Effect of charge-transfer complex on the energy level alignment between graphene and organic molecules

We performed density-functional theory calculations to study the electronic structures at the interfaces between graphene and organic molecules that have been used in organic light-emitting diodes. In terms of work function, graphene itself is not favorable as either anode or cathode for commonly used electron or hole transport molecular species. However, the formation of charge transfer complex on the chemically inert sp(2) carbon surface can provide a particular advantage. Unlike metal surfaces, the graphene surface remains non-bonded to electron-accepting molecules even after electron transfer, inducing an improved Fermi-level alignment with the highest-occupied-molecular-orbital level of the hole-injecting-layer molecules.This work was supported by the 2011 Research Fund of the UNIST. N.P. and H.J. were supported by the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (MEST) (Grant No. 2009-0087731). G.B. and W.P. were supported by the Basic Science Research Program (No. 2011-0006268). S.H. and J.P. were supported by Priority Research Centers Program (No. 2011-0018395) and the Converging Research Center Program (No. 2011K000620)