**5. Roadmap**

The Perovskite solar cell (PSC) field has now become an emerging field and reports on fur‐ ther improvement in performance are expected in the near future, achieving PCE of more than 30% efficiency has now become a realistic goal. Furthermore, PSC can be used as top cells in two‐level tandem configurations using crystalline silicon or copper indium gallium selenide‐based photovoltaic devices as bottom cells. It is expected that by using silicon‐ based tandems, PCEs of 28–30% can be achieved. Yet there are issues related to the stability and toxicity, hysteresis in perovskite solar cells, which has to be solved. Experimental and theoretical investigations have demonstrated that that halide perovskites exhibit a series of superior electronic and optical properties for solar cell applications, such as proper band‐ gap and band alignment, high optical absorption, bipolar carrier conductivity, tunable doping ability, and benign defect properties. A lot of studies are required to optimize the material properties and to find new perovskite candidates for high‐efficiency, stable solar cells. Band structure engineering of CH3 NH3 PbI3 needs to be extensively investigated by replacing organic cations, Pb or I, with other choices. Furthermore, the mechanisms of per‐ formance degradations have to be resolved in a more prominent manner. Water‐corroded perovskites as rapid degradation occur in moist environments. So the reaction mechanism between H2 O and the perovskite surface could be carefully studied, leading to the develop‐ ment of new methods for stabilizing perovskites. Although some groups have fabricated the long‐term stable perovskites in the laboratory through chemical composition engineer‐ ing [32, 88], the fundamental reason for alloy stabilization of the structures requires more study. However, it is predicted that the study should converge to the p‐i‐n planar hetero‐ junction perovskite solar cell to understand the device structure and properties from single crystal.
