Researchers at the Institute of Materials Science in Barcelona (ICMAB-CSIC) and the Helmholtz-Zentrum Berlin für Materialien und Energie (Germany) have used a unique microscopy technique to demonstrate that perovskites are not ferroelectric, as was thought.

The new technique, patented by CSIC in 2017, is the direct piezoelectric force microscopy (DPFM) which, for the first time, is used in lead halide perovskite solar cells.

In the case of lead halide perovskites, ferroelectricity could help to understand their potential for active materials in solar cells, and in fact, that was quite a plausible explanation so far. However, the study demonstrates, for the first time, that the fact that these materials are optimal for solar cells is not due to ferroelectricity. "This work is very interesting for understanding why these cells are so efficient," says Andrés Gómez, researcher at the ICMAB-CSIC and first author of the article. It seems that the final answer was not yet found.

The technique used to elucidate the non-ferroelectricity of lead halide perovskites is the DPFM (direct piezoelectric force microscopy) technique. "Until now there was only one advanced mode of atomic force microscopy (AFM) called piezoresponse force microscopy (PFM) to study the ferroelectricity of these samples. However, this mode has caused a lot of controversy, as it is not reliable enough to distinguish between a ferroelectric material and one which is not. Although it is possible to measure ferroelectricity with PFM, other effects can give a false signal, obtaining erroneous results", explains Gómez.

However, the DPFM technique, introduced in 2017 at the ICMAB-CSIC, complementary to PFM, measures the piezoelectric effect in a direct way and allows to clearly discern if a sample is ferroelectric or not. The technique does not produce spurious signals, since it excludes many measuring artifacts because via the piezoelectric effect a mechanical energy is directly converted into electrical energy in a strictly proportional way. This fact is fundamental for examining the existence of ferroelectricity in lead halide perovskites.

For this study, polycrystalline samples of lead halide perovskites and samples of other materials with known ferroelectricity used as control were analyzed, and experiments were conducted with perovskites with different properties (grain size, layer thickness, different substrates, different textures...) using PFM and DPFM, and even EFM (electrostatic force microscopy).

This is the first time that the DPFM technique was used in lead halide perovskite solar cells. "No other research group has been able, with nanometer-scale resolution, to elucidate whether these cells are really ferroelectric or not," says Gómez.