Organic–Organic Heterojunction Interfaces: Effect of Molecular Orientation

Organic–organic heterojunctions (OOHs) are critical features in organic light‐emitting diodes, ambipolar organic field‐effect transistors and organic solar cells, which are fundamental building blocks in low‐cost, large‐scale, and flexible electronics. Due to the highly anisotropic nature of π‐conjugated molecules, the molecular orientation of organic thin films can significantly affect the device performance, such as light absorption and charge‐carrier transport, as well as the energy level alignment at OOH interfaces. This Feature Article highlights recent progress in the understanding of interface energetics at small molecule OOH interfaces, focusing on the characterization and fabrication of OOH with well‐defined molecular orientations using a combination of in situ low‐temperature scanning tunneling microscopy, synchrotron‐based high‐resolution ultraviolet photoelectron spectroscopy and near‐edge X‐ray absorption fine structure measurements. The orientation dependent energy level alignments at the OOH interfaces will be discussed in detail.