Interface-Induced Band Bending and Charge Separation in All-Organic ZnPc/F x ZnPc Heterostructures

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 (F8ZnPc and F16ZnPc). We demonstrate that these interfaces exhibit not only a type-II band offset but also band bending. The band bending causes both the LUMO and the HOMO states to localize away from the interface. Therefore, band bending creates a strong driving force for charge separation. We used ultraviolet photoemission spectroscopy (UPS) to experimentally confirm this predicted band bending. The wave function 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 an improved performance in organic photovoltaic devices.