Effect of Cu Doping on the Properties of ZnTe: Cu Thin Films and CdS/CdTe/ZnTe Solar Cells

The effects of Cu doping concentration and post-deposition annealing treatment on the properties of ZnTe thin films were investigated in an effort to decrease the Cu doping concentration and improve the long-term stability of CdS/CdTe/ZnTe solar cells. The structural, compositional, and electrical properties were studied systematically using x-ray diffraction (XRD), electron microprobe, Halleffect and conductivity measurements. XRD measurements indicated that the crystalline phase of as-deposited and low-temperature annealed ZnTe films is dependent on Cu doping concentration. Low-Cu-doped films exhibited zincblende phase, whereas high-Cu-doped films showed wurtzite phase. After annealing at high temperature (.gtoreq. 350.deg.C), all films exhibited zincblende structure. Electronprobe microanalysis revealed a deficiency of cations in low-Cu-doped films and an excess of cations in high-Cu-doped films. Hall effect measurements revealed a dependence of hole mobility on Cu doping concentration with the highest mobility (20 cm2/V-s) obtained at a low Cu concentration. Carrier concentrations higher than mid-10 (sup 18) cm-3 were obtained at a Cu concentration of 2 at. % andrelatively low annealing temperatures. Studies of the activation energy of dark conductivity suggested that intrinsic defects (e.g., Zn vacancies) are the dominant acceptors for Cu concentrations lower than 4.5 at.%. Finally, ZnTe films with Cu concentrations as low as 1 at.% were used successfully as a back contact layer in CdTe based solar cells. Fill factors over 0.70 were obtained using ZnTefilms of low Cu concentrations.