Band-gap grading in Cu(In,Ga)Se2 solar cells

The quaternary system Cu(In,Ga)Se2 (CIGS) allows the band gap of the semiconductor to be adjusted over a range of 1.04–1.67 eV. Using a non-uniform Ga/In ratio throughout the film thickness, additional fields can be built into p-type CIGS-based solar cells, and some researchers have asserted that these fields can enhance performance. The experimental evidence that grading improves device performance, however, has not been compelling, mostly because the addition of Ga itself improves device performance and hence a consistent separation of the grading benefit has not always been achieved. Numerical modeling tools are used in this contribution to show that (1) there can be a beneficial effect of grading, (2) in standard thickness CIGS cells the benefit is smaller than commonly believed, (3) there is also the strong possibility of reduced rather than of increased device performance, and (4) thin-absorber cells derive more substantial benefit.