The Back Contact in CdTe/CdS Thin Film Solar Cells

A barrier-free back contact in CdTe/CdS solar cells is fundamental for obtaining high-efficiency devices. Generally, the back contact is made by depositing a few nanometres of copper onto an etched CdTe surface, where a thin layer of tellurium is formed. In this way, a CuxTe film is obtained forming an ohmic contact with CdTe. However, if x exceeds 0.4 the contact is not stable and the solar cell degrades. Several attempts were made to make a barrier-free contact by using low-resistivity p-type materials such as: Sb2Te3, As2Te3, Bi2Te3, MoOx and ZnTe but the results were not definitive. Still some copper seems to be needed. We used As2Te3, Bi2Te3 and ZnTe as buffers covered by ̴ 10 nm of copper deposited at 520 K substrate temperature. This kind of contact works very well but only if the final contact is made of a thin layer of platinum covered by a thicker layer of molybdenum. Platinum exhibits a high work function (5.8 eV) and it is thus suitable to make an ohmic contact to the p-type high-conductivity buffer layers.