# Overview on Different Types of Solar Cells: An Update
## 3 Perovskite Solar Cells  
### The Meaning of Perovskite  
Originally, the term \"perovskite\" referred to the calcium titanate crystal structure, which was identified in 1839 by German mineralogist [PERSON] and given the name by Russian mineralogist [PERSON]. Compounds with perovskite structures typically contain the formula ABX\({}_{3}\), where 'A' and 'B' stand for cations and 'X' is an anion joining the two cations. These metal halide perovskites, consisting of lead (Pb\({}^{2+}\)), methyl-ammonium CH3 NH\({}^{3+}\), an organic cation, and a halide anion such as iodide (I) or bromide (Br), have drawn much attention as prospective perovskite solar-cell materials in recent years. Figure 5 displays three-dimensional (3D) and two-dimensional (2D) schematic designs as well as the usual unit cell arrangement of a perovskite compound. Usually, structures can be built from a wide variety of unique component combinations. By changing the composition of these substances, researchers may produce perovskite crystals with a variety of properties, including physical, optical, magnetic, and electrical. In 1958, [PERSON] first discovered halide perovskites for cesium lead halides [117]. Following this, it was also observed that small organic molecules with effective radii less than 260 pm (such as MA (CH\({}_{3}\)NH\({}_{3}\)), FA (CH(NH)\({}_{2}\))) could also accommodate the inside of the lead halide octahedrons from the analysis of Goldschimdt's tolerance factor/octahedral factor; additionally, [PERSON], in 1978, successfully synthesized MAPbX\({}_{3}\) (X = halogens (I, Br, Cl or Mixtures of them) and superhalogen (BF\({}_{4}\), PF\({}_{6}\))) for the first time [118]. Thereafter, researchers started working on layered 2D perovskite systems (commonly, (R-NH\({}_{3}\))\({}_{2}\)MX\({}_{4}\)), where the extended inorganic lead halide octahedral cage is separated by large organo-ammonium cations [119]. The resurgent hybrid perovskite was initiated by the introduction of three-dimensional hybrid perovskites (CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) and CH\({}_{3}\)NH\({}_{3}\)PbBr\({}_{3}\)) as absorbers [120] in mesoporous solar cells by [PERSON] and co-workers in 2009. Since then, researchers across the globe have used them as absorber materials in photovoltaic cells and achieved PCE as high as 25.7% in sa ingle junction [121] and 29.8% in monolithic perovskite/Si tandem solar cells (Figure 6). Superior optoelectronic properties such as low bandgap with direct nature, high optical absorption both in visible and IR regions, ultrahigh diffusion length, high defect tolerance, large and balanced electron-hole mobilities, high photoluminescence quantum yield, and low surface recombination velocity make them attractive candidates for solar-cell application [122; 123]. Nowadays, the various structures of perovskite solar cells such as mesoporous, planar, and inverted structures have been developed. In the mesoporous solar cells, the device structure was composed of glass/fluorine-doped tin oxide (FTO)/dense electron transport layer (ETL)/mesoporous oxide layer/perovskite layer (\(\sim\)500 nm)/hole transport layer (HTL)/metal electrode [124; 125; 126]. The mesoporous TiO\({}_{2}\) is used as an ETL material allowing perovskites to penetrate into the pores to form an interconnected absorbing layer. Additionally, titanium dioxide (TiO\({}_{2}\)) also prevents the electron-hole recombination in the FTO by blocking the holes. Experimental results revealed that other oxide materials such as zinc oxide (ZnO), aluminum oxide (Al\({}_{2}\)O\({}_{3}\)), and zirconium dioxide (ZrO\({}_{2}\)) were also used as mesoporous oxide layers. In this structure, Spiro-OMeTAD (2,2',7',7'-Tetrakis [N, N-di(4-methoxyphenyl) amino]-9,9'-spirobifluorene) is the most used HTL and noble metals such as Au, Ag, and Pt are used as the counter electrodes. In the case of planar structure, the mesoporous ETL is removed and only two sharp interfaces are observed between perovskites and two transport layers (ETL and HTL).