Formation and Stability of Dopants and Defects in CdTe-Based Solar Cells

CdTe is a key semiconductor material for thin-film solar cells, with a record efficiency of 23.1%. Increasing the efficiency toward the theoretical limit of 32-33% requires a better understanding and control of doping and defects in CdTe and CdSeTe. Doping efficiencies in CdTe absorbers are quite low (~1 hole per 100 dopants for ~10¹⁷ cm^-3 dopants) compared to typical semiconductors, such as Si and GaAs. The source of compensation and possible causes of non-radiative recombination have been widely discussed in the literature. There is a lack of consensus on which native defects are most abundant and more impactful in single crystals and thin films. In this presentation, we discuss our most recent results of first-principles calculations based on density functional and hybrid functionals for dopants and defects in CdTe and CdSeTe, focusing not only on defect formation energies and transition levels but also on migration barriers to discuss the incorporation and stability of defects in different stages of the solar cell fabrication, from deposition to post-deposition treatments at high temperatures and bringing the material down to room temperature. We argue that the chemical environment during the last stages of film preparation, including the chloride treatment, dictates which defects or complexes are incorporated and, thus, the impact on the electrical and optical properties of the films.

This work was performed in collaboration with I. Chatratin, I. Evangelista, W. Shafarman, and B. McCandless, and supported by the US DOE EERE Solar Energy Technologies Office, grant no. DE-EE0009344, and made use of NERSC supercomputer facility.