Band Edges of Hybrid Halide Perovskites under the Influence of Mixed-Cation Approach: A Scanning Tunneling Spectroscopic Insight

Mixed-cation perovskite thin films are prepared and characterized by introducing formamidinium (FA⁺) and cesium (Cs⁺) ions at the methylammonium (MA⁺) site of the traditional methylammonium lead triiodide (MAPbI₃) perovskite structure. From scanning tunneling spectroscopy and density of states spectra thereof, the exact band edges of the perovskites with all possible binary cation combinations have been estimated. From the obtained band locations, we could correlate the change in band energies with the size of the cation and have discussed the results in terms of orbital overlap. The band edge(s) responsible for the change in band gap in the multication systems could be identified and correlated with the origin of the bands. The obtained band energies provided the band gap of mixed-cation perovskites that are commonly derived from optical absorption spectroscopy. From the perspective of device applications, the results are hence useful in drawing band diagrams and designing band-engineered mixed-cation hybrid perovskites for solar cell applications.