Enthusiastic Discussions on Halide Perovskite Materials beyond Photovoltaics at Sungkyun International Solar Forum 2017 (SISF2017) 22.1%,2 pointed out that the efficiency enhancement was mainly achieved by the evolution of device structure, from improvements to the perovskite deposition process for obtaining a uniform and dense thin layer, band gap engineering, and control of defect concentrations. Yang Yang (UCLA, United States) mentioned that the careful engineering of intermediate phases resulted in significantly enhanced crystallinity with reduced defects, which increased carrier lifetime, consequently leading to more than 19% PCE. The high-efficiency cell up to 22% was introduced by Anders Hagfeldt (EPFL, Switzerland).3 He showed the importance of the cation mixed perovskite film called the “triple cation” composition, i.e., Cs/formamidinium (FA)/methylammonium (MA). This material exhibited an outstanding open-circuit voltage of 1.24 V. Tsutomu Miyasaka stated that he could achieve a stable efficiency up to 21.6% with a high Voc of 1.18 V by improving the continuous interfacial structure between TiO2 and perovskite, based on low-temperature solution processes (
