Derived Conduction Band Offset by Photoelectron Yield Spectroscopy and Its Quantitative Number for Efficiency Enhancement of Flexible, Cd-Free, and All-Dry Process Zn1–xMgxO:Al/Zn1–xMgxO/Cu(In,Ga)(S,Se)2 Solar Cells

The actual conduction band offset (CBO) values of flexible, Cd-free, and all-dry process sputtered Zn0.88Mg0.12O:Al/sputtered Zn1–xMgxO buffer/Cu(In,Ga)(S,Se)2 solar cells on stainless steel were estimated using photoelectron yield spectroscopy. It is disclosed that the change of Mg content of the buffer from 0 to 0.41 yields the variation of its band gap (Eg) from 3.29 to 4.14 eV, thereby adjusting the CBO at the CIGSSe/Zn1–xMgxO interface from −0.20 to +0.60 eV. It is theoretically predicted that the derived CBO values are quantitatively optimized in a region from approximately −0.18 to +0.23 eV for high cell performance. The optimized and quantitative CBO values were then applied into the real devices. Consequently, high experimental conversion efficiencies up to about 15.0% (the highest one of 16.6%) are realized under the derived CBO values in a range from about +0.11 to +0.23 eV (or Mg content of Zn1–xMgxO buffer from 0.16 to 0.23), corresponding to the improved carrier lifetime and reduction of carrier recombination. On the other hand, the decreased CBO to −0.2 eV (using ZnO buffer) or lower and the increased CBO over +0.23 eV (Zn1–xMgxO buffer with an Mg content of over 0.23) generate large cliff-like and spike-like pn junctions, respectively, thereby increasing carrier recombination and reducing cell performances. Ultimately, the optimized derived CBO range was experimentally disclosed for the efficiency enhancement, which was numerically confirmed.