CH3NH3PbBr3 is not pyroelectric, excluding ferroelectric-enhanced photovoltaic performance

Abstract

To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr3). We find that MAPbBr3 is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr3 semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr3 solar cells.

Tetragonal CH 3 NH 3 PbI 3 is ferroelectric

“…This phenomenon is called "pyroelectricity" and is direct evidence for existing polarity. Earlier, we showed clear experimental evidence that at room temperature (RT), cubic MAPbBr 3 is not pyroelectric (22). Here, we prove experimentally that tetragonal MAPbI 3 , the stable RT phase of this material, is pyroelectric and thus polar.…”

“…As in our earlier work on MAPbBr 3 (22), we used periodic pulse pyroelectric measurements (45)(46)(47) to determine whether tetragonal MAPbI 3 is polar along its c axis. Pyroelectric currents can be distinguished from other possible thermally stimulated electric response (TSER) (thermoelectricity or flexoelectricity) (22,48) by heating once one, and once the other side of the crystal periodically. Pyroelectric currents will reverse their sign once a crystal, together with its electrical leads, is flipped (by 180°o n its other face).…”

“…the domain structure that typifies a ferroelectric material are found to be quite contradictory (22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). To analyze the question whether HaPs are actually ferroelectric, we need to test for the presence/absence of several fundamental phenomena that are tightly related to it.…”

“…The existence of polar domains induces charge separation, lowers charge recombination, and allows high conductivity due to local degeneracy of the SC along the domain walls (19,20). If this is so, then it can explain the remarkably high voltage efficiency, that is, low voltage loss in HaP-based solar cells [estimated as the difference between, or ratio, of open-circuit voltage and band gap: (E G − V OC ) and (V OC /E G ), respectively (21,22)], especially for those with iodide as the halide. A further indication for a low recombination rate of photogenerated carriers is the respectable voltage and current at maximum power that these HaP-based devices possess, which makes the question whether HaPs can be ferroelectric of more than fundamental scientific interest.…”

Absence of ferroelectricity in methylammonium lead iodide perovskite

“…19,27,28 Recently, there have been numerous conflicting reports regarding ferroelectricity in perovskites. [29][30][31][32] The controversy over the ferroelectricity of methylammonium lead iodide (CH3NH3PbI3) perovskite layer closely resembles the dispute over the ferroelectricity of bananas 33 and highlights the extra care that should be taken in the interpretation of electrical polarization loops and the claim of ferroelectricity. 34 The prerequisite of ferroelectricity of a compound is that the crystal structure must be non-centrosymmetric hence exhibiting a strong optical second harmonic generation (SHG).…”