Tsunehiro Unno

Characteristics of GaAs-based concentrator cells

GaAs-based cells, including GaAs single-junction cells, AlGaAs/GaAs two-junction cells, and InGaP/GaAs two-junction cells grown on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) are examined in various levels of concentration and backside cooling temperature. All types of cells have shown boost of efficiency in low and medium ranges of concentration. The cell efficiencies obtained are 31.5% at 20-suns of AM1.5 for InGaP/GaAs tandem cell, and 29.2% at 7-suns of AM1.5 for AlGaAs/GaAs tandem cell, respectively. The GaAs single-junction cell is also examined as the reference. A new equivalent circuit model reveals that increase of apparent leakage current is responsible for a rapid efficiency drop in the high-concentration region. It is possible to improve it by reducing contact resistance and using uniform concentrated illumination.

Analysis of impurity diffusion from tunnel diodes and optimization for operation in tandem cells

Abstract The tunnel diode has been applied as an interconnector in monolithic devices such as tandem solar cells. However, thermal degradation due to impurity diffusion is often observed due to growth at above about 600°C. In this study, the impurity diffusion from highly doped tunnel junctions after annealing has been analyzed, and it has been suggested that carbon has the advantage of a low diffusion coefficient as the p-type impurities. Furthermore, the thermally stable double hetero (DH) structure GaAs tunnel diodes which have been proposed in our previous work have been optimized. The thermal degradation is greatly suppressed by using a DH-structure which consists of a GaAs tunnel diode sandwiched between Al x Ga 1− x As layers, and as a result, a higher tunnel peak current density can be achieved by optimizing the impurity concentration and DH composition.

Characteristics of GaAs solar cells on Ge substrate with a preliminary grown thin layer of AlGaAs

Abstract Characteristics of GaAs solar cell on Ge substrate with a new buffer layer structure is reported. The buffer layer structure, which consisted of a preliminarily grown thin layer of A1xGa1−xAs and a 1 μm thick GaAs layer, was designed to obtain a high quality GaAs layer on Ge substrate by metalorganic chemical vapor deposition (MOCVD). Performance of a GaAs solar cell fabricated on Ge substrate with the buffer layer structure was compared with that fabricated on Ge substrate with a conventional GaAs buffer layer and also that fabricated on GaAs substrate. A conversion efficiency of 23.18% (AM1.5G) was successfully obtained for the cell fabricated on Ge substrate with the new buffer layer structure, while it was 20.92% for the cell fabricated on Ge substrate with the conventional GaAs buffer layer. Values of Voc and Jsc, for the cell fabricated on Ge substrate with the new buffer layer structure were approximately comparable to those of a 25.39% efficiency GaAs solar cell fabricated on GaAs substrate.

Improvement of life time of minority carriers in GaAs epi-layer grown on Ge substrate

Abstract A buffer layer structure on Ge substrate was studied for MOCVD growth of a high-quality GaAs layer. The buffer layer structure was designed taking into consideration both lattice constants and thermal expansion coefficients of GaAs and Ge. It consisted of a preliminarily grown thin layer of Al x Ga 1− x As and a GaAs layer. Photoluminescence (PL) decay of a GaAs layer in an Alo 0.2 Ga 0.8 As-GaAs-Al 0.2 Ga 0.8 As double-hetero (DH) structure, which was grown on the buffer layer structure, was observed by time-resolved PL method to estimate the quality of epilayers in the DH structure. The PL decay time strongly depended on Al content ( x ) of the Al x Ga 1− x As preliminary layer, and the highest value was obtained when the x was 0.25. A PL decay time above 20 ns was successfully obtained for the DH structure grown on the buffer layer structure, which consisted of a 0.05 μm thick Al 0.25 Ga 0.75 As layer and a 1 μm thick GaAs layer. Although this value was half of that for the DH structure grown on GaAs substrate, it was much longer than the value of 3 ns for the DH structure grown on Ge substrate with a conventional GaAs buffer layer 1 μm thick.