Effect of back contact metallization on the material stability of CdTe/CdS solar cells

The electrical contact on CdTe is important for the high efficiency and long term stability of CdTe/CdS solar cells. As most of the metals form a Schottky barrier on p-type CdTe, a buffer layer is needed to reduce the effective barrier height and enhance the acceptor concentration at the back contact. In order to obtain stable but highly efficient cells we investigated different back contact materials and etching procedures. Accelerated life time tests revealed the stability of the cells. CdTe/CdS cells with Al or Au metallization showed degradation, whereas Mo metallization produced very stable cells with Sb2Te3 and Sb buffer layers. These cells have up to 10 % efficiency after 9 months accelerated life time tests which corresponds to 75 years under normal conditions. The barrier height and the doping profile have been measured with C-V characteristics