Titanium-Doped In2O3: A High-Mobility, Thermally Stable Back Contact for Bifacial Chalcopyrite Solar Cells

Abstract

This study evaluates In2O3:Ti as a transparent back contact (TBC) material in bifacial (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells with a band gap of 1.1 eV and compares it to commonly used In2O3: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, In2O3:Ti retains its exceptionally high mobility (>100 cm2/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/cm2 higher short-circuit current density is measured under rear illumination for cells with an In2O3:Ti back contact as compared with devices using highly doped In2O3: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.