**5. Electron diffraction of CH3NH3PbI3**

When the sample amount, sample area or film thickness is smaller, it is difficult to obtain the necessary diffraction amplitude by XRD. Since the amount is enough for the TEM observation, only TEM observation may be applied to obtain the structure data. To obtain the information on the fundamental atomic arrangements, electron diffraction patterns should be taken along the various directions of the crystal, and the fundamental crystal system and lattice constants may be estimated. Then, high-resolution TEM observation and composition analysis by energy dispersive X-ray spectroscopy are performed, and the approximate atomic structure model is constructed. Most of the materials have similar structures to the known materials, and the structures will be estimated if the database on the known structures is available. For example, lots of new structures were found for high-Tc superconducting oxides, which have basic perovskite structures, and the approximate atomic structure models can be constructed from the high-resolution TEM images, electron diffraction patterns, and composition analysis of the elements [17, 19].

If a structure of the TEM specimen is known, observation direction of the crystal should be selected, and electron diffraction pattern along the direction should be estimated. Any regions selected by the selected area aperture can be observed in electron diffraction patterns, and the structure can be easily analyzed by comparing TEM images with electron diffraction patterns. When electron diffraction pattern is observed in the selected area, the diffraction pattern is often inclined from the aimed direction, which is noticed from the asymmetry of the electron diffraction pattern. The sample holder can be usually tilted along two directions, and the specimen should be tilted as the diffraction pattern shows center symmetry. Atomic structure models of cubic CH3NH3PbI3 observed along varioius directions are shown in Figure 7. Note that the atomic positions of CH3, NH3 and I are disordered as observed in the structure models. Corresponding electron diffraction patterns of cubic CH3NH3PbI3 calculated along the [100], [110], [111] and [210] directions are shown in Figure 8.

Atomic structure models of tetragonal CH3NH3PbI3 observed along [001], [100], [021], [221] and [110] are shown in Figure 9, which correspond to [001], [110], [111], [210] and [100] of cubic phase in Figure 8, respectively. Atomic positions of I are fixed for the tetragonal phase, and only atomic positions of CH3 and NH3 are disordered. For the tetragonal phase, the crystal symmetries are lowered as indicated by arrows in Figure 9(c) and 9(e). Several diffraction spots in Figure 9 have different diffraction intensities compared with Figure 8, which would be due to the different crystal symmetry of the CH3NH3PbI3 compound.

High-resolution TEM observations have been performed for the perovskite materials [20], and the nanostructures were discussed. Although TEM is a powerful tool for nanostructured materials, sample damage by electron beam irradiation should be avoided, because the CH3NH3PbI3 are known to be unstable during annealing at elevated temperatures. Several TEM results have been reported for the CH3NH3PbI3 and CH3CH2NH3PbI3, and the structures were discussed by electron diffraction and high-resolution images in these works [1, 9, 28].

Crystal Structures of CH3NH3PbI3 and Related Perovskite Compounds Used for Solar Cells http://dx.doi.org/10.5772/59284 93

**5. Electron diffraction of CH3NH3PbI3**

92 Solar Cells - New Approaches and Reviews

[110], [111] and [210] directions are shown in Figure 8.

to the different crystal symmetry of the CH3NH3PbI3 compound.

elements [17, 19].

When the sample amount, sample area or film thickness is smaller, it is difficult to obtain the necessary diffraction amplitude by XRD. Since the amount is enough for the TEM observation, only TEM observation may be applied to obtain the structure data. To obtain the information on the fundamental atomic arrangements, electron diffraction patterns should be taken along the various directions of the crystal, and the fundamental crystal system and lattice constants may be estimated. Then, high-resolution TEM observation and composition analysis by energy dispersive X-ray spectroscopy are performed, and the approximate atomic structure model is constructed. Most of the materials have similar structures to the known materials, and the structures will be estimated if the database on the known structures is available. For example, lots of new structures were found for high-Tc superconducting oxides, which have basic perovskite structures, and the approximate atomic structure models can be constructed from the high-resolution TEM images, electron diffraction patterns, and composition analysis of the

If a structure of the TEM specimen is known, observation direction of the crystal should be selected, and electron diffraction pattern along the direction should be estimated. Any regions selected by the selected area aperture can be observed in electron diffraction patterns, and the structure can be easily analyzed by comparing TEM images with electron diffraction patterns. When electron diffraction pattern is observed in the selected area, the diffraction pattern is often inclined from the aimed direction, which is noticed from the asymmetry of the electron diffraction pattern. The sample holder can be usually tilted along two directions, and the specimen should be tilted as the diffraction pattern shows center symmetry. Atomic structure models of cubic CH3NH3PbI3 observed along varioius directions are shown in Figure 7. Note that the atomic positions of CH3, NH3 and I are disordered as observed in the structure models. Corresponding electron diffraction patterns of cubic CH3NH3PbI3 calculated along the [100],

Atomic structure models of tetragonal CH3NH3PbI3 observed along [001], [100], [021], [221] and [110] are shown in Figure 9, which correspond to [001], [110], [111], [210] and [100] of cubic phase in Figure 8, respectively. Atomic positions of I are fixed for the tetragonal phase, and only atomic positions of CH3 and NH3 are disordered. For the tetragonal phase, the crystal symmetries are lowered as indicated by arrows in Figure 9(c) and 9(e). Several diffraction spots in Figure 9 have different diffraction intensities compared with Figure 8, which would be due

High-resolution TEM observations have been performed for the perovskite materials [20], and the nanostructures were discussed. Although TEM is a powerful tool for nanostructured materials, sample damage by electron beam irradiation should be avoided, because the CH3NH3PbI3 are known to be unstable during annealing at elevated temperatures. Several TEM results have been reported for the CH3NH3PbI3 and CH3CH2NH3PbI3, and the structures were discussed by electron diffraction and high-resolution images in these works [1, 9, 28].

**Figure 7.** Atomic structure models of cubic CH3NH3PbI3 observed along (a) perspevtive view, (b) [100], (c) [110], (d) [111] and (e) [210].

**Figure 8.** Calculated electron diffraction patterns of cubic CH3NH3PbI3 along (a) [100], (b) [110], (c) [111] and (d) [210].

Crystal Structures of CH3NH3PbI3 and Related Perovskite Compounds Used for Solar Cells http://dx.doi.org/10.5772/59284 95

**000**

**<sup>110</sup> <sup>011</sup>**

**c d**

**101**

**000 000**

**Figure 8.** Calculated electron diffraction patterns of cubic CH3NH3PbI3 along (a) [100], (b) [110], (c) [111] and (d) [210].

**b**

**000**

**001**

**110**

**120**

**121**

**111**

**001**

**001 010**

**011**

94 Solar Cells - New Approaches and Reviews

**a**

**Figure 9.** Atomic structure models of tetragonal CH3NH3PbI3 observed along (a) [001], (b) [100], (c) [021], (d) [221] and (e) [110] and (f) perspevtive view.

**Figure 10.** Calculated electron diffraction patterns of tetragonal CH3NH3PbI3 along (a) [001], (b) [100], (c) [021], (d) [221] and (e) [110].
