Total band alignment in theoretical and experimental aspects for enhanced performance of flexible and Cd-free Cu (In,Ga)(S,Se)2 solar cell fabricated by all-dry process

Total band alignment parameters, which are CBO, Delta EC-TA, and Delta EC-TB values, in the Cu (In,Ga)(S,Se)(2) (CIGSSe) solar cells are theoretically and experimentally optimized. Conduction band minimum (E-C) difference between the CIGSSe absorber and buffer is represented by CBO, and that between absorber and transparent conductive oxide (TCO) is denoted by Delta EC-TA. The E-C difference between buffer and TCO is represented by Delta EC-TA. Based on the measurement of photoelectron yield spectroscopy, the actual CBO, Delta EC-TA, and Delta EC-TB values are disclosed. According to the numerical simulation, the useful overview information about the appropriate CBO, Delta EC-TA, and Delta EC-TB values is disclosed for the selections of the suitable buffer and TCO materials to reduce the carrier recombination, thus enhancing the photovoltaic performances of the CIGSSe solar cell. As a result, the sputtered Zn0.84Mg0.16O buffer and sputtered Zn0.88Mg0.12O:Al TCO layers are experimentally applied for the appropriate CBO of +0.11 eV, Delta EC-TA of +0.16 eV, and Delta EC-TB of +0.05 eV, thus giving rise to the flexible, Cd-free, and all-dry process CIGSSe solar cell on stainless steel substrate with the increased conversion efficiency up to 15.3% (and the highest one of 16.5%) as the eco-friendly and low-cost solar cell. It is ultimately suggested that the experimental results are well predicted by the overview numerical results, which are useful for the improvement of the photovoltaic performances through the total band alignment.