**2.4. Thermal evaporation technique**

The thermal evaporation method was firstly used by M. Era et al in 1997. They performed the dual-source vapor deposition by using lead iodide PbI2 and organic ammonium iodide RNH3I, in particular, the 2-phenylethylammonium iodide C6H5C2H4NH3I.

As it is shown in Figure 7 organic and inorganic source were co-evaporated and deposited on fused quartz substrates. The pressure of evaporation chamber was about 10-6 Torr. In the preparation, the substrates were allowed to stand at room temperature. The spectrum of the vacuum deposited film corresponds well to those of single crystal and spin-coated films of the layered perovskite. Appearance of the strong exciton absorption and sharp exciton emission proves that the layered perovskite structure is organized in the vacuum deposited film [17].

**Figure 7.** Schematic representation of the two-step dipping technique.

The benefits of this technique are that it is possible to precisely control the thickness and smoothness of the thin-film surfaces. However, this method has some disadvantage. It is often difficult to balance the organic and inorganic rates, an important criterion for achieving the correct compositions of the resulting perovskite films. Because each organic component easily contaminates the inside of the evaporation equipment is expected to limit the preparation of various perovskites using different organic components. In addition, in some cases, the organic salt might not be thermally stable up to the temperatures required for evaporation, making this approach impracticable for a certain number of systems.

without organic salt and dried in vacuum. Two-step dip-processing is a convenient method which can be used for a variety of organics and inorganics, even if they have incompatible

The thermal evaporation method was firstly used by M. Era et al in 1997. They performed the dual-source vapor deposition by using lead iodide PbI2 and organic ammonium iodide RNH3I,

As it is shown in Figure 7 organic and inorganic source were co-evaporated and deposited on fused quartz substrates. The pressure of evaporation chamber was about 10-6 Torr. In the preparation, the substrates were allowed to stand at room temperature. The spectrum of the vacuum deposited film corresponds well to those of single crystal and spin-coated films of the layered perovskite. Appearance of the strong exciton absorption and sharp exciton emission proves that the layered perovskite structure is organized in the vacuum deposited film [17].

The benefits of this technique are that it is possible to precisely control the thickness and smoothness of the thin-film surfaces. However, this method has some disadvantage. It is often difficult to balance the organic and inorganic rates, an important criterion for achieving the correct compositions of the resulting perovskite films. Because each organic component easily contaminates the inside of the evaporation equipment is expected to limit the preparation of

in particular, the 2-phenylethylammonium iodide C6H5C2H4NH3I.

**Figure 7.** Schematic representation of the two-step dipping technique.

solubility characteristics [26].

230 Solar Cells - New Approaches and Reviews

**2.4. Thermal evaporation technique**

Furthermore, another method was developed to deposit perovskites thin films by using a single evaporation source. Mitzi et al. (1999). The apparatus for this single source thermal ablation (SSTA) technique consists of a vacuum chamber, with an electrical feed-through to a thin tantalum sheet heater, as shown in Figure 8.

**Figure 8.** Schematic cross section of a single source thermal ablation chamber

Crystals, powder, or a concentrated solution of starting charge is placed on the heater. A suspension of insoluble powders in a quick-drying solvent are placed on the heater, because this enables the powder to be in better physical and thermal contact with, as well as more evenly dispersed across, the sheet. Under a suitable vacuum condition, the sheet temperature reaches approximately 1000 o C in 1-2 second, the entire starting charge ablates from the heater surface well before it incandesces. After ablation, the inorganic and organic parts reassemble on the substrates to produce optically clear films of the chosen product.

The key point to this procedure is that the ablation is quick enough for the inorganic and organic compounds to evaporate from the source at basically the same time and before the organic portion has had an opportunity to decompose. In many instances (particularly with comparatively simple organic cations), the as-deposited films are crystalline and single phase at room temperature [26].

As show in Figure 9 Mingzhen Liu et al. compare the X-ray diffraction pattern of films of CH3NH3PbI3-xClx both vapour-deposited and solution-cast onto compact TiO2-coated FTOcoated glass. The main diffraction peaks, assigned to the 110, 220 and 330 peaks at 14.12 °, 28.44 ° and, respectively, 43.23°, are in same positions for both methods of films preparation, demonstrating that both techniques have produced the same organic-inorganic perovskite with an orthorhombic crystal structure [17]. Remarkably, focusing on the region of the (110) diffraction peak at 14.12 °, there is only a small peak at 12.65 ° (the (001) diffraction peak for PbI2) and no observable peak at 15.68 ° (the (110) diffraction peak for CH3NH3PbCl3), indicating a high level of phase purity.

**Figure 9.** X-ray diffraction spectra of a solution-processed perovskite film (blue) and vapour deposited perovskite film (red) [17].

Figure 10 shows high-resolution scanning electron micrographs of CH3NH3PbI3 perovskite film spin-cast on a glass/ITO/PEDOT:PSS substrate by Jun-Yuan Jeng et al. The crystal sizes in the cluster-domain regions of the perovskite are around 100–150 nm in CH3NH3PbI3 perovskite film from butyrolactone solution and around 150–200nm in DMF solution [29].

Sanjun Zhang et al. performs atomic force microscopy (AFM) measurements for each spincoated (R-(CH2)nNH3)2PbX4 in order to examine the ability of the molecules to self-organize and define the surface roughness. Several examples of the obtained images are given in Figure 11.With the phenyl based semiconductor (2-phenylethanamine lead iodide), it was possible to cover the whole surface of the glass substrate; however, this was not the case for Cyclohexyl‐ methanamine lead iodide, Myrtanylamine lead iodide and Cyclohexanamine lead bromide. It is clear that the surface roughness of the 2D phenyl-based is lower than that of the others [30].

comparatively simple organic cations), the as-deposited films are crystalline and single phase

As show in Figure 9 Mingzhen Liu et al. compare the X-ray diffraction pattern of films of CH3NH3PbI3-xClx both vapour-deposited and solution-cast onto compact TiO2-coated FTOcoated glass. The main diffraction peaks, assigned to the 110, 220 and 330 peaks at 14.12 °, 28.44 ° and, respectively, 43.23°, are in same positions for both methods of films preparation, demonstrating that both techniques have produced the same organic-inorganic perovskite with an orthorhombic crystal structure [17]. Remarkably, focusing on the region of the (110) diffraction peak at 14.12 °, there is only a small peak at 12.65 ° (the (001) diffraction peak for PbI2) and no observable peak at 15.68 ° (the (110) diffraction peak for CH3NH3PbCl3), indicating

**Figure 9.** X-ray diffraction spectra of a solution-processed perovskite film (blue) and vapour deposited perovskite film

Figure 10 shows high-resolution scanning electron micrographs of CH3NH3PbI3 perovskite film spin-cast on a glass/ITO/PEDOT:PSS substrate by Jun-Yuan Jeng et al. The crystal sizes in the cluster-domain regions of the perovskite are around 100–150 nm in CH3NH3PbI3 perovskite

Sanjun Zhang et al. performs atomic force microscopy (AFM) measurements for each spincoated (R-(CH2)nNH3)2PbX4 in order to examine the ability of the molecules to self-organize and define the surface roughness. Several examples of the obtained images are given in Figure 11.With the phenyl based semiconductor (2-phenylethanamine lead iodide), it was possible to cover the whole surface of the glass substrate; however, this was not the case for Cyclohexyl‐ methanamine lead iodide, Myrtanylamine lead iodide and Cyclohexanamine lead bromide. It is clear that the surface roughness of the 2D phenyl-based is lower than that of the others [30].

film from butyrolactone solution and around 150–200nm in DMF solution [29].

at room temperature [26].

232 Solar Cells - New Approaches and Reviews

a high level of phase purity.

(red) [17].

**Figure 10.** High-resolution scanning electron micrographs of CH3NH3PbI3 perovskite film from (a) butyrolactone sol‐ ution and (b) DMF solution [29].

**Figure 11.** AFM images of 2D organic–inorganic semiconductor films: (a) 2-phenylethanamine lead iodide, (b) Cyclo‐ hexylmethanamine lead iodide, (c) Myrtanylamine lead iodide and (d) Cyclohexanamine lead bromide. The scales are 20 μm × 20 μm. Color coding of height is shown in the bar [30].
