Ferroelectric materials are characterized by a switchable macroscopic polarization. A wide number of perovskite oxides have ferroelectric behavior. In contrast, the existence of ferroelectricity in organic-inorganic perovskite thin films has been matter of intense debate over the past few years.
If proven to exist, its magnitude in comparison to other known processes occurring in perovskites, such as electronic and ionic conductivity, will determine its importance for their optoelectronic properties and, in particular, for their unprecedented high photovoltaic performance. In this work, we aimed at contributing to the understanding of this topic by extending the range of measurements beyond what is conventionally carried out.
In particular, we employed three techniques, namely, direct piezoelectric force microscopy (DPFM), piezoelectric force microscopy (PFM), and electrostatic force microscopy (EFM), which we applied to two perovskite materials, prepared with control of film thickness and grain sizes, enabling the evaluation of morphological differences. Based on our experimental data and by comparing to well-known ferroelectric materials, we concluded that lead halide perovskites are ferroelectricity-free. Hence, further work is necessary to pinpoint the physical reasons for the unprecedented success of this new class of photovoltaic materials.
Ferroelectricity-free lead halide perovskites
Andrés Gómez, Qiong Wang, Alejandro R. Goñi, Mariano Campoy-Quiles, Antonio Abate
Energy and Environment Science 12, 2537−2547, 2019
Figure: (a) Scheme of the DPFM measurement on a ferroelectric sample with antiparallel domain configuration. The current signal recorded in DPFM should reverse its sign when the tip crosses different domains, depending upon scan direction. (b) DPFM images obtained for periodically poled lithium niobate (PPLN), for which the current sign is reversed as the scan direction changes, exactly as expected for conventional ferroelectric materials. (c) DPFM images of the CsFAMA tri-cation perovskite scanned under similar conditions to those of PPLN. The CsFAMA perovskite does not show any sign reversal, resembling typical current-sensing AFM mappings.