Influence of Surface Ligands on Energetics at FASnI₃/C₆₀ Interfaces and Their Impact on Photovoltaic Performance

Interfacial chemistry and energetics significantly impact the performance of photovoltaic devices. In the case of Pb-containing organic metal halide perovskites, photoelectron spectroscopy has been used to determine the energetic alignment of frontier electronic energy levels at various interfaces present in the photovoltaic device. For the Sn-containing analogues, which are less toxic, no such measurements have been made. Through a combination of ultraviolet, inverse, and X-ray photoelectron spectroscopy (UPS, IPES, and XPS, respectively) measurements taken at varying thickness increments during stepwise deposition of C₆₀ on FASnI₃, we present the first direct measurements of the frontier electronic energy levels across the FASnI₃/C₆₀ interface. The results show band bending in both materials and transport gap widening in FASnI₃ at the interface with C₆₀. The XPS results show that iodide diffuses into C₆₀ and results in n-doping of C₆₀. This iodide diffusion out of FASnI₃ impacts the valence and conduction band energies of FASnI₃ more than the core levels, with the core level shifts displaying a different trend than the valence and conduction bands. Surface treatment of FASnI₃ with carboxylic acids and bulky ammonium substituted surface ligands results in slight alterations in the interfacial energetics, and all surface ligands result in similar or improved PV performance relative to the untreated devices. The greatest PV stability results from treatment with a fluorinated carboxylic acid derivative; however, iodide diffusion is still observed to occur with this surface ligand.