Titanium‐Doped In ₂ O ₃ : A High‐Mobility, Thermally Stable Back Contact for Bifacial Chalcopyrite Solar Cells

This study evaluates In 2 O 3 :Ti as a transparent back contact (TBC) material in bifacial (Ag,Cu)(In,Ga)Se 2 (ACIGS) solar cells with a band gap of 1.1 eV and compares it to commonly used In 2 O 3 :Sn. Both TBC layers were processed with a sheet resistance ≤10 Ω/sq, as required in a monolithically series‐connected ACIGS module. In contrast to several other high‐mobility TBCs previously tested in ACIGS solar cells, In 2 O 3 :Ti retains its exceptionally high mobility (>100 cm 2 /Vs), low resistivity (2.4·10 −4 Ωcm), and minimal near‐infrared absorption (<5% at ACIGS bandgap) after high‐temperature absorber deposition. As a result, an up to 3 mA/cm 2 higher short‐circuit current density is measured under rear illumination for cells with an In 2 O 3 :Ti back contact as compared with devices using highly doped In 2 O 3 :Sn. The best cell reaches an efficiency of 10.2% at rear illumination with a bifaciality factor of 68%. At front illumination, the cell performance is on the same level for both TBCs.