Probing Ferroelectricity in Hybrid Halide Perovskites through Tunneling Spectroscopy

The existence of ferroelectricity in the archetypal perovskite, namely, methylammonium lead iodide (MAPbI₃), remains in debate due to overlapping effects from ion migrations, charge accumulation, and ferroelasticity. We introduce the measurement of tunneling current in scanning tunneling microscopy (STM) and surface potential in Kelvin probe force microscopy (KPFM) upon voltage-induced poling to probe polarization dynamics in the perovskite films. While a poling-dependent tunneling current was observed in the STM, measurements with KPFM reveal a flip in the surface potential upon poling, indicating the presence of remnant polarization at the nanoscale of the films. Transient tunneling current and KPFM responses provide compelling evidence of ferroelectricity in unstrained MAPbI₃ films, ruling out contributions from ferroelasticity or ion migration and establishing them as powerful techniques to probe dipole-mediated functionalities in a broader sense. The findings thereby open pathways for the integration of polarization phenomena into optoelectronic devices, including perovskite solar cells.