Influence of Surface Termination on the Energy Level Alignment at the CH3NH3PbI3 Perovskite/C60 Interface

The impressive photovoltaic performance of hybrid iodide CH3NH3PbI3 perovskite relies, among other factors, on the optimal alignment of the electronic energy levels of the semiconductor with respect to conventional hole transporting (HTM) and electron transporting (ETM) materials. Here, we first report on density functional theory electronic structure calculations of slab models of the (001) surface aiming to assess how the perovskite valence and conduction band edge (VBE and CBE) energies depend on the nature of the surface exposed to vacuum. We find that the surface termination plays a critical role in determining the energies of the frontier crystal orbitals, with PbI-terminated surface showing VBE and CBE energy similar to 1 eV below the corresponding levels in the methylammonium-terminated surfaces. We next build perovskite/C60 interfaces based on two such surfaces and discuss the associated electronic structure in light of recent experimental data. The two interfaces are rather inert showing limited band bending/shifts with respect to the isolated components, in line with photoelectron spectroscopy data. They, however, yield very different electron extraction energies, possibly explaining the different behaviors reported in the literature.