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Abstract:
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
(Cd2+) 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.
