Using electroabsorption to measure changes in exciton binding energy of x-phenylethylammonium tin iodide (PESI, x=2-F, 3-F, 4-F, and 2-CF3)*

The influence of octahedral tilt in 2D metal halide perovskite materials on exciton binding energy is studied using x-phenylethylammonium tin iodide (x-PESI) compounds, with "x" being 3-F, 4-F, 2-CF3, and 2-methyl. In 2d these materials, 2D layers of inorganic metal and halogen atoms are separated by organic molecules. The "3-F", etc., nomenclature indicates the position of the fluorine or trifluoromethyl group on the benzene ring of the phenylethylammonium molecules. These molecular substitutions affect the structure of the inorganic metal halide layers by inducing octahedral tilting. We use electroabsorption (EA) spectroscopy to measure changes in absorption with and without an applied electric field to determine exciton binding energies. A red-shift in the 1s exciton absorption peak with field is observed, coming from the Stark effect, producing an oscillation in the EA signal as a function of photon energy. The Franz-Keldysh (FK) effect further changes the bandgap absorption, producing additional oscillations in the EA signal, with the bandgap energy can be then determined by the first zero crossing of these FK oscillations. The exciton binding energy is calculated as the difference between the bandgap energy and the 1s exciton absorption peak energy. Octahedral tilt appears to affect binding energy, with a transition observed around 100-150 K, potentially due to phase transitions or other effects.