Defect properties of Sb- and Bi-doped CuInSe2: The effect of the deep lone-pair s states

摘要

Bi or Sb doping has been used to make better material properties of polycrystalline Cu(In,Ga)Se as solar cell absorbers, including the experimentally observed improved electrical properties. However, the mechanism is still not clear. Using first-principles method, we investigate the stability and electronic structure of Bi- and Sb-related defects in CuInSe and study their effects on the doping efficiency. Contrary to previous thinking that Bi or Sb substituted on the anion site, we find that under anion-rich conditions, the impurities can substitute on cation sites and are isovalent to In because of the formation of the impurity lone pair s states. When the impurities substitute for Cu, the defects act as shallow double donors and help remove the deep In level, thus resulting in the improved carrier life time. On the other hand, under anion-poor conditions, impurities at the Se site create amphoteric deep levels that are detrimental to the device performance.