**1. Introduction**

Lead halide perovskites materials have been well known for many years [1], but the first incorporation into photovoltaic applications was reported by Miyasaka et al. in 2009 [2]. The lead halide perovskites, CH3NH3PbBr3, and CH3NH3PbI3 were coated on a mesoporous TiO2 electron-collector as photosensitized dyes and generated 3.8% power conversion efficiency (PCE), which was based on a dye-sensitized solar cell (DSSC) architecture. However, the cells

© 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. © 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.

were only stable for a matter of minutes because of a liquid corrosive electrolyte. In 2009, using the same dye-sensitized concept to improve upon the PCE, achieving 6.5% PCE [3].

In general, the word "perovskite" is used to describe any material with the same structure as inorganic CaTiO3. Organic halide perovskites present a general formula of AMX3, where A and M are monovalent and bivalent cations, respectively, and X is a monovalent anion that binds to both cations. M is coordinated to six X anions, and A is coordinated to 12 X anions (**Figure 1**). Consequently, they form anionic M-X semiconducting frameworks and chargecompensating cations [1]. In this case of lead halide perovskites, M is Pb atom and X is a halogen atom (Cl, Br, I, or a combination of them). The PbX6 octahedra consists of a three-dimensional (3D) framework and small-sized organic or inorganic cations, which can fit into the PbX6 framework, such as CH3NH3 + , HC(NH2)2 + , and Cs+.

**Figure 1.** Schematic representation of the 3D inorganic framework of organic halide perovskites.

A breakthrough came in 2012, Michael Grätzel and Park [4] contacted CH3NH3PbI3 perovskites with a solid-state electrolyte, spiro-OMeTAD, as a hole-transporting layer (HTL) to improve the device stability. The device structure is shown in **Figure 2**. The all-solid-state mesoscopic solar cells showed the PCE exceeding 9% and began a new perovskite solar cell (PSC) subject in the photovoltaic researches. Subsequently, Lee et al. [5], from the University of Oxford, replaced the mesoporous TiO2 with an inert Al2O3 scaffold, resulting in increased open-circuit voltage and a relative improvement in efficiency of 3–5% more than those with TiO2 scaffolds, as shown in **Figure 3** [4]. One cell of Al2O3-based cells exhibited high efficiency (red solid trace with crosses) and one exhibiting Voc > 1.1 V (red dashed line with crosses); for a perovskitesensitized TiO2 solar cell (black trace with circles); and for a planar-junction diode with structure FTO / compact TiO2 / CH3NH3PbI3-xClx / spiro-OMeTAD / Ag (purple trace with squares). They showed that the efficiencies of almost 10% were achievable using the 'sensitized' TiO2 architecture with the solid-state hole transporter, but higher efficiencies, above 10%, were attained by replacing it with an inert scaffold. This showed the PSCs may not require the mesoporous TiO2 layer in order to transport electrons and the hypothesis that a scaffold is not needed for electron extraction was proved later. A thin-film type PSCs, with no mesoporous scaffold, of >10% efficiency were achieved [6–9].

were only stable for a matter of minutes because of a liquid corrosive electrolyte. In 2009, using

In general, the word "perovskite" is used to describe any material with the same structure as inorganic CaTiO3. Organic halide perovskites present a general formula of AMX3, where A and M are monovalent and bivalent cations, respectively, and X is a monovalent anion that binds to both cations. M is coordinated to six X anions, and A is coordinated to 12 X anions (**Figure 1**). Consequently, they form anionic M-X semiconducting frameworks and chargecompensating cations [1]. In this case of lead halide perovskites, M is Pb atom and X is a halogen atom (Cl, Br, I, or a combination of them). The PbX6 octahedra consists of a three-dimensional (3D) framework and small-sized organic or inorganic cations, which can fit into the PbX6

, and Cs+.

the same dye-sensitized concept to improve upon the PCE, achieving 6.5% PCE [3].

+

framework, such as CH3NH3

204 Nanostructured Solar Cells

+

, HC(NH2)2

**Figure 1.** Schematic representation of the 3D inorganic framework of organic halide perovskites.

scaffold, of >10% efficiency were achieved [6–9].

A breakthrough came in 2012, Michael Grätzel and Park [4] contacted CH3NH3PbI3 perovskites with a solid-state electrolyte, spiro-OMeTAD, as a hole-transporting layer (HTL) to improve the device stability. The device structure is shown in **Figure 2**. The all-solid-state mesoscopic solar cells showed the PCE exceeding 9% and began a new perovskite solar cell (PSC) subject in the photovoltaic researches. Subsequently, Lee et al. [5], from the University of Oxford, replaced the mesoporous TiO2 with an inert Al2O3 scaffold, resulting in increased open-circuit voltage and a relative improvement in efficiency of 3–5% more than those with TiO2 scaffolds, as shown in **Figure 3** [4]. One cell of Al2O3-based cells exhibited high efficiency (red solid trace with crosses) and one exhibiting Voc > 1.1 V (red dashed line with crosses); for a perovskitesensitized TiO2 solar cell (black trace with circles); and for a planar-junction diode with structure FTO / compact TiO2 / CH3NH3PbI3-xClx / spiro-OMeTAD / Ag (purple trace with squares). They showed that the efficiencies of almost 10% were achievable using the 'sensitized' TiO2 architecture with the solid-state hole transporter, but higher efficiencies, above 10%, were attained by replacing it with an inert scaffold. This showed the PSCs may not require the mesoporous TiO2 layer in order to transport electrons and the hypothesis that a scaffold is not needed for electron extraction was proved later. A thin-film type PSCs, with no mesoporous

**Figure 2.** Solid-state device and its cross-sectional mesostructure. (a) Real solid-state device. (b) Cross-sectional structure of the device. (c) Cross-sectional SEM image of the device. (d) Active layer-underlayer-FTO interfacial junction structure.

**Figure 3.** (a) Schematic illustrating the charge transfer and charge transport in a perovskite-sensitized TiO2 solar cell (left) and a noninjecting Al2O3-based solar cell (right). (b) Current density-voltage characteristics under simulated AM1.5 illumination for Al2O3-based cells, one cell exhibiting high efficiency (red solid trace with crosses) and one exhibiting VOC > 1.1 V (red dashed line with crosses); for a perovskite-sensitized TiO2 solar cell (black trace with circles); and for a planar-junction diode with structure FTO/compact TiO2/CH3NH3PbI2Cl/spiro-OMeTAD/Ag (purple trace with squares).

In 2013, both the planar and mesoscopic architectures, **Figure 4**, saw a large amount of developments. Burschka et al. [10] and Bi et al. [11] demonstrated a deposition technique for the mesoscopic-type architecture, exceeding 15% efficiency using a two-step solution processing; Liu et al. [12] showed that it was possible to fabricate planar-type PSCs; using thermal evaporation method at a similar time, over 15% efficiency was achieved. A number of new deposition techniques and even higher efficiencies were reported in 2014 [13, 14]. A reversescan efficiency of 19.3% was claimed by Zhou et al. [15] at UCLA using the planar thin-film architecture. In November 2014, a device by researchers from KRICT achieved a record with the certification of a non-stabilized efficiency of 20.1% [15, 16]. In December 2015, a new record efficiency of 21.0% was achieved by EPFL [15]. Subsequently, in March 2016, researchers from KRICT and UNIST created the highest certified record for a single-junction perovskite solar cell with 22.1% [15].

**Figure 4.** Structure diagram of (a) mesoscopic perovskite solar cell and (b) planar perovskite solar cell.
