**2.3. Two-step dip-coating**

temperature or rapidly than that needed for evaporation of the metal halide component. Despite these evident difficulties, organic-inorganic perovskites represent a number of significant opportunities for thin film deposition or crystal growth of organic-inorganic hybrid perovskites, such as two-step dipping technique, spin coating, stamping, Langmuir-Blodgett (LB), two source thermal evaporation, solution evaporation and so on, which make possible the applications of perovskites as organic-inorganic electronic or photonic devices [27,28]. This section will offered a selected compilation of recent progress in this topic, demonstrating that a number of simple and effective methods can be utilized for the deposition of this considerable

Spin-coating is a very convenient technique widely applied to uniform thin film deposition. As it is shown in Figure 5, an amount of solution is dropped on the substrate which is fixed on the spin-coater, and then it is rotated at high speed in order to spread the fluid by centrifugal force. It can be considered as a special case of solution crystal growth, which allows the formation of highly oriented layered perovskites on a substrate, while the solvent is evapo‐ rating off. On the other hand, Spin-coating enables deposition of hybrid perovskites on various substrates, including glass, plastic, quartz, silicon and sapphire. Selection of the substrate, the solvent, the concentration of the hybrid in the solvent, the substrate temperature, and the spin speed are relevant parameters for this technique. In some cases, the wetting properties of the solution on the chosen substrate improved by pretreating the substrate surface with a suitable

adhesion agent. In addition, post deposition low-temperature annealing (T < 250o

in quite short time (several minutes) in room environment.

**Figure 5.** Schematic of the spin-coating process

hybrid films is sometimes employed to improve crystallinity and phase purity. Mitzi et al. (2001b) comparing with the traditional deposition technique for inorganic semiconductors, spin-coating method doesn't require cumbersome equipment while it gives high-quality films

Actually, in order to realize a layer with the desired thickness, can modify the concentration of perovskites solution and keep the other spin-coating parameters (spin speed, acceleration and spin duration) fixed. Generally, homogeneous 2D layered perovskites films with a thickness from 10 nm to 100 nm can be obtained by carefully selecting the parameters: less

C) of the

class of materials.

**2.2. Spin-coating technique**

228 Solar Cells - New Approaches and Reviews

In a sequential deposition procedure, a metal halide film is first deposited by vacuum evapo‐ ration or spin-coated from solution. Subsequently this film is transformed into the perovskite by dipped into a solution including the organic cation. Proper selection of solvent for the dipping solution is important. So that the organic salt is soluble in it, but the starting metal halide and the final organic-inorganic perovskite are not soluble.

In this case, the organic cations in solution intercalate into and rapidly react with the metal halide on the substrate and form a crystalline film of the desired hybrid, as it is described in Figure 6.

**Figure 6.** Schematic representation of the two-step dipping technique. In (a), a film of the metal halide is deposited onto a substrate using vacuum evaporation. The metal halide film is then (b) dipped into a solution containing the organic cation. The resulting film after dipping has the characteristic room temperature photoluminescence spectrum.

For the perovskite family, (R-NH3)2 (CH3NH3) n-1 MnI3n+1 (R = butyl or phenethyl; M = Pb or Sn; n = 1 or 2), toluene/2-propanol mixture is a suitable solvent for the organic salt. The dipping times are relatively short: several seconds to several minutes, depending on the system. For example, a film of (C4H9NH3)2PbI4 was composed of a vacuum deposited film of PbI2 (See figure 6 (a)) by dipping it into a butylammonium iodide solution, the reaction time was 1-3 min, which depends the PbI2 film thickness (200-300 nm). After dip-coating, the films were instantly immersed in a rinse solution of the same solvent ratio as the initial dipping solution 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 solubility characteristics [26].
