Development of a Zn-based n-layer in Cuprous Oxide Top Cells for High-efficiency Tandem Photovoltaics

We fabricated transparent Cu
₂
O top cells consisting of a pn junction between a Cu
₂
O p-layer and a Zn-based n-layer for tandem photovoltaics. The open-circuit voltage (V
_oc
) exceeded 1.0 V and efficiency was 6.6% in a top cell using zinc tin oxide (ZTO) for the n-layer. A tandem solar cell combining this top cell with a Si bottom cell demonstrated a tandem efficiency of 24.2%, exceeding the efficiency of Si alone. Device simulations showed that V
_oc
can be further improved by reducing discontinuity in the conduction band, and that both top-cell efficiency and tandem efficiency can be improved.