A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric

Organic–inorganic hybrid perovskite,
[CH3NH3]­PbI3, holds a great potential
for next-generation
solar devices. However, whether the ferroelectricity exists in [CH3NH3]­PbI3 and results in the ultrahigh
performance is not at present clear. Beyond that, no hybrid lead iodide
perovskite ferroelectric has yet been found. Here, using precise molecular
modifications, we successfully designed a room-temperature hybrid
perovskite ferroelectric, [(CH3)3NCH2I]­PbI3. Because of the high-symmetry and nearly spherical
shape of [(CH3)4N]+ cation, [(CH3)4N]­PbI3 crystallizes in a centrosymmetric
space group P63/m at
room temperature and undergoes a structural phase transition at 184
K. Accompanied by the introduction of halogen atoms on the cation
from F to I, the phase transition temperature gradually increases
to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH3)3NCH2I]–I and the largest volume
of [(CH3)3NCH2I]+ among
these compounds might be possible reasons for the stabilization of
ordered [(CH3)3NCH2I]+ cation array and further reservation of its ferroelectricity at
relatively high temperature. This work provides an efficient molecular
design strategy toward the targeted harvest of room-temperature organic–inorganic
perovskite ferroelectrics, and should inspire further exploration
of the interplay between structure and ferroelectricity. The discovery
of lead iodide perovskite ferroelectric also offers a foothold to
the possibility for the existence of ferroelectricity in [CH3NH3]­PbI3.