**6. Summary**

Perovskite has been the most initiative and promising material in terms of energy harvesting among organic, inorganic, and organic‐inorganic SCs [8]. In case the difficulties, which are mainly about stability, are overcome, the humanity would face to a new and affordable solar energy harvester for future use. Therefore, the novelties in PSC technology attract a large amount of attention in our recent world due to its significance on its effectiveness on electricity production from free and abundant sunlight.

The main prospects for the future cover the strategies in terms of reducing the band gap, enhancing the light absorption capacity, and improving the conversion efficiency. In addition, exposing the charge transport properties and improving interfacial engineering methods in device fabrication are important. Especially, the chemistry work on chemical composition has great value. Hence, the lion's share for future work should be explaining the photo‐physical mechanism of solid‐state SCs with respect to different chemicals.

Unfortunately, the present information obtained on "how PSCs operate?" is rather insufficient at least for now. Because the complete working principle is not well explained [18, 19]. Therefore, deriving novel clues on (1) light absorption, (2) charge separation, (3) charge transport, and (4) charge collection are needed during research since these four key issues are general SC parameters to identify the principal working of solar conversion process.

The PSC is mostly accompanied with a HTM layer. In view of the available HTMs, the inorganic ones are rather less progressed reaching an applied efficiency of 11%. On the other hand, these applications are promising in view of their improved stability against ambient conditions. With respect to these, polymeric HTMs offer tunable oxidization potential and better surface morphology. In addition, the researchers also attained advanced HOMO levels [78, 91–93]. The efficiency of polymer HTMs‐based SCs is reported to reach more than 16% [31, 94, 95]. The third kind of HTMs is small molecules. Among these, spiro‐OMeTAD has been the most used HTM in PSCs till now since it has already been evaluated in solid‐state DSSCs and OLED devices [67, 83, 112] (that is, spiro‐OMeTAD has already been a standard HTM for researchers). The use of hybrid lead halide with spiro‐OMeTAD‐based HTM is reported to give an efficiency of around 15% PCE [82, 84]; however, the cost of spiro‐OMeTAD remains high due to its difficult synthesis process. Therefore, novel small molecules should be designed and synthe‐ sized in order to develop cost‐effective PSC devices.

We note that PSCs evolved rapidly and the semiconducting phenomena occurred within the cell remained shaded until now. Hence, the semiconducting behavior is continuously investi‐ gated benefitting from the current‐voltage characteristics of SCs in some research groups [113]. Deriving an increased efficiency level in PSCs is also related to the way how deposition is carried out. The perfection within the selected coating method is also important [114, 115]. Besides all discussion presented here, we note that the use of completely wet processing method during the production of SCs is especially important for an easy fabrication. Therefore, studies made on "all‐solution production process" are highly valuable since vacuum produc‐ tion is expensive and highly energy dependent [116].
