“…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. In substrate configuration Cu is commonly added to the devices using the same approach as developed for devices in superstrate configuration, that is, the use of a Cu-containing layer with an equivalent Cu thickness of several nanometres in the electrical back contact structure 15,16,18 .…”