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.