**2. Synthesis of inorganic–organic solar cells materials**

The synthesis of perovskites is the main and important procedure among perovskites study. Compared with the common semiconductors, the fabrication of perovskites samples is relatively easy. Because the crystals of perovskites molecules can form spontaneously via selfassembly process and they need neither intricate equipment nor confined environment condition but can be synthesized and deposited simply by soft chemical methods at room temperature. Generally there are two steps for synthesis: synthesis of ammonium salts and preparation of perovskites solution.

In the first step, for the perovskites that are mainly in form of (R-NH3)2MX4, the as-prepared amines transform to ammonium salts by reacting the amines with halogen acid. This neutral‐ ization reaction where the salts are generate is described in formula 1:

$$\text{R - } NH\_2 + HX \rightarrow \text{ R - } NH\_2 \cdot HX \tag{1}$$

The halogen acids used to produce corresponding ammonium salts are HI 57 wt%, HBr 48 wt % or HCl 37 wt% aqueous solution. After several days of drying in desiccator, the salts are dry. These dry ammonium salts are used to prepare perovskites solution. In this step, *R* - *N H*<sup>2</sup> . *HX* ammonium salt is mixed with lead halide PbX2 in stoichiometric amount in mole, and dissolved in solvent. This is a coordination reaction and it can be described by the chemical formula: 2:

$$2\{\text{R}\cdot NH\_2\cdot HX\} + M\,X\_2 \rightarrow \{\text{R}\cdot NH\_3\}\_2M\,X\_4\tag{2}$$

The solution is then put under agitation or in ultrasonic bath until the solutes are totally dissolved and the solution appears limpid. From the perovskites solution, 2D crystals can be obtained by evaporation of solvent by self-organization process. The solvent containing *R* - *N H*<sup>2</sup> . *HX* and MX2 is first spin-coated on the substrate. 2D layered perovskites crystals are then obtained upon solvent evaporation. In the absorption spectra of 2D layered perovskites crystals, a sharp peak appears at room temperature, which is characteristic of the formation of 2D layered perovskites crystal structure [25].

Another method to synthesis of perovskites is two-step based on the layer-by-layer technique. For example, thin films of microcrystalline (C8H17NH3)2PbBr4 are prepared by the two-step growth process by Kitazawa et.al as follows: (1) precipitation of nanometer-sized PbBr2 particles on substrates by vapor deposition and then (2) growth of (C8H17NH3)2PbBr4 films by exposing PbBr2 particles to C8H17NH3Br vapor. A simple vacuum chamber is used as a deposition apparatus with about 8×10-6 Torr as Background pressure. This chamber is attached to a vacuum system, two-independent thermal evaporation sources, a shutter and a substrate holder. The thermal evaporation source consists of a quartz cell coiled with a tantalum wire. First of all, PbBr2 particles are deposited on Si (100) substrates by vapor deposition. Next, PbBr2 particles are exposed to C8H17NH3Br vapor for growing of (C8H17NH3)2PbBr4 films. Exposure time is varied from 0 to 600 s. The substrate temperature is kept at room temperature during deposition. Thin films of microcrystalline that prepared by the two-step growth process and their optical properties are dependent on the exposure time [12].

Perovskites in form of luminescent nanoparticles are another remarkable kind of crystals which has attracted excessive attention recently. Between bulk materials and atomic or molecular structures, the nanoparticles show very specific properties with potential applica‐ tions in various fields such as sensing or LEDs. Nanoparticles often have specific optical properties as they are small enough to confine their electrons and produce quantum effects. Thus, the fabrication method which chooses the size of nanoparticles is very vital. For the first time Audebert et al. have realized luminescent nanoparticles by a spray-drying method. In brief, the ammonium salts and PbBr2 or PbI2 semiconductors are firstly dissolved in DMF solvent and used for the nanoparticles spray drying. The experimental spray drier is composed of an aerosol generator and an evaporation chamber which is settled in an oven maintaining at 250 °C. (Figure 4)

assembly process and they need neither intricate equipment nor confined environment condition but can be synthesized and deposited simply by soft chemical methods at room temperature. Generally there are two steps for synthesis: synthesis of ammonium salts and

In the first step, for the perovskites that are mainly in form of (R-NH3)2MX4, the as-prepared amines transform to ammonium salts by reacting the amines with halogen acid. This neutral‐

The halogen acids used to produce corresponding ammonium salts are HI 57 wt%, HBr 48 wt % or HCl 37 wt% aqueous solution. After several days of drying in desiccator, the salts are dry. These dry ammonium salts are used to prepare perovskites solution. In this step, *R* - *N H*<sup>2</sup> . *HX* ammonium salt is mixed with lead halide PbX2 in stoichiometric amount in mole, and dissolved in solvent. This is a coordination reaction and it can be described by the chemical formula: 2:

The solution is then put under agitation or in ultrasonic bath until the solutes are totally dissolved and the solution appears limpid. From the perovskites solution, 2D crystals can be obtained by evaporation of solvent by self-organization process. The solvent containing *R* - *N H*<sup>2</sup> . *HX* and MX2 is first spin-coated on the substrate. 2D layered perovskites crystals are then obtained upon solvent evaporation. In the absorption spectra of 2D layered perovskites crystals, a sharp peak appears at room temperature, which is characteristic of the formation of

Another method to synthesis of perovskites is two-step based on the layer-by-layer technique. For example, thin films of microcrystalline (C8H17NH3)2PbBr4 are prepared by the two-step growth process by Kitazawa et.al as follows: (1) precipitation of nanometer-sized PbBr2 particles on substrates by vapor deposition and then (2) growth of (C8H17NH3)2PbBr4 films by exposing PbBr2 particles to C8H17NH3Br vapor. A simple vacuum chamber is used as a deposition apparatus with about 8×10-6 Torr as Background pressure. This chamber is attached to a vacuum system, two-independent thermal evaporation sources, a shutter and a substrate holder. The thermal evaporation source consists of a quartz cell coiled with a tantalum wire. First of all, PbBr2 particles are deposited on Si (100) substrates by vapor deposition. Next, PbBr2 particles are exposed to C8H17NH3Br vapor for growing of (C8H17NH3)2PbBr4 films. Exposure time is varied from 0 to 600 s. The substrate temperature is kept at room temperature during deposition. Thin films of microcrystalline that prepared by the two-step growth process

Perovskites in form of luminescent nanoparticles are another remarkable kind of crystals which has attracted excessive attention recently. Between bulk materials and atomic or molecular structures, the nanoparticles show very specific properties with potential applica‐

2(*R* - *N H*<sup>2</sup> . *HX* ) + *M X*<sup>2</sup> → (*R* - *NH* 3)2

and their optical properties are dependent on the exposure time [12].

*R* - *N H*<sup>2</sup> + *HX* → *R* - *N H*<sup>2</sup> . *HX* (1)

*M X*<sup>4</sup> (2)

ization reaction where the salts are generate is described in formula 1:

preparation of perovskites solution.

226 Solar Cells - New Approaches and Reviews

2D layered perovskites crystal structure [25].

**Figure 4.** Schematic of the spray-drying method to preparation of organic-inorganic perovskite nanoparticles [25].

Droplets with initial mean diameter of 0.35 μm are carried by dry air from the aerosol generator to the evaporation chamber. Dried particles are collected onto a 0.2 μm cutoff Teflon filter and are stored at ambient temperature [25]. Transmission electron microscopy measurements show that these particles are spherical and their sizes are of the order of 50 to 500 nm.
