Tandem solar cells with Cu(In,Ga)S2 top cells on ZnO coated substrates

We have prepared transparent top cells for stacked tandems with wide gap Cu(In,Ga)S2 absorbers on ZnO:Al/soda-lime-glass (SLG) and Mo/ZnO:Al/SLG substrates. Their overall transmission increases with the absorber band gap. The band gaps were 1.63, 1.72, and 1.79 eV. We investigated the properties of mechanically stacked tandem solar cells using these top cells and a CuInSe2 bottom cell. The short circuit current density of the tandem cells is typically limited by that of the bottom cell illuminated through the top cells. Our best tandem efficiency was estimated to be 4.4% with an open circuit voltage of 1.08 V.