Simple Wafer-Scale Growth and Transfer of Graphene Film Converted from Spin-Coated Fullerene Derivative Ji Hoon Seo, Jae-Wook Kang, Dong-Ho Kim, Sungjin Jo, Seung Yoon Ryu, Hyung Woo Lee and Chang Su Kim ECS Solid State Lett. 2013, Volume 2, Issue 2, Pages M13-M16. doi: 10.1149/2.003302ssl Email alerting service

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© 2012 The Electrochemical Society

ECS Solid State Letters, 2 (2) M13-M16 (2013)

2162-8742/2013/2(2)/M13/4/$28.00 © The Electrochemical Society

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Simple Wafer-Scale Growth and Transfer of Graphene Film Converted from Spin-Coated Fullerene Derivative Ji Hoon Seo,a,b Jae-Wook Kang,a Dong-Ho Kim,a Sungjin Jo,c Seung Yoon Ryu,d Hyung Woo Lee,b,z and Chang Su Kima,z a Advanced Functional Thin Films Department, Korea Institute of Materials Science, Changwon 641-831, Korea b National Core Research Center for Hybrid Materials Solution, Pusan National University, Busan 609-735, Korea c School of Energy Engineering, Kyungpook National University, Daegu 702-701, Korea d Department of Information Display, Sunmoon University, Asan 336-708, Korea

We report a simple method for growing wafer-scale graphene films using PCBM (phenyl-C61 -butyric acid methyl ester), a fullerene derivative, as a solid carbon source. PCBM films spin-coated on nickel catalyst were easily converted to graphene films having a thickness of a few layers by thermal annealing without any reactive gas. This method of converting PCBM to graphene is safe, unlike the chemical vapor deposition (CVD) method that uses explosive precursor gases. PCBM-derived graphene films were also transferred through a simple process to plastic substrates with a supporting layer for use in organic solar cells. The power conversion efficiency (PCE) of bulk heterojunction organic solar cells prepared on the PCBM-derived graphene electrode was 0.98%. This study indicates that PCBM-derived graphene films can serve as an inexpensive, flexible alternative to indium tin oxide (ITO) films, and therefore, they can improve the economic viability and flexibility of organic solar cells. © 2012 The Electrochemical Society. [DOI: 10.1149/2.003302ssl] All rights reserved. Manuscript submitted October 11, 2012; revised manuscript received November 5, 2012. Published November 20, 2012.

Traditionally, indium tin oxide (ITO) has been widely used as a standard transparent electrode in various types of optoelectronic devices.1,2 There has thus been a constantly increasing demand for ITO for use in consumer electronics. However, ITO has a number of drawbacks and is unlikely to be the material of choice in future optoelectronic devices. First, the cost of ITO thin films is very high, mainly because ITO thin films must be vacuum deposited at rates orders of magnitude slower than coating processes. Second, indium is a relatively scarce element. Third, the brittleness of ITO renders it susceptible to mechanical damage, making it unsuitable for use in flexible electronics.3–5 Therefore, considerable industrial and research efforts have been directed toward finding a replacement for ITO. In this regard, many alternatives such as conducting polymers,6,7 metal nanowires,8,9 and carbon nanotubes10,11 have been studied. In recent times, graphene has attracted considerable research interest as a next-generation transparent electrode because of its uniformly high transparency and excellent electrical conductivity.12,13 In addition, graphene is more mechanically robust than ITO and shows superior bending performance.14,15 Graphene can be fabricated by several methods, including the exfoliation of highly oriented pyrolysis graphite (HOPG)16 and SiC film decomposition at high temperatures in vacuum.17 The chemical vapor deposition (CVD) method has attracted particular attention as a promising method for synthesizing large-area graphene films for practical applications.18,19 However, this method requires the use of a sophisticated reactor and/or vacuum system, and this tends to increase the cost of fabrication. In addition, the use of toxic, corrosive, flammable, and/or explosive precursor gases (typically, CH4 or C2 H2 ) can cause chemical and safety hazards.20 Herein, we report a simple method for growing wafer-scale graphene films using PCBM (phenyl-C61 -butyric acid methyl ester), a fullerene derivative, as a solid carbon source. We chose PCBM in this study because graphene can be formed through the thermally induced decomposition of PCBM in combination with a metal catalyst.21,22 In addition, PCBM is used more widely than any other material as an electron acceptor for organic electronics because of its high solubility in most common solvents.23,24 PCBM films spin-coated on nickel catalyst were easily converted to graphene films having a thickness of a few layers by thermal annealing without the use of any reactive gas. This method of converting PCBM to graphene is safe, unlike the CVD method that uses explosive precursor gases. PCBM-derived graphene films could also be easily transferred to plastic substrates with a supporting layer for use in an organic solar cell.

z

E-mail: [email protected]; [email protected].

Figure 1 shows the steps required to fabricate graphene films. First, a ∼300-nm-thick layer of nickel was sputter-deposited onto SiO2 /Si substrates. At the same time, a PCBM solution was prepared by dissolving PCBM in chlorobenzene. Then, a PCBM film was formed by spin-coating solutions with different weight percents (0.1–1 wt%) at 5000 rpm for 40 s. Next, the sample was thermally annealed at 800◦ C under vacuum (1 Torr) without any reactive gas for 5 min. It was then rapidly cooled to room temperature at a maximum rate of 50◦ C/min depending on the annealing temperature. PCBM-derived graphene films were then obtained on the sample surface, as shown in Fig. 1. Raman spectroscopy and optical microscopy were the imaging techniques used in this study. Raman spectroscopy was conducted using 633 nm laser excitation. The optical transmission properties of graphene were determined by UV–visible spectrophotometry. The sheet resistance was measured using a four-point probe system. To evaluate the application of PCBM-derived graphene films as anodes in organic solar cells, we fabricated conventional bulk heterojunction organic solar cells. After the patterning of the graphene electrode, a ∼40-nm-thick PEDOT:PSS (Baytron P) layer was spin-coated on the graphene electrode and dried at 150◦ C for 1 min on a hot plate inside a glove box. A poly(3-hexylthiophene) (P3HT):PCBM blend solution was prepared as a photoactive layer with a mass ratio of 1:1 in dichlorobenzene. A 240-nm-thick photoactive layer was deposited by spin-coating in a glove box. A 120-nm-thick cathode electrode with aluminum was prepared on the photoactive layer by thermal evaporation. The active area of the device was 0.38 cm2 with an island-type electrode geometry. The current density–voltage (J–V) characteristics of the organic solar cell devices were measured under AM1.5 simulated illumination with an intensity of 100 mW/cm2 . The J–V curves were recorded automatically using a Keithley SMU 2410 Source Meter by illuminating the devices. Figure 2a shows Raman spectra of the PCBM spin-coated on nickel catalyst before and after thermal annealing at 800◦ C without any reactive gas for 5 min. The as spin-coated PCBM film exhibits a broad Raman peak over the 1000–1700 cm−1 range but not at ∼2700 cm−1 . In contrast, the Raman spectrum after thermal annealing at 800◦ C exhibits characteristic graphene fingerprints of the G band (1580 cm−1 ), and 2D band (2700 cm−1 ).25–27 For graphene formation, carbon atoms are incorporated into the bulk of a metal catalyst such as nickel to form a solid solution at high temperatures. Then, as the metal cools to room temperature, the solubility of carbon in nickel decreases. The segregation of the carbon atoms leads to the formation of the graphene films.28–30 Our results suggest that the catalytic effect of transition metals such as nickel enables the growth of graphene on metal surfaces when appropriate PCBM films are used as carbon sources. The inset in Figure 2 shows optical microscope image before and after

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Figure 1. Schematic illustration of the method for growing graphene films. A 300-nm-thick nickel film was sputter-deposited on SiO2 substrate, following which PCBM was spin-coated on nickel catalyst. After the samples were thermally annealed at 800◦ C without any reactive gas for 5 min, graphene films were obtained on the nickel.

thermal annealing. The large-area smooth surface was a very important factor because it enabled the growth of high quality, uniform graphene films. The surface of the as spin-coated PCBM film was mainly rough because of the rough surface of polycrystalline nickel catalyst. On the other hand, we found that 800◦ C is the optimum temperature range, consistently giving large area and uniform graphene films. The content of the carbon source is one important factor that affects the segregation of carbon atoms and the quality of the supported graphene films. Figure 2b shows Raman spectra of the PCBM films

Figure 2. (a) Raman spectra of the PCBM spin-coated on nickel catalyst before and after thermal annealing at 800◦ C without any reactive gas for 5 min. Inset shows optical microscope image of the sample before and after thermal annealing. (b) Raman spectra of the PCBM films formed by spin-coating solutions (obtained by dissolution of PCBM in chlorobenzene) with different weight percents (wt%) on nickel catalyst. All samples were thermally annealed at 800◦ C for 5 min.

formed by spin-coating solutions (obtained by dissolution of PCBM in chlorobenzene) with different weight percents (wt%) on nickel catalyst. All samples were thermally annealed at 800◦ C for 5 min. Raman spectra of films formed by solutions with less than 0.1 wt% PCBM did not show any peaks related to graphene or amorphous carbon. Our results indicate that most compounds formed by the dissociation of very thin PCBM films were removed from the substrates during thermal annealing because of vaporization in the furnace and then evacuated without re-adsorption on the substrate. The films formed by solutions with 0.5 wt% PCBM show well-defined G and 2D features. In contrast, films formed by solutions with more than 1 wt% PCBM show broad peaks around the D peak (1350 cm−1 ) and G peak. The ratio of 2D to G peaks in the Raman spectra shows that PCBM-derived graphene films did not have only one layer, and a higher ratio of 2D to G peaks would imply that fewer layers are present.25–27 From the viewpoint of device fabrication, graphene grown on metal catalyst substrates must be transferred to a target substrate without damage, which would otherwise degrade its properties. The controlled transfer of graphene films from metal catalyst substrates remains a challenge for many applications.31–33 Figure 3a shows an alternative strategy to transfer graphene directly to a plastic substrate using a low-viscosity, photo-curing polyurethane (NOA 74, Norland Optical Adhesive). NOA 74 is a clear, colorless, liquid photopolymer that cures when exposed to ultraviolet light. Because it is a one-part system and is completely solid, it offers many advantages when applied to bonding with plastic substrates.34,35 In the first step, NOA 74 was rolled on graphene films combined with polyethylene terephthalate (PET) substrates and cured under 320-nm UV light for 10 min. UVcured NOA 74 shows good adhesion and easy bonding with the top graphene film and the bottom plastic substrate. Next, a FeCl3 aqueous solution was used to etch away all the nickel layers. Subsequently, only graphene films were transferred onto the PET substrate with the NOA 74 supporting layer. The graphene films maintain their well-defined G and 2D features after transfer, as confirmed by Raman spectra. The graphene films are apparently well preserved after transfer onto insulating substrates. Figure 3b shows graphene films on a 4-inch Si wafer and on PET substrates, demonstrating the transferability of graphene films from metal catalyst substrates and their high potential for use in flexible electronics. Figure 4a shows the optical transmittance of graphene films on the PET substrate with the NOA 74 supporting layer. It has been reported that single-layer graphene films absorb ∼2.3% of light.36,37 PCBM-derived graphene films absorb ∼9% of light, suggesting that our obtained graphene films are a few layers thick, which is in good agreement with the Raman spectra. The sheet resistance of PCBMderived graphene films was found to be ∼1 k/sq using a four-point probe system. PCBM-derived graphene films show fairly similar distribution of sheet resistance across the sample area with dimensions of approximately 2 × 2 cm. Figure 4b shows the J–V characteristics of the organic solar cell fabricated with the PCBM-derived graphene electrode. This device showed an open circuit voltage (VOC ) of 0.64 V, short circuit current density (JSC ) of 5.30 mA/cm2 , fill factor (FF) of 28.5%, and PCE of 0.98%. In contrast, the organic solar cell fabricated on the ITO electrode (∼10 /sq) showed a VOC of 0.63 V, JSC of 7.16 mA/cm2 , FF of 45.5%, and PCE of 2.01%. Generally, the JSC and

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Figure 3. (a) Schematic illustration of the process for transferring graphene onto the PET plastic substrate with the NOA 74 supporting layer. (b) Digital camera images of the graphene films on a 4-inch Si wafer and on PET substrates having high transparency and flexibility.

Figure 4. (a) Optical transmittance of graphene films on the PET substrate with the NOA 74 supporting layer. (b) Current density–voltage curves of an organic solar cell with a graphene electrode in the dark and under 100 mW/cm2 AM 1.5 G simulated solar illumination. The inset shows the schematic structure of the organic solar cell.

FF values of solar cells are critically dependent on the sheet resistance of the electrode.38,39 Therefore, the lower efficiency of the device on graphene is mainly due to a reduced JSC and FF compared to the cell on ITO, both of which are due to the higher sheet resistance of the graphene film.40,41 For further improvements to the PCE, the sheet resistance of graphene should be reduced. This can be accomplished either by more effective doping or by increasing the carrier mobility via interface control.42,43 The importance of our findings will be further strengthened with such improvements. In conclusion, we have demonstrated a simple method for growing wafer-scale grapheme films and for then transferring them from the viewpoint of use as a transparent electrode for manufacturing organic solar cells. We fabricated PCBM-derived graphene films by thermal annealing and transferred them through a simple process to plastic substrates with a supporting layer. The PCE of bulk heterojunction organic solar cells prepared on the PCBM-derived graphene electrode was 0.98%. Further research aimed at improving the graphene film quality and optimizing the device structure should lead to graphene

films being extensively used as an alternative material to ITO films in organic solar cells. Acknowledgments This research was supported by R&D Program grants (2012PNK2970) from the Korea Institute of Materials Science. This research was also supported in part by the Basic Science Research Program (2011-0014709) and National Core Research Center program (2011-0006257) from the National Research Foundation of Korea. Prof. S. Jo gratefully acknowledges support from the Human Resources Development Program (20094010200010) from the Korea Institute of Energy Technology Evaluation and Planning. References 1. D. S. Hecht, L. B. Hu, and G. Irvin, Adv. Mater., 23, 1482 (2011). 2. Q. Wan, E. N. Dattoli, W. Y. Fung, W. Guo, Y. B. Chen, X. Q. Pan, and W. Lu, Nano Lett., 6, 2909 (2006).

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