Theoretical Study on the Diffusion Mechanism of Cd in the Cu-Poor Phase of CuInSe2 Solar Cell Material

摘要

We have employed first-principles static and molecular dynamics (MD) calculations with semilocal and screened-exchange hybrid density functionals to study the diffusion of Cd in bulk CuIn5Se8, a copper-poor ordered vacancy compound of CuInSe2. The diffusion mechanism and the underlying kinetics/energetics were investigated by combining ab initio metadynamics simulations and nudged elastic band (NEB) calculations. We found that the migration of Cd occurs via a kick-out of Cu atoms, assisted by the pristine vacancies that are constitutive of this compound, and follows a double-hump energy profile. The rate-limiting step has a barrier of about 1 eV at 0 K but reduces to 0.3 eV at 850 K, pointing out non-negligible dynamical effects. Hybrid functional calculations reveal that Cd impurities are doubly positively charged (Cd~(2+)) in p-type and intrinsic conditions. The position of the 0/2+ charge transition level explains why Cd impurities do not constitute deep traps for carriers, making them not harmful for the solar cell device.