The effect of excitons in AlxGa1-xAs/GaAs superlattice solar cells has been investigated. We have shown that the superlattice active layers are effective to improve the solar cell performances because of the exciton enhanced photo-absorption. External quantum efficiency spectra show sharp and intense increase at the absorption edge due to excitonic absorption. This result indicates that excitonic photo-absorption can be stabilized at room temperature by using a superlattice structure. Optical properties of superlattice solar cells depend on the superlattice parameters because they determine the excitonic confinement effect, the tunneling effect and the sub-band structure. In this study, we compare external quantum efficiency for solar cells with different superlattice parameters to optimize the structure. The optimal barrier layer thickness is determined to be 1 nm for the Al0.5Ga0.5As/GaAs superlattice solar cell with 2-mu m-thick active layer. (C) 2015 Elsevier B.V. All rights reserved.