Non-oxidized graphene nanoplatelets as an efficient hole transport layer in organic light-emitting diodes

In this work, we have synthesized and applied non-oxidized graphene nanoplatelets (GNPs) with high work function of 5.61 eV into the aluminum tris(8-hydroxyquinoline) (Alq₃)-based organic light-emitting diodes (OLEDs) as an efficient hole transport layer (HTL). By the assistance of Py ⁺ ions from the pyridinium tribromide salt (Py⁺Br3-), the exfoliation of GNPs can be easily obtained via sonication. After proper filtering, the concentration of GNPs dispersed in water-ethanol mixture is about 0.2 mg/mL. To be the HTL, we presumed that a nearly full coverage of GNPs on ITO substrates can be achieved by multiple spin-coat process. Thereby, low turn-on voltage of 4.1 V (@100 mA/cm²) and high luminance of 36,000 cd/m² (at 8.4 V) were obtained, which corresponded a luminous efficiency (2.33 cd/A) about 200 times and 1.4 times higher than that in OLEDs without any HTL and with a N,N-bis-(1-naphyl)-N,N-diphenyl-1,1′-biphenyl- 4,4″-diamine (NPB) HTL, respectively.