Research ArticleMATERIALS SCIENCE

Revealing the detailed path of sequential deposition for metal halide perovskite formation

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Science Advances  02 Feb 2018:
Vol. 4, no. 2, e1701402
DOI: 10.1126/sciadv.1701402
  • Fig. 1 XRD analysis of PbI2 crystallization and perovskite formation.

    (A) XRD spectra of PbI2 samples dipped in MAI for 2, 4, 8, 25, 50, 200, and 400 s, showing the (001) reflection of the 2H polytype of PbI2 and the (002) and (110) reflections of the tetragonal perovskite. (B) FWHM of the (001) reflection of PbI2 and of the (110) reflection of the perovskite versus time for the spectra shown in (A). a.u., arbitrary units.

  • Fig. 2 Schematic depicting the stages of the reaction in sequential deposition.

    Dashed arrows indicate mass transfer. (A) Nucleation and growth of PbI2. (B) Intercalation of MAI and structural reorganization to form CH3NH3PbI3 perovskite. (C) Ostwald ripening where perovskite from the mesoporous layer is transported to the capping layer. (D) Further Ostwald ripening at longer dipping times where perovskite from the small crystals in the capping layer is transported to larger ones. (E) Gibbs free energy shown as a function of the reaction coordinate.

  • Fig. 3 Perovskite formation with time studied using SEM.

    (A) Unreacted PbI2 film. (B) Sample dipped for 2 s showing the growth of nuclei as hexagonal platelets of PbI2. (C) Sample dipped for 4 s showing hexagonal platelets of PbI2 on the crystal surfaces. (D) Sample dipped for 8 s showing the textured crystals. Scale bars, 0.5 μm (A to C) and 1 μm (D).

  • Fig. 4 CL study on films of intermediate conversion.

    (A) SEM image of sample dipped for 10 s with clusters visible on the crystals. Scale bar, 0.5 μm. (B) CL spectra taken at two different points on the same sample. PbI2 emission between 480 and 550 nm is assigned green, and perovskite emission between 720 and 810 nm is assigned red. (C) Pseudocolor CL image overlay on a SEM image of the same sample showing mixed crystalline aggregates composed of perovskite and PbI2. Scale bar, 0.5 μm. (D) Pseudocolor CL image overlay on a SEM image. The crystal is perovskite, whereas the clusters (indicated with arrows) on it are identified as PbI2. Scale bar, 0.5 μm.

  • Fig. 5 CLSM of samples with a 2.5-μm mesoporous Al2O3 layer.

    Emission between 500 and 550 nm attributed to PbI2 was assigned green, and emission between 700 and 800 nm attributed to perovskite was assigned red. The color saturation scales with the emission intensity. (A) Schematic showing planes parallel to the sample surface and the cross section, showing the different parts of the sample being imaged. (B) Composite pseudocolor image of the surface of a sample of intermediate conversion dipped for 60 s. Scale bar, 5 μm. (C) Composite pseudocolor cross-sectional image of the same sample showing the perovskite capping layer over the unconverted PbI2 in the mesoporous layer. Scale bar, 1 μm. (D) Normalized emission intensity showing horizontally averaged values of PbI2 emission and perovskite emission across the depth of the sample in (C). (E) Composite pseudocolor cross-sectional image of a sample of nearly complete conversion showing trapped, unconverted PbI2 in the mesoporous layer. Scale bar, 2 μm.

  • Fig. 6 In situ kinetic data of perovskite formation.

    (A) Isothermal kinetic data at 10°, 15°, and 25°C shown as the conversion fraction (α). (B) Isothermal reaction rate (dα/dt) shown as a function of conversion fraction (α) for the data in (A). (C) Kinetic data for samples of low and high PbI2 loading dipped under light and in the dark, shown as the conversion fraction (α).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/e1701402/DC1

    Supplementary Text

    fig. S1. XRD spectra of unreacted PbI2 sample and sample dipped in MAI solution for 4 s in the dark.

    fig. S2. CL study on a sample dipped for 60 s for nearly complete conversion.

    fig. S3. SEM images of perovskite films with increasing dipping times showing Ostwald ripening.

    table S1. FWHM of the (001) reflection of the 2H polytype of PbI2 and the corresponding height of the platelet from the Scherrer equation.

    Reference (32)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. XRD spectra of unreacted PbI2 sample and sample dipped in MAI solution for 4 s in the dark.
    • fig. S2. CL study on a sample dipped for 60 s for nearly complete conversion.
    • fig. S3. SEM images of perovskite films with increasing dipping times showing Ostwald ripening.
    • table S1. FWHM of the (001) reflection of the 2H polytype of PbI2 and the corresponding height of the platelet from the Scherrer equation.
    • Reference (32)

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