Chalcopyrite Thin-Film Tandem Solar Cells with 1.5 V Open-Circuit-Voltage

We have fabricated two-terminal mechanical stacked chalcopyrite-based tandem devices to identify the technical issues required to achieve high performance solar cells in the near future. A tandem device with a Ag(ln_0.2Ga_0.8)Se₂ (AIGS) top cell and a Cu(ln,Ga)Se₂ (CIGS) bottom cell showed an open-circuit-voltage (V_oc) of 1.46 V. It was, however, found that the short-circuit current (J_sc) of a two-terminal tandem device was limited by the low J_sc of the filtered bottom cell because of a current mismatch between the top and bottom cells. In order to improve the Jsc of the filtered bottom cell, we have tried two process options, replacing ITO back contacts with high-mobility (Mo-doped ln₂O₃) IMO and the use of low-Ga content CIGS bottom cells. The J_sc of the filtered bottom cells increased significantly using these techniques. As a result, an 8 % total-area (0.5 cm²) efficiency 2-terminal mechanical stacked tandem device was achieved using a CIGS (15% Ga) bottom cell with an AIGS top cell fabricated with an IMO back contact