Yuki Nagato

Growth of InGaAsP solar cells and their application to triple-junction top cells used in smart stack multijunction solar cells

The authors report on high-quality InGaAsP (1.61–1.65 eV) solar cells grown on a GaAs substrate; their study is the first to grow these using solid-source molecular beam epitaxy (SS-MBE). A temperature of 430 °C was found to be suitable for the growth of the InGaAsP solar cells. The properties of these InGaAsP solar cells were found to be better than those of AlGaAs solar cells that had the same bandgap energy, and it was found to be suitable for use as the second cell in a triple-junction top cell used in a smart stack multijunction solar cell. The authors also developed an InGaP/InGaAsP/GaAs solar cell and found that it had an impressive open-circuit voltage of 3.16 V. This result indicates that high-performance InGaP/InGaAsP/GaAs triple-junction solar cells can be fabricated using SS-MBE.

Effects of substrate miscut on the properties of InGaP solar cells grown on GaAs(001) by solid-source molecular beam epitaxy

The growth of ternary InGaP alloys is often susceptible to atomic ordering, which leads to an anomalous bandgap reduction as well as the formation of antiphase boundaries (APBs). The effect of substrate miscut on the performance of lattice-matched In0.52Ga0.48P solar cells grown on GaAs(001) substrates by solid-source molecular beam epitaxy (SS-MBE) is investigated. A B-type miscut enhanced single-variant atomic ordering even with SS-MBE, resulting in a bandgap (E g) reduction from 1.87 eV for an alloy grown on an exact substrate to 1.85 eV for that grown on the substrate miscut 6? toward (111)B. Conversely, an A-type miscut suppressed the formation of atomic ordering, resulting in the E g widening of the alloy grown on the substrate miscut 6? toward (111)A to 1.89 eV. With regard to solar cell performance, InGaP solar cells grown on A-type miscut substrates enhanced the open-circuit voltage (V OC) and W OC (= E g/q ? V OC) because of the low degree of atomic ordering. Large improvements in W OC and efficiency to 0.58 V and 10.93%, respectively, were obtained for the cell grown on the substrate miscut 2? toward (111)B. A reduction in the number of APBs due to single-variant atomic ordering was related to this latter result.

InGaP-based InP quantum dot solar cells with extended optical absorption range

In the quest for an efficient optical absorption of broad-band solar irradiation, intermediate-band solar cells composed of wide-bandgap semiconductors have attracted attention. In the present study, we developed and investigated the performance of wide-bandgap InGaP-based InP quantum dot (QD) solar cells. The solar cells were fabricated by solid-source molecular beam epitaxy, and their optical absorption range was found to be up to ~850 nm, which is larger than the ~680 nm optical absorption range of the host InGaP solar cells. Through the measurements of the voltage-dependent quantum efficiency, the photocarriers generated in the InGaP host were determined to be captured into the InP QDs, rather than expelled from the solar cells. The findings of this study highlight the need for the development of an optimized structure of intermediate-band solar cells to mitigate the capture of the photocarriers.

A proposal for wide-bandgap intermediate-band solar cells using type-II InP/InGaP quantum dots

We propose the use of type-II InP quantum dots (QDs) in wide-bandgap InGaP host for the wide-bandgap intermediate-band solar cells (IBSCs). We demonstrate that the type-II InP QDs in the InGaP host exhibits carrier lifetimes greater than 30 ns. In addition, we find that while the valence band offset for holes is negligible, a large confinement potential for electrons (i.e., ~0.35 eV) is formed in these type-II InP QDs. This indicates that type-II InP QDs can simultaneously suppress thermal carrier escape while enhancing the second optical excitation in the InGaP host, thus leading to a highly efficient IBSC.