**Conflict of interest**

*Nanostructures*

**5. Conclusions**

**4.2 Ternary transition metal oxides**

ties. The 3.6 eV bandgap and 10–15 cm<sup>2</sup>

and wearable DSSCs and PSCs in the future.

Future Planning (2013M3A6B1078874).

**Acknowledgements**

Besides the simple binary metal oxide systems, ternary metal oxide systems such as Strontium titanate (SrTiO3), Zinc Stannate (Zn2SnO4), and Barium Stannate (BaSnO3) have also been considered as photoanode materials in the DSSCs and PSCs (**Figure 4b**). The SrTiO3 is a semiconductor with bandgap similar of 3.2 eV. However, its conduction band is relatively at higher position than that of TiO2, which results in a higher *Voc* [29]. A high dielectric constant makes SrTiO3 as electrically mesoporous even with a large particle size of ~80 nm [30]. In addition, Zn2SnO4 is particularly interesting because of its physical and electrical proper-

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

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

Author SFS would like to thank University Grants Commission, New Delhi, for awarding Dr. D. S. Kothari Postdoctoral Fellowship scheme (F.4-2/2006 (BSR)/ CH/16-17/0015) and author KHK would like to thank Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT &

made it stable against UV light with high, electrical conductivity, and low visible absorption over TiO2 [31]. The ternary BaSnO3 is an *n*-type semiconductor with a wide bandgap of 3.1 eV, and its band structure and electrical properties can be controlled easily by atomic substitution or doping into the Ba or Sn site for better performance when used in DSSCs' application [32]. In this sense, as the electrode

materials in DSSCs, the ternary oxides are better than the binary.

electron mobility of Zn2SnO4 have

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The authors declare no competing interests.
