Revisiting the role of group-V acceptor impurities and their AX centers in CdTe

CdTe thin-film solar cells are commercially available technology, with active layers that are just a few microns thick and record power conversion efficiency of over 22%. Further improvements will rely on increasing hole concentration and carrier lifetimes, and reducing recombination rates.  At the  heart of this problem is the behavior of acceptor impurities such as Sb, As, and P.  While early theoretical results indicated a rather high ionization energy of 230 meV for Sb substituting on the Te site, and likely formation of AX compensation centers, leading to low hole concentrations, recent experimental observations indicate a much lower ionization energy of ~100 meV, and put into question the hole compensation by the formation of AX centers.  Understanding the behavior of these group-V impurities is crucial for designing the growth and processing of devices. Using electronic structure calculations based on hybrid density functional theory, we investigate formation energies and acceptor transition levels of P, As, and Sb in CdTe, and explore the formation of related AX centers. We pay special attention to the inclusion of spin-orbit coupling in the calculations and the size of the supercells to describe these acceptor impurities and related AX centers.  Our results indicate lower ionization energies than previously reported, and that AX centers are unlikely to play important role in the compensation of p-type conductivity.  Compensation by acceptor native defects are proposed to explain the experimental data.

This work was supported by the US DOE EERE Solar Energy Technologies Office, grant no. DE-EE0009344, and made use of NERSC supercomputer facility.