Effect of Intermixing at CdS/CdTe Interface on Defect Properties

We investigated the stability and electronic properties of defects in CdTe_1-xS_x that can be formed at the CdS/CdTe interface. As the anions mix at the interface, the defect properties are significantly affected, especially those defects centered at cation sites like Cd vacancy, V_Cd, and Te on Cd antisite, Te_Cd, because the environment surrounding the defect sites can have different configurations. We show that at a given composition, the transition energy levels of V_Cd and Te_Cd become close to the valence band maximum when the defect has more S atoms in their local environment, thus improving the device performance. Such beneficial role is also found at the grain boundaries when the Te atom is replaced by S in the Te-Te wrong bonds, reducing the energy of the grain boundary level. On the other hand, the transition levels with respect to the valence band edge of CdTe_1-xS_x increases with the S concentration as the valence band edge decreases with the S concentration, resulting in the reduced p-type doping efficiency.