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

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 CuIn 5 Se 8 , a copper-poor ordered vacancy compound of CuInSe 2 . 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 ptype 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.

Investigation of the potassium fluoride post deposition treatment on the CIGSe/CdS interface using hard X-ray photoemission spectroscopy – a comparative study

“…They concluded that the increased deficiency of Cu at the CIGSe surface leads to an enhanced Cd diffusion into the absorber during the chemical bath deposition (CBD) of the CdS buffer layer even at low process temperatures. This phenomenon is also supported by theoretical studies conducted by Kiss et al 8,9 Recently, we also demonstrated with hard X-ray photoelectron spectroscopy (HAXPES) that the amount of Cd diffusing into the interface region of the absorber during the CBD of CdS is dependent on the Cu concentration. 10 The penetration of Cd into the CIGSe absorber becomes even more pronounced upon annealing in ultra-high vacuum (UHV) at 350 1C indicating that the Cd diffusion in CIGSe might be correlated with the Cu migration in this material which is directly connected to the formation of Cu vacancies (V Cu ).…”

Cd and Cu Interdiffusion in Cu(In, Ga)Se2/CdS Hetero-Interfaces

“…Additionally, Lei et al, also using TEM, have observed the presence of Cu in the CdS layer, but have only seen CdS present at the GBs of the CIGS [4]. Although it has been shown that Cd diffuses in the CIGS mostly in the bulk and not in the GBs [34], a theoretical study has shown that the most likely diffusion mechanism is based on two steps by creating a Cu vacancy in the CIGS followed by a displacement of Cd into the vacancy [38]. These processes have energy barriers of 1 eV and 0.5 eV, respectively [38], in accordance with experimental data [34].…”

Point contacts at the copper-indium-gallium-selenide interface—A theoretical outlook

“…An optimum buffer layer, in addition to appropriate band line-up, should also generate shallow defects at the interface carrying the required positive charge. The standard CdS buffer layer might do so via ionized Cd within the Cu-free surface reconstruction of CIGS [23]. As our model includes charged defects only at the PL but not at the contact interface, it may be argued that the benefit of the PL layer is exaggerated.…”