“…For example, first-principle calculations suggest that Cu dopants in CdTe can occupy the Cd atomic site acting as an acceptor, as well as the interstitial positions acting as a donor 20 . Originally, Cu has been added to the back contact of CdTe solar cells with the purpose of improving the contact properties of CdTe by forming a degenerate semiconductor layer, for example, in the form of Cu x Te, Cu-doped ZnTe, HgTe:CuTe-doped graphite paste or As 2 Te 3 :Cu 5, [21][22][23][24] . During back contact processing, part of the Cu has also been found to diffuse into CdTe 25,26 , where it increases acceptor concentration and affects carrier lifetime.…”