EBSD Analysis of Lead Halide Perovskites using Symmetry S3

Methyl Ammonium Lead Halides (MALHs) are organic crystal compounds with perovskite structures with applications in fields such as solar cells, LEDs, LASERs and photodetectors. The grain size and texture of materials belonging to this group such as methylammonium lead iodide (MAPbI3) can have a significant effect on their performance. These materials are very sensitive to analysis with an electron beam, making it difficult for them to be characterised using SEM based techniques. However, recent improvements to Electron Backscatter Diffraction (EBSD) detectors provide a solution — for the first time allowing characterisation of grain size and texture in MAPbI3 by Electron Backscatter Diffraction.

Introduction

Methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) is sensitive to visible light and can be used in solar cells to increase conversion efficiencies. The photosensitive properties of MAPbI3 materials also make them useful as layers in light-emitting diodes. These performance characteristics have meant that MAPbI3 materials have become the subject of significant research effort.

MAPbI3 is a type of organic crystal with a perovskite structure (Fig. 1). The sample has previously been analysed using XRD to determine the crystal structure and found to have a tetragonal structure that is very close to being cubic — for simplicity it was treated as a cubic structure when analysed by EBSD.

Fig. 1 — MAPbI3 with a perovskite structure

The photosensitive properties of MAPbI3 materials are affected by the grain structure and preferred orientations. EBSD is a commonly used technique for the characterisation of grain structure and orientations in a variety of crystalline materials and therefore offers a potential solution for the characterisation of MAPbI3 and similar materials. MAPbI3 is composed of organic molecules and can be easily damaged by an electron beam, making it difficult to characterise in the SEM. This can result in EBSD patterns that are unstable and rapidly disappear if the electron dose is too high (a function of electron beam current and pixel dwell time) making it difficult to collect good quality patterns for EBSD analysis.

In order to be able to acquire mapping data from beam sensitive samples, it is important to keep the electron dose low in order to prevent beam damage of the material before the diffraction pattern has been acquired. High beam currents can be used, but only with extremely short dwell times.

For these reasons, it has, until recently, not been possible to collect EBSD maps from MAPbI3 and only limited orientation analysis has been performed based on patterns collected manually from discrete points. Only being able to collect patterns from discrete points manually is not only inconvenient but also limits the amount of information which can be extracted, restricting our understanding of the relationships between texture, grain size and material performance.

Conventional CCD Based EBSD System

EBSD patterns and maps from a MAPbI3 sample collected using a conventional charge coupled device (CCD) based EBSD detector are shown in figure 2. The pattern quality is very poor and, although patterns from some individual points can be indexed, the resulting indexing rate during mapping was only 16% and as a result the mapping data is barely usable. The IPF maps on figure 2 (d and e) include large black areas, which indicate that either no EBSP was detected from that point or that the pattern was too poor to be indexed. For grain size measurements it is important that the boundaries are well defined — typically an indexing rate above 90% is required.

The beam sensitive nature of the material is evident from electron images which show changes when the beam is on the sample. It is also clear that charging is a problem. Many researchers have encountered these issues and as a result there is almost no published literature on this material containing orientation data.

Fig. 2 — Data collected at 20kV, 1nA probe current and 240ms exposure using a conventional CCD based system showing examples of point analysis and mapping. a) SE image b) Example of a good quality EBSP c) SE image with mapping area overlay d) Inverse pole figure (IPF) map viewed in the X direction, non-indexed points coloured in black e) IPF map viewed in the Z direction, non-indexed points coloured in black

Symmetry: CMOS Based EBSD System

The Symmetry EBSD detector offers a solution to the problems described above. It uses a complementary metal oxide semiconductor (CMOS) sensor to enable extremely high acquisition speeds whilst still maintaining good pattern resolutions. Importantly, the conventional inefficient lens system has been replaced with fibre optic coupling between the phosphor screen and the CMOS sensor, giving a significantly improved detector sensitivity. This makes it possible to work at very low electron doses, enabling successful analysis of beam sensitive materials such as MAPbI3.

Analysis of the same sample with Symmetry shows a significant difference. Figure 3 shows an example of a mapping EBSP at a resolution of 622x512 pixels collected at 20kV, 1nA probe current and 35ms exposure time, corresponding to a mapping speed of 30Hz. The EBSP is of good quality and as a result the indexing rate during mapping is high while using a significantly lower electron dose.

Fig. 3 — Example EBSP from MAPbI3 collected using the Symmetry EBSD detector. Patterns were collected at 20kV, 1nA probe current and an exposure time of 35ms.

Results

In order to get statistical information for grain size and texture analysis a large area was analysed. This was done at 20nA with a reduced exposure time to keep the electron dose constant. The Symmetry detector's far greater sensitivity means that the speed of the analysis can be increased further by reducing exposure time without significantly decreasing pattern quality. An area of 1.26mm x 0.95mm was covered using a step size of 5μm in 23 seconds, corresponding to a mapping speed of above 2000Hz.

Figure 4 shows data collected from the sample. The maps show that the sample contains small deformed grains and as well as some larger grains with a diameter of approximately 200 μm. Grain size analysis (based on defining a critical boundary angle and a minimum number of pixels to constitute a grain) was performed and the location of the individual grains can be seen in Figure 4f. Statistical data can be easily extracted from this data — here the mean grain diameter was determined to be 48 μm with the largest grain having a 319 μm diameter.

The IPFz map in figure 4e is dominated by a red colour, indicating that the majority of the grains have the same orientation — in this case the <100> direction is parallel to the surface normal. The single dominant colour indicates that the material is strongly textured, and the strength of the texture can be quantified if the data is plotted using pole figures as shown on figure 5.

Fig. 4 — Raw EBSD data acquired from a MAPbI3 sample at 2032Hz using Symmetry EBSD detector. a) Forescatter image of the sample b) Band Contrast map c) Inverse pole figure (IPF) map viewed in the X direction d) Inverse pole figure (IPF) map viewed in the Y direction e) Inverse pole figure (IPF) map viewed in the Z direction and f) Grain map

The pole figures in figure 5 clearly show a strong preferred orientation with the poles to {100} planes aligned parallel to the sample z axis (surface normal) at a density 44 times stronger than if the orientations had been random. By combining the pole figure data and the maps it is easy to see where the grains with the {100}||z are located.

Fig. 5 — {100}, {110} and {111} pole figures.

Conclusions

MAPbI3 is an organic crystal that generates only weak electron diffraction patterns. The analysis requires an extremely high sensitivity EBSD detector to detect diffraction patterns at the low electron dose needed to avoid extensive sample damage and charging. The Symmetry EBSD detector, using a combination of fibre optics and a customised CMOS sensor, is perfect for the analysis of MAPbI3 samples, with high sensitivity and high speed delivering improved pattern quality and thus high-quality data.

The increased sensitivity of the Symmetry detector has enabled a leap in the EBSD analysis of MAPbI3 samples from single point analyses to automated mapping, thereby providing a solution for characterising grain size and texture and improving our understanding of these exciting yet challenging materials.

References

1. Khoram et al., Phy. Chem. C, 2015
2. Khoram et al., Phys. Chem. C, 2016
3. Zhao et al., SciBull, 2017
4. Hettiarachchi et al., Cryst. Eng. Comm., 2018
5. Huang et al., arxiv.org/abs/1801.08305, 2018

Acknowledgement

We would like to thank Mr Haochen Fan and Dr Kejian Jiang from the Institute of Chemistry of the Chinese Academy of Sciences, for the samples provided.