Professor Tsutomu Miyasaka has majored in excited state electrochemistry involving photo-sensitized metal oxide electrodes and created photoelectric conversion systems based on self-organization of photosensitive organic molecules and molecular assemblies. His first study of dye monomolecular films immobilized on metal oxide semiconductors (1979) is recognized as the basic structure of dye-sensitized solar cell (DSSC), which he has later developed into a lightweight flexible structure by establishing low temperature process (<150℃) for preparation of mesoporous metal oxide layers on plastic substrates. He also studied immobilization and orientation of a photosensitive biomolecule, bacteriorhodopsin, on metal oxide to develop an optical image sensor that can simulate the function of human retina as an artificial photoreceptor. These unique studies, despite different goals of applications, are equally achieved by creating the method to control self-organized immobilization of visible light-absorbing active molecules on the oxide surface in order to elicit efficient photoelectric response.
These research backgrounds led him to discover the organic inorganic hybrid perovskite material as visible light absorbers in photovoltaic cell. Although the organic-bound perovskite materials had been extensively investigated by Dr. Mitzi in the 1990's focusing on the luminescent properties of two-dimensional (2D) perovskite, three-dimensional (3D) materials had not been studied for power device applications. In 2006 Dr. Miyasaka first employed 3D perovskite crystals and discovered their photovoltaic ability in fabrication of photoelectrochemical cells. Nano-crystals of lead halide perovskite, represented by APbX3 (A: organic cation, X: halide anion), was employed as a solution-deposited visible light sensitizer of titanium oxide semiconductor, which he found to achieve power conversion efficiency over 3% in 2009. His pioneering study elucidated rare advantages of the organic inorganic perovskite as a rare solid-state semiconductor in three points, that are, strong sunlight harvesting ability of the perovskite endowed by narrow band gap absorption, low-cost solution-based processability of the material in preparing photovoltaic films, and tunability of the band gap wavelength by chemically changing the kind of halide anion. In 2008 he had also succeeded in fabrication of all-solid-state perovskite photovoltaic cell by replacing electrolyte layer with carbon-polymer conductive composite materials. As the first solid-state perovskite cell, this cell structure is recognized as the basic model of the present extensively studied perovskite solar cells. After his collaboration study with an Oxford university group, he could enhance the conversion efficiency of the perovskite solar cell beyond 10%, which triggered world-wide active research of perovskite solar cell. The above-described achievements have led to today's research and development of high performance perovskite solar cells, which has demonstrated the remarkable level of efficiency, >22%, approaching those of silicon and GaAs.
Dr. Miyasaka has continued construction of perovskite solar cells focusing on low temperature preparation of metal oxide semiconductors (TiO2, SnO2, ZnO, etc.), including amorphous materials, as electron transporting and hole-blocking layers that are essential for efficiency enhancement. He demonstrated that low temperature-processed cells are capable of efficiency as high as high temperature sintered metal oxide-based cells (>20%). For industrialization, this work significantly contributes to the design of low cost manufacture processes and development of light weight flexible devices. Further, Dr. Miyasaka made efforts in creating perovskite photovoltaic materials without containing lead in his aim to develop environmentally benign devices. Based on solution process, he could develop a method to improve photovoltraic ability of lead-free Bi-based perovskite by tuning the morphology of 3D crystalline layer and succeeded in enhancement of cell performance. Such challenge to create new solution-processable hybrid materials will be a significant contribution to future progress of the perovskite-based photovoltaics.
As mentioned above, Dr. Miyasaka constructed many photoelectric conversion systems based on photoelectrochemistry and physicochemistry and raised excellent research results by unique methods of thin film synthesis and device fabrication. In particular, his discovery of the photovoltaic property of chemically processed organic inorganic perovskites and the establishment of a new research field of solar energy conversion led by "high efficiency solar cells made by chemistry" have greatly contributed to the science and engineering of chemistry. His achievement is worthy of the Japan Chemical Society Award.