I would like to ask this question by the example of the CdS buffer layer in CIGS solar cells. One paper (https://pubs.rsc.org/en/content/articlelanding/2017/se/c7se00348j#!divAbstract) says that CdS is deployed, because it has "excellent properties for better device performances such as favorable band alignment both to CIGS and ZnO, protection of the absorber surface by complete coverage of its surface (due to close lattice matching) from the subsequent layer deposition process, and removal of natural oxides from the absorber surface by the CBD process".
I am particularly interested in the part about "favorable band alignments". So far I didn't really understand what was written about it and while I wait for answers, I will further read the Wikipedia article on "Band offsets" and the book "Heterojunction band offset engineering" (Franciosi, A.; Walle, C.).
If someone could explain to me (with only basic knowledge in Fermi-Dirac distribution, and a little bit more detailed knowledge in solar cell physics), what "favorable band alignments" mean, I would be super happy. :-)
Also, how relevant is this reason compared to other ones stated in the quote above?