Quantum Well Energetics of an n = 2 Ruddlesden–Popper Phase Perovskite

A comprehensive investigation of the electronic energy levels of an n = 2 Ruddlesden–Popper phase perovskite is presented. Ultraviolet and inverse photoemission spectroscopies are used to probe the density of states in the valence and conduction bands, respectively, of the quasi-2D perovskite, butylammonium cesium lead iodide (BA₂CsPb₂I₇). By comparing experimental spectra with calculated projected density of states, the contributions from Cs, Pb, and I to the quantum well states are identified, and distinguished from those of the organic ligand barrier layer. The ionization energy, electron affinity, and exciton binding energy of this material are derived. The energetics of the quantum well structure are discussed in terms of the number of Pb-halide layers. The resulting energy diagram suggests that a type-I heterojunction would be formed with the n = 1 BA₂PbI₄. Finally, surface photovoltage performed via Kelvin probe force microscopy is used to evaluate band bending at the surface of the BA₂CsPb₂I₇ thin films.