**1. Introduction**

The fast‐paced industrial development and population growth has increased the consump‐ tion of global energy to such an extent that it has become the ultimate necessity to use the renewable energy resources for long‐term sustainable development. Now it has become a challenge for both scientists and technologists to generate the cost‐effective and environmen‐ tally friendly renewable energy resources [1, 2].

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Although solar cells based on the photovoltaic effect have attracted great attention due to the advantage of decentralization and sustainability, yet they suffer low cost effectiveness. Another emerging class of thin‐film energy devices based on amorphous silicon also tried to capture the market, making headway by processing of costs per unit area [3–5]. The manu‐ facturing of inorganic thin‐films solar cells needs high‐temperature and high vacuum‐based techniques [6]. In addition, these techniques are limited and due to the inclusion of toxic ele‐ ments, they are limited to large‐scale production and wide applications [7].

In 1991, a new breakthrough emerged in the form of dye‐sensitized solar cells (DSSCs) that have attracted considerable attention due to their potential application in low‐cost solar energy conversion [8–16]. A high efficiency exceeding 12% was obtained by using 10 μm mesoporous TiO2 film sensitized with a cobalt redox electrolyte and an organic dye [17]. Furthermore, solid‐state DSSCs were also investigated where the liquid electrolyte was replaced by a solid hole‐transporting material (HTM) [e.g., poly(3‐hexylth‐iophene)(P3HT),2,2′,7,7′‐tetra‐ kis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′spirobifluorene (spiro‐MeOTAD)], polyaniline, and polypyrrole [8] to increase the open circuit voltage and stability of solar cells [18–22]. However, these ss‐DSSCs also suffer from faster electron recombination dynamics between electrons (TiO2 ) and holes (hole transporter), which results in the low efficiency of ss‐DSSCs [23]. So attempts were made to design various types of cells to increase the efficiency of solar cells [24].

This efficiency criterion was increased by the introduction of the perovskite sensitizer ABX3 (A = CH3 NH3 , B = Pb, Sn, and X = Cl, Br, I), introduced by Prof. Grätzel and team, which has opened a new era in the field of DSSCs due to the excellent light‐harvesting capabilities [24–37]. These materials are composed of earth abundant materials, inexpensive, processable at low temperatures (printing techniques), generate charges freely (after absorption) in bulk materials, which qualify them as low energy‐loss charge generators and collectors [38–40]. Methylammonium lead trihalide (CH3 NH3 PbX3 , where X is a halogen ion such as I− , Br− , and Cl− ) have an optical bandgap between 2.3 and 1.6 eV depending on halide content, while formamidinum lead trihalide (H2 NCHNH2 PbX3 ) also have a bandgap between 2.2 and 1.5 eV. The minimum bandgap is closer to optimum for a single‐junction cell than methylam‐ monium lead trihalide, which enhance to higher efficiencies [41]. The power conversion effi‐ ciency (PCE) of perovskite cells was improved from 7.2 to 15.9%, which is associated with the comparable optical absorption length and charge‐carrier diffusion lengths, making this device the most outperforming relative to the other third‐generation thin‐film solar cell tech‐ nologies. Although two different configurations using CH3 NH3 PbI3 perovskite in a classical solid‐state DSSC and in a thin‐film planar configuration with CH3 NH3 PbI3−*<sup>x</sup>* Cl*<sup>x</sup>* , having effi‐ ciency exceeding 16%, have been reported [26, 42], provided few issues related to the stability and hysteresis are to be solved effectively [43].

Here, it is necessary to mention that the lack of hysteresis that was an obstacle for stable operation in perovskite was observed recently using thin films of organometallic perovskites with millimeter‐scale crystalline grains with efficiencies approximately equal to 18% [44].

The three recent reports have given high hopes in the field of solar cells as EPFL scientists have developed a new hole‐transporting material FDT that can reduce the cost and achieve the power conversion efficiency of 20.2% [45]. Another study by Hong Kong University claims that they have achieved the highest efficiency of 25.5% by perovskite‐silicon tandem solar cells [46]. In the meantime, it has been claimed that the efficiency of more than 30% can be achieved by tandem solar cells based on silicon and perovskites [47].
