This paper reviews fundamentals of solar cells and materials and present recent R&D status of various solar cells and materials and their future. In order to realize high efficiency and highly reliable solar cells, understanding and controlling imperfections (defects) are very important. There are several loss mechanisms to be solved for realizing high-efficiency and highly reliable solar cells. 1) bulk recombination loss, 2) surface recombination loss, 3) interface recombination loss, 4) voltage loss, 5) fill factor loss, 6) optical loss, 7) insufficient –energy photon loss. Key technologies for reducing the above losses are high quality film growth, reduction in density of defects, optimization of carrier concentration in base and emitter layers, double-hetero (DH) junction structure, lattice-matching of active layers and substrate, surface and interface passivation, reduction in series resistance and leakage current, anti-reflection coating, photon recycling and so forth.
The multi-junction solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 31-32%. In order to realize high efficiency multi-junction (MJ) solar cells, understanding and controlling imperfections (defects) are also very important. The key issues for realizing super-high-efficiency MJ solar cells are 1) sub cell material selection, 2) tunnel junction for sub cell interconnection, 3) lattice-matching, 4) carrier confinement, 5) photon confinement, 6) anti-reflection in wide wavelength region and so forth. For concentrator applications by using MJ cells, the cell contact grid structure should be designed in order to reduce the energy loss due to series resistance, and tunnel junction with high tunnel peak current density is necessary. Because cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells, effectiveness of double hetero structure tunnel diode is also presented in this paper.
Present status of various solar cells efficiencies are presented: 46.0% for concentrator III-V compound 4-junction solar cell, 38.8% for 1-sun III-V compound 4-junctiion cell, 25.6% for single-crystal Si cell, 21.7% for CIGS cell, 21.5% for CdTe cell, 20.1% for Perovskite cell, 13.6% for a-Si based 3-junction cell, 11.9% for dye-sensitized cell and 11.1% for organic cell. Efficiency potential of various solar cells is also discussed.