With respect to an efficient tandem construction the transmission of the top cell merits highest attention. For a cell on the basis of CuGaSe2 we present here a detailed optical characterization. The initial focus is on the investigation of the individual layers making up the cell stack, followed by their combination. Results from considerations on light trapping in the CuGaSe2 are shown as well as modifications of the transparent conducting back contact related to the absorber growth. Material parameters obtained in this way serve as input for an optical model. As an outlook we present a loss diagram for the short circuit current density of the bottom cell related to the optical properties of the CuGaSe2 top cell.
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