A two-terminal monolithic InGaP/GaAs tandem solar cell with a new efficiency record of 30.28% is realized with a practical large area of 4 cm2 under one-sun air-mass 1.5 global illumination. We report improvements of the tandem cell performance by introducing a double-hetero (hereafter DH) structure InGaP tunnel junction, in which the InGaP layers are surrounded by high band gap AlInP barriers. The DH structure by AlInP barriers increase the peak current of InGaP tunnel junction. The AlInP barrier directly below the InGaP top cell, which takes the part of a back surface field (hereafter BSF) layer, is found to be considerably effective in reflecting minority carriers in the top cell. The AlInP BSF layer does not only form a high potential barrier but also prevents the diffusion of zinc from a high doped tunnel junction toward the top cell during epitaxial growth. Furthermore, an InGaP tunnel junction reduces the absorption loss, which exists in a GaAs tunnel junction, and increases the photogenerated current in the GaAs bottom cell.

A world-record efficiency of 30.28% has been attained for two-terminal monolithic In0.5Ga0.5P/GaAs tandem solar cells under one-sun air-mass 1.5 global illumination. The cell area has a practical size of 4 cm2. At first, high efficiency In0.5Ga0.5P single junction cells had been developed by improving the minority carrier lifetime. Second, the GaAs single junction cells had been investigated to obtain higher open-circuit voltage. Third, the tandem cell performance had been improved by using an InGaP tunnel junction with AlInP barriers which constitute a double-hetero structure and increase the peak current of the tunnel junction. In addition, the AlInP barrier located beneath the InGaP top cell have been found to be effective for reflecting minority carriers in the top cell and for suppressing the diffusion of zinc from the highly doped tunnel junction toward the top cell during epitaxial growth.

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