Global climate change is triggering numerous natural disasters, causing heat waves, super storms, floods, and wildfires all over the world. Due to enormous CO₂ emissions caused by human activity, the global temperature rises and the resulting changes to the climate are predicted to become worse in the near future. This is a serious threat for all species living on the earth which needs to be solved quickly through the development of clean‐energy technology. Although the historic Paris Agreement was signed in 2016, temperatures have already risen beyond 1 °C, compared with a preindustrial level, and the threshold of 1.5 °C, targeted in the Paris Agreement, could be reached as early as 2030. Accordingly, there is a vital need to push forward the introduction of renewable energy as soon as possible.

Currently, solar electricity is generated primarily by large‐area Si solar panels. Over the last 50 years, the fabrication cost of Si solar cells has been reduced drastically and, in some countries, the power‐generation cost of Si photovoltaics becomes lower than those of coal and nuclear power plants. Nevertheless, to meet the ambitious target of the Paris Agreement, CO₂ emissions should ultimately be suppressed to zero by 2050 and, therefore, continued reductions in solar‐panel cost are indispensable.

Hybrid perovskite solar cells, developed first by Prof. Miyasaka's group in 2009, could play a key role in realizing such a clean energy world in the future. In fact, the development of hybrid perovskite solar cells has revolutionized the prospects of next‐generation photovoltaics and is now entirely reshaping the course of solar cell research. “Hybrid perovskite” specifically means that this material consists of an exceptional combination of organic and inorganic components in the form of perovskite crystals. This unique material has allowed us to develop high efficiency devices at a pace never before seen in solar cell history; only 10 years have been needed for the fabrication of a hybrid perovskite solar cell with a high efficiency of 25.2%, even though a half century has been required for Si solar cells to reach a 26.7% efficiency. Importantly, the device structures of hybrid perovskites are quite simple and high‐quality hybrid perovskite absorbers can be formed at low temperatures (∼100 °C), making the perovskite devices extremely attractive. Now, hybrid perovskites have become the most exciting field of solar cell research, with the largest number of submitted papers among all solar‐cell research fields.

High conversion efficiencies realized in hybrid perovskite solar cells need to be understood as a consequence of many favorable characteristics of hybrid perovskites as solar cell materials. In particular, hybrid perovskites satisfy all critical requirements necessary as semiconducting light absorbers. Moreover, by the vast amount of research efforts, better control of material properties and superior device designing have been established, enabling the fabrication of over 20% efficiency devices relatively easily.

This book provides comprehensive descriptions of the fundamental properties of unique hybrid perovskites as well as the principles and applications of various hybrid perovskite photovoltaics. Both basic and advanced concepts of hybrid perovskite devices are covered thoroughly in this book. To serve readers at different levels of expertise, this book consists of two major parts: characteristics of hybrid perovskites (Part I) and hybrid perovskite devices (Part II). In addition to these two parts, an introduction to the whole book (Chapter 1) and an overview of hybrid perovskite solar cells (Chapter 2, by Prof. Miyasaka) are provided.

Importantly, intensive research efforts that have taken place over the last decade have led to remarkable breakthroughs in the understanding and interpretation of hybrid perovskite properties. In Part I of this book (Chapters 3–10), a wide variety of fundamental hybrid perovskite characteristics, including optical, carrier transport, photoluminescence, ferroelectric, and grain boundary properties, are explained. Furthermore, the chapters in Part I attempt to bring some consensus on several controversial issues about band gap structures (direct/indirect), absolute values of absorption coefficients, mechanism of carrier mobility limit and nature of grain boundary.

The Part II of this book (Chapters 11–18) focuses on important aspects of hybrid perovskite solar cells. In Part II, in addition to the overall descriptions for Pb‐based hybrid perovskite devices, more specific topics of mixed‐cation perovskites and Sn‐based perovskites are discussed. The serious problems of hybrid perovskite solar cells (device instability and hysteresis characteristics) are further covered in detail. Within the perovskite research field, new research areas of tandem solar cell applications are growing rapidly. In particular, a tandem device with a perovskite top cell and a crystalline Si bottom cell has attracted significant attention as this device opens a new path for further improving current Si solar‐cell technology. Part II of this book will attempt to answer one simple but important question: “why are hybrid perovskite solar cells so special among all solar cell devices?”

For the publication of this book, I would like to thank all chapter authors who put great efforts into preparing manuscripts in the middle of coronavirus pandemic that occurred during 2020. I am especially grateful to Dr. Martin Preuss (Wiley‐VCH) who has encouraged me to edit this book. I would like to thank Drs. Taisuke Matsui and Akira Terakawa (Panasonic) for helpful discussion. I wish to express my sincere gratitude to my students at Gifu University, Mitsutoshi Nishiwaki, Yukinori Nishigaki, Yoshitsune Kato, Masayuki Kozawa, Tatsuya Narikuri, Tomoya Kobayashi, Ryo Ishihara, Yosuke Kinden, Sara Maeda, Yusuke Tani, and Ryoki Toda who helped me greatly to edit this book. I sincerely hope that this book will contribute to suppressing CO₂ emissions and bring a clean‐energy world in the very near future.