NOVELTY - The strain-balanced quantum well tunnel junction (SB-QWTJ) device comprises a QWTJ that is situated between n++ and p++ layers on a substrate and includes an n-type quantum well structure adjacent to a p-type quantum well structure. The n-type quantum well structure comprises an n-type quantum well layer disposed between corresponding n-type barrier layers, and the p-type quantum well structure comprises a p-type quantum well layer disposed between corresponding p-type barrier layers, where the n-type barrier layer is provided adjacent to the p-type barrier layer.    USE - The strain-balanced quantum well tunnel junction device is useful in metamorphic solar cells.    ADVANTAGE - The device improves power conversion efficiency, tunneling performance and optical transparency of the solar cells.    DETAILED DESCRIPTION - The strain-balanced quantum well tunnel junction (SB-QWTJ) device comprises a QWTJ that is situated between n++ and p++ layers on a substrate and includes an n-type quantum well structure adjacent to a p-type quantum well structure. The n-type quantum well structure comprises an n-type quantum well layer disposed between corresponding n-type barrier layers, and the p-type quantum well structure comprises a p-type quantum well layer disposed between corresponding p-type barrier layers, where the n-type barrier layer is provided adjacent to the p-type barrier layer. One of the n- and p-type quantum well layers exhibits a first strain with respect to the substrate, and one of the corresponding n- and p-type barrier layers exhibits a second strain with respect to the substrate. A composition and a thickness of the quantum well layers and the barrier layers are configured to balance a net strain in the device. The quantum well layer exhibits a compressive strain with respect to the substrate, and the corresponding barrier layer exhibits a tensile strain with respect to the substrate. The quantum well layer and the barrier layer are configured such that the tensile strain in the barrier layer balances the compressive strain in the device. The substrate comprises indium phosphide, the quantum well layer is configured to exhibit the compressive strain with respect to the indium phosphide substrate, and the corresponding barrier layer is configured to exhibit a tensile strain with respect to the indium phosphide substrate. The QWTJ structure comprises: an n-type indium gallium arsenide (InxGa1-xAs) quantum well layer disposed between corresponding n-type indium aluminum arsenide (InyAl1-yAs) barrier layers; and a p-type indium gallium arsenide quantum well layer disposed between corresponding p-type indium aluminum arsenide barrier layers. The n-type indium aluminum arsenide barrier layer is provided adjacent to the p-type indium aluminum arsenide barrier layer.