single layer shows a high leakage current (10–4–10–7 A cm–2), whereas the gate leakage current of GO/PS bilayer is one order or more less than that of the GO film; this indicates the excellent electrical strength of the GO/PS bilayer. As mentioned in the introduction part, since the hydrophilic hydroxyl and/or epoxide functional groups on the GO surface can function as trap sites and reduce the device characteristics, it is necessary to modify the GO surface with the hydrophobic layer. The leakage current reduction can be attributed to the surface passivation of the hydrophilic GO with the hydrophobic PS, which could facilitate the use of GO as a dielectric of the OFETs.

(a) (b) 90 GO 10-4 GO ) 2

) GO/PS

GO/PS 2 80

70 10-6

60

50 10-8 Au 40 Dielectric p-Si Capacitance (nF/cm Capacitance 30 Density(A/cm Current 10-10 10k 100k 1M average 0.0 0.2 0.4 0.6 0.8 Frequency (Hz) Electric Field (MV/cm)

Figure 3. Analysis of dielectric properties of GO and GO/PS films: (a) frequency dependence of capacitance of GO and GO/PS films and (b) current density–electric field characteristics of GO and GO/PS films. The inset shows a schematic diagram of the metal–insulator–metal structure that induces the current density–electric field characteristics.

3.2. Molecular Ordering and Crystallinity Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 8

Appl. Sci.Figure2019 ,2.9, Height-mode 2 AFM topographs of (a) GO film, (b) polystyrene (PS)-coated GO (GO/PS) film, 4 of 8 and (c) pentacene film deposited on GO/PS bilayer dielectric.

ToTo measure measure the the gate gate dielectric dielectric properties properties of the GO of and the GO/PS GO bilayer and GO/PS films, metal–insulator– bilayer films, metal–insulator–metalmetal capacitors (herein, capacitors Au–GO–heavily (herein, Au–GO–heavilyp-doped Si wafer p-doped or Au–GO/PS–heavily Si wafer or Au–GO/PS–heavily p-doped Si wafer) p-doped Si wafer) were fabricated. Figure3 shows the measured C and electric-field-dependent gate were fabricated. Figure 3 shows the measured Ci and electric-field-dependenti gate leakage current leakage current density. The C value of GO decreases from 85 to 62.4 nF cm−2 (average: 73.3 nF cm−2) density. The Ci value of GO idecreases from 85 to 62.4 nF cm–2 (average: 73.3 nF cm–2) and that of −2 −2 andGO/PS that decreases of GO/PS from decreases 40.3 to 34.8 from nF 40.3 cm to–2 (average: 34.8 nF cm 37.8 nF(average: cm–2), as 37.8 the nFfrequency cm ), increases as the frequency from 10 increases from 10 kHz to 1 MHz, which agrees well with the reported C value of a GO film with kHz to 1 MHz, which agrees well with the reported Ci value of a GO filmi with a similar thickness a similar thickness [13]. Although the C value of the GO/PS bilayer is smaller than that of the [13]. Although the Ci value of the GO/PS ibilayer is smaller than that of the GO film because of the GO film because of the low dielectric constant of PS, the former C is high enough for GO/PS to low dielectric constant of PS, the former Ci is high enough for GO/PSi to serve as a dielectric layer for serve as a dielectric layer for low-voltage operation of OFETs. As can be observed from the data low-voltage operation of OFETs. As can be observed from the data in Figure 3a, the Ci values of the inGO Figure film and3a, thethe GO/PSCi values bilayer of the decrease GO film by and about the 27% GO/PS and bilayer14%, respectively, decrease by with about an 27%increase and in 14%, the respectively,frequency towith up to an 1 increaseMHz; this in indicates the frequency that the to up GO/PS to 1 MHz; bilayer this has indicates fewer mobile that the impurities GO/PS bilayer on the hasdielectric fewer mobilesurface impuritiesthan does onthe the GO dielectric single layer. surface Figure than 3b does shows the GOthe singleelectric-field-dependent layer. Figure3b shows gate theleakage electric-field-dependent current. The GO single gate layer leakage shows current. a high The leakage GO single current layer (10 shows–4–10–7 a A high cm leakage–2), whereas current the −4 −7 −2 (10gate leakage–10 A current cm ), of whereas GO/PS the bilayer gate leakageis one order current or more of GO/PS less than bilayer that is of one the order GO film; or more this lessindicates than thatthe excellent of the GO electrical film; this strength indicates of the the excellent GO/PS bilayer. electrical As strength mentioned of the in the GO/PS introduction bilayer. Aspart, mentioned since the inhydrophilic the introduction hydroxyl part, and/or since epoxide the hydrophilic functional hydroxyl groups and/or on the epoxideGO surface functional can function groups as on trap the sites GO surfaceand reduce can functionthe device as characteristics, trap sites and reduceit is necess theary device to modify characteristics, the GO surface it is necessary with the to hydrophobic modify the GOlayer. surface The leakage with the current hydrophobic reduction layer. can The be leakage attributed current to the reduction surface canpassivation be attributed of the to hydrophilic the surface passivationGO with the of hydrophobic the hydrophilic PS, GOwhich with could the hydrophobicfacilitate the use PS, whichof GO couldas a dielectric facilitate of the the use OFETs. of GO as a dielectric of the OFETs.

(a) (b) 90 GO 10-4 GO ) 2

) GO/PS

GO/PS 2 80

70 10-6

60

50 10-8 Au 40 Dielectric p-Si Capacitance (nF/cm Capacitance 30 Density(A/cm Current 10-10 10k 100k 1M average 0.0 0.2 0.4 0.6 0.8 Frequency (Hz) Electric Field (MV/cm)

FigureFigure 3.3. AnalysisAnalysis ofof dielectricdielectric propertiesproperties ofof GOGO andand GO/PSGO/PS films:films: ((aa)) frequencyfrequency dependencedependence ofof capacitancecapacitance ofof GOGO andand GO/PSGO/PS filmsfilms andand ((bb)) currentcurrent density–electricdensity–electric fieldfield characteristicscharacteristics ofof GOGO andand GO/PSGO/PS films.films. TheThe inset inset shows shows a schematic a schematic diagram diagra ofm the of metal–insulator–metal the metal–insulator–metal structure structure that induces that theinduces current the density–electric current density–electric field characteristics. field characteristics. 3.2. Molecular Ordering and Crystallinity 3.2. Molecular Ordering and Crystallinity The molecular ordering and crystallinity of the pentacene layers on various dielectrics (GO, GO/PS, and PS) were investigated by out-of-plane XRD and 2D-GIXD analysis. Figure4 shows the XRD patterns of the GO film and pentacene films on the various dielectrics. As can be seen from the black and blue graphs in Figure4, the diffraction peaks of the GO film correspond to (001) and (002) planes, and an interlayer spacing (d001) of 8.3 ± 0.1 Å, and the diffraction peaks of the pentacene layer on the PS substrate correspond to (001), (002), and (003) planes with d001 of 15.5 ± 0.1 Å; these results are in good agreement with literature values [23,24]. Diffraction peaks of only GO are observed in the XRD patterns of pentacene on the GO film; on the other hand, in the XRD patterns of pentacene on the GO/PS bilayer film, thin-film-phase diffraction peaks of pentacene are observed, as seen in Figure4. This indicates that the pentacene film is more ordered and has more aligned crystalline on the GO/PS bilayer film than on the GO film. Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 8

The molecular ordering and crystallinity of the pentacene layers on various dielectrics (GO, GO/PS, and PS) were investigated by out-of-plane XRD and 2D-GIXD analysis. Figure 4 shows the XRD patterns of the GO film and pentacene films on the various dielectrics. As can be seen from the black and blue graphs in Figure 4, the diffraction peaks of the GO film correspond to (001) and (002) planes, and an interlayer spacing (d001) of 8.3 ± 0.1 Å, and the diffraction peaks of the pentacene layer on the PS substrate correspond to (001), (002), and (003) planes with d001 of 15.5 ± 0.1 Å; these results are in good agreement with literature values [23,24]. Diffraction peaks of only GO are observed in the XRD patterns of pentacene on the GO film; on the other hand, in the XRD patterns of pentacene on the GO/PS bilayer film, thin-film-phase diffraction peaks of pentacene are observed, as seen in FigureAppl. Sci. 4.2019 This, 9 ,indicates 2 that the pentacene film is more ordered and has more aligned crystalline5 on of 8 the GO/PS bilayer film than on the GO film.

(001) GO 0.5 Pentacene on GO Pentacene on GO/PS 0.4 Pentacene on PS (ref.)

0.3 (002) (003) 0.2 Intensity (a. u.) Intensity

0.1

5 10152025 2θ (deg.)

FigureFigure 4. 4. X-rayX-ray diffraction diffraction (XRD) (XRD) patterns patterns of of GO GO film film and and pentacene pentacene films films deposited deposited on on various various dielectricsdielectrics (GO, (GO, GO/PS, GO/PS, and and PS). PS). Since the crystal structure of organic semiconductors affects the carrier transport properties, Since the crystal structure of organic semiconductors affects the carrier transport properties, analysis of the crystal structures of thin-film organic semiconductors is crucial for understanding the analysis of the crystal structures of thin-film organic semiconductors is crucial for understanding the electrical and physical properties of OFETs. Figure5 shows 2D-GIXD scan images of the GO film electrical and physical properties of OFETs. Figure 5 shows 2D-GIXD scan images of the GO film and and pentacene films deposited on the various dielectrics. In Figure5a, a broad (001) plane of GO is pentacene films deposited on the various dielectrics. In Figure 5a, a broad (001) plane of GO is observed in the qz direction (direction parallel to the substrate), which corresponds to the XRD result observed in the qz direction (direction parallel to the substrate), which corresponds to the XRD result of GO in Figure4. The 2D-GIXD patterns of pentacene in Figure5b,c reveal that the pentacene film on of GO in Figure 4. The 2D-GIXD patterns of pentacene in Figures 5b,c reveal that the pentacene film the GO dielectric contains both a “thin-film phase” and a “bulk phase” in the qz and qxy directions, on the GO dielectric contains both a “thin-film phase” and a “bulk phase” in the qz and qxy directions, whereas the pentacene film on the GO/PS bilayer shows a relatively highly oriented “thin-film phase” whereas the pentacene film on the GO/PS bilayer shows a relatively highly oriented “thin-film phase” in the qz and qxy directions. In the case of carrier transport in pentacene, the π–π overlap depends in the qz and qxy directions. In the case of carrier transport in pentacene, the π–π overlap depends largely on the molecular alignment along the in-plane direction, qxy, and not the out-of-plane direction, largely on the molecular alignment along the in-plane direction, qxy, and not the out-of-plane qz. It has been reported that the bulk phase corresponds to a tilted molecular orientation and the direction, qz. It has been reported that the bulk phase corresponds to a tilted molecular orientation poorest degree of π–π overlap [25]. Therefore, the 2D-GIXD results indicate that the pentacene grains and the poorest degree of π–π overlap [25]. Therefore, the 2D-GIXD results indicate that the are well-stacked with fewer mismatched and tilted crystalline domains on the GO/PS bilayer than on pentacene grains are well-stacked with fewer mismatched and tilted crystalline domains on the theAppl. GO Sci. film. 2018, 8, x FOR PEER REVIEW 6 of 8 GO/PS bilayer than on the GO film.

FigureFigure 5. Two-dimensional5. Two-dimensional grazing grazing incidence incidence X-ray X-ray diffraction diffraction (2D-GIXD) (2D-GIXD) scan scan images images of (ofa) ( GOa) GO film, film, (b)( pentaceneb) pentacene films films deposited deposited on on GO, GO, and and (c) GO/PS(c) GO/PS dielectric. dielectric. Subscripts Subscrip “T”ts “T” and and “B” “B” represent represent “thin-film“thin-film phase” phase” and and “bulk “bulk phase,” phase,” respectively. respectively. 3.3. Device Characterization 3.3. Device Characterization Figure6a shows the transfer characteristics in the saturation regime ( VD = −5 V) for a Figure 6a shows the transfer characteristics in the saturation regime (VD = −5 V) for a pentacene-based OFET with a GO/PS bilayer dielectric (W/L = 1000/100 µm). In the case of OFET pentacene-based OFET with a GO/PS bilayer dielectric (W/L = 1000/100 μm). In the case of OFET device, using GO as a dielectric layer, the transistor characteristics were not shown despite the high Ci device, using GO as a dielectric layer, the transistor characteristics were not shown despite the value of GO film, which is due to the high surface roughness and high leakage of GO film, and the high Ci value of GO film, which is due to the high surface roughness and high leakage of GO poor crystallinity of pentacene on the GO dielectric. By contrast, the OFET with the GO/PS bilayer film, and the poor crystallinity of pentacene on the GO dielectric. By contrast, the OFET with the GO/PS bilayer dielectric exhibits a high μ of 1.05 cm2 V–1 s–1, an on/off ratio exceeding 102, and Vth of −1.4 V with Ci of 37.8 nF cm–2. Figure 6b shows the output characteristics (ID vs. VD curves at VG = 0, −1.25, −2.5, −3.75, and −5 V) of the OFET with the GO/PS bilayer dielectric. Although the OFET shows some leakage current at VG = 0 V, the device can be operated within −5 V with clear saturation behavior. It is not easy to fabricate reproducible OFET devices (yield less than 50 %), however, this study successfully enabled the use of GO as a dielectric layer of low-voltage OFET with a high mobility.

Figure 6. (a) Transfer and (b) output characteristics of OFET with GO/PS bilayer dielectric under a single sweep mode (VG = 0, −1.25, −2.5, −3.75, and −5 V from black circles to cyan triangles). The inset in (a) shows a schematic of the device structure used in this study.

4. Conclusions In summary, we have successfully demonstrated a high-performance OFET with a GO/PS bilayer gate dielectric, which shows μ of 1.05 cm2 V–1 s–1 and operates within a low voltage of −5 V. The high performance of the device is attributed to the significant reduction in the surface roughness and gate leakage current through overlaying of the hydrophilic GO surface with hydrophobic PS, as Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 8

Figure 5. Two-dimensional grazing incidence X-ray diffraction (2D-GIXD) scan images of (a) GO film, (b) pentacene films deposited on GO, and (c) GO/PS dielectric. Subscripts “T” and “B” represent “thin-film phase” and “bulk phase,” respectively.

3.3. Device Characterization

Figure 6a shows the transfer characteristics in the saturation regime (VD = −5 V) for a pentacene-based OFET with a GO/PS bilayer dielectric (W/L = 1000/100 μm). In the case of OFET device, using GO as a dielectric layer, the transistor characteristics were not shown despite the high Ci value of GO film, which is due to the high surface roughness and high leakage of GO film, and the poor crystallinity of pentacene on the GO dielectric. By contrast, the OFET with the Appl. Sci. 2019, 9, 2 6 of 8 GO/PS bilayer dielectric exhibits a high μ of 1.05 cm2 V–1 s–1, an on/off ratio exceeding 102, and Vth of −1.4 V with Ci of 37.8 nF cm–2. Figure 6b shows the output characteristics (ID vs. VD curves 2 −1 −1 2 dielectricat VG = 0, exhibits −1.25, − a2.5, high −3.75,µ of and 1.05 − cm5 V)V of thes OFET, an on/off with ratiothe GO/PS exceeding bilayer 10 ,dielectric. and Vth of Although−1.4 V with −2 Cthei of OFET 37.8 nF shows cm .some Figure leakage6b shows current the output at VG = characteristics 0 V, the device ( I Dcanvs. beV Doperatedcurves at withinVG = 0,−5− V1.25, with− 2.5, −clear3.75, saturation and −5 V) behavior. of the OFET It is withnot easy the GO/PSto fabricate bilayer reproducible dielectric. OFET Although devices the (yield OFET less shows than some leakage50 %), however, current at thisVG study= 0 V, thesuccessfully device can enabled be operated the use within of GO− 5as V a withdielectric clear layer saturation of low-voltage behavior. It is notOFET easy with to fabricate a high mobility. reproducible OFET devices (yield less than 50 %), however, this study successfully enabled the use of GO as a dielectric layer of low-voltage OFET with a high mobility.

FigureFigure 6. 6.( a(a)) Transfer Transfer and and (b ()b output) output characteristics characteristics of of OFET OFET with with GO/PS GO/PS bilayer bilayer dielectric dielectric under under a a singlesingle sweep sweep mode mode ((VVGG = 0, 0, −−1.25,1.25, −−2.5,2.5, −3.75,−3.75, and and −5 −V5 from V from black black circles circles to cyan to cyan triangles). triangles). The Theinset insetin (a) in shows (a) shows a schematic a schematic of the of device the device structure structure used used in this in thisstudy. study. 4. Conclusions 4. Conclusions In summary, we have successfully demonstrated a high-performance OFET with a GO/PS bilayer In summary, we have successfully demonstrated a high-performance OFET with a GO/PS gate dielectric, which shows µ of 1.05 cm2 V−1 s−1 and operates within a low voltage of −5 V. The high bilayer gate dielectric, which shows μ of 1.05 cm2 V–1 s–1 and operates within a low voltage of −5 V. performance of the device is attributed to the significant reduction in the surface roughness and gate The high performance of the device is attributed to the significant reduction in the surface roughness leakage current through overlaying of the hydrophilic GO surface with hydrophobic PS, as well as to a and gate leakage current through overlaying of the hydrophilic GO surface with hydrophobic PS, as significant increase in the crystallinity of the pentacene film on the GO/PS bilayer surface. We believe that this work provides a facile route for fabricating low-power-consumption electronic devices by taking advantage of the thinness and excellent insulating properties of GO, which have not been satisfactorily studied in the past.

Author Contributions: S.N. and J.J. conceived and designed the experiments; S.N. and Y.J.J. performed the experiments; all authors analyzed the data, discussed the results and wrote the manuscript. Funding: This work was supported by the Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIT) (2018-0-00202, Development of Core Technologies for Transparent Flexible Display Integrated Biometric Recognition Device) and also by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1A02050420). Conflicts of Interest: The authors declare no conflict of interest.

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