Wide bandgap Cu2ZnSn1−xGexS4 fabricated on transparent conductive oxide-coated substrates for top-cells of multi-junction solar cells

•Cu2ZnSn1−xGexS4 (CZTGS) is fabricated on TCO-coated glass substrates.•Bandgap is controlled by the Ge/(Sn + Ge) composition ratio.•Efficiency is 2.4% with an optimal bandgap for double-junction solar cells.

Multi-junction solar cells made up of only non-toxic and earth-abundant elements can solve both energy and environmental problems. We fabricated Cu2ZnSn1−xGexS4 (CZTGS) solar cells on transparent-conductive-oxide (TCO)-coated glass substrates in order to use them as top-cells. We investigated the relationship between the bandgap and the Ge/(Sn + Ge) composition ratio of the CZTGS thin films. The bandgap increased with an increase in the Ge/((Sn + Ge)) composition ratio. We also fabricated a CZTGS solar cell with a bandgap of 1.8 eV with a Ge/(Sn + Ge) composition ratio of 0.69, which is close to the optimal bandgap for the top-cells of double-junction solar cells. A maximum conversion efficiency of 2.4% was exhibited by the cell fabricated on TCO with a bandgap of 1.8 eV.

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