Interface-induced band bending and charge separation in all-organic ZnPc/F$_x$ZnPc heterostructures

TL;DR

This study combines theoretical and experimental methods to show how interface-induced band bending in ZnPc-based heterostructures enhances charge separation, offering insights for improved organic photovoltaic device design.

Contribution

The paper reveals that interface-induced band bending in ZnPc heterostructures promotes charge separation, supported by DFT calculations and UPS measurements, advancing understanding of organic photovoltaic interfaces.

Findings

Band bending causes HOMO and LUMO localization away from the interface.

Type-II band offset confirmed in ZnPc heterostructures.

Wavefunction shapes resemble particle-in-a-box states in stacked molecules.

Abstract

Organic semiconductors are attractive building blocks for electronic devices due to their low cost and flexibility. Furthermore, heterostructures with type-II band alignments can efficiently separate photogenerated charges via a charge transfer and separation process. Here, we use density functional theory (DFT) to investigate model interfaces formed by zinc phthalocyanine (ZnPc) and its fluorinated derivatives (F$_8$ZnPc and F$_{16}$ZnPc). We demonstrate that these interfaces not only exhibit a type-II band offset, but also band bending. The band bending causes both the LUMO and HOMO states to localize away from the interface. Therefore, the band bending creates a strong driving force for charge separation. We used ultraviolet photoemission spectroscopy (UPS) to experimentally confirm this predicted band bending. The wavefunction envelopes of vertically-stacked molecules resemble particle-in-a-box states, but this shape is lost when the molecules are staggered. These results elucidate how interface-induced band bending facilitates charge separation in all-organic heterostructures and suggest a design pathway toward improved performance in organic photovoltaic devices.