Extremely efficient flexible organic light-emitting diodes with modified graphene anode

Although graphene films have a strong potential to replace indium tin oxide anodes in organic light-emitting diodes (OLEDs), to date, the luminous efficiency of OLEDs with graphene anodes has been limited by a lack of efficient methods to improve the low work function and reduce the sheet resistance of graphene films to the levels required for electrodes1–4. Here, we fabricate flexible OLEDs by modifying the graphene anode to have a high work function and low sheet resistance, and thus achieve extremely high luminous efficiencies (37.2 lm W in fluorescent OLEDs, 102.7 lm W in phosphorescent OLEDs), which are significantly higher than those of optimized devices with an indium tin oxide anode (24.1 lmW in fluorescent OLEDs, 85.6 lmW in phosphorescent OLEDs). We also fabricate flexible white OLED lighting devices using the graphene anode. These results demonstrate the great potential of graphene anodes for use in a wide variety of high-performance flexible organic optoelectronics. Graphene is a flexible two-dimensional sheet of sp-hybridized carbon atoms5–12 that has potential applications in a variety of electronic devices1–4,13–19. Finding a way to replace the conventional brittle indium tin oxide (ITO) electrode with a flexible graphene electrode is a major research topic20. ITO has been used widely in optoelectronic devices such as organic light-emitting diodes (OLEDs) and liquid-crystal displays, but its raw materials are becoming increasingly expensive, and it is brittle, rendering it unsuitable for flexible devices20. It is therefore desirable to develop alternative transparent and flexible electrodes such as graphene. Despite its strong potential as a transparent conductor, the practical application of graphene as the anode of organic optoelectronic devices has been limited because of its relatively low work function (WF) ( 4.4 eV, Supplementary Table S2) and high sheet resistance (.300 VA in refs 3,4) compared with ITO ( 4.7≤WF≤ 4.9 eV and 10 VA). The low WF of graphene causes the hole injection between the graphene anode and the overlying organic layers to be unfavourable because of the high injection barrier at the interface (Fig. 1a) (for example, the ionization potential of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB) is 5.4 eV). As a result, graphene-based OLEDs have shown poorer current efficiencies (CEs, units cd A) than ITO-based devices3,4 (Supplementary Table S3, Fig. S3). In addition, the low conductivity of pristine graphene films limits the luminous (power) efficiencies (LEs, units lm W) of the devices because it gives rise to high operating voltages. To achieve practical graphene anodes, a method to eliminate these disadvantages of graphene (low WF and high sheet resistance) needs to be developed. Here, we demonstrate a method to increase the surface WF and reduce the sheet resistance of graphene films to 5.95 eV and 30 VA, respectively, by using conducting polymer compositions to modify the surface, thus creating a WF gradient from the graphene to the overlying organic layer, and by doping with p-dopants HNO3 or AuCl3 (Supplementary Tables S4,S5). The higher WF enables holes to be injected easily into the organic layer despite the high hole-injection barrier at the interface between the graphene anode and the organic layer. We used WF-tunable polymeric conducting polymers to improve hole injection from the anode to the organic layer. Without a holeinjection layer (HIL), hole injection from the graphene anode to the overlying hole-transport layer (HTL; for example, NPB) is unfavourable because of the huge hole-injection energy barrier ( 1.0 eV) at the interface (Fig. 1a). To achieve a high CE in OLEDs that have graphene anodes, the efficiency of hole injection from the graphene electrode to the overlying organic layers must be increased. To meet this requirement, we incorporated a self-organized gradient HIL (which we term ‘GraHIL’) composed of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulphonate) (PEDOT:PSS) and a tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid copolymer, one of the perfluorinated ionomers (PFI; Supplementary Fig. S6), which provides a WF gradient through the layer (surface WF1⁄4 5.95 eV; Fig. 1b) and thus enables holes to be injected efficiently to the overlying organic layer (ref. 21; Supplementary Table S4, Fig. S7). To use graphene films as electrodes in flexible electronics, large-area synthesis and an efficient transfer method are essential1–4,8,18. Large-scale synthesis of graphene films can be achieved using chemical vapour deposition (CVD)8–12. To form a graphene anode on a flexible poly(ethyleneterephthalate) (PET) substrate (Fig. 1e), multilayered graphene films were transferred to a PET substrate using a poly(methyl methacrylate) (PMMA) polymer support or a thermal release tape. The transferred multilayered graphene films were then etched using a pre-patterned shadow mask and reactive ion etching with O2 plasma. We used our polymeric HILs (GraHIL) to modify the patterned graphene anode. We fabricated a series of small-molecule fluorescent OLEDs, including a control device with an ITO anode and several experimental devices with multilayered graphene anodes (two, three or four graphene layers). The graphene films were p-doped with HNO3 or AuCl3 to decrease their sheet resistance. We measured the CEs and LEs of the devices fabricated using different kinds of HILs. For the ITO-based devices with our selforganized polymeric HILs, GraHIL, the CE was 18.4 cd A (LE≈ 24.1 lmW), which was much higher than that of the device with a conventional small-molecule hole-injection material,