Versatile p-Type Chemical Doping to Achieve Ideal Flexible Graphene Electrodes

We report effective solution-processed chemical p-type doping of graphene using trifluoromethanesulfonic acid (CF3SO3H, TFMS), that can provide essential requirements to approach an ideal flexible graphene anode for practical applications: high optical transmittance, low sheet resistance (70% decrease), high work function (0.83 eV increase), smooth surface, and air-stability at the same time. The TFMS-doped graphene formed nearly ohmic contact with a conventional organic hole transporting layer, and a green phosphorescent organic light-emitting diode with the TFMS-doped graphene anode showed lower operating voltage, and higher device efficiencies (104.1 cdA/C01, 80.7 lmW/C01) than those with conventional ITO (84.8 cdA/C01, 73.8 lmW/C01). The widely used indium tin oxide (ITO) transparent electrode is not appropriate for use in flexible electronics, because it is brittle, increasingly expensive, and causes diffusion of impurities into devices. Therefore, flexible transparent electrodes must be developed before the use of flexible electronics is practical. Graphene is a one-atom thick sheet of sp2 hybridized carbon atoms that has unique electrical properties and mechanical robustness. However, pristine graphene has suffered from low charge carrier density, leading to higher sheet resistance compared to ITO, thus necessitating doping to improve its conductivity.