Development of high-efficiency solar cell modules and new application fields are significant for the further development of photovoltaics (PV) and creation of new clean energy infrastructure based on PV. Especially, development of PV-powered EV applications is desirable and very important for this end. This paper shows analytical results for efficiency potential of various solar cells for choosing candidates of high-efficiency solar cell modules for automobile applications. As a result of analysis, Si tandem solar cells are thought to be some of their candidates. This paper also overviews efficiency potential and recent activities of various Si tandem solar cells such as III-V/Si, II-VI/Si, chalcopyrite/Si, perovskite/Si and nanowire/Si tandem solar cells. The III-V/Si tandem solar cells are expected to have a high potential for various applications because of high efficiency with efficiencies of more than 36% for 2-junction and 42 % for 3-junction tandem solar cells under 1-sun AM1.5 G, lightweight and low-cost potential. Recent results for our 28.2 % efficiency and Sharp’s 33% mechanically stacked InGaP/GaAs/Si 3-junction solar cell are also presented. Approaches to automobile application by using III-V/Si tandem solar cells and static low concentration are presented.

We review our work on combinatorial search and investigation of morphotropic phase boundaries (MPBs) in chemically substituted BiFeO3 (BFO). Utilizing the thin-film composition spread technique, we discovered that rare-earth (RE = Sm, Gd, and Dy) substitution into the A-site of the BFO lattice results in a structural phase transition from the rhombohedral to the orthorhombic phase. At the structural boundary, both the piezoelectric coefficient and the dielectric constant are substantially enhanced. It is also found that the observed MPB behavior can be universally described by the average A-site ionic radius as a critical parameter, indicating that chemical pressure effect due to substitution is the primary cause for the MPB behavior in RE-substituted BFO. Our combinatorial investigations were further extended to the A- and B-site cosubstituted BFO in the pseudoternary composition spread of (Bi1−xSmx)(Fe1−yScy)O3. Clustering analysis of structural and ferroelectric property data of the fabricated pseudoternary composition spread reveals close correlations between the structural and ferroelectric properties. We show that the evolution in structural and ferroelectric properties is controlled solely by the A-site Sm substitution and not the B-site Sc substitution.

BaTiO3/SrTiO3(001) epitaxial thin films were prepared at various growth rates by pulsed laser deposition, and their heterostructures were evaluated by synchrotron x-ray diffraction measurements and cross-sectional scanning transmission electron microscopy observations. In a film grown at a low deposition rate (0.01 nm/s), misfit dislocations are found near the interface and a fully relaxed BaTiO3 thin film grows epitaxially on the substrate. On the other hand, a film grown at a high deposition rate (0.04 nm/s) consists of strained and relaxed BaTiO3 lattices. Our results showed that the critical thickness of BaTiO3/SrTiO3(001) epitaxial thin films can be controlled by the deposition rate and that the critical thickness increases with increasing the deposition rate, and by adjusting the deposition rate we were able to prepare epitaxial thin films consisting of fully strained BaTiO3, partially strained BaTiO3 or fully relaxed BaTiO3. We have also achieved the growth controlling of BaTiO3 thin films on SrTiO3(001) substrates with SrRuO3 bottom electrode layer.