Mg Content Impact of a Sputtered Zn1–xMgxO:Al Transparent Electrode on Photovoltaic Performances of Flexible, Cd-Free, and All-Dry-Process Cu(In,Ga)(S,Se)2 Solar Cells

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Development of flexible, Cd-free, and all-dry-process Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cells on stainless-steel (SUS) substrates has been conducted with sputtered Zn 0.84 Mg 0.16 O buffer and sputtered Zn 1– x Mg x O:Al transparent conductive oxide (TCO). The conduction band minimum ( E C ) difference between the CIGSSe absorber and Zn 1– x Mg x O:Al TCO layer, which is called Δ E C-TA, is examined as the promising parameter to enhance conversion efficiency (η). According to photoelectron yield spectroscopy, actual valence band maximum ( E V ) of Zn 1– x Mg x O:Al is almost constant from −7.54 to −7.57 eV under the increased Mg contents from 0 to 0.19, where the band gap ( E g ) is changed from about 3.5 to 3.9 eV, respectively. Namely, Mg content variation in Zn 1– x Mg x O:Al primarily changes the E C value, thereby giving rise to the adjustment of Δ E C-TA . It is disclosed that Δ E C-TA plays an important role in increasing η. In theoretical results, optimized Δ E C-TA for high performances is in a range from +0.07 to +0.57 eV. In experimental results, the η values are clearly enhanced from 13.7% with a Δ E C-TA of −0.06 eV (for ZnO:Al TCO) to 16.1% (and the highest η of 16.5%) with the optimal Δ E C-TA from +0.16 eV to +0.22 eV (for Zn 1– x Mg x O:Al TCO) under an optimal Mg content from 0.12 to 0.16, well consistent with the theoretical results.