Effects of defects and non-coordinating molecular overlayers on the work function of graphene and energy-level alignment with organic molecules

To elucidate the features of graphene as an electrode material, we studied the effect of defects and molecular overlayers on the work function of graphene using density-functional theory. We found that in-plane geometrical deformations (such as Stone-Thrower-Wales defects, carbon vacancies, and hydrogenated edges) have only a marginal effect. In contrast, intercalated alkaline atoms (K or Li) and overlayers of superhalogen species (BF4 and PF6) radically change the work function. We show that the geometry of the sp(2) carbon surface remains robust after electron transfer to superhalogens, and the Fermi level could be well aligned with the energy levels of organic molecules. These methods for work function control can be used for the application of graphene materials as transparent electrodes for organic light-emitting devices. (C) 2011 Elsevier Ltd. All rights reserved.