Low work function Ca doped graphene as a transparent cathode for organic opto-electronics and OLEDs

Graphene is an excellent candidate for a transparent electrode since it has high lateral conductivity and optical transparency, but due to its high work function, between 4.5 and 5eV, it can serve as a good transparent anode but poor transparent cathode due to a barrier for electrons injection. Reducing graphene's work function is thus essential to make it an ideal transparent cathode as well. Naturally, n-type doping of graphene increases its Fermi level and thus reduces the work function. Among many choices of n-type dopants alkaline element can be good candidate, here we present a study of calcium (Ca) n-doped graphene to be serve as a transparent cathode. Several measurements were carried out on Ca doped graphene and compared them with pristine graphene as a reference, such as: Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) Field effect transistor (FET), transparent electron microscopy (TEM), and photoemission spectroscopy. Our results show that doping graphene with 1 nm of evaporated Ca on the surface reduces its work function by nearly 1eV. We further performed current-voltage characteristics of graphene/Alq3/Ag structures having undoped and doped graphene cathodes. The structures with doped graphene showed an increase of two orders of magnitude in current under the same applied bias due to the contacts barrier reduction. The results confirm the effectiveness of Ca-doped graphene for reducing the barrier for electron injection from graphene to next adjacent organic layer. It demonstrates the feasibility of Ca doped graphene to serve as a transparent cathode in organic devices in general and in OLEDs in particular.