**1. Introduction**

Organic-inorganic hybrid perovskites have risen as promising materials for a new generation of solar cells because of their ease of manufacturing and good performance, competing with the best photovoltaic devices based on silicon [1–5]. The introduction of CH3NH3PbX3 (X = Br and I) as the absorber material in an electrolyte-based solar cell structure established the beginning of perovskitebased photovoltaics [2]. However, the power conversion efficiency (PCE) and cell stability were low due to the corrosion of the perovskites by the liquid electrolyte. The replacement of the liquid electrolyte with a solid hole-transporting material led to a key progress in 2012, resulting in both higher PCE and cell stability [3]. Subsequently, great efforts have been devoted to the improvement of these hybrid

perovskite-based cells, including the development of a wide range of architectures, which allowed the PCEs to increase up to 23% within the past years [6]. 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 the photogenerated species [11].

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

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 octahedra, determining large cages where CH3NH3 + units are located. In order to correlate 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).

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

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

**95**

**Figure 1.**

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron…*

and finally orthorhombic, at 120 K, with MA units fully oriented in the (101) plane [32]. Regarding to Cl-containing phases, a systematic study of the structural properties of hybrid mixed perovskites CH3NH3Pb(Br1−xClx)3 by combining synchrotron X-ray diffraction and UV-vis spectroscopic analyses reveals that the band gap can be chemically tuned according to the Br/Cl ratio. The orientation of the organic MA units may also play an important role in the optoelectronic properties of these materials. By neutron powder diffraction, we found at RT three different orientations depending on the chlorine content and, therefore, on the unit-cell size. At lower temperatures, we unveiled that the halide disorder prevents the cooperative rearrangements needed to drive the octahedral PbX6 tiltings in intermediate Br/Cl ratios; only CH3NH3PbCl3 underwent conspicuous phase transitions (cubic at room temperature, evolving to tetragonal and orthorhombic at 120 K) [33]. H-bond interactions with the halide ions stabilize these conformations, in accordance to reported theoretical calculations [34].

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

CH3 NH2 + HX → CH3 NH3X (from X = Br to Cl) (1)

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

CH3 NH3X + PbX2 → CH3 NH3 PbX3 (from X = Br to Cl) (2)

*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.*

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

HCl, respectively, according to the following reaction:

*DOI: http://dx.doi.org/10.5772/intechopen.91421*

**2. Crystal growth**

ing reaction:

*Structural Phase Transitions of Hybrid Perovskites CH3NH3PbX3 (X = Br, Cl) from Synchrotron… DOI: http://dx.doi.org/10.5772/intechopen.91421*

and finally orthorhombic, at 120 K, with MA units fully oriented in the (101) plane [32]. Regarding to Cl-containing phases, a systematic study of the structural properties of hybrid mixed perovskites CH3NH3Pb(Br1−xClx)3 by combining synchrotron X-ray diffraction and UV-vis spectroscopic analyses reveals that the band gap can be chemically tuned according to the Br/Cl ratio. The orientation of the organic MA units may also play an important role in the optoelectronic properties of these materials. By neutron powder diffraction, we found at RT three different orientations depending on the chlorine content and, therefore, on the unit-cell size. At lower temperatures, we unveiled that the halide disorder prevents the cooperative rearrangements needed to drive the octahedral PbX6 tiltings in intermediate Br/Cl ratios; only CH3NH3PbCl3 underwent conspicuous phase transitions (cubic at room temperature, evolving to tetragonal and orthorhombic at 120 K) [33]. H-bond interactions with the halide ions stabilize these conformations, in accordance to reported theoretical calculations [34].
