Three-Dimensional Optical Tomography and Correlated Elemental

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Letter

Three Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Micro-Structures: an Insight Into Defect-Related Lattice Distortion and Photo-Induced Ion Migration Juan F. Galisteo-López, Yuelong Li, and Hernan Miguez J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02456 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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The Journal of Physical Chemistry Letters

Three Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Micro-Structures: an Insight Into Defect-Related Lattice Distortion and Photo-Induced Ion Migration Juan F. Galisteo-López,§ Yuelong Li§ and Hernán Míguez* Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Científicas (CSIC)-Universidad de Sevilla, C/Américo Vespucio 49, 41092 Sevilla, Spain

Corresponding Author § Both authors contributed equally to this work. *e-mail: [email protected]

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ABSTRACT

Organic lead halide perovskites have recently been proposed for applications in light emitting devices of different sort. More specifically, regular crystalline microstructures constitute an efficient light source and fulfill the geometrical requirements to act as resonators, giving rise to waveguiding and optical amplification. Herein we show three dimensional laser scanning confocal tomography studies of different types of methylammonium lead bromide microstructures which have allowed us to dissect their photoemission properties with a precision of 0.036 µm3. This analysis shows that their spectral emission presents strong spatial variations which can be attributed to defect-related lattice distortions. It is also largely enhanced under light exposure, which triggers the migration of halide ions away from illuminated regions eventually leading to a strongly anisotropic degradation. Our work points to the need of performing an optical quality test of individualized crystallites prior to their use in optoelectronics device and provides a means to do so.

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Hybrid organic inorganic perovskites have demonstrated an enormous potential for optoelectronic applications over the past few years.1 The interest in these materials was triggered by initial studies of methyl ammonium lead iodide (MAPbI3) leading to photovoltaic devices presenting unprecedented efficiencies for a solution processed material, with current values rapidly approaching those of well-established crystalline silicon cells.2 In parallel with the outstanding properties from the point of view of light harvesters came evidences of an excellent performance as light emitter.3,4 The latter, together with the possibility of tuning the material bandgap and hence its emission band throughout the entire visible range via changes in the stoichiometry of the halide component,5 has paved the way for its use in light emitting applications.6 Recent studies have focused on two technologically relevant applications from the point of view of light emitters: light emitting diodes7,8 and lasers. In the latter approach the perovskite can be used as a gain material to be incorporated into an optical cavity4,9 following the conventional lasing approach or, due to the possibility of crystallizing these structures into polygonal shapes, use nano and microstructures as lasing resonators themselves profiting from the feedback provided by internal reflections.10,11,12,13,14,15,16,17,18 The use of microstructures is particularly appealing from the point of view of developing components for micro and nanoscale photonic and optoelectronic devices. Whether its use is intended as active elements for LEDs or coherently emitting lasers a precise knowledge of the emissive properties of the micro structure is essential. Herein we propose the use of correlated optical (laser scanning confocal microscopy - LSCM) and structural (scanning electron microscopy - SEM) characterization together with elemental information (provided by energy dispersive spectroscopy - EDS) as a tool to evaluate the


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viability of hybrid organic inorganic based perovskite micro structures for light emitting purposes. In particular we concentrate on methyl ammonium lead bromide (MAPbBr3) structures which have demonstrated the most efficient emission in the VIS region of the electromagnetic spectrum leading to lasing.12,13,14,16,17,18 This combined approach provides a bulk spatially and spectrally resolved map of the PL of these structures which reflects its homogeneity and allows tackling certain fundamental aspects of the emission of these structures such as the role of lightinduced ion migration in their optical properties. Further the structural and optical implications of light induced material degradation, a fundamental issue for this material as many applications will demand external illumination, are discussed evidencing the role of the migration of the halide component in this type of materials. Samples were prepared according to a previously described method for the fabrication of perovskite-based microstructures which have successfully demonstrated lasing16 as well as on a modification of the latter (see Methods) in order to obtain different crystal shapes. In particular, micro-cubes, wires and platelets having dimensions of a few microns were obtained. Next the luminescent properties of individual micro-structures, comprising the spatial distribution of the PL as well as spectral information, were studied in a LSCM (Zeiss LSM 700 Duo) equipped with a variable intensity CW laser probe operating at a wavelength of λ=405nm. Figure 1 shows results for each of the three types of micro-structures under consideration, namely cubes, wires and platelets. While under inspection by electron microscopy all structures present a homogeneous aspect (see Fig.1a-c), PL images, obtained integrating all the emission in the 505580nm range when illuminated using low pump intensities (0.7 W/cm2), evidence how the outer part emits more efficiently than the inner one with the edges showing the highest intensity (Fig.1d-f). If we compare the PL images with the corresponding SEM micrographs it is evident


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that a perfect morphology under electron microscopy inspection does not necessarily translate into a homogeneous luminescent behavior. This effect was also observed in polycrystalline films deposited by spin coating of a MAPbBr3 precursor on a zero-fluorescence glass substrate following a posterior annealing process (Fig.S1) indicating that it is likely related with the nature of the material and not due to guiding effects by the micro-structure. This point was further corroborated by simulating the mode distribution within the micro-structures (Fig. S2). While simulations evidence a complex mode distribution due to constructive interference of light emitted within the system, which pattern changes as one considers different emission wavelengths, the PL map retrieved from LSCM retains a fixed shape for all frequencies with the above mentioned efficient PL taking place towards the sample edges. Next, information regarding the emitting properties of the bulk of the structure was obtained by combining PL images from different sections of the samples. In these images (Fig.1g-i) a fact already visible in the individual sections becomes clearer: the PL intensity is highly inhomogeneous across the micro-structures, the edge emitting more efficiently than the center for all the micro-structures under consideration. When studied by X-ray diffraction, all samples herein considered showed distinctive features of the cubic crystalline phase (Fig. 1j and 1k), the one expected for MAPbBr3 at room temperature,19 with a clear preferential orientation of {001} planes parallel to the substrate in the case of wires and platelets (Fig. 1k).


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Figure 1. Scanning electron microscopy images (a-c), PL images (d-f) and 3D PL reconstructions (g-i) of the three micro-structures under study; namely cubes, wires and platelets. (j) and (k) correspond to x-ray diffraction data for samples comprising microcubes and a combination of microwires and platelets respectively. All scale bars correspond to 2µm.

Spectral information can also be recorded with the present measurements from the volume of the studied microstructures. Figure 2 shows results for the particular case of the micro-cube where PL spectra were collected from different regions (300x300 nm2) within each vertical section of the structure, providing spectral information on volumes of 0.036 µm3. As the focus moves away from the first microstructure facet it encounters, the substrate-cube interface (z=0), images become slightly less sharp and dimmer (Fig.2a). The former is related to the cube acting as a scattering object itself, due to the large refractive index contrast between the cube and the surrounding air, which directs light away from the collection, and the latter is due to the cube


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absorbing part of the pump intensity. PL spectra were fitted to a Gaussian function and we extracted the center of the peak and its maximum. If we collect information from a diagonal of each vertical section, the PL peaks retain its full width at half maximum but the spectral position undergoes variations which correlate with the emission intensity. In particular, the PL presents a slight peak blueshift (1.5-2 nm) towards the sample edges, where intensity becomes higher (Fig. 2b,c). Further, as the focus of the objective moves away from the substrate-cube interface (i.e. for increasing z values), an additional redshift of all the PL spectra across the diagonal takes place. A plausible explanation for the redshift observed as the focus of the objective is moved towards the micro-structure is the reabsorption of the emission by the part of the sample it must traverse before exiting the system. On the other hand spectral changes across a single PL image are expected to have a different origin, related with lattice distortions, as discussed below.

Figure 2. (a) 3D PL reconstruction of a MAPbBr3 cube together with PL images taken at several vertical positions of the micro-structure. . Scale bar corresponds to 2µm. Spectral position (b) and PL intensity (c) extracted from a Gaussian fit of the PL spectrum obtained from 0.036 µm3 regions taken at adjacent positions across a diagonal of the different vertical sections highlighted as a dashed arrow in (a). Purple, orange and green data correspond to measurements collected for vertical sections at z=0.2, 1.0 and 1.6 µm respectively.


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In order to gain further insight into the origin of the asymmetry present in the PL of the microstructures, the above study was complemented with elemental information provided by EDS. In what follows we focus on the micro-platelets which present the smallest sample thickness (around 1µm) and thus elemental information from the whole structure is obtained. Figure 3 shows results for a micro-platelet comprising structural and emission characterization. PL images (Fig.3a) show the characteristic asymmetry in PL, which can be better appreciated and quantified as we consider PL profiles taken along different directions of the image (Fig.3b). Here it is evident how the PL intensity is enhanced by a factor of 4 (1.5) at the platelet edges (sides) with respect to the center of the structure showing a dimmer PL intensity. No correlation is apparent between this PL inhomogeneity and the structure of the platelet (Fig.3c) or its composition (Fig.3d-f).

Figure 3. PL image (a) together with profiles extracted along different directions of the platelet (b) highlighted as dashed lines 1 (grey) and 2 (red line) in (a). (c) Scanning electron microscopy


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image. (d-f) EDS maps corresponding to different elements characteristic of MAPbBr3, namely Br (d), Pb (e) and C (f). All scale bars correspond to 2µm. Increasing the pump intensity by a factor of 10 led to a permanent photo-darkening of the systems. This darkening occurs within tens of seconds and no further effects appeared for up to 5 additional minutes of illumination (Fig. S3). Here two distinct darkening behaviors take place depending on the sample region considered. Highly luminescent regions, present at the structure edges, undergo a strong drop in PL (up to 90% of the initial intensity) while dimmer regions near the platelet center present a less pronounced PL darkening (ca. 50%). Such difference in darkening behaviors is reflected by structural changes in the platelet. The PL changes at the edges, showing an initial efficient emission, are accompanied by a physical photo-induced degradation, the material physically crumbling. On the other hand, poorly emitting regions towards the structure center retain its structure. Further increasing the pump intensity increases the degradation process which always starts at the bright sample edges and advances towards the center (see Fig. S4). To shed some light into the mechanism behind this photo-induced degradation we performed a high-power (14.2 W/cm2) spatially selective illumination on a region of a platelet (Fig.4) and collected low power PL images of the initial and final structure. After illuminating for 60 seconds, the original structure, presenting the characteristic inhomogeneous PL (Fig. 4a), undergoes strong changes in its emission, with the illuminated region showing an irreversible darkening and the rest of the platelet brightening up (Fig.4b), something which can also be appreciated in the tomographic images constructed from several vertical sections (Fig.4c,d). This PL redistribution can be quantified from PL profiles extracted along different lines in the platelet (Fig.4e,f). Here sample regions non exposed to the high pump beam show an increase in the PL


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intensity of 20-50% depending on the region. On the other hand, the PL intensity in the exposed quadrant nearly vanishes at the degraded part and presents a drop of 30% in the PL in the remaining exposed part.

Figure 4. 2D (a,b) and 3D PL images (c,d) of a platelet before and after being selectively illuminated with a power of 14.2 W/cm2 in the region indicated by the dashed square. All scale bars correspond to 2µm. (e,f) PL profiles along vertical lines indicated by arrows in (b) with x (e) and y (f). Black (orange) curve corresponds to PL after (before) the selective illumination. Vertical dashed lines correspond to sample edges. Blue shaded region corresponds to the part of the sample illuminated with high pump power. Map of the PL spectral position for a platelet before (g) and after (h) selectively exposing it to a high pump power (14.2 W/cm2). (i) Spectral


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variation of the PL peak across the platelet upon the selective illumination. Dashed square indicates the illuminated region. If we now map the spectral distribution of the PL throughout the platelet, strong changes can be observed which correlate with the modified PL intensity (Fig.4-i). Prior to selectively exposing one region of the structure to a high pump power, the platelet presents spatial variations in the PL peak in accordance with previous observations for the cube (see Fig.2). In particular the edges present a slight (ca. 2nm) blueshift of the emission with respect to the center (Fig.4g). After selectively illuminating the sample, strong spectral changes take place which correlate with the spatial changes in PL intensity observed. In particular, the region exposed to the high pump power presents a blueshifted PL peak with respect to the unexposed one (Fig.4h). Here it must be noted that the slight blueshift present at the structure edge with respect to the center remains in the region which has not been exposed to the high pump power. To evaluate the overall change in the spatial distribution of the PL peak, we can subtract the spectral position of the PL before and after selectively exposing the platelet (Fig.4i). Here it is evident how the drop in PL intensity in the exposed region is accompanied by a strong blueshift while the rise in PL in the remaining structure has undergone a redshift. The correlation between spectral and intensity changes can be better appreciated if we plot the PL intensity change, extracted from the Gaussian fit of the spectra, across the structure upon illumination (Fig.S5); regions brightening up upon sample illumination present a redshifted PL spectrum while those darkening show a blueshift. To better understand the processes taking place during the high power selective illumination of the platelet, structural information comprising SEM and EDS images was collected (Fig.5). As expected for such pump conditions, a physical degradation process has taken place at the illuminated region which accompanies the above mentioned darkening of the sample PL. As


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observed when the whole structure was illuminated, the photo-induced degradation starts at the sample edge, as can be clearly appreciated in the electron microscopy image of Fig.5a. If we now extract elemental information by collecting EDS maps from the platelet it becomes evident that the photo-physical and structural changes described above are accompanied by compositional ones. In particular, EDS maps for elements which are representative of the MAPbBr3 structure (see Fig.5b-d) clearly show how the concentration of the halide component is drastically reduced in the illuminated region, mainly in the part showing physical degradation. This fact becomes more evident as we compare line-profiles traced along different parts of the halide map (Fig.5ef). Such compositional changes, accompanied by spectral ones, become clearer in samples illuminated under very high pump powers (see Fig.S6). Here, where the photo-induced degradation is taken to the extreme, only a few luminescent grains are left over from the original structure. An elemental study of the remains of the structure now evidence how the bromide component has completely vanished from the material.

Figure 5. SEM image (a) and EDS maps for Pb (b), Br (c) and C (d) carried out in the platelet after the illumination. All scale bars correspond to 2µm. EDS profiles taken across vertical lines


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x (grey) and y (black line) in (b) and (c) for the Br (e) and Pb (f) cases. Vertical dashed lines indicate the sample edges. Blue shaded region corresponds to the part of the sample illuminated with high pump power. Recently inhomogeneous spatial distributions in the emissive properties of metal halide perovskite films and micro-structures, having different morphologies and composition, have been associated with changes in elemental concentration across the structure.20,21,22,23 While some authors have observed a correlation between MA-rich regions and high cathodoluminescence towards the edges of CH3NH3PbI3-xBrx micro-cubes,22 other studies focusing on MAPbI3 have found evidence of a correspondence between iodide-rich regions with good emissive properties23 or a depletion of the iodide content in illuminated regions caused by ion migration.21 For the case of the micro-structures under consideration in this study, the presence of a strong PL inhomogeneity under low power excitation conditions cannot be explained solely by a compositional gradient. While such gradient may be present, it is likely too small to be detected in our EDS maps. Further, if variations in composition were responsible for such emissive changes, poorly emitting regions are expected to exhibit a redshifted PL spectrum,20,22 contrary to what we observe. Beyond compositional changes, two other factors affecting the PL of the micro-structures are the presence of defects, whose concentration is expected to be larger at the surface,24 and the interaction with the surrounding atmosphere where oxygen has been demonstrated to foster photo-induced processes leading to an enhanced PL quantum yield (QY).25,26,27 Recent studies on MAPbBr3 single crystals of mm3 dimensions have demonstrated how bulk emission is redshifted with respect to that coming from the surface and presents longer lifetimes, as expected for low defect densities responsible for non-radiative recombination paths, and is less affected by the surrounding atmosphere.27 Also Grancini and co-


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workers28 observed a blueshifted emission at the edges of MAPbI3 single crystals which they could associate with a lattice distortion coming from the interaction between the perovskite surface and H2O and O2 molecules in the atmosphere. The spectral variation of the PL we observe across the micro-structure (see Fig.4) is likely caused by the local lattice distortion taking place at the edges of the crystal as described in ref. 28, where our measured spectral distribution is reflecting the map of the lattice distortion across the sample. On the other hand, the enhanced PL at the sample edges responds to that sample region being more affected by the surrounding atmosphere which causes the above mentioned PL QY enhancement. Finally, the overall redshift we observed for the case of the micro-cube as we move the focus into the structure (Fig.2) could also reflect the effect of lattice distortion being less pronounced as we probe the bulk properties, although reabsorption of the emitted light is certainly affecting the 3D PL measurement as mentioned above. As the pump intensity increases, other processes come into play and cause the observed photophysical and structural changes including sample degradation; mainly the migration of ions within the MAPbBr3 lattice. The issue of photo-induced degradation is particularly relevant for hybrid organic-inorganic perovskites as most applications envisaged for this kind of material, mainly photovoltaics and light emission, involve its illumination with different degrees of intensity. This type of material degradation has been recently related with ion-migration in MAPbI3 films,23,29,30 a process also thought to be linked with other phenomena such as photocurrent hysteresis in photovoltaic devices or a giant static dielectric constant.31,32 While some studies hypothesized on the migration of MA ions29,30 being responsible for a relaxation of the perovskite structure, having associated an increased bandgap and the eventual collapse of the crystalline structure, no evidence for modified concentration of the organic cation was provided.


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On the other hand, recent reports of MAPbI3 films have shown how iodide rich regions present a higher emission and related it to ion migration of the halide under light21 and electron beam irradiation.23 Light-induced halide migration has been further proved to be responsible for phase segregation in mixed structures of the form CH3NH3PbBrxI3−x.33,34 In the MAPbBr3 micro-structures under study, as the pump intensity goes above ca. 5 W/cm2 the first signs of material degradation appear (see Fig.S4). Regardless of the pump intensity material degradation starts at the sample edges and moves towards the center. A more detailed picture of the process can be extracted from the experiments where a partial illumination of the structure is carried out (Figs.4 and 5). Here, gathering photophysical information together with SEM and compositional maps, it becomes evident that under these pump conditions a photo-induced migration of halide ions takes place away from the surface. The illuminated regions, where the bromide population is depleted, undergoes photophysical changes comprising a drop in PL as well as a blueshift of the spectrum. On the other hand, the rest of the structure experiences a rise in its PL intensity accompanied by a spectral redshift. Such changes correspond to previous observations in polycrystalline MAPbI3 films21,23 and corroborate that under visible light illumination a photo-induced halide migration takes place. The migration of Br ions can affect the stability of the lattice structure, either by introducing strain35 or modifying the halide-leadhalide bond36 as recently proposed for CsSnI3 and MAPbI3 respectively, causing an increased bandgap, which is consistent with the blue-shift of the luminescence peak we observe from photo-darkened regions. These changes can eventually make the structure instable and cause its collapse, as it happens in our case, being the origin of photo-induced degradation of lead halide perovskites. Similar selective illumination of the platelets employing pump intensities below the onset of degradation gave rise to changes in the PL intensity in agreement with the proposed


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mechanism of photo-induced ion migration. However, it did not yield spectral or compositional changes which are only detectable after degradation has started, probably indicating that such variations are too small for the sensitivity of the employed experimental techniques. The ultimate mechanism behind photo-induced halide migration in lead halide perovskites, and the photo-physical changes it involves, is currently under debate. DeQuilettes and co-workers21 have proposed that photo-generated charges trapped at defects, present mainly in surfaces and grain boundaries, end up inducing electric fields within the micro-structures which favor the migration of halide ions. Recently Mosconi et. al37 have provided experimental and theoretical evidence which point towards the annihilation of VI+/Ii- Frenkel pairs by the migration of optically excited ions as the route towards PL enhancement commonly observed in these structures. While the present study, performed on single crystals with a well-defined structure, fits with these two proposals it provides evidence of strongly anisotropic migration which could help determining the final mechanism. The origin of such anisotropic behavior could be in the surface termination of the fabricated structures. Our XRD analysis points to the [001] planes being parallel to the platelets surface. First principle calculations for MAPbI338 have shown that such planes in the cubic structure are non-polar and thus expected to be more stable. On the other hand, crystal terminations at edges where two [001] planes meet can present different orientations and involve other planes such as the [111] or [110] which are more prone to present defects. Mosconi and co-workers have carried out simulations where they demonstrate how the presence of defects at the surface of MAPbI3 can indeed accelerate water-induced degradation in this kind of material.39 This difference in crystallographic termination could explain how the micro-structure edges are more prone to interact with the surrounding atmosphere influencing both the PL anisotropy and the photo-induced processes eventually leading to material


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degradation. The role played by the environment could also explain why optical measurements performed with similar pump powers on MAPbI3 films isolated from the atmosphere40,41 did not lead to emission or structural changes like the ones reported here. In summary we have presented the first three dimensional tomography description of different MAPbBr3 micro-structures employing scanning luminescence confocal microscopy. The structures present an anisotropic distribution of both its PL intensity and spectral position which is likely related with the presence of a defect-related lattice distortion as well as the interaction with the environment. Further, upon exposure to higher optical power excitation, the emission redistribution and structural changes of the material are shown to be dictated by the migration of the halide ions in the lattice. Such migration eventually induces material degradation which takes place in a highly anisotropic manner pointing to the surface termination having a critical role in the process. While the mechanism of ion migration is nowadays acknowledged as key actor in several processes taking place in hybrid organic inorganic perovskites, this study highlights the need of considering factors such as crystal termination or the role of the environment in order to be able to draw a clear picture. As many applications envisioned for this kind of structures demand external illumination as a means to excite them, the present results should be taken into account prior to plan a prolonged exposure of the material to external visible radiation. Mechanisms to prevent ion migration have been proposed31 so it remains to be demonstrated whether avoiding such migration could prevent or at least minimize material degradation under external illumination. Further, beyond avoiding material degradation, a spatially inhomogeneous spectral distribution is certainly undesirable for applications such as lasers, as it implies changes in the gain spectra which will hamper its use as active material.


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AUTHOR INFORMATION The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial support of the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007−2013)/ERC grant agreement n° 307081 (POLIGHT) and the Spanish Ministry of Economy and Competitiveness under grant MAT2014-54852-R is gratefully acknowledged. YL acknowledges the financial support from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement n° 622533. Juan Luis Ribas at the Centro de Investigación Tecnología e Innovación de la Universidad de Sevilla (CITIUS) is acknowledged for assistance with correlative microscopy expriments.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Sample fabrication, details of optical and structural characterization, LSCM images of polycrystalline samples, simulations of total field intensity distribution within platelets, PL and elemental information of photodegraded platelets, PL and SEM images of platelets at different stages of photo-induced degradation. (PDF)

REFERENCES


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Three-Dimensional Optical Tomography and Correlated Elemental

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