■ Example of research topics ■
Strain-balanced multiple quantum wells for efficiency enhancement of multi-junction solar cells
K. Toprasertpong, B. Kim, H. -H. Huang, K. Watanabe, Y. Nakano, and M. Sugiyama
Multi-junction solar cell can improve the energy conversion efficiency of conventional solar cells, from < 30% to > 40%, by stacking materials with different bandgaps, which efficiently absorb a wide range of the solar spectrum. A set of materials with the optimal bandgap combination, the lattice constant matched with the underlying substrate, and high material quality are necessary to maximize the efficiency of multi-junction solar cells. For instance, bandgap in a range of 0.7 - 1.4 eV and 1.5 - 1.9 eV is required in the Ge-based and 1.2 - 2.0 eV is required in the Si-based multi-junction cells. We introduce InGaAs/GaAsP, GaAsP/GaAsP, and InGaP/InGaP strain-balanced multiple quantum wells as materials satisfying the above requirements: they can be kept pseudo-lattice-matched with the underlying substrate while their bandgaps are adjustable. Furthermore, the high luminescence efficiency of quantum wells is expected to boost the open-circuit voltage of the cell. Our research ranges from the device modeling, simulation of electrical properties, crystal growth of thin-film solar cells by the metal-organic vapor phase epitaxy technique, to the evaluation of device performance.
Fig. (a) Efficiency estimation of multi-junction solar cell including GaAsP/GaAsP strain-balanced quantum wells. Measured (b) quantum efficiency and (c) current-voltage characteristics of InGaAs/GaAsP strain-balanced quantum well solar cells.