Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell

The interfacial band alignment among boron subnaphthalocyanine chloride (SubNc), boron subphthalocyanine chloride (SubPc), and α-sexithiophene (α-6T) is explored using ultraviolet, inverse, and X-ray photoemission spectroscopies (UPS, IPES, and XPS, respectively). With these tools, the ionization energy (IE) and electron affinity (EA) for each material are determined. Layer-by-layer deposition of SubPc and SubNc on α-6T as well as SubPc on SubNc, combined with UPS and IPES, allows for the direct determination of the energy level alignment at the interfaces of interest. A small dipole is found at the α-6T/SubNc/SubPc interface, expanding the donor-LUMO to acceptor-HOMO gap and explaining the large open circuit voltage obtained with these devices. However, there is a small electron barrier between SubNc and SubPc, which may limit the efficiency of electron extraction in the current device configuration. Excess chlorine may be responsible for the high IE and EA found for SubNc and could potentially be remedied with improved synthetic methods or further purification.