Interface-induced band bending and charge separation in all-organic ZnPc/F_xZnPc 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 (CT) process. All-organic heterostructures combine the benefits of improved CT, while keeping low cost and flexibility. However, precise details of the CT mechanisms for all-organic heterostructures are still missing.

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 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. When the molecules are stacked vertically, the wavefunction envelopes 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.