**3. Optimal conditions for hybrid perovskite crystallization**

Having understood the importance of high-quality perovskite films as an active layer in a device, we now turn to outline the generality of the protocol for preparation of perovskite material. This will increase reproducibility and enhance the stability.

There are various techniques for regulation and determination of optimal conditions for crystallization as listed in **Figure 2**; they are solvent selection, regulation of solvent evaporation rate, special additions including antisolvents and additives, annealing by solvent and temperature annealing.

#### **3.1 Solvent selection**

In the preparation of perovskite, the choice of solvent is a crucial parameter influencing the crystallization and morphology of perovskite. In the selection of preferred solvent, the key perquisite/condition is that the solvent must be polar for easy dissolution of precursors and its physical properties like boiling point and vapor pressure, must to be taken into consideration with respect to the desired crystallization mechanism, that is, rapid or slow. Polar *solvents* such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), γ-butyrolactone (GBL), *N*-Methyl-2-pyrrolidone (NMP) are the generic solvents used in dissolving precursor and mixing these solvents is a window of opportunity to optimize the crystallization [21].

It is important to carefully control the solvent evaporation as a means to form stepwise crystallization. The dissimilarity in the crystallization process is observed when employing different solvents: (DMF > DMSO > NMP from fast to slow crystallization).

DMF which has high vapor pressure evaporates rapidly during spin coating leading to short drying window for formation of perovskite thin films. While the other solvents like DMSO and NMP have lesser vapor pressure and are tricky to be evaporated, therefore, it highly prolongs the drying window. To that effect, DMF is generally not utilized alone, but it is mixed with other solvents so as to widen/prolong the antisolvent window.

The preferred solvent in these series for formation of qualitative perovskite film is DMSO. Nevertheless, perovskite produced from this solvent has several drawbacks like incomplete conversion (non-reacted PbI2) and large polydispersity in crystal size. These limitations can be resolved by mixing the solvents [7].

*Optimal Conditions for Preparation of Perovskite Materials for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.107992*

**Figure 2.** *Parameters to optimize for up-scaling [20].*

#### **3.2 Anti-solvent treatment**

A solution is a mixture of solute dissolved in a solvent. An anti-solvent is a liquid that does not dissolve the solute but is miscible with the solvent. During preparation of perovskite via solution technique, perovskite precursor is dissolved in solvents.

Antisolvent is commonly added to the perovskite film after its formation or during its growth in order to avoid decomposition and reaction with perovskite. During spin coating procedure, most of the solvent is removed owing to the centrifugal force produced by spinning. But, there is still leftover solvent in the film made by spin coating due to film wetness. For qualitative crystallization, this residual solvent needs to be removed by thermal annealing. But, these solvents evaporate gradually during annealing which may lead to poor film morphology, hence influencing the overall performance of perovskite based devices. To overcome this issue, the use of antisolvent was introduce during spinning operation so as to quickly lessen the solubility of perovskite precursor and facilitate the rapid crystallization, which enhances the performance of devices.

Various anti-solvents were reported in literature [6, 7, 21] such as chlorobenzene (CB), benzene, xylene (XYL), toluene (TL), ethyl acetate etc. The properties of these anti-solvents particularly the boiling points and polarity, occupy an important place in the quality of the films. For instance, if the anti-solvent polarity is as strong as usual solvent, it will dissolve perovskite. The optimal values for suitable antisolvent

polarity fall within 2–4.5 [22], above 4.5 is detrimental to the growth of perovskite film. Also, the antisolvent polarity ascertains the miscibility of antisolvent to solvent, which initiates the effect of removal of antisolvent on solvent. The high polarity increases miscibility of the antisolvent with the solvent, which give rise to the high solvent removal rate. However, antisolvent with too low polarity will result in poor solvent removal effect [6]. Moreso, favorable antisolvent depends also on the boiling point. The boiling point of antisolvent checks the rate of perovskite crystal growth [19]. The drying tempo of high boiling point antisolvent is delay in spin coating procedure, which lengthens the period of crystal growth. The presence of antisolvent in the film offers adequate fluidness, which enables the neighboring nuclei bigger and enhances the size of the grain. If the boiling point is small, the antisolvent will melt away too quick, which give rise to poor removal effect of solvent [23]. In sum, the fundamental role of antisolvent are listed below:

