Low-temperature direct wafer bonding is a promising technique for fabricating multijunction solar cells with more than four junctions in order to obtain high conversion efficiencies. However, it has been difficult to reduce the bond interface resistance between a GaAs-based subcell wafer and an InP-based subcell wafer. We found that a novel bonding structure comprising heavily Zn-doped (1 × 1019 cm−3) p+-GaAs and S-doped (3 × 1018 cm−3) n-InP had an interface resistance of 2.5 × 10−5 Ω·cm2, which is the lowest value ever reported. This result suggests that the newly developed room-temperature wafer bonding technique has high potential to realize high-efficiency multijunction solar cells.

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We report the initial results of GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy (MBE) technique. For GaAs single-junction solar cell, with the application of AlInP as the window layer and GaInP as the back surface field layer, the photovoltaic conversion efficiency of 26% at one sun concentration and air mass 1.5 global (AM1.5G) is realized. The efficiency of 16.4% is also reached for GaInP solar cell. Our results demonstrate that the MBE-grown phosphide-contained III-V compound semiconductor solar cell can be quite comparable to the metal-organic-chemical-vapor-deposition-grown high-efficiency solar cell.

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We report a GaAs tunnel junction grown by all-solid-state molecular beam epitaxy (MBE), using tellurium (Te) and magnesium (Mg) as n- and p-type dopants, respectively. The growth conditions, including V/III ratio, and growth rate, growth temperature, were optimized. Through these optimizations, Te- and Mg-doped GaAs with high carrier concentrations as well as good mobilities were obtained. A GaAs tunnel junction with a peak current density of 21 A/cm2 was demonstrated.