Resonance and localization effects at a dipolar organic semiconductor interface.

The image state manifold of the dipolar organic semiconductor vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite is investigated by angle-resolved two-photon photoemission (AR-TPPE) spectroscopy in the 0-1 monolayer regimes. Interfacial charge-transfer from the image potential state of clean graphite populates a near-resonant VONc anion level, identifiable by the graphite image potential state by its distinct momentum dispersion obtained from AR-TPPE. This affinity level is subject to depolarization by the neighboring molecules, resulting in stabilization of this state with coverage. Near a coverage of one monolayer, a hybrid image potential/anion state is also formed, showing progressive localization with coverage. Intensities for all these features develop rather differently with molecular coverage, pointing towards the different types of charge-transfer interactions at play at this interface.

Influence of electrostatic fields on molecular electronic structure: insights for interfacial charge transfer.

Molecular and interfacial electronic structure at organic semiconductor interfaces shows a rich and subtle dependence on short- and long-range electrostatic interactions. Interface dipoles can be controlled making use of the anisotropic charge distribution at the interface, often with direct consequences also for the molecular electronic structure. In this Perspective, we will discuss the emerging understanding of how local and collective electrostatic effects control energy level alignment and molecular electronic structure at organic semiconductor interfaces and highlight some of the ramifications for interfacial charge-transfer dynamics. Attention is paid to the validity of the underlying assumptions inherent to the classical electrostatic approach.

Image states at the interface with a dipolar organic semiconductor.

Image states of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite are investigated in the submonolayer to few monolayer regime. The presence of a significant molecular dipole in the organized thin films leads to a strong modification of the image states with coverage. In the 0-1 ML regime, we observe successive stabilization of the image state with increasing coverage. Above 1 ML, a new image state develops, corresponding to the screened interaction at the organic semiconductor/substrate interface. We show that the evolution of the observed image states can be understood on the basis of resonance-enhanced anion formation in the presence of strong electric fields. These data represent a step toward understanding the influence of electrostatic fields on electronic structure at organic semiconductor interfaces.