UC Berkeley

Triiodide perovskites CsPbI3, CsSnI3, and FAPbI3(where FA is formamidinium) are highly promising materials for a range of optoelectronic applications in energy conversion. However, they are thermodynamically unstable at room temperature, preferring to form low-temperature (low-T) non-perovskite phases with one-dimensional anisotropic crystal structures. While such thermodynamic behavior represents a major obstacle toward realizing high-performance devices based on their high-temperature (high-T) perovskite phases, the underlying phase transition dynamics are still not well understood. Here we use in situ optical micro-spectroscopy to quantitatively study the transition from the low-T to high-T phases in individual CsSnI3and FAPbI3nanowires. We reveal a large blueshift in the photoluminescence (PL) peak (∼38 meV) at the low-T/high-T two-phase interface of partially transitioned FAPbI3wire, which may result from the lattice distortion at the phase boundary. Compared to the experimentally derived activation energy of CsSnI3(∼1.93 eV), the activation energy of FAPbI3is relatively small (∼0.84 eV), indicating a lower kinetic energy barrier when transitioning from a face-sharing octahedral configuration to a corner-sharing one. Further, the phase propagation rate in CsSnI3is observed to be relatively high, which may be attributed to a high concentration of Sn vacancies. Our results could not only facilitate a deeper understanding of phase transition dynamics in halide perovskites with anisotropic crystal structures, but also enable controllable manipulation of optoelectronic properties via local phase engineering.