Sputtered Cd1-xMgxTe Films for Top Cells in Tandem Devices

Cd_1-xMg_xTe is a potential candidate for the top cell in two-terminal tandem solar cells. The close match of the lattice constant of MgTe with CdTe and the apparent complete miscibility of MgTe in CdTe and the rapid increase in band gap with Mg content gives flexibility to prepare material with the appropriate band gap for current-matching in a tandem solar cell. We have chosen to deposit Cd _1-xMg_xTe thin films by RF sputtering to maintain access to low growth temperatures. Substrate temperatures greater than 250degC produced highly adhering polycrystalline films with grain size in the range of 100-150 nm and a strong preferential (111) orientation. All the films were p-type and resistive. Energy Dispersive X-ray Spectroscopy (EDS) analysis showed that the Mg concentration in the film is significantly lower than that of the source target. Films were chloride-treated at temperatures in the range 350 to 387degC in a variety of ambients and the band gap showed narrowing at higher annealing temperature and time duration, particularly in oxygen containing ambients. XRD and AFM studies of the Cd_1-xMg_x Te films showed evidence of re-crystallization including grain growth and a more random crystallographic orientation of the grains. Prototype CdS/Cd_1-xMg_xTe solar cells were fabricated