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

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 Zn0.84Mg0.16O buffer and sputtered Zn1-xMgxO:Al transparent conductive oxide (TCO). The conduction band minimum (E-C) difference between the CIGSSe absorber and Zn1-xMgxO:Al TCO layer, which is called Delta EC-TA, is examined as the promising parameter to enhance conversion efficiency (eta). According to photoelectron yield spectroscopy, actual valence band maximum (E-V) of Zn1-xMgxO:Al is almost constant from -7.54 to -7.57 eV under the increased Mg contents from 0 to 0.19, where the hand gap (E-g) is changed from about 3.5 to 3.9 eV, respectively. Namely, Mg content variation in Zn1-xMgxO:Al primarily changes the E-C value, thereby giving rise to the adjustment of Delta EC-TA. It is disclosed that Delta EC-TA plays an important role in increasing eta. In theoretical results, optimized Delta EC-TA for high performances is in a range from +0.07 to +0.57 eV. In experimental results, the eta values are clearly enhanced from 13.7% with a Delta EC-TA of -0.06 eV (for ZnO:Al TCO) to 16.1% (and the highest eta of 16.5%) with the optimal Delta EC-TA from +0.16 eV to +0.22 eV (for Zn1-xMgxO:Al TCO) under an optimal Mg content from 0.12 to 0.16, well consistent with the theoretical results.