Insights into defects and their diffusion in CdTe from hybrid functional calculations

Cadmium Telluride (CdTe) has emerged as a leading thin-film technology for solar cells. The main favorable characteristics, including an optimal band gap of ~1.5 eV, a high absorption coefficient, and a simple manufacturing process have been driving extensive research efforts to enhance its photoconversion efficiency. Despite a respectable efficiency record of 22% in the laboratory and 18.6% in commercially available modules, these are still far below the theoretical limit of 33%. It is expected that higher hole concentration is crucial to improving efficiency, however, the limitations in achieving higher doping efficiencies are still unknown. To address this issue, we performed hybrid density functional calculations to investigate the formability and diffusivity of native defects in CdTe, analyzing their impact on the electronic structure of the material. Our study reveals that most native defects are donors and act as compensation centers in p-type material. Yet, some exhibit low migration barriers and are expected to be unstable at room temperature. Cd interstitial is the lowest energy donor defect in p-type CdTe. But its migration barrier of 0.5 eV makes it highly mobile and unstable at room temperature. The Te vacancy is the next lowest formation energy donor. The migration barrier of 1.4 eV indicates that the Te vacancy is stable at room temperature, and likely an important compensation center. The antisite Cd on Te is also a donor, with a low formation energy and a much higher migration barrier, potentially limiting the hole concentration in samples grown under Cd-rich conditions. Our results, therefore, provide insights into potential compensation centers and offer guidance to mitigate their concentration and their impact on CdTe-based solar cells.