Synthesis of graphene from non-gaseous carbon sources and its work function engineering by charge transfer doping for efficient PLEDs

Abstract

Graphene has been considered as a potential candidate for transparent conductive electrodes due to its high transparency and electrical conductivity. In order to apply graphene as the transparent conductive electrode for organic or polymer light-emitting diodes (OLEDs or PLEDs), it is important not only to synthesize large-area and uniform graphene but also to tune its work function. In this study, we have demonstrated a simple method for the synthesis of large area and mono- to few-layer graphene on a nickel foil. A cellulose acetate laminate film was coated on a nickel foil as a solid carbon source. Compared to previous synthesis methods using solid carbon sources, the mentioned carbon source can be easily coated on the metal catalyst layer. The uniform graphene layer could be grown on the nickel catalyst layer due to the uniformity of the solid carbon source layer, which was made of a cellulose acetate laminate film coated by the cold press lamination process. The graphene was grown from the cellulose acetate laminate film without explosive gaseous carbon sources usually used in the growth of graphene by CVD methods. A series of processes mentioned above could start and end with the lamination process. In order to control the thickness of the gra-phene layer, the graphene samples were annealed at various temperatures and durations. The sheet resistance and transmittance of the transferred graphene on a PET substrate were $4240-100 \Omega sq^{-1}$ and 85-60 %, respectively. To demonstrate the practical use of graphene, a flexible PLED was fabricated on the graphene electrode synthesized through the method mentioned above. The maximum luminance and luminance efficiency were $4040 cd m^{-2}$ at 6.8 V and $8.2 cd A^{-1}$ at 4.0 V. To our knowledge, this is the first report on a typical PLED fabricated on the graphene electrode from a solid carbon source. Graphene has been considered to be a potential alternative transparent and flexible electrode for replacing commercially available indium tin oxide (ITO) anode. However, the relatively high sheet resistance and low work function of graphene compared with ITO limit the application of graphene as an anode for OLEDs or PLEDs. Here, flexible PLEDs made by using bis(trifluoromethanesulfonyl)amide (TFSA,$[CF_3SO_2]_2$ NH) and $MoO_x$ doped graphene anodes are demonstrated to have low sheet resistance and high work function. The gra-phene is easily doped with TFSA and $MoO_x$ by means of a simple spin-coating process. After doping, the sheet resistance decreased and work function increased without significant decrease of transmittance, which resulted in efficient hole injection from the graphene anode to HTL. Therefore, efficiency of PLEDs on the doped graphene anode improved compared with that of PLEDs on a pristine graphene anode and is comparable to that of ITO anode. Our demonstration of TFSA-and $MoO_x$ -doped graphene as a flexible and transparent electrode of PLED show that the TFSA-and $MoO_x$ -doped graphene electrode could be a promising flexible and transparent electrode.