Conversion of solar energy to electrical power using photovoltaic (PV) devices is one of the most important sources of renewable energy. Solar cells based on organic–inorganic perovskites, referred to as perovskite solar cells (PSCs), have recently drawn global interest 166because their power conversion efficiencies (PCEs) have increased dramatically, from 3.8% to more than 20%, over the course of only five to six years (NREL 2016). Specifically, in 2009, Miyasaka et al. used methylammonium lead halide, CH3NH3PbX3 (MAPbX3, where X = Br or I), as a light-absorbing layer in liquid-electrolyte–based dye-sensitized solar cells (DSSCs), and reported a PCE of 3.8% (Kojima et al. 2009). However, this type of PSC had a drawback: instability of the deposited MAPbI3 in the liquid electrolyte. This problem was solved by replacing the liquid electrolyte with solid-state, hole-transporting materials (HTMs), leading to an efficiency as high as 9.7% and devices exhibiting long-term stability (Kim et al. 2012). Up to that point, the PSC architectures were identical to those of conventional DSSCs. Light-harvesting dyes or organic–inorganic perovskite nanoparticles were regarded to play the role of sensitizer, which injects the excited electrons into a ­mesoporous-TiO2 (mp-TiO2) scaffold or the holes into the HTM.