Multi-junction solar cells and novel structures for solar cell applications

The present status of R&D program for super-high efficiency III–V compound multi-junction solar cells in the New Sunshine Project in Japan is presented. As a result of InGaP top cell material quality improvement, development of optically and electrically low-loss double-heterostructure InGaP tunnel junction, photon and carrier confinements, and lattice matching between active cell layers and substrate, InGaP/InGaAs/Ge monolithic cascade 3-junction cells with an efficiency of 31.7% at 1-sun AM1.5 and InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with the highest (world-record) efficiency of 33.3% at 1-sun AM1.5 have been realized. As an approach for low-cost and high-efficiency cells, better radiation resistance of GaAs thin-film solar cells with novel structures fabricated on Si substrates has also been demonstrated. Novel structures such as Bragg reflector and super-lattice structures are found to show a better initial cell performance and radiation resistance since those layers act as buffer layers to reduce dislocations, and act as a back-surface field and back-surface reflector layers.