Pressure-Induced Polymorphic, Optical, and Electronic Transitions of Formamidinium Lead Iodide Perovskite

Formamidinium lead iodide (FAPbI₃) perovskite as a superior solar cell material was investigated in two polymorphs at high pressures using in situ synchrotron X-ray diffraction, FTIR spectroscopy, photoluminescence (PL) spectroscopy, electrical conductivity (EC) measurements, and ab initio calculations. We identified two new structures (i.e., Imm2 and Immm) for α-FAPbI₃ but only a structural distortion (in C2/c) for δ-FAPbI₃ upon compression. A pressure-enhanced hydrogen bond plays a prominent role in structural modifications, as corroborated by FTIR spectroscopy. PL measurements and calculations consistently show the structure and pressure dependences of the band gap energies. Finally, EC measurements reveal drastically different transport properties of α- and δ-FAPbI₃ at low pressures but a common trend to metallic states at high pressures. All of these observations suggest strongly contrasting structural stabilities and pressure-tuned optoelectric properties of the two FAPbI₃ polymorphs.