**3. Crystal structures of CH3NH3PbX3 (X=Cl, Br, or I) compounds**

The crystals of methylammonium trihalogenoplumbates(II) (CH3NH3PbX3, X=Cl, Br, or I) have perovskite structures and provide structural transitionsupon heating [24], 22]. The crystal systems and transition temperatures are summarized in Table 1, as reported in the previous works [22, 21]. Atomic sites were indicated from the space group table [6]. Although the CH3NH3PbX3 perovskite crystals have a cubic symmetry for the highest temperature phase, the CH3NH3 ion is polar and has C3v symmetry, which should result in disordered cubic phase [14]. In addition to the disordering of the CH3NH3 ion, the halogen ions were also disordered in the cubic phase, as shown in Figure 1(a) and Table 2 [14]. Site occupancies were set as 1/4 for I and 1/12 for C and N. The CH3NH3 ion occupies 12 equivalent orientations of the C2 axis, and hydrogen atoms have two kinds of configurations on the C2 axis. Then, the total degree of freedom is 24 [21].

**2. Synthesis of methylammonium trihalogenoplumbates (II)**

were formed upon cooling from 100 °C.

78 Solar Cells - New Approaches and Reviews

There are various fabrication processes for the methylammonium trihalogenoplumbates (II) (CH3NH3PbI3) compound with the perovskite structures. Two typical synthesis methods for the CH3NH3PbI3 (MAPbI3) were reported [1]. MAPbI3 could be synthesised from an equimolar mixture of CH3NH3I and PbI2 using the reported method [8]. CH3NH3I was synthesised at first by reacting a concentrated aqueous solution of hydroiodic acid with methylamine, and the cleaned precipitant was mixed with PbI2 in gamma-butyrolactone to obtain the MAPbI3 product. Crystalline MAPbI3 was obtained by drop-casting the solutions on glass substrates, and annealed at 100 °C. Polycrystalline MAPbI3 could be also prepared by precipitation from a hydroiodic acid solution [22]. Lead(II) acetate was dissolved in a concentrated aqueous HI and heated. An HI solution with CH3NH2 was added to the solution, and black precipitates

A typical fabrication process of the TiO2/CH3NH3PbI3 photovoltaic devices is also described here [28]. The details of the fabrication process is described in the reported paper [2] except for the mesoporous TiO2 layer [16]. The photovoltaic cells were fabricated by the following process. F-Doped tin oxide (FTO) substrates were cleaned using an ultrasonic bath with acetone and methanol and dried under nitrogen gas. The 0.30M TiOx precursor solution was prepared from titanium diisopropoxide bis(acetyl acetonate) (0.11 mL) with 1-butanol (1 mL), and the TiOx precursor solution was spin-coated on the FTO substrate at 3000 rpm for 30 s and annealed 125 °C for 5 min. This process was performed two times, and the FTO substrate was sintered at 500 °C for 30min to form the compact TiO2 layer. After that, mesoporous TiO2 paste was coated on the substrate by a spin-coating method at 5000 rpm for 30 s. For the mesoporous TiO2 layer, the TiO2 paste was prepared with TiO2 powder (Aerosil, P-25) with poly(ethylene glycol) in ultrapure water. The solution was mixed with acetylacetone and triton X-100 for 30min. The cells were annealed at 120 °C for 5min and at 500 °C for 30min. For the preparation of pigment with a perovskite structure, a solution of CH3NH3I and PbI2 with a mole ratio of 1:1 in γ-butyrolactone (0.5 mL) was mixed at 60 °C. The solution of CH3NH3I and PbI2 was then introduced into the TiO2 mesopores by spin-coating method and annealed at 100 °C for 15min. Then, the hole transport layer (HTL) was prepared by spin coating. As the HTLs, a solution of spiro-OMeTAD (36.1 mg) in chlorobenzene (0.5 mL) was mixed with a solution of lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI) in acetonitrile (0.5 mL) for 12 h. The former solution with 4-tert-butylpyridine (14.4 μL) was mixed with the Li-TFSI solution (8.8 μL) for 30min at 70 °C. Finally, gold (Au) metal contacts were evaporated as top electrodes. Layered structures of the photovoltaic cells were denoted as FTO/TiO2/CH3NH3PbI3/HTL/Au.

**3. Crystal structures of CH3NH3PbX3 (X=Cl, Br, or I) compounds**

The crystals of methylammonium trihalogenoplumbates(II) (CH3NH3PbX3, X=Cl, Br, or I) have perovskite structures and provide structural transitionsupon heating [24], 22]. The crystal As the temperature decreases, the cubic phase is transformed in the tetragonal phase, as shown in Figure 1(b) and Table 3 [10]. The isotropic displacement parameters B were calculated as 8π<sup>2</sup> *Uiso*. For the tetragonal phase, I ions are ordered, which resulted in the lower symmetry from the cubic phase. Site occupancies were set as 1/4 for C and N for the tetragonal CH3NH3PbI3. As the temperature decreases lower, the tetragonal phase is transformed in the orthorhombic systems, which is due to the ordering of CH3NH3 ions in the unit cell, as shown in Figure 1(c) and Table 4 [1].

Energy gaps of the CH3NH3PbI3 were also measured and calculated [1], as summrized in Table 5. The energy gap increases with increasing temperature from the *ab-initio* calculation, and the measured energy gap of ~1.5 eV is suitable for solar cell materials.


**Table 1.** Crystal systems and transition temperatures of CH3NH3PbX3 (X=Cl, Br, or I).


**Table 2.** Structural parameters of cubic CH3NH3PbI3. Space group *Pmm* (Z=1), *a*=6.391 Å at 330 K. B is isotropic displacement parameter.

**Figure 1.** Structure models of CH3NH3PbI3 with (a) cubic, (b) tetragonal and (c) orthorhombic structures.

Crystal Structures of CH3NH3PbI3 and Related Perovskite Compounds Used for Solar Cells http://dx.doi.org/10.5772/59284 81


**Table 3.** Structural parameters of tetragonal CH3NH3PbI3 at 220 K. Space group *I4/mcm* (Z=4), *a*=8.800 Å, *c*=12.685 Å. *B* is isotropic displacement parameter.


**Table 4.** Structural parameters of orthorhombic CH3NH3PbI3 at 100 K. Space group *Pnma* (Z=4), *a*=8.8362 Å, *b*=12.5804 Å, *c*=8.5551 Å. All occupancy factors 1.0. B is isotropic displacement parameter.


**Table 5.** Energy band gaps of CH3NH3PbI3.

**Pb**

**NH3**

**c**

**Pb**

**<sup>I</sup> NH CH3 <sup>3</sup>**

**a a**

**I**

**Figure 1.** Structure models of CH3NH3PbI3 with (a) cubic, (b) tetragonal and (c) orthorhombic structures.

**CH3 NH3**

**Pb**

**(b)**

**Pb**

**I**

**I**

**I**

**Pb I**

80 Solar Cells - New Approaches and Reviews

**a**

**(a)**

**a a**

**b**

**c a**

**CH3**

**Pb**

**(c)**


**Table 6.** Structural parameters of cubic CH3NH3PbCl3. Space group *Pm3m* (Z=1), *a*=5.666 Å at 200 K. B is isotropic displacement parameter.

Structural parameters of cubic CH3NH3PbCl3 and CH3NH3PbBr3 are summarized as Table 6 and 7, respectively [14, 15]. They have similar structure parameters compared with the cubic CH3NH3PbI3, except for the lattice constants. Lattice parameters of these compounds are strongly depedent on the size of halogen ions, as shown in Figure 2. As summarized in Table 8, ion radii of halogen elements increase with increasing atomic numbers, which affect the lattice constants of CH3NH3PbX3, as observed in Figure 2.


**Table 7.** Structural parameters of cubic CH3NH3PbBr3. Space group *Pm3m* (Z=1), *a*=5.933 nm at 298 K. B is isotropic displacement parameter.


**Table 8.** Ion radii of halogen and 14 group elements.

**Figure 2.** Lattice constants of CH3NH3PbX3 (X=Cl, Br, or I).
