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Scanning Photocurrent Microscopy in Single Crystal Multidimensional Hybrid Lead Bromide Perovskites

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Abstract

We investigated solution-grown single crystals of multidimensional 2D–3D hybrid lead bromide perovskites using spatially resolved photocurrent and photoluminescence. Scanning photocurrent microscopy (SPCM) measurements where the electrodes consisted of a dip probe contact and a back contact. The crystals revealed significant differences between 3D and multidimensional 2D–3D perovskites under biased detection, not only in terms of photocarrier decay length values but also in the spatial dynamics across the crystal. In general, the photocurrent maps indicate that the closer the border proximity, the shorter the effective decay length, thus suggesting a determinant role of the border recombination centers in monocrystalline samples. In this case, multidimensional 2D–3D perovskites exhibited a simple fitting model consisting of a single exponential, while 3D perovskites demonstrated two distinct charge carrier migration dynamics within the crystal: fast and slow. Although the first one matches that of the 2D–3D perovskite, the long decay of the 3D sample exhibits a value two orders of magnitude larger. This difference could be attributed to the presence of interlayer screening and a larger exciton binding energy of the multidimensional 2D–3D perovskites with respect to their 3D counterparts.

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This open-access article is distributed under the Creative Commons Attribution 4.0 licence; the journal version is the authoritative record. Official MDPI article record · CC BY 4.0 licence · MDPI rights and permissions.

Figures

Figure 1. Schematic representation of the electric contact on the microcrystalline samples, showing the irradiation objective at the bottom part controlled with a piezoelectric stage.
Figure 1. Schematic representation of the electric contact on the microcrystalline samples, showing the irradiation objective at the bottom part controlled with a piezoelectric stage.

Source: E. Segura-Sanchis et al., Nanomaterials 13, 2570 (2023), CC BY 4.0.

Figure 2. X-ray diffraction patterns for 3D and 2D–3D perovskite samples, including the low-angle inset.
Figure 2. X-ray diffraction patterns for 3D (A) and 2D–3D perovskite (B). The inset shows a magnification of the low-angles region characteristic of the layered phase. Red peaks correspond to the 3D phase and blue peaks to the 2D phase (PbBr4)2−.

Source: E. Segura-Sanchis et al., Nanomaterials 13, 2570 (2023), CC BY 4.0.

Figure 3. FESEM images of crystalline 3D and 2D–3D perovskites.
Figure 3. FESEM image of crystalline 3D (A) and 2D–3D perovskite (B).

Source: E. Segura-Sanchis et al., Nanomaterials 13, 2570 (2023), CC BY 4.0.

Figure 4. Photoluminescence, diffuse-reflectance and transmittance spectra of 3D and 2D–3D perovskites.
Figure 4. (A) Photoluminescence emission spectra of polycrystalline 3D (a) and 2D–3D (b) samples. (B) Diffuse-reflectance UV–Vis absorption spectra of (a) 3D and (b) mixed 2D–3D perovskite; the inset magnifies the absorption edge. (C) Photoluminescence emission of single crystals measured with SPCM. (D) Transmittance spectra of (a) 3D and (b) mixed 2D–3D single crystals.

Source: E. Segura-Sanchis et al., Nanomaterials 13, 2570 (2023), CC BY 4.0.

Figure 5. Scanning photocurrent microscopy images and profile analysis of 2D–3D and 3D perovskite samples.
Figure 5. Scanning photocurrent microscopy images and profile analysis. (a) PL and (b) ISC images of a multidimensional 2D–3D sample. (c) PL and (d) ISC images of a 3D sample. (e,f) Edge extraction and (g,h) photocurrent sections of the 2D–3D and 3D samples, respectively. (i) Effective-like photocurrent length extracted from the dashed lines (e−1) of (g,h).

Source: E. Segura-Sanchis et al., Nanomaterials 13, 2570 (2023), CC BY 4.0.

Figure 6. Model fits to experimental photocurrent profiles for 2D–3D and 3D perovskite samples.
Figure 6. Model fit to experimental photocurrent profiles. (a,b) Experimental (red) and fitted (black) profiles for the 2D–3D sample corresponding to a short and a long distance to the collection tip electrode. Dashed sections indicate experimental points disregarded for fitting. (c,d) The same for the 3D sample.

Source: E. Segura-Sanchis et al., Nanomaterials 13, 2570 (2023), CC BY 4.0.

Research fields

Bottom-up

Solution-grown single crystals provide matched three-dimensional and multidimensional 2D–3D lead-bromide perovskites. Controlling the incorporated organic cation changes the layered character while retaining crystals large enough for spatially resolved transport measurements.

Simulations & fits

Photocurrent decay away from the probe is analysed with exponential transport models. A single regime describes the multidimensional crystals, whereas the 3D material requires fast and slow components, exposing a long-range contribution that differs by roughly two orders of magnitude.

Characterization

Scanning photocurrent microscopy with a movable tip and back contact maps charge collection across individual crystals. Photoluminescence, diffuse reflectance, X-ray diffraction and electron microscopy provide the optical, structural and morphological context needed to interpret border recombination and dimensionality-dependent transport.

RESEARCH TOPICS

EmissionThe optical comparison begins with spectra measured on both polycrystalline films and selected single crystals. The films emit at 545 nm for the conventional 3D perovskite and 535 nm for the multidimensional 2D–3D material, whereas individual crystals give narrower, better-defined peaks (Figure 4). Time-resolved measurements add a second distinction: the carrier lifetime is 9.70 ns in the 3D crystal and 18.39 ns in the 2D–3D crystal, showing that a longer luminescence lifetime does not by itself imply more efficient photocurrent collection.HarvestingThe key comparison is spatial rather than purely spectral. Figure 5 maps photoluminescence and short-circuit photocurrent across one 2D–3D and one 3D crystal, then extracts current profiles from the probe towards different crystal edges. In the 2D–3D sample the fitted decay length increases from 4.08 to 10.71 μm when the edge-to-probe distance grows. The 3D crystal requires an additional long-decay component, D2 = 328.88 ± 13.73 μm, which the authors identify as a possible fingerprint of its more efficient charge harvesting.MaterialsThe experiment separates two bromide perovskites whose dimensionality is changed through the organic cation: a conventional three-dimensional lattice and a mixed 2D–3D structure. Their different charge-transport profiles are therefore interpreted in relation to interlayer screening and the higher exciton binding energy expected in the multidimensional crystal. Crystal geometry also matters: the samples used for the optical analysis are about 4 μm thick for the 3D case and 13 μm for the 2D–3D case, producing different Fabry–Pérot oscillation periods before the photocurrent maps are compared.SetupsScanning photocurrent microscopy is performed on a single crystal placed on ITO-coated glass, which acts as the back electrode, while a movable conductive probe provides the local collecting contact. A 405 nm picosecond laser integrated into the microscope excites the crystal point by point at room temperature. Recording photoluminescence and short-circuit current over the same area makes it possible to compare where light is emitted with where photogenerated carriers are actually collected; Figures 5 and 6 then relate those maps to profiles taken at controlled distances from the crystal edges.ChemistryBoth materials are lead-bromide hybrid perovskites, but the organic-cation formulation changes how the inorganic framework is connected. That compositional choice produces either the conventional 3D structure or the multidimensional 2D–3D structure studied here. The paper deliberately uses MAPbBr3-based materials because bromide perovskites are more stable under ambient conditions than their iodide counterparts, reducing degradation as a competing explanation for the measured differences. The resulting contrast can therefore be discussed in terms of dimensionality, carrier binding and interlayer transport rather than only sample ageing.
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