Perovskite Large Scale Synthesis

Research has shown that the variation of composition PbBr2:MAmBr:OAmBr and reaction conditions (injection speed, temperature and growth time) results in various morphologies, predominantly influenced and controlled by long alkyl-chain cations.[8][9][10][11][18] The ligand-assisted synthesis^[8] was modified to a suitable recipe wherein the perovskite formation could be detected in an adequate time window. The perovskite crystal formation as well as the buildup of ligand-stabilized surfaces/facets can be demonstrated.

The synthesis of CH3NH3PbBr3 perovskites was carried out with a fixed solvent/anti-solvent ratio of DMF 1:5 Toluene at room temperature. The precursors were dissolved in DMF and the slowly injected into toluene. Systematical tests by adjusting the molar ratio between PbBr2, MAmBr and OAmBr to 0.1:0.1:0.3 (P03), 0.1:0.3:0.1 (P02) and 0.3:0.1:0.1 (P01) give an overview of the perovskite formation Instantaneous precipitation and a color change to yellow/orange was observed. The synthesis shows a gradual aggregation of the product arising from larger aggregates which precipitate out from the toluene solution. The dispersion was placed on carbon-coated copper grids. After solvent evaporation in air, the detection of products at advanced growth stages was observed. The sample P03 is shown infigure 30.

Figure 30: TEM images ((a)–(c)) show the formation of nanocrystal islands prepared with ratio PbBr2:OAmBr:MAmBr0.1:0.3:0.1 (P03) in toluene (5.3 nm ± 1.0 nm in diameter).

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We obtained islands of well-separated spherical nanospheres with 3-fold excess of OAmBr. The nanocrystals show a size distribution of 5.3 nm ± 1.0 nm surrounded by weak contrasted regions due to organic ligands. Two-dimensional organometallic halide perovskite nanoplatelets could be obtained by TEM analyses of MAmBr rich samples (P02). The produced platelets present, showed a mixture of small- and large-sized particles with varying thickness (figure 31). Smaller spots decorate the nanoplatelets. These results coincide with that one reported by Zhu et al.^[8]

Figure 31: TEM images ((a)–(c)) show the formation of ultra-thin nanoplatelets of various size prepared with ratio PbBr2:OAmBr:MAmBr0.1:0.1:0.3 (P02) in toluene.

An increase in the lead bromide content leads (P01) to three-dimensional nanocube formation. The ligands cannot effectively restrain the growth process in three dimensions but OAmBr is still able to coordinate at the surface of the perovskite crystals and contain smaller nanocrystals (figure 32).

Figure 32: TEM images ((a)–(c)) show the formation of three-dimensional nanocubes prepared with ratio PbBr2:OAmBr:MAmBr0.3:0.1:0.1 (P01) in toluene.

Study of Nucleation and Growth Kinetics of Perovskite Nanocrystals with In-situ Experiments

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It is clear that the ratio PbBr2:OAmBr:MAmBr influences the perovskite morphology and its associated structural development. The extreme ratios depicted above, are similar to the results reported elsewhere. The ligand ratio plays an important role in promoting the growth process and determining the final structure and optical properties. For a more detailed understanding of the shape-controlled synthesis, anisotropic nanocrystals were synthesized. Their structure-property relationship was studied directly after the preparation and one day after. The perovskites were synthesized with a fixed sample composition of PbBr2:OAmBr:MAmBr 0.1:0.16:0.24 (P04) allowing their ex-situ optical detection and their morphology in the earlier growth stages and after 24 h (figure 33). PbBr2:OAmBr:MAmBr0.1:0.16:0.24 (P04) in toluene (λecx=350 nm) over 2 days.

Absorbance signals could be detected at 385 nm, 411 nm, 455 nm and 473 nm immediately after the injection (figure 33, left). After 24 h, the signals are slightly red-shifted towards 505 nm.

Structures of lower dimensionality are mainly responsible for the broad absorbance in the lower wavelength region. The long scattering tail at higher wavelength indicates the presence of larger structures (e.g. superstructures). These observations are in agreement with the literature[8][25][42]

where the blue-shift of smaller-dimension nanocrystals is explained by the quantum confinement effect in nanometer dimensions. The presence of 0-dimensional spherical nanocrystals of 4.6 nm ± 1.7 nm is depicted in figure 34.

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Figure 34: TEM images during the formation of smaller nanocrystals ((a)–(c)) in the early growth stages (4.6 nm ± 1.7 nm in diameter) prepared with the ratio PbBr2:OAmBr:MAmBr0.1:0.16:0.24 (P04) in toluene.

The perovskite formation at room temperature leads in the early growth stages to the instantaneous formation of small particles (colloidal solutions of 3–5 nm nanocrystals) and can be visualized by TEM. The nanocrystals are embedded in an amorphous material.^[6][42] In contrast to MAmBr, the OAmBr ligands are not able to be incorporated into the perovskite crystals because of their long hydrocarbon tail. With further reaction time, the formation of nanosheets with varying lengths and widths of 100–600 nm (figure 35 (a–c)) could be visualized via TEM.

Figure 35: TEM images during the formation of nanocrystals and nanosheets, and smaller nanocrystals prepared with the ratio PbBr2:OAmBr:MAmBr0.1:0.16:0.24 (P04) in toluene after one day.

Study of Nucleation and Growth Kinetics of Perovskite Nanocrystals with In-situ Experiments

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Due to the low contrast in TEM imaging compared to the nanocrystals, the sheets are estimated to be much thinner. On the carbon layer the pseudorectangular-shaped quantum platelets are separated by an OAmBr ligand layer. This confirms that the long hydrocarbon chain of amine ligands is on the flat (001) plane of the cubic CH3NH3PbBr3.^[25] At longer reaction times the formation of perovskites with a higher dimension framework are stabilized by a ligand-layer. The corresponding photoluminescence spectra (figure 33, right) of the perovskites show the beginnings of the perovskite formation with multipeak emission at 456 nm, 475 nm, 488 nm and 518 nm (λ_exc = 350 nm). The large blue shift is indicative for the quantum confinement effect of the unpurified sample consisting of various nanostructures.^[19] The emission wavelength maximum after one day shifted towards 518 nm with the corresponding peak position in the absorption spectrum at 505 nm (Stokes shift = 13 nm). The symmetrical band edge emission at 518 nm has a full width at half-maximum (FWHM) of 24.3 nm. The small Stokes shift of perovskites originates from the direct band gap recombination.^[33]

The absorption spectra, the PL peaks, and the corresponding TEM images all suggest, that perovskite growth starts with the formation of subunits in form of spherical nanocrystals. The effect of aging time leads to the formation of higher dimensional structures.^[42] While this large scale approach allows ex-situ characterization, the structural development will be addressed in more detail later on with in-situ experiments.