**4. Summary and outlook**

The main point of this review is to establish the optimal conditions/control for increasing the repeatability/reproducibility of perovskite thin films fabrication especially for large scale production. The thin film quality synthesized by convectional solution procedure could be poor, which is characterized by irregular grain size, voids, low coverage and high roughness. The application of optimal conditions will greatly solve these problems and increase the reproducibility of preparation. Proper annealing temperature optimizes the nucleation and perovskite crystal growth. Controlling the crystallization of perovskite by the extraction of solvent using antisolvent treatment is very important. The boiling point and polarity of the preferred antisolvent must be moderated that is, not too high or low, otherwise, it will lead to poor morphology of the film. Mixing antisolvents with different polarities can be employed to neutralize the polarity, and hence the optimal crystallization rate will be achieved.

From a practical perspective, actualization of the essential features in the cells and devices directly depends on perovskite film quality (uniformity, absence of voids, high degree of crystallinity and coverage, optimum sizes of crystallites). The considerations and optimal conditions stated in the content of the write up are applicable to all the available techniques of film fabrication with indication of optimization mechanism of perovskite formation and crystallization.
