Entropy-driven structural transition and kinetic trapping in formamidinium lead iodide perovskite.
A challenge of hybrid perovskite solar cells is device instability, which calls for an understanding of the perovskite structural stability and phase transitions. Using neutron diffraction and first-principles calculations on formamidinium lead iodide (FAPbI3), we show that the entropy contribution to the Gibbs free energy caused by isotropic rotations of the FA+cation plays a crucial role in the cubic-to-hexagonal structural phase transition. Furthermore, we observe that the cubic-to-hexagonal phase transition exhibits a large thermal hysteresis. Our first-principles calculations confirm the existence of a potential barrier between the cubic and hexagonal structures, which provides an explanation for the observed thermal hysteresis. By exploiting the potential barrier, we demonstrate kinetic trapping of the cubic phase, desirable for solar cells, even at 8.2 K by thermal quenching.
Department of Physics, University of Virginia, Charlottesville, VA 22904, USA.
Tianran
0000-0003-2961-6493
T
Chen
Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22904, USA.
Benjamin J
BJ
Foley
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Changwon
0000-0002-1788-045X
C
Park
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Craig M
0000-0002-9637-9355
CM
Brown
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Leland W
LW
Harriger
Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14853, USA.
Jooseop
0000-0002-4413-5412
J
Lee
Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14853, USA.
Jacob
J
Ruff
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Mina
M
Yoon
Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22904, USA.
Joshua J
JJ
Choi
Department of Physics, University of Virginia, Charlottesville, VA 22904, USA.
Seung-Hun
SH
Lee
Organic-inorganic perovskite; neutron scattering; photovoltaic perovskite