**5. Conclusions**

The DSSC and PSC solar cells have attracted scientific and technological importance as an alternative to conventional Si-based solar cells. A market feasibility of the solar cells will be a part of the manufacturing cost, durability, fabrication time, chemical stability, mechanical robustness, and power conversion efficiency. The design strategy, preparation method, and surface chemistry of transition metal oxides with excellent electrical and optical properties will also have an impact. An era of nanotechnology has opened a door to tailing transition metal oxide materials for DSSCs' and PSCs' applications. In this chapter, we briefly have discussed four basic topics about the DSSCs and PSCs. Initially, background, motivation, and present needs of DSSCs and PSCs are covered. The required photovoltaic parameters including short-circuit current density, open-circuit voltage, fill factor, and incident photon-to-current conversion efficiency to develop good DSSCs and PSCs are emphasized in brief. The historical background has been presented to get an idea regarding the new investigations taking place to replace dye molecules through perovskite absorber layer. Information on the theoretical and practical details has also been provided to obtain DSSCs and PSCs with high solar-to-electricity power conversion efficiencies. Working principle of the DSSCs is explored by considering electron and hole pair generation, charge transportation, and charge separation and recombination etc. In transition metal oxide in binary and ternary forms like zero, one, two, and three dimensions such as nanoparticles, nanotubes, nanodisks, and nanoflowers, their implication is proposed as good candidates in developing a smart and wearable DSSCs and PSCs in the future.
