The role of gap states in the energy level alignment at the organic-organic heterojunction interfaces.

The interface properties of organic-organic heterojunctions (OOHs), such as interface energy level alignment (ELA), interfacial charge transfer, interface nanostructuring, molecular orientation and so on, play an essential role in determining the device performance for some technologically important organic electronic devices, encompassing organic solar cells, bipolar organic field-effect-transistors, and organic light-emitting-diodes. The aim of this article is to provide a balanced assessment on the understanding of the ELA at the small-molecule based OOH interfaces with well-defined molecular orientation, with particular emphasis on the role of gap states in organic thin films. A generalized picture of gap states determined ELA at the OOH interfaces is provided and their implications in relevant organic electronic devices have been discussed.