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As an effort to improve the conversion efficiency of InGaP/InGaAs/Ge triple junction solar cells, we investigated the effect of AlGaAs barrier layer on the cell performance under concentrated light condition. Three different solar cells having upper tunnel junctions (TJs) as n ++ -GaAs/p ++ -AlGaAs layers, n ++ -GaAs/p ++ -GaAs layers and n ++ -GaAs/p ++ -GaAs/p ++ -AlGaAs layers were prepared. Under concentrated light condition, open-circuit voltage ( V OC ), fill factor (FF) and conversion efficiencies were higher for the sample with an AlGaAs barrier layer than the samples without an AlGaAs barrier layer. For the sample with an AlGaAs barrier layer, external quantum efficiency was higher than other two samples. Most of all, the sample with a TJ as n ++ -GaAs/p ++ -GaAs layers showed a very poor electrical performance, which was associated with an imperfect crystalline quality of the InGaP top cell layers. [ABSTRACT FROM AUTHOR]