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All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Hot-Carrier Photoluminescence in Low-Temperature CsSnI3 Nanocrystals

In the halide perovskites, the B-site metal lone-pairs electrons are strongly suspected to underpin many of the interesting properties of the inorganic halide perovskites. Recently, a stable low-temperature monoclinic polar phase was predicted for CsSnI3, opening the possibility of direct investigation of both these lone pair electrons and a ferroelectric distorted structure. To date there are no other reports of such a structure in CsSnI3, and a known low-temperature monoclinic structure in CsSnBr3 remains unexplored. We have found optical evidence of a transformation occurring around 240 K in CsSnI3 nanocrystals, with several changes in optical behavior below this transition point, including novel high-energy photoluminescene and new states in the transient absorption spectrum. We have successfully characterized the optical properties of this low-temperature phase and found evidence for a polar, monoclinic structure. Discovery of a stable monoclinic polar structure in the halide perovskites opens many new potential directions for further research and electronics applications.

14 SOLAR ENERGY↗
DOE OSTI2020

Materials Data on CsSnI3 by Materials Project

CsSnI3 crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Cs1+ is bonded in a 9-coordinate geometry to nine I1- atoms. There are a spread of Cs–I bond distances ranging from 3.98–4.33 Å. Sn2+ is bonded to six I1- atoms to form edge-sharing SnI6 octahedra. There are a spread of Sn–I bond distances ranging from 3.04–3.45 Å. There are three inequivalent I1- sites. In the first I1- site, I1- is bonded in a 5-coordinate geometry to two equivalent Cs1+ and three equivalent Sn2+ atoms. In the second I1- site, I1- is bonded in a 5-coordinate geometry to four equivalent Cs1+ and one Sn2+ atom. In the third I1- site, I1- is bonded in a 5-coordinate geometry to three equivalent Cs1+ and two equivalent Sn2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CsSnI3 by Materials Project

CsSnI3 is Orthorhombic Perovskite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Cs1+ is bonded in a 7-coordinate geometry to seven I1- atoms. There are a spread of Cs–I bond distances ranging from 4.05–4.43 Å. Sn2+ is bonded to six I1- atoms to form corner-sharing SnI6 octahedra. The corner-sharing octahedra tilt angles range from 6–21°. There are two shorter (3.16 Å) and four longer (3.17 Å) Sn–I bond lengths. There are two inequivalent I1- sites. In the first I1- site, I1- is bonded in a distorted see-saw-like geometry to two equivalent Cs1+ and two equivalent Sn2+ atoms. In the second I1- site, I1- is bonded to three equivalent Cs1+ and two equivalent Sn2+ atoms to form a mixture of distorted corner and edge-sharing ICs3Sn2 square pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CsSnI3 by Materials Project

CsSnI3 is (Cubic) Perovskite structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Cs1+ is bonded to twelve equivalent I1- atoms to form CsI12 cuboctahedra that share corners with twelve equivalent CsI12 cuboctahedra, faces with six equivalent CsI12 cuboctahedra, and faces with eight equivalent SnI6 octahedra. All Cs–I bond lengths are 4.45 Å. Sn2+ is bonded to six equivalent I1- atoms to form SnI6 octahedra that share corners with six equivalent SnI6 octahedra and faces with eight equivalent CsI12 cuboctahedra. The corner-sharing octahedral tilt angles are 0°. All Sn–I bond lengths are 3.14 Å. I1- is bonded to four equivalent Cs1+ and two equivalent Sn2+ atoms to form a mixture of distorted corner, edge, and face-sharing ICs4Sn2 octahedra. The corner-sharing octahedra tilt angles range from 0–60°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CsSnI3 by Materials Project

CsSnI3 crystallizes in the tetragonal P4/mbm space group. The structure is three-dimensional. there are two inequivalent Cs1+ sites. In the first Cs1+ site, Cs1+ is bonded in a body-centered cubic geometry to eight I1- atoms. There are four shorter (4.10 Å) and four longer (4.41 Å) Cs–I bond lengths. In the second Cs1+ site, Cs1+ is bonded in a distorted body-centered cubic geometry to eight I1- atoms. There are four shorter (4.10 Å) and four longer (4.41 Å) Cs–I bond lengths. There are two inequivalent Sn2+ sites. In the first Sn2+ site, Sn2+ is bonded to six I1- atoms to form corner-sharing SnI6 octahedra. The corner-sharing octahedra tilt angles range from 0–18°. There are two shorter (3.15 Å) and four longer (3.16 Å) Sn–I bond lengths. In the second Sn2+ site, Sn2+ is bonded to six I1- atoms to form corner-sharing SnI6 octahedra. The corner-sharing octahedra tilt angles range from 0–18°. There are two shorter (3.15 Å) and four longer (3.16 Å) Sn–I bond lengths. There are two inequivalent I1- sites. In the first I1- site, I1- is bonded in a distorted see-saw-like geometry to two equivalent Cs1+ and two Sn2+ atoms. In the second I1- site, I1- is bonded to four Cs1+ and two equivalent Sn2+ atoms to form a mixture of distorted corner and edge-sharing ICs4Sn2 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Dual Photoluminescence in Low-Temperature Phase of CsSnI 3 Nanocrystals

The expression of metal lone-pair electrons is hypothesized to underpin many of the interesting properties of inorganic halide perovskite semiconductors. Recently, a stable low-temperature monoclinic polar phase was predicted for CsSnBr 3 and CsSnI 3 , opening the possibility of direct investigation of a ferroelectric distorted structure compared to the undistorted structure. To date, there have been no experimental reports of such a structure in CsSnI 3 , and the low-temperature optical properties of CsSnI 3 nanocrystals have remained unexplored. Here we report optical and structural evidence of a phase transition around 240 K in 8.9 nm CsSnI 3 nanocrystals. Several changes in optical behavior occur below this transition point, including high-energy photoluminescence (PL) that emits concurrently with the exciton PL. The emergence of this high-energy PL is correlated with X-ray diffraction (XRD) and differential scanning calorimetry (DSC) supporting a phase transition from the orthorhombic structure between 240-200 K. Transient absorption measurements show an increase in the excited state lifetimes, i.e., slowed carrier cooling, at 200 K when photoexciting with photon energies above the high-energy state, consistent with slowed carrier cooling and emergence of high-energy PL. We hypothesize that the slowed carrier cooling is distinctive to this phase transition that modifies both the electronic and phonon structures that dictate excited-state carrier dynamics, and we discuss these changes.

14 SOLAR ENERGY↗
DOE OSTI2021

Phase transition dynamics in one-dimensional halide perovskite crystals

Triiodide perovskites CsPbI 3 , CsSnI 3 , and FAPbI 3 (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 CsSnI 3 and FAPbI 3 nanowires. We reveal a large blueshift in the photoluminescence (PL) peak (~38 meV) at the low-T/high-T two-phase interface of partially transitioned FAPbI 3 wire, which may result from the lattice distortion at the phase boundary. Compared to the experimentally derived activation energy of CsSnI 3 (~1.93 eV), the activation energy of FAPbI 3 is 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 CsSnI 3 is observed to be relatively high, which may be attributed to a high concentration of Sn vacancies. Furthermore, 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.

36 MATERIALS SCIENCE↗