Charge injection and transport across metal-C60 and C60-petacene interfaces: A first-principles study

Recent experiments demonstrated that [60]fullerene (C$_{60})$ molecules adsorbed on metal surfaces provide favorable energy level alignment for both electron and hole injections in the context of light-emitting diode applications [1,2]. The efficient hole injection across C$_{60}$ layers is rather surprising, since C$_{60}$s are highly electron-accepting molecules and should behave as a hole blocking (rather than hole injection) layer. To provide a microscopic understanding of these seemingly contradictory finding, we consider Au--C$_{60}$--pentacene--C$_{60}$--Au molecular junctions using a first-principles computational approach. We find the Fermi level pinning at the Au--C$_{60}$ interfaces and the strongly configuration-dependent charge transport efficiency at the C$_{60}$--pentacene interfaces. The former finding is in agreement with a recent experimental report [2] and our earlier conclusion from the study of polymerized C$_{60}$ wires [3]. We will explain the latter observation based on the nature of charge tunneling across $\pi $--$\pi $ orbitals [1] Lee, J.Y., Appl. Phys. Lett. \textbf{88}, 073512 (2006) [2] Wang, Z.B \textit{et al.}, Appl. Phys. Lett. \textbf{95}, 043302 (2009). [3] Lee, G.I., Kang, J.K., {\&} Kim, Y.-H., J. Phys. Chem. C \textbf{112}, 7029 (2008).