**2. Crystal growth**

*Perovskite and Piezoelectric Materials*

the photogenerated species [11].

of CH3NH3PbBr3 solar cells [7, 18, 19, 22–24].

dra, determining large cages where CH3NH3

perovskite-based cells, including the development of a wide range of architectures,

These halide perovskites have been shown to be finer and to exhibit more suitable optical and electrical properties, like the adaptability of the band gap by introducing different halides (i.e., bromide, chloride, iodide) and its relative proportion in the perovskite [7–9], the high absorption coefficient [10], and the long lifetime of

CH3NH3PbI3-based perovskite solar cells have been more extensively investigated, due to their fast charge extraction rates and their near-complete visible light absorption, related to a relatively low band gap, around 1.6 eV [11, 12]. However, the poor stability of CH3NH3PbI3 and rapid degradation in humidity has remained a drawback for its practical application, as a commercialized product [13–16]. CH3NH3PbBr3 constitutes a promising alternative, which presents a good charge transport in devices due to its long exciton diffusion length [17]. In addition, its cubic phase and low ionic mobility lead to a better stability under air and moist conditions, compared to the pseudo-cubic CH3NH3PbI3 phase, [7, 17–19]. Nevertheless, there are some undesirable features to be concerned about in these bromide-based perovskites; it is the case of a larger band gap (2.2 eV), which decreases the solar light absorption [20, 21], the relatively large exciton binding energy and the reduced light absorption beyond its band edge at 550 nm (linked to its, previously indicated, larger band gap), associated with more limited efficiencies

In parallel with the evaluation of the influence of a particular chemical composition of the perovskite, it is mandatory to determine the crystallographic structure under conditions in which the sample will be used. As other perovskite structures, CH3NH3PbBr3 consists of a framework constituted by corner-sharing PbBr6 octahe-

+

MAPbBr3 was previously studied by diffraction techniques in single crystal form by X-ray or in deuterated samples by neutron beams [25–27]. On the other hand, CH3NH3PbCl3 is also an alternative material that presents a wider band gap (3.1 eV), being also sensitive to the UV region. Furthermore, this compound exhibits a fast photoresponse and long-term photostability, having a charge carrier concentration, mobility, and diffusion length comparable with the best-developed crystal structures of CH3NH3PbI3 and CH3NH3PbBr3 [28–30]. However, both CH3NH3PbCl3 and

In this chapter, we review our previous work on the CH3NH3Pb(Br1−xClx)3 system, where we have investigated the crystallographic features in powdered, non-deuterated samples, using the mentioned state of the art techniques, neutron and synchrotron X-ray diffraction [32–34]. The crystal structure of MAPbBr3 has been determined and refined at different temperatures, describing the evolution of the orientation of MA group in the 120–295 K temperature range. We found a partial delocalization in the cubic phase (at room temperature), where C and N atoms present large multiplicity positions, becoming progressively localized across the sequence cubic-tetragonal

the mixed anion CH3NH3Pb(Br1−xClx)3 have been less investigated [31].

the crystal structure of the perovskites with their electro-optical properties, it is necessary to exhaustively study the structural details, including the orientation of the methylammonium (MA) units within the perovskite cage in the course of phase transitions. Although synchrotron XRD data are essential, this can only be fully accomplished using neutrons as a probe due to the presence of protons. Usually, the MA configuration is linked to the rotation or tilting of the PbX6 octahedra; the more symmetric the octahedral framework is, the more delocalized appears the organic unit inside the inorganic cage. This information is essential to establish relationships between these structures and the macroscopic phenomenology (optical and physical properties when these compounds are used as optoelectronic materials).

units are located. In order to correlate

which allowed the PCEs to increase up to 23% within the past years [6].

**94**

The crystal growth of CH3NH3Pb(Br1−xClx)3 (x = 0, 0.33, 0.5, 0.67 and 1) [32–34] was made from stoichiometric amounts of CH3NH3X and PbX2 (X = Cl, Br). Previously, the methyl ammonium bromide and methyl ammonium chloride were synthesized from methyl amine (CH3NH2) and the corresponding acid HBr and HCl, respectively, according to the following reaction:

$$\text{CH}\_3\text{NH}\_2 + \text{HX} \rightarrow \text{CH}\_3\text{NH}\_3\\\text{X (from X = Br to Cl)}\tag{1}$$

Then, the obtained methyl ammonium halides were reacted with the lead halide in stoichiometric amounts in dimethyl formamide (DMF) according to the following reaction:

$$\text{CH}\_3\text{NH}\_3\text{X} + \text{PbX}\_2 \rightarrow \text{CH}\_3\text{NH}\_3\text{PbX}\_3 \text{ (from X = Br to Cl)}\tag{2}$$

From this procedure, the mixed halide perovskites were obtained as well crystallized materials, showing crystals of variable sizes and colors, varying from orange for CH3NH3PbBr3 to white for CH3NH3PbCl3, adopting progressively paler hues of yellow as Cl content increases, as shown in the optical microscope images included in **Figure 1**. The effect of halide composition on the morphology and structure of

#### **Figure 1.**

*SEM images of the mixed perovskites and optical microscope images of as-grown CH3NH3PbX3 (from X = Br to Cl) perovskites. The insets show the color variation as Cl is introduced.*

crystals was observed in SEM images as is also shown in **Figure 1**. In all cases, the obtained perovskites show cuboid-type microcrystals. The content of chloride induces a decrease in the size of the crystals of the mixed perovskites.
