**2.2. Spin-coating technique**

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 C) of the 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 in quite short time (several minutes) in room environment.

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

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 concentrated solutions give thinner layers. The choice of the solvent is important because we need to consider the solubility for both the organic ammonium and the inorganic lead halide. Dimethylformamide (DMF) or Dimethyl sulfoxide (DMSO) are good solvents in which the perovskites usually have very high solubility. Some other solvents such as acetone, or acetonitrile can also be used. But solubility of perovskites in them is relatively poor (less than 5 wt%), and it takes too long to completely dissolve the solutes.

With the help of a profilemeter or an Atomic force microscopy to measure the layer thickness, can draw a calibration curve (thickness as a function of concentration) and adjust the concen‐ tration of solution in order to produce the desired thickness. The spin-coated 2D layered perovskites films are very reproducible, and therefore they are appropriate to be deposited on devices.
