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Clean Graphene Electrodes on Organic ThinFilm Devices via Orthogonal Fluorinated Chemistry Jonathan Hutchinson Beck, Robert Barton, Marshall Cox, Konstantinos Alexandrou, Nicholas Petrone, Giorgia Olivieri, Shyuan Yang, James Hone, and Ioannis Kymissis Nano Lett., Just Accepted Manuscript • Publication Date (Web): 16 Mar 2015 Downloaded from http://pubs.acs.org on March 16, 2015

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Clean Graphene Electrodes on Organic Thin-Film Devices via Orthogonal Fluorinated Chemistry Jonathan H. Beck,∗,† Robert A. Barton,‡ Marshall P. Cox,† Konstantinos Alexandrou,† Nicholas Petrone,¶ Giorgia Olivieri,§ Shyuan Yang,† James Hone,¶ and Ioannis Kymissis† Department of Electrical Engineering, Columbia University, New York 10027, Department of Physics, Columbia University, New York 10027, Department of Mechanical Engineering, Columbia University, New York 10027, and Dipartimento di Fisica, Università di Trieste, Trieste, Italy E-mail: [email protected]

Abstract Graphene is a promising flexible, highly transparent, and elementally abundant electrode for organic electronics. Typical methods utilized to transfer large-area films of graphene synthesized by chemical vapor deposition on metal catalysts are not compatible with organic thin-films, limiting the integration of graphene into organic optoelectronic devices. This article describes a graphene transfer process onto chemically sensitive organic semiconductor thin-films. The process incorporates an elastomeric stamp with a fluorinated polymer release layer that can be removed, post-transfer, via a fluorinated solvent; neither fluorinated material adversely affects the organic semiconductor materials. We used Raman spectroscopy, ∗ To

whom correspondence should be addressed of Electrical Engineering, Columbia University, New York 10027 ‡ Department of Physics, Columbia University, New York 10027 ¶ Department of Mechanical Engineering, Columbia University, New York 10027 § Dipartimento di Fisica, Università di Trieste, Trieste, Italy † Department

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atomic force microscopy and scanning electron microscopy to show that CVD graphene can be successfully transferred without inducing defects in the graphene film. To demonstrate our transfer method’s compatibility with organic semiconductors, we fabricate three classes of organic thin-film devices: graphene field effect transistors without additional cleaning processes, transparent organic light-emitting diodes, and transparent small-molecule organic photovoltaic devices. These experiments demonstrate the potential of hybrid graphene/organic devices in which graphene is deposited directly onto underlying organic thin-film structures.

KEYWORDS: Graphene, Organic Photovoltaic, Organic Light Emitting Diode, Organic Electronics, Electrodes, Thin Films Graphene, first isolated by Geim and Novoselov in 2004, 1,2 has garnered widespread interest for its excellent electrical, 3–7 mechanical, 8–10 and optical 11 properties. Recent work shows that graphene can be grown in large areas 12 and used as a transparent and flexible electrode for organic optoelectronic devices. 10,13,14 Graphene is a useful electrode material in organic thin-film devices because it is highly transparent, compatible with commercially scalable roll-to-roll processing, and can be deposited in ambient conditions. 15–17 A typical method for producing large-area graphene films is chemical vapor deposition (CVD) growth on metal substrates, which typically requires transfer onto a dielectric substrate for device applications. Recent reports have demonstrated the growth of wafer-scale single-crystal monolayer graphene on reusable growth substrates. 18 Significant research has been invested into improving the graphene transfer process; however, the transfer of CVD graphene on top of underlying organic semiconductor films has been difficult due to incompatibility with chemicals used to process and clean graphene, especially organic solvents. Typical methods utilized to transfer CVD graphene from the growth substrate to the process substrate implement a sacrificial or intermediary polymer layer, such as poly-methylmethacrylate (PMMA), which provides mechanical support to the graphene during the process and is ultimately removed after transfer. 19–22 While multiple transfer polymers have been demonstrated, the methods require a combination of solvent, water, etching and annealing steps during processing and post-processing which are incompatible with organic semiconductors. In another previously re2 ACS Paragon Plus Environment

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ported method, graphene is adhered to an elastomeric PDMS stamp which provides mechanical support and pick-and-place control when stamping onto the process substrate. 9 The stamp transfer method fails on organic thin-films because graphene tends to adhere to the stamp rather than the organic films when the stamp is removed. 23 Recent methods to stamp graphene onto organic thinfilm devices rely on additional functional interlayers such as graphene oxide and PEDOT:PSS to increase the graphene adhesion. 24,25 Residual polymer and PDMS siloxane can be electrically insulating and negatively affect device performance, 26 increase the contact resistance to graphene, 27 and decrease flexibility of graphene films. Song et al. introduced a solution to the adhesion and cleanliness problem by coating PDMS with a set of novel polymer release layers and demonstrated transfer to organic insulators to make gated FETs and capacitors. 28 However, no study has yet demonstrated a transfer method for CVD graphene onto organic thin-films to enable successful fabrication of organic optoelectronic devices. In this report, we describe a process to transfer single-layer, CVD-grown graphene onto organic thin-films, demonstrating the first implementation of graphene as a transparent top electrode for organic optoelectronic devices. The transfer process incorporates an elastomeric stamp with a heavily fluorinated polymer release layer. The orthogonal polymer release layer and residue are removed, post-transfer, with only orthogonal solvents which are fully compatible with the underlying organic semiconductor films. The polymer release layer also reduces the PDMS-graphene adhesion. The transferred graphene film may be stacked to improve electrode conductivity and is sufficiently clean to be directly contacted as an electrode. We transfer graphene via an elastomeric polydimethylsiloxane (PDMS) stamp treated with OSCoR fluorinated photoresist ("fl-resist", Orthogonal, Inc., Rochester, NY, USA). A schematic of this process is depicted in Figure 1a. The OSCoR orthogonal photoresist and solvents are immiscible with most organic semiconductors. Miscible materials mix homogenously while immiscible, or orthogonal, materials remain heterogenous when mixed. Most oleophilic and hydrophilic organic materials are miscible in the presence of aromatic and halogenated solvents. 31,32 The heavily fluorinated photoresist release layer and solvents do not dissolve or infiltrate the structures of non-

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copper etching the stamp is rinsed in DI water, blown dry with N2 , and left overnight to dry in a covered dish. We maintain the PDMS/graphene stamp in a filtered laminar flow hood in order to minimize particulate contamination. Previous work has not shown graphene transfer directly from stamps or thermal release tape to organic materials because the low organic thin-film surface energy causes incomplete transfer of graphene with many rips and tears. 24,28 The fluorinated photoresist modifies the surface adhesion of the PDMS stamp so graphene preferentially adheres to the organic thin-film process substrate. We place the transfer stamp onto the organic thin-films which were previously deposited onto the substrate, (Figure 1a) and apply light pressure while heating the substrate to 50◦ C to promote adhesion between graphene and the substrate. After 30 seconds the stamp is lifted away, leaving behind a single layer of graphene. Residual fluorinated photoresist is removed by three subsequent spray/puddle spin cleans using the fluorinated photoresist stripper. The photoresist stripper conTM

tains an fluorinated solvent from the Novec

(3M) family of hydrofluoroethers (example shown

in Figure 1b). We complete the fl-resist-transfer process by spinning the substrate to remove any residual stripper. The transfer process may be repeated to transfer additional graphene films. We transferred graphene films up to 1 cm2 in area onto 300 nm SiO2 /Si substrates, as shown in Figure 2a, for Raman spectroscopy, AFM, and SEM analysis. We inspected the transferred graphene films using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM micrograph in Figure 2b and AFM image in Figure 2c show that the fl-resist transfer method successfully transfers CVD graphene with minimal structural defects, such as rips, tears or wrinkles, and minimal transfer-related contamination, such as particulate residue. The AFM measurements show a thin layer of polymer residue on the surface of the transferred graphene; however, the graphene RMS roughness of 0.526 nm over a 1.5 by 1.5 µ m area indicates that any residue is uniform, and comparable to other transfer polymer residues before high-temperature annealing. 36 We inspected the fl-resist-transferred graphene films using Raman spectroscopy and confirmed that the graphene is continuous with a low defect density. We transferred graphene onto 300 nm

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We demonstrated the compatibility and conductivity of transferred graphene by fabricating organic light emitting diode (OLED) devices with an fl-resist-transferred graphene cathode. We deposited the OLED thin-films onto ITO glass and used the fl-resist-transfer method to laminate a graphene cathode on top of the organic thin-films. We repeated the fl-resist-transfer technique to stack an additional graphene sheet in order to reduce the sheet resistance of the graphene electrode. 12 The graphene OLED structure is shown in the inset of Figure 3b. Graphene has previously been incorporated in OLEDs as bottom electrodes, 10,13 but has never been integrated as a top-electrode because there was no compatible process to transfer graphene onto underlying organic layers. The fl-resist transfer enables the use of graphene as a laminated electrode, and is an alternative to thermally deposited metals or sputtered indium tin oxide (ITO). Figure 3b shows a photograph of the graphene OLED. The transparency of the device is directly attributable to graphene, which transmits ∼97.7% of white light, in conjunction with a transparent ITO anode. 11 Figure 3c shows the radiance of the graphene OLED device compared to the control OLED device. The graphene OLED requires a higher turn-on voltage than the control OLED due to a low shunt resistance and cathode work function mismatch; the magnesium-silver cathode in the control device has a better match to the Lowest Unoccupied Molecular Orbital (LUMO) energy level of the TPBi electron transporting material. The performance of the graphene OLED device may potentially be improved by decreasing the graphene work function, increasing the shunt resistance by reducing parasitic conduction, and decreasing graphene sheet resistance by using additional graphene layers. 12,43,44 We also fabricated semi-transparent organic photovoltaic (OPV) devices with top-laminated graphene cathodes. The fl-resist-transfer process can be used to integrate graphene with highperformance, state-of-the-art small-molecule and polymer OPV materials. OPV top electrodes are typically opaque metals such as aluminum, but have recently been integrated with graphene. 25,45 We used the fl-resist-transfer process to successively laminate two layers of graphene onto a SubPc/C60 small-molecule, vacuum-deposited OPV device on an ITO/glass substrate. We stacked two graphene sheets in order to reduce the sheet resistance of the graphene electrode. 12 The OPV

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device structure is shown in the inset of Figure 3e. The control OPV devices consist of a planar heterojunction with a SubPc donor and C60 acceptor, with an overall power conversion efficiency of 1.51%. The control devices have an identical device structure with a thermally-deposited aluminum cathode in place of graphene. The graphene OPV device has a power conversion efficiency of 0.31% and similar open-circuit voltage to the control device, shown in Figure 3d and Table 1. The graphene OPV has a lower short-circuit current (Jsc ) than the control device because it lacks a metallic cathode, which increases optical absorption by acting as a backside reflector of visible light (micrograph shown in Figure 3e). The fill factor of the graphene OPV device is limited by a high series resistance, which suggests that increasing the conductivity of the graphene cathode by laminating additional graphene layers would increase performance. 12,45 The performance of the graphene device may potentially be improved by decreasing the graphene work function and decreasing the sheet resistance. 44,46 Table 1: Performance characteristics of OPV devices made with fl-resist-transferred graphene cathodes and aluminum cathodes. Cathode Type Voc [V] Jsc [mA cm−2 ] FF [%] Laminated graphene 0.89 1.57 23 Aluminum 1.09 3.00 48

η p [%] 0.31 1.51

For the first time, we fabricated OLED and OPV devices with clean graphene top electrodes by laminating the graphene directly onto the devices. We successfully transferred graphene to organic thin-films via a modified PDMS stamp with an orthogonal photoresist release layer. The SEM and AFM micrographs show that the graphene has a low surface roughness, high continuity and low contamination. Raman spectroscopy indicates that the transferred graphene has a low defect density. We demonstrated that the orthogonal photoresist transfer technique produces continuous, high-quality graphene by fabricating GFETs. As the quality of CVD graphene improves, this transfer process may advance large-scale, graphene transfer to organic substrates to enable commercialization of flexible, semitransparent OLEDs and OPVs with graphene. This novel transfer technique potentially enables new hybrid organic-graphene device structures, such as tandem OPVs and all-carbon organic optoelectronic devices; the transfer technique may also be 9 ACS Paragon Plus Environment

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incorporated in promising high-volume, roll-to-roll production of organic electronics.

Experimental Preparation of Chemical Vapor Deposition Graphene films: Large-area films of graphene were grown using CVD processes on copper foil. The complete synthesis is reported in the supporting information of a previous study. 40 Measurement and Characterization: We mixed Sylgard 184 PDMS (Dow Corning) as instructed, vacuum degassed the mixture, and cured in a 80◦ C oven for 1 hour. Parylene-C was deposited in a Specialty Coating Systems Labcoter. We recorded SEM images in a Hitachi S-4700 field emission scanning electron microscope operated at 2 kV. We obtained AFM images in a Dimension Icon which was operated in non-contact mode. We performed Raman spectroscopy with a Renishaw inVia Raman Microscope using a 532 nm laser. The resolution of spatial Raman images is 500 nm. Transistor fabrication: Single-layer graphene previously grown on 25 µ m copper foils (99.999%) was spin coated with fluorinated resist. Unwanted graphene from the back of the copper foil was removed by etching with an oxygen plasma. PDMS stamps were cut into small pieces (1 cm x 1 cm) in order to produce PDMS/parylene-c/fl-resist/graphene/copper stamps. The prepared stamp was etched in ammonium persulfate (Transene APS-100) for five hours to remove copper from the backside. After etching, the PDMS/parylene-c/fl-resist/graphene stamp was rinsed thoroughly with DI water and dried overnight before transfer to the substrate. The target substrate, 300 nm SiO2 on silicon, was cleaned in piranha solution, rinsed in DI water and dried. The PDMS/parylenec/fl-resist/graphene stack was placed face-down on top of the wafer, on an 80◦ C hot plate. After stamp removal, the silicon/graphene/fl-resist sample was rinsed with fluorinated stripper. We used thermal physical vapor deposition to deposit Cr/Au (5 nm / 45 nm) source/drain contacts using a shadow mask. The transistor geometry has a W/L=20 (2000 µ m/100 µ m). Electrical characterization of the fabricated devices was performed with a Keithley 4200 parameter analyzer. All

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electrical measurements were performed at atmosphere and room temperature using three needle probes. OLED fabrication: OLED devices were evaporated on pre-patterned ITO substrates (LumTec) with a sheet resistance of 9-15 Ω −1 . Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (HC Stark) was filtered with 0.45 µ m syringe filters and spin-coated on cleaned substrates at 3000 rpm to a thickness of approximately 80 nm. The PEDOT:PSS film was subsequently annealed at 120◦ C for 1 hour. Following PEDOT:PSS deposition the substrates were taken into the N2 glovebox. The OLED consists of 50 nm of N,N’-Bis(3-methylphenyl)-N,N’-diphenyl-9,9spirobifluorene-2,7-diamine (Spiro-TPD, LumTec), 40 nm of Tris(8-hydroxyquinolinato)aluminium (AlQ3 , LumTec), 30 nm of 2,2’,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, LumTec) and a cathode. Control devices have a 50 nm 10:1 Mg:Ag / 25 nm Ag cathode. All OLED devices were kept in nitrogen for the duration of the experiment. OLED devices were driven by a Keithley 2602 sourcemeter and current-voltage data was collected. We measured radiance with a calibrated Newport 818 photodetector. The OLED peak emission is approximately 530 nm, as measured with an Ocean Optics USB4000 Fiber Optic Spectrometer. OPV fabrication: The OPV devices were evaporated on pre-patterned ITO substrates (LumTec) with a sheet resistance of 9-15 Ω −1 . The OPV consists of a 10 nm MoO3 (molybdenum oxide) hole injection layer, 20 nm boron subphthalocyanine chloride (SubPc) electron donor, 40 nm C60 electron acceptor, 10 nm TPBi (2,2’,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) exciton blocking layer, and cathode. For graphene devices the cathode is two successively stamped single layers of graphene transferred using the fl-resist process. Control devices have a 100 nm aluminum cathode. All depositions were performed at 1.0 Å sec−1 at less than 2 · 10−6 Torr. The graphene device area was 2.58 mm2 while the control device area was 1 mm2 . The illumination of the control device was from the bottom (through the ITO anode) while the illumination of the graphene device was from the top (through the graphene cathode). J-V characterization is performed using a Keithley 2400 source meter and a Newport solar simulator with an AM1.5 global tilt filter in a N2 environment. 11 ACS Paragon Plus Environment

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Graphene transferred onto Si using traditionally PMMA methods was analyzed with Raman spectroscopy. Figure 4d shows the spatially resolved G-peak scan of PMMA-transferred graphene over a 16 by 14 µ m area. The PMMA transferred graphene was rinsed with dichloromethane and acetone before measurement. Based on the G-peak shift, the PMMA residue causes about 0.1 eV more doping than the fl-resist residue. We then analyzed graphene transferred onto TPBi with Raman spectroscopy. The background-corrected Raman spectrum is shown in Figure 4c. The background-corrected Raman spectrum was calculated by subtracting the Si/TPBi spectrum from the Si/TPBi/graphene spectrum. The presence of a strong signal from the G and 2D-peaks, at 1580 cm−1 and 2700 cm−1 , respectively, indicate that the graphene is successfully transferred by the flresist-method onto the organic semiconductor film. 37,38 The fl-transferred graphene on TPBi has the expected peak positions, relative heights, and full-width half-maximums (FWHM) relative to the measured Raman spectrum of graphene transferred onto SiO2 using the same process (Figure 2d). The close agreement of the Raman spectra demonstrates the effectiveness of the fl-transfer process onto inorganic and organic substrates.

Graphene OLED I-V curve

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Figure 5: The current-voltage characteristics of OLED devices with graphene and magnesiumsilver cathodes. Figure 5 shows the Current-Voltage characteristics of the graphene OLED device in comparison to a control device with a magnesium-silver cathode. The graphene OLED has a lower shunt resistance than the control device due to current passing through shorts between the graphene cathode 13 ACS Paragon Plus Environment

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and formation of a parasitic PEDOT:PSS anode. We estimate the sheet resistance of a two-layer graphene electrode to be 102 -103 Ω −1 , based on previous studies of graphene sheet resistance as a function of number of layers. 12,22,44 We estimate the sheet resistance of the magnesium-silver cathode to be 0.36 Ω −1 . The series resistance of the graphene OLED is higher because the sheet resistance of the graphene cathode is higher.

Acknowledgement We thank Philip Kim for access to graphene transistor measurement equipment. We thank John DeFranco at Orthogonal, Inc. for assistance with OSCoR materials. This material is based upon work supported as part of the Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001085. The authors declare no competing financial interest.

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Clean graphene electrodes on organic thin-film devices via orthogonal fluorinated chemistry.

Graphene is a promising flexible, highly transparent, and elementally abundant electrode for organic electronics. Typical methods utilized to transfer...
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