Significant reduction in the hole-injection barrier by the charge-transfer state formation: Diindenoperylene contacted with silver and copper electrodes

Abstract

Charge-carrier injection in organic electronics is critically influenced by the interface electronic states formed at the organic semiconducting thin films and metal electrode contacts. A practical solution for the issue is to form a charge-transfer (CT) state at the interfaces, which will produce a substantial density of gap states in the vicinity of the Fermi level of metal electrodes and thus reduce significantly the charge-injection barrier at the interfaces. In this study, we use ultraviolet photoelectron spectroscopy and demonstrate the formation of a CT state at diindenoperylene (DIP) and Ag (or Cu) electrode contacts. Remarkably, the CT state occurs irrespective of the electrode crystallinity, crystal grain orientations, or the molecular orientations in the DIP film, hence both the top- or bottom-metal contact conditions result in the formation of the CT state. Theoretical calculation simulations show the sign of strong interaction between those metal atoms and DIP leading to the CT state formation, analogous to the surface-induced aromatic stabilization proposed by Heimel et al. in a literature [Nat. Chem., 5(2013)187.]. Our results aid in understanding the mechanism of the CT state formation and propose the DIP-CT interlayers being novel candidates for the efficient hole-injection layers used in organic electronics.