Molecular orientation and electronic structure at organic heterojunction interfaces

Due to the highly anisotropic nature of π-conjugated molecules, the molecular orientation in organic thin films can significantly affect light absorption, charge transport, energy level alignment (ELA) and hence device performance. Synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy represents a powerful technique for probing molecular orientation. The aim of this review paper is to provide a balanced assessment on the investigation of molecular orientation at the organic-organic heterojunction (OOH) interface by NEXAFS, as well as the gap-states mediated orientation dependent energy level alignment at OOH interfaces. We highlight recent progress in elucidating molecular orientation at OOH interfaces dominated by various interfacial interactions, gap-states controlled orientation dependent energy level alignments at OOH interfaces, and the manipulations of molecular orientation and ELA in OOH.