Unraveled Face-Dependent Effects of Multilayered ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43105−43112

Unraveled Face-Dependent Effects of Multilayered Graphene Embedded in Transparent Organic Light-Emitting Diodes Jong Tae Lim,† Jaesu Kim,‡,§ Hyunkoo Lee,† Jaehyun Moon,† Byoung-Hwa Kwon,† Seongdeok Ahn,† Nam Sung Cho,† Byung-Wook Ahn,‡,§ Jeong-Ik Lee,*,† Kyuwook Ihm,*,∥ and Seong Chu Lim*,‡,§ †

Flexible Device Research Group, Electronics and Telecommunications Research Institute, Daejeon 34129, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea § IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Sungkyunkwan University, Suwon 16419, Republic of Korea ∥ Beamline Research Division, Pohang Accelerator Laboratory, Kyungbuk 37673, Republic of Korea ‡

S Supporting Information *

ABSTRACT: With increasing demand for transparent conducting electrodes, graphene has attracted considerable attention, owing to its high electrical conductivity, high transmittance, low reflectance, flexibility, and tunable work function. Two faces of singlelayer graphene are indistinguishable in its nature, and this idea has not been doubted even in multilayered graphene (MLG) because it is difficult to separately characterize the front (first-born) and the rear face (last-born) of MLG by using conventional analysis tools, such as Raman and ultraviolet spectroscopy, scanning probe microscopy, and sheet resistance. In this paper, we report the striking difference of the emission pattern and performance of transparent organic light-emitting diodes (OLEDs) depending on the adopted face of MLG and show the resolved chemical and physical states of both faces by using depth-selected absorption spectroscopy. Our results strongly support that the interface property between two different materials rules over the bulk property in the driving performance of OLEDs. KEYWORDS: graphene electrodes, organic light-emitting diodes, surface states, luminescence properties, near-edge X-ray absorption fine structure

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substrate phonons, and organic impurities.23−25 Among them, chemical doping of the layered materials has been used to control the optical properties of graphene as the electrical properties of these materials are tightly correlated with their optical properties.26 In multilayered graphene (MLG) flakes used in various devices, the distinction between the surface and bulk properties is not trivial as they are in contact with neighboring elements. Furthermore, conventional characterization tools for the flakes such as Raman spectroscopy and sheet resistance measurements measure the overall response of the material, in which the surface and intrinsic bulk properties are blended together. To clarify the morphology and properties of the surface of the layered materials, a technique with a limited probing depth is required. Surface-sensitive characterization is expected to help to reveal the role that the interface plays in various devices and to identify specific strategies to regulate and tune the properties of the surface/interface, leading to enhanced device performance.

INTRODUCTION Hexagonal single atomic carbon layers, known as graphene, have shown unique electronic properties that stem from the linear energy−momentum relation in their energy band.1−3 In addition to electrical properties, nearly uniform optical absorption of graphene within the visible range, increasing by approximately 2.3% per layer, adds another advantage for the application of graphene.4−6 It can be used as a transparent conducting electrode with superior flexibility and stretchability compared to tin-doped indium oxide (ITO) and zinc-doped indium oxide and increased chemical inertness compared to PEDOT:PSS.7−12 For these reasons, the introduction of a graphene electrode into optoelectronic devices has been performed for organic light-emitting diodes (OLEDs) and solar cells.13−17 The improvement of the electrical conductivity of graphene has been conducted in parallel using surfacetransfer doping, substitution doping, and auxiliary electrodes embedded in graphene.8,18−20 Graphene and transition-metal dichalcogenides (TMDs) possess a layered structure and a large surface-to-volume ratio. Although their surface is self-terminated, the intrinsic properties of these materials are considerably affected by the environment.21,22 Hence, many studies have been devoted to understanding and suppressing the effects of charged puddles, © 2017 American Chemical Society

Received: September 14, 2017 Accepted: November 21, 2017 Published: November 21, 2017 43105

DOI: 10.1021/acsami.7b13973 ACS Appl. Mater. Interfaces 2017, 9, 43105−43112

Research Article

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION MLGs were grown at different temperatures using chemical vapor deposition (CVD) for use as the top electrode in an OLED. To compare the effect of the bulk and surface properties of the MLG on device performance, we grew MLGs at different temperatures, resulting in the different sheet resistances. To study the interface, the face-selective transfer method was used, while investigating face-dependent effects of the applied MLGs on device performance. When the front side of the MLG (the first grown graphene layer) was contacted with the organic layer, the luminescence pattern was uniform regardless of the MLG growth temperature. However, a high driving voltage and low luminous current efficiency (LCE) were observed at low growth temperatures. The difference due to the MLG growth temperature on such operational parameters is not remarkable when the front side of MLG is engaged in the hole injection layer. In contrast to the front side, when the rear side of the MLG (the later grown graphene layer) contacted the organic layer, the emission pattern was extremely heterogeneous. The driving voltage and LCE were susceptible to the growth temperature, and the overall device features degraded considerably at low growth temperatures. The uneven luminance of the device originates from the surface functional group on the rear side of the MLG, which was confirmed by the sensitive near-edge X-ray absorption fine structure (NEXAFS) spectra. The top and bottom faces of the single-layer graphene (SLG) were physically indistinguishable in most cases, and the same surface states have always been assumed for MLG because it is difficult to obtain highresolution data of the front or rear side of the MLG by using Raman or ultraviolet spectroscopy (transmittance and reflectance) and overall sheet resistance. In the prepared MLG with a thickness of 2−4 nm, most characterization tools are not able to separately probe the bulk and surface properties. A 300 nm nickel (Ni) film was deposited on a 4 in. Si/SiO2 substrate using an e-beam evaporator at a base pressure of 1 × 10−6 Torr. Prior to the growth, to improve the adhesion of Ni on the SiO2 surface, the substrate was baked at 150 °C on a hot plate for 1 h. Then, it was cut into approximately 2 cm × 2 cm squares and placed inside a ceramic tube 2 in. in diameter, forming a CVD chamber. The growth tube was evacuated using a rotary pump and purged with argon (Ar) gas before the growth procedure. The temperature of the growth chamber was increased to 1000, 1050, and 1100 °C with 200 sccm Ar and 100 sccm hydrogen (H2) over 20 min. When the desired growth temperature was reached, 2 sccm of acetylene and 45 sccm of H2 gas were additionally supplied for 20 min.27 After finishing the deposition of MLG, the gas supply was terminated. The chamber was then cooled to 500 °C. The MLG layers synthesized at 1000, 1050, and 1100 °C were referred to as 1MLG, 2MLG, and 3MLG, respectively. An elastic graphene bonding structure (EGBS), which has the structure of PET/bonding layer (BL) (DMS-R22 or PFPE)/MLG, was fabricated as shown in the Supporting Information, Figure S1. Optical microscopy (Zeiss, Axio Imager 2) was used to examine the uniformity of the surface morphology and physical voids of the graphene films before and after transferring to PET/DMS-R22 (or PFPE). The observed MLG films showed no mechanical damage (Supporting Information, Figure S2). Raman spectroscopy (Renishaw RM-1000 Invia) with a laser excitation wavelength of 532 nm was used to investigate differences in the MLGs induced by the

growth temperature. The optical images of the front and rear side of the MLGs did not show any remarkable difference. In addition, the Raman spectra were taken from either side of MLG. The thickness of MLG varies locally owing to inconsistencies in the Ni grains. For this reason, Raman spectra were taken inside an individual grain, as shown in Figure 1a,b.

Figure 1. Raman spectra taken from the front and rear side of (a) thin grain and (b) thick grain in 3MLG. (c) Sheet resistance measured from the front and rear side of 1, 2, and 3MLG.

In a thin grain, the 2D peak, at 2700 cm−1, shows a much stronger intensity than that of the G-band peak, resembling the Raman spectrum of the monolayer graphene.27,28 However, in a thick grain, the peak intensity of the G-band is much higher than that of the 2D band, showing a similar result to graphite. Overall, the population of thick MLG grains was observed to be higher in the MLGs grown at 1100 °C compared to those grown at 1000 °C, but the crystal quality of each grain was not distinguishable between growth conditions. Atomic force microscopy (Hitachi Nano Navi) was used to investigate the surface roughness of the MLGs on PET/DMSR22 and PET/PFPE substrates (Supporting Information, Figure S3). The root-mean-square surface roughness on the rear side of the MLG on both PET/DMS-R22 and PET/PFPE was less than 1.2 nm. Thus, no corrugations were generated while the front-sided MLG was deposited onto the EGBS of the PET/PFPE/MLG. A uniform topography of MLG on the 43106


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ACS Applied Materials & Interfaces elastomer of the BL is a prerequisite for achieving uniform light emission after the formation of the OLED with an MLG top electrode. Differences in the transfer process of the rear-sided MLG do not alter the results when the rear-sided MLG is used for the EGBS.7 In addition to Raman spectroscopy, the sheet resistance of the MLG samples, grown at different temperatures, was measured by a multimeter (Keithley 2000) using four-terminal measurement, and the results are shown in Figure 1c. The MLGs synthesized at high temperatures have lower resistance than those grown at low temperatures. This is likely due to a larger number of thick grains in the MLG grown at a high temperature. In addition to the temperature dependence of the sheet resistance, we compared the sheet resistance measured from the front and rear side of the MLG. Surprisingly, a higher sheet resistance of the MLG was observed when resistance was measured from the front side. The same trend was observed consistently, regardless of the growth temperature. The difference in the sheet resistance between the front and rear side is approximately 10%. The mechanism behind the higher observed sheet resistance is, when electrical leads contact the front side is, not entirely clear. A lower sheet resistance on the rear side leads us to speculate that the surface graphene layers on the rear side may not be identical to the front side. For the side-dependent study, the incorporation of the MLG with either front- or rear-sided contact in the transparent OLED (TOLED) was performed (Supporting Information, Figure S1). The insertion of the MLG with different organic layer contact sides required a different BL for the transfer process. Therefore, the devices with front-sided contacts and rear-sided contacts have different elastomers; PFPE for the front-sided MLG and DMS-R22 for the rear-sided MLG were used. For this reason, there is a chance that both devices would differ in their transmittance and reflectance in the visible region. The devices integrated with PET/PFPE/F-3MLG (front-sided 3MLG) and PET/DMS-R22/R-3MLG (rear-sided 3MLG) were compared, and the results are shown in the Supporting Information, Figure S4a. The transmittance and reflectance at the wavelength of both devices are uniform within the visible range. At 550 nm, the transmittance and reflectance for both devices are as high as 72% and as low as 10%. In addition, electroluminescence spectra of both devices at the front direction are almost identical (Supporting Information, Figure S4b). We observed similar transmittance and reflectance from other devices fabricated with 2MLG and 1MLG. The current density−voltage−luminance (J−V−L) characteristics of the devices with 3MLG are shown in Figure 2a. The J−V−L characteristics of four pixels in a single device were examined over multiple devices. In the emission test shown in Figure 2a, the driving voltage (V1000) of the TOLED with 3MLG was defined as the voltage where the luminance reached approximately 1000 cd m−2. The V1000 was studied for top emission (TE) and bottom emission (BE) and at the front and rear side of MLG for each device. The variations of V1000 were compared among the prepared devices (Supporting Information, Figure S6a,b and Table S1). With increasing growth temperature, V1000 for both TE and BE decreases, which can account for the sheet resistance of the MLG observed in Figure 1c. High growth temperatures give rise to a low sheet resistance. The key feature in Figure 2a is the side dependency of V1000. For F-3MLG, V1000 for both TE and BE of the TOLEDs was approximately 5.7 V. However, the V1000 for R3MLG ranges from 7.8 V for TE to 8.7 V for BE. The higher

Figure 2. (a) Current density (Y1) and luminance (Y2) as a function of voltage curves and (b) LCE−luminance curves of the BE and TE from F-3MLG-TOLED and R-3MLG-TOLED, respectively. Black and red symbols denote the BE and TE characteristics of the F-3MLGTOLED, respectively. Green and blue symbols represent the BE and TE characteristics of the R-3MLG-TOLED, respectively. The TE patterns of (c) F-1MLG, F-2MLG, and F-3MLG and (d) R-1MLG, R2MLG, and R-3MLG.

V1000 from the rear-sided TOLED is repeated in R-2MLG and R-1MLG (Table S1, Supporting Information). Lower V1000 from the front-sided device compared to the rear-sided one is quite contradictory because a higher sheet resistance was obtained from the front-side MLG. From the viewpoint of sheet resistance, a higher V1000 would be expected from the front-sided TOLED rather than the rear-sided one. Thus, the results imply that in addition to the sheet resistance and bulk electrical properties, the interface states are an additional parameter whose role must be clarified for understanding the luminescent behavior of MLGs. LCE curves from 3MLG are shown in Figure 2b and those of the other devices are shown in the Supporting Information, Figure S6c,d. A quantitative comparison of LCE values was made at 5.7 V, at which the TOLED with 3MLG emits approximately 1000 cd m−2. In general, the emission properties of the TOLED follow the same trend as the J−V characteristics. That is, higher emission intensity was observed from the 3MLG than the 2MLG and 1MLG, implying that the crystalline quality 43107


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Figure 3. NEXAFS spectra taken from the (a) front side and (b) rear side of 3MLG at the incident angles of 30°, 55°, 70° to the MLG. The best fit of the π* resonance peaks depending on the X-ray incident angle for the front side (inset of a) and rear side (inset of b). (c) NEXAFS spectra taken at the magic angle (55°) for the front- and rear-sided 1, 2, and 3MLG. (d) UPS spectra of R-3MLG/HATCN (5 nm) [open red circles], F-3MLG/ HATCN (5 nm) [open blue circles], R-3MLG/HATCN (5 nm)/TAPC (10 nm) [red solid circles], and F-3MLG/HATCN (5 nm)/TAPC (10 nm) [blue solid circles].

interface effect rather than the overall film property as the source of the discrepancy, whose mechanisms will be discussed later. The results in Figure 2 indicate that the overall film properties, characterized by sheet resistance, Raman spectra, and surface roughness (see the Supporting Information for AFM, Figure S3), are limited to assess the differences of the front and rear sides of the MLG film in the emission of the prepared TOLEDs. Sheet resistance and Raman spectra cannot provide sufficient interface descriptions. In addition, only topographic information is available from AFM studies. The probing depth of conventional analytical tools used in MLG research was not shallow enough to discern the different characteristics of the two faces of MLG. Therefore, another intrinsic property of the MLG, like surface chemical states, may not be measured by these methods, which may also explain the poor emission pattern of rear-sided devices. NEXAFS spectroscopy is one of the most surface-sensitive analytical tools, which enables the probing of the energy structure near the Fermi level and spatial distribution of the molecular orbitals of 2-D materials. The signals in NEXAFS, that is, resonance peaks, are generated by the emission of loosely bound electrons in atoms induced by the relaxation energy of the transition events of interest.29 For electrons with a kinetic energy of 100−300 eV, the mean free path, λ, is approximately 2−3 atomic layers. In principle, e−d/λ electrons of all the electrons generated at a depth, d, are inelastically scattered. This means that only 37% of the signal generated from depth λ is detected by this NEXAFS setup, which enables

of MLG is the major factor governing the emission efficiency. With the front-sided device, the total luminance decreased to 83.4% for 2MLG and 80.8% for 1MLG, compared to 3MLG. In addition, the front-sided device emits more light than the rearsided one, as shown in the Supporting Information, Figure S6a,b and Table S1. A luminescent study could not be performed with the rear-sided devices, as they are completely nonemissive at 5.7 V. After characterizing the curves relating to LCE, the emission patterns of all the devices were examined. As shown in Figure 2c, the light emission of the front-sided devices was observed to be slightly better with 3MLG than with both 2MLG and 1MLG. Dark spots are randomly distributed in the devices with F-1- and F-2MLGs, but the degree of luminescence heterogeneity weakens in the device with F-3MLG. Among the front-sided devices, a slight difference in luminance can be seen. However, a significant unevenness can be observed in the luminescent pattern of the rear-sided devices. In Figure 2d, the emission patterns are segmented into many domains by dark lines. The similar transmittance and reflectance of the device are shown in the Supporting Information, Figure S4, which suggests that the luminescence properties of all three TOLEDs are mainly affected by the intrinsic electrical properties of graphene. The low V1000 in F-3MLG can be rationalized by the low sheet resistance of the 3MLG, as shown in Figure 1c. A low film resistance reduces the IR drop across the MLG, resulting in a lower turn-on threshold for the device. However, the large discrepancy in the luminance between the front- and rear-sided devices cannot be explained either by the sheet resistance or by the Raman spectra. Thus, the results suggest a dominant 43108


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substrate.29 The model system described by eq 1 assumes the π* orbital as a vector perpendicular to the surface plane. The real π* orbital is spread along the polar angle without a clear boundary, which gives rise to a broad minimum averaged angle even in ideal graphene. For F-3MLG, the prominent angle dependence of the π* resonance peak on the X-ray incident angle yields an averaged surface angle of 25°, implying that the effective surface angle is below 25° (inset of Figure 3a). For R3MLG, as expected from the weak angle dependence and high density of chemical functional groups, the averaged surface angle increased unfavorably to 44.5° (inset of Figure 3b). The causes of the increased surface angle on the rear side estimated by NEXAFS can be attributed to atomic imperfections of the MLG that are widely distributed on the rear side. Long-range heterogeneity of the luminescence (Figure 2d) indicates that the functional groups are ubiquitous, causing deterioration of the average surface angle and nonuniform spread of the applied potential at a micron scale, when used as an electrode. For the case of R-3MLG, the increased heterogeneity in the pixel image in Figure 2d is an indication of the broken uniformity of sp2 carbon bonding, provoked by local vacancy defects and chemisorbed contaminants at defect sites that have low reaction barriers.33 The atomic defects result in the significant deterioration of charged carrier transport properties owing to the nonlinear band structure near the Fermi level as well as increased carrier scattering by the defects. The chemical composition of each MLG surface was determined to further investigate these properties. From eq 1, when the incident angle of the X-ray is approximately 51°, the so-called magic angle in NEXAFS, the intensity dependence of the orbital on the X-ray incident angle vanishes. This allows a qualitative comparison of the chemical density on the surface. Figure 3c shows the NEXAFS spectra taken at the magic angle, 55°, from the front (blue) and rear side (red) MLGs grown under different conditions (1, 2, and 3MLG). In the 2 and 3MLG spectra, a sharp contrast between the front and rear side MLG is clear. For instance, on the rear side MLGs, the lower peak intensity of the π* transition and high density of unexpected chemical species are prominent, whereas the π* transition shows stronger peak intensity than the σ*(CH, COH) peak at 287.6 eV. Rear-side MLGs are composed of the last-born graphene layer, which likely includes a premature area with defects. These defects become energetically stabilized by interaction with contaminants and release lattice strain by long-range deformation. Further, in Figure 3c, it is clear that F-1MLG exhibits a strong σ*(CH, COH) peak at 287.6 eV, which is much weaker in F-2- and F3MLG. This is likely because F-1MLG is thin, enabling the signal from the bottom layer to be detected through the MLG. The thickness of the MLG was confirmed by transmittance [1MLG (85.4% at 550 nm); 6.7 layers = 23.5 Å, 3MLG (77.2% at 550 nm); 11 layers = 38.5 Å, (see the Supporting Information, Figure S7)]. Figure 3d shows the shift of the Fermi level of the hole transfer layers, which is consisted of either F-3MLG (or R3MLG)/hexaazatriphenylenehexacarbonitrile (HATCN) (5 nm) or F-3MLG (or R-3MLG)/HATCN (5 nm)/TAPC (10 nm). The hole transfer materials were applied on the front (blue circles) and rear sides (red circles) of the same MLG. The onset of the highest occupied molecular orbital (HOMO) of HATCN at the thickness of 5 nm on the front-side MLG is at 1.5 eV, which is 0.3 eV lower than that on the rear-side MLG. This difference is enlarged to 0.8 eV in 1,3,5-triazo-2,4,6-

the probing of only the chemical states near the surface that are likely to be dissimilar to those deeper within the MLG. The carbon K-edge NEXAFS spectra in Figure 3 obtained from the front and rear sides of 3MLG are shown by the partial electron yield (retarding background electrons below 100 eV) mode setup at incident angles of 30°, 55°, and 70° (from the vertical direction of the MLG surface plane). The resonance peaks at 285.2 and 292.0 eV were attributed to the electron transition from C 1s to the sp2 π* orbital and σ* of the C−C bond in the graphene layers, respectively.30 Because the π* orbital is oriented in the normal direction of the C−C basal plane, while the σ* orbital is parallel to the basal plane, depending on the X-ray incident angle, the intensity of these two peaks will alter in complementary. The resonance peaks between the π* and σ* orbitals of the C−C bond at 287.6 and 288.5 eV, respectively, are known to originate from the σ* bonds of hydrocarbon functional groups such as −CH, −OH, and −COx, which can be found at defective graphene sites.31,32 In ideal graphene, the hexagonal structure of carbon atoms has out-of-plane π orbitals with a three-fold symmetry, leading to the exotic electronic properties of the graphene.1−3 Therefore, with the high crystallinity of graphene comes a sharp angle dependence of the π* resonance peak. This is because of the anisotropic nature of the π orbital. In the NEXAFS spectra of F-3MLG in Figure 3a, as the incident angle decreases, that is, when the polarization angle of the incident Xray becomes parallel to the sp2 orbital, the intensity of the π* peak should grow, whereas the σ* peak should gradually decrease. This strong dependence of the π* resonance peak on the X-ray incident angle indicates that the crystalline graphene layer is well-preserved during the transfer onto the target substrate without postgrowth defect formation. The hydrocarbon peak detected for the front side of the 3MLG may be owing to the grain edges of the graphene, at which oxygen and hydrogen-related chemical groups adsorb at some terminated carbon atoms. Figure 3b shows the NEXAFS spectra of R-3MLG. Contrary to F-3MLG, the relative peak intensity of the π* orbital is much weaker than that of the σ* orbital even at an incident angle of 30°. This implies that the surface of R-3MLG includes a high density of defects such that the unique three-fold symmetry of the π* orbital is disturbed. Moreover, the weak angle dependence of the π* intensity indicates its spatial unevenness caused by the lattice strain due to high density of chemical groups. The defects on the rear side of the 3MLG tend to interact with contaminants for stabilization, resulting in a strong peak observed at 287.6 eV attributed to −OH, −CH, and −Ox functional groups. A much stronger peak intensity at 287.6 eV than the π* and σ* orbital peaks reflects a heavy functional group occupancy on the rear side of the MLG, which weakens angle dependence. The insets in both Figure 3a,b show the averaged angle between the surface of the graphene layer and the substrate, calculated by the equation with the Fermi golden rule as follows Iπ* = |⟨f |E ·T |i⟩|2 ⎤ 1⎡ 1 = P· ⎢1 + (3 cos2 θ − 1)(3 cos2 α − 1)⎥ ⎦ 3⎣ 2 1 + (1 + P) · sin 2 α 2

(1)

where P is the degree of polarization, θ is incident angle of the X-ray, and α is the averaged surface angle of the MLG to the 43109


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Figure 4. Schematic diagrams of the formation of grains and layers of MLG depending on the growth temperature (a) 1000, (b) 1050, and (c) 1100 °C. In each figure, the temperature of the carbon atoms at the moment of the graphene formation is represented by three different colors. The figure described carbon atoms that were dissolved into an Ni substrate and their diffusion to the Ni surface the moment the chamber is cooled from the growth temperature. As the substrate cools rapidly, the temperature of the precipitated carbon atoms drops as well. The color indicates the temporal and spatial distribution of the carbon atoms at different temperatures.

edges.36 At higher growth temperatures, the carbon atoms have higher migration energy, resulting in an MLG with higher crystallinity. The formation of graphene gradually slows until the substrate temperature drops to 550 °C. Below this temperature, some carbon atoms inside Ni may remain as Ni carbides, rather than migrate to form the graphene layer. Thus, the etching of the underlying Ni layer to facilitate the transfer may leave some carbon dangling bonds or imperfections that provide adsorption sites for the chemicals used in device fabrication. This is probably why a higher population of organic functional groups is found on the rear side of the MLG. If the emission pattern in Figure 2d results only from the MLG grains, the same emission patterns are expected in Figure 2c. Because we did not see such an emission pattern in Figure 2c, the chemical functional groups adsorbed at the atomic defects at the rear side of MLG, resulted from the separation of MLG from the Ni layer for the transfer process, provoke the dark lines in the emission pattern in Figure 2d. Furthermore, we can prepare two different multilayer graphenes; one is MLG grown from CVD and the other is MLG-stacked with SLG multiple times. In CVD-grown MLG, the chemical properties between the front and the rear side are distinguishable. The chemical properties of the rear side CVDgrown MLG is severely affected by the transfer process, whereas the pristine chemical properties of graphene is observed from the front side. In stacked MLGs, each layer is chemically functionalized by etchant and organics used for the transfer process. Stacking this graphene layer to form MLG homogenizes the chemical properties of both the front and the

triphosphorine-2,2,4,4,6,6-tetrachloride (TAPC) at the thickness of 100 Å on HATCN prepared on both sides of the MLG as marked by blue and red solid triangles in Figure 3d. In the hole transfer layers, the density of the hole as a major carrier is proportional to exp{−(EHOMO − EF)/KBT}, implying that the device performance could be changed over the energy difference of the HOMO level in an exponential manner.34 This explains the origin of the decrease in the luminance and LCE observed in the device with R-3MLG (Figure 2a). The growth mechanism of the MLG is responsible for the higher defect density on the rear side of the MLG. The growth of MLG layers is described in Figure 4, which presents the diffusion of carbon atoms inside Ni at different temperatures. During preheating of the 300 nm Ni substrate, the chamber temperature is raised to 1000−1100 °C and the Ni recrystallizes to form grains. The higher substrate temperature leads to the formation of a larger-sized grain with lower density.35 Because of the catalytic activity of Ni, the introduced hydrocarbon breaks on the surface of the Ni and carbon atoms dissolve into the Ni layer. When the growth chamber heating is turned off, the solubility of the carbon atoms drops, and the carbon atoms in the Ni diffuse to the surface and precipitate, forming a hexagonal graphene structure, which is shown in Figure 4. The graphene structures are colored differently depending on the temperature at the moment of graphene formation: the red symbols are the hottest, yellow indicates intermediate, and green indicates the coldest carbon atoms. Once the graphene layers form on the top of the Ni surface, the excess carbon atoms emerge to the grain boundaries and step 43110



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rear side. Therefore, with stacked-MLGs, face-dependent performance of OLED is not expected to occur. In case of stacked-MLGs, because the front and rear side are contaminated by chemical functional groups, the device performance with stacked-MLG is expected to be worse than the performance of the device with CVD-grown MLGs. In addition, with the chemical properties of stacked-MLGs, the conformal contact between each transferred SLG is not easy to achieve, possibly resulting in the creation of defects during the transfer process. For these reasons, the CVD-grown MLG is more advantageous over stacked-MLGs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13973. Fabrication of EGBS and TOLED with an EGBS, optical investigation of the MLG on PET/DMS-R22 and PET/ PFPE substrates, surface morphology of MLG during various fabrication steps, optical properties of TOLEDs, J−V−L properties of TOLEDs, thicknesses of 1MLG and 3MLG, summary of device characteristics (PDF)

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CONCLUSION In summary, MLGs grown at different temperatures gave rise to different sheet resistances when incorporated into inverted TOLEDs to study the effect of electrical bulk properties on the device. In addition, face-selective fabrication of graphene TOLEDs, with front- and rear-MLG contact sides was performed to compare with the bulk property study. The sheet resistance and electrical bulk properties among the different MLGs cause only minor differences in the lightemitting properties of the device. More stark differences in the device performance were observed when the contact side of the MLG was changed. This implies that the bulk characteristics were fully masked by the interface properties. Common characterization tools such as Raman spectroscopy and sheet resistance were not able to measure the intricate surface differences of the thin front and rear layers. Various layer materials were chosen for a target application according to their bulk properties and among them, interleaving dissimilar layers are accompanied for diverse heterostructures.37,38 To achieve the intended performance, the control of the surface and interfacial interaction properties of each layer is necessary. We believe that our approach can be widely applied to correlate the device characteristics to the interfacial properties, and hence giving a guideline in designing high-performance devices bearing two-dimensional materials.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-I.L.). *E-mail: [email protected] (K.I.). *E-mail: [email protected] (S.C.L.). ORCID

Jaehyun Moon: 0000-0002-8625-6562 Byoung-Hwa Kwon: 0000-0002-5435-6323 Seong Chu Lim: 0000-0002-0751-1458 Author Contributions

J.T.L., J.-I.L., S.C.L., and K.I. initiated the research. S.C.L. and J.K. synthesized the multilayered graphene and conducted various analyses. The transfer of MLG and the bonding process of TOLEDs were fabricated by J.T.L. The characteristics of TOLEDs were measured by J.T.L., H.L., J.M., and N.S.C. K.I. and J.T.L. analyzed soft X-ray spectroscopic data. The J.T.L., S.C.L., K.I., J.-I.L., and S.A. organized and analyzed all of the data and prepared the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIT) (B0101-16-0133: The core technology development of a light- and space-adaptable energysaving I/O platform for future advertising service; and no. 2017-0-00048: Development of Core Technologies for Tactile Input/Output Panels in Skintronics (Skin Electronics)). The authors from SKKU are grateful for financial support from IBSR011-D1. K.I. was supported by National Research Foundation of Korea (NRF) (grant no. NRF-2015M2A2A6A01045343) through the Ministry of Science, ICT, and Future Planning, Korea. Experiments at PLS-II were supported by MSIP-R. O., Korea.

MATERIALS AND METHODS

NEXAFS. NEXAFS analysis was carried out at the 4D PES beamline of the Pohang Accelerator Laboratory in Korea. The as-prepared samples were packed in a vacuum-sealed container and unpacked in an-N2 overflowing booth. The samples were attached to a Mo-heatable holder and loaded into a vacuum chamber. The analysis chamber (base pressure: 5 × 10−10 Torr) was equipped with an electron analyzer (R3000, Scienta) and an X-ray absorption spectroscopic detector with a retarding filter for low-energy background electrons. Device Fabrication. Inverted TOLEDs were fabricated by using the yellowish-green phosphorescents in the structure: glass/ITO/Lidoped TRE/TRE/(Ir(ppy)2(m-bppy))-doped PGH02/TcTa/TAPC/ HAT-CN/F-MLG or R-MLG/DMS-R22 or PFPE/PET (see the Supporting Information 5 for the detailed device structure). The depositions of all organics for the TOLED on patterned ITO-coated glass substrates were sequentially performed in situ inside a thermal evaporation chamber at a base pressure below 5 × 10−7 Torr. The emissive pixel area was approximately 1.5 mm × 1.5 mm. The TOLEDs with an F-MLG or R-MLG top electrode were fabricated to transfer an EGBS onto the glass/cathode/EIL/ETL/EML/HTL/HIL stack via gapless vacuum lamination. The fabricated TOLEDs were transferred to a glovebox, inside which the devices were encapsulated using a UV-curable epoxy and a glass cap containing a moisture absorbent.

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