Challenge beyond Graphene: Metal Oxide ... - ACS Publications

Subscriber access provided by UOW Library

Article

A Challenge Beyond Graphene: Metal oxide/Graphene/ Metal oxide Electrodes for Opto-electronic Devices Sungjun Kim, Ki Chang Kwon, Jae Yong Park, Hyung Won Cho, Illhwan Lee, Soo Young Kim, and Jong-Lam Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12443 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

A

Challenge

Beyond

Graphene:

Metal

oxide/Graphene/Metal oxide Electrodes for Optoelectronic Devices Sungjun Kim†, Ki Chang Kwon‡, Jae Yong Park†, Hyung Won Cho†, Illhwan Lee†, Soo Young Kim*‡ and Jong-Lam Lee*† Department of Materials Science and Engineering, Pohang University of Science and

†

Technology (POSTECH), Graduate Institute of Advanced Materials Science, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea ‡

School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Heukseok-

dong, Dongjak-gu, Seoul 156-756, Korea

KEYWORDS ITO replacement, transparent electrodes, graphene, stability, microcavity, OLEDs

ACS Paragon Plus Environment 1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

ABSTRACT Graphene have shown strong potential to occupy the transparent electrodes replacing indium tin oxide (ITO). However, the commercialization of graphene is still limited because of its poor chemical and electrical stability from reaction with environmental factors or essential materials such as poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Here, we have demonstrated a multilayered electrode, in which graphene is sandwiched between metal oxides (MOs) that have high stability and optical properties. The MOs over-coated graphene, and thereby protected it froms desorption of chemical dopants. Because of the resulting, chemical and electrical stability, the electrodes maintain low sheet resistance for 2.4 times longer than that does bare graphene and for 36 times longer than does PEDOT:PSS-coated graphene. Based on optical simulations, we derive the design rules for highly transparent MO/graphene/MO stacks and demonstrate optimized structure with TiO2 and WO3 electrode that has high transmittance (96%) which exceeds those of ITO (87%) and graphene (90%). Using an TiO2/graphene/WO3 electrode in organic light-emitting diodes (λ = 520 nm) instead of ITO or graphene anodes, increases cavity resonance and thereby increases power efficiencies by up to 30%. The MO/graphene/MO stacks designed will provide opportunities for commercialization of flexible electronics with graphene electrodes.


Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Transparent electrodes are crucial for opto-electronic devices such as organic lightemitting diodes (OLEDs), solar cells, and liquid-crystal displays1. Conventionally, indium tin oxide (ITO) has been used as the electrodes, but the increasing cost of indium and its brittleness make it unsuitable for future opto-electronic devices2. Therefore, alternative transparent electrodes should be developed. Graphene has theoretically excellent optical, electrical, and mechanical properties, and therefore as potential as a replacement for ITO3-13. Despite graphene’s ideal properties, pristine graphene film is not appropriate for use in electrodes because it has a relatively low work function (~4.4 eV) for p-type electrodes and high sheet resistance (Rs > 300 Ω/sq)3-5. Extensive research has been carried out to increase the work function and the electrical conductivity. The work function and conductivity can be improved by doping or chemical functionalization using various dopants from halogens to acidic liquid dopants such as HNO3, H2SO4, or SOCl2, and redox dopants such as FeCl3, AuCl3, or IrCl34-5. Another approach involves the use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on graphene as an additional layer14,15. Despite enhanced electrical properties of these electrodes, commercialization is still infeasible because the dopants are not stable against environmental factors. In addition, PEDOT:PSS is corrosive and acidic, so it can accelerate the degradation of graphene by reactive exchange of chemical dopant to PSS. Further, the contrast between the hydrophobicity of graphene and the hydrophilicity of PEDOT:PSS makes the composite unreliable16. Therefore, commercialization of a graphene electrode requires that it have high stability.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

Here, we demonstrate highly stable and transparent graphene-based electrodes that consist of multilayered structure of a graphene sandwiched between metal oxides (MOs). The MO over-coat prevents desorption of chemical dopants, so the graphene shows enhanced electrical and chemical stability even in harsh environment, and had 2.4 times and 36 times higher electrical stability than pristine graphene and PEDOT:PSS-coated graphene, respectively. For application as transparent electrodes, we establish the design rules for highly transparent MO/graphene (G)/MO stacks based on optical simulation, and demonstrate optimized transparent electrode using TiO2/graphene/WO3 (T/G/W) structure. The T/G/W electrode shows higher optical transmittance (96%) than ITO (87%), graphene (90%), and other previous metalbased electrodes. Furthermore, when T/G/W electrode is applied to organic light-emitting diodes (λ = 520 nm) to replace ITO or graphene anodes, the T/G/W electrodes can enhance the microcavity resonance, thereby increasing power efficiency by 30% relative to a device with ITO. This investigation provides new insights for commercialization of graphene electrodes and flexible electronics.

Results and Discussion The physical and electrical properties of CYTOP-assisted graphene sheet. Figure 1 shows the physical and electrical properties of fluoropolymer (CYTOP)-assisted graphene sheet. When a CYTOP layer was used as a polymer supporting layer instead of poly (methylmetharcrylate) (PMMA) to enhance the electrical properties of graphene sheet17-18, its Rs gradually decreased from 470 Ω/sq. to 80 Ω/sq. as the additional graphene layer was transferred onto graphene layer as shown in Figure 1a. The transmittance value at 550 nm of graphene sheet is about 97.3 %, which indicates that the graphene is close to a monolayer graphene sheet. The transmittance


Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


value at 550 nm decreased from 97.3 % to 82.5 % as the number of graphene layers increased from 1 to 5.

Figure 1. The physical and electrical properties of CYTOP-assisted graphene sheet. (a) Sheet resistance (bar) and transmittance value at 550 nm (dot and line) were displayed as a function of number of graphene sheet. (b) Raman spectroscopy of CYTOP-assisted and PMMA-assisted graphene sheet. The blue dashed line is G and 2D peaks of PMMA-assisted graphene. (c) SRPES C 1s spectrum of CYTOP-assisted graphene. The specific C-Fx of O-C-Fx peaks indicates that the CYTOP is well-attached with carbon atoms in graphene networks. (d) The SRPES secondary cut-off spectra of PMMA-assisted and CYTOP-assisted graphene. The red dashed line displays the typical value of PMMA-assisted graphene. The calculated work-function value is 4.7 eV. (e) The TEM image of CYTOP-assisted graphene. SAED pattern was well-defined and showed the crystalline structure of CYTOP-assisted graphene.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

In Figure 1b, the blue dashed line in Raman spectra indicates the positions of G and 2D peaks of PMMA-assisted graphene sheet. In Raman spectra, the G and 2D peaks of CYTOP-assisted graphene shifted to higher wavelength; this change indicates that the p-type doping of graphene had occurred19-20. The C 1s spectrum of CYTOP-assisted graphene in synchrotron radiation photoemission spectroscopy (SRPES, Figure 1c) shows four distinct features that correspond to the structural elements of CYTOP; C-F (289.8 eV), O-C-F (291.1 eV), C-F2 (291.8 eV), and OC-F2 (293.0 eV) groups17. This result indicates that the mild annealing had caused CYTOP to combine well with carbon atoms in graphene. In the secondary cut-off spectrum in SRPES was shown in Figure 1d. The typical onset of the secondary cut-off of graphene located at 4.3 eV as shown in red dashed line. In case of CYTOP-assisted graphene, the onset of the secondary cutoff was shifted to higher work function, 4.7 eV; this shift indicates that graphene had become ptype doped due to the CYTOP layer. The transmission electron microscopy (TEM) of CYTOPassisted graphene revealed a typical wrinkled structure with corrugation and scrolling, which are intrinsic properties of graphene (Figure 1e). The well-defined diffraction spots in the selected area electron diffraction (SAED) patterns confirm the crystalline structure of the graphene. The fast Fourier transform image shows the honeycomb lattice structure of CYTOP-assisted graphene (Figure 1e, inset). Stability of graphene sheet with hole-injection layer. We compared the electrical and chemical stability of graphene sheet with PEDOT:PSS or several MOs in Figure 2. Graphene was synthesized using chemical vapor deposition. To form a graphene anode, graphene film was transferred layer-by-layer to a glass substrate with a CYTOP supporting material18. To enhance the electrical properties of the graphene sheets, they were subjected to additional HNO3treatment, which decreased their Rs to one-third of its initial value5,15 (Figure S1). Exposure to air


Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increased Rs of graphene, but the rates depended on its coating (Figure 2a). The Rs of graphene is normalized with respect to that without air exposure.

Figure 2. Stability of graphene sheet with hole-injection layer. (a) Normalized sheet resistance of graphene, graphene/PEDOT:PSS (1:2.5), graphene/PEDOT:PSS (1:6), graphene/WO3, graphene/V2O5, and graphene/MoO3 as a function of exposure time in air. (b) 2-point resistance of graphene, graphene/PEDOT:PSS (1:2.5), and graphene/WO3 as a function of exposure time in high temperature (60 °C) and humid condition (90 %). (c) The Raman spectrum of bare graphene, graphene/PEDOT:PSS (1:2.5), and graphene/WO3 samples. The spectrum was normalized to G peak of bare graphene. (d) Scatter plots of position of G and 2D peak in Raman spectra with data points adapted from (c). The closed black square, open red circle, and closed blue triangle symbols indicated the bare graphene, graphene/PEDOT:PSS (1:2.5), and graphene/WO3, respectively. The distribution of position of G and 2D peaks are displayed in error bars. (10 measurements data)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

For pristine graphene, the normalized Rs increased to 2.1 times its initial value after exposure to air for 7.4 h (degradation rate r = 0.28 h-1), but then stopped increasing, probably because the graphene had degraded fully. PEDOT:PSS coatings caused increase in the rate at which of Rs increased. Total degradation occurred in 30 min (r = 5.4 h-1) when the PEDOT:PSS ratio was 1:6, and in 1h (r = 2.2 h-1) when ratio was 1:2.5. In addition, the degradation rate of graphene was accelerated by a factor of 2.5 when the concentration of PSS was increased by a factor of 2.4. These results suggest that PSS, which has an SO3 functional group, replaced the chemical dopant, i.e. NO3-, decreased the hole concentration and accelerated the increase in Rs. The replacement of functional group may be a result of partial charges on the S and N atoms in the functional groups. Because the difference of electronegativity between S (2.6) and O (3.5) is larger than between N (3.0) and O, the S atom in the SO3- have larger partial positive charge than does the N atom in NO3-. Therefore electrons in graphene may react more strongly with SO3- ions than with NO3- ions. Thus, SO3- ions replace NO3- in graphene/PEDOT:PSS and cause increase in Rs. On the other hands, when the MOs are coated on graphene, the degradation rates decrease to 0.09 h-1, and the fully degraded time become longer over 11 h. In addition, MOs thicker than 30 nm form a continuous film and further increase the electrical stability of graphene (Figure S2a). The enhanced stability in MO-coated graphene is also observed even at high temperature (60 °C) and high humidity (90 %) (Figure 2b). Graphene coated with a 60-nm WO3 layer had lower resistance (510 Ω) and slower degradation rate than pristine graphene (650 Ω). These results demonstrate that the MOs prevented desorption of chemical dopant, and thereby increased the electrical stability of the graphene, regardless of environmental conditions. To investigate the barrier effect of MOs on a bare graphene layer, Raman spectroscopy was used to compare the


Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


position of the G and 2D peaks of bare, PEDOT:PSS-coated, and MO-coated graphene (Figure 2c). The hole doping to graphene has been reported to show the shifts in both G and 2D peaks to higher wavenumbers. In the bare graphene, the G peak occurred at 1590 cm-1, which agree with previously reports14,32. The D peak was small, which means that the transfer step induced only a few defects. The graphene/PEDOT:PSS had a relatively high D peak due to defects caused by the PSS molecules, and the G and 2D peaks were shifted to lower wavenumbers than those of bare graphene (Figure 2d), these shifts indicate that the PEDOT:PSS layer degraded the graphene layer, and this result concurs with the degradation over time of the resistance of the graphene sheet by the PEDOT:PSS layer. The replacement of NO3- with SO3- from the PSS molecules decreases the hole concentration, which results in shift of the G peak position to a lower wavenumber. In the MO-coated graphene, the D peak intensity did not change, and the peaks did not shift, this result means that the MO layer formed a good layer that protected the graphene from exposure to air and thus prevented chemical reactions with it21,22. Optical simulation and properties of multilayered graphene structures.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

Figure 3. Calculated and experimentally measured optical properties of multilayered graphene structures. Calculated contour plots of transmittance (λ = 520 nm) as a function of (a) WO3 and graphene thickness, (b) refractive indices of inner dielectric and outer dielectric and (c) WO3 and TiO2 thickness. Transmittance spectrum of (d) ITO and T/G 3L/W with different WO3 thickness and (e) ITO, graphene, and T/G/W with different number of graphene layers. Photographs of (f) ITO, (g) graphene (h) T/G/W of 3 to 5 graphene layers. Although the over-coated MO layer improved the stability of graphene, the unmatched refractive indices of MO and graphene degraded its optical properties. Figure 3a shows the calculated contour plots of transmittance (λ = 520 nm) for glass/G/W as a function of graphene and WO3 thicknesses, based on transfer matrix method23. The transmittance decreased from 83% to 74% when 50-nm-thick WO3 was coated on 5-layered graphene. Based on Maxwell’s Equations, the larger complex refractive index n of graphene (2.72 + 1.37i) than that of glass (n = 1.45) makes the admittance Y of glass/graphene increase as graphene thickness increases, thereby decreasing the transmittance and increasing the reflectance24. For instance, Y of glass increased from 1.45 to 1.68+0.06i when the glass was coated with five layers of graphene (Figure S3, open circle). To meet the requirement for zero reflection condition, the over-coated materials should have n ≤ 1.3 which is much smaller than n of most of MOs (Figure S3a). In contrast, the additional MO layer


Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between graphene and glass satisfies the zero reflection condition with most MOs, and enhances the optical properties (Figure S3b). To satisfy the zero reflection condition, n of the additional MO should match the n of protective MO. Figure 3b shows the simulated transmittance of the glass/MO/G/MO structure as a function of n of both dielectrics. The number of graphene layers fixed at 3 and the thicknesses of the MOs varied to maximize the transmittance at each point. The result indicates that the transmittance of this multilayered structure can be maximized when the over-coated MO and the under-coated MO have the same n, which is the condition that makes the total Y of the structure equal to Y of air (n = 1). WO3 (n = 1.9) and TiO2 (n = 2.5) were adopted as over-coated MO and under-coated MO, respectively. The calculated contour plot of transmittance (λ = 520 nm) of the glass/T/G/W (Figure 3b) shows that the transmittance of T/G/W can be optimized at ~95 % with 62-nm-thick TiO2 and 60-nm-thick WO3, in this configuration, Y = 1.09 + 0.02i (Figure S4e, solid circle), which is much smaller than that of glass/ITO (1.69 + 0.24i) or glass/graphene (1.68 + 0.06i). Figure 3d shows the transmittance of the T/G/W structure as a function of WO3 thickness. The thickness of TiO2 and the number of graphene layers were fixed at 62 nm and 3 layers respectively, following the highest optical transmittance in previous simulation results in Figure 3c. The TiO2/G had lower transmittance (80 % at 520 nm) than that of the ITO (87 % at 520 nm) structure. However, the transmittance at 520 nm of the multilayered structure increased to 87 %, 95 %, 95 %, and 86 % as the WO3 layer on graphene had thickness of 40, 50, 60, and 70 nm, respectively. Furthermore, the wavelength at which transmittance was highest gradually shifted from 480 nm to 610 nm to satisfy the zero reflection condition25. The transmittance spectra of glass/ITO, glass/G, and glass/T/G/W structures, as a function of number of graphene layers (L) are shown in Figure 3e. When the thicknesses of TiO2 and WO3 were fixed at 62 nm and 60 nm respectively, which are the optimal



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

thicknesses according to the previous results. The transmittance value at 520 nm of three layers of graphene on glass was ~ 90 %, which is same as the highest transmittance of glass/ITO. The transmittance decreased from 90 % to 84 % as the number L of graphene layers increased from 3 to 5. Interestingly, the transmittance value at 520 nm increased to 96 % in the T/G 3L/W configuration. These results indicate that T (62 nm)/G 3L/W (60 nm) is the optimal condition in this experiment. Comparing T/G 3L/W with previous MO/metal/MO or T/metal/MO, it shows comparable optical transmittance, but broader wide angle window (Figure S5). Figure S5 shows the simulated transmittance spectrum of metal (Ag 10 nm, conventionally used26,27), graphene, WO3(30 nm)/Ag(10 nm)/WO3(30 nm)26, T/Ag(10 nm)/W and T/G/W with varied incident angle. It is shown that the transmittance of Ag and Ag based multilayers are dropped by 2-folds than graphene and T/G/W. Figure 3f–h show the photographs of glass/ITO, glass/G, and glass/T/G/W as a function of number of graphene layers. The glass/T/G/W structure is greenish which means that the multilayers preferentially transmit light from the green region (500 - 550 nm). This result concurs with transmittance measurements. Electrical and luminous characteristics of green OLEDs on T/G/W electrode.


Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Performance of green OLEDs on T/G/W electrode. (a) Current density–voltage (b) luminance–voltage (c) power efficiency–voltage characteristics and (d) EL spectrum of OLEDs on ITO, G 3L and T/G/W.

We fabricated OLEDs using ITO, G 3L, and T/G/W with 3L, 4L, or 5L graphene as anodes. High leakage current often occurs due to the surface roughness of graphene. This could be due to the residual CYTOP during the graphene transfer process. To accurately evaluate the optical and electrical properties of T/G/W electrodes, we used a thick hole transport layer (HTL, ~ 450 nm) to reduce the leakage current of graphene-based OLEDs. No leakage current was found in all graphene-based OLEDs. Even though the leakage current could be minimized by adjusting the thick-HTL layer, there is still room to improve its performance. Figure 4a and b show the current



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

density–voltage characteristics of the devices. For reliable values, each experiment was repeated at least 5 times. The devices showed identical current density–voltage characteristics. The luminance value of OLEDs based on ITO was 48750 cd/m2 at a current density of 100 mA/cm2. The luminance value increased to 51900 cd/m2 in OLED using G 3L anode. The OLEDs with T/G 3L/W showed the highest luminance value of 60300 cd/m2 at a current density of 100 mA/cm2. The luminance value of OLEDs based on graphene was gradually decreased with increasing the layers of graphene. The decrease of luminance in T/G 5L/W sample can be explained by the effect of light absorption in the graphene layer (Figure S6b). Graphene has a high refractive index (n = 2.72) and extinction coefficient (k = 1.37). Therefore, the absorption is rapidly decreased by increasing their thickness23. Figure 4c shows the power efficiency–current density and luminance–current density characteristics of the devices, respectively. The power efficiency value of OLEDs with ITO is 20 lm/W (at 5 mA/cm2). In the case of T/G 3L/W device, the value increased to 23 lm/W. The OLEDs with T/G 3L (4L)/W showed an enhanced power efficiency value of 26 lm/W because of enhanced out-coupling efficiency of devices. The enhancement is originated from the micro-cavity effect. Figure 4d shows the normalized electroluminescence (EL) spectra at 50 mA/cm2 differed among the devices. Devices with ITO showed a main emission peak at ~ 525 nm with a full width at half maximum (FWHM) of 74 nm. With G 3L, the intensity of the EL spectrum increased, and the FWHM slightly decreased to 69 nm. The T/G 3L/W device had a very intense EL spectrum with a narrow FWHM of 48 nm, which suggests a micro-cavity effect26. Although the transmittance of graphene itself can be increased by inserting metal oxides to satisfy the zero-reflection condition, MO/G/MO does not satisfy this condition when it is used as the anode of an OLED (Figure S8a). In this case, the transmittance of light incident from the organic medium to glass through this MO/G/MO is more


Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


relevant than transmittance from air to glass. As the air is replaced by an organic medium, the reflection at the interface of organic layer and MO/G/MO assembly can be increased (Figure S8b). However, this increased reflection causes resonation of light in the cavity that occurs between the anode and the cathode, and causes increased light extraction by OLED devices28. These results concur with the other evidence in Figure S8c, d and calculations performed using the 3D finite-difference time-domain (FDTD) method.

Figure 5. FDTD simulation. Calculated Poynting vector magnitude of OLEDs (λ = 520 nm) on (a) ITO, (b) 3 layers graphene (G) and (c) TiO2/3 layers graphene/WO3 electrodes by FDTD method. (d) Integrated magnitude of Poynting vector in active layer and glass of each devices normalized to those of ITO device. (e) Simulated angular emission pattern of OLEDs with each electrode. (f) The far-field intensity difference between MO/G/MO and ITO (black) or G (red) as a function of detecting angle. FDTD simulation. To understand how graphene and MO/G/MO improve the performance of OLEDs, we used 3D FDTD simulation to calculate wave propagation in devices29. Because the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

dipoles in OLEDs are a statistical mixture of horizontal and vertical dipoles, both modes for each electrode were simulated to characterize the effects of transverse electrical and transverse magnetic waves (Figure S9). The simulation results showed enhanced out-coupling of photons in devices that have the MO/G/MO structure. Total internal reflection occurs, and light is trapped in the ITO layer, because the ITO has a large n than do glass and the organic layer (Figure 5a)30,31. In contrast, graphene-based OLEDs trap virtually no light because they contain a very thin graphene layer and out-coupling intensity is higher than in ITO (Figure 5b). Besides, the outcoupling intensity is further increased by insertion of metal oxide layers (Figure 5c)18. For indepth understanding of the optical manipulation of light in the devices, the integrated magnitudes of Poynting vectors in the active layer and glass of each device were compared to those of ITO devices (Figure 5d). Relative to integrated magnitude of Poynting vectors in the active layer of ITO devices, graphene device shows higher intensity (1.13) than that of ITO (1), the increase is induced by elimination of wave-guided loss in electrodes (Figure S10). In the MO/G/MO structure, the integrated intensity in the active layer was 27 % higher than in devices with ITO and with graphene indicates light resonation between the anode and the cathode. As a result of this micro-cavity effect, the integrated intensity in glass is increased from 1.13 to 1.23. Angular far-field intensities of devices were also simulated (Figure 5e). The graphene device shows Lambertian emission like ITO, but increased intensity by elimination of light trapping that occurs in ITO. The MO/G/MO device shows significant increase in emission in the normal direction, this change indicates micro-cavity resonance occurs in the device. Although the MO/G/MO device has lower light extraction than does graphene angles ≥ 28o to the normal, the MO/G/MO device has higher total light extraction efficiency than do the ITO and graphene devices. These


Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


simulation results suggest that the MO/G/MO electrodes can increase the out-coupling efficiencies of devices.

Conclusion We demonstrated a multilayered graphene electrode with MO as an alternative to ITO. The new electrode has high stability and transmittance. The G/MO can maintain low Rs 2.4 times longer than pristine graphene and 36 times longer than PEDOT:PSS coated graphene. In PEDOT:PSS coated graphene, the G and 2D peaks in Raman spectra shifted to lower wavenumbers, this change indicates that the graphene layer was degraded by the PEDOT:PSS layer. However, in the G/MO layers, the D peak intensity did not change, and the peaks did not shift, this result means that the MO layer forms a good protective layer to protect desorption of chemical dopants and to increase the stability of graphene in air. To maximize the optical properties of G/MO, MO/G/MO stacks were introduced to satisfy the zero-reflection condition. 3D FDTD simulation results showed enhanced micro-cavity effects in the devices with the MO/G/MO structure, and suggest that the MO/G/MO electrodes can increase the out-coupling efficiencies of devices. TiO2 and WO3 were used for demonstration. At λ = 520 nm, the T/G/W device showed higher transmittance (96 %) than did ITO (87 %) and graphene (90 %). The optimal T/G/W electrode structures were predicted by optical simulation and experimental measurement. By replacing ITO with T/G/W electrodes in OLEDs (λ = 520 nm), high optical transmittance, and the micro-cavity effect were achieved, which resulted in an increase in device power efficiencies from 20 to 26 lm/W at 5 mA/cm2. This T/G/W electrode may allow development of highly efficient ITO-free OLEDs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

Methods Electrode fabrication with a structure of MO/G/MO. The graphene samples were grown on 25-µm-thick copper foil in a quartz tube furnace, using a chemical vapor deposition method involving methane (CH4) and hydrogen (H2) gas. The CYTOP-coated foil was heated on a hot plate at 160 °C for at least 20 min to facilitate a bond between CYTOP and carbon atoms. After curing of the CYTOP, the residue of CYTOP on the other side of the graphene-grown copper foil was cleaned by using a cotton bud with acetone. The sample was immersed in a ferric chloride bath at room temperature for 12 - 18 h to etch away the copper foil. Then, the remaining CYTOP-assisted graphene was carefully dipped into the DI water bath for about 7 - 9 times to remove any residual etchant. The CYTOP-assisted graphene was then transferred to 62-nm-thick TiO2 deposited glass. After the graphene transfer process, CYTOP was removed by a PF-5056 (performance fluid, supplied by 3M) bath at room temperature for 5 min. Then, 60-nm-thick WO3 was deposited on graphene, as an outer dielectric, by thermal deposition system. The sheet resistance was measured in a standard state using a four-point probe technique (Keithley 2612A multimeter, U.S.A.). Device fabrication. The ITO (Commercial ITO Glass, transmittance in visible region ~ 90 %, sheet resistance ~ 10 ohm/sq.), G, T/G/W samples were loaded into the thermal deposition chamber. The organic layers and the Al cathode for the OLEDs were deposited under high vacuum (10-6 torr) by thermal evaporation onto all samples at the same time to ensure consistent results. The structure of the OLEDs consisted of WO3 doped (4,4’-N,N’-dicarbazole)biphenyl (CBP) (450 nm, 5 wt%), tris(1-phenyl-isoquinolinato-C2,N)iridium(III) doped CBP (15 wt%, 40


Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nm), 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP, 10 nm), LiF (1 nm), and Al (150 nm). The active area of the device was 0.5 × 0.5 mm2. Measurement. The current density–voltage and luminescence–current density characteristics of the devices were measured with a HP-4156A semiconductor parameter analyzer and a YOKOGAWA 3298F in a nitrogen ambient. The refractive index and the transmittance value of the refractive index modulation layer were measured by using an ellipsometer (J. A. Woollam Co., Inc M-44), a tungsten-halogen lamp, and a monochromator. Simulation. In order to study the effect of transparent electrode for OLEDs, 3-dimensional (3D) finite-difference time-domain (FDTD) simulations were conducted by commercial software (Fullwave and LED Utility, Synopsys Inc.). The simulation structure consists of an aluminum cathode 150nm, LiF (refractive index (n) = 1.39, extinction coefficient (k) = 0) 1nm, BCP (n = 1.70, k = 0.01) 10nm, WO3 doped CBP (n = 1.75, k = 0) 480nm, Ir(ppy)3 doped CBP (n = 1.74, k = 0) 40 nm, transparent electrode and glass (n = 1.52, k = 0) 1um. The transparent electrodes consist of ITO 150nm for ITO (n = 1.95, k = 0.04) device, graphene 1nm for graphene device, and WO3 60nm (n = 1.91, k = 0), graphene (n = 2.72, k = 1.37) 1nm, TiO2 (n = 2.34, k = 0.0003) 62nm for metal oxide/graphene/metal oxide device. The lateral size of the structure is 20 um, which is enough to saturate extraction efficiency as a function of lateral width. The lateral boundary conditions for the FDTD set to perfect matched layer (PML) to avoid the unwanted reflected electromagnetic wave at the edge of the structure. The vertical boundary conditions are PML for bottom layer inside of glass and perfect electric conductor (PEC) was employed to bottom layer to approximate the metallic mirror. The PML layer is required for elimination of reflected waves from bottom of glass surface, which indicates thick glass substrate compared with thin device layers. The grid size set to 25 nm, which is smaller than a 20 of wavelength, in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

bulk and non-uniform grids were employed as every layer has more than two grid layers to resolve thin layer such as graphene and LiF. The light emitted by vertical and horizontal dipole source in the interface between WO3 doped CBP and Ir(ppy)3 doped CBP with wavelength of 520 nm. The cross-sectional discrete Fourier transformation (DFT) monitor was used to obtain spatial pointing vector magnitude distribution. The light extraction efficiency is calculated from light output power normalized by the power of excitation source.

Supporting Information: Additional properties of the CYTOP-graphene; details of experiments, calculations and optimization processes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected], [email protected] Present Addresses † Department of Bio Engineering, UC Berkeley, 442 Stanley hall, Berkeley, California 94720 United states Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡ S. Kim, K. C. Kwon and J. Y. Park contributed equally. ACKNOWLEDGMENT


Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1402-14. REFERENCES 1. Nam, S.; Song, M.; Kim, D.H; Cho, B.; Lee, H. M.; Kwon, J.-D.; Park, S.-G.; Nam, K.-S.; Jeong, Y.; Kwon, S.-H.; Park, Y. C.; Jin, S.-H.; Kang, J.-W.; Jo, S.; Kim, C. S. Ultrasmooth, Extremely Deformable and Shape Recoverable Ag Nanowire Embedded Transparent Electrode. Sci. Rep. 2014, 4, 4788. 2. Wang, Z. B.; Helander, M. G.; Qiu, J.; Puzzo, D. P.; Greiner, M. T.; Hudson, Z. M.; Wang, S.; Liu, Z. W.; Lu, Z. H. Unlocking the Full Potential of Organic Light-Emitting Diodes on Flexible Plastic. Nat. Photonics 2011, 5, 753-757. 3. Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192-200. 4. Li, N.; Oida, S.; Tulevski, G. S.; Han, S.-J.; Hannon, J. B.; Sadana, D. K.; Chen, T.-C. Efficient and Bright Organic Light-Emitting Diodes on Singe-layer Graphene Electrodes. Nat. Commun. 2013, 4, 2294. 5. Han, T.-H.; Lee, Y.; Choi, M.-R.; Woo, S.-H.; Bae, S.-H.; Hong, B. H.; Ahn, J.-H.; Lee, T.W. Extremely Efficient Flexible Organic Light-Emitting Diodes with Modified Graphene Anode. Nat. Photonics 2012, 6, 105-110. 6. Geim, A. K. Graphene: Status and Prospects, Science 2009, 324, 1530-1534. 7. Hassoun, J.; Bonaccorso, F.; Agostini, M.; Angelucci, M.; Betti, M. G.; Cingolani, R.; Gemmi, M.; Mariani, S.; Panero, S.; Scrosati, B. An Advanced Lithium-Ion Battery on a Graphene Anode and a Lithium Iron Phosphate Cathode, Nano Lett. 2014, 14, 4901-4906.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

8. Quesnel, E.; Roux, F.; Emieux, F.; Faucherand, P.; Kymakis, E.; Volonkis, G.; Giustino, F.; Martin-Garcia, B.; Moreels, I.; Gursel, S. A. Graphene-based technologies for energy applications, challenge and perspectives, 2D Mater. 2015, 2, 030204. 9. Kakavelakis, G.; Konios, D.; Stratakis, E.; Kymakis, E. Enhancement of the Efficiency and Stability of Organic Photovoltaic Devices via the Addition of a Lithium-Neutralized Graphene Oxide Electron-Transporting Layer, Chem. Mater. 2014, 26, 5988-5993. 10. Liu, J.; Xue, Y.; Gao, Y.; Yu, D.; Durstock, M.; Dai, L. Hole and Electron Extraction Layers Based on Graphene Oxide Derivatives for High-Performance Bulk Heeterojunction Solar Cells, Adv. Mater. 2012, 24, 2228-2233. 11. Novoselov, K. S.; Geim, A. K. Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene, Nature 2005, 138, 197-200 12. Schwierz, F. Graphene Transistors, Nat. Nanotechnol. 2010, 5, 487-496 13. Konios, D.; Petridis, C.; Kakavelakis, G.; Sygletou, M.; Savva, K.; Stratakis, E.; Kymakis, E. Reduced Graphene Oxide Micromesh Electrodes for Large Area, Flexible, Organic Photovoltaic Devices, Adv. Funct. Mater. 2015, 25, 2213-2221. 14. Kwon, K. C.; Choi, K. S.; Kim, S. Y Increased Work Function in Few-Layer Graphene sheets via Metal Chloride Doping. Adv. Funct. Mater. 2012, 22, 4724-4731. 15. Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574-578.


Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16. Jung, G. H.; Hong, K.; Dong, W. J.; Kim, S.; Lee, J.-L. BCP/Ag/MoO3 Transparent Cathodes for Organic Photovoltaics. Adv. Energy Mater. 2011, 1, 1023-1028. 17. Lee, W. H.; Suk, J. W.; Lee, J.; Hao, Y.; Park, J.; Yang, J. W.; Ha, H.-W.; Murali, S.; Chou, H.; Akinwande, D. Simultaneous transfer and doping of CVD-grown graphene by fluoropolymer for transparent conductive films on plastic. ACS Nano 2012, 6, 1284-1290. 18. Kwon, K. C.; Kim, S.; Kim, C.; Lee, J.-L.; Kim, S. Y. Fluoropolymer-Assisted Graphene Electrode for Organic Light-Emitting Diodes. Org. Electron. 2014, 15, 3154-3161. 19. Lee, W. H.; Suk, J. W.; Lee, J.; Hao, Y.; Park, J.; Yang, J. W.; Ha, H. W.; Murali, S.; Chou, H.; Akinwande, D.; Kim, K. S.; Ruoff, R. S. Simultaneous Transfer and Doping of CVDgrown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic. ACS Nano 2012, 6, 1284-1290. 20. Beams, R.; Cancado, L. G.; Novotny, L. Raman Characterization of Defects and Dopants in Graphene, J. Phys.: Condens. Matter 2015, 27, 083002. 21. Kwon, K. C.; Kim, S. Y. Extended Thermal Stability in Metal-Chloride Doped Graphene Using Graphene Overlayers. Chem. Eng. J. 2014, 244, 355-363. 22. Kwon, K. C.; Dong, W. J.; Jung, G. H.; Ham, J.; Lee, J.-L.; Kim, S. Y. Extension of Stability in Organic Photovoltaic Cells Using UV/Ozone-Treated Graphene Sheets. Sol. Energy Mater. Sol. Cells 2013, 109, 148-154. 23. Macleod, H. A. Thin-Film Optical Filter, Taylor & Francis: New York, 2001. 24. Bruna, M.; Borini, S. Optical Constants of Graphene Layers in the Visible Range. Appl. Phys. Lett. 2009, 94, 031901.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

25. Kim, S.; Cho, H. W.; Hong, K.; Son, J. H.; Kim, K.; Koo, B.; Kim, S.; Lee, J.-L. Design of Red, Green, Blue Transparent Electrodes for Flexible Optical Devices. Opt. Express 2014, 22, A1257-A1269. 26. Betancur, R.; Romero-Gomez, P.; Martinez-Otero, A.; Elias, X.; Maymo M.; Martorell, J. Transparent Polymer Solar Cells Employing a Layered Light-Trapping Architecture. Nat. Photonics 2013, 7, 995-1000. 27. Tao, C.; Xie, G.; Meng, F.; Ruan, S.; Chen, W. Tailoring Spatial Distribution of the Optical Field Intensity in Semitransparent Inverted Organic Solar Cells. J. Phys. Chem. C 2011, 115, 12611-12615. 28. Hofmann, S.; Thomschke, M.; Lussem, B.; Leo, K. Top-Emitting Organic Light-Emitting Diodes. Opt. Express 2011, 19, A1250-A1264. 29. Chutinan, A.; Ishihara, K.; Asano, T.; Fujita, M.; Noda. S. Theoretical Analysis on LightExtraction Efficiency of Organic Light-Emitting Diodes Using FDTD and Mode-Expansion Methods. Org. Electron. 2005, 6, 3-9. 30. Koo, W. H.; Jeong, S. M.; Araoka, F.; Ishikawa, K.; Nishimura, S.; Toyooka, T.; Takezoe, H. Light Extraction from Organic Light-Emitting Diodes Enhanced by Spontaneously Formed Buckles. Nat. Photonics 2010, 4, 222-226. 31. Cho, S.-H.; Song, Y.-W.; Lee, J.-G.; Kim, Y.-C.; Lee, J. H.; Ha, J.; Oh, J.-S.; Lee, S. Y.; Lee, S. Y.; Hwang, K. H.; Zang, D.-S.; Lee, Y.-H. Weak-Microcavity Organic LightEmitting Diodes with Improved Light Out-Coupling. Opt. Express 2008, 16, 12632-12639. 32. Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman spectroscopy in Graphene. Phys. Rep. 2009, 473, 51-87.


Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of contents


Challenge beyond Graphene: Metal Oxide ... - ACS Publications

Recommend Documents