Fig. 1. (Color online) Schematic diagram of FAPbI₃ phases at different temperatures. Reprinted with permission from Ref. [21], Copyright 2018, American Chemical Society.

Fig. 2. (Color online) (a) XRD patterns with different periodic alignments for α- and δ-FAPbI₃ single crystals and powder. (b) Different Raman shift for α- and δ-FAPbI₃. (a) and (b) Reprinted with permission from Ref. [44], Copyright 2023, American Chemical Society. (c) Temperature-dependent steady-state PL spectrum of α-FAPbI₃. (d) The extracted FWHM from steady-state PL spectrum of α-FAPbI₃, and the well fitted red line indicates contributions from inhomogeneous broadening and Fröhlich coupling. (c) and (d) Reprinted with permission from Ref. [38], Copyright 2016, published under the terms of the Creative Commons CC BY license.

Fig. 3. (Color online) (a) Different structures of photoactive α-FAPbI₃ and photo-inactive δ-FAPbI₃. (b) Phonon density of states for the cubic, tetragonal, and hexagonal structures of FAPbI₃, where the blue and red peaks represent the vibrations of the PbI₃ octahedrons and FA⁺ cations, respectively. (a) and (b) Reprinted with permission from Ref. [33], Copyright 2022, American Chemical Society. (c) Schematic diagram of energy bands for STEs. Reprinted with permission from Ref. [24], Copyright 2020, Springer Nature. (d) PL spectra of α- and δ-FAPbI₃. Reprinted with permission from Ref. [44], Copyright 2023, American Chemical Society.

Fig. 4. (Color online) (a)−(c) Real part of the permittivity of FAPbI₃ measured at 100 kHz during temperature cycles at different conditions. Reprinted with permission from Ref. [46], Copyright 2019, American Chemical Society. (d) Thermal diffusivity test of α- and δ-FAPbI₃. Reprinted with permission from Ref. [44], Copyright 2023, American Chemical Society.

Fig. 5. (Color online) (a) Schematic diagram of Gibbs free energy for α- and δ-FAPbI₃. Reprinted with permission from Ref. [58], Copyright 2024, Elsevier. (b) The kinetic diagram with oriented and isotropic FA⁺ for cubic α-FAPbI₃ and hexagonal δ-FAPbI₃. Reprinted with permission from Ref. [59], Copyright 2016, published under the terms of the Creative Commons CC BY−NC license. (c) Phase stability comparison of α-FAPbI₃ films with/without internal strain. Reprinted with permission from Ref. [60], Copyright 2020, Springer Nature. (d) Phase transition diagram of α-FAPbI₃ during compression and decompression. Reprinted with permission from Ref. [22], Copyright 2018, American Chemical Society.

Fig. 6. (Color online) (a) Schematic diagram of phase transition between α- and δ-FAPbI₃. Reprinted with permission from Ref. [58], Copyright 2024, published under the terms of the Creative Commons CC BY−NC−ND license. (b) Schematic diagram of the micro-area δ-to-α phase transition using direct-laser-writing. (c) Relationship between laser power and irradiation time of direct-laser-writing to realize δ-to-α phase transition. (d) Hybrid α-/δ-FAPbI₃ under visible light and UV light constructed by direct-laser-writing, and the relationship between linewidth of the phase transition region and laser power. (b)−(d) Reprinted with permission from Ref. [44], Copyright 2023, American Chemical Society.

Fig. 7. (Color online) (a) Above-gap oscillations in the absorption spectra of FAPbI₃ films at different temperatures. The inset illustrates two mechanisms that may result in the oscillations: quantum confinement in deep wells and periodicity of the superlattice confining potential. Reprinted with permission from Ref. [67], Copyright 2020, Springer Nature. (b) Schematic diagram of α-FAPbI₃ single-crystal photothermal detector array, which denotes as device-α(SC) array, and the crosstalk of the nearest-neighbor and next-nearest-neighbor detection units in the device-α(SC) array. (c) Schematic diagram of hybrid α/δ-FAPbI₃ single-crystal photothermal detector array by direct-laser-writing, which denotes as device-α array, and the crosstalk of the nearest-neighbor and next-nearest-neighbor detection units in the device-α array. (d) Terahertz photothermal proof-of-concept imaging of the device-α(SC) array. (e) Terahertz photothermal proof-of-concept imaging of the device-α array. (b)−(e) Reprinted with permission from Ref. [44], Copyright 2023, American Chemical Society.

Fig. 8. (Color online) (a) SEM images of micro-grating on α-FAPbI₃ film constructed by direct-laser-writing. (b) Schematic diagram of polarization photodetector with micro-grating. (c) Polarization performance of the α-FAPbI₃ photodetector with micro-grating, including angle-dependent photocurrent and linear sensitivity. (a)−(c) Reprinted with permission from Ref. [82], Copyright 2022, John Wiley and Sons.

Fig. 9. (Color online) (a) Schematic diagram of the fabrication process of the α/δ-FAPbI₃ phase junction. (b) HRTEM of the α/δ-FAPbI₃ phase junction. (c) Schematic diagrams of the energy levels of pure α-FAPbI₃, pure δ-FAPbI₃, and the α/δ-FAPbI₃ phase junction. (d) Schematic diagram of the fabrication process of the bilayered δ-FAPbI₃/perovskite films. (e) Current density−voltage curves for devices based on pristine perovskite films and bilayered δ-FAPbI₃/perovskite films. (f) Statistics of PCEs and J_sc for devices based on pristine perovskite films and bilayered δ-FAPbI₃/perovskite films. (g) Evolution of PCEs for devices based on pristine perovskite films and bilayered δ-FAPbI₃/perovskite films under 40% ± 5% relative humidity (RH) at room temperature. (a)−(c) Reprinted with permission from Ref. [87], Copyright 2017, published under the terms of the Creative Commons CC BY−NC license. (d)−(g) Reprinted with permission from Ref. [88], Copyright 2022, John Wiley and Sons.

Fig. 10. (Color online) (a) Schematic diagram of the fabrication process of δ-FAPbI₃ crystals, and the fabrication process of α-FAPbI₃ thin films using δ-FAPbI₃ crystals. (b) SEM image of the synthesized δ-FAPbI₃ crystal. (c) Current density−voltage curves for devices based on intermediate δ-FAPbI₃ single crystals (target) and powder sample (control). (d) Evolution of PCEs for target and control devices under continuous 1 sun illumination. (a)−(d) Reprinted with permission from Ref. [89], Copyright 2023, John Wiley and Sons.