Alkali Atoms Diffusion Mechanism in CuInSe₂ Explained by Kinetic Monte Carlo Simulations

Abstract

Adaptive kinetic Monte Carlo simulation (aKMC) is employed to study the dynamics and the diffusion of point defects in the CuInSe 2 lattice. The aKMC results show that lighter alkali atoms can diffuse into the CuInSe 2 grains, whereas the diffusion of heavier alkali atoms is limited to the Cu-poor region of the absorber. The key difference between the diffusion of lighter and heavier alkali elements is the energy barrier of the ion exchange between alkali interstitial atoms and Cu. For lighter alkali atoms like Na, the interstitial diffusion and the ion-exchange mechanism have comparable energy barriers. Therefore, Na interstitial atoms can diffuse into the grains and replace Cu atoms in the CuInSe 2 lattice. In contrast to Na, the ion-exchange mechanism occurs spontaneously for heavier alkali atoms like Rb and the further diffusion of these atoms depends on the availability of Cu vacancies. The outdiffusion of alkali substitutional atoms from the grains results in the formation of Cu vacancies which in turn increases the hole concentration in the absorber. In this respect, Na is more efficient than Rb due to the higher concentration of Na substitutional defects in the CuInSe 2 grains.

Effectiveness of an RbF Post Deposition Treatment of CIGS Solar Cells in Dependence on the Cu Content of the Absorber Layer

“…On the other hand, recently published Monte Carlo simulations show that for heavy alkali metals (like Rb) the diffusion via V Cu has much lower energy barriers than diffusion via interstitial positions in the lattice, while in case of Na these barriers are comparable [26]. Therefore, it seems likely that Rb preferably occupies Cu sites in Cu-poor material near the GBs or at the surface of the CIGS [26], but is rather unlikely to replace Cu in a stoichiometric CIGS lattice. In samples with a high number of available V Cu , Rb may hence not occupy Na sites at the GBs but directly diffuses into the Cu-poor material at the surface of the CIGS lattice [cf.…”

“…The positive effect of Rb on V OC is-among other mechanisms-generally attributed to the following effects: an Rb-Na exchange mechanism, in which Rb pushes Na into the grain interior leading to improved n CV [15]- [18] as well as a subsequent passivation of the GBs by the Rb itself [19]- [24] leading to improved carrier lifetimes [3], [18], [25]. On the other hand, recently published Monte Carlo simulations show that for heavy alkali metals (like Rb) the diffusion via V Cu has much lower energy barriers than diffusion via interstitial positions in the lattice, while in case of Na these barriers are comparable [26]. Therefore, it seems likely that Rb preferably occupies Cu sites in Cu-poor material near the GBs or at the surface of the CIGS [26], but is rather unlikely to replace Cu in a stoichiometric CIGS lattice.…”

Insights into the Effects of RbF‐Post‐Deposition Treatments on the Absorber Surface of High Efficiency Cu(In,Ga)Se2 Solar Cells and Development of Analytical and Machine Learning Process Monitoring Methodologies Based on Combinatorial Analysis

“…[ 19 ] For light alkali metals such as Na and Li, the diffusion via copper vacancies (V Cu ) and the diffusion via interstitial positions in the lattice have comparable energy barriers, thus it was shown that Na can either replace vacant Cu places or occupy interstices in the crystal lattice of CIGSe. [ 25 ] However, for heavy alkali metals, such as rubidium, the energy barrier is much lower for the diffusion via V Cu than for the diffusion via the interstitial positions. [ 25 ] As a consequence, under Cu‐poor conditions, Rb will have a preference to occupy vacant Cu sites, in particular the ones located near the grain boundaries (GBs) or at the surface of the CIGSe absorber, since the large atomic radius of Rb prevents its deeper penetration into the CIGSe grains.…”

“…[ 25 ] However, for heavy alkali metals, such as rubidium, the energy barrier is much lower for the diffusion via V Cu than for the diffusion via the interstitial positions. [ 25 ] As a consequence, under Cu‐poor conditions, Rb will have a preference to occupy vacant Cu sites, in particular the ones located near the grain boundaries (GBs) or at the surface of the CIGSe absorber, since the large atomic radius of Rb prevents its deeper penetration into the CIGSe grains. In a stoichiometric CIGSe lattice, it is not probable for Rb to replace Cu due to the low availability of vacancies, and it will remain outside the CIGSe grain.…”

“…In a stoichiometric CIGSe lattice, it is not probable for Rb to replace Cu due to the low availability of vacancies, and it will remain outside the CIGSe grain. [ 12,25 ] Taking this into account, it has been proposed that the positive impact of Rb on the V oc for CIGSe with [Cu]/([Ga]+[In]) (CGI) ≥ 0.8 can be attributed to a Rb‐Na exchange mechanism in which Rb is accumulated at the GBs and displaces Na toward the interior of the grain, thus leading to improved carrier concentrations. [ 26 ] In addition, the passivation of the GBs by Rb itself would lead to increased carrier lifetimes.…”