the CADMIUM TELLURIDE SOLAR CELLS (CdTe) technology, which currently is the thin-film technology with the lowest demonstrated cost per Wp. We start with discussing the physical properties of CdTe, which is a II-VI semiconductor because it consists of the II valence electron element cadmium (Cd) and the VIvalence electron element tellurium (Te).
The band gap of CdTe is 1.44 eV, a value which is close to the optimal band gap for single junction solar cell. CdTe is a direct band gap material, consequently only a few micrometres of CdTe are required to absorb all the photons with an energy higher than the band gap energy. If the light-excited charge carriers should be efficiently collected at the contacts, their diffusion has to be in the order of the thickness.
CADMIUM TELLURIDE SOLAR CELLS CdTe can be n-doped by replacing the II-valence electron atom Cd with a III-valence electron atom like aluminium, gallium or indium. n-doping can be achieved as well by replacing a VI-valence tellurium atom with a VII-valence electron element like fluorine, chlorine, bromine and iodine atoms. The III- and VII-valence atoms act as shallow donors. Also a tellurium vacancy acts like a donor
p-doping of CdTe can be achieved by replacing Cd with a I-valence electron atom like copper, silver or gold. It can be achieved as well by replacing Te atoms with Vvalence electron elements such as nitrogen, phosphorus or arsenicum. These elements act as shallow acceptors. But also a Cd vacancy acts as an acceptor. In solar cells, p-doped CdTe is used. However, it is difficult to obtain CdTe with a high doping level.
above Figure shows the structure of a typical CdTe solar cell. First, transparent front contact is deposited onto the glass superstrate. This can be tin oxide or cadmium stannate, which is a Cd-Sn-oxide alloy. On top of that the n-layer is deposited, which is a cadmium sulfide layer, similar to the n-uffer layer in CIGS solar cells . Then, the p-type CdTe absorber layer is deposited with a typical thickness of a few micrometres. Making a good back contact on CdTe is rather challenging because the material properties of CdTe do not allow a large choice of acceptable metals. Heavily
doping the contact area with a semiconductor material improves the contact qualities, however, achieving high doping levels in CdTe is problematic. Copper containing contacts have been used as back contacts, however, on the long term they may face instability problems due to diffusion of Cu through the CdTe layer up to the CdS buffer layer. Nowadays, stable antimony telluride layers in combination with molybdenum are used.