SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INORMATION Supplementary Information Extremely Efficient lexible Organic Light-emitting Diodes with Modified Graphene Anode Tae-Hee Han 1, Youngbin Lee 2, Mi-Ri Choi 1, Seong-Hoon Woo 1, Sang-Hoon Bae 2, Byung Hee Hong 3, Jong-Hyun Ahn 2* and Tae-Woo Lee 1* 1 Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk , Republic of Korea, 2 SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT) and School of Advanced Materials Science and Engineering,, Sungkyunkwan University, Suwon, Gyeonggi-do , Republic of Korea, 3 Department of Chemistry, Seoul National University, Seoul , Republic of Korea *To whom all correspondence should be addressed: twlee@postech.ac.kr, ahnj@skku.edu nature photonics 1

2 SUPPLEMENTARY INORMATION Materials and Methods Graphene growth and fabrication of patterned graphene anodes Table S1. The process condition of graphene growth using CVD. Temperature rising Annealing of Cu foil Growth Cooling H 2 flow 10 s.c.c.m 10 s.c.c.m 10 s.c.c.m 10 s.c.c.m CH 4 flow s.c.c.m - Pressure 80 mtorr 80 mtorr 1.6 Torr 80 mtorr Temperature RT 1000 C 1000 C 1000 C 1000 C RT Time 30 min 30 min 30 min 150 min Electrical and optical characterization The I V L characteristics and electroluminescence spectra were obtained using a Keithley 236 source measurement unit and a Minolta CS2000 Spectroradiometer, which were controlled by a computer. The dark injection space-charge-limited-current (DI-SCLC) transient measurement was performed by using a pulse generator (HP 214 B) and a digital oscilloscope (Agilent Infiniium 54832B). Secondary ion mass spectroscopy Dynamic secondary ion mass spectroscopy measurements were performed using an IMS 6. Depth profile for each sample was acquired using a 12.5 kv O 2 + ion source, +5 kv secondary voltage and 15 na source current. Determined ions were 12C, 19, 23Na, 12C19, 2 nature photonics

3 ITO/ PE HN3/PEHN/PET3HN/PET3CNHN3/PETSUPPLEMENTARY INORMATION 32S, 39K, 115In and 120Sn. The raster size and examined area was 250 μm 250 μm and 60 μm 2 for each sample. SUPPLEMENTARY IGURES AND DESCRIPTION ig. S1 shows optical transmittance of multi-layered graphene, carbon nanotube (CNT), and ITO anodes deposited on PET as a function of wavelength. As the number of graphene layers increases, transmittance tends to decrease gradually (2.3 % per a layer). The graphene films showed over 90 % transmittance at the 550 nm wavelength. Especially, graphene anodes have more homogeneous transmittance behavior compared with the ITO anode in visible range, which provides a great advantage especially for blue, red, and white OLEDs as well as organic solar cells. The transmittance of the CNT film we used was much lower than graphene films and ITO. 100 Transmittance(%) G2G3G4TTOOOOT Wavelength (nm) igure S1. Transmittance of multi-layered graphene, carbon nanotube (CNT), and ITO anodes on a PET substrate. G2, G3, and G4 denote the two-layered, three-layered, and fourlayered graphene sheets. The graphene and CNT were doped with HNO 3. nature photonics 3

4 2L HNO33LHNO34L_HNO3SUPPLEMENTARY INORMATION The binding energy tended to increase as the number of graphene layers increased in ultraviolet photoelectron spectroscopy (UPS) spectra (ig. S2). Gradual increase of the binding energies in UPS spectrum indicates that the work function of graphene films gradually increased with the increasing the number of graphene layers (Table S2). Additionally, the work functions of graphene films increased with doping with HNO 3 or AuCl Intensity (a.u.) Binding energy (ev) igure S2. UPS spectra of multilayered graphenes doped with HNO 3. Table S2. Work functions of multi-layered graphene anodes measured by UPS Undoped (ev) HNO 3 (ev) AuCl 3 (ev) 2 Layers Layers Layers nature photonics

5 SUPPLEMENTARY INORMATION Table S3. Comparison of our work with previous literature. Previous literature Anode (CNT or Graphene) Emission type Emission Colours Maximum Efficiency (cd A, lm W ) Efficiency at 500 cd/m 2 (cd A, lm W ) Maximum Efficiency with ITO (cd A, lm W ) Ref. S1 CNT luorescent Green ~1.4 cd A ~0.333 cd A ~1.9 cd A Ref. S2 CNT luorescent (polymer) Green ~1.6 cd A ~0.4 cd A 157 cd A Ref. S3 CNT luorescent Green ~2.3 cd A ~1 cd A cd A Ref. S4 CNT luorescent Green ~1.19 cd A ~1.1 cd A [a] ~4.0 cd A Ref. S5 CNT luorescent (polymer) Green ~0.85 cd A 0.7 cd A 157 cd A Ref. S6 CNT Phosphorescent Green ~1.2 cd A ~1 cd A [b] ~40 cd A Ref. S7 Graphene luorescent Green ~0.35 lm W ~0.3 lm W ~0.5 lm W Ref. S8 Graphene Phosphorescent Green ~0.75 cd A ~0.7 cd A ~40 cd/a [b] ~0.53 lm W ~0.25 lm W Our work Graphene_ HNO3 Phosphorescent Green 98.1 cd A lm W ~77.2 lm W 85.6 lm W ~ 90 cd A 81.8 cd A Our work Graphene _HNO 3 luorescent Green 30.2 cd A 37.2 lm W ~ 28.3 lm W lm W ~ 28.4 cd A cd A Our work Graphene _AuCl 3 luorescent Green cd A lm W ~27.28 lm W lm W ~27.09 cd A cd A Our work CNT_HNO 3 luorescent Green 15.8 cd A lm W ~9.7 lm W lm W ~15.2 cd A cd A [a] The typical current efficiency data taken from the common green fluorescent OLED device using the same device structure with an ITO anode (Ref. S9). [b] The typical current efficiency data taken from the common green phosphorescent OLED device using the same emitting dopant and an ITO anode (Ref. S10). nature photonics 5

6 SUPPLEMENTARY INORMATION To the best of our knowledge, in 2010, there have been two research papers regarding OLEDs using graphene sheet anodes. Although the two reports in Table S3 demonstrated that graphene anode can act as an anode, the reported luminous efficiencies were quite low compared with the state-of-the art OLED devices. The major reason comes from the lack of efficient methods to improve the low work function and high sheet resistance of graphene films at the quality required for electrodes. In addition, all the reported paper regarding OLEDs using carbon nanotubes as an anode to date have exhibited relatively poorer luminous efficiency compared with the ITO-based device. This can be also attributed to the high hole injection barrier and insufficient sheet resistance. ig. S3 clearly show that our device performance lie well beyond all the previous OLEDs using CNTs and graphenes as an anode. Current efficiency (cd/a) Progress of green fluorescent OLEDs CNT Our Work Graphene Graphene Our Work CNT Year igure S3. Progress of green fluorescent OLEDS with carbon nanotubes (CNTs) and graphene anodes. 6 nature photonics

7 SUPPLEMENTARY INORMATION We performed additional experiments to fabricate fluorescent green OLEDs using carbon nanotube (CNT)-based anode which otherwise have the same device structure with graphene anode-based devices. irst of all, we patterned commercially available CNT electrode on flexible poly(ethylene terephthalate) (PET) substrate (TOP NANOSYS, Inc., the sheet resistance: ~373.7 Ω, thickness: ~60 nm). Then we characterized the OLED devices using the GraHIL on the CNT-based anode to compare with those using graphene-based anodes. The fluorescent green OLED device with the CNT anode exhibited a current efficiency (~15.8 cd A ) which is even much higher than that of ITO-based device with conventionally used small-molecule HIL material, 2TNATA (~13.7 cd A ) (ig. 2a). Therefore, it is demonstrated that the relatively lower work function of CNT (~4.55 ev) than ITO anode can be overcome by applying GraHIL which has gradient and high work function on top of the anode. However, compared with graphene anodes, the CNT anode has three major drawbacks in order to be employed in OLED as an anode: i) insufficient sheet resistance of CNT anode which is even lower than two-layered graphene anode (Sheet resistance~ Ω ), ii) significant surface roughness (RMS roughness ~15.8 nm, please refer to the following AM and SEM images)(ig. S4 & S5) iii) lower transmittance than four-layered graphene (The UV/Vis transmittance spectrum is shown in ig. S1). The rough surface of the CNT anode can provide local protruding regions which make more current to pass through the local regions than other non-protruding regions. Therefore, due to this local thinning effect in the device, a high leakage current in a low voltage regime can be observed, which apparently degrades the device efficiency. nature photonics 7

8 SUPPLEMENTARY INORMATION igure S4. Atomic force microscope (AM) image of CNT film on poly(ethylene terephthalate) (PET) substrate. The root-mean-square (RMS) surface roughness is ~15.8 nm. igure S5. Scanning electron microscope (SEM) image of CNT film on poly(ethylene terephthalate) (PET) substrate. Our GraHIL is composed of PEDOT:PSS and a tetrafluoroethylene-perfluoro-3,6-dioxa-4- methyl-7-octenesulfonic acid copolymer (PI) (CAS number: ) (chemical structure is shown in ig. S6). 8 nature photonics

9 SUPPLEMENTARY INORMATION C C m C O C C C C 3 n O C C O S O OH igure S6. Chemical structure of a tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7- octenesulfonic acid copolymer (PI). Since PI is more hydrophobic and has lower surface energy (~20 mn m ) than PEDOT:PSS, PI preferentially tends to be located toward the surface of the HIL film. That is PI which has higher ionization potential energy than PEDOT:PSS concentration increase gradually from bottom to top of the HIL film making gradient work function in HIL film (ig. S7). urthermore, we achieved a very high work function of GraHIL surface by optimizing PI concentration relative to PEDOT:PSS ratio (Table S4). Table S4. The conducting polymer compositions and their surface work functions. Sample code PEDOT/PSS/PI (w/w/w) Work function (ev) [a] AI / 6 / GraHIL 1 / 6 / [a] The work functions were measured by a photoelectron spectrometer in air (Rieken Keiki Co. Ltd, AC-2) (Ref. S11). nature photonics 9

10 PEDOTSuloncacidSuloneldeC2SUPPLEMENTARY INORMATION To find out the surface composition and molecular distribution versus sputter time (i.e. film depth), X-ray photoelectron spectroscopy (XPS) was used. Deconvoluted S2p peaks for PEDOT (164.5 ev), sulfonic acid (168.4 and ev), sulfide (162 ev), and sulfone (166.6 ev) concentrations and C1s peak at ev for the PI concentration were used. The PI is rich at the surface in GraHIL and molecular concentration of PI gradually decreased with a depth (ig. S7). 80 Molecular concentration (%) Sufffii Sputter time (sec) igure S7. Molecular depth profiles for the hole injection layer (GraHIL) with a work function gradient. The sheet resistance of the graphene films decreases as the number of stacked layer increases, because the randomly stacked layers can behave independently without considerable change in the electronic band structures. The graphene anodes were p-doped with HNO 3 or AuCl 3, leading significant reduction of their sheet resistances. In case of 4- layered graphene films, sheet resistance of HNO 3 and AuCl 3 doped graphene films is reduced about 38 % and 61 % compared with undoped graphene films. 10 nature photonics

11 SUPPLEMENTARY INORMATION Table S5. Sheet resistance according to the number of transferred graphene layers and different p-dopants Undoped (Ω ) HNO 3 (Ω ) AuCl 3 (Ω ) 2 Layers Layers Layers Although AuCl 3 doped graphene shows lowest sheet resistance and highest work function out of doped graphenes, Au clusters are formed on top of the surface of the graphene anode during reduction of Au atoms. s12 Because the agglomeration of Au atoms reaches more than 50 nm-sized metal clusters (ig. S8), the devices with this kind of graphene anode will have a lot of local protruding regions, which makes the effective thickness shorter in local region than in other flat region. This localized thinning effect caused by metal clusters induces a high leakage current in a low voltage regime. Therefore, the current and luminous (power) efficiency of device with AuCl 3 doped four-layered graphene anode shows relatively lower than that of the device with HNO 3 -doped four-layered graphene in lower voltage regime compared with the device efficiency at voltages higher than 3V. nature photonics 11

12 SUPPLEMENTARY INORMATION a b 1 μm 1 μm c d (nm) (μm) (nm) (μm) igure S8. a, b. SEM image of both four-layered graphene films doped with HNO 3 and AuCl 3 on PET. Doping with HNO 3 maintains the graphene films clean which means smooth surface without particles or residues. The AuCl 3 doping remains gold nano particles on graphene film. c, d. AM images of four-layered graphene films doped with HNO 3 and AuCl 3 on PET. The graphene film doped with HNO 3 represents root-mean-square surface roughness of around 3.4 nm which abide by the roughness of PET. Otherwise, the graphene doped with AuCl 3 has particles showing nm size distribution. 12 nature photonics

13 SUPPLEMENTARY INORMATION We performed bending test of the fluorescent green OLED device with four-layered graphene anode and ITO. The bending radius is 0.75 cm and the strain is estimated 1.25 % according to the previous literature. s13 We found that the graphene device maintained almost the same current density even after 1000 times bending, while the ITO device was completely failed after 800 times. Therefore, excellent bending stability of the device with graphene anode is demonstrated. 1.0 Normalized current (a.u.) Number of cycles ITO G4_HNO 3 Break igure S9. Bending stability of OLEDs with graphene and ITO anode. The current at 3V were measured as a function of bending cycles. The sheet resistance tended to increase gradually under ambient conditions as shown in the following figure. The sheet resistance of graphene electrode at ambient condition is stabilized after 30 hours. However, we found that the sheet resistance was maintained without a considerable change in sheet resistance under vacuum conditions, indicating the stable doping of HNO 3 into graphene without vaporization. Therefore, when the graphene experiments were carried out under a vacuum, the sheet resistance will not be changed significantly. During the HNO 3 doping process, the electrons in graphene are transferred to protons, resulting in strong nature photonics 13

14 SUPPLEMENTARY INORMATION disappears as the excessive charges react with water or oxygen molecules in air, which can be minimized by preventing from the direct contact with air in vacuum. The initial small variation of the sheet resistance under vacuum condition can be ascribed to the experimental situation that it takes some time until the vacuum condition is saturated Ambient condition Vacuum condition 1.3 Rs/Rs Time (h) igure S10. Change of sheet resistance of graphene electrode doped by nitric acid as a function of time. The sheet resistance of graphene electrode under ambient condition is stabilized after 30 hours. On the other hand, graphene under vacuum condition exhibits stable performance without a considerable change in sheet resistance. (The first and second points indicate the data measured just before and after air extraction, respectively). The vacuum condition can prevent the excessive charges in doped graphene from reacting with water or oxygen molecules in air and as a result, sustain the doping effect. We calculated the hole injection efficiency (η) of each device. J = 1. 2 J transit η (1) SCL where J transit is the current density measured at the peak of the DI SCLC transient formed by 14 nature photonics

15 SUPPLEMENTARY INORMATION ohmic contact and J SCL is theoretical space charged limited current (equation (2)). 9 E = ε ε 0 μ 0 exp( β E ) (2) 8 d J SCL 2 Where ε is the dielectric constant, ε 0 is the vacuum permittivity, E is the electric field, d is the film thickness, μ 0 is the mobility when electric field is zero (E = 0), and β is the pool-frenkel constant that implies electric field dependent carrier mobility. References and Notes S1. Aguirre, C. M. et al. Carbon nanotube sheets as electrodes in organic light-emitting diodes. Appl. Phys. Lett. 88, (2006). S2. Li, J. et al. Organic light-emitting diodes having carbon nanotube anodes. Nano Lett., 6, (2006). S3. Williams, C. D. et al. Multiwalled carbon nanotube sheets as transparent electrodes in high brightness organic light-emitting diodes. Appl. Phys. Lett., 93, (2008). S4. Wang, Y. et al. Optimizing single-walled carbon nanotube films for applications in electroluminescent devices. Adv. Mater. 20, (2008). S5. Hu, L., Li, J., Liu, J., Grüner, G. & Marks, T. J. lexible organic light-emitting diodes with transparent carbon nanotube electrodes: problems and solutions. Nanotechnology 21, (2010). S6. Chien, Y.-M., Lefevre,., Shin, I. & Izquierdo, R. A solution processed top emission OLED with transparent carbon nanotube electrodes. Nanotechnology 21, (2010) S7. Wu, J. et al. Organic light-emitting diodes on solution processed graphene transparent electrodes. ACS Nano, 4, (2010). nature photonics 15

16 SUPPLEMENTARY INORMATION S8. Sun, T. et al. Multilayered graphene used as anode of organic light emitting devices. Appl. Phys. Lett. 96, (2010). S9. Yang, T.-H., Juang,.-S., Tsai, Y.-S., Kuo, W.-K. & Yokoyama, M. Improvement in Luminance Efficiency by Insertion of Buffer Layers in lexible Organic Light-Emitting Diodes. Jpn. J. Appl. Phys. 45, (2006). S10. Liao, L. S., Klubek, K. P. & Tang, C. W. High-ef ciency tandem organic light-emitting diodes. Appl. Phys. Lett. 84, (2004). S11. Lee, T.-W., Chung, Y., Kwon, O. & Park, J.-J. Self-organized gradient hole injection to improve the performance of polymer electroluminescent devices. Adv. unct. Mater. 17, (2007). S12. Gűnes,. et al. Layer-by-layer doping of few-layer graphene film. ACS nano. 4, (2010). S13. Suo, Z., Ma, E. Y., Gleskova H. & Wagner, S. Mechanics of rollable and foldable filmon-foil electronics. Appl. Phys. Lett. 74, (1999). 16 nature photonics