Structural and Thermal Properties in Formamidinium and Cs-Mixed Lead Halides

Abstract

Formamidinium [FA, HC­(NH2)2 +] lead iodide and its cation mixture have attracted interest as potentials in applications for efficient solar cells superior to well-known methylammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA1–x Cs x PbI3 for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α-FA0.9Cs0.1PbI3 as well as α-FAPbI3 crystallize in a cubic Pm3̅m structure with orientationally disordered FA molecules confined in the nondistorted Pb–I framework. In FAPbI3, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between β- and γ-phases instead of the previously known single β–γ transition. After doping with 10% Cs, the multiple first-order transitions disappeared, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.

Impact of Orientational Glass Formation and Local Strain on Photo-Induced Halide Segregation in Hybrid Metal-Halide Perovskites

“…With decreasing temperature from T = 300 K, the PL spectra narrow and start to redshift from T = 150 K, with additional anomalous broadening ( Supporting Information section S4, Figure S8a), likely connected to the low-energy rattling of the MA cation. 54 We observe no anomalous broadening of the PL for the x = 0.2 and x = 0.5 compositions ( Figure S8b,c ), in agreement with a reduction in MA cation motion as a result of preferential alignment and a smaller unit cell volume compared to x = 0. In contrast to x = 0, the x = 0.5 composition shows a sharp jump in the spectral peak position around T = 180 K ( Figure 6 c), which can be attributed to emission from Br-rich regions following photo-induced halide segregation.…”

Advances to High‐Performance Black‐Phase FAPbI3 Perovskite for Efficient and Stable Photovoltaics

“…Reproduced with permission. [ 26 ] Copyright 2019, American Chemical Society. e) Scheme on the phase and polymorphism behavior of FAPbI 3 .…”

“…[ 24,25 ] Figure 1d showed the comparison between the temperature dependence of heat capacity for FAPbI 3 and FA 1– x Cs x PbI 3 with different phases. [ 26 ] It can be observed that there are additional two first‐order transition between β‐ and γ‐phases in FAPbI 3 . The disappearance of multiple transitions in FA 1– x Cs x PbI 3 indicates that the cation doping can affect the structural transitions of FA‐based perovskite.…”

Intrinsically Stable Wide Band Gap Br-Free CsxFA1-xPbI3 (x=0.5-0.9) Perovskites Overcoming Phase Segregation

“…(25-28) However, depositing high quality CsxFA1-xPbI3 (x>0.5) with both high PV performance and no phase segregation is still challenging because of the large lattice mismatch and different phase transition temperature between FAPbI3 and CsPbI3 perovskites. (29,30) Herein we demonstrate intrinsically stable Br free phase pure CsxFA1-xPbI3 (x=0.5-0.9) perovskites with tunable wide band gap, which did not exhibit any phase segregation under thermal or light stress. Specifically, the Br free Cs0.7FA0.3PbI3 perovskite with 1.65 eV band gap exhibits intrinsically thermal and photo stability in comparison with the typical I-Br mixed halide perovskites of MAPb(I0.8Br0.2)3 and Cs0.25FA0.75Pb(I0.8Br0.2)3.…”