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

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.