Resolving the energy alignment between methylammonium lead iodide and C60: an in-situ photoelectron spectroscopy study

Understanding and controlling the energy level alignment at interfaces between lead halide perovskites and electron transport layers is crucial for optimizing charge extraction by minimizing recombination losses in high-efficiency perovskite solar cells. In this work, we investigated the energy level alignment of C60 on in-situ cleaved MAPbI3 single crystals in multiple repeat experiments using photoelectron spectroscopy aiming to resolve inconsistencies reported in earlier studies. Our results show that both materials remain chemically stable upon interface formation, the strong reactions typically seen when metals contact perovskites. By analyzing Pb 4f and C 1s core level positions in detail, we determined that C60 consistently exhibits a downward energy shift toward MAPbI3, which works against efficient charge extraction. The magnitude of this shift, however, is highly sensitive to the surface composition, highlighting that small variations may lead to significant differences in results. At higher C60 coverages of more than 5 monolayers, a constant HOMO-valence band offset of 0.52 eV was obtained. Assuming a 1.86 eV HOMO-LUMO gap, the C60 LUMO is 0.25 eV below the MAPbI3 conduction band, a value favorable for charge extraction. These findings underscore the decisive role of surface chemistry on interfacial energetics, explain performance variability in perovskite devices, and demonstrate the need to control and accurately measure surface properties. Furthermore, the observed energetic alignment can explain why further interface modification by charge blocking layers or surface passivation is needed for optimized device efficiencies.